question regarding plywood selection and thickness

[ajangiuli]

Okay as i am pealing away the deck on my project I am starting to consider the manner in which i plan to put it back together. unfortunately there are certain sections of the deck that need a bit more attention then others and with that i decided to remove the entire deck from the transom to the cuddy doorway. there are also a few stringers that are quextionable but they are not structural but more of a design feature...okay blah blah blah all that said let me ask the question i came to ask.

the original deck of the boat was 1/2 inch plywood covered with what appears to be glass and gell coat. so when i look to replace this i was considering using 3/4 plywood for the base of the deck with a glass and non slip surface finish.

now looking at the boat i was considering a couple of options and would love some feed back as to there practicality.......

1. i was considering the 3/4 plywood running perpendicular to the hull. basic woodworking tells me this is opposite of the hull therefore making it stronger

2. would it be wiser to run 2 layers of 3/8 plywood rather then the one layer of 3/4 with overlapping seams. this also makes for a tighter and stronger finish.

3. type of plywood. I know i didnt take marine grade plywood out of the boat during the demolition process so do i need to put marine grade back? or can i apply a coat of epoxy to the underside then secure it to the stringers then glass? what i notice about any time of exterior lumber that i have used in the past they always seem to be soaked with moisture, would sealing it with glass only cause the same rot problem down the road????

thanks for the help
anthony

 

[drewpster]

Re: question regarding plywood selection and thickness

1) If you had structural problems with the hull then build it as strong as you can. If the hull is ok, and the wood is just rotten, stick to the original design. After all, it lasted for how long?

2) Refer to number 1- its your personal preference really. But consider the original design before you spend the extra dough.

3) Boats rot because of neglect far more often than from design problems. Marine grade will rot even if it is marine grade. I researched this subject both online and with local repair shops. (one has been in the bizz for 30 years) The consensus was to use good quality AC grade ply. It has good veneers, and good working surfaces. Seal it as best you can and take care of the boat. It will likely outlast you. My opinion is that true marine ply is best used in boats completely made of ply. Unless you are building one of those floating pianos, marine ply is unnecessary in my view. Structurally speaking, quality plywood is stronger than dimensional wood. As a fiberglass core it is at best a compromise. The only way to guarantee against rot is to use composite cores. Composites are extremely expensive and unnecessary as well. Unless of course you want to win the Americas Cup. Again before you over build consider how long the boat lasted before you came to own it. (mine was 32 yrs old before the cores came out, and I used it twice a week until the restore started)

 

[Lion hunter]

Re: question regarding plywood selection and thickness

1. i was considering the 3/4 plywood running perpendicular to the hull. basic woodworking tells me this is opposite of the hull therefore making it stronger

Doesn't really matter as about 1/2 the plys will run one way and the other 1/2 will run perpendicular.

2. would it be wiser to run 2 layers of 3/8 plywood rather then the one layer of 3/4 with overlapping seams. this also makes for a tighter and stronger finish.

Unless you have a way to put the 2 pieces of 3/8's under pressure and 400 degree heat the 3/4 will be stronger. Joining 2 3/8 may make a stronger seam but the rest of the deck will be weaker than 1 piece of 3/4.

3. type of plywood

Marine plywood is expensive because it uses a high grade (ususally b) face and back and tighter core gaps. Sanded Exterior plywood that is rated for underlayment is the best comprimise and most cost effective thing to use. It will usually be a B face and C back. All knot holes in the face should be plugged. The down fall of this grade is the gaps allowed in the cores and the exposed knot holes on the back. This doesn't effect strength but does allow for water intrusion. This is easily remedied with glassing on the face, sides and back. Plywood will not have the moisture problem that lumber does. The veneers are dryed before assembly. If there is to much moisture in the veneers it will "blow" from the steam build up during the gluing and pressing process. These voids from blows are detected by a machine when the panels are sent through the saws and are rejected. Although some get missed by the operator in the stacking process the thing to look for is if the panel has a big red ink spot on it a blow was detected, don't buy that piece.

 

[berry79]

Re: question regarding plywood selection and thickness

Check out marine-tech plywood from plum creek. Cheaper then your basic marine ply. Localy for me its about $38.00 for 3/8 and $44.00 for 1/2".
http://www.glen-l.com/wood-plywood/marine-tech.html

 

[Lion hunter]

Re: question regarding plywood selection and thickness

I'd bet if you compared the specs on his vs a BC underlayment you'll find that they are rather close (or the same). Like I said the biggest difference is the core gaps and the backs. And it doesn't matter if since you can't see the back and whether you use marine or not it all needs to be glassed. The matrine-tec guarantees soid core throughout where as a BC only gaurantees a solid core against the face. This is just protecttion against a high heel going through it since it is rated as underlayment. Many times a mill will make a product for years such as BC and figure out that with one change such as adding a solid center instead of C grade it meets the standard for marine grade. They just lay up the panel, have APA inpect it and create a new market with a higher cost panel. Also as much as I hate to shop there the HD has very strict standards when buying wood. Usually companies are allowed 5% defect per order. HD is much different. They buy so much quantity they control what the mill does. If they uncover a tarp and don't like the looks of the core gaps in the 1st load it all goes back to the mill. Spent many hours filling core gaps by hand and sanding to make loads acceptable.

 

[PiratePast40]

Re: question regarding plywood selection and thickness

I just don't understand why people spend so muct time arguing and stressing over a few dollars extra for marine ply for a 5 to 10 thousand dollar boat. If you're only using 3 or 4 sheets of ply, why in the world would you not spend the few extra dollars for marine rated ply with a lifetime guarantee?

 

[erikgreen]

Re: question regarding plywood selection and thickness

Because A) I have a $2000-$3000 dollar boat, not a 5k-10k boat and B) I hate getting taken for a ride by plywood companies... I'll use what I need, and not fall into paying more than I should to have someone else do my thinking for me.

See also: Anything labeled "marine"

Erik

 

[thrillhouse700]

Re: question regarding plywood selection and thickness

Yeah Im using the birch stuff that redfury is using. I am not worried about it at all. This boat will last for at least 10 years or longer if fiber glassed the iboat way. More than likely I will be dead in 10 years time anyways.

 

[Lion hunter]

Re: question regarding plywood selection and thickness

Alot of times it is the availabilty of marine ply. None of the lumber yards in my area sell it. You pay a special order fee or drive 60 miles to the nearest big town to get it. Also I don't think anyone is arguing or stressing over it. Just trying to provide some information so people can make an informed purchase.

 

[109jb]

Re: question regarding plywood selection and thickness

I used CDX and ACX for my sea ray restore, but if I had to do it over again I'd use pressure treated ply from the local lumber yard. I also would ony fiberglass to join it to the hull using 1708 and use truck bedliner on top. Just my opinion.

 

[robert graham]

Re: question regarding plywood selection and thickness

I would use a 3/4" exterior plywood, then finish it with a penetrating sealer that would give it even better water resistance, then just slap another coat of sealer on every year or so and the stuff will last forever, kind of like a well-maintained deck on your house. I think fiberglass/gel-coating definitely seals in the moisture and leads to rot. If you don't like looking at the natural wood color then lay down a black rubber non-slip mat or something similar. just my opinion
Good Luck!

 

[robert graham]

Re: question regarding plywood selection and thickness

you might also consider adding some stainless steel flush mount hinges on all or certain parts of the deck for future access, inspection, repairs, storage,etc. run a heavy bead of marine silicone sealer on the sub-flooring/stringers, lay the plywood down and screw it down with stainless steel screws. easy fix! Good Luck!

 

[Mark42]

Re: question regarding plywood selection and thickness

I can't see spending the money on marine grade ply when the boat probably wasn't even built with it in the first place. Most pleasure boats are not built with marine grade ply. Using plain old CDX and save the money for buying resin and glass. My choice would be to use pressure treated, like the APA suggests. Remember, Marine grade ply rots just as fast as cdx, but pressure treated will last many times longer.

 

[ajangiuli]

Re: question regarding plywood selection and thickness

WOW.....lol...thanks for all the good input and the experience.....all in all i decided to use 3/4 fir ac grade plywood for the deck. plan on installing in this manner after the installation of the 2 non-structural stringers that run the length of the deck.
1. re-glass any and all areas on the underside of the deck and stringers

2. dry fit all pieces of 3/4 plywood, pre-drill countersinks for fasteners and prep.

3. seal underside and edging of 3/4 plywood deck with penetrating sealer, when dry apply beads of marine grade silicon to stringers and re-fit plywood and secure

4. fill fasteners, sand, fiberglass seams and edges where the deck and hull meet, then complete fiber glassing to entire deck.

5. apply finish (not decided yet but i kind of like the bed liner idea or non stick surface)

now my question would be is it wise to fiber glass the underside of the deck or will a penetrating sealer be sufficient?

oh and the reason i chose to go with the ac grade 3/4 fir plywood.....

A) it is a spotless smooth finish, much easier to work with in my book
B) generally flat and without the imperfections that come with lesser grade plywood
C) most importantly was the cost........FREE!!!!! i happen to have 5 sheets left over from a job i am doing and it just seems a sin to use it for shelves in the garage or building a doghouse.....lol.....

thanks again for all the great input everyone..im sure i will be coming back often to re-read and ask many more questions

A.J.

 

[Mark42]

Re: question regarding plywood selection and thickness

Don't apply a water sealer. Glass both sides, or at a minimum, roll on a coat of resin on the bottom side.

 

[ajangiuli]

Re: question regarding plywood selection and thickness

Don't apply a water sealer. Glass both sides, or at a minimum, roll on a coat of resin on the bottom side.

thanks mark thats exactly what i was curious about.....will resin alone be sufficent? it certainly becomes more cost effective if there is no glass mat but it seems like a small cost in retrospect to the finish product.

thanks again