J 34 we extended rudder

[J 29 Guy]

We have a old IOR boat at our club a J 34 and it got noticed it has a deeper rudder then OEM. It has been measured and 7 inches has been added to it. What kind of performance gain could be gained from a deeper rudder? I understand why they did it ,these old IOR boats could get wobbly going down wind. I would have to think it there would be a performance gain on all points of sail for this boat but what kind of gains?

J 29 Guy

 

[Ilan Voyager]

The main and lone purpose of this rather common modification on old IOR is yaw stability downwind while being with a spi. Most of the SORC and IOR boats were notoriously unstable while sailing with the round balloon spis and just one spinnaker pole. I remember very thrilling times when the boat suddenly made a half turn towards the wind, the mast almost in the water and the spi dragging in green water like a fishing net while being in the middle of the herd during a regatta. You could hear a lot of screams.
Before deepening the rudder you have to check if the shaft would be strong enough. There are surprises on very light built boats...
You won't gain a lot directly on speed, as the IOR boats, because of the philosophy of the jauge, have never been very fast (the joke in the seventies clubs: the purpose of the IOR jauge is to make to make win a boat a bit less slow than the opponents by adding well placed bumps at the points of measure), but surely in handling and safety thus a bit in mean speed as I said already the IOR were not brilliants in yaw stability downwind.
The rudder deepening maybe changes the rating of the boat, that depends on what rule you are racing now. The jauge has to be recalculated.

 

[J 29 Guy]

We race under PHRF, I wonder what perforce gains would be accomplished? I understand why it was done on days where we round up going down wind on our J 29 they seem to just keep going with out any issues.

 

[tlouth7]

If you have a decent amount of weather helm then in theory the increased rudder aspect ratio would reduce lift-induced drag. OTOH the deeper foil would have greater drag when unloaded, especially the extra surface area in very light winds.

 

[Ilan Voyager]

It's truly minimal. The deepening is only 7 inches, I doubt that changes the center of drift in a significant way. So maybe it just stops the rudder of losing "grip" in a reach when the bow is going down by the pressure of the spi, thus the stern goes up, sometimes the rudder may suck air and ventilate so a bit longer rudder keeps at least the lower part in green water. Maybe that makes a bit more sensitive and effective rudder, acting better with less movement of the tiller.
It's impossible to predict a percentage of improvement, but in general rule a boat that stops yawing in a reach, that you can get better control with less floppy movements of the rudder, that you can feel better the good angle of rudder for going upwind is a better boat. At least it is an easier boat to push hard in regatta and to get its full potential.
Look at the size of the rudders of modern racing sailboats like the mini 6.5 meters which has 2 deep rudders and you'll understand what I mean. Things have changed in design of racing sailboats (and also cruising). A racing mini 6.5 skipped by a lone guy cross the Atlantic at 7 knots of mean speed, and in best conditions can cross the Channel between Brest and Portsmouth at 18 (eighteen) knots. A cruising mini 6.5 for a couple can get and maintain easily more than 10 knots downwind without effort nor acrobaties. That was unthinkable on a bigger IOR racing sailboat 40 years ago.