Daggerboard vertical angle.

[waynemarlow]

Farrier has his dagger boards running at quite a slack negative angle from in front of the mast, to beyond the CoE of the sail area ( to get the mean effect of the dagger board to be at CoE ), the Pulse 600 runs a true vertical as with most beach cats, the moths are now running it at a positive 3 or 4 degrees to prevent air bubbles running down the board at speed.

Is there any reason why we couldn't angle the dagger board at say 10 degrees forward at the bottom, speeds are probably in the 8 - 20 knot range.

The reason I ask is that with an increasing number of Tris, the main and mast is getting back further and further in the boat with a small jib. We then add a large Screacher or Code O at the front. Now when the Code O is up and working, the CoE is way way forward and the dagger board is now in the wrong place as its set up for the main and jibs CoE. This then causes huge lee helm on the rudder.

If the daggerboard is angled forward then by simply making the board longer the effective CoE of the board moves forwards to the correct position. Thus a crew could adjust the CoE of the board to match that of the boats requirements.

Comments please.

 

[John Perry]

On our small monohull we have the centreboard, when down, angled aft about 15 degrees whereas the rudder blade is angled slightly forward to partially balance the steering. We do sometimes get weed caught on the rudder but not I think on the centreboard, although it is hard to be certain of that since we cannot see it under the boat. That may answer your question!

I have read that angling a keel/rudder forwards does avoid air entrainment, an advantage, perhaps even a necessity, for a Moth, but not relevant to keels that protrude through the bottom of an immersed hull.

I assume that the trend to move the mast further aft is motivated by a desire to set larger screechers, and maybe to some extent by styling. I have heard it said that it reduces the tendancy to pitchpole but I dont think that is true, the longitudinal position of the rig on the hull makes no difference to the pitchpoling moment caused by the foreward thrust component of the rig, at least not until the stern is high in the air by which time it is too late to worrry about it.

Ian Farrier does place the rig and daggerboard a bit further forward than most other designers but his more performance oriented boats can still set pretty big screechers just by having a long bowsprit.

 

[Stumble]

There are some structural issues with a forward swept lifting surface, in that they can become aerodynamically unstable. But a slight amount probably isn't a huge concern. The question is will doing it really effect the boat enough to make it worth the trouble.

On Farriers new boats I think he used a swept back board to get around the trunk/mast support issues, basically it was a room thing, two pieces of the boat needed to be in the same spot, and raking the board was an easy way to accomplish that for minimal cost.

But using longer boards to effect CoE sound like a really bad idea to me. You would need them maximally forward as the wind picks up, which means the board needs to be stiff enough to handle maximum preassure with a much longer lever arm. I would think building a trunk that allows for rake adjustment would be a lot cheaper. You could pretty much just borrow the design for foiling A-Cats beefed up to handle a larger boat for a lot less.

 

[Doug Lord]

Some more info here: http://www.boatdesign.net/forums/multihulls/forward-raked-daggerboards-53079.html

 

[Steve W]

Malcolm Tennant designed some cats many years ago with fwd angled boards but I don't know if any were built.

Steve.

 

[Richard Woods]

raked forwards is good in theory - albeit for marginal gain. But catches weed easily - major boat stopper!

Usually the daggerboard is in front of the mast and angled aft so that it can be withdrawn or even raised with the mast up and boom still in position

Richard Woods of Woods Designs

www.sailingcatamarans.com