# Flat or round bottom for a foiler "board"

Discussion in 'Sailboats' started by mm7, Jan 21, 2014.

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### mm7Junior Member

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I think one of those guys mentions this but it's just something to keep in mind: the "theoretical hull speed" of the Moth hull using an old formula is 1.34X square root of the waterline length(hull length)= 4.4 knots. So, theoretically, below this speed the hull only has frictional drag and above this speed wave making resistance comes into play(applies only on hulls with a low enough L/B ratio for this to be close to true). The problem with this is at higher L/B ratio's this changes-it's truer the lower the L/B ratio. A Moth hull with an 11/1 L/B ratio will plane as it approaches 15 knots where a much wider hull(like a board) will plane at 6-7 knots. 6-7 knots is also the speed at which a properly designed Moth foiler can take off. The skinny hull will have a much wider speed range where wavemaking resistance doesn't come into play and where wave penetration is excellent.
Again, I think if you're designing a boat to foil, optimizing the hull for the quickest takeoff makes sense, which seems to me to be a skinny ,high L/B shape so that wavemaking drag doesn't come into play before takeoff...Also, the skinny hull will penetrate waves with the lowest drag compared to a wider hull.
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If you get to the point you don't believe anything anyone says go to the Moth on Foils! thread and look closely at the videos of the Moth hulls over the last couple of years. You won't find a wide foiler hull in the bunch.......

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### mm7Junior Member

I totally agree, and I was not going to make a wide one from very beginning.
My question was - round vs flat bottom. It is just bottom, the long and narrow bottom that can be U shape or |_| shape. I suggested that |_| shape could provide some help in pre-take-off phase with its lifting ability, and I was seeking a confirmation or disproof of this idea and also other opinions - all are very appreciated.

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### daiquiriEngineering and Design

I agree with that, Doug.
At the end it all boils down to the conclusion - which was stated multiple times inside the first half of the page 1 - that in the pre-foiling speed range it makes no sense to count on any significant contribution from the hydrodynamic (read: planing) forces generated by the hull. The foils will lift the boat up much sooner than the hull will start generating dynamic lift force.
If the foils are removable (allowing skipper a choice between going displacement or foiling, for example in light winds) then it makes sense to shape the hull for stability and minimum resistance in light winds. If the foils are fixed, then the boat will be penalized in light winds and off the foils regardless of the hull shape.
In any case it is then advantageous to use a slender hull (which Doug calls "high L/B ratio") and choose the cross-section shape by considering other, non-hydrodynamic constraints.

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If you're locked into a high L/B ratio then the choice of rounded or flat bottom is 6 of one, half dozen of the other. A skinny hull like that will produce negligible lift at pre foiling speeds even with a flat bottom and should not be considered in foil lift calcs.
I'd go with rounded chines forward and sharp chines aft and that should do it....
==============
Thanks, Slavi!

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### daiquiriEngineering and Design

If the boat has fixed-foils, the shape you describe will give you just a bigger constructive complexity with no effect on the pre-foiling characteristics. From the aesthetic point of view, well yes - it would look nice.

Knowing that:
- high L/B (slenderness) hull gives almost no dynamic lift at pre-foiling speeds, regardless of the cross-section shape;
- the hydrodynamic resistance of fixed foils will offset any eventual (and really small) advantage of a round cross-section over a rectangular one (or vice-versa, if you believe so);
- the boat will foil 99% of the time,
the immediate conclusion is that all you need is a slender (high L/B) hull. The cross-section is nearly irrelevant.

One instance in which the cross section shape comes into consideration is the operation in choppy seas. For a given weight and a total draft of the boat, a flat-bottomed hull will have smaller canoe-body draft (which is the draft of the hull without appendages). It means that the vertical strut of the foil can be made longer, which gives more margin against waves touching the hull in foiling mode. That could give some advantage, considering that touching the water surface means slowing the boat down. The less frequent the contact with waves, the higher the average speed.

Cheers

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Looks nice and works 1/100th of a percent better at prefoiling speeds. Many dinghies detail the boat like that to the extent possible under the class rules.
I learned about it when racing Windmills and from the Star class guys when making a detailed Star model.
It certainly isn't a critical detail for a production boat but I would personally detail my own boat this way-too many good people have made a point of telling me about it to ignore it in a race boat(as permitted by the class rules, of course).

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### daiquiriEngineering and Design

Come on Doug, I know that you are aware of the fact that Windmill and Star dinghies (L/B = 5-6) cannot be compared to moth-style hull (L/B=9). On more beamy hulls the aft chine works because it promotes generation of dynamic lift in upright position and favourably modifies volume distribution when the hull is heeled. Now add the drag of the foil to a L/D=9 hull and you have lost that 1/100th of a percent advantage.

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You may be right but if I was racing my own boat I'd sure think about the detail of the chine-more or less regardless of L/B ratio. Maybe it's borderline superstition?
Moth hull: 11/1 L/B

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### daiquiriEngineering and Design

Noted, thanks. Cheers

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### mm7Junior Member

I've calculated lift in XFoil for a long thin flattish symmetric foil chord 3m, beam 0.4m. on speed 2m/s.
Yes its Cl is 20 times less than Cl of foil. But still it exists.
Let say if foil creates 1000N lift, bottom will create 50N. Not too much but still something. But I agree, I cannot reduce foil based on such little flat bottom help.

Wait a second. I should compare lift forces not just Cl. Lift force is proportional area, and area of bottom is ~10 times more than area of foil. So Lift force of the bottom will be about a half of lift force of foil.
This changes the things. If foil creates 500N, bottom will add 250N. If it is true, I can reduce foil area relying on bottom lift.

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Somethings wrong there......Invest in Ray Vellinga's book "Hydrofoils Design Build Fly" -lots of good data including the foil areas of about 8 different Moths.

13. ### Baltic BanditPrevious Member

<removed> How many chined windsurfer boards do you see? they were experimented with and discarded because at speed they are slow and at slow speed they do not perform better than a flat board.

Given how quickly even a stock low volume "short board" starts planning, the notion that at low speeds you won't get hydrodynamic lift out of a floater board is just ignorance.

Remember a "wake board" which has dramatically less surface area for hydrodynamic lift plane fully at 10 knots http://culture.evo.com/2013/05/wake...eed-rope-length-weighting-your-boat-properly/ . So a board that has more than twice the surface area will be planning at 5 knots and generating substantial lift before then..

So to say that you won't get much hydro lift at slow speeds is just wrong

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### mm7Junior Member

I calculated based on:
- foil: span 1m, chord 10sm, Area 1000sm2
- hull bottom: length 3m, beam 0.4m, plan area: 6000sm2

Yes, it is not 1/10, it is 1/6 but still according to this, hull should create substantial lift.

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### daiquiriEngineering and Design

As far as I know, XFoil calculates aerodynamic characteristics of 2D airfoils, not of 3D wings with a finite span. Perhaps I have missed some info about it.
Or perhaps you have used the Xflr5 software? Anyways, unfortunately, it will not tell you anything useful when it comes to the lift created by the hull. Firstly because the aspect ratio of the wing you have modeled is so low (AR=0.13) that it falls outside any of the physical models implemented by these software. And secondly because neither Xfoil nor Xflr5 can deal with the case of a body which lays on the interface between water and air.

If you still want to use them, one way to get a bit closer to the reality is to take take the results from the Xfoil/Xflr5, set the Cp over the upper surface to zero and recalculate the lift force created by the bottom pressure only. In that way you'll see that the lift coefficient will drop down in a drastic manner. That's because the upper surface of the airfoil is the one which carries the biggest part of the load on an airfoil, while the upper surface of the planing board is exposed to the air and hence it's contribute to the lift is is some 825 times lower (ratio of water/air densities). So nearly all of the lift of a planing surface comes from the bottom only.

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