Rudder and Lateral Plane Area

g'day fella's, hoping you could give me THE answer... Should rudder area been included for lateral plane area calculations? In my (self) studies, some of my texts say no, others yes, others say use 30-50% rudder area. Which is it or is the jury still out? Does it make a difference if the rudder is completely submerged (keel hung) or transom hung?

 

Hello,

If you talk about the total Lateral Plane Area (Alp), then it is defined as the lateral projection of the underwater body, rudder included.

The usefulness of that number basically depends on the hull and keel type. Since the Alp has been "invented" in the times of full-keel yachts as a mean of calculating the underwater hull area which serves to counteract the sail lateral force, it's applicability is dubious when it comes to modern fin-keel yachts.

The shoal, rounded-bottom modern hull creates nearly no lateral force, so the task of keeping the forces balanced is all upon the fin keel and on rudder.
So instead of using the Alp (which comprises the hull), it is more appropriate to use just the fin and rudder area as correspondent design parameters.

The rudder serves as a stabilization and steering device, and can create both positive and negative lift (with respect to the keel), depending on your design.

Since it is generally desireable to have a moderate weather helm, it means that the rudder will be creating a certain amount of positive lift (having the same direction of the lift force generated by the keel), so it can be included in this new Alp definition, if you really want to use that number.

A partially submerged rudder generates less lift than a fully-submerged one, because of the ventilation. A same rudder's lift distribution will go from nearly semi-ellyptical when fully-submerged (under the hull), to ellyptical when partially-submerged (transom hung), with a correspondent decrease in the net lift force. The same is valid for a partially-submerged keel, a condition which can happen at large heel angles.

 

Daiquiri gave a very good insight and knoweldgeble information.
For a traditional long keel, rudder attached to the keel, I use 5% of the rudder area which I include with lateral plane area and the total rudder area been 10% to 12% of the total lateral plane area, without the 5% aforesaid.

 

THE answer does not exist. There is no magic solution to all boat designs. You have to calculate everything together. For example,turbulence, specially on shallow, wide boats will decrease the amount of lateral resistance of the foils.

 

I agree with you Gonzo, but at the end of the day the boat has to sail, so I think a pragmatic approach is necessary.
I know that some in a canoe body do not use the body, just the fin and rudder in the lateral resitance. It works too.
As you say the answer is not carve in stone. Some trial and error is necessary, and I find a sailing scaled model very useful when designing a new boat with some unusual features.

 

Pragmatic approaches are not bad. I think that dividing boats in groups that have similar characteristics helps narrow the design questions. However, an answer for all boats, as much as it is a dream we all have, doesn't yet exist.