newbie questions

[jclays]

Got some questions for you guys. Hope you dont mind my asking because i'll have a bunch as I go on.
1. Ive got my questions answered regarding veneering and exterior surface. Project looks like i'll be able to remove the whole panel and replace it. Hunted down the builders of the boat and got all the info I needed. Thanks to all that gave their input.
2. I'll be building a tender/errand runner to use in the harbor/marina. First boat build for me. Reviewing plans from a few sources. One plan from Old Warf Dory calls for underlayment plywood. What is underlayment plywood?? I have access to Fir marine plywood. I'll have lots more questions as I go thru plans and start building.
Thanks
Jim

[Stop Water]

Got some questions for you guys. Hope you dont mind my asking because i'll have a bunch as I go on.

2. I'll be building a tender/errand runner to use in the harbor/marina. First boat build for me. Reviewing plans from a few sources. One plan from Old Warf Dory calls for underlayment plywood. What is underlayment plywood?? I have access to Fir marine plywood. I'll have lots more questions as I go thru plans and start building.
Thanks
Jim
underlayment has the same water proof glue as marine ply, the big difference is that underlayment has voids and marine should not have any voids. i use underlayment on decks and bulkheads that are going to be fiberglassed on both sides, if going to glass it you should be fine, but if you don't glass i'd use marine ply. underlayment can save alot of money on big boat but if it's just a few sheets you might just get marine ply, its nicer to work with. hope this helps.

[Cuyahoga Chuck]

The most common underlayment is the luan that all the bigbox stores sell. You don't want any of that stuff. It will require glass/epoxy on both sides which translates into increase cost and excessive weight. There is supposed to be some properly made luan but I have never been able to find any. Whatever you use needs to contain 3 approximately equal verneers, waterproof glue and an absolute minimum of voids in the center layer.
There are better brands of underlayment but most of the stuff I have seen came from upscale floor covering retailers. It ain't cheap. Most was in excess of $40 a sheet
When I built the pram in my avatar I got some "Multiply" made at that time by Weyerhauser. Nice thick veneers, waterproof glue, only an occasional quarter-size void and as light as okoume. I backlighted each sheet, marked each void and filled it with epoxy. The downside of Multiply is it only comes in half sheets so a lot of scarfing was involved. Later I found that whatever type of wood was involved did not suffer microbe-contaminated water gladly. Any place the epoxy coat was breached turned black in a hurry. I was able to head off trouble because I kept the boats in varnish and could head off any rot. No real harm but it looks bad.
I built a Cheap Canoe with Mulitply 8 years ago and the bottom got so covered with black scars I had to cover it with 6 ounce glass. A recent run down a local river had me hitting bottom on about 15 rapids. Plenty of abrasions but no more worries.
Capsual judgement is I'm going to pay the freight and use BS1088 okoume from now on.
Fir will work but it will "check". The grain will open up in a gazillion little cracks over time and ruin whatever finish is applied. The only salvation for that is a layer of lightweight glass. If weight is a consideration for you the weight difference between okoume and fir will be considerable. Fir ply is 1/3 heavier per sheet.
Sorry I couldn't simplify your choices. The plywood world is a complicated place.