Thread: How much floatation does a wooden boat need?

Hey Andrew,

The foam like Space invading stuff, actually hold moisture and encourage Dampness and rot in wooden boats. Some of the boats that have been re done at work have had Polystyrene Blocks wrapped in Plastic sheets stowed under the floor, that can be removed and dried, If they get wet. Depending on where you are using the boat, In shore or outside, as to how long you can tread water, but it should be enough to keep the boat above the water till help arrives (If needed), maybe have a yarn with the local ShipWright or boat builder.

HazzaB

There are several types of "foam" you can use, but lets dispel some myths. Polystyrene is usually not the foam you want in a boat, because of it's open cell construction. This construction type (closed or open cell) is the dividing line for marine use foams (for the most part). As a rule, you'll want closed cell foam, which doesn't absorb moisture and this type of foam is usually a polyurethane.

Foam can cause issues by the way it's installed too and once you use foam, you've pretty much insured the space can't be used for much else.

Floatation or buoyancy bags do a much better job then foam, mostly because they can be deflated and removed, if for nothing else then to clean out the compartment. On your boat, I'd recommend around 600 to 700 kilos of floatation, which assumes a full up weight, some equipment, gear and crew are aboard a fully swamped boat and trying to bail it out. This is a lot of floatation, which is why, except for small craft compartmentalization is used, rather then actual foam or buoyancy (think of how much foam it would that to keep an air craft carrier afloat!) bags.

Lastly, If a compartment is sealed up, then it doesn't matter what is inside, because it's working as a floatation chamber anyway. My point is foam and bags are great, but usually redundant, when installed inside a compartment. Unless you take a torpedo hit or happen to run over pointy logs and the like, which just happen to puncture the compartment (the bag will likely get it too), then floating stuff inside it is redundant. An air bubble or pocket is just as effective as foam or dozens of empty 2 litter bottles jammed into a locker and doesn't rattle or cause other issues.

Thanks.

The boat doesn't have any enclosed compartments, which is why I was looking at whether it needed additional bouyancy.
It looks to have been designed such that if you hosed anywhere on the inside, the water would drain down to the floor and to the back of the boat. Lots of air circulation to allow it to dry off!
Might put an inflatable air mattress under the floor, and across the boat under the motor well - at least they can then be removed for cleaning and drying off.

That's generally the way I design wooden boats too, lots of open spaces, so ventilation can be easily accommodated, which is the most important thing in wooden boat longevity. In a small day boat, where capsize is a real and possibly frequent occurrence, then floatation is a must. Hell, I'll intentionally capsize one of these just for the relief of the dunking in the drink. On an 18' cabin cruiser, I'm more concerned about maintenance and where to put the extra cooler of beer. If you get into a situation where floatation is necessary, you will probably have known for a while and should have done something about it.

For example, last year I ran a buddy's bass boat over a log at speed (about 40 knots), which naturally made a big hole and a mes off the lower leg on the engine. My first reaction was to stop the boat and find the hole, which was quite big, then I restarted the engine which was shaking itself to pieces and blasted as fast as practical, with the lower leg trying to self destruct, up onto the nearest land mass a couple miles away.

My point is though the boat had some floatation, my situation would have left me stranded, without likely rescue for some time and the boat, if not eventually sinking was going to be bow high and impractical as a rescue device, if permitted to swamp in gator infested water. I elected to use what I had left in the boat to get me and my buddy to shore. The engine was damaged, the boat damaged, I wanted to make sure we came out fairly undamaged. Yep, when we hit shore at about 25 knots, it tore up the boat and engine some more, but I saved the beer and my friend, which are the important points (in that order too) . . . The boat was fixed the engine replaced and we now have a cool story to brag about to unsuspecting bystanders at the launch ramp.

The moral of the story is floatation is way over rated and very rarely necessary on an 18' power cruiser. Having a cool head and making good decisions when the crap hits the fan, are the things that permit you to have another shot at life the follow day.

You may want to check the relevant section of Australian standard AS1799 which you can download at :: National Marine Safety Committee ::: National Standards ::

Actually, what you want is ISO 12217-3 for basic flotation. You'll also need a few other ISO standards, plus relevant ABYC, AS, CE or other standard accordance guidelines.

Having jumped through these hoops many times, I can assure you that the home builder doesn't have to do this, the designer does. The average person doesn't have a clue about ISO 13590 and shouldn't be expected to.

The document listed above is the NS for the ABP for recreational boats. It's used to guide though some of the pertinent ISO, ABYC (etc.) recommendations and of course the layout and disclaimers associated with the ABP template. Section 2.1.2 of AS-1799.1 is what you really want to look at. I'm fairly sure these new regulations are not retroactive, so you're safe on your 18' Hartley.