Full-length keel

[daiquiri]

Hello,
I'm designing a wood sailing cat boat (with the option for a small outboard motor, electric or gas) for a friend. The actual (not definitive) dimensions are LOA = 5 m (16.4 ft) and B = 1.75 m (5.8 ft). Will try to mantain operative weight at around 600-650 kg (around 1300-1400 lbs).
The sail will be gaff-type, between 16 and 20 sq.m (170 and 215 sq.ft, respectively).
It is intended for short, very relaxed weekend trips near the shore (in a lake or in a not too choppy sea), so I don't need high Vmg's. The structure will be a mixed ply-on-frame and stitch n' glue.
Now, I don't want to complicate the construction and to steal the precious space in the cockpit with the slot for retractable daggerboard.
Hence, a choice of full-length keel.
The problem:
I have no data about full-length keels, so don't know what would be the correct size or lateral plane are for a boat of this size. I could simply copy some of existing designs - we have the example of the traditional "passera" or "pasara" boats here in Adriatic sea, which somewhat resembles this one both in shape and in dimensions.
But I would also love to hear opinions from people who have made boats with full-length keels and from those who are sailing them.
I would basicaly need to know what is, in your opinion, the most appropriate value for either of the two dimensions ("A" and/or "B") I've highlighted in the attachment (which shows the boat at the actual design stage).
Any opinion is appreciated, thanks.

[Ike]

I would think that a keel would be overkill on a boat this size. Since you ruled out acenterboard, I would opt for leeboards. They don't take up interior room, rotate up so you can get in shallow areas, and can be completely removed when you don't need them. They can be just as efficient as a centerboard or keel.

[daiquiri]

Thanks for the reply, Ike.
I agree with you that the keel will be a weight and drag penalty for this boat. The fact is that the person who will sail it would like to give it a look of the traditional small workboats from the Adriatic sea, while using the modern and simple boat building techniques (and a simplified structure). So it will have to be a compromise between the two needs.
Leeboards are very effective, we agree on that, but are not traditionally used in this part of the Mediterranean.

I'm sorry, I should have specified it in the initial post. :\

P.S.
This is the passera (in Italian) or pasara (in croatian) workboat, just to give a gross idea:
http://xoomer.virgilio.it/histria/ci...no/passera.htm

[tspeer]

Originally Posted by daiquiri

...It is intended for short, very relaxed weekend trips near the shore (in a lake or in a not too choppy sea), so I don't need high Vmg's. The structure will be a mixed ply-on-frame and stitch n' glue.
Now, I don't want to complicate the construction and to steal the precious space in the cockpit with the slot for retractable daggerboard.
...

Why not leeboards? They would be easier to build than a keel and preserve shallow draft, but not take up any space inside.

If you don't want to take up too much space, bilge boards would free up space in the middle and support the seats. Twice as complex to build as a centerboard, though.

[Ad Hoc]

daiquiri

I have a lovely old book with some plans.
There is a plan of a boat almost like yours, maybe this will help?

[terhohalme]

You should make the keel lower at bow. Otherwise CLR will stay too much forward and your sailing balance bad (too much windward helm).

[daiquiri]

Hello, thanks to everyone for their replies.

@tspeer:
The two reasons why I woudn't like to use the leeboard (but no decision is definitive yet) are aesthetics and the search for a traditional hull form... Yes, I know it is crazy from a purely practical point of view, but from time to time we should let the heart win over the reason...

@Ad Hoc:
Thanks for that page (and for the 64 series, but I'll write you in pvt on that one ). I have noted that it is a motor boat, and in fact it's keel looks a bit underdimensioned to counteract the sail forces.

I understand this is an argument on which very little reliable data exists, so if no quantitative advice arrives about the correct dimensions of that keel I guess that there will be two choices here, at the end:
- take the dimensions from some traditional boat, without asking myself why it (the keel) has to be just like that, or
- (craziness in action!) perform a cfd, with Ad Hoc's favorite software , just for the pleasure of better understanding the forces on such a super-extra-low-aspect keel.

The only problem is that I'm working on it in my spare time, so it will slow the design time quite a lot...

[daiquiri]

Originally Posted by terhohalme

You should make the keel lower at bow. Otherwise CLR will stay too much forward and your sailing balance bad (too much windward helm).

Hmmmmmph... Shouldn't it be quite the opposite?
If I make it lower (deeper) at the bow, I'll have more keel area at the bow and the center of pressure will shift forward. Is there something I'm missing about how this kind of keel works?
I see it as an extremely-low-aspect-ratio cousin of a delta wing, with a correspondant hydrodynamic behaviour...

[Ad Hoc]

Daiquiri
If you're more concerned about the lateral area, is this for directional stability?
If so, treat it like a rudder. Most rudders have an area of about 2% of the total underwater later area. Then look at the attached, calculate a rough idea from this "similar" boat. Then go about half way....?

[terhohalme]

If CLR is forward and CE of sail backward windward helm exist. In a long keeler CE should be about 10-15% fore CLR for a decent balance. Look at exísting long keelers.

[daiquiri]

@Ad Hoc:
That Gimcrack figures do help, indeed. It allows me to (guess)timate lift and drag forces vs. AoA for a very low AR Lateral plane form. It's a risky job, I know. My immersed hull form is different, but at least I have some shaky numbers to grasp on. Thanks.

[Ad Hoc]

Daiquiri

Isn't that what good naval architecture is all about?..best guesses....it is in my book!

[daiquiri]

Originally Posted by terhohalme

If CLR is forward and CE of sail backward windward helm exist. In a long keeler CE should be about 10-15% fore CLR for a decent balance. Look at exísting long keelers.

I just saw the explanatory pic you have enclosed. I had mis-understood the phrase "make the keel lower at the bow" in your first post. It is all clear now.
I will take your advice into consideration, also during the sailplan design. Thanks!

I remain open for any other good advice.

[PAR]

I believe what terhohalme is trying to say is cut down the forefoot. I would use a different approach, I'd incorporate a stub keel with as little wetted surface as necessary and house the centerboard within it, below the sole. You get a shoal draft keel, an appendage that will dramatically help sailing ability and this appendage doesn't intrude into the cockpit or cabin spaces.

I wouldn't use a full length, shallow keel, just a stub of a keel, which is enough to house the centerboard. Why place wetted surface under the boat when you don't need it. As a powerboat it'll help straight line tracking, as a sail boat it'll offer the shoal option with the board retracted.

[alan white]

MY boat is about 1000-1200# and 15 ft, full keel and no board. 5 ft beam.
You are over-canvassed for most anywhere and should reduce to at least 160-180 sq ft.
I have a cutaway forfoot, otherwise a 22" draft with moderate drag. this is not a boat that points high. She'll go to weather but not like a boat with a higher aspect board or keel.
Add six inches to the keel depth and cut the forefoot away, and even scoop in the after-keel ahead of the rudder, and it will go okay. The boat will track like a dream, and the weight will pull you through a tack. It's my favorite kind of boat.