rudder size

[Samnz]

Does anyone know if there are gains to be made upwind by using a larger than required rudder combined with weather helm to counter leeway? kind of like a trim tab?

taking it a step further if you pulled the daggerboard up halfway and used mainly a large rudder for leeway?

I have a rudders way longer than needed, and wondering whether to cut them or try the long version first?

[bruceb]

SNZ, I had a 23' mono that we raced in a class (MORC) that penalized draft. Both the keel and rudder were vertical lifting, so of course, we experimented with raking the mast back and loading the rudder, while trying the keel in different positions. It was not very effective. The boat would start to make excessive leeway at about the same point of keel extension, and we went slower. This was a slow boat (about J-24 speed) and we did not change the shapes of the foils, so not everything was tested, but we found that loading the rudder over about 20% didn't work very well. An extreme example is a Hobie 16 set to point with the mast raked back too far. It will go up wind somewhat better, but gives up speed the rest of the time- and they used to break a lot of rudders trying. Still, I would not cut those rudders until I had tried them B

[Samnz]

Originally Posted by bruceb

but we found that loading the rudder over about 20% didn't work very well.

20% of what? how do you calculate that? one rudder is 2m long. for a 28ft tri with float rudders its very long

[thudpucker]

I wonder if the Shape of the Rudder, and the Thickness at the Front vs the Rear had as much to do with steering as the Size and Balance of the Rudder.

[catsketcher]

Gday Sam

Monos have been doing this for a couple of decades. Its what makes them a pain when a rudder falls off. I think you could look at your rudder size and foil shape, find its drag profile and then get it to be taking load at the best combination of lift/drag.

As for pulling up the daggerboard - I wouldn't think so. I used to get lots of rudder loading if I didn't have enough board down and the boat went sideways.

My humble opinion is that multis have incredibly small foils, case in point is tacking and slow speeds. Multis are hard to sail because the foils are usually too small. Larger foils and then the boat becomes really lovely - just like a mono.

I would have the rudder area determined by slow speed and heavy weather handling rather than a formula. Then your boat will be super sweet to sail.

cheers

Phil

[HASYB]

Sam, if you cut them now you certainly won't know the difference.

Cheers,

Hielan

[bruceb]

Sam, 20% is a "guestimate" from memory about 30 years ago- but, the rudder was about 1/3 the area of the keel with both all the way down, the foils were good for the time (the manufacture also built airplanes), and 20% is an estimate of the loading split between the foils.
Phil is correct, we broke the rudder one ugly night offshore, and yes, it was a handful My tri, both by design and large foils, does handle as well or better than most monos, and doesn't seem slower for having them- but I do raise the dagger as speed picks up. Next season, I will have a lifting rudder also, so I can experiment with less down. Float rudders are FAR more effective than center hull ones, so I would expect you to need about half of those 2 meter ones?? My floats are not strong enough aft to support rudders or I would already have them. I notice Ferrier's latest 32R has three daggers plus float rudders.
IMO, any foil has an angle of attack that is it's lowest drag/lift, rudders are usually thicker percent of cord than daggers, so the rudder needs to operate at just under its best attack angle up wind, a balance between rig loading and rudder size. A cruising tri doesn't usually lift its main hull, so the dagger needs to be big enough to prevent much leeway- two hulls going sideways presents far more drag than a little larger dagger. Rudder total size is a compromise between low total drag and having enough control with a big downwind sail. I prefer a rudder sized to "never" stall. B

[oldsailor7]

Bruce said:- "IMO, any foil has an angle of attack that is it's lowest drag/lift"

On an average foil section (say NACA 0008) the best L/D ratio is obtained at 4 deg AOA. Since most daggers and keels are fixed on the mainhull CL this means the boat will have an angle of leeway of 4 deg also. I have always thought (for racing at least) it would be beneficial to have an instrument which showed the boats angle of leeway at all times. I did invent one, but being a hater of patents, I didn't patent it. With that information you could always set the right sails, the best foil position and the best course thru the water, for most efficient sailing. IE: Speed to the mark.

[bruceb]

OS, that sounds about right, and 4% leeway stinks I guess that is why most of the fast monos and multis are using assm boards. So where does that leave Sam? B

[Samnz]

thanks for the info guys. Im really keen to try 2 different lengths before cutting them down. see if there is a speed and vmg difference. I will be able to calculate the ideal angle of attack for the section.

If the leeway is 4 degree and the rudder was say 4 degree angled to counter weather helm the rudder angle of attack would be 8 degree?

If a 2x larger rudder was 2 x the wetted surface area but only needed 2 degress to counter the same weather helm making the total 6 degrees which is possibly inside the ideal foil angle of attack maybe it works out nearly as good drag wise?

My original intention was to make them as small as possible whilst having control but friend mentioned this leeway countering aspect which is why im looking into it some more.

[oldsailor7]

Samnz said:-"If a 2x larger rudder was 2 x the wetted surface area but only needed 2 degress to counter the same weather helm making the total 6 degrees which is possibly inside the ideal foil angle of attack maybe it works out nearly as good drag wise?"

Given the impossibility of calculating the EXACT areas and centres of effort of the underwater foils against the EXACT areas and centres of pressures of the sails, the centres of lateral areas of these air and water foils are generally used to make these general calculations. There is always a degree of "Suck it and See" involved.

Therefore I would say the above statement of Samnz is about right.

If a boat (any boat) has a very light---but distinct ---pull to weather on the tiller then the boat is considered to be safely balanced.

[Gary Baigent]

hi Sam, your powerful, new 8.5 tri is highly buoyant with the very large, semi-circular? cross section floats, the boat sits high on top of the water - and will want to skid sideways under sail loads; therefore if you thought of sailing with main dagger lifted somewhat plus compensating by using increased depth lee float rudder to get to windward, then I think the boat will want to fall off and you'd be compensating by pushing helm over, lee helming and the boat would still be going sideways. IMO you'll always need that elliptically tipped?? big main hull dagger completely down ... but because your rudders are float mounted, and therefore pressed down from sail power, they'll be working in clean water ... and therefore do not have to be as large as a conventional tri main hull rudder (which can aerate). Your powerful trimaran is really a three hulled cat ... and I'm guessing, the float rudders would need to be equivalent in size and depth to other proven 8.5 class cats.

[bruceb]

I noticed on the F-boat site that the new twin rudder F-32R is using F-22 blades on each float- and they seem to work well. B

[Samnz]

Originally Posted by bruceb

I noticed on the F-boat site that the new twin rudder F-32R is using F-22 blades on each float- and they seem to work well. B

So no one knows whether its always faster to have the smallest possible rudder on a raceboat? (provided you can steer properly)

[oldsailor7]

Originally Posted by Samnz

So no one knows whether its always faster to have the smallest possible rudder on a raceboat? (provided you can steer properly)

It's the proviso that proves the statement.