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Old 02-13-2019, 08:21 PM   #21
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Here's another good article on roll attenuation.
Roll Attenuation and Bilge Keels
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.
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Old 02-13-2019, 08:48 PM   #22
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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
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Old 02-13-2019, 09:23 PM   #23
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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.
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.
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Old 02-13-2019, 09:32 PM   #24
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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/b...ilge-keels.pdf
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.
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Old 02-13-2019, 09:50 PM   #25
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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.
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Old 02-13-2019, 10:27 PM   #26
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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?
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Old 02-13-2019, 11:25 PM   #27
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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.
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Old 02-13-2019, 11:55 PM   #28
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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.
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Old 02-14-2019, 02:46 AM   #29
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Quote:
Originally Posted by AusCan View Post
Here's another good article on roll attenuation.

Roll Attenuation and Bilge Keels
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.Click image for larger version

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Old 02-14-2019, 04:59 AM   #30
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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.
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Old 02-14-2019, 08:24 AM   #31
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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.
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Old 02-14-2019, 09:15 AM   #32
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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.
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Old 02-14-2019, 09:32 AM   #33
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Didn't TF'S Dirt Doc install them for his new build North Pacific? He has not mentioned them for nearly a year now.
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Old 02-14-2019, 09:37 AM   #34
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He did, and I have talked with him, I think he is satisfied with them.
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Old 02-14-2019, 09:42 AM   #35
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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.
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Old 02-14-2019, 09:56 AM   #36
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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.
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Old 02-14-2019, 10:21 AM   #37
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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.

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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.

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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.
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Old 02-14-2019, 10:34 AM   #38
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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
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Old 02-14-2019, 10:51 AM   #39
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I just re-read my second post, and it was Snarky. Apologies.
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Old 02-14-2019, 10:56 AM   #40
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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.
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