So as to not alienate the individual webmasters, much of these discussion will likely appear on two separate forums.
I seek to get a discussion going of smaller units as utilized on vessels in the 10-30' foot range.
Originally Posted by Capt J Aside from the shallower draft as mentioned earlier, jets have two major disadvantages to props.
Originally Posted by YachtForums You'll notice as speed and RPM's increased, the efficiency of the pump increased exponentially and quickly caught up to the propped hull.
Originally Posted by Capt J The reason the jets are not as efficient at lower speeds is because they have to be designed to achieve 2350 rpms at full throttle and the impellor has to have the correct pitch to allow the engine to achieve full throttle rpm, also the pump nozzle has to be large enough for that volume of water. At the Miami show this year I took a look at JetPac, a relatively new patented marine propulsion system. Most engine failures are due to over gearing or overpropping, my estimate is that over 90% of boats are over-propped after delivery, most due to overweight, dirty bottoms or bad center of gravity (?C of G?). If the engines have the capacity to be overloaded (as in all prop driven boats) a simple C of G change (like a large load in the front of a bowrider) can grenade the engine in a short period of time, with no verifiable installation fault This results in a full warrantee problem, dissatisfaction, expense and ruined reputation. Although we have regularly experienced similar performance with equally powered planing boats, conventional inboard jet boats have a bow down attitude that robs them of a considerable advantage. Observing this has made it hard to understand the prevailing opinion that jet drives are less efficient than propellers.
A study in 1992 conducted by Naval Architects Donald Blount and Robert Bartee dispelled Kim?s analysis in ?Design of Propulsion Systems for High speed Craft? (Marine Technology, SNAME, Oct.
3) The lower thrust line also makes the boat more stable by cutting down the boat teeter caused by directional changes of the nozzle and this would reduce wandering at all speeds. 4) Inlet size would be reduced; this would enhance the efficiency of the boat by reducing the hook effect caused by putting a large hole in the most critical part of the hull. It starts with the use of an automotive engine because most of us find that our automobile is ready to go anytime we need to use it. A propeller puts a heavier load on the engine if the boat is heavier and can easily overload the engine leading to premature engine failure. Water jet systems typically are more reliable than propeller systems because they are less complex and the engines are protected from overloads more adequately. JetPac? can be an attractive choice for you if your family goes with you in the boat or it is a commercial boat. Originally Posted by YachtForums I have no problem with the preceding hypothesis or statements.
Let me give an example? in IHBA Drag racing (using jet boats), when the pump unloads at high speed, you?re essentially driving a kite. With their exposed, prop driven counterparts, when the prop unloads? the downforce on the hull remains constant.
Inboard water jets, while having larger diameter jets and moving more water, have a disadvantage because the steering nozzle is usually at or just behind the boat transom. In recent years, I think Bombardier adapted Mercury power plants for their jet-boats, but I believe these are also, high output 2-strokes.
My point is, comparing a high output 2-stroke, that is requiring more RPM's (and fuel) to sustain the torque of a large displacement 4-stroke is not a fair comparison.
If it were not for these operating range restrictions virtually all boats would be water-jet powered.
The intelligent inlet duct adjusts to recover the velocity head of the incoming water at all planing speeds and at all throttle positions. This combination of larger jet size, efficient inlet duct, and variable nozzle allows a 50 to 80% increase in low speed thrust, while increasing top speed and maintaining higher propulsion efficiency at all boat speeds and accelerations.
The intake gullet is the recessed area within the hull leading up to the entrance of the jet pump. Originally Posted by YachtForums Not really, although it is possible to mechanically reduce the volumetric area of the cavity without having having an adverse effect on laminar flow.
Originally Posted by Codger There was a variable intake for waterjets in development during the mid 90s. As speed increases sufficiently, water can not make the abrupt turn into the intake gullet as quickly as it can at slower speeds.
Of all the components that make up a jet pump, the venturi is by far the most critical component in dimension, shape and size. My electric powered personal watercraft prototype was commended by the 2005 International Concept Boat Competition, but it was powered by trolling motors which require too much draft.
It appears that electric power is not sufficient enough to drive jet pumps and they don't work well at low speed anyway. The craft currently drafts only 5" of water and I would prefer to have propulsion that will work in the same depth. If the width of the wheels are about the same as the depth of the tunnel, and the top of the tunnel is below the waterline, it should not cavitate and should have propulsion wherever the the boat can draft. The exiting size of the venturi's orifice is generally half the size of the dimensional area of the intake gullet footprint, or a 2-to-1 reduction. Increasing the venturi's expulsion size will decrease backpressure, and allow water to be processed more rapidly, thus moving the hull (mass) forward at a faster rate due to more available thrust, but sacrifices top speed because of reduced compression. The real reason that an adjustable venturi is necessary and holds so much value is that because pumps do not run fully loaded at higher speeds. Since it was a pliant type of material, was there some upper range of HorsePower that might limit the utilization of this particular material?? Originally Posted by Originally Posted by YachtForums One of our first VGV?s was an adjustable venturi that utilized inner bowl ?feathers? actuated by an aperture that closes concentrically.
Originally Posted by Originally Posted by YachtForums A properly designed venturi can yield significant acceleration gains and top speed gains.
However, current generation jet pump configurations and placements are the real limiting factor. A more important area to examine for now, regarding current generation impellers, has not been addressed by any of the manufacturers. 1) The pressure differential between the area located immediately in front of and directly behind the leading edge of every blade at the root. One exception is the jet drive, though, where the propeller (actually impeller) is so efficient because it hardly has any blade tip losses. It certainly appears as though the rim-driven propulsor concept has a lot of potential as a component in a jet pump drive system??ie, a rim driven impeller (?a new era of impeller design? as noted by YachtForums). One variation of the rim-driven impeller itself might consist of a fixed-bladed model where the blades were ?fixed? to the inner circumference of the ?rim cylinder?. Over-lapping blades are one of the most beneficial ingredients in pump efficiency, along with radial edge (swept) blade designs. So I assume these fixed blades might best appear a bit more elongated or ?screw like? to obtain some overlap? My next thought went to why not add a progressive pitch to these fixed blades, so that the water was accelerated some additional amount. The rim-driven impeller may offer some alternatives to this equation that might allow for progressive pitch of the blades.
But wait a minute, both of those solutions restrict me to only processing the same volume of water that I would with the non-progressive pitched blades.
3) provide a path of less resistance for ?excessive? flow that the inlet gullet might have allowed, thus cutting down on resistance to the boat?s forward progress, and making adjustments to the inlet gullet volume less critical. Besides the hydrodynamics of the situation, the rim-driven impeller obviously offers a lot less potential for fouling at both its hub-less center and at the zero-gap, blade-to-rimwall area of a jet pump unit. Location: Nova Scotia Thanks Ian , I would gladly give that prop a try if you think it is appropriate for my boat.
Location: Nova Scotia I brought the the frame down about 5 inches using short pieces of square tube and hose clamps.
Lots of noise compared to a sea kayak so I can see myself making some sort of enclosure for the drive stuff as well as keeping a little of the splash away, mostly a noise thing. Originally Posted by GMR I brought the the frame down about 5 inches using short pieces of square tube and hose clamps. If you add a diagonal brace to your original shaft support design, it might hold the prop in position. Then I switched to a 16x14 pattern prop and found that with one person on the boat it took a lot of work to keep anywhere near the cadence I like and I don't know how long I could do that. Scrounge parts from used bikes at Charity donation stores, angle grinder gears, model RC props, electricians flex drill spring steel drive shaft or 10mm Al etc. When you added a heavy deck and a passenger you dropped your speed and increased your required thrust. Porta yeah can get my hands on all the other parts, just that gear to drive shaft ( sorry again not sure what to call it haha. I am going to do some testing using the second chain ring (yes, 42 teeth)Coach and try using the larger props. Tom, when you talk about trimming the trailing edge of the prop, do you have a suggestion about what persentage to remove? Should have the lowered frame extentions done tomorrow and I don't think I will bother with plasic bushings until I test them. My gearbox is a 90 degree unit with a 2:1 ratio and half inch shafts that I purchased directly from Mitrpak.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals.
Surface sterndrive systems for workboats, pleasure boats, cruisers, high performance and raceboats made in Germany.
Location: San Francisco Bay Area Prop size calculator I'm thinking of purchasing and re-powering a 1977 31' Searay Weekender. I'm thinking of going to 320 HP EFI Mercruisers, also gas, and am wondering how I would figure out a couple of things.
Is there a way to figure out what changes to the prop I would need to make with the new higher HP? Location: Bridgewater NS Canada Generally, the higher the prop rpm the smaller the prop diameter, and the less efficient the prop. A quick check using my proprietory software indicates that the boat at 22 kts is running with about 2 x 150 hp. Location: Bridgewater NS Canada Craig, as you probably know, there are a ton of factors involved in getting a boat to run at the peak of it's performance capability, and props are just one of them. I presume that you have trim tabs mounted on the transom; these can be a source for as much drag as they are benefit. Also, the condition of your bottom is really important - it should be clean & smooth, and all edges should be quite sharp.
Just got a fresh bottom job this week, and the performance numbers were from the ride back to the marina Wednesday.
Originally Posted by 67-LS1 I'm thinking of purchasing and re-powering a 1977 31' Searay Weekender. Note that in resistance calculation the power on the graph is delivered power and in your case it is nearly breake power.
I calculated all three propellers for max diameter you measured, as bigger diameter results with better efficiency.
You mention that the optimum RPM is 2300-2600 RPM and the prop is close to cavitating at 2800.
Location: Australia Still a fair bit of extra weight, and a different shaft angle , plus the prop tunnels, will be a different boat.
Some people say you can not run a prop close to the transom and you need a long shaft to clear transom disturbances. I think this may be a case where abusing one particular person, namely, the best prop guy you know, will get you there without messing with the hull. Location: Beaufort, SC and H'ville, NC The 375hp TA was an option as well at some point in time. A 32 foot boat weighing in at 8500 seems light.Might be worth confirming Barry, not the same boat. I've gotten a picture from a eBay seller that shows the underside of one of these pedal boats, so now I have a good idea how the hull is fashioned.
I've gotten to thinking that maybe steering could be more efficient and the pedaling mechanism made more stable (higher quality design), so that the plastic (or whatever hull material) and bearings almost NEVER wear out. If you want an efficient boat, you will have to build it on your own like the rest of us here in this thread. Location: kent UK I have long been a bike rider living by water, i have always wanted to combine the two.
1.the prop shaft will have to leave the sprocket at an angle so that it can exit the hull in the water.
If you feel uncomfortable using a steel ball bearing at the stern, there is a company selling affordable ball bearings in Delrin with ceramic balls.


The sidewinder is a simple way of doing a pedal drive system on a boat, that is if you want to have a crick in your neck. Rick did make a 90 deg gearbox using magnets instead of gears, was heavy and slipped a bit under high load.
Location: Amersham bucks uk Waveless Concept I am always searching for the perfect pedal boat which should be fairly fast,manuverable, light and easy to transport. This initially appears to be full of new ideas but is in reality is a small pedal boat application of a displacement glider, that are normally motor driven boats around 10 m long.
With the upper hull section similar to a planing hull, at maximum speed would this boat glide on the water surface. The short boat length would help achieve planing but can this concept achieve the impossible.
Having posted on another thread most feedback indicated that this is most likely a semi displacment boat and possibly could be a planning hull. Guest625101138 Previous Member   Do you have any idea of the weight of the boat? The old engine maxed out about 2000 rpm never got to 2500 rpm and this got it to max hull speed.
On prop I can only go to 14'' max no more room, but Im still confused as to whether it might get into a semi displacement position with the the new engine, give the shaft rpm at 1620 . Originally Posted by Gilaroo Rick, it's about 1400kg fully loaded That is much lighter than it looks.
Location: Ireland Thanks Rick that's a bit more encouraging, I'll talk to the prop supplier also to get their advice on the prop size they would recommend. I spoke with an experience prop shop in the UK and they are recommending a 14X11 prop, so I think I'm just going to go with that and hope I can pull 1 or may 2 knots on the displacement speed of 5.35K.
For the money Ive spent on this little boat I could now have had bought a small 18-20ft new fisher boat with everything spanking new, but there is something special with an old wee boat and the unique challenges it offers + you learn a lot more hands on stuff which is all part of the fun. Location: Finland How did the old propeller change from 12.78x7 to 14x17??? A 14x11 propeller will be somewhat more efficient and less prone to cavitation, but the thrust will not be over 2000 N, thus not a dramatic change and I would not expect a dramatic change in speed. Frosty Previous Member   If its too big and too close the the hull it will damage the hull and will vibrate.
As for the new prop , well clearly will be the same as old prop so keeping 14'' makes sense to go with. I sought to try and put most of the key words in the title such that a web search of this subject matter would pick it up.
I didn't spend that much time in their booth, but I done a little research since I got home, and I present here some quotes from their literature and website that might deserve more discussion. This opinion has been accepted as gospel for many years based on an analysis ?Hydrodynamics Aspects of Internal Pump Jet Propulsion? (University of Michigan, 1964) H.C. This means the jet drive shaft has to be higher than ideal because of the engine crankshaft height. These approaches are unacceptable to us, because, it has been established over many years, that a larger diameter, slower turning, low-pressure jet performs much more efficiently. This is a major part of the outboards performance advantage (a bracket increases this advantage). This advantage can be lost if the water jet components are made of aluminum and are more subject to erosion, corrosion and wear.
One drive setup (propeller, gear ratio, etc) usually cannot achieve maximum performance across the full range of speeds in a specific boat.
We have compromised top speed (usually by a few mph) to give you excellent acceleration, powerful towing capability, very attractive fuel economy, and high reliability. Two cycle water jets are not as responsive as any of the other systems because the small diameter water jet operating at high rpm and pressure does not move enough water to provide crisp response to steering changes.
It presents a predictable load to the engine and that load does NOT change with changes in the boat.
A jet driven boat CAN turn faster because it does not have an appendage protruding beneath the surface that will hold a designated track. In either case, jet-pumps need torque to create pressure, not RPM's, however both must be present to reach reasonable levels of efficiency.
Although they are fast and maneuverable, their initial acceleration is so poor they can barely pull water-skiers out of the water.
And, as the inlet duct becomes more efficient, it increases pressure on the nozzle, which results in higher flow through the system. As a blade rotates, it pushes water back (and outwards due to centrifugal and accelerated force).
To further enhance velocity, water passes through the venturi before finally exiting the pump as thrust. I've done a considerable amount of analysis in this area and it yields great improvements in efficiency, but the mechanical means of altering the gullet would not prove economically viable. All that I saw of it was a paper-napkin drawing during a conversation that I probably shouldn't have been having. In the end, we developed an inner liner that was positioned on the top of the gullet (and only the top). It seems too logical and I can't find where any one is using this, so please tell me where my thinking is flawed.
Quite simply, the venturi is a reducer or compressor, and in the case of water, which can not be compressed, it is an accelerator.
Decreasing the venturi's expulsion size will create more backpressure, which results in less water being processed, but increases the velocity at which it exits. The reason that I mention flex is because it is conceivable to utilize a material with built in flex to accomplish some desirable characteristics.
While it was mechanically a very cool-looking contraption, much like the afterburning tail-feathers on a fighter jet, it was hydrodynamically incorrect. This can accomplished via mechanical means and activated by variables in hydrodynamic pressure.
I wonder when, or if they'll come up with an electrically driven rim-drive waterjet propulsion.
Now we know there is considerable work being done on rim-drive technology, particularly those incorporating permanent-magnet electric motor components to power-up the rim. I would imagine it would not be too difficult to design up a kevlar-belt driven unit similar to either of those depicted at Peripheral Journal Propeller Drive utilizing suitable bearings and minimal water seals as the whole ?rim cylinder? and belt-drive mechanism might be contained in a ?water box? as noted in AIR Fertigung product info.pdf. My first thought was that the inner circumference of the rim drive ?cylinder? might be constricted in diameter along its length to compensate for the change in volume between the progressively pitched blades.
How about making use of that that ?center channel stream? of water that would exist in the free space between the inner blade tips of the rim drive impeller? There may be some arguments for variable-pitch blade impellers, but that complicates matters for the ?small jet pump units?.
Building an above surface RING through with the shaft goes allows free movement within the boundaries set by the ring.
Then when I added a heavy temporary 2 x 6 spruce deck and another 200 pound person it got significantly more difficult. I wanted to put a keyway into some of the couplings and hubs so making it the same size as the gearbox shafts makes sense. One, to get it to sit between the frame tubes instead of on top of them which will drop the height another two inches. I can also see that I will need a comfy office chair with armrests and cup holders but I may elect to remove the wheels. Extrapolating this out to twin 255's at full throttle, the boat ought to max out at around 26-27 kts.
This is optimized for cruising speed, so the engines may be overloaded if you try to max the throttles. ChrisCraft is not known for it's stellar peformance, but more as a comfortable family boat. Although the boat is out of the water, I wouldn't be able to get a picture until Wednesday.
You also mention that via the resistance meathod, the most economical cruise is 28-30 knots?
To absorbe more power for the same diameter you have corresponding pitch and area of blades.
And with a diameter of 18", still enough extra area that getting on a plane shouldn't be a problem.
Currently my living room and kitchen are renovated and the boat has to wait for a few more weeks. The sideforces are here limited by the strength in your arm, as waterflow of the unbalanced rudder tries to push it back. Worth, Tx, USA There probably is an answer in the 1623 posts before yours in this thread. I'm probaly biased here but the simplest and least messy way is to use Ricks layout that he has on his V series of boats. Its Ricks V15 design, constuction is of 3mm plywood skin and frames using fibreglass for holding it together.
Propcalc rutines are very simple and have been shown to produce very inaccurate resulsts in several cases.
A 14x17 propeller requires over 16 HP at 1000 rpm shaft speed and delivers a thrust of about 1600 N (with cavitation). From these posts and other reserach I've done including buying the book you recommended by Dave Gerr , my I instincts tell me that a 12'' pitch would be the optimum prop for new engine. Secondly, I did a search within this forum and quite a number of different threads on this subject, but divided out into a great number of individual ones. To me, this indicated the pump may not have been the best choice for the operating speeds of this hull. A jet works great with a turbine because a turbine is designed to spin a certain rpm all of the time.
After this point, they will become less efficient due to aeration and lacking the vacuum to bring water up to the intake gullet at higher speeds. They are actually more efficient in many applications, especially when the craft will be operating at or near their optimum output. I would particularly be interested in Carl's observations as he appears to have lots of experience with 'water jets'.
Kim claimed the efficiency from a water jet is far less then that of a well-designed propeller system. This analysis revealed that a propeller-driven boat will have a hull efficiency of 92%, while the water-jet driven boat will have a hull efficiency of 110% at speeds over 25 knots.
Although jets should be fitted with a reduction to be efficient, most are fitted directly to the engine. Because the outboard is completely over the transom, in a bracket design, the passenger?s location is further aft also, further enhancing the performance. Two cycle inboard water jet systems are made of aluminum and use an outboard power head through a complex drive system leading to degraded reliability. Jet pumps will encounter a greater degree of loading and unloading because they are recessed inside the hull, as opposed to a prop placed underneath the hull.
The forward velocity remains relatively unchanged for the moment and the hull still has the same level of wind speed passing under it. Both of these scenarios, at the speeds these boats are traveling, can result in the boat swapping ends. A greater distance can provide increased leverage, but this does not equate to faster turning.
Jet boats generally don't have an appendage of this type, allowing more slip at the stern and thus a quicker change in heading. They can be designed to either go fast with poor acceleration, or to provide acceleration at the trade-off of low top speed.
Their innovative technologies improve jet performance by up to 80% at low boat speeds, while also increasing top speed, fuel efficiency and cruising range. The venturi works on the principle that a restriction or reduction in line size will cause water to accelerate if the same volume is to be realized at the other end of the restriction.
It used a soft durameter plastic that was flexible enough to pull away from its seated housing as vacuum increased, creating a bubble shape.
This negative pressure zone pulled the skin away from its seated position, thus reducing the volumetric area of the intake cavity, which ultimately increases vacuum and therefore efficiency at higher speeds. The venturi, for those of you not familiar, is the shroud located just after the stator blades (directing vanes) and the part of the jet pump that the steering nozzle or thrust deflectors are most commonly connected to.
This results in higher speeds, but does not give the mass of water necessary for greater acceleration.


In 1987, a very unique material was made available, current regulated (electrical stimuli), that lined the inner walls of a venturi (or bowl) and controlled exact camber and orifice dimension.
The reason is simple, while it reduced orifice size it also increased the rate of compression while failing to control trajectory.
In most cases, engine modifications will dictate the need for an impeller better matched for the torque and rpm available from said modifications. It is the impact of water against the inner liner wall that substantially reduces the over-all efficiency of current single stage axial-flow pumps. Centrifugal force can activate blade rotation and hydrodynamic pressure can control the angle of attack. The input drive shaft would exit the ?water box? to be hooked up to whatever motor was chosen to drive the jet pump unit. Next I thought, maybe some sort of inner hub attached to the blade tips configured as a cone to account for the volume changes. Could this be the source of my extra ?feed water? to fill the void crated by the progressive pitch?? That old prop I've got made of fibreglass is still on offer if you can handle the cost of shipping. I was holding the boat while Jane pedalled and the prop moved at least 8 inches forward as the shaft bowed down enough to touch rocks on the bottom! So I didn't have another 16" prop so I tried a 16x 12 that had been cut down to about 13 inches and that worked much better for two people.
You can use very large electric props which have lower pitch and they don't seem to bend as much, or perhaps switch to large wood (XOAR?). Swapping the 225's for 320's should put the max speed at around 31 knots, which may be on the upper limits of controlability of the hull. Lightly loaded in calm water she'll do 25 knots at 4400 RPM, and she has more throttle left (I've had them up to 5500 RPM) No room for any more diameter, prop shop suggested 20P, maybe with cup.
The shape of a hull has a lot to do with how fast it can go with a given power installation.
They were probably sized as they are as a compromise to get out of the hole and up on plane quickly at the expense of top end and efficiency at speed. Looking at what others may have done in a similar vein with the same hull, would be helpful.
Even if this information is the same as the original factory specs, boats are sometimes modified. These rental boats are built in such way, that part count is minimized and parts are simple. If the bore is precise, it will seal enough and run smooth without too much additional drag. A bike crank with chain running to a 90 deg gearbox then a flexible shaft running down the side, no holes in the hull. However, will there be any chance I can get to a SLR ratio somewhere between displacment and semi displacment situation with 21 Hp to improve on the max displacement speed of about 5.35 knots? It takes only ~4 HP to turn a 12.78x7 propeller at 1300 rpm shaft speed, which is also enough for more than 5 kn speed for the boat you have. At 12 kn the consumptions were roughly doubled, thus 40 hp would probably not get those boats to plane and 20 hp would not get them over 7 kn.
When I get out and about I'll drop a n update on this thread to say what I actually managed and how the ride was. The boat probably did over 5 kn at 1200 rpm (engine) and the rest was never planned to be used. I then used a manual measurement procedure to measure the pitch using a straighline method I found and protractor which give me circa 17 pitch.
But who am I to argue with a guy who has made props for 30 years and has a lot of happy customers ?? Thought I might try to bring a number of these discussions together into one information pile.
When a pump is combined with a diesel engine it is not an efficient system because of the nature of the diesel engine. As speed increases and the hull developes more lift, (riding higher on the water), pumps are prone to inducing more air, resulting in a reduction in vacuum. Kim?s analysis was even reproduced in the 1988 version of ?Principals of Naval Architecture? by the Society of Naval Architects and Marine Engineers (SNAME).
JetPac? water jet components are ALL 316 stainless steel and are highly resistant to erosion and corrosion.
Smaller diameter water jets operate at higher speeds and higher pressures and do not move as much water as larger diameter water jets. Conventional rudders generally offer greater deflection, because they have greater travel than the typical jet pump steering nozzle allows. Again, the hull plays an equally critical role in the performance of either propulsion system, or it?s form of thrust vectoring. Vectored thrust under high pressure, will more than make up for the leverage lost from a further forward exit point. They are more maneuverable than outdrives because the jet outlet is directionally controlled.
These patented methods mark the most significant advance in marine propulsion systems in many years. This parallels engine development in commercial aircraft where high-bypass turbofans move more air through a larger jet for shorter takeoffs.
This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect on the blade face and a negative pressure, or pulling effect, on the backside of the blade.
For instance, a larger vessel with greater displacement may choose an intake gullet design with a more gradual rake leading up to the jet pump entrance. The water seal incorporated at the drive shaft?s exit point would guarantee the ultimate water integrity of the boat in case of failure or slow leak of a seal at the rim itself. A shrouded drive with an expulsion point far aft of a stator section resulted in an unturbulated flow that was hard to detect. I could go with a fixed shaft, or at least a fixed bearing close to the prop so that the motion is limited to fore and aft as the shaft bows (but not as much as it used to).
The gearbox will now sit just forward and below the crank but behind the forward main frame tube. I'd do some searching about (try contacting SeaRay) to determine if this hull runs well at that speed.
Also check your weight balance; if you tend to ride bow-high at cruising speed you may want to see if shifting some weight a bit forward might get the nose down and present a more efficient planing surface to the water. My numbers show that you should be looking at props in the 16D x 19P range to wring out top speed. So I guess I'll go with 20P 3 blade, and if they're too much, the prop shop can always pitch them back some, and forget about the cup.
I can't imagine that they could get away with the 17" diameter wheels that boat has now and the hull won't allow anything larger.
Thus, for safety and other reasons, it is a good idea to verify information here to make sure it matches up with your boat.
The shaft can be either 8mm spring steel or 10mm aluminium, the prop size would depend on the gear ratio and amount of power you can produce. I have to change my prop too as it is Left hand direction while the new Yanmar is clockwise.
The propellers of these stern drives are typically more than the 13" max here, thus there will probably be no gain in efficiency. He made a good point which was not to get too obsessed with the maths, within 2 seconds of giving him the data he was very convinced of a 14X11 as the best option, at the end of the day it's not going to be miles off.
This is compounded by intake gullet vacuum, which is essentially artificial weight? and is QUITE significant with jet pumps. The reason pump nozzles have limited travel is because vectored thrust can hydraulic at the venturi?s orifice under extreme angles and high pressure.
Certainly the mass of water being moved is important, but pressure and deflection play important roles as well. But, historically, the jet was sized for speed and lacked the low speed thrust required for docking and acceleration.
And the water box concept would allow for the removal of the whole impeller drive unit for inspection or repair even while the vessel was afloat. But, worm drives are not good accelerators, because you cannot progressively increase the pitch.
I have a kite that I tried out on dry land last night and it seems to pull quite hard but I don't expect to try it on the boat yet. Experiment using weights such as plastic water tanks laid on the forecabin sole to see if there is an improvement. Given that these numbers are 'guestimates', and you want to get as much blade area as possible in a given apeture, your prop guy's estimate of a 17D x 20P blade is probably on the money. And also it suggests a 14x10 propeller, when I have a 14x8 propeller and I'm not getting the very full rpm. I'm curious, how did you arrive at the stats of jet pumps being 40% less efficient than conventional props? This is a matter of controlling hydraulics and optimizing the connection between the venturi and the nozzle. In this scenario, intake gullet vacuum is not as critical because the weight of the hull (and the depth of the pump) will keep the intake cavity primed. We certainly have agreement from all parties that a variable inlet and outlet can remarkable improve the jet-pump performance!! You may want to look at a higher-ratio gearbox to allow a slower rpm, bigger diameter prop, if there is room under the hull to fit it. Also, when seas are rough, I run about 14-16 kt, turning 3200 rpm, with the boat dragging her tail.
If so, consider moving permanent objects such as batteries, FW tanks, genset, etc., forward a bit.
The boat is 5m long with a 300mm beam and two 2.4m outriggers, another advantage is the shaft can be lifted out of the water and the boat sit flat on the beach.
In contrast, a light, high speed hull that rides closer to the water's surface, may use an intake gullet with a more aggressive rake and a reduced intake gullet area.
Unlike a turbine that can compress air via a system of increased blade rpm and pitch, water cannot be compressed? only accelerated. In that case I could use a u-joint as well but that means the prop will no longer be lined up with the current at any time. Four blade props absorb more power, but at high rpms they run the risk of cavitating in the wash of the blade 'in front' as they rotate.
The bow has to be the opposite of sharp, so that it does less harm in case of a collision or hard docking. This decrease in cavity size, increases the vacuum (or negative pressure zone) at the intake, which helps reduce ventilation brought on by a higher speed planing hulls that operate near the water's surface. And in the case of venturis, that pressure can control a multitude of variables? and this entire process can be executed with NO MOVING PARTS! However, it is conceivable to ?feed? or draw in water at specific points on the exterior of the worm drive to fill the void.
Her most efficient speed is full out (mostly) 24 kt at about 4000-4200 RPM burning 20 gph (all by GPS and Flowscan), which is usually impossible to do offshore. The SR-71 Blackbird used a similar system to draw air into the afterburner, because the amount of air present at higher altitudes was reduced.
By comparison with a Bertram 28 with the same engines, gears, (props unknown), that outweighs my boat by more than a ton.
I bet that if you clean out lockers, get rid of unused items, and swap out a few old things for lighter, more modern stuff that you can shed a couple of hundred pounds off of your displaced weight. These are quite far from this boat, but gives you some idea of the needed power for a planning hull of this size and weigh.
I think 28-30 kt should be about right given the weight difference, and my Chris doesn't have near the deadrise of the Bert. My after body is nearly flat (she's shaped rather like a lobster hull, but with a shallower forefoot (don't know the deadrise).




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