Outrigger balance

[jdoorly]

I am considering adding a sail and and outrigger to a kayak but am concerned about the imbalance of hydrodynamic forces between the smaller outrigger hull and the main hull. Doesn't this imbalance cause huge lee helm? Is this overcome by balancing the center of effort of the sail against the total hydrodynamic drag?

There shouldn't be any problem with helm balance.
Triamarans have just two hulls wet at a time, and the helm balance is superbe. Multihulls really don't develop helm problems like monohulls.

[sorenfdk]

Originally Posted by d munkelt

There shouldn't be any problem with helm balance.
Triamarans have just two hulls wet at a time, and the helm balance is superbe. Multihulls really don't develop helm problems like monohulls.

I'm not an expert when it comes to multihulls, but the reason why they don't develop helm problems might just be because the designers have balanced the center of effort against the hydrodynamic drag and the center of lateral resistance.

I can't believe it's just a part of their nature...

[Stephen Ditmore]

What I've heard is that the lead (leeed) should be about zero, which means to me that the 25% chord line of the daggarbord/centerbord/leeboard/or keel should be right under the rig's center of effort. I've never calculated whether this is the case on my Catapult catamaran, but my mast rake is easily adjusted, and I know what setting works.

An interesting thing about the Catapult is that the mast can be canted to windward. The effect of doing so on the helm is pronounced, however.... it causes lee helm. If the rig is raked aft to compensate it becomes tough to get the boat to come out of stays when tacking (with the mast rake reduced the boat tacks easily).

If you plan to carry a big rig, diagonal stability is important to consider. Trimarans generally carry the floats as far forward as weight distribution will allow. Kayaks don't generally pile on the canvas, though, so they have more latitude where float placement is concerned.

[garydierking]

You haven't said whether you're adding a single or double outrigger. With a double there is no problem and both tacks will be the same, but with a single outrigger I use a pivoting leeboard to shift the center of lateral resistance when changing tacks from ama to windward to ama to leeward.
Don't automatically think that you need two amas as there are some advantages to a single such as less weight, expense and a much easier recovery in case of capsize.
I agree that most multihulls don't need much or any lead with respect to center of effort. I feel that this is mostly due to the fact that there is less heeling and therefore less distortion to the waterplane shape.

gary.dierking.net

[tspeer]

Originally Posted by garydierking

...I agree that most multihulls don't need much or any lead with respect to center of effort. I feel that this is mostly due to the fact that there is less heeling and therefore less distortion to the waterplane shape....

John Letcher makes a great point in his book on self-steering, that it's not so much the distortion of the waterplane shape that matters, but the lateral displacement of the center of effort with heel that makes a monohull round up. Of course, the reduction in heeling with a multihull helps tremendously in this regard.

As for the change in waterplane, the multihull experiences a far greater change with heel than the monohull - that's the whole point in having multiple hulls! If the change in waterplane shape were that important, a catamaran with asymmetrical hulls, like a Hobie 16, would experience significant lee helm as the load shifted to the single cambered hull.

I have seen a tendency toward some lee helm at high speeds with trimarans, but it seems to be more related to changes in the sail plan. The asymmetrical spinnakers, screechers, Code 0's, etc. all add a lot of area forward of the normal foretriangle.

[garydierking]

[quote=tspeer] If the change in waterplane shape were that important, a catamaran with asymmetrical hulls, like a Hobie 16, would experience significant lee helm as the load shifted to the single cambered hull.

Except that by the time the windward hull is out of the water, the waterplane of the leeward hull will have changed to a symetrical shape.

[tspeer]

Originally Posted by garydierking

...Except that by the time the windward hull is out of the water, the waterplane of the leeward hull will have changed to a symetrical shape.

No, just the opposite. With two Hobie 16 banana hulls in the water, you have opposing camber and no net moment. With one hull out of the water, you have one asymmetrical hull.

With a multihull, at low angles of heel, the hydrodynamic drag and rig's center of effort are aligned. As the heel increases, the drag shifts toward the outside hull, but so does the center of effort of the rig. So the impact of the yawing moment due to drag is partially compensated.

For example, take this trimaran's footprint plot:

The main hull is nearly flying at 15 degrees of heel. At the same heel angle, the center of effort of the rig has moved 7 ft (2.1m) to leeward, or more than half the distance to the ama. So the net yawing moment due to drag is half of what one might expect.

But the drag moment is a comparatively minor actor. The lift/drag ratio of the hull is probably 5 or better, and more like 7 if you leave out the drag of the board, which is on the centerline (see http://www.john-shuttleworth.com/Dogstar50-article.html for representative numbers on a performance cruising cat's hydrodynamic drag components). So the remaining 6 ft (1.8 m) of equivalent drag moment arm is comparable to about -1 (0.3m) ft of lead. And that's for the limiting case of flying a hull.

As the boat is driven harder, it's quite likely that the center of lateral resistance of the ama moves forward. The majority of the lateral resistance will come from the board, but the ama bow could have a significant moment for the comparatively small load that it carries.

The net effect is a fairly balanced helm over a wide range of lateral loading for the mulithull, and the shift in drag doesn't necessarily cause a huge shift to lee helm.

Naturally, the precise balance for jdoorly's kayak will depend on the details of the ama, board, rig, etc. I think I'd be inclined to provide for adjusting the longitudinal position of the board so as to experiment with the right lead. I did this with my first boat, a canoe converted for sail. I mounted the leeboard thwart on rails between the center twart and front seat, using J bolts. Once I found the right location, I never had to move it again. With the kayak rig, one could do much the same with a temporary leeboard mounting, then come up with a more elegant permanent solution once the proper location was known.

[threehullbob]

There is an entire book deicated to sailing canoes and kyacks Amazon . com has it.

The only way it works is by putting on lee boats.

Go to the proa web sites. There are a good number of photos of the process.