posted 01-09-2005 10:37 AM ET (US)        

Question(s): Should I replace with 3/4 plywood as in the original or solid Mahogany? Why would BW use plywood instead of solid in the orginal? Is this a weight or cost issue?

Thanks for any insight!

posted 01-09-2005 11:21 AM ET (US)         

I am facing somewhat of the same issue. The seats in my 1970 13' Sport are shot and I want to replace them. I am going to use marine grade mahogany plywood. This is actually much stronger than mahogany planks and I am sure that is why BW used it. Probably cheaper too.

posted 01-09-2005 01:44 PM ET (US)            

I would certainly rely on high quality marine ply however, be certain to finish properly esp edge grain. The rule of thumb for recoating bright work in marine environment is when componets lose 50% of gloss - scuff sand, do a few coats w/ marine spar varnish

posted 01-10-2005 07:38 AM ET (US)            

The part number is 12-118-201 on BW drawing number 62011800 taken from the 'jkirz' plans on this website (16 foot).

I've been consulting with John Perry who recently announced on this website that he can make BW wooden replacement parts. He says he would make the seat support out of solid mahogany because he doesn't have a good source for true marine plywood.

The impression I get from your comments is that there is no question but that I should go with marine plywood in this case.

Thanks again for your input.

JS

posted 01-10-2005 12:05 PM ET (US)            

Yes, you should use plywood for your Nauset "Church Pew" seat support.

Richard,

No! Plywood is NOT the appropriate material for a thwart seat. It will flex far too much and break.

Folks, let's look at what these types of material are, and are not, good at:

Solid wood will always be stiffer along its length than an identically sized piece of plywood. All of it fibers are running in the same direction whereas plywood, being a lamination of wood veneers that alternate direction, will have only about half of it wood fibers running in one direction.

"So what" you ask? Wood fiber has tremendous tensile strength. But put a bunch of wood fibers together and you will find they are relatively easy to separate from one another.

This is why solid wood is such a poor choice for a component like the under gunwale rod racks found on many Boston Whalers. Those are complex shapes with thin wood that crosses both directions. If they are made of solid wood, there will certainly be a point where the grain of the wood is only an inch or so long that that is where it WILL crack and fall off.

By using plywood, there is reinforcing grain in both directions and the part is more likely to stay together.

In the case of a thwart seat in, say, a 1970 Sport 13, the stress applied to the seat is a bending stress. The thwart seat is a plank of 5/4 mahogany and it has to span from one side of the hull to the other and carry (potentially) a lot of weight. The seat is essentially a beam to hold up your butt.

If you use plywood in that application, only about half of the wood fibers are running across the with of the boat and contribute any stiffness. Furthermore, the fibers that do run the length of the seat are subject to greater tensile and compressive forces.

I would also like to point out that (almost all) plywood has a "grain" to it as well. There are typically an odd number of veneers in the composition of a piece of plywood and the face veneers are not the same as the core veneers.

In practice, a piece of plywood will be significantly stiffer along the length of its face veneers that across it. This difference of greater than you might think and can be tested very easily.

To do so take a square of plywood, maybe a half sheet (4' x 4') and put it on a pair of saw horses. Apply a load to it (a bucket of water, small outboard motor, child, etc.) and measure the deflection. Now rotate the plywood 90 degrees and repeat. There will be quite a difference.

To digress even further, Boston Whaler used to test the stiffness of each and every thwart seat they produced for their boats. They had a jig that would apply a load and they would make sure the piece of mahogany did not deflect to much or break. "Soft" thwarts seats were rejected.

posted 01-10-2005 06:17 PM ET (US)            

The three boomerang shaped uprights on the the Eastport transom seat that I added to the Nauset were made from ply. all other horizontal slats and seat back were from 5/4 solid mahogany ply. given the seat design solid wood on the boomerang supports would have split under load as the three brackets do not support or carry load from the cockpit sole as a pilot seat does.

posted 01-10-2005 08:10 PM ET (US)         

Thanks for the input. Tom, I should have waited for your response first. This is (one of) your area of expertise.

posted 01-11-2005 10:05 PM ET (US)            

Therefore, I am curious why you would recommend Gorilla over epoxy and, in fact, have little direct experience with Gorilla. Pls advise, I am interested.

In any case, the cool thing about wood is there are almost no limits as to what can be done or innovated with high quality wood, fastners and adhesives.

posted 01-12-2005 12:07 PM ET (US)            

SpeedBump states that the pilot seat supports weren't always made with plywood and that solid mahogany was sometimes used. Can anyone confirm this? The orignal drawing for this part indicates "rib grain, 3/4 plywood mahogany, Royal Marine" and is dated 6/2/61, the year the Nauset first went into production.

Thanks,

posted 01-12-2005 07:23 PM ET (US)            

BW records tell me the hull was shipped bare from the factory to the dealer and the dealer installed the consol, forward hatch cover and pilot seat to the customer specifications. Keep in mind the boat could have been destined to be a sakonnet, eastport or nauset given the hull was shipped bare.

Pete- You are correct about the west system , it is an excellent product but two part and must be mixed with care even using their dispenser system. It is also significantly more expensive, so for a single piece or small glue up fresh poly seems to make sence.