Altering shoal draft keel?

First of all - hello everyone! What an interesting site this is. Thank you all for your input on the various topics - I'm already learning.

My "design exercise" has many answers, I'm sure. Given the information below, I would like your input as to what would be the best modification strategy for me to pursue. The answer may even be none-at-all.

Background: I own a Com-Pac 23D monohull. I am considering modifying the shape of its keel in order to increase windward ability when heeled beyond 10 or 15 degrees. Currently, the keel is a simple 8 foot long, flat bottomed NACA foil, and the boat draws approximately 2.5 feet. As is common with shoal draft vessels, the more I heel, the more I sideslip. I am considering an add-on modification to the current keel shape in order to accomplish this.

For the sake of simplicity, I would like the add-on to the keel to be composite, and to attach in the same manner. I think that the best way to do this will be to design a "shoe" or "cup" (in the shape of the bottom of my current keel) at the top of the add-on, into which the current keel will be glassed.

The boat is ballasted quite well (1340 lbs ballast in keel, displacement of 3000 lbs +/- (there is an inboard diesel)), and is regarded to have a conservative and strong rig (237 sq ft sail area w/105% working jib) for boats of this type. My intention is not to add any significant amount of weight to the ballast ratio, rather I want it to improve the lift produced by the keel. Although I like the current shoal draft, I am wondering if a 6-18" increase in draft would be well worth the trade-off in both effort and resultant draft increase.

I do understand that if I change the keel shape, I will also change the forces and efforts on the rig, but with some improvement to strengthen the chainplate attachment scantlings, I think the rig is well capable of handling a bit more stress. I do not plan on making this additional keel area a large amount, and I am not looking for instant 200% performance improvement - simple elimination of most of the sideslip would be quite enough for me.

I would like to hear your opinions on these questions, and I'll be glad to provide any other information that I have if necessary. I'm sure more questions will follow. Thanks in advance!

1) Would a simple "endplate" that stopped water from flowing off of the bottom of the keel when heeled offer any significant improvement to the lift which is already being produced by this shape?

2) A simple extending of the current keel shape seems to be the 2nd easiest way to make this modification - would another 12-18" of keel depth create enormous stresses? Any recommendations on how one would calculate this? Any way to tell how this would improve the amount of lift produced - is it a simple surface-area ratio (ie; 2x surface area = 2x lift produced)?

3) The winged keel concept makes a lot of sense, but it will be harder to make, harder to get off if run aground, and may not be as strong in that scenario. Is it the best way for me to get the improvement I am seeking, though? I also wonder approximately how wide the wings would be (from tip-to-tip), and approx. where the best placement (fore and aft) on the keel length for them would be. Also a calculation project, I assume...

I'm considering all this because I like the boat, and if the modification could be done for under even US$5000, I would save a ton of money over simply buying and outfitting a different vessel.

 

You should have a look at a 'Scheel keel'

 

Hi Brian - Thanks for the reply. If I do the "keel extension", it would be easy to incorporate that sort of shape into it. Using a microballon slurry with a putty consistency, I could actually build up the shape the keel body while the boat was on stands, then lay glass over it after it was well faired. If it didn't work well, it would be easy to remove, also.

The Scheel keel is an option from the builder on the Com-Pac 35.

Something like this:

http://www.designinc.com/Larina/keel.htm

PS - I thought I recognized your name - I spoke with you on the phone a year or so ago about your mast-aft rig ideas - you are responsible for me knowing about this site.

 

You can glue high density foam to the keel, shape it and then glass over it.

 

Hi gonzo -

I thought about using shaped foam as a core, but discarded the idea primarily because even high-density foam in that position would be anti-ballast. There would be other concerns as well: it wouldn't be as resistant to a crushing impact, even a closed-cell foam would probably wind up absorbing water in the event of puncture or cracks, which would be bad for the keel laminate underneath, result in delamination, etc etc...

If I use a microballoon or colloidal silica resin paste built up in layers, I could bring the density down closer to that of water, and wouldn't have the water penetration worries if the skin was breached. It will take longer to shape and fair. I may be able to insert some lead into hollows carved into the paste when it has dried, burying that under later layers of paste, to offset any buoyancy that is created by the addition.

Tradeoffs, always tradeoffs...

Thanks for the input, gonzo.

 

Take a look at just how much buoyancy there would be in a foam keel extension - I bet it's not a very significant hit to your stability.

It's not easy to design an end plate or wings that will have a net reduction in drag. Especially for a boat that heels because of the changing flow angles. I think a simple extension is your best bet.

 

Could ferrocement be used?

 

I think ferrocement would change the displacement minimally. It is a great structural material, and cheap too.

 

Tspeer - I think you are right - there wouldn't be *that* much buoyancy added with foam, and the more I thought about it the more I think I'll add some weight in the "flares".

My understanding of the Scheel keel is that the point of the flared shape is not as a drag-reducing feature (like wingtip vortice generators), but instead as a pressure generator, creating higher pressure on the downwind side of the keel, increasing it's power as a lifting surface. However, my information comes from what I have been able to find on the web, so I may well misunderstand this.

Regarding the use of ferrocement - my boat is made of FRP, so I do not know if it could be used, unless you mean as the core material, with glass laid over it? I have never worked in ferrocement. I do have quite extensive experience in FRP composites.