Bilge keels for 48’ steel displacement F/V

[FishermanTravis]

Hi everyone,
I am hoping someone can offer advice on the most effective angle and how to choose the attachment point of bilge keels. I will be installing these myself on a 48 foot steel vessel. Beam is 13’6” and draft 6.5 feet. The vessel has a very round chine like a wine barrel and has a very active roll motion. I have two bilge keels that were already made for something else. They are 12 feet long and 18 inches wide.
Is there a rule of thumb for the angle of attachment for most effectiveness? Also what is the best way to lay them out for the least resistance to forward motion? I’ve heard parallel to the keel and also parallel to the waterline, but the shape of the hull would make them have a curve if measured that way Thanks

 

[comfisherman]

Favorite postion for the ones we've placed have been, as wide as possible but tucked under enough that they don't catch pots or pilings. Fair amount of folks try and figure out the best layout to also not heavily catch travel lift straps.... not sure what's best as it's a least worst kinda deal.

We've always done parallel to keel with a 3 axis laser to mark the hull.

 

[TANSL]

We've always done parallel to keel with a 3 axis laser to mark the hull.

Well, in my opinion, that is not a good practice.
In everything else I agree with you.

 

[FishermanTravis]

Well, in my opinion, that is not a good practice.
In everything else I agree with you.

what would you suggest?
I do imagine the front ends of the chocks would be wide and scoop water if parallel to keel, unless they started back where the beam was widest. I can also imagine the same effect though opposite with the suggestion to parallel the waterline. Maybe the answer is to split the difference….

 

[comfisherman]

The laser replaced "just eyeballing it" and is a pretty easy way to mark left right symmetry after tracing the hull out with a floating head level. Then lock the laser and transcribe whatever alignment looks like it works. Usually we then would make a jig on the line out of thin cut ply strips hot glued together. From there we'd do the final tweaks and test fit. Then use the jigs to cut the plate before tacking together and test fit. Prior to that we did a similar setup with a bucket full of plumb bobs.

Years ago the trend was shallower and longer that followed the hull contours. Seems like the last decade has had shorter but deeper versions.

 

[TANSL]

what would you suggest?

The bilge keel, which should be approximately 2/3 of the length, is usually placed (as I have always done on large boats) so that it more or less follows the lines of water flow through the submerged hull. I know that it is not possible to know the shape of these lines well, unless tests are carried out in a tank test, but a sufficient approximation can be obtained.

 

[Tad]

Typical modern configurations.....

 

[FishermanTravis]

Typical modern configurations.....
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I have seen these pictures and I agree they look effective and sturdy. Also they look like there’s a lot of edges for hooks to catch on while longlining halibut and sablefish. Thanks for the response!