# Hull Drag of Small Dingies/Skiffs

Discussion in 'Sailboats' started by Matth, Apr 23, 2007.

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### Tim BSenior Member

Hummm, I could see what was being got at with the takeoff analogy, but it's easy to look at it too closely. 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.

"No-hump" hulls are a little bit of a myth. I personally think it is more to do with the running trim than the hull shape (but obviously hull-shape makes a difference). It is a case of running the hull at the optimum angle of attack to get on the plane.

Vertical (sharp) bows undoubtedly are better through waves, and are also likely to lower wave drag (though it's difficult to quantify it directly). see my Lark/420 example (yet again, though there are any number of similar boats). 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.

When thinking about the CG of the boat, consider where the crew is. they will have a VERY large effect.

Hope this helps to clarify things.

Tim B.

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### froshSenior Member

Hump in the transition, is it a myth?

Hi Matt and Tim, If your experience in dinghies is something less than a 49er in the performance and hydrodynamic areas, then I fully understand why you might expect that induced hull drag increases dramatically when approaching the Froude number for that hull. 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 is quite likely that some-one like Van Essen (the FD designer) described his radical new design this way in the early 1950's.
As far as the aeroplane example goes, I am not sure that you are right. Maybe you are only partly on track with that analogy. My aeroplane aerodynamic maths is now a bit rusty and I would rather leave this to a real expert.
I would love to hear Tom Speer's analysis, and opinion on this one.
http://www.bethwaite.com./designs/49er/
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. 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 matter for a Moth as all the competitive ones are fully flying now on foils. 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.
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. This of course assumes that lifting foils are not fitted to either example.
This is the area that I am currently exploring. 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.
(1) Pacific proa on port tack and Atlantic proa on starboard. Too much for the brain and body to adapt to in strong winds, as handling for each mode is almost a world away.
(2) Easily capsized to windward when in Atlantic mode in light wings as crew was hiked out on wings.
(3) Very tricky to keep the ama from burying too deep into the water when in Atlantic mode.
(4) Total buoyancy of the ama which was borrowed from my padding/sailing OC2 was around 80 litres, much too low for 5.6m long proa with 2 adults aboard, sailing in 20 knots of wind.

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### MatthQuestionable

Sigurd: The drawing was done on SolidWorks, a mechanical CAD package. It doesn't do hulls or ropes very well (that I know of) but it's great for mechanisms and moving parts, and making drawings for machinists. The guy in the drawing I pulled off the web, it's a copy of a little wooden toy that IKEA sells, with articulated joints etc. It can be manipulated by just grabbing hands, legs, etc with the mouse and moving them around. All of the relationships between parts are defined geometrically, for example I set the curve of his back to be tangent to the seatback, his feet to be parallel to the footrests, etc.
The sail can be sheeted and rig rotated just with the mouse, or parts can be fixed in place.
I think it's one of the cheaper commercial CAD packages, but still a few thousand\$. I'm using some connections from an old job to use a laptop after hours...

Matt

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### Phil StevoJunior Member

I detect a few misconceptions:

1. The "humpless" hull was around a long while before the 49er. I once had a 1980 vintage NS14 which accellerated smoothly onto the plane without significantly raising of the bow. It was typical of the time when the class went for more rounded bow sections and straighter lines. FB has a tendency to ignore all NS14 development after he left the class and developed the Taser.

2. Narrow moths with canoe sterns were consistanly faster in all conditions than any wide stern / planing hull moths and that is why the wide ones died out, long before foils.

3. A canoe stern or narrow transom does not mean a boat will not plane well. The nethercott IC is the best example but the new DC designs will be a lot faster. 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.

4. A planing boat does not rely on a wide stern to lift. In fact if it did they would all be nose divers, before they achieved planing. The lift must be centred midships, where the combination of the weight and downloads from the rig are positioned. 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.

Fast boats with good transition have narrow topsides, rounded underbody at the bow, straight buttock lines and if narrowed aft need sharp chines. This description fits all the latsest fast boats in classes where development has continued without restrictions, NS14, I14s, some 12s, Development Canoes.
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.

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### PI DesignSenior Member

Spot on Phil. Just one thing - you may not start planing on the stern, but as you pick up speed it's the only part in the water. So it makes sense to optimise the stern for high speed planing i.e. flat and wide.
I'm looking forward to getting my NS on the water and testing out this humpless thing for myself.

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### Tim BSenior Member

well, sort of yes and no.

point 4 -- read Savitsky's paper if you will. I think you've understood it fairly well, essentially it all comes down to equilibrium.

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). 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. This doesn't mean that it's not there, just that you may not notice.

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. 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.

Tim B.

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### froshSenior Member

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. I don't claim that F. Bethwaite or JB was the pioneer of hull design where the transition was almost humpless.
Matt and Tim, I don't really think that the aircraft take-off is a close analogy at all. 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. 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.
Going back to boat hulls now, classic displacement to planing transitions that have to cross over a hump are no longer always the case. Phil also agrees with this.
Back now to answers to Phil's posting.
I understand the evolution of Moth hulls well having been a Moth sailor, quite some time ago.; (a few different scow designs including one own design), and have been following the changes and progression through wide skiffs, narrow skiffs etc. with much interest.
By the time that Moth hulls became around 12" wide (max. beam), planing or non planing design was not really the main issue any more. It was about winning on the race course in a wide range of conditions. There is no doubt that a Hungry Tiger could eat the Moth design that spun off the Europe dinghy for breakfast, lunch and dinner.
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 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.
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.
Formula sailboards have ingredients of design and concept (particarly a freesail of very high L/D ratio), promoting their incredible performance in medium winds that will never be possible in dinghys except for perhaps the very finest foiler designs.
Anyway Phil, it is a pleasure to discuss this sort off stuff with you.

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### CT 249Senior Member

Well said Phil!

Tim B wrote "No-hump" hulls are a little bit of a myth. I personally think it is more to do with the running trim than the hull shape (but obviously hull-shape makes a difference). It is a case of running the hull at the optimum angle of attack to get on the plane."

Yes, but assuming both boats are sailed at the optimum angle of attack to get on the plane, I think it's fairly well accepted that the "humpless" or small-hump boat will slip onto the plane more easily.

Matt wrote; "For the boat, this might mean reducing the 'jacking up'. 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. 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.
Does this make any sense?"

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. Julian says that he tries to stop the boat rising out on the plane too early and therefore he gains more top speed.

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.

Re Formula boards; Phil, what conditions did they need to hold or beat the foilers? I've heard the Formula sailor's side of the story.....they are impressed by the Moths and Canoes but reckon the boards are a bit faster in planing winds.

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. 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.

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### PI DesignSenior Member

Low DLR has multiple benefits - long hull has higher 'hull speed', light hull produces less waves (drag), light hull planes more easily. These factors combine to help increase displacement speeds and reduce planing speed, which helps reduce the drag hump. I suspect high L/B also helps (fine entry = less wave making).
Does the depth of forefoot have an effect? If you accept that planing can't occur until the bow is out the water (how else can you get a stagnation point?), then having less bow in must help get planing earlier, but that doesn't necessarily mean you have a humpless transition.

Hope that makes sense, had to rush this post!

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### froshSenior Member

To have a hump or not!

Chris,
"No-hump" hulls are a little bit of a myth. QUOTE
It is a matter of degree in my opinion. What difference will a very minor hump make in the real world anyway? This is not about passing some sort of theory exam in hydrodynamics.
No-one has discussed much about rocker so far except to say that an efficient planing hull needs fairly straight tail rocker. OK I go along with that. 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. I realize that these hulls are not meant to plane and do not need to transition thru a hump whatever speed they achieve. However even without planing, it doesnt make them slower say than a 29er which is an all out planing hull.
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. hull beam, and the rocker almost all in the front half of the hull. Then lets have soft chines forward gradually changing to very sharp chines in the rear half. Also lets flatten the roundness of the hull bottom steadily from about the mid point to the transom.
Let us then assume that such a cat sails predominantly in fully powered up conditions.
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. 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!

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### MatthQuestionable

messabout: Thanks for sharing your towing results--what are the units for the S/L ratios you mention? Knots/ft? And have you scaled those numbers for 1/4 scale models?

Leo: On further research, I think my comment about skin drag being linear was just plain wrong. I was basing it on 1) a spreadsheet I had found called Baileyspeed.xls which is based on the on the Savitsky formula, and showed a skin drag rising sort-of linearly. And 2) I thought that skin drag was a caused by viscosity, therefore the drag force would be proportional to speed. It seems that I was wrong about 2), and the only reason the skin drag seems to increase only slowly is that the wetted area calculated by the spreadsheet decreases with rising speed. I will keep reading.

On the second link Sigurd gave there is a picture of a hull idea (attached). Looks a lot like the SailRocket design; two planing surfaces at front and rear, with the trim angle set by the hull shape and not changing with speed and jack-up. I like his idea of making a shallow step to ventilate the hull at higher speeds.

Matt

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### Phil StevoJunior Member

In the "BOOK" FB claims that JB invented the "Humpless" hull, so I did not accuse you of this litterary anachronism, Frosh.

I try not to read too much theory especially recent stuff with computerised calculations. 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.

It does seem pretty simple:
1. 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. This speed is determined mostly by length.
2. Most boats will plane if pushed fast enough. If they are reasonaly flat and balanced this planing will be manageble. Some shapes will be unstable.
(If the boat is lighter it will sink less and there will be less spring, and have straigheter buttocklines. If it is narrower it will have straighter water lines too.)
3. The hump speed is increased if the boat is light and narrow. 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.
4. 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.
5. So if the hump speed ends up faster than the planing speed the hump is never reached and the boat transitions onto the plane without needing to "climb over its bow wave".

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. But the do this about double the theoretical Hump speed. This narrowness and lightness can be measured as DLR if you wish. Modern materials just make it easier now.

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.

Re spring profile in cat hulls. The sping aft was an creation of the 60s. A few UK C Cats had it and the Tornado made it popular. The F18s maybe have made it more obvious. Moths and A Cats to a lesser extent do not employ it. Maybe because they are lighter and need less spring anyway. 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.

Frosh, there have over the years been may attempts at making planing cat hulls. The best was probably the 12ft Kitty cat. 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. So you are taking on a brave challenge.

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### Paul ScottSenior Member

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. The effect looked like ball bearings.

Phil Bolger did a planing cat too. I can't find the article, but it's pretty cool, and as written, pretty funny. If I remember right it was C cat.

I got obsessed about the hump a few months ago, and from my digging, it seems to revolve around 2 main things- 1) friction drag, which is, as noted above directly related to how much hull is in the water and how fast it is going, and 2) wave drag, which gets bigger with speed and then, depending
on the weight and shape can get smaller. But I found if you add friction and wave resistance together for a light planing hull, you always find a resistance rise and fall, and it made me wonder what a small hump is, say, on a 16' by 30" class legal flat bottomed ICD with me (180lbs) on board. 10lbs? 20lbs? 40lbs?

And, frankly, I'm wondering if there is really just one hump, but many?

Paul

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### froshSenior Member

Theories vs. observations.

What has become very interesting is that as this thread progresses the guys that are making most the contributions have separated off into two quite distinct camps.
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.
This is not meant to be a criticism and I am NOT wanting to offend anyone in either camp.
It is just interesting where different people are coming from, thats all.

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### Paul ScottSenior Member

frosh, if you add us all together, we almost make up one whole giant online brain!

Paul

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