Yes, I realize I misspelled the name of the sup. Take that as a sign of how much help I require. 

Chris,

If you're going to foam the inside, make sure that you use an appropriate foam. That spray-in stuff sold at DIY stores and the stuff sold for packing bulky objects is not it. You need closed cell, 2-part polyurethane expanding buoyancy foam. It's about $80.00 to fill 8 cubic feet of volume. Unlike the DIY store foam, it is not affected by epoxy and will not act like a sponge in case of a leak. It also stands up to flexing and vibration better.

As far as should you do it goes, here's some issues to think about. First, foam adds weight (about 2 lbs.cubic foot) and expense. It's also messy to work with. It likes to bubble over and when it 's cured it's a bear to remove. If the space is not open enough, it will tear the structure apart as it expands. The only way to get a completely filled space is to fill it before you close it up, but this requires the excess foam to be removed and the remaining foam to be shaped to be conformal with the designed shape of the compartment. This creates a lot of really nasty dust that is sharp and sticks to everything. The liquid components are a stinking toxic mess (once it cures they're inert with no smell to speak of).

The Kaholo is wood, so even if it's completely filled with water it will float. If you do get a puncture somehow, it's a lot easier to fix when you don't have to remove the foam. But honestly, I have yet to hear of anyone puncturing their Kaholo.

Just to be fair, here's some good things about foam. It will keep large quantities of water from entering a punctured hull, so the flotation characteristics will not change. It also makes a hollow structure stiffer and more resistant to compression. With proper design, it can even be used in place of stringers and bulkheads to stiffen a hollow structure, resulting in a net decrease in labor and materials cost (but this requires the structure to be specifically designed for this). With all the hollow spaces, it's a reasonably good insulator and sound deadener.

As far as your original question goes, It won't hurt anything to put the foam in. Your board will be a little heavier and cost more and you'll have a bunch more work to do, but at the end of the day you'll have a nice SUP. On the other hand, the Kaholo doesn't really need the foam in most circumstances. Unless you're going to be using it in really rough surf with lots of very sharp rocks, you'll be fine without it.

Have fun with your build,

Laszlo

Laszlo,

Thank you for the thorough response. The instructions make a lot of comments about ways to save weight. Is that just for carrying, or is there some reason to be lighter on the water?

I'm definitely having fun and gaining great respect for you guys who have done so many of these.

Chris

I would forget the foam. Foam will continue to expand for awwile and will mkost likey blow out the seams. Plus, any foam absorbs water so in addition to extra foam weight you will wind up with water weight. 

BTW - wood floats.  

Also - foam core boards (SUPs/surfboards) require breathers, so foaming the Kaholo won't eliminate the breather. 

The designer has spoken, no foam is necessary.

Regarding foam use in general, expanding foams will only blow out your seams if you misuse them. Putting foam into an uncovered compartment, then installing the cover a day or so after the foam is put in is totally safe for the seams.

Expanded polystyrene is meant for use as insulation and packing material. Mechanically it's not as strong as expanding polyurethane and it's more sensitive to solvents and fuels. It's not Coast Guard approved for use as buoyancy foam.

Laszlo

Thanks, guys!  I had a break from the shop for a few days (pneumonia) but will head over today. Unless he went happy with foam in my absence (he's retired and likes to tinker) I'll nix that plan. Last time we talked he was thinking about throwing in those pool noodles from the dollar store. 

Stick to the plans Ctwiggs.   That is what we pay the money for....these guys in the shop working all the senarios and coming up with the best options.

Pool Noddles??? :)  LOL.   You might want to get that board away from your friends house before he does something really odd to your board!!   You don't need any more floatation - all the air trapped in the board by the design is clearly all anyone else has needed.