Fixed Bilge Keels...Effective?

[Jim Cooper]

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!

 

[psneeld]

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.

 

[Diesel Duck]

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

 

[kulas44]

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".

 

[Jim Cooper]

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.

 

[psneeld]

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.

 

[Sailor of Fortune]

Fast Fred on here is the only member that I can think of that has them.

 

[FlyWright]

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?

 

[SCOTTEDAVIS]

Can be a problem at haul-out on a travel lift, straps may not play well with the darn things.

 

[kulas44]

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.

 

[Jim Cooper]

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.

 

[BruceK]

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.

 

[kulas44]

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.

 

[FF]

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.

 

[Mako]

The are good threads on TF about this subject. This one I remember lately:
http://www.trawlerforum.com/forums/s3/static-fins-22721.html

 

[kulas44]

FF, that is one of the best reasons for bilge keels. And they can be configured to protect twin props.

 

[AusCan]

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)