wooden boats and bouyancy

[yotphix]

Just curious if a wooden sailboat of cruising dimensions typically has what i think is called positive bouyancy. By this I mean to ask if they would float at the surface even filled with water as a rule. Is this true for plank on frame or even ply construction?

[Raggi_Thor]

Just. Maybe :-)
Many species of wood used in boatbuilding have a density of 0.4 to 0.6 (water is 1). As a rule of thumb you could say that each kg (or pound :-) of wood in a boat can carry itself and the same weight of lead (or other ballast). That means that a boat weighing 1 ton with 40% ballast will theoretically float. But that is if the wood is dry! Modern plywood boats covered with epoxy and with less ballast will float better.

When I was young I had an 18 feet lapstrake sailing boat with 160kg of iron in the keel. I had lots of flotation (Styrofoam) under deck because this kind of boat had demonstrated it could sink...

[yotphix]

That answers my question perfectly Raggi. Thanks so much!

[PAR]

An object of any material can be made to float, if it's mass is less then the dent in the water it makes when placed in it. A boat of lead, if it's shape is sufficient to support it's mass, will float.

This dent is displacement or the exact amount of water that is pushed out of the way when the object is placed in it. Just as the bath tub water rises up the side of the tub as you get in, an object placed in the ocean "displaces" the water that once occupied the area. Knowing the weight of water, we can calculate the amount of materials and equipment aboard a prospective boat design and engineer this into a shape that will permit it to float this weight, maybe with a healthy margin to handle a press of wind in the sails or other eventualities, like keeping your feet dry underway.

In smaller designs it's possible engineer unsinkable craft, but as the sizes go up, this quickly becomes much less likely. There are a lot of reasons for this, but the sacrifices of usable space becomes a big problem. The designer must resort to water tight compartments to divide up the interior volume of the vessel, if reasonable accommodations and safety can be expected. I used to own a 21 ton 44' ketch. Can you imagine the amount of foam and lose of interior space it would have required to keep it from sinking when swamped. It may have had the cabin space of a Catalina 22, but it might have floated if swamped. If swamped it would have trapped several large bubbles of air below decks for a while, maybe several hours, possibly days, but eventually it would have sunk like a 42,000 pound stone.

[MikeJohns]

Originally Posted by yotphix

Just curious if a wooden sailboat of cruising dimensions typically has .......... positive bouyancy. ...........

Its one of those compelling myths that smaller vessels should be made of materials that float.

I have often had people express concern that a steel of ferro-cement boat would be unsafe because it would lack positive bouyancy.

No ballasted cruising sailboat of any usable size will float when holed unless specifically designed to do so and then it takes careful design and placement of sealed cavities or bulkheads.

Once a boat gets to cruising boat size the volume of the timber tends to reduce relative to her displacment then there's maybe 30% ballast sitting there along with all the other extraneous denser than water gear like the engine rigging fastenings anchors winches steering gear prop and shaft .....etc etc. They all add up to a tidy mass . Even wooden boats that have been holed and lost their ballast tend to sink in my experience regardless of the timber from which they were made.

Some of our boat building timbers here are denser than water when seasoned and dry.

Cheers

[Paul Scott]

I doubt if my 40' 10,000 lb (4600# keel, 8' down) cruising sled would, even if it is C.O.V.E..

Paul

[Raggi_Thor]

Even if a wooden boat with a little ballast don't sink, it will be impossible no navigate and you can only sit on top of it. A large portion of the wood is in the deck so this will be in the water.

But I remember an old rule, "stay at your boat as long as it floats!"..