Do I really need all my chambers filled with flotation foam

Good hot afternoon everyone
I am building a catamaran style shallow water scooter boat hull design all put together and taped up and 1st coat of resin applied, just to harden up everything so I can get ready to apply 3 layers of fiberglass(woven 10%). My question is this:
Each hull side(catamaran style), side will hv 9 cavities, total of 18 both sides, front and rear very small, because of angles needed to get the style of sponson I wanted. Money is limited right now, my question is: should I fill every compartment with flotation foam or just every other one, or just one compartment at each corner of boat & 1 in center. Less than 550lbs total on boat batteries, trolling motor and myself, poles lures stuff like that. I will hv 3 layers of woven fiberglass, and 2 layers of gel coat all are name brand products. I’m looking at about 35.approx 1 1/2cubic squares per chamber cept for front and very back. Thinking maybe 3 per side, not over thinking this cause won’t hv very long less than a year and will give it away. The kayak I want rigged out is over 5 grand plus electronics. Saving fir down payment for another boat thanks for any Input, please no smart alkecs comments thank you fir your time great boating everyone

 

Sounds like you get to do whatever makes you comfortable. You aren't planning to carry paying passengers, right?

I'd be incline to do the ends where a collision might hole the bow or stern, then just a couple of inches against the underside of the deck. But you're a catamaran, so she won't right herself, no need to keep the flotation high like that.

 

Are you allowed to not have any foam filled compartments at all?
If so, and noting that you will have 9 compartments in each hull, I would be inclined to just build them well, and keep them as watertight void spaces.
You will still have a much better chance of staying afloat if a compartment (or even a few) is holed, compared to the average 'small' yacht which has no watertight bulkheads, apart from maybe a collision bulkhead in the bow.

How big is your cat scooter?
It sounds like you are building it with plywood(?) and then sheathing it with fibreglass? If so, are you using polyester or epoxy resin? I presume polyester, as you mention finishing with gelcoat (?)
Can you post some photos please of your progress so far?
We always love to see construction photos of projects underway.

 

Are the compartments sealed or open lockers?

Foam open lockers to just above dwl, otherwise, a closed chamber is fine for a private boat.

 

A point that you might give some consideration to is how you will drain any leakage from the hulls.It isn't the simplest of tasks to create that many totally watertight chambers and in regions that have large temperature fluctuations you may need to also ease pressure variations.Putting a boat in to cool water after it has been on a trailer in the sun may well encourage the entry of a little water through screw holes and you will need to drain it.18 drain plugs on the outside of the hulls would look messy.

 

The Workstar 17 is an interesting kit boat, with multiple underfloor compartments -
Workstar 17 - Fyne Boat Kits https://www.fyneboatkits.co.uk/kits/motor-boats/workstar-17-workboat/

She appears to have drains between the small compartments (perhaps draining aft to a bilge pump?) and they note in the link above that she has watertight compartments (that are accessible) fore and aft to provide positive buoyancy.

 

I would NOT limber the watertights at the bottom. If they are indeed intended to be watertight, I'd either install submersible vents at the top or just a breather hole. Others have also used a piece of hose. One issue with the watertights I see is water vapor, even in a submersible vent, can build inside them, so they probably need occasional drying out. Edit (also, if plywood inside, better plan for water)

McMaster-Carr https://www.mcmaster.com/4505N11/

McMaster-Carr https://www.mcmaster.com/vents/

 

Been a long while since I posted here. Been working on my website. Anyway, the simple answer to your question is no. You don't have to fill them with foam. Foam is not a requirement. Flotation is. How you achieve it is up to you. The USCG standard. is a performance standard not a specification. see my page on level flotation. Boat Building Regulations | Level Flotation https://newboatbuilders.com/pages/flot2.html

 

Some time ago, I was helping with work on a small, portable, sealed medical gadget. It had membrane switches on the front. When ascending in an airplane, the pressure difference would pop off, or at least unseal, the membrane switches. The solution was a little Gore Tex patch that would pass gases and vapors, but not liquids. I don't know which flavor of Gore Tex; they have a bunch, but it might be an easy solution. Of course, it's a good idea to ventilate the chambers from time to time.

 

I use Gore Tex breather valves on my watertight bulkheads.

 

I could use some snap in Gore Tex vents. Where do you get them?
My Googlefoo didn't find any in stock.

 

Search gore tex breather valve

 

Thanks, I tried that but no retailers with anything in stock came up.
I'll keep trying.

 

Mcmaster carr submersible vents. I can link if u need. Too tired now.

 

Thank you.