Forums: Whisstock Designs: centerboard Design 055 New Topic Post Reply
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sante
Posted: Oct 29th, 2013 at 16:50   |   Subject: centerboard Design 055
Hi George,

I am gettimg forward with my boat. The frames, transom, rudder are almost finished. I am also working on the ballasted ply centerboard and I have a question concerning its weight. In your drawings the centerboard has about 10 kg lead in it. Now, on the George Blog I read about the addition of 100kg of internal ballast. Does it make sense to put this additional ballast (or a part of it) into the centerboard?

Regards
Sante
debenriver
Posted: Oct 30th, 2013 at 23:59   |   Subject: centerboard Design 055
Hi Santé

First - let me say how much I appreciated your photos - they are great - many many thanks!

Originally the boat was designed with an aluminium alloy centreboard. The ply centreboard with 10kg of lead encapsulated towards the bottom gives pretty much the same moment (weight x distance) as the alu board, so it's contribution to stability is about the same. Additionally, because ply and timber are buoyant you need some weight to help it go down and make sure it does't float up again (until you want it to!).

You could put more ballast weight in it, but only to the extent that you can haul it up – though that is relatively easy to fix by increasing the power of the lift purchase.

There is also a limit to the strength of a ply centreboard and a lot of weight low down will cause it to break. I think if you added a substantial amount extra weight you would need a 6mm stainless steel core – not all stainless steel - just a rectangular core made from, say, 50 x 6 flat bar welded up.

Because the original centreboard was metal, the case is not very wide, so it is difficult to get a wood/epoxy board thick enough to be strong, and there is a limit to the additional weight you can incorporate into it.

But you are perfectly correct – if you could get extra weight into the bottom end of the board, it would be more effective kilo for kilo than ballast under the floorboards.

Just simple moments: 100 kg x 0.2m (distance of the ballast below the dwl) = 20 kgm. 20kg x 1.0m (distance of the centre of gravity of the ballast in the lowered centreboard below the dwl) = 20 kgm. So if you increase the amount of ballast in the centreboard from 10kg to 30 kg (a 20kg increase) that would be about the same as 100 kg of ballast under the floorboards (when the centreboard is down).

Which is indeed worth considering.

I would be happy to draw up a scheme if you want me to?

Best regards

George
sante
Posted: Nov 1st, 2013 at 18:30   |   Subject: centerboard Design 055
Hi George,

I am happy that you like the photos. I will send you some more from time to time.

Thank you for your answer concerning the centerboard.
I think I will try the method of putting 20kg additional weight into the centerboard.
I appreciate that in this case the boats weight will be 80 kg less compared to the method of puttig 100 kg under the floorboards.

Regards Sante