Though the hull speed is just a reference or a value of comparison, not a barrier of any hull form.
Many displacement vessels now have enough power to exceed hull speed, usually with the stern wave above the funnel:-) I hate to think how much fuel they are wasting to get that extra few knots.
In applying this formula you have to use consistent units so for metric, wavelength is in metres and g is 9.8. For any given LWL the hull speed in deep water DOES NOT CHANGE irrespective of the hull shape.
Location: Whangarei Hull speed I do not recall saying hull speed is a speed limit.
This was a study on why multihulls and very narrow vessels appear to dis regard the Froud number. The conclusion was, hull speed, defined as the point where wave making resistance is such that power required to increase speed slightly increases exponentially. Hull speed, as Froude observed it, has more relevance for wide hulls than for slender hulls.
One observation I have made with very light displacement hulls is that the hull speed corresponds with the design speed when the hull is designed for minimum drag. I have attached drag curves for two hulls with the same length therefore the same hull speed.
Point is that hull speed still has significance for very light displacement hulls as it provides the length that results in the minium overall drag for that speed. The curve for the light displacement boat also shows that there is no point where the wave drag is increasing exponentially. Originally Posted by Kerry Thomas Also we can get higher power these days with the same displacement. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Location: Michigan, USA A real drag When you talk about drag, you need to specify which kind. A general purpose sailboat, with limited Sail Area (SA), the goals are to limit drag by reducing, first, frictional drag, then wave making drag, then aerodynamic drag. Most modern sailboats have flattish bottoms, in that whatever curves they have save little surface area over a dead flat bottom.
2.) The flatter hull tends to make less waves when going through the water at the higher speeds that the increased SA allows, furthering the advantage. Naturally, if you are going to go to that extent, aerodynamic drag must be dealt with ruthlessly, hence the low, streamlined houses and the very limited rigging. Wave making drag increases with the square of the increased speed (twice as much speed means four times as much drag) where frictional drag increases in direct proportion to the increase in speed (twice as much speed means twice as much drag).
Location: Arlington, WA-USA generally for the same size sailboat, wide beam flat bottom hulls are designed to plane. Location: Milwaukee, WI You need enough clearance for the rudder to be able to swing. Location: Eustis, FL The wedge rudder is fairly common on high speed craft and nothing new, except to the design I updated after it's first racing season. Originally Posted by PAR Ideally, you want as little if any at all gap, between the hull and rudder top. If you want the least resistance at high speed, then toss that rudder in the trash, as it's not remotely close, for a high speed rudder.
About 60 years ago a Rudder with Gottingen Profile under a high speed vessel designed by Vertens suffered during test trials for the first time with a desastous result under cavitation. The rudder therefore became abrubt overbalanced --100%---and the boat described immediatly a curve and run up the shore.
At first sight one put the blame on the helmsman, but , after repair a few days later it happened again. Further developments lead to asymmetric deltaprofiles , counteracting the propwash 's helice. Location: Eastcoast -The rudder will NOT be like the one in the original picture.
Location: Eustis, FL If that's the hull you're trying to make go 50 MPH, you got a lot more to worry about than the miniscule amounts of drag the rudder brings to the table.
Originally Posted by PAR If that's the hull you're trying to make go 50 MPH, you got a lot more to worry about than the miniscule amounts of drag the rudder brings to the table. This boat is 38' long; being built light as possible (roughly 18,000lbs) and will have 1200 hp.
My question is concerning the rudder, and opinions on how to make the most efficient rudder design possible. Location: Milwaukee, WI An efficient rudder is one that will steer at all speeds. Location: Eustis, FL Agreed, the rudder's efficiency, behind that built down hull is marginal at best, regardless of plan form and to a lesser extent it's section.


All this said, your real speed (I played with these some years back) will come from the fair and smoothness of the hull and the most possible wetted surface reduction, you can get away with. The Gunboat is indeed a quick machine, but it would be quicker, pitch less and float higher when pressed if it had no rocker, the same as the ORMA 60 floats.
Location: Wellington, New Zealand The Wharram Tiki series are notorious for pitching.
All the fastest beach cats don't have much rocker but are notorious for pitchpoling, but then they don't have much buoyancy or lift at the bows. The stick when sharpened would cleave the water, but the long parrallel sides would have a lot of drag due to the wetted surface area, and tacking or turning would be a bugger.
The hulls are cylindrical in section below the water line for almost their entire length.-- Combined with a gentle uniform rocker along their length, this serves to dampen the pitching motion of the boat and with no vee forward forward, she is much easier to tack. To everyone who has contributed, my grateful thanks and keep those comments and ideas coming.
Guillermo, when we have completed the family move to Newhaven, East Sussex, we'll be about 30 kms from John Shuttleworth, so I shall be consulting him in due time. If you base it purely on passage making time, then you have to consider how much risk you want to take, and in what sort of weather. Guest625101138 Previous Member   The 4 tonne 12m GODZILLA hull ends up quite fat. I did a bit of fiddling above the waterline, bow and stern, along the lines discussed in earlier posts.
Location: Wellington, New Zealand Rick, nothing like a picture to express a point of view.
Location: Earth Richard, the usual practise is to retract daggerboards as speed increases. Guest625101138 Previous Member   Some good observations and question. Pushing a 4 tonne hull to 20kts takes reasonable grunt even if the hull is optimised for it.
Anyhow the keel needs to be quite strong and I did not go into detail on its stresses just made it look like a section commensurate with the forces.
I thought your VPP software was limited to propellor power, but obviously you have progressed since then. Could you give me the name of a good VPP program so I could look at sticking a rig on my design?
Location: Florida Cover the existing boxes with 12 mm plywood lids, this is probably what they used to have. Beef up the transom and make it strong , scuff out the inside with a grinder and reglass all the stuff thats inside the hull , 4 mm Ply over the tops ,glass both sides so it will last longer . Find a mean little 9.9 johnson or evinrude ,set up the motor to a good height ,do some measuring when you do the transom to make sure you can mount the motor at the right height , extend the tiller handle about a couple of feet with a piece of plastic pipe , then buy a really good life jacket and a pair of diving gloves , mount on the gunwhales each side a pair of good strong grab handles and you will be flying for sure !!.and i mean flying !! Originally Posted by Paul A Cover the existing boxes with 12 mm plywood lids, this is probably what they used to have.
I have a similar boat we build and sell as a one man boat with 30lb Minkota transom mount trolling motors and they do not sell well due to lack of stability. It is a stripped bass boat hull with the upper casting decks, holding tanks etc etc removed that make boats expensive and impractical if you ask me, although it looks cool?? It is light weight - 155kg without motor - since it has no framing, stringers etc but solid poured in closed cell PU foam sandwiched between the hull and floor, making it immensely strong, solid and unsinkable. In the original post you envisioned two elevated fishing seats and no apparent reservation about engines up to 10 HP. Location: Florida Just because you've had therapy over early boating activity in your lives, doesn't mean you should deprive the same from another, looking at the same path in boating life. Location: Sydney, Nova Scotia, Canada What I would do If other tests did not work out well, this is what I would do.
If the fiberglass boat can be put in usable shape easily, I would try any type of square or round pontoons.
I find it hard to understand the implications and the usefulness of this chart since whilst sailing one is always striving to align the sails correctly to the apparent wind angle except perhaps for angles above 150 degrees.
Conversely the drag created by the hull and the mast is shown to be constant whatever the angle yet surely is must be a lot higher when the wind is coming from Broadside than when it comes on the nose.
Location: North Texas How does the text differentiate between induced v viscous drag? It looks like induced may be a function of heeling while viscous drag is that of the surrounding water the boat's wave system is pulling along with it.
Location: Port Orchard, Washington, USA I think you need to re-read the text to get you head around the vector directions of apparent wind, lift, and drag. A vessel is nothing but a bunch of opinions and compromises held together by the faith of the builders and engineers that they did it correctly. IMP-ish View Public Profile View IMP-ish's Gallery Find all posts by IMP-ish « Lead Paint???


It is the speed where the boat speed matches the phase velocity of a wave having the same wavelength as the hull waterline length.
The velocity corresponding with hull speed results in rapidly rising wave drag with wide hulls.
To determine this I just load the hull coordinates into Michlet and it gives me the answer over my selected speed range. Such hulls can surf when running before large waves and, if extreme enough, can even plane like a powerboat.
So even if a flattish hull had twice the surface area of a deeper rounder hull, which, by the way it usually doesn't, it would have only 33% more drag.
Of course as Gonzo mentions, hull shape can play a big role in this, but a bustle or pad can be employed to permit the rudder to sweep the bottom at the same distance. There's a few trains of thought in this regard, but a recent race boat I designed, has showed a marked improvement with a new wedge style of rudder, I drew up, as a replacement for the more traditional foil section. You have a huge deadwood assembly or built down hull, either of which will generate tremendous amounts of drag, compared to the rudder. At 50 MPH, the wedge will start to rear it's head, but with these flow numbers, you'll still need a bit more speed to possibly justify the wedge. Everyone I know that designs these puppies, has a specific set of angles they'll employ, based on several factors.
Most of these puppies are on the verge of longitudinal stability at these speeds, so control is pretty import, but there are many other factors that can make a slightly slower boat win consistently, simply because it gets up quicker, turns more precisely and accelerates faster. Exchanging information this way is about the rapid evolution of new designs, which benefits us all. Want to build a Trimaran » Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Search this Thread Advanced Search Similar Threads Thread Thread Starter Forum Replies Last Post Underwater viewing pane in hull ?
It's a planning hull and a 10 HP outboard will make it run well, don't be put off by the negative posts, you're the one that has to live with your decisions. It doesn't need a casting deck that's 125 mm taller then the bottom of the boat, it doesn't need built in ice boxes or other devices, it's just a little boat that will blast along just fine if you keep it light weight, but will wallow like a tub if over loaded. I had noticed a small, about ten foot, jon boat with a tall seat, a big motor and a fat man. If that were the case I'm not sure why values at all below 30 save for trying to deal with tacking through the wind.
Simply put, a good foil section on the same plan form will likely be on the edge of cavitating, while the wedge will just be coming effective. It will be fun with a 10 HP on it and you should try it, but I'd try to find a 2 stroke to save weight on the stern. She goes like clappers with only a 15hp outboard and with that motor you do not need a skipper license or COF all any of the money making crap the new boating laws requires.
Clean out the interior, put thin decks on the boat that will enclose all but a small place for the driver. Are there any advantage or disadvantage to keeping the rudder as close to the bottom of the hull as possible.
It's true that the boats that turn the fastest, typically win, but there's usually more to it than this observation.
It should result in an underwater shape with gentle rocker rising towards the stern and stem.
I do however already own a kayak and a float tube and have been on many boats, deep sea and on the dam. Try to stand up straight in that boat, or just sit and turn around and I guarantee you bad moments. Some times people are unfairly taken, but I've noticed that more often then not, these are people everyone else admits were aiming directly at challenging mortality in most of their ventures. It may roll in any direction as you would expect, but pitching is also about being rolled back in the opposite direction by same wave passing under the ball. What I looked at was two winged sails located inboard of either hull as I can produce meaningful design forces. If you have a portable power saw, then use it to destroy that boat, if you do not have such a saw then borrow one and destroy the boat.
I was obvioulsy not going to put a high deck, just one at water level with short swivel chairs. Bungee jumper dies in PWC incident, local sky diver drives boat into bridge, ex-navy seal breaks neck attempting 60 MPH turn in overpowered Jon boat, etc.
I don't know if the boat was made by Barracuda boats, or if the name Barracuda was a model from a boat manufacturer.



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