Canard fin

[sighmoon]

What is the canard fin for on this boat?



It is from a brokerage advert and the advert says the fin is unballasted and lowered only to windward. The boat also has a very deep, high aspect, bulbed keel, but it is a rigid keel, it does not cant.

The only purpose seems to be to move the centre of lateral resistance (a long way) forward. But why would you want to do that to go upwind? Can anyone explain the thinking behind it?

If it worked well, why didn't it catch on?

[daiquiri]

Several things come up to my mind:

1) the main keel might have some extreme laminar foil section, designed to minimize friction drag at downwind (i.e. planing mode) tacks. In that case a flow separation over the keel surface might be occuring at upwind tacks, when it works at positive angles of attack. That can be even more true if you say that the keel is long and high aspect ratio.
In that case a canard (with that nice classic NACA 000X foil section) induces flow velocity components at the keel which tend to reduce keel's angle of attack and to reattach the flow.

And you are right about your consideration on the lateral resistance. By doing that job, the canard takes pretty much of hydrodynamic load from the keel (which, in turn, becomes less loaded), thus shifting the overall center of pressure forward.

2) The rudder might be under-dimensioned, either in area or in foil thickness, for the same reason seen above.
In that case the canard, by shifting the center of pressure forward, possibly reduces the lever arm between the sail and appendage forces, which allows for less weather helm and a smaller rudder.

During downwind tacks both fixed keel and rudder are drag-makers, so it is advantageous to minimize their drag impact, by promoting the laminar flow and by reducing as much as practicaly possible their area and/or foil thickness. That makes them inadequate to counter-act the sail forces during upwind tacks (it also increases a danger of broaching), so an additional lifting surface is needed.

3) The hull form is such that under high heel angles either the rudder or the keel work close to the water surface. That condition decreases their hydrodynamic effectiveness. The canard acts as an additional surface to use only during tacks which increase heel considerably, which means upwind tacks.

4) A combination of the above.

That's my consideration based just on that photo, without knowing anything else about that boat's shape, buoyancy, C.G. distribution and hull form.
Maybe we find out that the designer of the boat is a member of this forum, so he might give a completely different explanation.

[sighmoon]

Thank you daiquiri,

I don't know about 1) and 2), but for 3), you are right, I think, that the hull form would bring the rudder and the keel close to the surface of the water - it's very beamy and shallow.

The designer is Julian Everitt.

[phum]

any idea why I can't see the photo?
Peter