Twin Centreboards

[Ginny Harvey]

Hi,
I have plans for a flat bottomed daysailer dinghy. It is designed with an offset centreboard.

As this design impinges on the position on which the helmsman can comfortably sit on the side with the centreboard I have been thinking of trying dual centreboards, but moved further out towards the gunwales. This I am hoping will also increase the stability of the boat.

My question is: would the area of the combined centreboards need to be the same as the single, or greater, and if the latter how would the area be calculated.

Thank you!

 

[Robert Biegler]

I have been thinking of trying dual centreboards, but moved further out towards the gunwales. This I am hoping will also increase the stability of the boat.

I don't know how that would affect stability enough to be noticeable.

My question is: would the area of the combined centreboards need to be the same as the single, or greater, and if the latter how would the area be calculated.

It depends on how you expect to sail. If you expect to heel enough that the windward board will only be halfway immersed or less, then ventilation can decrease its effectiveness below what you would expect from the immersed span. Then the leeward board needs to be large enough to do the job on its own. On the other hand, if you expect the windward board to do little, you can make the boards asymmetric, which gives a bit more lift, and you can decrease size a bit.

If you expect to keep the boat flat, then the same area as a single board should be fine, which would be 71% of the linear dimensions. However, linear scaling would mean that you have half the cross section and 71% of the thickness, and that would mean much reduced strength. Your boards need to be strong enough to support your weight when you stand on one to right the boat after a capsize. So perhaps keep chord and thickness the same as a single board.

I learned to sail on an Atlanta Koralle, a model with twin centreboards, rather than twin daggerboards. I found a picture:

I think that is about two thirds the draft you would expect from a single centreboard. If I remember correctly, the centreboards had an asymmetric profile, though so crude (just sharpened edges) that I doubt it made much difference.

I found another picture of what I assume is an earlier model with daggerboards, which appear to be larger:

 

[Blueknarr]

I suspect that you will be alternating boards while tacking. Therefore two full sized boards.

 

[Ginny Harvey]

I don't know how that would affect stability enough to be noticeable.


It depends on how you expect to sail. If you expect to heel enough that the windward board will only be halfway immersed or less, then ventilation can decrease its effectiveness below what you would expect from the immersed span. Then the leeward board needs to be large enough to do the job on its own. On the other hand, if you expect the windward board to do little, you can make the boards asymmetric, which gives a bit more lift, and you can decrease size a bit.

If you expect to keep the boat flat, then the same area as a single board should be fine, which would be 71% of the linear dimensions. However, linear scaling would mean that you have half the cross section and 71% of the thickness, and that would mean much reduced strength. Your boards need to be strong enough to support your weight when you stand on one to right the boat after a capsize. So perhaps keep chord and thickness the same as a single board.

I learned to sail on an Atlanta Koralle, a model with twin centreboards, rather than twin daggerboards. I found a picture:

I think that is about two thirds the draft you would expect from a single centreboard. If I remember correctly, the centreboards had an asymmetric profile, though so crude (just sharpened edges) that I doubt it made much difference.

I found another picture of what I assume is an earlier model with daggerboards, which appear to be larger:

I don't know how that would affect stability enough to be noticeable

Thank you @Robert Biegler for your help and photos. I had thought that if the lateral pivot point was actually the leeward centreboard, rather than in the centre of the boat, then you would have most of the weight of the hull keeping you upright, and therefore increasing stability. Does that sound right?

 

[Ginny Harvey]

I suspect that you will be alternating boards while tacking. Therefore two full sized boards.

Thank you @Blueknarr,
Would it work to increase the aspect ratio of the boards, so they are deeper and narrower, as the further out the cases are moved the less height in the hull there is to accommodate them?

 

[clmanges]

If your plans can accommodate it, gunnel-mounted leeboards would free up all of the interior space.

 

[philSweet]

You can let the boards stick out the bottom of the hull a couple inches. This is actually very helpful for centering the boat on a trailer that has side bunks - you just position the trailer bunks slightly inboard or outboard of the boards. It is also handy for running downwind. This allows the board case to live under the side seats.

Yes, both boards need to be full sized, and you only sail with the lee board down.

 

[Robert Biegler]

I had thought that if the lateral pivot point was actually the leeward centreboard, rather than in the centre of the boat, then you would have most of the weight of the hull keeping you upright, and therefore increasing stability. Does that sound right?

I think the pivot point around the longitudinal axis would be the centre of buoyancy. Using only the leeward board would shift the centre of buoyancy a little to lee, but it would be unlikely to have enough volume to make that noticeable. The main advantage of bilgeboards is to free up the cockpit. It could be a big benefit if you plan on dinghy cruising.

If you want righting moment, you could mount them further out and angle them with the tip in, like so:

That would mean you need a beam, but on the plus side, no moving parts.

 

[Ginny Harvey]

I think the pivot point around the longitudinal axis would be the centre of buoyancy. Using only the leeward board would shift the centre of buoyancy a little to lee, but it would be unlikely to have enough volume to make that noticeable. The main advantage of bilgeboards is to free up the cockpit. It could be a big benefit if you plan on dinghy cruising.

Thank you @Robert Biegler, that makes complete sense.

 

[Ginny Harvey]

You can let the boards stick out the bottom of the hull a couple inches. This is actually very helpful for centering the boat on a trailer that has side bunks - you just position the trailer bunks slightly inboard or outboard of the boards. It is also handy for running downwind. This allows the board case to live under the side seats.

Yes, both boards need to be full sized, and you only sail with the lee board down.

Thank you @philSweet that is very helpful.