Composite or marine grade ply wood?

[wnybear]

Trying to post this for a second time let's see if this works. I am getting ready to start my rebuild on my Grandfather's old 70's rinker tri hull. I was wondering if anyone here had ever tried Coosa composite board for their transom or decking? The reviews I have found seem biased(mostly from the manufacturer or retailers) and was wondering want actual people thought. Does it still need glassed over completely or just where it meets the sides of the boat and seems? any help would be GREATLY appreciated!!!!

 

[SkiDad]

Re: Composite or marine grade ply wood?

i would stick with marine plywood, it's proven so why mess with it - it's about 70-80 bucks a sheet so not too bad.

 

[rallyart]

Re: Composite or marine grade ply wood?

Composite (ie. reinforced plastic) is heavier and does not have the structural strength of plywood (ie. a wood laminate composite). The Coosa composite is foam with reinforcing and is lighter than plywood but does not have its strength. I would prefer a marine grade plywood that was completely covered with an epoxy coat. You can find really good specialty plywood that has a lifetime guarantee against rot without even being coated. It is also laid up to provide the most structural strength.
You can use a board like the Coosa product but you need to rely on the external fiberglass to give it real strength. Many boats do that. I prefer the advantages of wood. (In my alpine skis too)

 

[crabby captain john]

Re: Composite or marine grade ply wood?

Many builders have stopped using wood completely. Many use Coosa in transoms-- have seen it on twin rigging too. I did not know it was available to the public. If builders are using it instead of wood it would be my choice too

 

[Bamaman1]

Re: Composite or marine grade ply wood?

I too say that there's nothing better than regular marine grade plywood. It's not always easy to find, but if you look close you may have a plywood dealership in a middle or large size city.

If new boats are being made with Coosa, they're engineered to have the stuff in them. It's doubtful it'll be cheaper than plywood.

 

[ajax5240]

Re: Composite or marine grade ply wood?

May be an ignorant question, and I'm sure I could google it. But, what is the difference between marine grade and good ol lowes ply wood? I coat both sides in fiberglass resin anyway, so why not use the stuff that is $20/sheet?

Most of my boats I have for a year or two and flip for an upgrade, not to sound like a hack, but I think myself and probably the next two owners won't have a problem with the floors I install. If it was a family heirloom all wood Chris craft or some other fine collectors boat, no doubt I would use the best. But for a 1984 Bayliner Capri Fishing boat. The regular stuff has worked just fine, and my floor is solid as a rock!

Is the marine stuff pressure treated?

 

[pckeen]

Re: Composite or marine grade ply wood?

Hi Ajax,

There are a couple of key differences between marine grade and 'ordinary' ply wood. First, marine grade plywood has more plys of higher quality wood, than 'regular' construction plywood. This makes it both stronger and more stable than 'regular' plywood. Marine ply is suppposed to have fewer or no voids in it, making it structurally more sound. Second, marine plywood is glued together with waterproof glues, so it will stand up much better than when it wet. Exterior grade 'regular' plywood will also use waterproof glue however.

Key thing to know - avoid pressure treated plywood, as although it will stand up very well against water, it contains chemicals that can corrode aluminum and other materials.

There is nothing wrong with using exterior grade plywood. A number of restorers will use it to save money. Many restorers on this site will use Arauco-ply,, which is a little more expensive than cheap exterior grade ($30-35 a sheet), but is of higher quality, with more plys. Marine is the most expensive, and will run about $65+ from Menards.

 

[ajax5240]

Re: Composite or marine grade ply wood?

Thanks for the explanation Peter! I appreciate it!

 

[pckeen]

Re: Composite or marine grade ply wood?

No worries - just one further comment. I agree with mitzsyman's comment about Marine Plywood above but I don't know enough about Coosa to reccomend for or against it. However, assuming that you will going with some sort of plywood....

If this is an heirloom project - a boat you plan to leave to your kids, you would be best off using Marine. It is the best there is. If $$$ is an issue, however, and the project won't get done at that price....then Marine is not an option, so use the exterior grade or Arauco.

Best of luck!

 

[ajax5240]

Re: Composite or marine grade ply wood?

Just as long as you don't use OSB! Haha

 

[metriccrescentwrench]

Re: Composite or marine grade ply wood?

If $$$ is an issue, however, and the project won't get done at that price....then Marine is not an option, so use the exterior grade or Arauco.

I wouldn't be comfortable using any substitute material in the transom. The boat was designed with marine grade plywood, and that's what I'd use. More than likely the transom will just take one sheet of 3/4 plywood, the $60 or $70 for that isn't going to make or break any rebuild.

Now, with that being said, IMO exterior grade plywood should be fine for the deck.

 

[matt167]

Re: Composite or marine grade ply wood?

Marine grade is also sold as A/A grade exterior at some places. Keep that in mind when considering the costs. Another thing is, common exterior grades are scant ( 1/32" under nominal ) thicknesses because they lack 1 ply and all marine grades are at Nominal thickness. That said Plenty of boats came right from the factory with no better than ABX ply for wooden structure.. The only time plies and voids would really become a safety issue is when used for a boat hull itself.

 

[she won]

Re: Composite or marine grade ply wood?

Biggest differences in plywood--

Good old Lowe's plywood( or 84 lumber's- because i work there) is called a CDX grade. It is an interior plywood with exterior glues. The "C" and "D" refer to the grades of the plies. BCX or ACX are a good exterior plywood with exterior glues. Marine (as stated in earlier posts) grade is either A/A or A/B also refering to the grade of the plies. PRESSURE TREATED PLYWOOD IS NOT TRULY PRESSURE TREATED! It is a CDX grade that has been dipped in treatment, not using pressure to get treatment into cores.

Spend the money for bcx, acx, or marine grade

 

[she won]

Re: Composite or marine grade ply wood?

P.S. wnybear-
I will do some looking into coosa from my vendors for more info, and post what i find

 

[wnybear]

Re: Composite or marine grade ply wood?

Thanks all for the replies. The main reason I was interested was strength vs weight. Coosa is said to have as much strength with 30-45% less weight. It would also be roughly 5 times the cost but worth it if their rot resistance is what they say. Thanks again for all the information, I knew I came to the right place.

 

[pckeen]

Re: Composite or marine grade ply wood?

The weight difference will be trivial when compared with the boat/motor/people/gas/equipment/anchor etc. in the boat.

 

[wnybear]

Re: Composite or marine grade ply wood?

The weight difference will be trivial when compared with the boat/motor/people/gas/equipment/anchor etc. in the boat.

I have to disagree there. 3/4 marine ply weighs 75 lbs a sheet, put a total of 3 on the boat and its 225 lbs. Coosa being 30% lighter brings that down about 70 lbs which would give me more leeway for excess gear or person weight.

 

[metriccrescentwrench]

Re: Composite or marine grade ply wood?

I have to disagree there. 3/4 marine ply weighs 75 lbs a sheet, put a total of 3 on the boat and its 225 lbs. ....

Just an educated guess based on the boats I've rebuilt, but I'd estimate the total weight of a plywood transom core at around 50 to 60 pounds total.

And decking isn't going to require 3/4" plywood, 1/2" at the most. Your weight estimate is very high.

 

[ondarvr]

Re: Composite or marine grade ply wood?

Most boat builders use treated KD plywood, it's not really rated as Marine plywood though. They use it because it's low in cost and easy to work with, plus rather rot resistant. Coosa and other similar products aren't used more often due to cost, it's as simple as that.

Depending on the "other than wood" product being used it be may lighter or heavier than wood in the final product. While it is true that a sheet of plywood is heavier than a sheet of Coosa board, the plywood is about as strong as the structure needs to be without a skin of glass on each side, it really only needs a few coats of epoxy to be waterproof. A typical foam board like Coosa needs to have a layer of glass on each side to be strong and durable enough to survive. Some kinds come with skins and others are constructed with different materials that may not need a skin, but this adds cost and weight.



What this means is that for a floor you may need about an 1/8th inch of glass to protect the Coosa board, 1/8th inch of glass weighs about 1 lb per square foot, so you are adding about 32 lbs of glass for the top side of each 4x8 sheet and maybe a little less for the bottom. So the weight difference isn?t as great as first thought, you can pay more and get lighter and stronger ?other than wood? boards?but, the cost goes up. The plywood only needs to be waterproofed to hold up, But thin glass skins are typically used because epoxy is pricy and mixing epoxy and polyester on the same boat can be tricky.

If the cost is worth the no rot factor then Coosa and similar products are great, for production boats built to certain price points the cost of these products would kill the sales, people don?t or won?t normally pay extra for this type of construction.

PS I don't use wood in anything boat related. Although I have used the PT marine plywood in the past.

 

[pckeen]

Re: Composite or marine grade ply wood?

I have to disagree there. 3/4 marine ply weighs 75 lbs a sheet, put a total of 3 on the boat and its 225 lbs. Coosa being 30% lighter brings that down about 70 lbs which would give me more leeway for excess gear or person weight.

I get that Coosa will be lighter - but really, 70 lbs weight is trivial when compared to the weight of a 70's rinker tri-hull.