FYI: I've started a sister thread over at the Hoverclub of America, where you have to be a member to participate. We compare hovercraft designs from around the world and analyze them for strengths and weaknesses. These efforts may lead to a description or list of desirable features in future design exercises. If the hovercraft bag side skirt is made to be of such high pressure that it acts as a inflatable pontoon - or RIB (rigid inflatable boat), then it would act like one, right?
The wave impact on a RIB is turned into mechanical energy by the air-spring effect which then allows for higher speeds, right? NOTE: My design started out as a catamaran type as above, however it soon became apparent that it should be develop into a tri-hull because of it's pointed bow. I have chosen the above designs because they seem to hold the most promise for rough sea conditions, and yet are still fully amphibious.
Location: Michigan Wow, I finally got a bite on this thread, kind of like fishing in a boat. When I was 13 years old (1973) I saw the inablity of the traditional hovercraft to overcome tall obstacles (as viewed by me) as a serious short coming.
In retrospect the mass and weight of the craft never changes, so my partially undeveloped brain (unbiased mind) and lack of understanding eventualy grew to understand this (I hope). Location: cornfields On the helo, I would worry about the relatively wide top surface of the vehicle taking a lot of the pressure. Note: These landing systems were very high pressure, operated with tiny holes all over the bottom of a closed bag skirt allowing escaping air to lubercate the path. Howe was testing a new air-cushion landing gear (ACLG) developed by Textron's Bell Aerosystems Co.
Bell's ACLG permits landings on the most rudimentary runways and also on ice, water, sand, swampland, and terrain dotted with obstacles, such as rocks half the height of the inflatable bag.
Originally Posted by tom kane ,and safe form of cheap transport to take the family for a picknic.The roto system is not the usual helecopter design and is simple and the mechenics have been used for years. Location: Michigan Tom, I don't want to try to change your mind on your design - my only intent in mentioning the gyrocopter is that is one of the cheaper answers to heavier than air flight.
The reason I like to make similarties between different designs is to learn what works and why. Looking at your project with a more critical eye, one may ask; What is the problem you are trying to solve? If I could project my own personal thoughts into this - just do it because it can be done, and has not been done before!
Well, that's good enough for me as long as it's not a cash or time hog impending on other worthy projects. In a practical sense your design even on a good day has limited applications, but then again so does a basic hovercraft. In the end, I can't see large whirling blades of a low flying, not very manuverable craft being safe or economical - sorry.
Hover people are a tight nit group (so few of us) perhaps you could get a tour or free test ride.
One thing that I know from watching the Universal WIG craft is that the constantly inflated skirt (while in flight) is a huge safety feature. People always say helicopters are more manuverable than fixed wing craft, I have to agree as I almost lost my lunch (felt sick) in a helicopter when I was 16 years old. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. I will put some camber in the deck, but with the program I'm using right now consistent curves are a challenge. I know this is going to have the displacement of a brick, that's somewhat intentional to maximize both interior space and deck space. Location: Bellingham WA What are the relative advantages and disadvantages of this kind of a hull form?
Location: Flattop Islands For a given structure and load, the more square the sections the higher (shallower) the vessel will float.
Cold-molded construction (this can take various forms) is a relatively lightweight construction method with commensurate increases in cost to achieve high strength with low weight.
Obviously the deep vee sections mean a narrower cabin sole but a lower VCG, (better high angle stability) It also means tanks could get pushed higher (outboard) so there's a tradeoff.
Until this is clear, it seems a bit soon to talk of cost, construction hard chine vs rounded hull - or even the need for a custom design in the first place.
Originally Posted by Tad Cold-molded construction (this can take various forms) is a relatively lightweight construction method with commensurate increases in cost to achieve high strength with low weight. Location: Bellingham, WA Do not forget that safe and functional access for entering and exiting the kayaks is critical.
Originally Posted by dcweed Do not forget that safe and functional access for entering and exiting the kayaks is critical. Indian communities would often have a narrow ditch looping from a river and running under the palisade of their village and back to the river.
Cold-molded is expensive because epoxy and bronze fastenings are expensive, and good quality wood is not cheap. As the man hours in steel can be far less, total project hours are far less and the cost (rented buildings, etc) comes down and you get your boat sooner.
A recent review from our friend Brent Gaskill at OnshoreOffshore Magazine does a great job at explaining our unique stepped hull design. The original 24 began with an idea jotted down on a napkin that eventually was turned into a wooden plug, from which a mold was laid up. Despite whatever Atkin claimed as a top speed, I would say that flat-bottomed hull would be horrible at 18mph.
There's a fellow up in the Boatbuilding section that just launched a Handy Billy and he's got trim problems at top speed. In three hulls we can see an illustration of the lineage of powerboat hull design; and the search for speed and handling in a sea.
I believe the hull form shown below would run fairly nicely with moderate fuel efficiency in the 18-25 knot range in a moderate chop. Personally I will add that I think you guys that are professionals in the boat field ae by no means using the board for self promotion in submitting hulls and boats in line with the discussion at hand. So are you telling me that things have changed; that we have learned; that science has advanced in the last 60 to 80 years? Although we are possibly skating on the edge of self promotion, still safe but getting close to the edge, I must say that information provided by all submitters is valuable.
New designs using modern thinking adapted to modern needs and available technology will indeed produce an ever broadening range of choices. I fully agree with Tad?s statement ? but I can?t help but wonder if that same statement isn?t true for most every pleasure boat regardless of its hull form ? displacement, semi-displacement or planning? And I believe that RonW and Pipefitter and perhaps others have touched upon this theme of where will Jeff spend most of his boating time so as to get the most enjoyment out a particular design. The question in my mind is whether or not one is willing to make compromises on the 80% usage so that the other potential 20% can be accessible? Certainly there have been leaps ahead in technology and its application, but aren?t the vast majority of design improvements merely incremental?
Regardless of the specifics, displacement hulls need to be (more or less) a certain shape to go through the water efficiently. I have no formal background in NA, but I'm surprised that there hasn't been further study of tunnel stern designs, ala RM. The tunnel hull looks like the void is filled with water as it moves forward, with little difference in speed between the fluid in the tunnel and the surrounding fluid. Burmese teak sandwich core with West epoxy and varnish (11 coats total), glossy like a piano. Is there any reason a "lobster boat" top (closed on port with sliding windows, open on starboard with canvas) couldn't be used?
To my knowledge true 3D CFD (computational fluid dynamics) computer modeling of small powerboats does not yet exist. There has been a great deal of tank testing of larger power hulls, fast ferries, military vessels , mono and multi-hulls.
The best tool currently available for comparing a typical vee bottom with a tunnel hull would be the test tank. Location: Bellingham WA Hull forms and design theories Now that I know what I want to go in it, I'm looking for advice on hull forms for my Kayak Tender project. I'm going to get a bit philosophical here in trying to explain why I'm making the design decisions I have, so please bear with me. The way most recreational designs get around this is to only have the cabin be full height in the center and create a narrow walkway around the periphery to create a more manageable freeboard (See image 1). On many commercial bots deck space, either for working or cargo, is more important and so they have a continuous flat deck.
On the kayak tender, deck space is important because of the need to securely store and handle up to 4 kayaks.
In my current working design, the spaces are stacked in such a way that one occupied space is above another. Finally, for you NA's out there, what would be the ballpark cost to convert this kind of a design concept into buildable plans? For discussion's sake lets say you can get this boat built for 100k (you can't but lets just say you can).
Location: spain Good way to start your design is to purchase a set of plans now. Originally Posted by michael pierzga Good way to start your design is to purchase a set of plans now. I've looked at the Selway Fisher designs and while they come close, they still have that cabintop - walkway design.


Intended voyages is inside passage with the possibility of near coastal to California or Mexico at some point. Beyond just plan prices though, what is the process for coming up with a hull form for something like this?
If you went to the design office of designer like Tad, almost certainly he would recommend suitable stock designs from his catalogue for your review.
The designer will be able to identify what you want, propose details that you have not thought about and recommend cost effective building methods. Do you lay awake at night thinking up absurd comments like this, or are they just the whimsical exasperations, of a clueless, inexperienced mind?
Location: spain Sure, a hard chine, plywood , epoxy sheathed hull form is easiest to build and design .
You need sea worthy craft with big deck space for handling small kayaks and a boat that is easily powered. Id go for a hard chined narrow forward hull conceived like an aircraft carrier, build it lightweight and power it with a 4 stroke outboard to eliminate the wasted interior volume space of an engine room and big fuel tanks.
Use a stubby keel to generate stability and Perhaps water ballasted for on station duty, then light for transit duty. Originally Posted by michael pierzga Sure, a hard chine, plywood , epoxy sheathed hull form is easiest to build and design . That said, I'm going to see if I can squeeze a couple more feet out of the beam to bring it down to 10'. Would be great to figure out how to eliminate the inboard engine room and go 4 stroke outboard or a standard IO drive leg aft. Location: Southern England Hummm, I could see what was being got at with the takeoff analogy, but it's easy to look at it too closely.
Vertical (sharp) bows undoubtedly are better through waves, and are also likely to lower wave drag (though it's difficult to quantify it directly). My Vaka has taken the humpless transition as far from the 49er onwards as the 49er has taken the humpless transition from a 420 dinghy.
Sure, a hull with no balancing keel that needs to be kept upright and sailed flat, by crew movement and outboard weight, with a length to beam ratio a bit greater than that of an International Moth is too radical by far for a monohull dinghy. This does not matter for a Moth as all the competitive ones are fully flying now on foils. If sailing winds were consistently greater than 15 knots and steady, then who knows, a planing Moth hull might be much quicker than the canoe like shapes we see in the Moth class now.
The sail can be sheeted and rig rotated just with the mouse, or parts can be fixed in place.
Fast boats with good transition have narrow topsides, rounded underbody at the bow, straight buttock lines and if narrowed aft need sharp chines. I think these are a step ahead of the **er, 18 and 16 designs, which are now over 10 years old.
Not withstanding all of the above, the fastest course racing small monohull is still the formula sailboard, which is a simple flat bottom, short, wide punt, but very light and with plenty of sail. I'm looking forward to getting my NS on the water and testing out this humpless thing for myself. It would be un-true to say that there isn't an increase in wave drag as one approaches the displacement limit (though this limit varies). I'd love to see good tank-test data on some older and newer dinghy hulls by reputable engineers, to see what these values actually are. Location: AUSTRALIA Hi Phil, thanks for your valuable input, but just as you talk about misconceptions in some of the earlier postings, so I detect some very minor distortions in your posting also. Going back to boat hulls now, classic displacement to planing transitions that have to cross over a hump are no longer always the case.
Now about the DC, particularly your design, which I happen to have a lot of admiration for, both the construction process, and the hull shape.
It has plenty of flattish buttock sections; the fact that it has a canoe stern does not detract much from it's planing ability. I agree 100% with your analysis that narrow topsides in the bow area, with rounded but not v'eed bow bottom sections are the ingredients for easy transition to planing, and this is exactly how my vaka is designed for my tri. Sharp chines to get clean break away of water flow in the stern half of the hull has been well recognized in racing sailboard design for at least 25 years.
That's sort of the way Julian describes it, although other designers will point out that a boat with a low DLR inherently has a smaller hump. Nothing comes for free, and some people feel that the Bethwaite shape suffers from too much wsa and is a nosediver, because of this shape. As Phil says, there are similar humpless boats around and many of them have a much higher DLR but may compensate by more rounded sections under the bow to reduce wsa and create more dynamic lift. One thing about the Formula boards - as soon as they aren't planing they're about as fast as a Heron, so they have a massive hole in their performance. Location: England Low DLR has multiple benefits - long hull has higher 'hull speed', light hull produces less waves (drag), light hull planes more easily. No-one has discussed much about rocker so far except to say that an efficient planing hull needs fairly straight tail rocker.
What if you had a cat that was a little beamier than the the usual (F18 or Tornado) at a similar length, and had flat keel line aft, transom width almost the same as max.
Does any one believe that this hull (two of them, its a cat) will be planing when it is travelling at over 20 knots?
Furthermore because the forefoot is roughly at the waterline because the rocker is in the front half, does any one who agrees that such a cat will be planing at 20 knots boatspeed, also believe that the hulls will not need to jack up to a higher AoA, when the transition from diplacement to planing occurs?
IMO such a transition must occur as my firm belief is that when the cat is travelling at say 8 knots or less it is purely displacement sailing; but at 20 knots it is clearly planing. Leo: On further research, I think my comment about skin drag being linear was just plain wrong.
The narrowness, if extreme like narrow moths and cats, results in inadequate volume and planing area under the load area amidships and they push the bow down rather than lift and plane.
A Hungtry Tiger style moth has a speed limit of about 17 kts, I expect a T or F18 go about 23, Steve Clark has stated even Cogito only gets to about 25 max. Location: Spokane, Wa Chris White did a 15' planing cat, went fast off the wind, wouldn't go upwind very well, saw pics of it planing, bow wasn't up much, if at all, was really rolling along on it's own spray. On one hand we have myself, Chris Thompson, Phil Stevenson, and Paul Scott and one or two more talking about practical design features and how they might impinge on some performance parameters. Then we have a few others notably Sigurd, Tim and Matt that have become more and more mathematical and theoretical in their discussions.
Location: Spokane, Wa frosh, if you add us all together, we almost make up one whole giant online brain!
Also I have many sketches and concept models which may never get built, but are still fun to theorize and dream about. So even a non-displacement hovercraft should be able to deflect more of the wave energy impact, or so it follows to think so. I will post model photos eventually, but want to explore concepts of others before picking away at my own work in public. Not that should stop you from having fun with a novel idea, always something to be learned. I mean the hovercraft could not crush or roll over things that got in the way like a heavy tank could. I don't know if they landed on water, they did land on ice & snow, and they did traverse puddles along the airfield with ease (and some spray).
Deflated in flight, the ACLG hugs the bottom of the aircraft without causing aerodynamic drag. I feel that if one only looks at the differences the larger picture gets lost in minor details. However horsepower limits (5 hp) are very common, hard to get a nice stretch without hitting a dam or low bridge in most cases.
Form strictly a design study viewpoint you may want to address some of the day to day operating problems I've described in my hovercraft experiences. Under what circumstances do kayaks need a custom-designed supply tender, where the kayaks will drive the design?
Nobody has even mentioned - unless I missed it - what kind of kayaks or what the kayaks will be used for, and with the range of hull forms and sizes found in kayaks that is crucial. The following is an excerpt from the fantastic article from Brent who also runs a successful guide service outfitted with an Avenger Pro Series. When release points are created, as with a step, the water breaks free of the surface allowing the craft to speed up creating efficiency. To get WoodenBoat delivered to your door or computer, mobile device of choice, etc, click WB Subscriptions.
In the envelope they are great, one side or the other they may be reasonable, too far outside and you have trouble. There have been major changes in naval architecture, construction methods, materials, and the availability of lightweight reliable horsepower.
We all visit and contribute because this is a community of like-minded people, folks who care about what we care about.
One has to think that the navy has looked at many, many designs for applications in warfare. We all know the advantages to testing a design concept before ever setting tool to material.
Whereas the fluid movement in the tunnel stern (RM) model might cause a significant venturi effect, perhaps actually sucking water into the prop area; with whatever other effects this has on the overall hull performance.
Some CFD work is done for sailing yachts at the Americas Cup level, but there is no interest in it for small powerboats. Once you finalize your concept run it by the naval architect for approval That's what I'm trying to do. What is said is really that there is a transition between displacement sailing (taxi-ing around on wheels) and planing (flying) in terms how the restoring force is being generated.


I personally think it is more to do with the running trim than the hull shape (but obviously hull-shape makes a difference). The other issue is that it is now generally accepted that the 12 inch wide Moth hull does not plane, especially as most designs are pin tails. This does not in any way negate the concept that a Moth hull of the typical dimensions without foils nor pin tail could be built as planing hull. Initial trials based on a proa configuration (non-shunting but tacking) showed huge promise in this direction, although the design was very compromised by the following problems, so I have since converted to a tri.
Too much for the brain and body to adapt to in strong winds, as handling for each mode is almost a world away. This description fits all the latsest fast boats in classes where development has continued without restrictions, NS14, I14s, some 12s, Development Canoes. What you really have is a situation in which the skin friction is a relatively large percentage of the total drag, with a relatively small change in wave drag.
In the first instance, Maybe someone could take data off some of these hulls and run them through a delft-series calculation (at least for upright, no leeway) I think the results could be quite interesting. If it was, then at the moment just prior to take off, a very considerable increase in thrust of the jet engines would be required to get the wheels off the ground. I have been in my round bottomed outrigger canoe that was flying a parachute spinnaker in a mild gale, and I swear that the hull picked it self up, and skimmed over the water surface at a speed that was way beyond what we have ever experienced before in this boat.
The obvious way to do this would be to have a wide, flat planing-type surface Far Behind the center of gravity of the boat, and no similar surfaces up front.
Julian says that he tries to stop the boat rising out on the plane too early and therefore he gains more top speed. And the RSX proves that if you try to make them more of an all-rounder, you end up with the worst of both worlds. Somewhere in between the hulls change from one mode to the other, but I don't think the point at which the change occurs will even be perciptible to the crew! I happen to believe that there were many more break throughs in boat design over the thousands of years before computers, than there have been in the last 10 years since their users have tried to make things too complicated. Any boat gains drag with speed up to its displacement speed, traditionally this is the speed limit, Froude speed or hump speed depending on your perferred expert.
The planing (initiation) speed will also be reduced if the boat is light and narrow, because the buttock lines are straighter and because it has less weight to lift. I've been building hovercraft study models since I was 11 years old, and have been aware of the existence of hovercraft since I was six years old - I'm 44 now and never grew up. I know that the solid sidewall wall cushion vehicles are a form of non-displacement cat, in a general way.
You should know that someone has posted a computer model of a similar concept on the internet - it's blue.
The hovercraft could not fly overhead like a helicopter and escape danger by going over the next hill or tree line. The varaiation on this was out board arms looking like the landing pipes of helicopters which would swing down and hop the craft up.
I did a quick web search and the only references to kayak tenders that came up are for the use in the para above, so I don’t know what we are discussing here.
Or is this just a cruising boat that will carry a few kayaks along for recreational purposes? This provides a super soft ride while also creating lift in the transom, which allows the boat to run extremely shallow on its padded keel. This older hydrodynamic theory has carried over into new technology for the Avenger 24 creating a very dynamic and fuel efficient planing surface.
I absolutely agree that John Atkin's Ninigret may be one of the most perfect boats ever, but only in a particular envelope of construction, material, power, speed, and use. We have formulas for analysis of certain parameters (VCG, the myriad of moments, prismatic coefficient, etc.) but what about the integration of them and ultimate performance without ever having the hull see water? This would be an exceptionally good price for a custom design on a vessel of this class and configuration.
The other notable effect is that because of the reduced resistance in waves, a vertical-bowed boat will be easier to keep on the plane in marginal conditions.
My aeroplane aerodynamic maths is now a bit rusty and I would rather leave this to a real expert.
I once had a 1980 vintage NS14 which accellerated smoothly onto the plane without significantly raising of the bow. Having flown many times, and being very aware of the take off process, (as this is my favourite part of flying) I have never noticed that there was a noticable increase in thrust required at that moment. Sure enough, these new hull designs like the 29er have narrow, steep-sided bows that don't widen out until fairly far back, but are very wide and flat at the stern. However if we look at at the current F18 cats, the rocker is all in the tail, and the keel line in the front half is almost straight.
Then lets have soft chines forward gradually changing to very sharp chines in the rear half. This is because its lines will be straighter both horizontally and vertically, and it will make less energetic waves, and pass through chop more easilly. All the others from Cunningham's 16ft XY, 20ft Attunga, right up to the recent stephull French record chaser were failures relative to their competition. There is a thread somewhere on this site about it - the boys bashed it around a lot - no takers. I know this is a crazy idea, but the weight of the craft is already floating on air (weightless in a sense), how much extra force would be required to gain a little altitude? And so last year, the Air Force awarded Bell a $99,000 contract for wind-tunnel tests of the ACLG. Wide rivers can become partialy blocked by fallen tress and have powerlines and bridges crossing them.
Of course, that was also for fresh water and security - the palisade had gates that could be dropped down into the water to prevent a water-born attack - but it was a neat system.
The sharp entry dead rise provides a big boat feel in open water on a boat that also performs in shallow depths suited for much smaller skiffs.
The ideas were so new that no one knew what to expect until the first boat was floated and tested.
When it is in the up position,the tunnel enables the water to raise ahead of the propeller and water intake which allows the boat to run shallower.Similar concept to Atkins with his half washed prop in his tunnel sterned boats. The sad reality is you'll be very hard pressed to build this boat for 100k and 200k would be a more reasonable figure and still very reasonable for a vessel of this class.
Classical but not so ancient description of overcoming the hump talks of the hull jacking up at the bow, suffering increasing drag of high magnitude, as the hull gradually climbs up onto its own bow wave, and then commences to plane with a very noticable increase in speed. It was typical of the time when the class went for more rounded bow sections and straighter lines. Some narrow stern designs do get a bit squirrely at speed if the lines are drawn up aft, but straight lines and a sharp chines allow any width stern to part the water cleanly and will not retard planing. The lift must be centred midships, where the combination of the weight and downloads from the rig are positioned.
There is no doubt that a Hungry Tiger could eat the Moth design that spun off the Europe dinghy for breakfast, lunch and dinner. I realize that these hulls are not meant to plane and do not need to transition thru a hump whatever speed they achieve. Also lets flatten the roundness of the hull bottom steadily from about the mid point to the transom. Eggington, the system employs an elastic bag made of laminated nylon and rubber attached to the underside of the plane. As it turns out, the ideas on the napkin were off the chart and an exceptional performing boat was created.
There is some tricky little things happening to modern hulls but I haven't really noticed any profound improvement and the quality isn't there so much in the production boats. In which the design fee would be in the 9k range and more appropriate for the level of complexity in a yacht of this scale.
It is quite likely that some-one like Van Essen (the FD designer) described his radical new design this way in the early 1950's. FB has a tendency to ignore all NS14 development after he left the class and developed the Taser. This is why boats with bows which are too narrow or too Veed need the crew extremely aft before they take off, They also are slow in transition because the V shape offers more wetted surface in displacement mode than a well designed rounded section. However even without planing, it doesnt make them slower say than a 29er which is an all out planing hull.
For takeoffs and landings, the bag is inflated through louvers in the plane's underbelly by a fan on board.
More often they fly along side terrified that they can't out run you and exhaust themselves until banking off (your design would chop them up in a worst case).
The concept was developed from seaplane pontoons that included a step to give a point of release from the water's friction. When I see the tunnelized modern boats,the familiarity to the concept comes from different times.Only difference really being is they can use sculptured moldable shapes to make them somewhat more efficient for dual purposes but I don't think it has really evolved beyond the obvious.
Air is forced through hundreds of openings on the underside of the bag, producing an air cushion that holds the aircraft off the ground for silky take-offs and gentle touchdowns. In fact my son and I built a nice moth hull with about 30mm spring in the back and it was distinctly fast in light stuff but had a reaching speed limit a few knots slower than the hulls with straight spring aft, and consequently was never really competitive.



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