# Flat or round bottom for a foiler "board"

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

1. Joined: May 2009
Posts: 16,664
Likes: 333, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

### Doug LordFlight Ready

---------------
Good stuff, Slavi.

2. Joined: Jan 2014
Posts: 17
Likes: 2, Points: 3, Legacy Rep: 30
Location: Toronto

### mm7Junior Member

Yes of course Xflr5. Sorry for misleading.

I thought it uses some generic formula.
I just divided Cl half. Because I assumed that for a flat simmetric profile, like a sheet of plywood, both surfaces, upper and lower, create same lift. Bottom one by deflection, upper one by suction.
I never done that. I'll try.

3. Joined: May 2004
Posts: 5,373
Likes: 247, Points: 73, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

In reality it is not so. The upper (suction) surface contributes the most to the lift. In this page: http://www.mh-aerotools.de/airfoils/velocitydistributions.htm you can see some nice illustrations of this fact, and a very easy to understand primer on airfoil aerodynamics.
Cheers

4. ### Baltic BanditPrevious Member

All this is true but consider another "real world" implication and data point: How many wings be they vertical or horizontal, are designed with curved "bottom" surfaces? In almost all cases the bottom surface is as flat as possible to maximize the vertically driving force vector by reducing energy lost to spanwise flow

5. Joined: May 2004
Posts: 5,373
Likes: 247, Points: 73, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

Talking about curvature in longitudinal or spanwise direction?

6. Joined: May 2009
Posts: 16,664
Likes: 333, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

### Doug LordFlight Ready

Slavi- did you get this: " possible to maximize the vertically driving force vector by reducing energy lost to spanwise flow."
Seems like techno gibberish...

7. Joined: Jan 2014
Posts: 17
Likes: 2, Points: 3, Legacy Rep: 30
Location: Toronto

### mm7Junior Member

I do not disagree with that. But there an airfoil section is illustrated. It has an aerodynamic profile and thickness. This thickness makes upper surface longer in chord direction, and accordingly makes lower surface shorter, when wing turned up on some AOA - because the point where solid air separates on upper and lower flows shifts down on the rounded front tip of foil. Upper and lower path from this point to aft tip of the foil, where both flows merge together, becomes different. Upper flow moves loger way and faster than lower one, that creates different Bernoulli suction on surfaces.

What I was talking about was a flat profile that does not have a thickness - a "sheet of thin plywood". Here upper flow goes same length as lower flow. Here is no Bernoulli suction difference. It works purely on deflection of mass. And this deflection has same value for lower and upper surfaces.
That is why I just divided Cl half.

8. Joined: May 2004
Posts: 5,373
Likes: 247, Points: 73, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

No, I'm affraid it doesn't work the way you said.
The long-lived myth of two flows meeting at the trailing edge seem to be really hard to eradicate, but there are some really nice resources which help make things more clear. Like this video on YouTube: https://www.youtube.com/watch?v=UqBmdZ-BNig

For the rest, and with all due respect, I would really warmly suggest you to do some more reading about airflow aerodynamics before venturing into flow analysis based on inconsistent theories. Otherwise, you will be at risk of taking wrong decisions based on uninformed assumptions.

Cheers and good luck

9. Joined: May 2004
Posts: 5,373
Likes: 247, Points: 73, Legacy Rep: 3380
Location: Italy (Garda Lake) and Croatia (Istria)

### daiquiriEngineering and Design

No, I didn't get it. That's why I have asked for a clarification.

10. Joined: Jan 2014
Posts: 17
Likes: 2, Points: 3, Legacy Rep: 30
Location: Toronto

### mm7Junior Member

Two flows still meet on leading edge. Not same particles though.
But it is not relevant here.
Still the trimmed bottom deflects water down. And reaction of water will push the hull up. The reaction will be proportional square of speed, area and AoA.
If such reaction was not exist or was very small then a narrow hull, being trimmed 10dgr, should experience same resistance as not trimmed one.
But I think it contradicts the reality.

11. ### Baltic BanditPrevious Member

Spanwise

12. Joined: Jan 2014
Posts: 17
Likes: 2, Points: 3, Legacy Rep: 30
Location: Toronto

### mm7Junior Member

Thanks, I am doing the reading.

Fritsh,Bertram, "Hydrodynamic Design Aspects for Fast Conventional Vessels"
Savitsky, D., "Hydrodynamic Design of Planing Hulls"

I've calculated lift according to Savitsky formulae for hull length 8ft, beam 1ft on speeds 1-8kn and trims 1-5dgr.
It shows ~9 pounds of lift on 5kn speed 5dgr trim.
It is for [] planform, for () planform should be even less.
You are right, though some lift will be created, it will not be significant.

13. ### Baltic BanditPrevious Member

I dunno since the all up weight of your board is probably around 7#, that's 120% of the lift you need for "planning". Now assuming you weigh say190# wet in full tog - that's 5% of your lift being generated by the board itself.

5% is non-trivial

14. Joined: Jan 2016
Posts: 22
Likes: 1, Points: 3, Legacy Rep: 10
Location: Europe

### AraJunior Member

I would like to restart this thread about slender displacement-type hulls for light wind windfoiling. It stopped a few years ago and I don’t know if the slender windfoil board has ever been built. Windfoiling has seen a lot of development since then, and the current trend seems to go to shorter, wider boards.

In summary, the earlier posts described:

Post #1: A narrow displacement mode hull should be more efficient in pre-foil mode. A comparison of flat board vs. kayak done in Free!Ship Plus showed advantages for the kayak hull.

Post #2: Take-off speed was estimated as 5-6 kts, the contribution of hydrodynamic lift of a flat bottom will be small at this speed. The issue of zero-speed stability was raised (uphauling the sail).

Post #3: Foils and mast should attenuate rolling motion, helping with the uphaul.

Post #4: Creating dynamic lift with a slender hull creates a lot of induced drag.

Post #5: A lot of discussion went into the title question “round or flat bottom”, and comparisons to Moth hulls were made. By looking at pictures of last years Moth worlds, I would conclude that the difference cannot be that big. (see e.g. pictures and ranking on Moth Worlds 2019 Day 4: Slingsby secures Title - Catamaran Racing , News & Design https://www.catsailingnews.com/2019/12/moth-worlds-2019-day-4-slingsby-secures-title.html ). Personally I prefer the look of rounded shapes like boat no. 044, 118, 109, or 110 over the flat-bottom boats like no. 111 or 60. As these worlds were sailed in a windy place, the results are not to be taken as a proof for the best hull shape in lightest wind.

Post #11: Waterline beam is important, high length/beam ratio leads to low resistance. (Note: the waterline beam of a square hull section will stay the same just until take-off. The beam of a round bottomed hull will become narrower with increasing speed, as the foil starts to lift. V-shape?).

Post #29: Get inspiration from D2-boards.

Post #36: The cross section is nearly irrelevant.

Attached is a first sketch of a windfoil board made in Free!Ship Plus. Length 2.2 m, width 85 cm, displacement 88 L with 16.5 cm draft. Total volume 144 L. For comparison there is a picture of a typical 2020 windfoil board shown in the plan view.

And now the question: is this worth to follow up, or should I just buy what all the others buy?

Best regards,
Ara

#### Attached Files:

• ###### Wingding_9.fbm
File size:
88.6 KB
Views:
40

15. Joined: Nov 2015
Posts: 508
Likes: 56, Points: 28, Legacy Rep: 16
Location: Earth

### OzFredSenior Member

I think you've got it mostly right. Round vs flat for same length and beam is likely irrelevant. Moths developed a round section forward, flat section middle and aft by the end of the low–rider days. It was best overall for them, but they also wanted plaining over over 12 kn or so downwind, which is not a consideration for a foil board. The same shape was carried through in Prowler—Bladerider—Mach 2 foilers. Mach 2 hull shape hasn't changed in 12 years and is essentially 20 years old or more, yet it's still in the top Moths. Newer foilers like Exocet, Rocket and Bieker are round (and more rocker), but I think overall it's irrelevant except maybe for the last 0.01% pre–foiling speed. Foils, sail, stiffness, controls, layout and more recently aero matter much more (given equal crew).

I think speed to get lift from foils is worth much, much more than lift from flat bottomed hull. What is the induced drag from hull lift vs foil lift? So make it flat or round, whatever you want.

There's a lot to be said for copying existing top boards. They likely work. But please keep going with yours, if it's even half decent you'll have scratched an itch and we'll have an opportunity to vicariously scratch one too.

Doug Halsey likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.