Boatbuilding Blog

November 3, 2006

Twin Keels

Filed under: Building - Considerations — tomlarkin @ 11:12 pm

I plan on adding twin keels to the boat instead of the deep single keel shown in the plans. The keel as designed accomplished two things – it contained the propeller shaft, and protected the prop from obstructions in the water. Using an outboard removes both of those considerations.  Here are the advantages of twin keels for this boat as I see them:

  • Less draft – probably 6-10 inches less than the original. I love going in places most boats don’t dare to.
  • Smooth water flow to the prop.
  • It can dry out at low tide. Puget Sound has large tides – 12 to 16 feet being pretty common. I want to sit on the mudflats in Padilla Bay and watch birds while the tide is out. 
  • Ability to sit flat on shore or haul on a flatbed truck.
  • Roll attenuation? Faster hull speed? Smaller wake? All suggested by the following articles.

My main concerns about this configuration are added drag (not too concerned), and interference with steering – it may not turn quickly. Both of these can probably be alleviated by good design.

Boojum’s Twin Keels – article about the advantages of twin keels on a small heavy displacemnt tug cruiser.

Roll attenuation strategies – for motor yachts and motor sailers

Some degree of roll attenuation is contributed by the single chine hull form itself.  A single chine vessel appears to have roughly twice the roll damping ability of a rounded hull form (per published model tests in Marine Technology, performed on vessels having similar hull forms).  Roll amplitude will be less; roll acceleration may be greater, rolling will decay more quickly.

Why Twin Keels?

But I think my main reason for thinking of twin keels may have been a desire for freer contact with the shore. A fin-keel boat is bound to deep water. Any contact with the bottom is so likely to result in a predicament, with the boat lying on her side for half a tide, that she must sail with a large margin of safety most of the time, keeping far from shore, or right in the middle of the channel. In essence, she must keep clear of all kinds of tempting places where the bottom just might be too close to the top.

With twin keels we go right on up through the fleet and usually find, between the first row of moorings and the shore, enough water for us to swing comfort ably. And if the tide should go out and leave us aground in the middle of the night, who cares? It will be back in the morning. Thus, almost everywhere we go, we get the best berth in the harbor closest to the landing, close enough to see what’s going on onshore, surrounded, by the local fleet instead of outcast on the fringes.

BoatDesign Blog – thread on twin keels

It would appear as though the fore-to-aft placement of twin keels would definitely favor the more aft placement.

This is a nice boat, by the way. Simple, clean lines inside, plywood construction.


1 Comment »

  1. […] Filed under: Uncategorized — tomlarkin @ 1:23 pm I had a previous post about twin keels. I finally found some pictures of something […]

    Pingback by Twin Skegs « Boatbuilding Blog — March 19, 2007 @ 1:23 pm

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