Follow-Up on Self-Draining Cockpits
Some additional thoughts...
by Mike Waters NA
One comment raised on this subject (see last article here), is the difficulty of building a watertight floor into an existing design, especially within a lapstrake (clinker) hull.
Due to all the lap-joints with a clinker hull, this can indeed be an issue. Still doable, but to some, just too much disturbance and loss of access to the hull itself.
In such a case, there are still other options to give improved stability when partly swamped, so let’s look briefly at these.
The instability may have some wondering. “Does adding weight inside a boat low-down not help? Like inside ballast after all, no?” The problem with using water is what is called the “free surface effect” or FSE. You may already know how hard it is to carry a tray of water across a room without spilling it. This is because the upper surface water can flow too easily from one side to the other, quickly moving its center of gravity over from side to side. With a typically open-floor flooded boat, this beam is nearly a match for the exterior waterline beam that gives it stability in the first place! A perfect match over the full boat length would effectively result in Zero stability, so you can see how vulnerable a swamped boat can become. Even losing a quarter of your stability would be VERY noticeable. So what to do ?
Free surface effect (FSE) is a direct function of the Mt of Inertia of the surface water, and this in turn is directly proportional to the Cube of the free water Width. So if you can divide up the bilge area below your floorboard into smaller widths with watertight lengthwise girders (of say plywood), then the ‘Cube of those narrower Widths’ will be far less. In fact, if you divide up your under bilge area into 3, your FSE will be only
1/9th of the full beam. If you divide it into 4, that then reduces your effect to 1/16th ! Tapping into my ship design experience, (which incidentally, I do more often that most), this is the ONLY way a Tanker can succeed in bringing liquids across the Ocean without capsizing!
Even without going to a fully self-draining floor, you can improve flooded stability even further by closing in some of these lengthwise corridors with watertight tops to totally remove some of those areas from the FSE, cutting down the instability effect significantly more.
So keep this in mind when finishing off your open boat. What you do under those floorboards could save you from many a capsize. Even if you will still have some water to scoop out between those girders, it will not feel the life or death situation you might sense if the whole area is open for flooding. Sail safe. •SCA•
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I think the idea of a self-draining cockpit is a bit over sold here.
There are alternatives. Especially for boats with gross displacements under say a thousand pounds.
The pdracer fleet took capsize recovery safety seriously since its very beginning.
The pdracer approach was what they called "air-boxes". These were sealed on all sides, but typically had a hatch on top, or an inspection plate on the inward side.
The air-boxes usually extended across the boat or along its length. The advantage of the latter was that the boat usually ended up free of water after it was righted. The disadvantage was that, if the sides were high, the boat was hard to get back into.
With the air-boxes extending across the hull, the advantage was that the boat would sit lower in the water, due to a considerable amount of water still aboard and would be easier to get back into.
My personal boat, a scow, but longer an narrower than a pdracer, does not have airboxes. Instead, it has 12, one gallon milk jugs lashed under the side decks. This is to make it easier to board after righting. I can pull the side down and swim over it.
Once aboard, I can bail the water out, as the water in the boat will be supporting most of my weight, not the 12 milk jugs.
I Sailed an old FJ for many years, and always wished I could sail the water out of the hull. I had seen Flying Dutchman and other boats with the flaps in the transom and wondered if that would work on my FJ. Did you retrofit the FJ you pictured. I also remember seeing a device in the bottom of the boat in the stern that looked like a one-way valve that would sail the water out of the hull.