Bilge keels on a Gartside double ender

Of course it would be simple, or best, to build it to Gartside's plan. And I've built CB boats before.
BUT:
I want it to stand up in the tidal flats in front of our house.
With 12" bilge keels we should be able to beach without much of an issue
I'd like not to have a CB occupying all that space, not to mention removing the potential for leaks
Having one less moving part is another advantage
Curiosity, why have so few bilge keelers been built on this side of the pond? I will find out.

Thanks,
Steve

 

These bilge keels should dampen any rolling. You are essentially using them as "beaching chocks" and should work fine, unless the mud is really soft. They will not help much in sailing to windward so get a good aux. engine.

 

First off. When discussing bilge keels or twin keels, we should first specify whether we are talking about a powerboat or a sailboat. Second, we should keep in mind that there is no central authority on nautical terminology.

For my purposes, I consider twin keels to be keel fins which are more or less vertically parallel to one another, but not necessarily perpendicular to the hull surface they project from. Bilge keels, on the other hand, project more or less perpendicular from the hull surface they extend from, but are never vertically parallel to one another.

When talking about twin keels, we are talking about an arrangement which is similar to that of a biplane. The good news is that both keels are effective most of the time. The bad news is that the 2nd keel adds only about 20% to the total lift over that of the 1st keel. This is why you will never see them on a racing sailboat. They add mostly drag.

with fixed bilge keels, the windward keel becomes almost totally ineffective once the boat heels as its lifting surfaces become more and more parallel to the waterline. Then it is adding almost exclusively drag.

I have considered the idea that when the boat is sailing almost upright, like in a gentle breeze, both bilge keels will be effective. This is when the most effective keel area is needed. In a stronger wind, the boat heels further but sails faster. The faster sailing boat presumably needs less keel area to get adequate windward lift, so the ineffectiveness of the windward keel not only doesn't matter but may provide less drag than it would have if it had been a twin keel instead because it is providing mostly just frictional drag but almost no induced drag.

Attached below are two sketches of a design of mine which uses bilge keels. It does so mainly because it is convenient. There are two cylindrically developed surfaces to mount them to.

Another issue to consider is that the bilge keels you are considering are long and shallow, not short and deep, like the board would probably have been. This means they have very low aspect ratios, so provide less horizontal lift per surface area. This almost certainly means that the boat won't point as high or sail as close to the wind as the board version.

This being said, they may still be justified to meet your requirements. Top performance isn't everything.

 

Sorry for any confusion. I'm interested in bilge keels. I only mentioned twin keels because that was the only study I could find and it showed how important placement was for performance. Here's a rough sketch of what I'm considering. A bulkhead and some frames could add more support to the stringer.

 

If you want anti rolling bilge keels, you should separate them as much as possible from the centerline of the vessel. If you look for another different effect, the thing changes.

 

you might want to:
(a) put a shoe on the bottom.... a 3/4" x 48" (36 sq. in.) is a small area & might 'settle' into the beach due to heel force loading, wave liquefaction, soft sand, etc....
(b) if the beach is 'solid' (pebbles, rocks, etc), check out the forces/loading on the hull. You may need inboard stringers along the floors.
(c) check any wave action (including boat washes) during the tide changes. Bouncing on the beach can be tough on structures.

 

When pulling the boat out of the water for maintenance, it was nice to not have to use a cradle to set it in, just a "dolly" to roll it about. This was our old Westerly.

 

Thanks for the responses.

The keels on the Westerly are aligned pretty much as I envision doing. Also aligning, as suggested, along the diagonal seems the way to go. Here's a shot of the build with the a batten on a diagonal:

I'm not sure if adding a shoe on the keels is needed. I asked on another forum about the possibility of the keels getting "stuck" in mud and folks from Britain responded that is not a concern.

I don't know if shaping to keels, adding a NACA foil shape to the inner side, would be worth the effort.

 

I don't think so.

 

The keels should be planar, not cambered.

The keels should be aligned with the centerline of the boat to slightly toed in. That means a line along a keel parallel to the plane of the waterline should point straight ahead or towards the bow by a couple of degrees or less.

The keels should not cross plank laps. The planking will need to lined off around the keel locations.

The structure of the keels and the internal attachements needs to be designed and sized for worst case loads which will occur when the boat bumps into a rock and is moved on land. The potential loads on the in those situations is much greater than the hydrodynamic loads when sailing.

The wider the top plates and the further the bolts are from the centerplane of the keels the better.

The top plates of the keels will need to be bent and twisted to conform to the outer surface of the planks - or - structural fillers will be needed between the top plates and the outer surface of the planks. Fillers would be simpler but would potentially add drag.

Sufficient internal structure will be needed to spread the loads across several planks and the keel. Twisting and deformation of the planks should be avoided. The stringer and floors shown in the sketch might be sufficient but I am skeptical.

Creating an full airfoil shape on might be beneficial, but half an airfoil shape isn't going to do much. At a minimum the leading edges of the keels should be rounded, and if possible the trailing edges should be tapered.

How will the keels and attaching bolts be protected from corrosion?

 

Thanks David, some good points.
The #1 plank, outside of the garboard, should be about 8" wide. In my drawings and on the model the keels fit on that plank.
I'll have a mold of the top plate made to ensure it matches the bevel of the plank. Local welder assures me that won't be a problem.
The bilge stringer will be further reinforced by frames and a bulkhead, I'm still contemplating the actual design. David Wyman said he'd help and should keep me honest.
Haven't got around to the bolts yet, but if it's a steel plate, possibly stainless? Then that would determine the fasteners.
As you can tell from the photo above I have many months of work ahead before the hull gets flipped and these issues become pressing.
Thanks again,
Steve

 

I'm back and have made some progress planking since September. It's time to firm up the keel decisions. Here I've place a crude mock up of a bilge keel in its approximate position. It is towed in 1 degree, canted 15 degrees. Moving the keel one plank up would not be my choice as it would complicate the reinforcing internal structure. Possibly expose the leeward keel to surface interference?

I've done internet searches, emailed and called designers, taken my friend and naval architect, David Wyman, to lunch several times, crunching numbers and discussing bilge keel designs and concepts. David drew this design for the keels. I am considering adding endplates as in the Bolger triple keel (Boat Design Quarterly 30)

The hull shape is very similar to my 16' melonseed that spent last summer on the tidal mooring. Its obvious that riding out low tides on the mud flats won't be an issue and the bilge keels will be primarily for sailing performance. I will be adding a shallow center keel, similar to Bolger's triple keel. (I spoke to her former owner and he was very pleased with the boat and performance) The triple keel had more like bilge fins vs keels and Bolger put end plates on them, stating that they effectively added up to 50% area.


Having flown the 737 for many years I saw winglets appear and grow in size and they did increase efficiency at cruise. I haven't come across a design for the end plates, any thoughts? Bolger's appear to be rather small and straight plates.

 

What on the keel is the toe-in measured relative to? Is it the flat on the side labeled "inboard"? Or is it to some sort of mean line of the section?

Simple end plates are most likely to add significant drag and probably have limited effect on side force, potentially degrading side force.

 

end plates probably have little improvement effect on a low aspect ratio foil like this.

 

End plates may make a 0 - 2 % positive improvement.
They'll get wrecked when grounding.
I guess they could be sacrificial.
Make them small.