keel fin thickness

[usa2]

how thick would a keel foil need to be for a canting keel that has an approximately 200 lb bulb on it? The part of the foil below the boat will be 4 feet deep, and the part in the boat will be 3 feet high. The fore and aft length of the fin is 1.5 feet. I know the fin has to be slightly fatter than a normal foil, but by how much?

[the_sphincter]

1.28432432 times thicker than a normal fin. What materials are you using? How do you want to construct it? How much of a safety factor do you want?

[usa2]

it will be a steel fin made of a solid steel plate. Overall it will be 7 feet, but 3 feet down will be the pin that it will rotate at the through hull fitting.
The safety factor should be 4-6 times normal load. This boat will not be raced in over 25 knots if we can avoid it.

Why does the fin HAVE to be thicker than a "normal" fin? If you use CBTF or a ripoff version it's true that the fin section CAN be thicker than a "normal" fin for the purpose of reducing wetted surface since it wouldn't be used for lateral resistance.But you don't start out with a preset chord for the foil in that case..Keep in mind when CBTF works well it's because the whole system is interrelated-and CBTFco could be a big help. If your lateral resistance uses a trim tab or gybes then you have patent trouble unless you talk to CBTF; if it is fixed then you need a fin that IS normal in it's t/c ratio. Guess you've made a decision on lateral resistance because you don't mention a passage thru the fin for hydraulic lines or a shaft.
Your fin sounds incredibly heavy and unwieldy to me-200lbs is less ballast than you might have on an 18'sport boat with a canting keel.Just guessing: I'll bet that fin could be made of carbon and wood....
I hope you're just having a (hopefully) interesting discussion and not seriously asking for an engineering solution here. You need to pay a naval architect/marine engineer to look at your whole keel design!
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edit: it is! it is! Just did a rough calculation
based on an 18% thick section(if you're using CBTF) and the fin weight is approximately 921pounds! Or 621 pounds @ 12% t/c roughly speaking. I'm sure you'll find that that is way too much-but hire an engineer!

[Shife]

Originally Posted by usa2

it will be a steel fin made of a solid steel plate. Overall it will be 7 feet, but 3 feet down will be the pin that it will rotate at the through hull fitting.
The safety factor should be 4-6 times normal load. This boat will not be raced in over 25 knots if we can avoid it.

Is the fin, pivot, and lever arm, going to be machined from one piece, or welded together? If welded, then there will be some serious stresses involved on those welds. Doug may be on to something with this. It may be cheaper and stronger to build the assembly out of carbon and wood. Machining a fin that size with a pivot and lever arm, out of a solid block of stainless, would be mucho, mucho bucks.

[usa2]

it will be one fin. I am talking to naval architects about this, i just thought it would be interesting to see what you guys had to say on the subject. I also sent an email to CBTFco....they havent replied.
If i had everything my way the fin would be carbon, but i had to look at a steel one for the time being. However if it is as heavy as you suggest and my own calculations have found, then we will be going a different direction. Or just make the strut around 200 lbs and forget about the bulb.


The reason you would make it thicker is that with a canting keel, it will want to flex side to side more than a normal keel, because it is always at 45-65 degrees or more, rather than 20-30 degrees.

2, at that amount of ballast the ONLY reason to go thicker than required for best performance would be to create a Schock 40 type strut that is not designed to create lateral resistance but to have the lowest wetted surface possible. But that will only work well with a CBTF type system.
You say one fin-does that mean you're not using any extra lateral resistance? What cant angle?

[usa2]

one of the naval architects just phoned me and is suggesting we ditch the canting keel idea.

If we do go with the canting keel, the designed cant angle is 45 degrees. we were going to use wings on the trailing edge of the bulb that had variable incidence-but not retractable.

Back to what i was just told on the phone, the revised plan is use a forward rudder and aft rudder, and a strut( non-lift producing) to hold a minor amount of ballast. The designer said (like some of you guys said i believe) that wth 5 crew on the rail the keel ballast is not the primary stabilizer.

Do you intend the boat to be self righting? I was wondering whether it would be with a normal size rig on a 28 footer and just 200lb.'s of ballast....Carbon mast?
One other thing: are you sure you wouldn't rather ditch two or three of the crew and add that weight to a 60° canting keel? Bethwaites 79er has a hand operated 60° cant angle keel on a 26 footer. Seems like it might be a problem to always have to have 5 crew...
Retractable wings might be better but probably aren't possible with the small amount of ballast;at any rate Andy Dovell thinks real highly of fixed high aspect wings and Bethwaites keel on the 26 is a wing like on Procyon:the bulb is fashioned into a wing shape.
When do you think you'll build it?

[usa2]

Selfrighting would be good-whether or not that will happen is questionable but i bet i can talk to the naval architect and make that a requirement and see how much extra weight that will be. The rig will be either a J80 mast and sails or an Etchells 22-to save costs. The rig will be modified to fly asymmetrical spinnakers from the masthead (The J80 already does this so we'd leave that alone).
For crew, we want at least 4 in case something gets out of hand and we are in close quarters with other real classic yachts. The owners do not appreciate it when varnish gets scratched. The plan so far is 1 person driving, one on mainsheet, one on jib/spinnaker trim, and the foredeck. There would have been another person controlling the keel and wing, but now we might use them to control the canard. Or we will link the canard to the rear rudder, but i dont think that will happen because of the need to have both "collective" abilities and the regular maneuvering abilitys. That will take some time to sort out.
Im not sure when/if construction will start....hopefully this coming summer (06) we will start constructing the hull out of wood, epoxy(lots of it) and fiberglass.

You might want to think about that. If you place the twin rudders in the position recomended by CBTFco to take advantage of a reduction in wavemaking drag BUT you don't cant the strut you may have a problem with the aft rudder at speed due to abnormal wake influence from the strut.Talk to Bill.
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2, if you use a strut which would be a "fin" not designed to develop lateral resistance then you MUST use collective or the fin will develop way too much drag. You should pay a small fee ,if any, and make it a point to talk to Bill-it would save you from having to reinvent the wheel and they are interested in special projects.Remember that one of the first boats Alberto Calderon modified with twin foils was a FIXED keel boat and Bruce Sutphen one of the guys at CBTFco did some model work with twin foils , fixed keels and collective.
Seems to me that you're looking in the direction of innovative solutions and given the money that this is going to cost if you want the most bang for the buck you should really make a point of checking into getting help from guys that have worked with this stuff-you might talk to CBTFco, or Alberto Calderon who is still around out in Kahlifornia. I would bet that you would wind up saving money,time, trial and error if you talked with any of these guys!

[usa2]

what do you suppose would happen if i used twin high aspect rudders on each side of the stern to avoid the wake influence?

See part "2" of my last post-and the last sentence of part 1.
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If you mounted two rudders on the transom you wouldn't take advantage of the reduction in wavemaking drag possible with a correctly positioned set of rudders, I mean twin foils..

[usa2]

im waiting for his reply to my email. its been a few days since i sent it.

[Raggi_Thor]

I'll be very careful here as I have embarassed myself before with wrong calculations. BUT we use an extruded aluminium profile with approx 200lbs lead bulb on our Backman 18. That's a naca 0012 (12% thickness) with a 300mm cord length. The walls are approx 3.5 mm and there are two cross-beams or stiffeners inside. See http://www.mboats.no/Hardware/hardware.htm
Last time I discussed this issue here, some people thought this profile was too weak. I think the English say "the proof of the pudding is in the eating", and the boats are sailing with these keels (C55 im Sweden and Backman 21 in UK)