Will a bilge keel improve rolling at sea?

Francois · 09-02-2006, 02:14 PM

Hi all.I am at present doing the woodwork on a 80 ft powerboat .The hull is steel and has an alluminium superstructure from deck level to the flybridge .And as you know in boatyards the word goes round that now the designer wants to add a bildge keel.I think he is worried of heavy rolling

Do you think this will work ? BTW ,I have done many projects before and its the first time that I have encounterd that. Looks like bad planing to me .Thanks.

Francois

AMG · 09-02-2006, 02:33 PM

Yes, bilge keels will prevent rolling.

Garry Hartshorn · 09-02-2006, 02:52 PM

Best and simplest form of passive stabilization, and it works at anchor as well.

Francois · 09-02-2006, 02:54 PM

Thanks AMG .Looks like I will be able to go out there without rolling and getting sick

Francois

Francois · 09-02-2006, 03:29 PM

Garry, thats why I prefer my workbench

.Yes on completion of projects I have been on a few offshore trials ,some rock some dont

Francois

Innomare · 09-05-2006, 04:45 AM

Francois,

Bilge keels do help a lot, especially while sailing. However, you want to have them positioned in the right place, or they can create a lot of drag. When positioned right, the added drag is minimal.

There are two ways to find out how they should be positioned:

a) a "streamline visualisation run" in tanktesting. Wet dots of paint become long lines following the stream lines on the hull. Instead of paint, sometimes tiny ropes are used wich are attached at one side. Pictures are then taken during the run.
b) a CFD calculation (computational fluid dynamics). This has the advantage that you know the flowlines of the stream not just at the hull surface but also at a distance from it. If you have a 3D-model but are not doing tanktesting, this would also be the cheaper option.

For some pictures of what I mean, have a look at: http://www.marin.nl/Onderzoeksprojec..._research.html

While not too common on yachts (don't know why that is), bilge keels are fitted to almost every other ocean-going (displacement) vessel. They don't prevent rolling, but you get some reduction. They also increase the directional stability of the hull.

One other important point to take into account: the welding of the bilge keel to the hull should be relatively weak. If you hit something with the bilge keel, you want it to come off, not to tear your hull open.

Good luck!

Bruno

Naval Architect

AMG · 09-05-2006, 05:22 AM

Quote:

Originally Posted by Innomare

While not too common on yachts (don't know why that is), bilge keels are fitted to almost every other ocean-going (displacement) vessel. They don't prevent rolling, but you get some reduction. They also increase the directional stability of the hull.

I think the reason we don´t see bilge keels on yachts is that they interfere with the active stabilizer fins. On a trawler design I have made the active fins as a part of the bilge keels, thus redirecting the flow in the middle of the keels which may be a good solution. Can´t find the drawing now, but it is posted here somewhere...

And I agree, the keels do not "prevent rolling", but I used this word as opposed to "improve rolling" as in the headline...

Innomare · 09-05-2006, 10:21 AM

Hi Lars,

I didn't mean to pick on your words. I just don't want to be in the situation where somebody has bilge keels installed on their boat and is surprised to find that it's still rolling and they are still getting seasick (and then sueing me for claiming otherwise).
To say it simple: if you want an appartement, don't buy a boat

MYCaptainChris · 09-05-2006, 10:57 AM

I suggested passive stabilization in our last refit, but the finances wouldn't cover it. I already have active stabs and I planned to run two passives on each side of the hull, aft and forward of the active but on the same plane. How will this affect the active stabs? Should I consider one longer passive each side below the lines of the active?

chris

AMG · 09-05-2006, 11:19 AM

Your idea is in essence what I had in the old design I just found. The moving part is also attached to a bar connecting the two fixed keels as a protection and to make all of it more stable. I think it should work really well if installed at the right positions...

Francois · 09-05-2006, 12:20 PM

Thanks guys for the responses .Some really great info there.
Bruno ,I will read through it now as it looks very interesting.Just got back from work

Lars,I can see in the picture that it shows the positioning of the keels and that will depend on the hull shape and length of the vessel.

Francois

MYCaptainChris · 09-05-2006, 05:31 PM

in the image the passive appear to run a little deeper than the active, is that what you would suggest? My idea wasn't to encase them the way you have but I could see how this would work well. They passive I'd planned to be about half as deep as the active.
Would you keep the passive straight or curve them to the shape of the hull?

AMG · 09-05-2006, 05:51 PM

These bilge keels are angled outwards about 30 degrees from the centre and must remain straight when you look at them from the front so they don´t act as rudders. As you probably know it can be hard enough to get your rudder(s) or rather the autopilote and the active stabilizers to work nicely together.

The moving fin is enclosed to not catch fishing gear so easy and this whole setup is also fine where you want to dock at ebbtide.

The shaft for the fin is a little forward, off centered.

For a pure yacht such as yours, I think bilge keels half as deep as your fins is a good start. It will not be so expensive to modify later if you so wish.

MYCaptainChris · 09-05-2006, 10:26 PM

thanks for the info, now lets see what happens with the budget for the next refit.

Billy1119 · 09-06-2006, 02:05 PM

When implementing both passive and active stabilization methods into one hull, wouldn't the passive effectively reduce the effectiveness of the active stabilization? Sure, I see that it could/would help as well because the active likely wouldn't have to work as hard because the boat wouldn't roll as easily, but at the same time it'd have to work harder to fight against the passive?

Point being, it seems to me you would need to carefully calculate the size and effectiveness of the passive/active combo so as not to eliminate the effectiveness of one or the other. Maybe I'm wrong and they pretty much always work together, helping each other out. Thoughts?

On a side note, I like the enclosure of the active stabilizers, if nothing else, as a way to protect the fin from getting ripped off if the captain were to, uh, get off course.