Catamaran Centreboard Location

G'day Folks,

I'm currently doing a new F18 cat design and it struck me that all the quick cats these days have their centreboard cases a long way aft. Anyone know why they're generally arranged that way? As far as I see it, if the boards were placed a little further forward and were gybing (even just a couple of degrees would help), the lift generated would be greater (due to greater circulation of flow about the boards) and applied more efficiently due to the forces counteracting leeway being applied closer to the Centre of Effort. Thus the boat would slip less to windward, perhaps point slightly higher and might tack quicker due to the longitudinal greater lever arm (distance) between the rudders and centreboards (provided that there is sufficient pace prior to the tack to force the bows around).

Cheers,
'Devil

"Shared Lift"

You might find the following interesting.The ARC 21 uses a daggerboard placed forward of the front beam; balance is managed by making the rudder larger in area than the daggerboard:
Aquarius Sail Inc. - ARC-21
Address:http://aquarius-sail.com/default/catamarans/arc21/index.htm Changed:6:19 PM on Saturday, January 7, 2006

Also, you might check Rob Denney's proa site since he is using twin foils similar to those on CBTF boats. I've always thought that should work like a million dollars on a cat because you would be able to use "collective"(turning both foils at the same time) to improve windward performance. They can be made retractable by putting daggerboard trunks inside rotating drums like the VARA system.

As cats are supposed to fly the windward hull just above the water to obtain top speeds, the boat may be better balanced by placing the centreboards further forward. The force vector of the sails over the leeward hull is actually quite far forward (in plan view draw the sail force as an arrow going from the mast to the leeward hull and see where it bisects the hull - crude, but it gives an idea). So, if the windward centreboard is pulled up so that it is clear of the water (a surface piercing foil isn't as effecient anyway) then the leeward board should be placed forward to avoid lee helm. Also, rather than gybing boards, have you considered asymmetric ones - flat on the outside, curved on the inside (a bit like the hulls of a Hobie 16)? If the windward board was always clear of the water the asymmetry would provide extra lift and so could either help reduce leeway or be smaller in size for the same effect (reducing drag and increasing stability). I hope this is all clear, a picture would help but I don't know how to insert one!

Toby

OK,

Two things spring to mind. Why do you want to go to the hastle of having gybing boards? To reduce leeway? Lee way is just the angle that the centre line of the hull meets the water. By gybing the board you reduce this angle but the boat won't point any higher. The sails have to be sheeted further outboard so that they meet the wind at the same angle. All that gybing boards do is allow you to reduce the drag caused by the hull meeting the water at a slight angle. For most efficient boats this angle is already very small anyway and so the drag is small. (less than 2 degrees)

On a cat you can have asymetric boards that do the same thing without the complexity as the windward board is lifted.

The other point to be remembered is that if the rudder is in the wake of the dagger board then it is likely to stall and have increased drag. For this reason the fact that the water flow is not directly parallel with the hull is a good thing. Both foils operate in undisturbed water.

Nobody

TaSSie_deVil said:

...all the quick cats these days have their centreboard cases a long way aft. Anyone know why they're generally arranged that way? ...

Click to expand...

The boards are aft because the center of effort of the rig is aft. The rig is aft to get the weight aft. The weight is aft so there's more reserve buoyancy forward to keep the bows from burying.

Think of it like being a 14' - 16' cat with a 2' to 4' bow extension, and a much taller rig.

Tom,

That's pretty much the answer I was looking for... however just for giggles the first of the new design will have the ability to have the cb cases slide fore/aft about 200mm either way to see if it helps (care of a square c/b case arrangement with a sliding insert). A production boat would wind up with a proper cb case in whatever location we find to be best/quickest in the widest range of conditions.

The boat that the new design is based upon (having sailed it today in quite light winds and blew away the current aus national hobie tiger champion seemed to have some handling problems (according to my skipper), mostly a ird tendency for the boat to want to keep travelling forwards and thus be really heavy on the helm... we're hypothesising that it could be due to some construction problems which caused the centreboard cases to be too far aft and not entirely straight (thus causing problems with toe-in), but I also get the feeling that it could well also be due to the CLR having too little a lead over the CE. Either way, experimentation should ultimately solve this problem.

Another question while I have the expert on the line... (Tom!!) Also, a lot of the current generation of F18s seem to cause a large amount of wake due to depression of the water beneath the submerged sterns, most of which have chines of some description for one reason or another (capricorn F18s apparently try to disperse the waves beneath the stern, so have a slight double chine and hollow beneath the transom). In theory, it would be best to try and alleviate the depression of the water, thus reducing the amount of energy the boat requires to pass through the water... so, are rounder sections best aft with a bit of rocker to try and lift the transom higher out of the water for light conditions?

Cheers,
'Devil

TaSSie deVil: are rounder sections best aft with a bit of rocker to try and lift the transom higher out of the water for light conditions?

It would have to be pretty light. Even sailing close-hauled in moderately light air, a high-performance cat is going to be pushing its hull speed. That means that getting water around a pointed or rounded stern will be just as hard as separating it cleanly off a planing-style edge. The boat has a wake because it's haulin' butt!