# Foil Ratio

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

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### caimanJunior Member

Can anyone tell me how to compare the lift to area on foils please?eg if a 2 square metre 0012 foil fully immersed produces X amount of lift,what size would a 0015 foil have to be to produce the same X?Everything else being equal.I am trying to figure out the relative sizes of a rudder/dagger board combination.
I apologise if this has been done before.
Cheers

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### Doug LordFlight Ready

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Your best bet would probably be to use X-foil. http://web.mit.edu/drela/Public/web/xfoil/

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### oldsailor7Senior Member

If you are considering a daggerboards sideways "Lift", rather than a lifting hydrofoil, there is a simple "Rule of Thumb".
Ignore the rudder because if the boat and rig are balanced, there should be no sideways pressure on the rudder except when turning.
The rule of thumb for any symetrical foil is 2% of the projected sail area,
given that the hull is not designed to provide significant leeward resistance..

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### daiquiriEngineering and Design

Errr... So you are changing the airfoil type, and a size of the keel/rudder. Maybe I'm missing something there, but - what is the "everything else" that remains equal?

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### caimanJunior Member

Thanks for the advice so far.Old Sailor,I don't think my boat is 'balanced' properly.I am begining to think that when tracking in a straight line, the sideways lift of the rudder, is greater than the sideways lift of the dagger board.This is 'lifting' the stern to windward,and is giving the boat the symptoms of lee helm.
I am considering making a new rudder for a couple of reasons,my reading so far points to using 0015 section in rudders.If my DB is 0012 and say 2 square metres total 'working' area,it will produce X amount of 'lift'.How do I work out the area I need for a 0015 foil to produce the same amount of lift?Angle of attack/speed/density of water/etc all being equal.Also,is there a thumb rule regarding dagger board/rudder size?
Cheers

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### oldsailor7Senior Member

Caiman.
You should NEVER use the rudder to "Cure" a lee or weather helm. That is not the rudders job and only causes extra drag and makes the boat "cranky".
When the boat is properly balanced it should have just the very slightest amount of weather helm for safetys sake.
The balance of the boat can be effected (or should be ) by the set of the sails, and can be a factor of several things. Position of the mainsail boom, (traveller), tension on the mainsail leech, (mainsheet), twist (too much of it). Excess sheeting of the jib. Jib sheet lead too far forward.(too tight a leech). Mast rake too much forward. (Lee helm). Daggerboard too far forward. (Lee helm). Plus quite a few other reasons.
If you are satisfied with the set of your sails I suggest you rake the mast back some. You will be suprised how much difference just a few inches makes.
But don't try to make the rudder do a job it is not intended for.
Just my 2c worth.

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### basilSenior Member

Good call oldsailor7. I feel caimans lee helm problem has been discussed either on this forum or somewhere else. If my assumption is correct what has been done to try and eliminate the lee helm issues?

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### caimanJunior Member

I understand the theory of the position of the Centre of Effort of the sails in relation to the Centre of Lateral Resistance.I used to windsurf and learnt on a non planning board before progressing to a 'sinker'.
At the moment the boat is a bit cranky but with constant lee helm.This is creating the same amount of drag as weather helm but as said is dangerous.
I have followed advice from this forum regarding traveller position and leech tension and it has helped.I have moved the mast about 18 inches further aft which has also helped,however the problem still persists.There is a foam pad between the DB and the case to keep the DB forward in the case.Surely by moving the CLR forward,this should increase weather helm?When I bought the boat the rig was adjusted as far aft as possible,and raked aft as far as possible,so I don't think this is a new problem.Everything has been done to try to move the CE of the sails aft.
The thoughts I am left with are-
Should I slack the rear waterstays,and try to 'pull' the bows of the amas down with the front waterstays?More ama in the water forward to get some lateral resistance from the ama hull?
Is the front beam to long?Causing the amas to 'toe out'?
Is the rudder providing more 'lift' than the dagger board,and causing the symptoms of lee helm?I could build a smaller rudder,lateral resistance would be reduced,but he CLR moves forward?I suppose that I could get the same results by extending the DB by adding say 6 inches to the top and pushing the board down further.Which brought me to the question,'is there a way to compare the lift of different types of foil shape?'.If my rudder is producing X amount of lift from a 0015 foil,what ratio determines the size I need to add/subtract to my 0012 DB to obtain the same lift?
Alternativly, how much bigger/smaller does the new build rudder need to be?
I appreciate the link from Doug Lord,unfortunatly I don't have the skill to make use of it,but thanks anyway.
I have had some great sailing from Caiman.I feel that this 'helm' problem is certainly holding the boat back,I would love to be able to release Her potential.
There are some You Tube videos on 888Caiman.Maybe you will spot what I'm doing wrong from them.I don't know how to post a link,sorry.Thanks for the advice so far.
Cheers

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### Gary BaigentSenior Member

Caiman, lee helm is a real *******, dangerous and slow, don't need to tell you that. Changing dagger/rudder thickness to 0015 is not going to make much change to your helm problems. There is nothing wrong with 0012, less drag but also a little less lift. But that is not your problem. Cranking the ama bows down might help but you have to watch out for the fine ama bows taking over and steering you willy nilly. In fact that might already be your problem. Do you get lee, then weather helm? If so the bows could be the culprit.
But you say continuous lee helm. I don't know the design of your boat but if the bows are blowing off, then there is too much sail area forward. Try shifting the forestay aft. What happens with a small headsail? Better or worse lee helm?
Failing that, maybe your dagger is too small in area and the boat is blowing sideways. Post some photographs of the boat in profile and show the appendage positions.

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### brucebSenior Member

other causes

Caiman, What kind of boat do you have? I know you have posted before but I have missed it. I can't imagine your rig/boards are that far off. Moving the top of a mast one foot really changes most boats' balance a lot. Have you measured your hull to float alignment? Having the bows splayed out or turned in can really mess up a boat as well as things like a really badly designed or built rudder. It sounds like something basic is wrong. Lets us see some pictures before you start any major changes. B

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### idkfaSenior Member

I am begining to think that when tracking in a straight line,

Do you have a problem when sailing or when tacking?

ths

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### caimanJunior Member

Thanks for the replies.I should be able to get to the boat at low tide tomorrow morning.I'll try to get some pics showing the relative positions of the DB and the mast,and also some pics to compare the sizes of the foils.The boat is slow in going about.I seem to remember my old H16 tacking more quickly and reliably,but I was considerably younger then!I have quite often missed tacks on Caiman and ending up backing the jib/rudder and doing a '3 point turn' when going about.The lee helm does reduce when the jib is rolled up,but with just the main up I would expect a lot of weather helm,this is not the case.I do not want to renew the jib if I don't have to.Cost is a major stumbling block,but as important,I don't want to loose performance.The boat is an 8m Kurt Hughes design.I think the boat was built in 1989.If I have to, will contact KH and pay for His advice,or even buy the plans if I have to,but as I say,cost is an issue.
The only time I get weather helm as such,is when the rudder 'takes over'.I feel that there is too much area ahead of the rudder stock,and this gives it a 'servo' action.This is one of the other reasons for renewing the rudder.I made a new DB after the old one broke,and so am fairly confident that I can do the work.The helm issue was also there with the old DB.Another reason for a new rudder is so that it can kick up more easily.
GB you are right,you've got to have tried it before you know how much brain power lee helm absorbs,you'r constantly on your guard.It is 'fail to danger' rather than 'fail to safe'.
Cheers

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### oldsailor7Senior Member

Caiman.
When you have your sails set normally. and you are sailing along with the tiller held still----if you move it to left or right, does it "Snatch". That is, suddenly pulling one way or the other. If so, that indicates you have too much rudder area ahead of the pivot point. I think we have talked about this before --with pictures of your Red rudder. Am I right ?
I do think you should contact Kurt. I doubt if he would charge you for advice. After all it is the reputation of his design which is at stake.

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### brucebSenior Member

foil section

Caiman, On my tri, I have a 8.5% section on a 15"x5' (exposed) tapered dagger board that is about .016% of my 370' working sail area. I built the board and then got a larger main- so much for planning. I couldn't use any thicker section because my trunk is only 1.25" wide, but the foil is fairly carefully shaped. I race, load it hard, point very well and go fast- it seems to work just fine. I certainly don't have lee helm unless I raise the board more than half way. I have to be careful not to stall the board in very light air coming out of tacks, but it is really very forgiving most of the time- thin and long boards do work. B

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15. Joined: May 2008
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### oldsailor7Senior Member

Bruce. I think you meant 1.6% of 370 sq ft, which seems about right for your immersed board area and your total sail area.

I use the figure of 2% of the projected sail area ie: the foretriangle +mainsail area, which would be less than your full sail area, but comes out about the same.

Does that make sense ?

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