1975 21' Mariner rebuild - replacement foam thickness?

[nitro_alltrac]

I'm rebuilding my Dad's 21' Mariner that I inherited. I've got to put new floor down and after reading about the old foam needing replaced I'm going to do that while I'm in there.

Question though, how think does the closed cell foam need to be? I know it comes in 1 and 2 inch thicknesses, but what is the best total thickness to put in?

I just got the boat over to my house this past week so I don't have pictures yet. I'll get some up soon though.

Looking forward to getting this thing back in the water.

 

[BWR1953]

I'm rebuilding my Dad's 21' Mariner that I inherited. I've got to put new floor down and after reading about the old foam needing replaced I'm going to do that while I'm in there.

Question though, how think does the closed cell foam need to be? I know it comes in 1 and 2 inch thicknesses, but what is the best total thickness to put in?

I just got the boat over to my house this past week so I don't have pictures yet. I'll get some up soon though.

Looking forward to getting this thing back in the water.

I'd say to get the thickest foam that you can, which means fewer pieces to cut. The only stuff available in my area was 3/4" and so I ended up having to cut many more parts.

 

[nitro_alltrac]

So one layer should be good?

 

[HypnoCraft]

So one layer should be good?

When I did my flotation I got 1.5" foam. one layer horizontally probably wont be enough foam to float the hull. I put mine in vertically and traced a line below where the deck will sit. In total it took about 5 or 6 sheets of 1.5 X 24 X 96" XPS foam to fill below the deck.

 

[Decker83]

You will need to put in as much closed cell foam as possible. If the floor is factory you can see how the factory did it.
There is a lot of threads showing how other people have done theirs.

:welcome: aboard,

 

[nitro_alltrac]

Thanks guys. This is very helpful. I'll check for the other threads.

 

[jbcurt00]

Get a mix of a few thicknesses. You need 1CuFt of foam for every 60lbs of boat. Your 21ftr weighs more then 1200lbs I'd bet, so more then 20CuFt. Thats a lot.

If you just have 2in foam to put in a 3 or 3.5in cavity, you'll be cutting it to fit. But if you have 2in, 1in and 1.5in you can stack them and fill the 3 or 3.5in cavity...

Most lay it down, BWR1953 and HypnoCraft both stood it on edge. Both ways work...

 

[jbcurt00]

In total it took about 5 or 6 sheets of 1.5 X 24 X 96" XPS foam to fill below the deck.

Those size sheets only give you about 120lb per sheet, or at max 720lbs (if 6 sheets)...

Is that enough? Dont remember what size SC....

Edit: the 22ftr listed in your profile? Not nearly enough w a motor....

 

[HypnoCraft]

Those size sheets only give you about 120lb per sheet, or at max 720lbs (if 6 sheets)...

Is that enough? Dont remember what size SC....

Edit: the 22ftr listed in your profile? Not nearly enough w a motor....

That's all that would fit under the floor. The buoyancy calculations I did say that I have ~300kg of excess flotation or ~660lb. That's also doing the calculations conservatively. I was planning on doing a full submersion flotation test, but that seems a bit crazy. currently I have just over 26 ft^3 of foam.

I will be sure to look at my calculations again, thanks for the heads up!

Edit: I see I mistyped 1.5"... its actually 3" X 24" X 96", Sorry!

 

[jbcurt00]

Edit: I see I mistyped 1.5"... its actually 3" X 24" X 96", Sorry!

Good, that doubles it to 1440lbs of flotation value

 

[BWR1953]

Yep, I stood mine on end, glued and taped the stacks together and covered the 6 rows closest to the stern with HDPE trash bags to seal them off from any potential gasoline spills. :joyous:

Like so.

 

[HypnoCraft]

Yep, I stood mine on end, glued and taped the stacks together and covered the 6 rows closest to the stern with HDPE trash bags to seal them off from any potential gasoline spills. :joyous:

Like so.

oh those HDPE bags are a great idea!

 

[nitro_alltrac]

BWR1953 thanks for the picture. That really helps. I see that I need to get quite a few sheets. I want to make sure that I have as much buoyancy as possible. I'm not sure at the moment that there's that much of the old foam in the boat right now.

 

[Watermann]

Truthfully, flotation foam no matter the type will in larger boats only slow the sinking event not keep it floating forever. Especially if you have multiple batteries and or large motors like an 800 lb V6 IO or an over half ton V8. There really isn't the space to add enough foam, even more so if you have a belly fuel tank taking up under deck space but even if full of gas it does give some flotation since gas weights only 6 Lbs a gallon.