Flat or round bottom for a foiler "board"

[mm7]

I want to build a hydrofoil windsurfing board.
But I do not like windsurfing boards as a foil starting platform, because WS boards are designed for planing.
I believe that a narrow displacement mode hull is more efficient in pre-foil period. It should have less resistance. I've analyzed a flat board and a kayak style hulls in Free!ship Plus. It shows that generally "kayaks" are more efficient than WS boards. So it should be a "kayak" or Moth style hull.
Such hull can be with a round bottom or with more or less flat one.
It looks like a round bottom hull should have less wet skin and less resistance.
However Moths have a flat bottom.

Here is my question. Does flat bottom help to create lift when it is trimmed up?
I think it should.
I believe that making bottom flat will allow to reduce foil area, because large area is needed on low speed when taking off. Then it can be reduced, and better to be smaller for higher speed.
Is my uderstanding that a flat bottom can work as a sort of "upper ladder foil" correct?

 

[daiquiri]

You are correct in saying that very slender hull is more efficient at low speeds than a wide and flat one (such as the WS board). It is also true that a wide and flat bottom gives more hydrodynamic lift than a narrow and round one. However, this has to be considered against the target take-off speed. At the speed of around 5-6 kts, which will likely be the point at which the foil will produce a lift sufficient to lift you off the water surface, the hydrodynamic lift of a slender hull (of whatever cross-sectional shape) will be negligible but it's drag will be considerably lower than the drag of a typical WS board - which is what you need. At higher speeds, you should be foiling well above the water, so the hull will have no hydrodynamic influence.
So, in the speed range you're interested in, choosing a round rather than a flat-bottom cross-section will make very little difference in terms of hydrodynamic lift produced by the hull, once you have opted for a slender hull shape. The choice of cross-section will be then mostly a matter of construction method, complexity and desired initial stability.

The reason why WS boards are wide and flat are essentially two:
- to provide hydrodynamic lift at high speeds, leave the displacement mode and enter the planing regime
- to provide sufficient zero-speed stability for the person standing on the board.
Since you want to go foiling, you don't really need the first requirement. What remains is the question of zero-speed stability, which could perhaps be addressed by adopting the configuration of a slender main hull and two small outriggers? That's just an idea, though. I have not seen it around yet - for a good reason, perhaps?
Cheers

 

[mm7]

...
So, in the speed range you're interested in, choosing a round rather than a flat-bottom cross-section will make very little difference in terms of hydrodynamic lift produced by the hull...

Here I do not understand. A foil with area 600sm2 should create enough lift to bring 120kg up. However flat bottom with area 6000sm2 will make very little difference? Could you please explain why?

The reason why WS boards are wide and flat are essentially two:
- to provide hydrodynamic lift at high speeds, leave the displacement mode and enter the planing regime
- to provide sufficient zero-speed stability for the person standing on the board.
Since you want to go foiling, you don't really need the first requirement. What remains is the question of zero-speed stability, which could perhaps be addressed by adopting the configuration of a slender main hull and two small outriggers?

I do not thing outriggers are necessary. Foils and "leg" create enough healing resistance.

That's just an idea, though. I have not seen it around yet - for a good reason, perhaps?

You can take a look at Flying Moth in Net.

It has 35sm beam, flat bottom, and it has outriggers, but these for a skipper to sit on. These have nothing to do with zero-speed stability. I am also going to attach something like that, but much less in size and made from glassed plywood. My vessel will look like a reduced Moth with free-standing windsurfing rig and standing-up rider. Foil will be "Air Chair" style, like people use in kite and wind foiling.

The only question is cross-section - round bottom or flat one.

 

[Baltic Bandit]

Not sure why you care about the hull form. The real issue with a planning windsurfer shape vs a non-planning one is that your speed gain will be somewhat limited

 

[mm7]

I've analyzed resistance of hulls in FreeShip! Plus. WS board is 1.5-2 times (depending on analysis method) more resistive on speeds 1 to 5 knots, where it does not planing yet.
A role of a hull in this case is to provide with buoyancy while creating minimum resistance during acceleration from 0 to 5-6 knots where foils will push the thing out of water.
What I am suggesting is to assign one more role to the hull - to assist foils in lifting by deflecting water down with its flat bottom. If my envision correct, the main foil can be reduced a little bit, that will allow it to run faster.

 

[lohring]

Lots of information here.

Lohring Miller

 

[daiquiri]

The real issue with a planning windsurfer shape vs a non-planning one is that your speed gain will be somewhat limited

Agreed, and that was the essence of my previous post. Since one will be foiling all the time, the cross-section shape of the hull is pretty much irrelevant.

By the way, the boxy shape of the Moth dinghy is a result of the evolution from the narrow scow-type dinghy (non-foiling), which Moths once were, to the actual foiling machines. Had the original boats had a round shape, today's Moths would probably be shaped the same way.

 

[daiquiri]

What I am suggesting is to assign one more role to the hull - to assist foils in lifting by deflecting water down with its flat bottom. If my envision correct, the main foil can be reduced a little bit, that will allow it to run faster.

If you intend to go for a slender hull (Moth-like), as said previously, the gain in terms of hydrodynamic lift at the speed of 5 kts (SLR=1.5) will be really marginal - regardless of the shape of the cross-section. And it will come at the cost of the so-called induced drag. So, don't count too much on the contribution from the flat bottom to the overall lift. Besides that, the resistance of a box-shaped cross-section will possibly be higher than a similar round-shaped (or with well-rounded), because of the bigger wetted area (which creates friction). But there have been some fierce debates about this point on this forum.

 

[Doug Lord]

L/B ratio trumps lower wetted surface

I want to build a hydrofoil windsurfing board.
But I do not like windsurfing boards as a foil starting platform, because WS boards are designed for planing.
I believe that a narrow displacement mode hull is more efficient in pre-foil period. It should have less resistance. I've analyzed a flat board and a kayak style hulls in Free!ship Plus. It shows that generally "kayaks" are more efficient than WS boards. So it should be a "kayak" or Moth style hull.
Such hull can be with a round bottom or with more or less flat one.
It looks like a round bottom hull should have less wet skin and less resistance.
However Moths have a flat bottom.

Here is my question. Does flat bottom help to create lift when it is trimmed up?
I think it should.
I believe that making bottom flat will allow to reduce foil area, because large area is needed on low speed when taking off. Then it can be reduced, and better to be smaller for higher speed.
Is my uderstanding that a flat bottom can work as a sort of "upper ladder foil" correct?

===================
Moths use a square hull section because it allows the maximum waterline beam to be as narrow as possible. Long ago they found the the higher L/B ratio(skinnier hull) waterline beam trumped the wider semi-circular, low wetted surface section at pre-foiling speeds. These narrow hulls will plane but require higher speeds to do so than the takeoff speed with foils. Pitching the boat up can be a very good idea to get going quickly(in enough wind):the hull bottom is a poor substitute for a foil for generating low speed lift- when the boat is pitched up the foils generate much more lift(and higher drag) but the time it takes to get foiling is shortened and makes it a net gain.
With well designed foils the boat will fly before it will plane......so I wouldn't suggest including bottom area in foil lift calculations at all.
========
Illustration below shows the advantage of the narrow, squarish hull section as compared to to the same area semi-circular, low wetted surface, section. L/B ratio for the square section is 11/1 compared to 6.9/1 for the semi-circular section. The same section area translates to very close to the same displacement given the same waterline length. This section has been proven many times over to be faster on a Moth.
click-

 

[gggGuest]

Had the original boats had a round shape, today's Moths would probably be shaped the same way.

Not the case at all. Plenty of round section Moths over the years. Its interesting too that whilst the best displacement C Class Cats have round sections underwater, the best foiling ones have flat sections.

 

[mm7]

===================
Moths use a square hull section because it allows the maximum waterline beam to be as narrow as possible. Long ago they found the the higher L/B ratio(skinnier hull) waterline beam trumped the wider semi-circular, low wetted surface section at pre-foiling speeds. These narrow hulls will plane but require higher speeds to do so than the takeoff speed with foils. Pitching the boat up can be a very good idea to get going quickly(in enough wind):the hull bottom is a poor substitute for a foil for generating low speed lift- when the boat is pitched up the foils generate much more lift(and higher drag) but the time it takes to get foiling is shortened and makes it a net gain.
With well designed foils the boat will fly before it will plane......so I wouldn't suggest including bottom area in foil lift calculations at all.
========
Illustration below shows the advantage of the narrow, squarish hull section as compared to to the same area semi-circular, low wetted surface, section. L/B ratio for the square section is 11/1 compared to 6.9/1 for the semi-circular section. The same section area translates to very close to the same displacement given the same waterline length. This section has been proven many times over to be faster on a Moth.
click-

Thanks Doug, your explanation looks very convinceable.
The only moment. I do not rely on planing of flat (and narrow) bottom. At least not on the surface planing. I understand that take-off speed of foils is less than a speed of surface planing of the bottom. What I meant it was like a "deep planing", like a submerged surface with some little trim is sliding up on an imaginary water "slope". Does such effect exists? If yes, should it be counted?

Also, I am not a hydrodynamics engineer, and it is not clear to me why only the waterline beam is important? For a semi-round or triangle or prismatic cross section the waterline beam will be wider, but an "average" "underwaterline" beam will be narrower. Why the only waterline beam is so important?
If to go extreme in this direction, we can create very narrow waterline beam, like 1 inch, making an underwater hull like a submarine or torpedo, and attaching it to above the water deck by a long and narrow vertical stand. See the pic.

 

[Doug Lord]

Thanks Doug, your explanation looks very convinceable.
The only moment. I do not rely on planing of flat (and narrow) bottom. At least not on the surface planing. I understand that take-off speed of foils is less than a speed of surface planing of the bottom. What I meant it was like a "deep planing", like a submerged surface with some little trim is sliding up on an imaginary water "slope". Does such effect exists? If yes, should it be counted?

Also, I am not a hydrodynamics engineer, and it is not clear to me why only the waterline beam is important? For a semi-round or triangle or prismatic cross section the waterline beam will be wider, but an "average" "underwaterline" beam will be narrower. Why the only waterline beam is so important?
If to go extreme in this direction, we can create very narrow waterline beam, like 1 inch, making underwater hull like a submarine or torpedo, and attach it to above the water deck by a long and narrow vertical stand. See the pic.

-------------
No, not that I know of-and it certainly can't compete with foil lift. If you want vertical lift concentrate on foils. If you want quick takeoff concentrate on foils and a narrow, high L/B hull with proven results-the Moth hull.
The submarine type hull you show IF it is compared with a Moth type hull with the same displacement will be slow at prefoiling speeds. Much of sailboat design is based on starting with what has been proven to work and improving on that ,if you can. Good Luck!

PS-waterline beam is critically important because it has been proven that a high L/B(length divided by beam) ratio hull(a narrow, skinny hull) has less resistance than a wider hull assuming both the same length and weight.

 

[mm7]

Thanks all!
I see that the most practical approach will be to get the proven Moth type shape.
The question is answered.

 

[Doug Lord]

Thanks all!
I see that the most practical approach will be to get the proven Moth type shape.
The question is answered.

===========
I hope you'll keep us informed as your development progresses-best of luck!

 

[Baltic Bandit]

===================
Moths use a square hull section because it allows the maximum waterline beam to be as narrow as possible. Long ago they found the the higher L/B ratio(skinnier hull) waterline beam trumped the wider semi-circular, low wetted surface section at pre-foiling speeds. These narrow hulls will plane but require higher speeds to do so than the takeoff speed with foils. Pitching the boat up can be a very good idea to get going quickly(in enough wind):the hull bottom is a poor substitute for a foil for generating low speed lift- when the boat is pitched up the foils generate much more lift(and higher drag) but the time it takes to get foiling is shortened and makes it a net gain.
With well designed foils the boat will fly before it will plane......so I wouldn't suggest including bottom area in foil lift calculations at all.
========
Illustration below shows the advantage of the narrow, squarish hull section as compared to to the same area semi-circular, low wetted surface, section. L/B ratio for the square section is 11/1 compared to 6.9/1 for the semi-circular section. The same section area translates to very close to the same displacement given the same waterline length. This section has been proven many times over to be faster on a Moth.
click-

<removed jab at other member>

the square hulls were already there for "low riders" since it resulted in a more stable boat