Bilge Keels

Has anyone fitted bilge keels to a trawler(let`s skip the semantics) or an SD power boat hull, with twins or single.

If so, what form did they take, where were they placed,how was it done and attached,why did you do it, and what was the result.

I would be interested in that also. I have been toying with the idea of putting them on my boat.

Bilge keel are also known as rolling chocks in some parts. Here is a fairly long thread on them that might be worth perusing.
http://www.trawlerforum.com/forums/s31/rolling-chocks-ve-stabilizers-3164.html

Quick cut...


In previous threads I believe "batwings" were recommended as successful on hard chine boats where tough to fit bilge keels.


Soft chines are successful with bilge keels.


Someone reported success with some sort of chine "lip" on a hard chine trawler.


All create wetted surface and drag...so sizing is important to get the best compromise of rolling and speed reduction.

I have read most of the articles on these in the past, and couldn’t draw any hard conclusions as to whether they made much of a difference.

From what I've gathered, rolling chocks (the ones that typically protrude less than a foot from the hull and run about 3/4's the length of a semi-displacement hull at the chine) reduce roll only about 30%. I think the most important change rolling chocks would offer is in going from 'snap rolls' typical with a semi-displacement hull, to a softer change of direction.

They're probably the cheapest and easiest to maintain, as well as offering the least amount of worry; always deployed, never get fouled in debris or kelp, and won't get knocked off by deadheads.

Had a chat with a retired commercial fisherman based off the north end of Vancouver Island who had rolling chocks put on his semi-displacement hull. He said he put them on more for fuel savings, insisting they provided a bit of lift at cruising speed. Not sure about that one...

Murray at what angle? Straight down, 45 degrees, 30 degrees ....
If not straight down it would seem they could be damaged from floats or pilings.

Nomad Willy said:

Murray at what angle? Straight down, 45 degrees, 30 degrees ....
If not straight down it would seem they could be damaged from floats or pilings.

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45 degrees from looking at them. They're attached at the turn of the hull on a displacement hull or the chines on a semi-displacement hull, so they shouldn't extend out further than the rub rail or gunwale.

Hi Eric,

Looking at the photos in the following post I can see how docks might be a concern, but then again, that's what fenders are for

Trawler Forum - View Single Post - Rolling Chocks make them on the cheap

When I was considering a Schucker and was a member of their Yahoo group, one of the members had installed them on his boat. The Schucker is basically a fat round sailboat hull with a modest keel. The owner was quite pleased with the roll reduction and speed of roll.

My guess is the more round your hull is with less keel, the bigger the difference they will make. My boat has hard chines, is quite flat in the stern, and has a large keel in the stern third. These attributes I believe, make the roll very reasonable compared to many other boats. While it would be nice to reduce it further, it's likely only active stabizers would significantly improve it.

Ted

I was once a partner in a steel 78' stern trawler and the other partners insisted we needed roll chocks. After having them designed by a naval architect and having them fabricated and welded to the hull they made very little difference in the vessels roll. They may have dampened it marginally but the difference from not having them was hardly worth the twenty thousand plus they cost. After we sold the boat the new owners put paravanes on it which by reports made a satisfactory difference. I had another smaller trawler that I put a steadying sail on and was quite happy with the result, especially for the minimal investment. My current boat will be getting a sail.

MurrayM said:

Hi Eric,

Looking at the photos in the following post I can see how docks might be a concern, but then again, that's what fenders are for

Trawler Forum - View Single Post - Rolling Chocks make them on the cheap

Click to expand...

I would never order or build chocks like those in the link.
I can’t belive they pulled that boat w the travel lift. Seeing that they are that strong I wouldn’t pay the weight penalty either. Something that shouldn’t exist IMO. But I’ve seen round bilge boats with similar but I don’t think they usually stick out beyond the hull.
And the chocks in the link may cause that boat to trip and capsize rather than remain upright and just broach.

I didn't add mine so it is tough to say how much roll reduction they provide. The boat still has significant roll in a beam sea, but with the sails up I get a 90% reduction in roll.
With any stabilising system, they are far more effective on a soft chine round bilge vessel in comparison to a hard chined SD vessel.

Here's a thread on retro-fitting a set of rolling chocks:
http://www.trawlerforum.com/forums/s40/rolling-chocks-make-them-cheap-35679.html

I have no concerns in the travel lift. My rolling chocks/bilge keels are about 2" thick and protrude about 8-10" from the hull, but are not far off vertical.

AusCan,
Looks good to me but not much style but out of sight.

I see your keel and it looks much like mine. Like the flat sided tear drop rudder and it looks like a very stout shoe too. May even up-stage the Willards there mate.
And I agree sails are the best anti rolling device.

Are you on a marine railway?

I believe Fast Fred has them on his converted Navy UTB "Lucy". The previous owner installed them I think..

One advantage of passive "bilge fins", as the Millkraft Yard here calls them in an Invoice,is that although they are likely not as effective as active fins, they don`t need power,hydraulic and/or electrical, and running a genset. I saw a stabilized late GB42 advertised with 1250 hours on the mains, and a rebuilt genset with 1000 hours. I suspect a connection.
According to an owner of a 40ft SD boat, if I understood correctly,the fins reduce roll after the first roll occurs.
Pretty sure bogranjac1/Brett made and fitted his own, as well as flopper stoppers.

Not a lot on the interwebs about proven performance but I did find this article about roll dampening vs stability interesting: https://seagrant.uaf.edu/bookstore/boatkeeper/bilge-keels.pdf

Fish53 has spent his life running boats and gave you his opinion. My impression is similar from I have read when I was researching Stab Systems about 2 years ago. But, others also say they help, so I suppose its up to you to determine if they are worth it and how much bang for the buck they could provide.

Nomad Willy said:

AusCan,
Looks good to me but not much style but out of sight.

I see your keel and it looks much like mine. Like the flat sided tear drop rudder and it looks like a very stout shoe too. May even up-stage the Willards there mate.
And I agree sails are the best anti rolling device.

Are you on a marine railway?

Click to expand...

Oh mate, Not much upstages a Willard, but thanks for the compliment.
The shoe is stainless. I treated it with prop speed as regular antifouling doesn't last long on it, and any buildup of growth disturbs the flow to the prop.

Yes , our little marina has a rail mounted slipping setup. It works well. There's no worries of a boat getting dropped.

Here's another good article on roll attenuation.

Roll Attenuation and Bilge Keels

AusCan said:

Here's another good article on roll attenuation.
Roll Attenuation and Bilge Keels

Click to expand...

Apart from important information about roll improvement from bilge keel fins, this is the second article referencing the vessel "Boojum".
This I think comes from the Lewis Carroll poem "The Hunting of the Snark". According to the story, you were ok hunting a sea creature called a Snark, but if your Snark turned out to be a Boojum instead, you could expect a world of trouble. There is an entire CD of the words, set to music specially written, and voiced by some eminent English actors.

So I should not be snarky and say they are not much use?

I am somewhat intrigued by Marlow's use of twin keels. I see a lot of benefits there. They look like they could have been copied from fins on surfboards, but it seems that they weren't.

Marlow Yachts advanced technology found in their luxury ocean capable yachts.

http://www.marlowyachts.com/Velocejet_Strut_Keels.html

Insequent said:

So I should not be snarky and say they are not much use?

I am somewhat intrigued by Marlow's use of twin keels. I see a lot of benefits there. They look like they could have been copied from fins on surfboards, but it seems that they weren't.

Marlow Yachts advanced technology found in their luxury ocean capable yachts.

Marlow Yachts advanced technology found in their luxury ocean capable yachts.

Click to expand...

It`s ok if you are dealing with a Snark and not a Boojum.
The Marlow innovation is interesting,quite a different multipurpose development.
A Clipper40 in Coomera had bilge keels fitted midships, and later aft as well. Wonder if the gap was more helpful than extending the existing? Design and fit was by Millkraft. When the original owner sold and bought a bigger Clipper, he had Millkraft do it again. Even so,I`m wary of major mods affecting in water behavior.

Airstream345 said:

Not a lot on the interwebs about proven performance but I did find this article about roll dampening vs stability interesting: https://seagrant.uaf.edu/bookstore/boatkeeper/bilge-keels.pdf

Click to expand...

I can see the possibility of bilge keels/rolling chocks adding an element of instability in storm conditions.

If a boat is laid on its side in a storm with a bilge keel above the water line and then hit by another large wave, the upper bilge keel would add force to cause the boat roll over completely.
Hopefully the anti-rolling effect of bilge keels would assist the boat to avoid getting in the position where they are above the waterline.

AusCan,
I’ve always thought chine tripping could happen but never thought of the up-side fin getting into the act. Good one.

And in regard to not needing any power ..... not true.
From what I’ve read in the past one looses 1/2 a knot or so. Unless you slow down and take more time getting to where you’re going or power up to your normal speed before you installed the fins and thus needing more power and burning more fuel. Yea I know ... bucket of water (or worse) at the party.

Eric,An article I read said it`s not unusual for sailboats on the wind to have the weather bilge fin break the water. I see the logic of what you and Auscan are saying. But,the phenomenon in a sailboat pressed over by sails is different to our engine powered boats.
And,wouldn`t reduced rolling and wallowing about allow more force to go into forward momentum, balancing any impedance from the fins?

Bruce,
Well with a boat about to broach the windward side is trying to push her over and given a normal hull she gets knocked down on her side. Given a fairly low CG she just lays on her side or slides down the face of the wave. But with a stab fin pointlng down like AusCan’s boat all should be well. But w a fin sticking out at 30-45 degrees off a hard chine (like the boat in the link) providing a brake on the boat about as low as possible the CG and windage will be in control of the boat more or less deciding if she’s going to trip on her chine and the chock fin. Never heard of it happening but the threat or posibility of it happening is very real.

Bruce, you may have a point about the reduced rolling making up for the extra drag from the fins; it's tough to say as every boat is different. Some bilge keels are parallel, some have a few degrees of toe-in, some are thin plates, some are fat wedges. Then there's the wide variation of sea conditions.
Who knows... It's so hard to make any blanket statement about boats and be right in all cases.

AusCan said:

Here's another good article on roll attenuation.

Roll Attenuation and Bilge Keels

Click to expand...

Good article, I used to have a Westerly Pageant years ago with twin keels and it did have a pleasant motion. Something I wondered about while looking at your photos was the difference between a round bilge boat and a multi chine steel boat such as my experience. Your hull appears to have a fairly straight run and possibly less draft to length. The hull in my experience which was quite similar to the attached picture had more belly and I think depth, My roll chocks may have been a little larger than those in the picture but not by much. I guess my thought is about the relationship between draft and the effectiveness of roll chocks with yours being closer to the CG.

Most boats cruise where the wave making and skin friction are near equal and each is about 1/2 the vessels total drag.


Roll chocks and bilge keels add only surface friction , sure some extra fuel might be required to maintain normal cruise , but far less than a foul bottoms drag.