Foam is more dense than air.  Therefore, in theory, an airtight space not filled with foam is more buoyant than the same space filled with foam.  The only reason to fill the space with foam would be to provide backup buoyancy in the event of the airtight space becoming non-airtight due to damage.

.....Michael

the difference in buoyancy is negligible....as gramps said above buoyancy is simple a function of the weight of an enclosed area/volume relative the the weight of water in an equal area/volume.

foam, is typically used as fail-safe to ensure a boat is unsinkable should its watertight compartments get compromized.

that said, there are a lot of downsides to putting foam in enclosed spaces as in your picture.  the main downside is moisture gets into these place and if you have foam in there, it will not dry out and eventually that moisture will lead to the breakdown of a compartment like this.  so my perspective is that compartments like this need to have an opening that allows for inspection and drying of moisture that will accumulate there absent an inspection plate.

if you are concerned about ensuring that the hull will float regardless of damage, there are other aproaches.  some dinghys hang/strap closed cell foam flotation under their seats and should the hull get flooded...to ensure it stays above water until help or bailing arives.

but filling a closed area like this with foam is not a good idea based on my experience with a lots of boats over the last 40+ years.

h

>> was wondering if a water-tight compartment [with or without gear] is as boyant as a compartment filled with foam?<<

Non-structural foam, as one would use for simple flotation, weighs about 2 pounds per cubic foot. That's either expanded polyurethane foam, that you pour in, or expanded polystyrene (aka styrofoam). Foam is available in heavier densities if you expect it to bear weight.

Given the picture in the OP, you're probably not talking about more than a cubic foot. But you'd have to take the measurements and do the math to really see.

An empty watertight compartment will be more bouyant than a foam-filled one by the weight of the foam.

A watertight compartment with gear will be less bouyant because it's pretty much a given that your gear is going to be heavier than the foam.

The final technical point - a foam filled compartment that gets wet will be a lot heavier than one that's dry. I know of a 12' outboard whose underseat floatation became soaked - the boat picked up 50 pounds!

   Thanks for the reponses.  My intent is NOT to use foam but rather ensure a watertight compartment as best as possible.  Not a fan of the bouyancy bags simply for the fact that they detract so much from the look of the vessel,  and I do like the addition of a small storage/forward step area.  Would just keep a few lines or a bumper in the enlcosed area and perhaps a CG safety kit.  

Oceanluvr,  

I have a dinghy that I put 2-part structural closed cell foam into some 14 or 15 years ago, then sealed the compartments with no ventilation or access ports. It's still going strong with no signs of rot or other dire consequences.

I had a specific strategy for where and how I placed the foam. It was at the lowest point of the boat, in the front and back. Above these points the compartment was empty air. The reasoning was that these were the points most likely to make violent contact with rocks and other obstacles. I also taped the seams inside and out to make sure that the compartments were properly sealed.

The bow compartment, which looks a lot like the one in your Skerry picture, I simply painted with epoxy. The stern compartment I glassed on the inside. This was done for stability. The bow, with all the triangles bracing each other, won't flex except in a catastrophe. The rear, which was a series of flat panels, needed to be stabilized against flexing.

Flexing is the enemy of closed compartments. It causes fatigue micro cracks which eventually join into cracks which let in water. Eliminate the flex or stabilize the surfaces and there's no cracks. A sheet of glass cloth stops fir plywood from checking, so it has no problem stopping micro cracks.

I think much of bad rap that foam-filled closed spaces have came from polyester resin power boats. Polyester resin is porous, only the gelcoat and/or paint keeps the water out. Power boats also have a lot more vibration and flexing than our sail and muscle powered light craft. All it takes is a scratch in the gelcoat and some period of high speed runs to provoke water intrusion. And if the builder used open cell foam, it's a sponge.

As far as the buoyancy goes, it's the same whether the compartment is empty or full. It's simply determined by the volume that the compartment displaces when the boat is flooded, about 64 pounds per cubic foot. What filling or leaving it empty does is change the total displacement and balance of the boat. As the folks above point out, an empty compartment results in the lightest boat.

Just another data point for you to consider. Have fun,

Laszlo

   Thanks, Laszlo, you always seem to 'put the cherry on the top' and provide a wealth of information.

   The way I understand it foam or no foam you are adding a compartment in the bow of the boat.  Whether it is for storage or bouyancy what balances it and floats the stern?

Grumpy - I will make the same type of modification on the stern seat, making it a watertight compartment with a small inspection plate,  plus the foam under the thwarts as per the plan.