Mast Top-Float

I want to make a mast top-float from blue foam insulation and fiberglass to keep my M16 Scow from turtling when I capsize.
I have already designed it on CAD but don't know how much (inch^3) I need. Are there any formulas I can apply to get the optimum size?

I've flipped it twice in the last few years in a deep lake and it went right over 180 deg. There's a shallow lake closer to home but I don't sail there because I'm afraid the mast will get stuck in the mud. Good seamanship would help but that's going to take awhile so..
Thanks in advance for any suggestions.
KB

 

I expect something in the size range of a soccer ball or slightly smaller would be sufficient. The lever arm is quite long so you wouldn't really need much. Take the boat to the lake and tie/tape a 20 oz bottle (capped and empty) to the top of the mast then capsize the boat. See if it supports it, if not go to a liter, then 2 liters etc until it keeps the boat from turtling. Make sure to have the sail on but not the boom...to keep it from blowing away on you. The sail will add weight when wet so you want to take that into consideration by using it too.

 

Divide the displacement of the boat, by the length of the mast above the deck and this will get you close, to the approximate volume you'll need at the masthead, with a small reserve. For example, if you have a 500 pound boat with a 20' stick, you'll need about 40% of a cubic foot of floatation.

As a rule and a good habit to get into when capsizing, is to grab a PFD and swim to the masthead and tie it on immediately. This is usually enough to keep you from going turtle, while you gather up the rest of the stuff that's floating about.

 

While it is still warm. Rig the boat, tip it over at the dock.
Take a fishing scale and weigh the tip.
Do the arithmatic and then tie that much floaty stuff to the head of he sail and see if it works for you.
Right the boat and go sailing.
Alternatively, guys like Gordy Bowers make foam headboards for scow sails.
Maybe you can do it the way they teach you in engineering school: Ask someone who knows.
SHC

 

Hmmmm. I used to have a wooden Melges M16, and never turtled it. Of course, it had a wooden mast, too, which may have helped. (We swamped it the first time we capsized, because my brother and I both tried to go over the top and onto the boards without going swimming. That was enough to push the cockpit coaming under water. I never did that again!)

Check out the E scows. They have nifty panels with pockets in the heads of their mainsails into which closed cell foam panels are inserted. I think that's the way to go for your M16. It should be easy for a sailmaker to basically double up the top panel of your sail on each side to hold the foam. Cannibalize some boat cushions if you need to to get the foam pieces to go into them - the cushions are filled with a stack of foam sheets.

Edit:
I see Steve already beat me to it! I should read the whole thread before responding!

 

I've used a plastic ball-cock (about 4" diameter) to give just a bit more time before my 14' dinghy (probably a lot beamier than your scow) turtles. Not the world's most elegant solution, I know, but very effective. You don't necessarily need it to float, sinking more slowly will be fine. You just need enough time to get to the centreboard if you couldn't get there as you went over.

Cheers,

Tim B.