Location: Sweden Behaviour of a stepped hull I?ve built a small boat with a stepped a hull. The first 2 m of the bottom from the fore is V shaped with a deadrise of 17? and ends in a step.
The boat is powered with a used 40 hp outboard from 1976, with slightly damaged propeller and several peculiarities that I?m discovering one at a time.
I estimate the weight of the empty hull to be approximately 300 kg, so with the outboard, fuel, driver and some small stuff the weight should be 500 kg.
The boat behaves rather well at speeds up to 20 kn, but at 25 kn in a small chop and no passengers the propeller tends to beat air every now and then. A behaviour that annoys me and I would like comments on is that the boat doesn?t bank when turning. Location: Oriental, NC I am not surprised that the boat does not bank in turns. It's an interesting concept but I wonder how you arrived at the unique design elements in both forward and aft sections. Location: Sweden The idea with the step and the pipe is to create a flow of air for lubricating the rest of the hull, where most of the lifting forces act.
I want the boat to travel well on plane also at low speed, which is the reason for the large flat bottom (low weight per square meter).
The aspects I have considered seem to work as I expect, though the positive effect of the air lubrication is not proven.
Location: Italy (Garda Lake) and Croatia (Istria) The prop is slipping, as Fanie pointed out, most probably because it is aerated by the flow from the pipe. And the boat is turning flat because it was designed to turn flat - though inadvertently, as it seems.
I don't think a long shaft outboard is going to cure it, the cavitation plate has to run on top of the water, long shaft short shaft the same.
Flat bottom boats does skid over the water sideways if you turn hard at speed, and it works, but only on flat water, a single wake even from your own boat may cause the hull to stick and you eiether get throwed out or the boat capsize.
Originally Posted by Fanie I'm sure you will see a slight performance increase if you block the pipe. You could mount a valve at the air intake of the pipe, and play a bit with the valve aperture until you find the correct ammount of air which will not block the step and will not aerate the prop. Location: Sweden When spring comes and it get?s warmer I?ll check the problem with aeration further with someone to help me in the boat.
Originally Posted by daiquiri The prop is slipping, as Fanie pointed out, most probably because it is aerated by the flow from the pipe.
I've seen small steps at the transom for smaller boats, with the idea being that the step would allow upwards flow of water behind the transom more quickly, thus allowing the motor to be bolted up higher onto the transom and gaining some shallow-water advantage.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Those rudders also can give more positive longitudal trim for more lift from the mainfoils while take-off , or in mid-wind conditions. Perhaps sailing in light winds (with the mainfoils up) only with the hulls touching the water up to the steps ?
Under those circumstances the foils can get a bad angle of attack, resulting in a loss of lift, or negative lift which occures in a pitchpole. Remember, the little, short floats of l`hydroptere haven`t had enough volume and could`nt generate enough dynamic lift in the forward sections to avoid the last pitchpole.
Now they`ve made these stepped hulls longer with the planing surface and a lot of volume pretty far forward.
In planing mode L`Hydroptere also could use his rudders for adjusting the trim angle for max.
Racing around the world the maxi will hardly sail in lightwind conditions, so the french don`t need round bilged hulls that are better for lightwind conditions.
The daggerboard of the l`Hydroptere.ch seems to exist only for lightwinds while sailing without foils. I can see that there might be an advantage if the boat was to come off the foils at high speed to have the step there-that's why it's on Hydroptere-but I don't get what their plan is for light air. It`s always a challenge to read between the lines given in their HP, i always got more info exploiting the pix. Location: St Augustine Fl, Thailand Twin Headsail Maxi Version Rather interesting that the newest maxi version for use around the world seems to be evolving towards a catamaran with a central nacelle and an almost masthead cutter looking rig. And there is still the 'Power' Analysis feature that gives all the performance data points PLUS Acceleration curves and Elapsed Time maps.
And see it all in Comparative Graphic Trend Charts, or detailed data Reports, or Printed Performance Reports. 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!
At 20 kn with 3 extra passengers I didn?t have that problem (my wife didn?t allow me to go faster with her on board.) The behavior above 20 kn is not a real problem as the intended cruising speed is 15 ? 20 kn. With the step I also get a lifting force in the forward section, which allows the CoG to be further forward. The bottom profile at the rear is mostly for channeling the air flow from the step for lubrication, but also to provide better grip to prevent the boat from sliding sideways. You could try to resolve it by using a long shaft outboard, or by changing the position of the outboard, in order to place the prop out of the airstream. Alone in the boat it was not possible to hang over the transom and watch the water and propeller.
It?s not a big deal; I don?t mind that the boat isn?t forgiving to every sort of mistreatment. According to my calculations the step should allow the CoG to be more forward than otherwise and also be more insensitive to a lengthwise shift.
She will be equipped with a centreboard to sail with the foils out of the water when conditions are not favourable for 'take-off' and no more with only one rear tail unit as on l'Hydroptere but with two rear tail units known as rudder-elevator.
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.
If my purpose was only to get a boat I would just have bought one or built a proven design. Even if I had understood it before I built the boat I still would have built a flat bottom, as I prioritise the low speed planing performance. I?ve not yet seen anything that contradicts this, and taking the step out is not worth the effort anyway. From L'hydraplaneur's experience we know that the steps are a major drag in lite air-below 20k according to Parlier. 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. The central 1 m is flat all the way from step to transom, but at the sides the bottom twists to a negative deadrise at the transom.
Some amount of ventilation is needed to prevent a vortex formation behind the step, which would create a big increase in drag. Experimenting with the air flow will be interesting; I will also try a fan to increase the air flow to see if I can notice any effect of lubrication.
When I use the Savitsky equations for calculating drag I get the result that a stepped hull, which I treat as two hulls in tandem, has lower drag than a normal hull. He had said at one time that they had a variable geometry solution for that drag-but I never saw it. 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. The idea is to channel the air flow from the step for lubrication and to get some grip in the water. At the end I?ll adjust the air flow for best performance at 15 ? 20 kn, and it is unlikely that I will complicate things by using a fan other than for a test. On the other hand, the effect on the rear part of the wake caused by the first part isn't considered. 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. It would have been interesting to have the same boat, but without the step, for comparision.
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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