Keel Spillplate?

[CatMonoceros]

I'm getting ready in a few weeks to haul out my French catamaran. Among my tasks are to correct the horrible keel shape, but I'm wondering if I could benefit from a spill plate added to the bottom without paying too big of a drag penalty.

Currently my keels are approximately 18 inches deep by 13 feet long. As you can imagine with that aspect ratio, leeway is a real problem. Going upwind at 38 degrees apparent wind, I'm making 12-14 degrees leeway. The factory also very kindly left me with blunt leading and trailing edges at about 1.5-2 inches wide. Correcting the leading and trailing edges will no doubt help a lot, but having recently seen the winglets appear on the new HH fixed keel boats, got me wondering if I couldn't benefit from something similar.

I downloaded any free design and analysis software I could find, but it would seem I'm not smart enough to figure out how to properly attach a spillplate or winglet, and if I did I can't figure out the analysis part of it. I think a properly shaped winglet is beyond what I can accomplish in the short time I'll be out of the water, but a spillplate may be doable if it's worth doing.

Help me smart people. Thumbs up or thumbs down on the spillplate? If it would help, how big should it be?

 

[gonzo]

If your boat is going about 50 degrees upwind, it is not great but neither terrible for a cruising boat. Can you post a photo? If the boat has a lot of windage, the keel may not be the major problem. Is what you call blunt a flat surface? Also, the interface between the keel and the bottom has a big influence on the flow.

 

[CatMonoceros]

It's a cruising cat designed mostly for the charter market (Fountaine Pajot Salina). I tried to add a photo, made a gallery and added a photo to it, but it still says I don't have any media. Sorry.

It certainly does have a significant amount of windage, but not nearly as much as many more recent designs. I'm not unhappy about the boat speed or apparent wind angle, but I would love to reduce the leeway. I feel like if I could reduce leeway I could sail a little lower and faster. And, dragging a keel sideways through the water at more than 10 degrees doesn't do the speed any favors.

The way the keels are manufactured is two halves that have a flat surfaces for leading and trailing edges, with rounded corners. This makes it easy for them to bond the two halves together but is far from a proper foil shape. Then they are inserted into a socket in the hull and secured with a high strength caulk. As such, there is no fairing at the keel interface, and it's not practical to add a fairing because the keel mounting is not rigid.

 

[gonzo]

Fairing will help. You can use foam with fiberglass over it. The forward side should be a bit rounded and the aft end finer.

 

[CatMonoceros]

Yes, fairing the keel to the hull would help, but like I said, it's not practical. The keel slots into a socket in the hull and is secured by caulk. It's designed for the keel to pull out of the socket in the event of a grounding. Good for the charter market, not so good for sailing. As such, it's not strictly rigid and would break any fairing or could possibly cause other problems. Fixing that would involve a redesign of the keel attachment and all the structural floors inside the bilges.

But, my question was much more about limiting vortex circulation around the bottom of the keels, and whether the drag penalty would be worth the additional lift.

 

[gonzo]

The fairing can be attached to the ends of the keel and the edges caulked to the hull. For a plate, you first need to calculate if the down force will not make the keel fall off the slot. Then, you would need an analysis to design the position and dimensions of the plate. This is assuming a plate would give you a significant advantage. A small gain in upwind performance may end up creating steering issues downwind. I believe fairing the keels is your best option. Otherwise, either contact the original designer, or procure the original plans for a Naval Architect to work with. The cost of it will be quite high. I looked at the photos of your boat, and it has a lot of windage.

 

[CatMonoceros]

I can understand how fairing the keel to the hull would reduce drag, but would it reduce leeway? AKA increase lift to windward? How?

 

[gonzo]

It would be a more efficient foil, which would increase lift. Blunt ends create turbulence and cavitation.

 

[Alan Cattelliot]

Hi CatMonoceros,

is this the type of keel we are talking about (salina 48) ? In that case, and due to the very low aspect-ratio of its plan form, spill plates won't be of a great help to diminish your leeway. This boat has a bad heading "by nature". Best VMG are reached with the sails always a little open, isn't it ? What Gonzo proposes is perfectly save. If you consider it not very practical, the only options left are :

- make a strut with a foam corner laminated at the trailing edge, in order to increase the keel surface and sharpen the trailing edge - the sharper the better -
- change the keels profile for a thicker one, by laminating directly on the extrado and intrado.

 

[wet feet]

I'd be tempted to try a variation between those offered in the preceeding posts.I'd wrap the leading edge and trailing edge with parcel tape or similar then cover with perhaps a 900gsm laminate and build foam fairings,with enough room for a sensible glass covering.I'd fair the surface to the existing keels and then remove the fairings from the boat for a bit of tidying.Then I would attach them with a good smear of Sikaflex or similar,knowing that it should hold for all normal use,but if the outcome is poor it isn't absolutely impossible to remove them.