CB foil project

Moderator: GreenLake

Re: CB foil project

Postby GreenLake » Mon Jul 13, 2015 1:36 pm

I did the edge modification on the original CB. (Like K.C. I missed a perfect occasion for making a replacement, because I needed to put the boat back together).

I took 3M "High strength marine filler" and spread it along the trailing edge of the CB. Then I took a piece of wax paper, folded to make a long strip forming a shallow V, and used that around the filler to form it into a V shaped edge. Taped the wax paper to the CB while the filler was curing. Then sanded off the bumps and in the rear, sanded it to a 1/8 square section (actually shouldn't be quite square, one side should be a little further aft to help shed the vortices). I then faired over the transition between the board and the about 1/2 to 3/4" extension and on the leading edge, I bulked it up a bit to have a nice circular cross section. (Added some filler and fairing compound to widen the front 1/3 a bit, all in an attempt to get a bit closer to a real foil shape).

Anyway, I haven't had any hum out of that CB since, but a replacement might give still better results.
~ green ~ lake ~ ~
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Re: CB foil project

Postby Lil Maggie » Tue Jul 14, 2015 9:42 pm

I think the issue with my foil is that the chord near the tip of the board is thicker than the rest of the board (see my picture on the previous page)...a proper NACA 012 must maintain that ratio as the length of the section decreases. In my case, the last 6" or so are more of a 009 than a 012, because I did shape the 1/8" trailing edge into the board, and the last 6" is where the radiused leading edge transitions to the 1/8" t.e. and I rushed that part of the board. Raising the board up some somewhat mitigates the issue, and I think the loss due to vortexing is more than made up by the improved upwind performance. KC, you should give the foil a whack. I would like to try another using slightly denser wood like mahogany and really take my time shaping the tip. GL, I might pull the board this winter and try to fill and reshape the trailing edge near the tip, but I don't think it will get rid of the hum completely.

Cheers
Mike
A crappy day sailing is better than a good one at home...
DS 1 #2313
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Re: CB foil project

Postby GreenLake » Wed Jul 15, 2015 2:14 am

What I read from the picture
1685
is that the maximum chord is not at 1/3 from the forward edge near the tip. That's possibly more significant.

However, much nicer than a stock CB! And better than my "retouching" job, even if I did get rid of the "hum".
~ green ~ lake ~ ~
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Re: CB foil project

Postby Lil Maggie » Sat Jul 09, 2016 9:43 pm

Wow! long time no post....yes GL, I lose the 1/3 chord near the tip of the board, but it's not an issue in light winds, and I only get the hum if I exceed 6 kts with the board down. BTW I did get another chance at that Flying Scott the other day...we were sailing even upwind in light winds but I crushed him on both reach legs (were racing a triangle), enough that it couldn't recover on the final upwind leg...for their benefit, I was racing solo that day and they were full crewed; it would have been another story with higher winds

The foil is holding up really well...good tracking upwind and so far staying in one piece
cheers
Mike
A crappy day sailing is better than a good one at home...
DS 1 #2313
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Re: CB foil project

Postby GreenLake » Sun Jul 10, 2016 12:13 am

Thanks for the update.
~ green ~ lake ~ ~
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Re: CB foil project

Postby talbot » Sun Jul 10, 2016 8:55 pm

I tried to follow the posts over the years. At some point, did we get an estimate of how much the wooden board weighs?
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Re: CB foil project

Postby Lil Maggie » Tue Sep 27, 2016 9:08 am

I think mine came in about 16 lbs, it is made from construction grade K-D spruce, West system epoxy and f/g cloth. It has a 1/2" thick aluminum insert where the lift handle goes in. My old board runs at about 19 lbs ...

Mike
A crappy day sailing is better than a good one at home...
DS 1 #2313
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Re: CB foil project

Postby GreenLake » Tue Sep 27, 2016 12:32 pm

For those worried about the class rules, the upper limit is 25 lbs (from memory).

A wooden board like this, without any weights added, may have net positive buoyancy, which is perhaps not totally desirable in the CB (in contrast, to say, a rudder). Whether net positive or negative, I don't expect it would be very far from neutral, in either case.
~ green ~ lake ~ ~
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Re: CB foil project

Postby talbot » Tue Sep 27, 2016 3:08 pm

I believe the replacement board I got from D&R Marine was more than 25 lbs. Forgive me if I don't run out to the boat and pull it out to check right now, but I weighed it when it arrived, and I recall it being >30lbs. Might be something to find out if you are racing and order a D&R replacement CB.

By the way, our Precision 21 is advertised as having a foil-section rudder. I wrote the company and asked if the same was true for the CB. They said yes, they use foil sections for all CB's and rudders. They don't understand why everyone doesn't do the same, as the foil section designs are free. It might be a question to ask Cape Cod &/or D&R. If foil-section control surfaces are class-legal and people are willing to pay for them, why not offer them as an option--or even as standard? Other than the cost of changing molds (which would be passed on to us), is there any reason not to use NACA foils for CB and rudder?
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Re: CB foil project

Postby GreenLake » Tue Sep 27, 2016 5:01 pm

The class association has in the past acquired molds for CBs with foil sections, these are available somewhere. (Perhaps someone can find the details online or in some old issue of the DSQ quarterly?)

A new CB is reasonably expensive, so if you buy one, I agree, get one with a foil section. Given that many people that purchase small older boats like the DS are doing that to keep the cost of their hobby down, building your own foil will remain attractive.
~ green ~ lake ~ ~
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Re: CB foil project

Postby Lil Maggie » Tue Sep 27, 2016 9:26 pm

All I know is I raced three "regattas" and have three first place trophies from it this year. My CB stays down but if you move the handle anywhere in between it rides up, so it may be a little on the buoyant side....nothing I can do about it
A crappy day sailing is better than a good one at home...
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Re: CB foil project

Postby talbot » Tue Sep 27, 2016 11:56 pm

First of all, congratulations. I suspect the victories had more to do with the seamanship and tactics of the skipper than with the shape of the CB.
Secondly, sure there's something you can do about it. Just install an CB downhall pennant on the handle. Then you can set the angle of the board anywhere you want.
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Re: CB foil project

Postby GreenLake » Wed Sep 28, 2016 1:05 am

talbot wrote:First of all, congratulations. I suspect the victories had more to do with the seamanship and tactics of the skipper than with the shape of the CB.
Secondly, sure there's something you can do about it. Just install an CB downhall pennant on the handle. Then you can set the angle of the board anywhere you want.


What he said. And: If you used velcro (and used it the right way) to make the loop around the handle, you probably still could get the CB to release in case of a grounding w/o needing to purchase an auto-release cleat.
~ green ~ lake ~ ~
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Re: CB foil project

Postby Lil Maggie » Sat Jul 15, 2017 1:29 pm

Bumping an old thread again but ....last fall I managed a few gouges/dings on my centerboard (trailer induced!). I immediately wet on the repair (though been pretty lazy posting recently). Also I have since replaced the offending rollers/brackets and replaced with a single 2x6 bunk to carry the centerline from the forward edge of the CB trunk aft....no more dings on the CB on trailering or launching...some pictures of the repairs and the new bunk:
2376
2375
2374
2373
2372
The arrow on the last picture indicates where I still need a roller to keep the bow from grinding against the trailer tongue on retrieval...
cheers
Mike
A crappy day sailing is better than a good one at home...
DS 1 #2313
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