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Old 03-31-2016, 05:53 PM   #1
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Fixed Bilge Keels...Effective?

In my search for a retirement boat I came across the Symbol 48 LR Pilothouse with bulbous bow and amidships fixed bilge keels. I assume the bilge keels cannot be as effective as a gyro or active stabilizers but does anyone have experience as to what percentage they compare to the other stabilization systems? My observation - bulbous bow, full length keel, fixed bilge keels & 11 degree deadrise @ transom...there is a lot going on with that hull below the waterline!
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Old 03-31-2016, 06:11 PM   #2
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I am pretty sure without actually riding the vessel...it would take a lot of number crunching by an experienced yacht designer (or computer) to answer your question to any degree....


Someone with a similar hull and setup might say yes or no to their effectiveness...but little differences could be all that makes it one way or the other....and there would have to be way more input into the number crunching.
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Old 03-31-2016, 06:21 PM   #3
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Jim:

If you haven't already read it, Michael Kasten from Kasten Marine Design has an informative write up on roll attenuation and bilge keels.

Here's a link to his article: Roll Attenuation and Bilge Keels
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Old 03-31-2016, 06:24 PM   #4
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My experience has been that bilge keels help to stop the roll cycle but also tend to inniate it. They want to "follow the water".
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Old 03-31-2016, 06:53 PM   #5
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Thanks for the article Diesel Duck, it helped. I was actually surprised to see how effective the bilge keels can be in comparison to other roll attenuation methods, possibly 75% that of active stabilizers! A lot less expensive, better reliability and low maintenance.
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Old 03-31-2016, 06:57 PM   #6
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But..a lot depends on overall design of the vessel....and a lot of bilge keels and bulbous bows were afterthoughts that weren't well designed...be careful.

If factory original...much better but still not necessarily ...as we all know marketing can trump engineering sometimes.
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Old 03-31-2016, 06:57 PM   #7
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Fast Fred on here is the only member that I can think of that has them.
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Old 03-31-2016, 07:32 PM   #8
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Great article, DD!

I wish I had 'em! Aligned to protect the running gear and attenuate roll underway at slow speeds. What's not to like?



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Old 03-31-2016, 08:29 PM   #9
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Can be a problem at haul-out on a travel lift, straps may not play well with the darn things.
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Old 03-31-2016, 09:32 PM   #10
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A lot is not to like. They are not even close to 75% as effective as ANY other accepted type of stabilization. They add to underwater wetted surface area,,all the time. They will make the boat roll in moderate conditions when it normally would not. They will "trip" a boat in a beam sea much faster. They are a total waste of time and $$$ IMO.
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Old 03-31-2016, 09:46 PM   #11
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Sounds like you have experience with them... I think I understand the potential for tripping in a large beam sea but pleas explain the roll in moderate conditions when normally it would not.
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Old 03-31-2016, 10:03 PM   #12
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A friend fitted bilge keels to a small half cabin cruiser which rolled like a drunken marshmallow, and reported huge improvement.
A large Symbol MY appeared at the recent and the previous Sydney Boat Shows, for sale. I`d be as concerned about the boat in general as the underwater design, be it original or subsequent addition, the boat was big for $, but appeared to me to lack the quality of comparable boats.
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Old 04-01-2016, 12:16 AM   #13
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As for the roll in moderate conditions, given that my experience has been with round bilge displacement type hulls with enough weight to be considered heavy, when a wave goes by the weight of the boat tends to make it lag behind the "slant" of the water going under it, letting the water slide by with the boat staying more vertical than if it were level with the water on both sides of the wave. Bilge keels tend to catch that water and keep the boat more level with the "slant". Hence bigger initial movement but quicker recovery from a wave, unless another wave is right behind that one. Higher speeds generally make bilge keels more effective. Very similar to hard chines.
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Old 04-01-2016, 07:12 AM   #14
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Many bilge keels are so the vessel will remin upright when taking the ground.

England has 20 ft tides , so "mud berths" are common , deep water moorings were spoken for before WWII.

The added keels on LUCY are about a foot deep and about 30 ft long.

They were to slow the roll as LUCY was used as a deep water lobster supply boat.

NO idea how well they work compared to a similar 50 ft Utility Boat.

In a bad beam sea we simply change course a bit, or UGH, open the throttle.
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Old 04-01-2016, 07:50 AM   #15
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The are good threads on TF about this subject. This one I remember lately:
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Old 04-01-2016, 07:56 AM   #16
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FF, that is one of the best reasons for bilge keels. And they can be configured to protect twin props.
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Old 04-01-2016, 07:56 AM   #17
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My boat very round bottom boat has 15 foot x 1 foot bilge keels. (although only 7 tonnes and no bulbous bow) Earlier models of the same boat didn't have bilge keels and had the reputation of being very rolly.

I haven't had the opportunity to directly compare the ride of the two designs, but the designer claims an improvement. The boat is still very rolly in a short interval beam sea, so I doubt the claim of a 75% improvement. My guestimate might be 25%. I can definitely tell you that raising my steadying sails reduces the rolling a huge amount with a >10 knot wind on the beam. (from a ride that requires holding on at all times when standing to almost completely steady)
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