Why are stub keels short?

[Heath68]

Hi everyone,

Our cat is our on the hard at the moment and it is sitting on its stub keels. Looking at it however I cannot work out why the designer didn't make them longer?

Surely that would increase the point of having them (stopping leeway drift) without sacrificing depth of draft. The only thought I had was it might make the boat less easy to turn?? But I know little about catamarans so I didn't know if that would be true.

The reason I liked the idea of longer stub keels is so that the boat's waste and fresh water or even fuel or batteries could be stored in them freeing up valuable space inside the boat itself.

Any ideas anyone??


Heath

[Tupaia]

Picture maybe! How long and deep are the keels on your boat?
Stub keels are always a performance compromise when compared to dagger boards. They have many advantages in terms of practicality but this brings with it another bunch of compromises.

[catsketcher]

Some interesting reason why NOT to go longer than usual.

One is foil section - it is easy to get daggerboards or centreboards to a nice section - a NACA 0009 or 0010 or even 0012 is a good section for a daggerboard or centreboard. Going too thin is a problem for stopping stall - so if you have a 4 metre chord (chord is how long a foil is from front to back) then you could make it about 45cm wide to make it a 0012 section. That would be way too much drag so you thin up the section and shorten the chord - get it to 3 metres chord and go for a 0009 (9 percent thickness) and you still have a thickness of 27cm - again too wide. So designers try and reduce thickness but that does have issues with stall so you don't want to go too thin.

Also getting increase in chord does not produce a linear increase in lift - in fact adding chord to an already low aspect foil does little to increase lift at all. BUt it does add wetted surface area - so you could increase chord and produce no more lift when going to windward and then slow down even more downwind and in light air.

AS well as this - the centre of pressure in a foil moves depending on the angle of attack - this is not much of an issue with a daggerboatrd boat - if the CP moves 5% of chord then that would be only 5% of 600mm on my boat. But 5% of 4 metres is a big number and could cause issues with helming downwind - some minkeel cats are a;ready bad enough downwind - with swaying or needing constant helm movements to stay on track - not such a thing on dagger board cats.

So hopefully your designer knew of all the issues with long chord keels and designed the thinnest strong keel he could with the best lift to drag ratio possible in with a short span. (Span is the depth of the keel)

As for putting stuff in the keels some of my favourite Aussie designers have done so. My absolute favourite designer - Robin Chamberlin (a real sailors designer) used the keels of his 13.5 designs to house the sump of the diesel so it could fit under the floor near the middle of the hulls. Robin is great at getting weight out of the ends. John Hitch went one further and his designs had alloy keels that were the watertanks. You bolted them onto the hull bottoms - strong and again, weight where needed.

cheers

Phil

[Kettlewell]

I had a 32-foot cat with long, shallow keels that were very useful ground bumpers since they were very strong and sealed off from he rest of the boat--the keels could have been destroyed and the boat would still float. She would sit very comfortably on her own two keels, which made going aground low stress. OTOH, the windward ability was not the best. Tacking was a bit slower than some boats too. Our boat was excellent downwind and ran off like on rails. I think long, shallow keels are a viable option for cruising cats, but they do reduce performance to windward.

[Heath68]

Hi, not very deep, I would guess that they are about 50cm.. they really should change this website so that I can upload a picture rather than it keep asking for a url.

It's not the depth that makes me think of a missed opportunity, it's that they are probably only 20% at best of the hull length. If they were twice as long but no deeper they could help more with leeway, provide better support for the boat on the hard, greater storage of the things i mentioned before, all without sacrificing draft.

[catsketcher]

Once you go to a certain chord - lengthening the chord won't help increase lift - or reduce leeway. And it is always less wetted surface area to increase hull volume for more storage rather than add onto a protuberance. So adding chord won't help increase lift and you get more storage by just adding some slight volume to the hull in the design stage - so apart some support for on the hard - adding extra chord to a large chord minikeel is not good designnwhich is why designers don't tend to do it.

[TeddyDiver]

Originally Posted by Heath68

Hi, not very deep, I would guess that they are about 50cm.. they really should change this website so that I can upload a picture rather than it keep asking for a url. ..snip.

Manage attachments down to below when writing a post

[Leadfree]

Suggest you read:

HIGH PERFORMANCE SAILING, Frank Bethwaite;
AEROHYDRODYNAMICS OF SAILING, C.A. Marchaj;
MULTIHULL SEAMANSHIP, leSueur;
THE CRUISING MULTIHULL, Chris White