# Foil Ratio

Discussion in 'Multihulls' started by caiman, Dec 26, 2010.

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### RHoughRetro Dude

Wow.

This thread is all over the place.

First the lift is proportional to area and varies with the square of speed. This is relatively independent of foil profile.

What exactly do you mean by "lee helm"?
That when the rudder is on centre the boat turns away from the wind?
That the rudder must be held to turn the boat into the wind?
That when the rudder is released the boat turns away from the wind?

A centred rudder has an angle of attack about equal to the angle of attack of the dagger board (it is actually a little greater due to the downwash aft of the daggerboard). If the dagger board is lifting, the rudder is lifting *even when centred*.

The total lift requirement to reduce leeway to a minimum is shared by both foils. You do not want a zero lift rudder. Zero lift = 100% drag. *IF* the sections are the same (or nearly so) the ideal would be for both foils to be operating at their best L/D ratio so the lift should be proportional to the area of the foils. If the daggerboard is 3 times the area of the rudder, you should have 75% of the lift from the daggerboard, and 25% from the rudder.

Sail area Centre of Effort effects trim much on a sailboat much like Centre of Gravity effects trim on an aircraft. With the CE/CG forward, the Rudder/Tail has to lift the opposite of the Daggerboard/Wing to hold course. This is very bad for upwind performance or minimum sink rate. When the CE/CG is moved aft, the Rudder/Tail must lift the same direction as the Daggerboard/Wing to hold course. This is good for upwind performance or minimum sink rate.

The devil is in the details.

It sounds to me like you need to find a way to move the CE back or the CLR forward. Reducing rudder area or increasing daggerboard area would be my first step.

R

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### oldsailor7Senior Member

RH said:- "Reducing rudder area or increasing daggerboard area would be my first step."

That is absolutely correct --Except when the daggerboard is too far aft.

Also a smaller rudder may compromise its ability to act as a turning device.

A stalled out rudder can spoil your whole day.

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### brucebSenior Member

Details

OS7, yes, 1.6%- I think I was sniffing epoxy. My board was a compromise of getting it in and out of the swept back Buc trunk and mostly clearing the boom with it retracted. The area is about the max I could make it, not what I would like. I really want to see where Caiman's board is- something drastic seems to be wrong. B

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### Gary BaigentSenior Member

Wow Randy, are you not be slightly pedantic? - lee helm is pretty obvious, the boat suffers an imbalance and wants to turn away from the wind ... and to compensate you push the helm over.
Until we see photographs I'll take a guess and say the dagger area is too small, boat is skidding and lee helm has to be introduced to get it back on course.

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### RHoughRetro Dude

Thanks mate!

The CE vs CLR thing as it relates to helm and performance is poorly understood IMO.

All the "rule of thumb" areas and leads (CE vs CLR) are based on 2D estimates of a 3D problem.

R

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### caimanJunior Member

Gentlemen,thank you for the replies.Firstly,with all respect,KH's reputation for the design is not at stake.This boat is 20 years plus old.I do not have the plans, and so who knows what modifications have been done along those years,or if the plans were followed in the first place.I'm not even sure if the rig is original.Irrelevent of who designed Caiman,I am asking please,for advice from the forum to try to cure a problem with 'a boat'.
OS yes we have discussed Caiman before.One of the reasons for the proposed new rudder came from the thought that there is too much area ahead of the rudder stock.I intended to build a new rudder last Winter,unfortunatly I found some rot in the deckhead balsa core which had to be attended to,and so had no time.Ultimatly I will probably have to contact KH,but asking through the forum is expanding my knowledge along the way.EG. Immersed board area 1.6% of total sail area,or 2% of projected sail area.Does 'immersed board area' include the rudder?
Bruceb I remember the DB pics when you first posted them.I've tried to get to a 0012 section on my DB,this being the figure that seemed to keep cropping up in the forums.Also through the forums,the figure for rudders seems to be 0015,if only for the fact of getting a bit of material around the stock at the thinnest point.By the way,my exopy does'nt small !!
RHough my boat does all three!!! Would the 3-1 DB /rudder ratio be typical for a boat of this type?Taking the 3-1 ratio,is this assuming that the DB and the rudder are the same foil section? Would the lift be balanced at 3-1 if the DB was 0012 and the rudder was 0015?If not,how much bigger/smaller would the rudder need to be?
Gary Baigent,yep,thats the feeling.I can easily increase the span a bit by screwing a block onto the top of the DB and pushing it further down the 'hole'.Or,if I made a smaller rudder,the DB and rudder would 'skid' to the same degree and balance the boat?The first idea is better,but there are other implications.
I'm having camera problems at the moment but will post the pics as soon as possible.
I really do appreciate all your input.I have followed this forum since buying Caiman,particularily the Bucc threads.I've seen photos of the work you guys dutstanding.
Cheers

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### daiquiriEngineering and Design

Caiman, as has been said before, "everything else being equal" you won't get any improvement by switching from 0012 to 0015, because at the same angle of attack they give the same lift coefficient, and have the same moment coefficient. It means that at the same speed they will give you the same lift, hence will not change the "center of lateral resistance" of the underwater hull and will not affect the balance. The only advantage of the 0015 section is that it has a somewhat higher stall angle, but for the ordinary tiller angles it will only give you a higher drag. So, don't throw your money away.

Another possible origin of your boat's behaviour is that your daggerboard could be in stall for some reason.

I believe that, at this point, it is really necessary for us to see some photos of your boat, or at least a sketch with the main dimensions of the hull, daggerboard, rudder and sails - as well as the longitudinal positions of the former three.

Cheers!

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### oldsailor7Senior Member

Make a new daggerboard to fit the existing boardbox, but swept forward to move the CE of the hull forward. That would also reduce any tendancy of the board to Stall.
Example:- Forward swept aircraft wings. ( Would be a great weed catcher). Lol.

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### caimanJunior Member

OS I've re read your post and see that the rudder is not included in the 'board' area.Thanks.
Daiquiri that answers my original question.The 'foil/lift ratio' that I asked about is 1-1?The difference in foil number does not alter the amount of lift it produces,just it's stalling point?I did not realise that,thanks.I can add the 'lift ratio' idea to my 'Edison's list'.:idea:
I've brought the foils ashore and will take some measurments pending the arrival of a memory card reader for the camera.
Cheers

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### daiquiriEngineering and Design

Please measure sails too, and the position of the mast, daggerboard and rudder from some reference point on your hull - the bow or the stern, for example.

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### Tim BSenior Member

The lift-curve slope (dCl/dAlpha) is pretty much the same for most aerofoils in the attached flow regime (lets say +/-8 degrees for the sake of argument). Thickness will have little effect on lift curve slope. It will however affect drag (greater thickness, greater drag). However, the leading edge radius (which for similar foils increases with thickness) will produce different stall characteristics.

Then, of course is the question about area and balance. Just to clarify, CLR/CE on a boat bears only a passing resemblence to AC/CG on an aircraft.

Digression:
RHough may be right in saying that downward-lifting tailplanes are detrimental to minimum sink rate, though the effect will likely be very small (Cd may be very similar at positive and negative alpha, depending on the foil). However: A stable aircraft (many full-size aircraft are not) will have a CG forward of the aerodynamic centre, necessitating a downward-lifting tailplane. This is true for any configuration (including canard, but there it's an upward-lifting foreplane). The stability issue drives the CG location, and with aircraft the safe recovery is deemed nose-down, increasing speed.

Yachts have similar stability issues, but we consider the safe condition to be head-to-wind. Hence the CE of the sail is AFT of the CLR, providing a head-to-wind moment, if not countered by the rudder.

A centred rudder will produce lift when at an angle of leeway, but this will be significantly influenced by the hull, as well as the keel (or daggerboard). In the case of twin rudders, this is very significant for the rudder down-stream of the keel. For single-rudder setups, the effect of the keel on the rudder reduces quite rapidly with increasing leeway, as the wake moves further away from the rudder.

The easiest way to investigate this is to get some foam (white or blue closed-cell type) and attach it to the existing foil. This way you will quickly see what improves performance and what degrades it. With lee helm, you will also benefit from moving the rig aft, or raking it aft (or both), for the reasons mentioned above.

Good luck,

Tim B.

Note: The effect of camber is to move the zero-lift angle, and generally improve the maximum Cl/Cd ratio. Consider this as an aside for daggerboards and rudders, or you'll only go fast on one tack!

Last edited: Dec 30, 2010
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### daiquiriEngineering and Design

Other mechanism that might play a role here (while waiting for the photos), some of them perhaps mentioned in the previous posts:
- Low aspect-ratio rig (gaff or lug sails, for example) being mounted instead of a higher aspect-ratio bermuda rig.
- Low heel angle. Zero or even negative. Mentioning this because I see no reference to wind or bow speeds in the previous postings. Try to play with the heel angle, more heel will give you weather helm. It is similar to what you have experienced with windsurfs. When you lay down the sail, the board turns to windwards.
- A too flat sail. The flatter the sail, the more will its' CE be shifted forward, thus contributing to lee helm.
Cheers.

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### Tim BSenior Member

Daiquiri, I agree in parts:

A lower-aspect rig will tend to move the CE further aft (by it's geometry), but most owners consider re-rigging an existing boat to be an extreme solution (best cured with a new boat).

Heel angle will have a pronounced effect on the yaw moment from the hull (though the sails will contribute to this as well). This is a secondary directional stability mechanism, though it incurs hideous amounts of drag, so we generally wish to minimise it as far as possible.

I fully agree with the sail shape comment, but remember that sail tuning should only be used for fine adjustment.

Cheers,

Tim B.

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### daiquiriEngineering and Design

Tim, I was refering to the fact that the center of pressure moves towards the leading edge of a wing or a sail as the AR becomes very small (usually below 2) as is the case of gaff or lug sails.
See these graphs whih show the movement of the CP (or CE) for an elliptical wing and for a rectangular rudder as the AR diminshes. Sure, a thick rudder or a wing is not the same thing as a thin sail, but at very low AR it is not so important because a planform shape dominates the lift generation, not the airfoil type.

Cheers!

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### RHoughRetro Dude

The effect on min sink is very large, because for the same glide slope the velocity is a function of area loading. A downward lifting tail effectively increases area loading on the main foil so flight speed is increase to reach equilibrium (causing higher sink rate). All I'm trying point out is that for boats the foils that provide lateral resistance should be lifting the same direction. You pay the same drag penalty for lift from the rudder no matter the direction of the lift. I makes sense to me to have the rudder lifting to windward to keep the boat on course rather than lifting to leeward.

When I was designing gliders, I was amazed at how far aft of the wing downwash effects are present. The relative distance between main foil and rudder measured in main foil average chord lengths is much like a relatively short coupled aircraft on many boats. 0-0 geometric Decalage is altered by downwash so the horizontal stab "sees" a non zero AoA.

For mono's I believe the lateral displacement of the CE due to heel is the greatest source of windward yaw. I am just starting looking at multi's but my feeling is that the leeward hull loading increases leeward yaw as the boat powers up. I would expect that the yaw moment is to windward below some heel angle and then to leeward above some heel angle. Placing the foils in the leeward hull (catamaran) probably reduces this effect compared to having the dagger board and rudder in the windward hull (trimaran) but I haven't spent much time looking at these relationships in detail.

Looking at the projected lateral area of a hull, foils, and rig the classic assumption is that the relationship between the centre of area of the rig and centre of area of the hull and foils controls yaw. There are some obvious errors in this method. The force from the rig is somewhat forward of the centre of area. The underwater area does not contribute equally to lateral resistance. If the CLR based on Centre of Lateral Area is correct you should be able to tow a boat sideways from that point, if you actually try this in most cases you will find the "balance point" is not in line with the centre of area.

Most boats have "lead" the Centre of Sail Area is forward of the Centre of Lateral Area below the waterline. This should produce a leeward yaw moment it should not produce windward yaw. I believe this built in leeward yaw is to balance the windward yaw from the rig when heeled such that when the boat is sailing heeled on her lines only a slight windward yaw remains. The range of "lead" from several published sources was something like 0 to 17% of waterline, designs that sail at higher heel angles tended to have more lead than boats that sail more upright.

I could have this all wrong, I'm always eager to learn more!

R

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