Don't judge me. I'm completely new to this. This is just a thought.
I'm refurbishing a Supercat 17 , and prepping the mast so that water cannot get into it. Would it be a stupid idea to fill it full of foam packing peanuts so as to displace any water that could possibly get in if it did? They weigh almost nothing and would allow drainage.
Saying it's stupid would be a little insensitive.. ;-)
Let's just say, not a good idea. Jeff has the right answer, just make sure the mast is empty of water and seal it so none can get in. Air is lighter than foam peanuts.
Welcome to the forum, don't be afraid to ask questions, we all had to start somewhere. The temptation is always there to invent a new way to do something, but understand these boats have been sailing since the 60's and there is generally a consensus "best" way to get things done.
Always start with the original owners manual from the designer, you can find a lot of good Supercat info and docs athttps://www.thebeachcats.com/pictures?g2_itemId=78177
Including the original owners manual and parts list.
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