Bilge Keels

[Wxx3]

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.

A very good point.
And while I didn't think of it, I read worried enough about the unknown that I decided against them.

By this time, (when i made the decision against) I'd had enough experience in very large beam seas, 20 to 25 feet, that I decided to stick with what i knew worked and got me to Ireland safe and sound.

If they were that beneficial, then every bost would have them.

Also, they seem very popular on work boats in areas with drying harbors.
Making it easier to work on your own hull.
Had i planned on staying in NW Europe, I was contemplating a stilt system.

 

[Rebel112r]

In regards to the fins attached to my NP42
They are not heavy, mine hollow, drilled fore and aft, so fill with water.
They have not been any problem with haul out, mine and another dozen or so NP’s have had no problems with haulout. Also have not heard any owners complaining about them.
They don’t present a problem with docking.
I have noticed no difference in speeds or fuel usage.
They were less than 4K installed, haulout and paint not included.
They slow down roll, don’t get rid of it.
They contain stern wave, trapped underneath, and I think add lift to stern, helping with pitch, although when conditions dictate that you need to slow down, then you do what most do, and wish you were elsewhere.
Active fins would do a better job, but at 10 times the cost, plus the space and structural requirements.
I would do it again, on a similar boat.

 

[sunchaser]

Didn't TF'S Dirt Doc install them for his new build North Pacific? He has not mentioned them for nearly a year now.

 

[Rebel112r]

He did, and I have talked with him, I think he is satisfied with them.

 

[sunchaser]

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. .

The infamous M/V catamaran trip. It indeed can occur and is much discussed by NZ and Australian cat builders and cruisers. The secret they tell me is avoid beam to situations in heavy seas when in a cat.

 

[Nomad Willy]

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.

FF,
I suspect that there will be some wave making losses but they are probably thought of as surface friction .. usually. Such as observing speed before anf after installation.
There would be zero wave making drag only if the fins were in perfect alignment w the usual direction of water flow along the bottom of the hull. And in places there’s probably quite a bit difference. And those boats w “toe in” probably have considerable drag and one could guess wrong and wind up w no toe in effect.

Haven’t ever heard of calculating or doing experiments to determine how the water flow along the bottom aligns itself directionally. One could attach tuff’s attached to the bottom and deploy some brave diver in clear water to take pictures as the boat ran above. But I don’t think there has been scientific experimentation to determine water flow direction. And it would be considerably different with each hull.

So there’s quite a bit opportunity to reduce drag of bilge keels but it’s not going to be easy .... IMO.

 

[Mako]

Haven’t ever heard of calculating or doing experiments to determine how the water flow along the bottom aligns itself directionally.

I had many conversations with the naval architect at one shipyard. They generally install them along the diagonals.

One could attach tuff’s attached to the bottom and deploy some brave diver in clear water to take pictures as the boat ran above.

Modeling using tuffs would be an excellent use of one's time if planning to retrofit an existing hull. This would need to be done in very smooth water, with calibrated grid lines or markers painted on, and holding a GoPro on a stick to video tape at each station. The diver idea is not so practical.

But I don’t think there has been scientific experimentation to determine water flow direction. And it would be considerably different with each hull.

Other than my first comment, designers do this type of modeling on water tank models, as well as computer simulations, but I doubt that applies to the crowd on TF. And you're absolutely right that each hull is different.

Alot of technical papers and articles that I have read generally state that the effectiveness of "rolling chocks" is less than 10%. However the naval architect I referred to absolutely swears on about 30%, which is really excellent. His fins were larger than normal though.

Someone previously commented that the chocks are likely less effective on hard chine hulls, which have a natural dampening effect, than on round bottom. I would think that this is correct.

 

[CptnPete]

A far cry from bilge keels, paravanes and seakeepers, but what appears to be a reasonable effort and cost for a DIY project we are considering is adding PVC rails below the waterline which reportedly helps reduce roll at anchor, add stability underway, and maybe even increase speed for SD hulls. Seems to provide some hard chine characteristics to round chine hulls. Like was done to boat in pics below.

Similar approach found here: https://www.integritymarinecorp.com/smart-rails

 

[Fletcher500]

I just re-read my second post, and it was Snarky. Apologies.

 

[Fish53]

I just re-read my second post, and it was Snarky. Apologies.

I didn't think that was "snarky" I've posted a few things I should have thought about a little more first, it is nice of you to show your concern however.

 

[sunchaser]

I just re-read my second post, and it was Snarky. Apologies.

If post #18 is snarky I sure missed it. Keep up being Fletcher!

 

[MurrayM]

A far cry from bilge keels, paravanes and seakeepers, but what appears to be a reasonable effort and cost for a DIY project we are considering is adding PVC rails below the waterline which reportedly helps reduce roll at anchor, add stability underway, and maybe even increase speed for SD hulls. Seems to provide some hard chine characteristics to round chine hulls. Like was done to boat in pics below.

Similar approach found here: https://www.integritymarinecorp.com/smart-rails

Interesting product...just have to heed the warning about protecting them from lifting straps!

 

[AusCan]

Alot of technical papers and articles that I have read generally state that the effectiveness of "rolling chocks" is less than 10%. However the naval architect I referred to absolutely swears on about 30%, which is really excellent. His fins were larger than normal though.

I don't know how effectiveness is measured, but with the rolling chocks on my boat, she still rolls like %^#! without the sails. Most of the boats by the same builder had no rollings chocks or sail configuration. I haven't yet been aboard one, but it's hard to imagine they would be much worse. If I was to guess the effectiveness of my rolling chocks, I'd lean towards the <10% improvement.

I agree with the comment by Rebel regarding bilge keels/rolling chocks containing stern waves and giving extra lift. My boat absolutely loves a following sea. In my limited experience, I've never been aboard another boat that handles a following sea better. The benefit is this regard may be as much or more than the anti-rolling improvements.

It's all guesswork without testing an identical boat extensively with and without rolling chocks.

 

[Fish53]

I don't know how effectiveness is measured, but with the rolling chocks on my boat, she still rolls like %^#! without the sails. Most of the boats by the same builder had no rollings chocks or sail configuration. I haven't yet been aboard one, but it's hard to imagine they would be much worse. If I was to guess the effectiveness of my rolling chocks, I'd lean towards the <10% improvement.

I agree with the comment by Rebel regarding bilge keels/rolling chocks containing stern waves and giving extra lift. My boat absolutely loves a following sea. In my limited experience, I've never been aboard another boat that handles a following sea better. The benefit is this regard may be as much or more than the anti-rolling improvements.

It's all guesswork without testing an identical boat extensively with and without rolling chocks.

Do you think the double end may be contributing to good following sea performance, mines double ended and also does well in those conditions? Mine also rolls but not in a fashion I find objectionable.

 

[sledge]

i don't think it was mentioned in this thread.... but the other obvious benefit of true bilge keels (Wxx3 did mention a drying harbor for commercial boats); but especially for folks using the shallow ICW, if you find yourself accidentally stuck in the shallows in a falling tide, many bilge keels can keep you upright safely until high tide returns.
I agree what i've read though is they tend to slow you a knot or two and therefore I'd think you would burn a bit more fuel in the long run.

 

[AusCan]

Do you think the double end may be contributing to good following sea performance, mines double ended and also does well in those conditions? Mine also rolls but not in a fashion I find objectionable.

Sure - the double end is a big plus. A big rudder also helps.

My boat can roll terribly in certain conditions; it's all matter of timing. It only gets objectionable when the boat's natural roll cycle equals, or is divisible by, the swell interval. While underway, it's easy to change how often the boat encounters the swell by adjusting direction and/or speed. (or using the sails)
At anchor, I can adjust direction I"m pointed, but if conditions change during the night things can get ugly. The rolling chocks seem to have less effect at rest compared to underway. I've been looking into flopper stoppers of various type to address this, although it is more crap to deal with, and I prefer simplicity.

 

[Nomad Willy]

If post #18 is snarky I sure missed it. Keep up being Fletcher!

Fletcher's post;
"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."

If that's snarky I wonder why I haven't been excommunicated.
I vote for being Fletcher too. And you could add a little "zing" to your posts now and then. thanks

 

[Fish53]

Sure - the double end is a big plus. A big rudder also helps.

My boat can roll terribly in certain conditions; it's all matter of timing. It only gets objectionable when the boat's natural roll cycle equals, or is divisible by, the swell interval. While underway, it's easy to change how often the boat encounters the swell by adjusting direction and/or speed. (or using the sails)
At anchor, I can adjust direction I"m pointed, but if conditions change during the night things can get ugly. The rolling chocks seem to have less effect at rest compared to underway. I've been looking into flopper stoppers of various type to address this, although it is more crap to deal with, and I prefer simplicity.

I used to have paravanes on my dragger and they were a pain, I took them off and never looked back. I second your vote for simplicity.

 

[MurrayM]

In a sea kayak this would be called a 'seal launch' and done bow first with no need for the bulldozer. Thought the rolling chocks on steroids were pretty interesting though...

 

[nwboater]

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.

My Selene had bilge keels added by the PO. I removed and installed hydraulic fin stabilizers. Before removing, the keels would cause loud and unexpected “thud” noise underway — after much examination, we determined that the keels where mounted too far forward and would trap air/water between keel and inside if hull. Something g to consider when planning location.

 

[Nomad Willy]

nwboater,
Makes me think of paravanes. One is supposed to mount them (fore and aft) where the boat rises and falls the least. That would be somewhere aft of amidships. If bilge keels are attached too close to the bow slaming can occur.

 

[Wxx3]

nwboater,
Makes me think of paravanes. One is supposed to mount them (fore and aft) where the boat rises and falls the least. That would be somewhere aft of amidships. If bilge keels are attached too close to the bow slaming can occur.

A few comments:
You don't get something for nothing. In a totally flat sea, if I throw one bird the water, the speed reduction will be negligible. On the other hand, in a beam sea, the windward bird will reduce the roll by 75%, but I'll lose 10%, speed or 10% increased fuel consumption.

Bilge keels have much more surface area than my little bird. They are also wide, so present a considerable cross section under all conditions. Thus, I'd guess, guess, mind you, that it's a 5% increase 100% of the time. In a rolling sea, that number would go up, just like the paravane bird.

When we did discuss installation on my KK42, they would have started under the pilot house door running aft to within a few feet of the stern. They would have been perpendicular to the hull and midway between the keel and waterline. About 6" in height, maybe 6" wide. (I'm recollecting from memory, don't hold me to those inches).

Again, I like my birds, since i know what they can do and if i find myself in a dangerous situation, 35+ ft seas, i can run them deeper, down to 40 ft AND could pull them altogether if need be. As i don't think they make the boat safer, just make me feel better.
I believe the cored hull adds buoyancy and allows the boat to bob when heaved to.

 

[Rick Hudson]

Static fins

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.

+++++++++++++++
There seem to be a lot of opinions but not a lot of actual owners in this thread. Here's my 2c. I added static fins on the hard chines of my PT38 a couple of years ago. Actually, Independent Marine in Coombs, Vancouver Island installed them.

Here were the big questions ahead of time:
1. Do they hit the dock? No, provided you have fenders between the vessel and the dock.
2. What happens when the boat is in slings? You hold your breath, but the installer assured me he's never had one fail during a lift. I don't hold my breath any more.
3. Do they affect the speed? No, not in my experience. Some even argue they improve speed because the vessel twists less in the water.
4. Do they work? Ah, the big question. Yes, they certainly did on our 13 ton boat. Of course, the boat will still rise and fall - that's physics - but when crossing wakes at an angle, or in a beam sea, the boat twists much less or not at all. The only time I noticed that old porpoise action was when a strong following sea was on the aft quarter. Otherwise, it's better, or much better than it was before.
5. Do they work at anchor? No, unless it's a very light swell.
6. Where are they placed? Ours were about 20' long, angled at about 45 degrees down off the hard chine. They were made of about 3/8" fiberglass sheet folded double, about 2" deep, and looked very flimsy before being installed (see Q2 above) but were very solid once installed. Two small holes allow the air to bleed out of them (you don't need extra buoyancy, but added inertia to roll is always good).

Would I install them again? Absolutely. They changed our enjoyment completely, and I don't get the grumbling from the Admiral that I used to.

IMO.
Rick Hudson
Raincoast Gypsy,
Sidney, Vancouver Island

 

[BruceK]

Rick,that is helpful. Especially as the "Clipper 40" boat I`m considering is quite similar. The Original Owner(OO) had them fitted by a highly reputable yard. Initially midship, later adding shorter fins aft. Seller(S),still friends with the OO(good sign!), says OO fitted same to a Clipper 48 replacing the 40. Unfortunately no pics available ,barring a swim I can`t eyeball them until haulout, if I get that far. S says movements after initial movement are well damped.

Got any pics you can post? I envisaged them attached to the hull below the chines. Yours extend outboard mounted on the chines, or did I misunderstand?

 

[dirtdoc1]

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 have them on my trawler. 45' North Pacific, 40k displacement, SD hull, placed on the back sides of the hull. Right at the most severe part of the bend or chine, just under the waterline. Face downward about 40 degrees? I can't remember the dimensions. I think they are about 18" wide and at least longer than 10'. They are glassed on to the hull. They can be removed without damaging the hull. They were installed in Vancouver Island. I think in Nanaimo. I could get that info if you want.

Upside:

The boat feels much heavier, even though I had them installed hollow. They can be added solid too. They didn't add a significant amount of weight. They actually raised the back by about 3" which, was needed. The boat reacts like a much heavier, bigger boat. They really help with rolling. In a beam sea the boat will start to roll, then slowly return. Also, almost no roll at anchor. I like the way they make the boat feel. I really noticed the difference. The captain that I hired to take the boat to have them installed really noticed the difference. He's going to have them installed on his NP43. I've been told that most of the fishing trawlers use them. That's what originally sold me. That was verified when I started noticing them on dry docked fishing boats. There are a lot of aluminum fishing trawlers that have them up around Bellingham.

Downside:

The boat is more difficult to maneuver while docking. It doesn't respond to throttle turning like it did before. Trevor (the owner of NP Yachts) noticed it too. That's the only downside that I have noticed. It's a pretty big downside though. If you decide to have them installed I recommend also installing bow and stern thrusters to help with the loss of control around the docks.

I would definitely use them again if I couldn't afford active stabilizers. I was told that I could add active stabilizers too.

Added:

I didn't notice a drop in top speed or an increase in gph. It's hard to tell because there are so many variables with tide, wind etc..

 

[Bill & Stella]

Bilge keel design should be done by a naval architect. They have to run parallel to laminar flow. If not they will lift or depress the vessel when under way and cause excessive drag.

 

[BruceK]

Thanks dirtdoc1. Helpful, especially as NP came to use the same Taiwan builder as Clipper were using,so hulls are likely similar.
These are on an existing boat, fortunately with thrusters both ends. I could wish people left their boats as built, but that`s used boats for you. In this case I trust the Yard that built them,being long established builders and repairers. Just wish I could see pics of the install.

 

[dirtdoc1]

Thanks dirtdoc1. Helpful, especially as NP came to use the same Taiwan builder as Clipper were using,so hulls are likely similar.
These are on an existing boat, fortunately with thrusters both ends. I could wish people left their boats as built, but that`s used boats for you. In this case I trust the Yard that built them,being long established builders and repairers. Just wish I could see pics of the install.

Sorry, no pics. I hired a captain to run the boat over to the yard. I was at home in Palo Alto at the time. I wonder if he took any pics? I never did ask him. I'll check.

Trevor Brice recommended the yard. He said that they were the only ones up North that made and installed them and that their track record was solid.

 

[dirtdoc1]

Bilge keel design should be done by a naval architect. They have to run parallel to laminar flow. If not they will lift or depress the vessel when under way and cause excessive drag.

Makes sense. So far so good.

 

[Jdoc]

Thanks dirtdoc1. Helpful, especially as NP came to use the same Taiwan builder as Clipper were using,so hulls are likely similar.
These are on an existing boat, fortunately with thrusters both ends. I could wish people left their boats as built, but that`s used boats for you. In this case I trust the Yard that built them,being long established builders and repairers. Just wish I could see pics of the install.

This is Coosa board and epoxy