Bilge Keels

[dirtdoc1]

I’ve read, somewhere in the distant past, that it’s not just pressure on the bilge keel / rolling chock that dampens roll, but also turbulence on the “non pressure” side comes into play as well.

I'd love to know of the mechanics that take place. I find it very interesting that my bilge keels took away some docking maneuverability. I used to be able to spin the boat around by going forward and backward. Let's say you are sideways to your slip. I would turn the wheel over hard while still gliding forward until it stopped turning. Then (while leaving the wheel hard over) put it in reverse. The boat would keep turning in the same direction until it stopped turning then put the throttle forward to keep it turning. By throttling back and forth the boat would keep turning in place. Once the bilge keels were installed the boat was not as responsive to that same procedure.

Very strange. Trevor (the owner of North Pacific Yachts) noticed it too.

Cheers!

 

[Comodave]

I think that in the docking scenario the bilge keels act like a regular keel does and resists turning. I am not surprised that the boat does not want to turn as easily with the bilge keels. If it is bad enough, get thrusters. Then it will turn on a dime and give you some change back.

 

[AusCan]

Yes, with 3 keels a boat tends to stay on its line. Most of the time this is a good thing perhaps it’s worth upsizing your rudder when installing bilge keels.

 

[MurrayM]

Yes, with 3 keels a boat tends to stay on its line. Most of the time this is a good thing perhaps it’s worth upsizing your rudder when installing bilge keels.

So...once having paid for bilge keels / rolling chocks, paying for a bow thruster for marina fairway control, then paying to modify the rudder to kick the arse end over...why not just buy a Seakeeper Gyro?

Ahhh, the slippery slope of big boat buck rationalization

 

[Fletcher500]

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

Sun, I made that comment with tongue in cheek after I read the replies following mine implying that it was “snarky” to suggest that BK’s didn’t work. Anyway, no harm, no foul. Have not logged in for awhile. Just read the thread, and it was an interesting discussion.

 

[koliver]

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

Reminds me that a number of Bayliner (32s?) had their soft chines modified with add-on hard chines, so that the rolling would be lessened. An article at the time in Pacific Yachting detailed the process. Alas, I know nobody that had it done, so no reports of its success.

 

[Comodave]

On the Bayliners the hard chine addition was called underhulls I believe. I understand they did work and help with the rolling. Since they are not building the 32, 38 and 45s anymore the company stopped producing them.

 

[ben2go]

Yes, with 3 keels a boat tends to stay on its line. Most of the time this is a good thing perhaps it’s worth upsizing your rudder when installing bilge keels.

Add in an articulated rudder for good measures.

 

[clyde]

bentogo...your right, we hauled out 2 yrs ago, in port towsend, and shoulda put the 6" x 45* hardwood additions to rear end of our rudder!! but we are going north to the bella bella area next summer, be our 21st yr, then maybe putting skybird up for sale! we`ve had her for 27 yrs, she had 1800 hrs then, and now has 10,600 hrs! the volvo 70B/no turbo, runs fine...in 98, we went around vancooover islnd, north to south, met a guy with a volvo in his troller, he had just re-built it, so i asked him the reason, and he replied that he just thot it was time, then i asked about the hours, he looked at the ceiling, scrinched up his eyes, and replied 38,000 i think!!...clyde

 

[ben2go]

bentogo...your right, we hauled out 2 yrs ago, in port towsend, and shoulda put the 6" x 45* hardwood additions to rear end of our rudder!! but we are going north to the bella bella area next summer, be our 21st yr, then maybe putting skybird up for sale! we`ve had her for 27 yrs, she had 1800 hrs then, and now has 10,600 hrs! the volvo 70B/no turbo, runs fine...in 98, we went around vancooover islnd, north to south, met a guy with a volvo in his troller, he had just re-built it, so i asked him the reason, and he replied that he just thot it was time, then i asked about the hours, he looked at the ceiling, scrinched up his eyes, and replied 38,000 i think!!...clyde

 

[DDW]

I have had passive stabilizers installed on our American Tug 34 for three years now. They were installed professionally by Independent Shipyards here on Vancouver Island, but they traveled to my marina to do the work, and brought everything pre-manufactured ready to go. Completed in one day and launched the next, the improvement in fwd.quarter and beam seas and wakes of passing yahoos is very much worth the relatively modest cost (under 6K). Rr. quarter seas are not affected as much.They start about 1/3 from the bow and at the stern are about 12" they are hollow, and fill with water. They are very rugged, and slings are no problem at all. Docks are not an issue, as they do not extend beyond the rub rail. There might be a small improvement in speed, but the comfort is noticeable. A 40 foot trawler travelling behind me around Cape Caution asked why we were not bobbing in the Pacific swells the way they were.

Do you have pictures you could share?

 

[Nomad Willy]

I'd love to know of the mechanics that take place. I find it very interesting that my bilge keels took away some docking maneuverability. I used to be able to spin the boat around by going forward and backward. Let's say you are sideways to your slip. I would turn the wheel over hard while still gliding forward until it stopped turning. Then (while leaving the wheel hard over) put it in reverse. The boat would keep turning in the same direction until it stopped turning then put the throttle forward to keep it turning. By throttling back and forth the boat would keep turning in place. Once the bilge keels were installed the boat was not as responsive to that same procedure.

Very strange. Trevor (the owner of North Pacific Yachts) noticed it too.

Cheers!

Dirtdoc,
Re the bilge keels and maneuvering agility loss I’m think’in the fins that angle out and down (45 degrees or so) just act much like bulldozer blades putting a big damper on the ability to yaw. Swinging the stern would be a real problem unless the fins were limited to the midships area.

 

[DDW]

Not that it's a popularity contest, but so far I'm counting 5 posters on this thread who have had these installed on their hard chine SD boats, all of which are happy with few downsides to report (one report of reduced low speed maneuverability). In linked threads, there are a number of others, again positive. So far I've not found a report of regret for installing them, or removing them after installation. (One post above about a Salene, not quite the same as a hard chine SD boat).

Any primary or second hand stories about putting them on this type of boat and regretting it?

I note that the ones installed by Independent on Van Isle are significantly larger than those typically pictured for round bilge yachts and ships. Perhaps that explains the difference in effectiveness? The placebo effect is strong when money is spent on a boat, but not universal, I'd expect some negative accounts if there was discontent.

 

[dirtdoc1]

Dirtdoc,
Re the bilge keels and maneuvering agility loss I’m think’in the fins that angle out and down (45 degrees or so) just act much like bulldozer blades putting a big damper on the ability to yaw. Swinging the stern would be a real problem unless the fins were limited to the midships area.

They start at midship and run all the way to the transom. They're about 18" wide and extend downward about 40 degrees. I think you may be on to something with the bulldozer effect. As I try to spin the boat around in the harbor I'll bet the fins act as big shovels pushing a lot of water as the boat tries to rotate. That makes a lot of sense. Although it's disappointing the benefits definitely outweigh the loss of docking maneuverability. I have bow and stern thrusters that take care of that.

Thanks for your input Willy!

 

[dirtdoc1]

Here's a 60' vessel built by Independent Shipwrights that has rolling chocks...last 7 photos show them quite well: Boat for sale - Independent Shipwrights Ltd LRC Pilothouse Trawler - 60'

Do they look like yours, salish ranger?

Mine look very similar to the ones on the Independent Shipwrights 60'. As I said in a previous post the bilge keels help make most conditions much more pleasant. There is no doubt that they work well. At around $6500 they are a no brainer. That said - they can be overwhelmed at some point. I used them for over 1300 nm last year all in fairly protected waters in the PNW Inside Passage. There was only one time that the boat rolled uncomfortably. I just don't know how much more and how many more times the boat would have rolled uncomfortably without them. My guess is a lot more.

Running down the coast in unprotected waters is a whole other thing. I haven't done that yet so they really haven't been tested.

One more thing - Mine were installed hollow because the boat was sitting low. I don't think water gets inside because they raised the back of the boat up about 3" and that has not changed.

Cheers!

PS. just received these from the captain who took my boat over to get these installed. Sorry about the sideways pics.

 

[MurrayM]

Here's some interesting bilge keels with 'bulbous' tips on a Sirius 40 sailboat. Makes sense if you plan to beach your boat occasionally... regular bilge keels would knife through soft sand or mud.

Scroll down to first photograph after the video:

https://distantshores.ca/boatblog_files/cool-keels-shallow-draft.php

 

[ben2go]

Here's some interesting bilge keels with 'bulbous' tips on a Sirius 40 sailboat. Makes sense if you plan to beach your boat occasionally... regular bilge keels would knife through soft sand or mud.

Scroll down to first photograph after the video:

https://distantshores.ca/boatblog_files/cool-keels-shallow-draft.php

Some of those boats with the bulbous bilge keels have a shoe on the bottom of the rudder stay to aide in grounding without damaging the bottom of the rudder.

 

[DDW]

Though, the bulbous tips are to lower the center of gravity. Any effect on sinking in mud is secondary.

 

[Art]

Complicated!

 

[ddw36]

Excellent discussion, albeit I'm reading very late. Missing in the discussion is the dual damping (not dampening!) effect bilge keels have. Yes, hard-chined boats resist rolling. However, consider: A boat rises on port, declines on starboard. The hard-chined boat will indeed resist on the starboard side, but there is nothing to resist on the port side! Bilge keels resist both against a rising side and a declining side. With the rising side, the water presses on the topside of the keel, resisting in that direction; on the declining side, the water presses on the underside of the keel.

 

[dirtdoc1]

I thought I'd come back to this thread since I've added active fin stabilizers to the passive bilge keels (rolling chocks, whatever). I was very happy with the bilge keels until I got in to some serious following seas. The boat was all over the place. I wouldn't have bothered to add Keypower Active Fin Stabilizers just for the occasional severe following seas but I was planning a trip down the coast. I heard that on the way down it's mostly following seas. Anyways, they performed wonderfully in following seas, beam seas, quartering, etc. Now the boat has both passive and active stabilizers. I don't know whether the bilge keels help the active fins or not. My guess is they don't do much to help. They do work while anchored. I've never been uncomfortable at anchor even in high winds. The fins don't work when the engine is off so they're no help at anchor.

 

[jimdavi]

Thanks for the post Dave.

 

[Mako]

Dirtdoc do you have a photo of how the fins are placed in relation to the rolling chocks?

 

[ben2go]

Dirtdoc do you have a photo of how the fins are placed in relation to the rolling chocks?

x2

 

[garbler]

Interesting topic and I know in the yacht world it’s a tough one to figure out. When I worked as crew on big charter yachts Vosper and Naiad were fitted on almost all the bigger vessels. Cooks and passengers loved. Before I would even contemplate fitting real bilge keels or keel chocks ( there is a difference ) I would have a NA who has hands on experience and not just equations evaluate your hull and make recommendations. It should always be considered when installing ‘ bilge keels ‘ that they represent a type of dry docking interference with yards who use travel lifts or those who block on railways. Synchrolift table systems are the safest normally but good divers and careful planning is a must. You don’t want to see some Travel Lift operator pushing his slings fore and aft to get around the BK’s leaving the hulls major weight unsupported.

Attached are drawings of the USN’s MCM and older Phil Rhodes designed MSO’s ( wooden minesweepers ) their bilge keel design and location. Lots of tank testing and thought went into these and though the ships still rolled in the right sea conditions you you didn’t loose your ‘ eyes teeth ‘ . The ships sound or acoustic signature was a major element also. Keep in mind a Vosper or fixed bilge keel must be sacrificial to the hull skin. Too strong = hull damage, too weak and they come off. When I worked at Merrill Stevens in Miami years ago we always beefed up framing and skin to install Vosper or Naiad’s. As the photo shows grounding damage can be significant

 

[ben2go]

Interesting topic and I know in the yacht world it’s a tough one to figure out. When I worked as crew on big charter yachts Vosper and Naiad were fitted on almost all the bigger vessels. Cooks and passengers loved. Before I would even contemplate fitting real bilge keels or keel chocks ( there is a difference ) I would have a NA who has hands on experience and not just equations evaluate your hull and make recommendations. It should always be considered when installing ‘ bilge keels ‘ that they represent a type of dry docking interference with yards who use travel lifts or those who block on railways. Synchrolift table systems are the safest normally but good divers and careful planning is a must. You don’t want to see some Travel Lift operator pushing his slings fore and aft to get around the BK’s leaving the hulls major weight unsupported.

Attached are drawings of the USN’s MCM and older Phil Rhodes designed MSO’s ( wooden minesweepers ) their bilge keel design and location. Lots of tank testing and thought went into these and though the ships still rolled in the right sea conditions you you didn’t loose your ‘ eyes teeth ‘ . The ships sound or acoustic signature was a major element also. Keep in mind a Vosper or fixed bilge keel must be sacrificial to the hull skin. Too strong = hull damage, too weak and they come off. When I worked at Merrill Stevens in Miami years ago we always beefed up framing and skin to install Vosper or Naiad’s. As the photo shows grounding damage can be significant

I would never build bilge keels or any other such structure to strongly. I would rather loose or damage one than have a hole ripped in the side of the boat. If bolt or screw pulls through, no big deal, bung it up. If the side rips open it's an all out fight to save the boat and it's crew.

 

[dirtdoc1]

Dirtdoc do you have a photo of how the fins are placed in relation to the rolling chocks?

The front of the bilge keels were cut away to make room for the fins. Dave must have cut off about 6'.

 

[jimdavi]

Happy with the fins? What is the roll reduction?

 

[DDW]

Attached are drawings of the USN’s MCM and older Phil Rhodes designed MSO’s ( wooden minesweepers ) their bilge keel design and location.

Those are tiny little things compared to what is being installed up in BC. Also, on a hard chine boat you don't have to guess much where they need to go. The way mine were put on, you don't need to worry about slings, hydraulic trailers or railways. As you can see in DirtDoc's pictures, they have no problem with slings. I think they would break off the skin if hit hard enough, but if hit that hard, the boat was going to have a hole in it anyway.

There is a huge range of appendages being called chocks or bilge keels. Discussion of a particular installation needs to be accompanied by pictures, or it may be meaningless.

 

[MurrayM]

To throw another monkey into the works, I’ve recently found some references to ‘bilge boards’ which are a trawlers cousin to daggerboards on catamarans. They are shorter than daggerboards and appear to be deployed and lifted by hand. Will try and sniff out a link with some detailed information.