Bilge keels on a Gartside double ender

First post here, so let me introduce myself. I'm a recently retired airline pilot, now spending my time playing as an amateur boatbuilder. We moved to Blue Hill Maine for the boating, beauty and lifestyle. My current project is a Paul Gartside Sjogin III.

We live on the north end of Blue Hill Bay where the 10' tide retreats twice a day to reveal acres of mud flats which got me thinking of how they handle that in England. The beamy, slack bilge of Sjogin III would seem suitable for bilge keels so I mentioned this to Paul and he gave me his OK if not his blessing. He comes from the land of bilge keels but has no experience designing or building them and is interested to see how they preform.
So I'm calling my project Sjogin IIIa. (link to my build) Adding the keels and removing the CB also opens up the boat. I'm also stretching it 10% to 20 1/2 feet, to add a small cabin similar to the original Sjogin design. I hope to mostly day cruise Downeast Maine with my wife and two large dogs, nothing too adventurous. The construction will be glued lap using Vendia marine planks.
I haven't been able to find a lot of design info on bilge keels but have looked at many photos. Also talking to Paul, and a local NA friend, this is what I'm planning: add bilge stringers to through bolt the keels and they would be captured by the floors. The keels could be made of 3/4" steel, with a 1/4" flange. If 1'x4' that would provide almost half of the estimated ballast. Mounted along a waterline and angled 13-15 degrees, which is what the NA determined would be the optimum heel is my first guess. Not sure if making a NACA foil on the inboard side is worth the effort.
This design has not built before so there is no real world data to work from. It will be an experiment, hopefully a fun and successful one.
Thanks,
Steve

 

Welcome to the forum.

I would use a bilge diagonal instead of a water line. Regarding the cross section of bilge keels, a flat bulbous bar would be ideal.

 

Welcome. Neat project. Bilge keels aren't far from foils ya' know...

 

Is this based on experience or do you have any references I can look up? As I mentioned I've not found much on bilge keel placement. I did find this article by Bray Yacht Design on twin keels, where they determined that placement was critical. There is not a rush as I've haven't even begun planking.
Thanks,
Steve

 

The balance keels are placed as far as possible from the center line of the ship (so that the righting moment generated by them is as large as possible) but so that they do not protrude from the ship's beam. Longitudinally they should cover, more or less, 2/3 of the length, centered on the length of the flotation. If you want to place "twin keels", not what I understand as bilge keels (against the rolling of the boat), the reasoning can be very different.
(See attached file)

 

Tansl is correct. Bilge keels are not twin keels: different size, proportions, location, and purpose.
Ted Brewer has done several twin keel designs.

 

Well to be fair/clear. The term are often mixed.

I've seen this referred to as both twin and bilge keels:


And this referred to as bilge keels or strakes




From context, I believe the OP is referring to the former not the later, while TANSL is referring to the later not the former.

 

The function of the two keels on Steve's project are to provide lateral resistance in place of a centerboard, provide ballast so that the boat has the designed displacement and also improve stability, and to keep the boat upright if it is anchored or moored in an area which dries out at low tide. This type of keels are usually called "twin keels" rather than "bilge keels".

While the keels are planned to be made of steel the primary material for the hull is wood with glued lapstrake planking.

 

Yes, I think you're right. Obviously I speak of the "later" but, once my comment was written, I realized that the OP was probably referring to the "former".

 

Yes, we're talking bilge keels. I mentioned the twin keel study because it showed that minor changes in position really effected the performance. As David said the primary purpose is stability at low tide, and then to provide lateral tracking while sailing. I carved a 1" scale model and to help determine where to position the keels for stability and am looking for the optimum positioning for sailing efficiency. It will be possible (although not easy) to reposition the keels along the bilge strake if needed after launching.

 

These photos were some of the many that I had found and studied. The keels on my model are very similar to the later.

 

Steve, are the two keels planned to be 13-15 degrees from horizontal or from vertical?

 

Vertical, to get the keel perpendicular at optimum heel is my thinking

 

Another thought. This is a 19' boat so why not keep it simple and build as is (centerboard.). The boat is more beach accessible (less draft) than a twin keel and towing a dinghy to get ashore might not be necessary.
Bilge keels might work for beaching & roll damping but they will be less efficient than the Gartside centerboard shown.

 

He wants to be rid of the centerboard box.