Keel Position

[thealliance]

I am building a boat and have a bulb keel, and a mast, but im not sure where to place them in relation to one another. can you help me?

[Raggi_Thor]

Are you serious?
No drawings?
No calculations?
I hope this is a small project ;-)

Normally the geometrical center of the sails should be a little bit forward of the center of the underwater profile, including the keel, looked from the side.

Do you have any books?

[thealliance]

None. dont wory, its very small. just big enough for one person.

Ok, but how far? Are there any proportions, any rules for that kind of thing?

No books. are there any books about that?

[Raggi_Thor]

OK, take a look at this thread:
center of effort of a gaff sail

And this "primer in yeach design" from a veteran, Ted Brewer:
http://www.tedbrewer.com/yachtdesign.html

Lead ("leed") is normally a few percent of your waterline length.
How much you need depends on how much your boat heels, if you have a high rig, narrow boat, deep keel. All those factors will produce more weather hemlm, so you need more lead to balance.

[Raggi_Thor]

Search this forum for books, teher are thousands :-)
John Teal has written a short and simple book, " How to design a boat".

[thealliance]

Thanks.

It has a roughly 1:3 ratio of boom to mast, and an equal size headsail. the boat is more like an america's cup boat, but much much smaller. the hull is thin and round, so it looks like it will heel quite a lot. i want it to be pretty hairy to keep up, and a not too deep keel.

I asume that the keel should be in the center of effort for the sail, but it looks like that will be too far foward, and will cause the boat to nosedive (and i dont know where to put my mast yet, either), but if i put the keel back a bit, wont the boat rotate around the keel, as the effort is at a different place from the keel ?????

Should i just place my mast where it looks apropriate, then place the keel acording to that.

Search this forum for books, teher are thousands :-)

Ok. thanks.

[Mikey]

Another factor that affect keel placement a lot is symmetry of the hull. It sounds in your case like it is a rather slender hull where the aft is also slender. This would reduce the lead further, or to put it simple, that would put the keel closer to the mast.

A good starting point is to put the mast at 40%.

Mikey

[thealliance]

So if you had a boat that was entirely symetrical ('+' smyretic?) the keel would be below the mast? or further away from the mast?

A good starting point is to put the mast at 40%.

That's 40% of the hull length from the bow?

[Mikey]

You don't want a hull that is entirely symmetrical. That would reduce rightening moment (the boats ability to withstand a sideways wind force) and many many other things.

But yes, if you did have a totally symmetric hull, then the keel would be under the mast.

All percentages are % of the length of the waterline. The "most forward point" of the waterline is called forward perpendicular and "the aftmost point" of the waterline aft perpendicular. To put the mast at 40% simply means that you start at the forward perpendicular and move aft 40% of the length of the water line.

Let's put it like this
The distance between mast and keel aims to counteract the weatherhelm caused by the yacht heeling

This is a very good article that is explaining how this works in a very simple way, read it
http://www.duckworksmagazine.com/05/...bj/3/index.cfm

Mikey

[Mikey]

Originally Posted by thealliance

...but it looks like that will be too far foward

Absolutely correct, you would get enormous lee helm

[thealliance]

So the further forward the keel, the bigger weatherhelm?

There should be an attachment:

For the kind of boat ( i know it looks normal, but its hard to draw), is the keel position more the green, red, blue, or black?

Hope the pic works

I have gathered from that artical that the keel should be slightly back from the center of effort, but the weight far enough back so that it is tilted upwards slightly when at rest, but level (looking from the side) in normal winds.

The Alliance

[Mikey]

Pic didn't work, click on "Manage Attachments", click Browse and then upload