Forums: Whisstock Designs: Integral keels, shoal keels and swing ballasted keels, swing unballasted centreboards and integral centreboards New Topic Post Reply
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renato
Posted: Mar 28th, 2012 at 20:38   |   Subject: Integral keels, shoal keels and swing ballasted keels, swing unballasted centreboards and integral centreboards
Hi Mr. Whisstock. I like so much your design (number 056) 26' Fast Sloop.I'd like to solve some doubts so that I can buy your project after your revisions as you have mentionned about that plans. Please, I would understand some questions about ballast.I read an article called 'virtual design' by jacques Mertens. He sad that 'Integral centerboard' is not so good as 'Integral Keel'. Integral centerboard would be when all ballast is the only single centerboard part - no ballast outside the centerboard, but Integral Keel, when the ballast would be bellow hull FIXED on its lenght like a retangle on side view. In this case, the centerboard would not contain any ballast. So Is really better the Integral Keel than Integral centerboard ? Second question: Is the centreboard of your design 056 ( 26' Fast Sloop ) ballasted or, all ballast is inside an Integral keel perhaps? Third question: So, Is it possible share the ballast between keel and centerboard in a single boat putting ballast at the tip (or from tip to the middle) of center board ? Maybe, I could draw a rough picturing this last question if you want. Sorry about any english grammar error. thank you so much sincerely, Mr.Whisstock (a pretty name!)
debenriver
Posted: Mar 30th, 2012 at 07:38   |   Subject: Integral keels, shoal keels and swing ballasted keels, swing unballasted centreboards and integral centreboards
Wow! That is a big question! And there is not a simple answer.

Sailboats get their stability (which is basically the ability to carry sails and stay reasonably upright) from two things: The shape of the hull; and the centre of gravity (CoG) of the boat.

The stability from the hull shape is called "form stability" Very wide hulls have more form stability than very narrow hulls. If you take this to extremes – like a catamaran or trimaran – then all the stability comes from hull form.

For monohulls, very wide hulls usually make less good seaboats than a rather narrower hull, so stability from the position of the CoG comes into play. They have a mixture of form stability and CoG stability.

It is easier to provide a low CoG by using a heavy material like iron or lead and put is as far down below the waterline as possible. Hence the ballast keel.

There is another function of a ballast keel or a centreboard or a daggerboard: they help the sailboat to sail close to the wind and not get blown sideways (leeway). And if they are shaped hydrodynamically, then they will additionally give "lift" to windward. The resistance to leeway and the lift to windward are basically nothing to do with the weight of the keel/centreboard/daggerboard but are a function of shape, position, depth and area. We can call this lateral resistance.

So, in simplest of terms, a ballast keel has two functions: it keeps the boat reasonably upright when sailing by virtue of its low CoG; and it helps the boat sail close to the wind by reducing leeway and providing lift to windward.

All other things being equal, a large lump of well-shaped lead as deep down as possible will give the best stability. And again, all things being equal, the more stable the boat the greater her ability to carry sail.

So there is no one answer to your question. It is indeed possible to share the stability/lateral resistance equation between say, a long shallow keel to give a good CoG, and a centreboard to provide lateral resistance. All our Pocket Cruisers have this arrangement.

Design 056 has two alternative keel arrangements. She can have a long shallow ballast keel with a centreboard; or she can have a fin keel with a lead bulb at the bottom.

It is also possible to have a ballasted centreboard, or a lifting keel as they are often called – the "Integral Centreboard" that you mention – and I have built several boats with this arrangement. Personally I do not like it. I prefer to have my ballast fixed so that if anything goes wrong with the lifting arrangements, then at least I have a stable sailboat, even if she isn't particularly good to windward. And to me, it is an overly complicated, prone to failure system.

I hope that I have answered your question in a untechnical way – it is rather a technical subject. And there are other considerations to do with the fore-and-aft position of a keel or a centreboard, so that the boat sails well, which I won't go into here.

Hope this helps!

George
renato
Posted: Mar 31st, 2012 at 02:16   |   Subject: Just for say Thanks
Thanks Mr. Whisstock. Thank you for your fast answer to me. I was able to understand everything of your clear words. So I would like to know a bit more about 056 20' Fast Sloop. Perhaps, an abstract like there are for the other sailboats ? But do not hurry, just when you feel good. Nice. Very nice designs !
debenriver
Posted: Apr 7th, 2012 at 08:43   |   Subject: Integral keels, shoal keels and swing ballasted keels, swing unballasted centreboards and integral centreboards
I will try to get to it. I would like to be able to include this design in the downloadable designs as she was (well still is) a very successful boat.

George