Bulb keel shape

The only keel bulb shape that I've had personal experience in developing is the one below. First, I read in one of my design books about the theory that a ridge running under a bulb would be beneficial in reducing the equalization of pressure from the high pressure side to the low pressure side. I didn't fully buy that so instead of adding a horizontal wing to the bulb I added the small vertical foil-the theory was the same as the ridge-to reduce the vortex associated with high and low pressure at the end of the keel fin at a lower wetted surface than a horizontal wing-thereby reducing drag. The only evidence I have that it worked was that the boats using it won all their races-but I think they won for a number of reasons.
In model racing boats now they are using a very long narrow bulb with no fins.
The shape I used has minimum wetted surface compared to some of the other shapes but all the shapes have in common that the designers were attempting to reduce drag.
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Make sure you get help with the amount of lead you use because it will determine righting moment(RM) which ,in turn, determines the size of the mast and rigging you will use as well as the structural requirements in the hull to support the rig loads and keel loads.

 

About wing keels and bulbs http://www.boatdesign.net/forums/sailboats/winged-keel-bulb-keel-40596.html

 

Very helpful information, especially about designing bulbs. The K1 is exactally the kind of design I was thinking of!

Yes, I believe they are a bit too tender as well. Originally they stuck a 30 lb chuck of lead in the dagger board at the bottom. That helps, i'm sure. I think what I will do for now, is try to cram about 40 pounds of lead into the bottom of my daggerboard, without doing a bulb. It will be easy enough to add a bulb in the future. I wouldn't mind making two different daggerboards. This gives up the advantage of cheaper bulb construction with concrete and steel, but I can remove the daggerboard out through the top.

 

Yup, the Sailability section of my club had a couple on test and they were a bit too short and tender for comfortable sailing even for able bodied people. The length meant it kept screwing round into the wind too much even though it was generally reasonably balanced. On a very puffy small lake this is not good. A few appendages were also 'lost' and had to be recovered as the design (as delivered) did not allow for retaining the daggerboard on knockdown!.

Biggest advantage was light weight, but they just were not quite developed enough to be satisfactory.

I can verify the K1 arrangement works pretty well - the concept was a single handed Firefly with self tacking jib. It is a pretty dry boat that sails well with few if any vices and takes off onto the plane without the least fuss. My arm was heavily twisted to try one a few years back, it turned out to be the designers own boat. Had a very interesting conversation with him about it alongside other craft.

 

Given a choice, I'd take the new board casting as the route to explore before delving into bulbs, and other contrivances. Up wind a 60 pound fin board, will crush a 60 pound bulb on the end of a fin shaped board, simply because of the frictional resistance and eddy making. Now, if you used a shorter bulb/fin arrangement, with some wings or beaver tail, you could gain reduced draft, while the appendage "thinks" it's deeper then it actually is. You'll still have the drag element to the equation, but if draft is a concern an advantage over a fin that could 20% deeper.

On one of me "test bed" sharpies I played with bulb, fin and beaver tail arrangements, just to get a better feel for cost/benefit/performance gain/lose studies. Repeatedly, the ballasted fin was the better choice, for several mostly already mentioned reasons. This assumed a level playing field with the same ballast, draft and general fin portion sections.

 

Well, the information seems pretty conclusive. If you are given a reasonably deep draft, the fin is the way to go. If you limit draft, you have to get creative.

 

Bingo, all things being equal, the fin is the ticket. It has the least amount of resistance for it's draft and though you can get a lower CG with the bulb, you also pickup a huge drag penalty, even on the cleverest of shapes. This said, there are some tricks you can employ on the fin to help lower it's drag, but these are specific speed related and some clever engineering (flow dynamics) will need to be utilized.

 

I'm not a naval architect, my opinion could be a mistake.

According to the literature.


1) Designs with wings / winglets generally do not work well. There are waves at the sea, the angles of pitch and roll of the yacht (sorry for aviation terms) are constantly changing. The wings on the keel have have a damping effect => produce drag.

2) Swept backward keels. According to the literature, these keels can cope with seaweeds / ropes / fishing nets.
The bulb shifted backward is determined by this idea.
Potential problems.
- If it were an aircraft, such a bulb design could result in flutter. I don't know whether it is important for yachts.
- If the keel is installed so the position of the hydrodynamic center is correct, the position of the center of mass could be shifted too backwards.

3) The bulb shifted forward. A little bit better hydrodynamics, better structural strength.
Potential problem - seaweeds.

4) Retractable inclined forward dagger boards. Perfect to design the cabin: the centerboard well is not in the cabin, but in the cockpit.

5) Tandem keels, the bulb is connecting forward and aft keels.
Approximately have the same hydrodynamics as (3).
If the angle of attack of the both components of a tandem keel were adjustable, than, according to Munk's theorem on biplane wing, the lift/drag ratio would be exactly the same as for a single keel.
If there are no adjustments in the tandem system, results are a little bit worse. http://www.boatdesign.net/forums/sailboats/tandem-keel-1058.html Post #10 by Tom Speer

Tandem keels can provide great structural strength, can cope with sea weeds and the cener of mass can be moved forward / aft as you wish.


6) There are lots of additional keel designs not mentioned in this brief review.

 

All good information. Thanks guys!

 

I think we all need to back up a little bit here,

The boat is so small and short that you are going to be limited by hull speed far more than a bit more or less drag on the keel. I would build whatever is easiest and do a decent job fairing it.

Seems wings where a dead end in keel development, never see them anymore except as a way to keep ballast low in a shoal draft boat.

 

Correct, The hull speed on it is 4.1 mph. I will be focusing on pointing performance at that speed, because there is nothing more than that to go to, except maybe a quick little surf on a big wave (1 or 2 feet tall). I'm not sure I'll want to be out in that thing in chop like that though.

 

Here's the route I ended up going: Sheet metal form, will pour 70 lbs of lead into it, so probably the bottom 18 inches will be solid lead, and then it will all be wrapped in fiberglass.

 

Well, It's all poured! I only did 50 lbs instead, and I'll see how that goes.



Next I'll stick it on the daggerboard and fair/glass the whole thing!

 

That looks like a good design. I'm curious to see how the rest of the foil comes together.

 

I did the same thing with a centreboard on an 18ft daysailor, 70lbs from memory and it was a good choice. The only issue is raising/lowering, you need a good setup.