Dinghy development: centreboards

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centreboards

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=341
Printed Date: 12 Aug 25 at 9:07pm
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com

Topic: centreboards

Posted By: tgruitt
Subject: centreboards
Date Posted: 18 Dec 04 at 6:51pm

gybing centreboards, good or not?

I think they're good, what does everyone else think

Replies:

Posted By: Bruce Starbuck
Date Posted: 18 Dec 04 at 10:24pm

Never used one, but my gut instinct would be to be very suspicious. The type of boats which use them (say 505s for example) would plane upwind in any decent breeze. In these conditions, you would be putting the bow down to generate speed and would be tacking through more than 90 degrees. The windier it gets, the lower a planing boat will go upwind. Height is secondary to going fast, and it often seems that height is obtained by going fast, so why have the extra drag of an angled centreboard?

On the other hand, it often seems to be the case in sailing that something just seems to work, even if you can't quite put your finger on why, so who knows?

Posted By: redback
Date Posted: 18 Dec 04 at 11:52pm

I've tried them but think its best to have the tightest fit in the case - the gybing board used to move about in a chop, which can't be good.

Posted By: Stefan Lloyd
Date Posted: 19 Dec 04 at 6:11pm

From what I remember, Merlins tried them but soon abandoned the experiement. "Like driving down the motorway on bald tyres" was one well-known helm's description of the downwind experience.

Posted By: JimC
Date Posted: 19 Dec 04 at 8:39pm

Originally posted by Bruce Starbuck

so why have the extra drag of an angled centreboard?

Why should there be any extra drag? The boat will travel at the same angle of attack to the board, no matter what. With a non pivoting board the boat travels slightly crabwise. With a gybing board of exactly the right angle (and how you work that out goodness knows) then the water is travelling exactly straight down the boat so there ought to be slightly less drag if anything. There will also be a tiny difference in the jib slot, shich should be the tiniest of fractions more open. On the other hand the rudder is operating straight in the wake of the daggerboard. If you tune your boat so that the rudder supplies some of the side force as well as the daggerboard this is bad news.

Posted By: Dave S
Date Posted: 19 Dec 04 at 9:42pm

Originally posted by Bruce Starbuck

why have the extra drag of an angled centreboard?

Because you have to have it anyway, you don't have a choice. Your centreboard needs an angle of attact to generate lift; this is why all boats with symmetrical foils have a certain amount of leeway, and also why you go sideways if you stall your foils out on the start line. If your foils are inefficient (eg a bilge-keel cruising boat) your leeway could be shocking; with modern high-performance foils on a fast dinghy it should only be a few degrees. Nevertheless, your board is angled slightly off the direction of travel, that's how it works. The concept of the gybing board is merely to point the hull in the direction of travel so that it's just the foil going sideways, not the whole boat.

I suspect the reason that gybing boards aren't universally used is either that the extra drag of a non-gybing system isn't that large, or else Jim's argument that you may not want to run the rudder in the wake of the board.

When I sailed 505s, gybing boards were all the rage (they stopped gybing when you pulled them up to go downhill) but maybe that was a fashion thing; the boats I sail now have daggerboards, and I'm not aware of anyone having even tried a gybing board.

Dave

Posted By: Bruce Starbuck
Date Posted: 19 Dec 04 at 11:00pm

Well **** my hat, I didn't know that!

Never heard that theory explained properly before, but it makes sense. The board will always travel the same angle through the water, so you're just "bearing away" the hull to be in line with the direction of travel. Like it.

Would you have to free the sails a bit in theory then, to be at the same angle of attack to the wind as on a non-gybing board boat?

Posted By: JimC
Date Posted: 20 Dec 04 at 8:16am

Originally posted by Bruce Starbuck

Would you have to free the sails a bit in theory then, to be at the same angle of attack to the wind as on a non-gybing board boat?

Yep, but remember dinghies run astonisihingly low leeway anyway, so probably hardly noticeable.