George Dewey

Keel trailing edge bevel

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My new (to me) 2004 Ben First 36.7 is briefly hauled and I see that there is no bevel on the trailing edge of the keel. The class rules allow up to a 30 degree bevel. Is it worth the effort to add it? I do plan to race the boat. My understanding, which may be wrong, is that the purpose would be to reduce the vortex and therefore drag off the keel. 

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If your trailing edge is very thin, 1-2mm, then leave it. Or improve it..

If it's bulky, put a bevel on.

I would put in the hours and make sure the t.edge(s) is symmetrical and as thin as possible.

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Couple of thoughts on this, although both arrive at similar conclusions.  First one is from PYD, second is from S Thompson:  Bimmers input is similar to Thompson's.

Basically for width >2mm, go with a radiused bevel.

 

principlesofyachtdesign6.35.jpg

keel-rudder.gif

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On 5/8/2018 at 4:29 PM, Itsabimmerthing said:

If your trailing edge is very thin, 1-2mm, then leave it. Or improve it..

If it's bulky, put a bevel on.

I would put in the hours and make sure the t.edge(s) is symmetrical and as thin as possible.

That is only true with some modification..  The boat is a cruiser/racer and will definitely not plane under normal circumstances and thus the nit-picky detail of what peeps do to planing sportboats and foiling craft does only to a very limited extent apply to the boat in question

Spending a lot of effort to get to a 1mil thin trailing edge is way overkill and may also be against class rules.  Symmetrical is usually a poor choice as the keel can develop carman vortices resulting in keel hum (and the associated drag) - which is why you see in some of the drawings posted in the thread are purposefully assymetric - this is to avoid "keel hum" and the drag that comes with it.

 

For a 36.7 it would probably make sense to get to about a 3mm thick trailing edge and bevel it 30 degrees.  Just remember - the thinner the edge - the more fragile it becomes and more of a PITA to repair

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We changed our trailing edge from symmetrical to asymmetrical and stopped all humming at speed. I don't know whether Karman vortices have a measurable effect on boat speed, but they can generate nasty resonances and annoyingly loud humming that really can't be good for structural integrity long term.

 

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Another thing to consider is what happens if you wrap a rope rode around the keel. A sharp edge can cut right through, I've seen it done.

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Oh no, you can totally hear keels hum. I sail in classes that don't allow weirdness at the trailing edge and every boat hums at some speed or another.

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Oh yeah I saw that rule, I just didn't know if it was worth the effort. As it turns out, I'm just out of time. The boat launches on the 17th and I am all booked up with projects until then. I'm considering doing a complete bottom job next season, so this can wait for that. This is my first season with this boat, so it won't be a lack of keel bevel that costs me races! 

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3 hours ago, fucket said:

Oh no, you can totally hear keels hum. I sail in classes that don't allow weirdness at the trailing edge and every boat hums at some speed or another.

How fast would I have to be going to hear the hum?

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32 minutes ago, jarcher said:

How fast would I have to be going to hear the hum?

I don't think I've ever heard a boat hum under 9 knots or so, but I've been told it happens. On J-105's, every boat I've sailed on will hum with 10+ knots.

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2 hours ago, fucket said:

I don't think I've ever heard a boat hum under 9 knots or so, but I've been told it happens. On J-105's, every boat I've sailed on will hum with 10+ knots.

Well then, my goal is to get my keel humming!

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Yeah, my goal is to get a hummer too.

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