Dan and I had a great discussion yesterday about twin keels/hulls verses mono keel/hulls. I was noticing that the amas (pontoons) on his Hobie Cat are asymmetrical. When attached to the trampoline as designed, the outer faces are flatter than the curved inner faces. Desire’s keels are shaped in a similar way. The reason for this is that a twin hulled or twin keeled boat usually has the wind hitting it from one side or the other. The upwind side wants to lift up and the downwind side wants to bite into the water as it becomes the fulcrum for a lever. Not only does the downwind keel/hull bite into the water, but because it is curved on the side towards the center line of the boat it behaves like an airplane wing (oriented straight down in the water) and wants to ‘lift’ towards weather (wind). This allows the hulls to more efficiently track in a straight line rather than being pushed sideways by the wind.
After running a few errands today in the BF2 (big fricken ford) I came home to find ‘Desire’s older brother nearby. ‘Good Measure’ is owned by a good friend of mine, Larry Casidy, and was built in the same yard by the same designer as Desire, Lauret Giles. After building several hundred ‘Snapdragon’ boats (Larry’s style), Giles was awarded a grant by the British Royal Navy to do some tank testing and refine the hydrodynamic properties of his twin keeled concept. He came up with the Westerly ‘Centaur’ and built about 2,500 of them between the late ’60’s and mid-late ’70’s.
Good Measure's keels
Desire's keels
When you look at the two boats side by side you can immediately see how Giles refined the hull shape and keel configurations. Larry added two feet to the transom and a 3 foot bow sprit, but other than that they were both 26 foot boats. The older Snapdragon design has straight down relatively flat keels and a more rounded hull. The Centaur hull has a flatter bottom between the splayed keels. It’s hard to see but the Centaur keels are flatter on the outside and more curved toward the centerline of the boat. The new design made the boat much more stable in heavy rolling seas as well as allowed her to point (track) closer to the wind.
Desire from the bow
One other interesting feature you will notice is that Desire’s forward hull has a chine, or ridge running parallel to the water line. Below this chine the hull is slightly concave to deflect small splasher waves away from the hull and thereby having less splashing and spray flying onto the decks and back into the cockpit.
Every displacement hull has a theoretical maximum speed for a moving through the water. This is because the wave created by the bow moves farther and farther back along the hull as speed increases. When it reaches the transom at it’s maximum hull speed it creates a hole in the water and starts sucking the transom down as more power/wind is applied. A planning hull, like a ski boat, overcomes this by ‘rising on a plane’ or skipping across the surface of the water like a flat rock.
I constantly exceed “Desire’s theoretical limit of around 5.4 knots. I think this is is largely due to the flat area between her keels. She is a semi-planning displacement hull! With engine alone she normally cruises at 6.5 knots. Surfing in heavy seas I’ve gotten her as high as 9.3 knots for short periods!
WARNING/Disclaimer!!! Aug 3, 2009: I re-read this post today and realized I need to point out how absolutely foolish it is to surf any boat in heavy seas! …unless you really know what you and your boat are doing and you want to while away a boring storm with some entertaining fun on the ocean…
Here are some things that have worked for me(but may not necessarily work for you, your boat or your sensibilities:
First: (and foremost): Never ever ever go into the trough of a wave 90 degrees to the wave! Several things will surely happen (some simultaneously), the wind will stop, the boat will slow down, the nose may dig in, the following wave can flip you end over end (pitchpole), if you make it home, you will surly be crying…
Second: You have to sense and hand steer every wave differently. Never ever ever use an autopilot or wind vane while surfing. If you absolutely have to sail/motor downwind/downwave in these conditions use drogues, anchors, warps, anything you can think of to hang over the stern to slow the boat down. The safest tack to make it through these kind of conditions is to stear 30-50 degrees off the prevailing (largest) wave sets.
Third: If you screw up or something unexpected happens stear into the wave. The strongest, most hydrodynamic area of the boat is the nose and it’s designed to bust into waves. Use it if you have too but try not to jibe unless you can control it.
Fourth: Learn how to make your boat heave too, with the jib/rudder in light winds and with just the main/rudder in heavier winds. Along with learning how to heave too you will have to learn how to get out of heaving too, preferably without doing a hard jibe. Heaving too will save your life, lunch and/or relationship one day, trust me…