Plywood deck thickness and fastenings

I am restoring a wooden Bristol Channel Cutter and would like to use plywood for the new deck. I have read everything I can get my hands on but can't seem to find an answer to two critical questions.

1. How thick should the plywood deck be. The deck frames are 2" sided and spaced 15" on center. The deck has a 5 1/2" arc in the 11 foot beam. I have seen a number of referance to high strength 1/2 marine plywood which would probably bend OK to the camber but seemed on the light side. I have thought about using two layers of 3/8" but am concened about laminating two layers as the reason I am replacing the whole deck is that water got in between layers on the old deck. The laminate was white cedar not plywood but it made the risks of "field" laminates quite clear.

2. In additon to epoxying the plywood to the frames what would be an appropriate size, type and spaicng for fasteners.

thanks

drew

 

You didn't say what thickness originally covered the deck but I'm guessing 7/8" solid wood? If so, double 3/8" would be completely adaquate. Epoxy between to adhere and to avoid any potential for rot. Generally you can go a bit thinner when going from solid planks to plywood. 1/2" seems a bit light for what is a pretty heavily built boat in that size range.

 

Generally, you can use the hull planking thickness as a guide to decking thickness. A workboat or an off shore cruiser may have decks as thick as the topside planking. Usually, you do not reduce the plywood thickness, because it's not as strong longitudinally, as the solid lumber it replaces. Since your previous deck was a softwood, you'd be okay with a 10% - 20% reduction in thickness if replaced with two layers of marine grade plywood.

A plywood deck with two layers of material is a very good approach. Of course stagger the seams well. I wouldn't epoxy the plywood to the deck beams, nor the sheer shelf. Bed and fasten, so you can remove it in the future if necessary. 2 layers of 1/4" is much stronger then a single layer of 1/2". So if 1/2" seems to get it done for you, then two layers of 1/4" will be the bee's knees.

If you epoxy the two layers together, you'll not have the inter-laminate rotting you had with the solid lumber deck. Use a notched trowel to apply the thickened goo and coat every inch of decking. Start at the center and work toward the edges in a spiral pattern with your fasteners, which should be spaced fairly closely. It's also very helpful if you lift the edges as you work, keeping them from touching the deck until you apply pressure with a fastener. This prevents air from being trapped between the layers.

One other important note is to always notch deck structures into the edges of the plywood sheets. Don't align the edge of a sheet with the end of a cabin or hatch opening (as examples), thinking you'll be saving some work. This makes the deck weaker and should be avoided. Ideally, the first layer should be oriented at a slight cant of 22 degrees to the centerline and laid in the fore and aft direction. The second layer is canted the other way and the reason is it provides additional longitudinal stiffness and cross grain strength to the deck.

 

Is this that cool old boat that was out by the gate at Scaranos down at the port of Albany? I took a bunch of photos of her when i worked there over the winter of 05/06.Id be interested to know the history.
Steve.

 

Plywood deck on Bristol Channel Pilot Cutter

Please don't use plywood if the rest of your hull is still built in the traditional way.
An epxoy sheathed plywood deck is a wonderful thing on the right boat but with an old traditional hull it is far too stiff.
Traditional wooden boat structures move a lot and putting a stiff deck on will put extra strain into the rest of your hull.
Sorry to put a damper on your idea but I work with a first class boat builder in Gloucester, England and he has restored several boats like yours and will only fit original style laid decks.
Good luck with your project.

Graham Westbrook
Naval Architect
Westbrook Marine Projects Ltd.

 

The change from a laid deck to a plywood one is, however, done all the time and I'd think additional stiffness would not negatively affect the hull. The deck is broken up by cockpit and house structures whose joints allow a great deal of give, nevermind the hull can certainly flex to a large extent under a diagonally stiff deck structure (in terms of twist).

 

Graham, one of the best things you can do for a traditional build is stiffen the "upper web" and insure it's watertight with plywood. In actuality, you really are just gaining a lot of cross grain rigidity, while the longitudinal stiffness remains very similar. I can sight you example after example of documented classic that has a plywood substrate under it's replaced laid decking. In fact, I as well as others have employed beaded plywood as a replacement for laid decking, with the beads facing inboard, simulating the laid decking from below, then placed a veneer on the plywood on the weather side to offer the laid decking experience to bare feet.

 

Thanks for all of the responses. All were very helpful. I am much more comfortable using two 3/8" layers. My plan had been to stagger them and offset the seams from the openings. The only unanswered question is what size, type and spacing of fasteners I should use as I apply the first and second layers?

Although I live in Albany, the boat was never at Scarano's yard.

Drew

 

PAR

Your use of beaded ply sounds wonderful. Do you have a specific product to recomend?

drew

 

Just thought I'd weigh in for a moment since I'm doing a somewhat different restoration but I'm just a few miles away from you at the SYC in Schenectady.

Although the deck on my 73 Silverton is attached to a fiberglass hull it's constructed of douglas fir that I bought from Boulter Plywood in Boston. 3 1/2 inch camber across 10 1/2' beam at it's widest point. I stayed true to the original design and used 1/2 inch douglas fir as the base, staggered the edges and used 5/8" for the upper lamination, 1 1/8" overall. In the original design the 1 1/8 inch deck was simply cut and screwed into a clamp that ran around the inside of the hull/deck flange. There were no beams supporting the span and the deck held up quite well for 30+ years. It did deteriorate where people screwed items into it (without sealing) but other than that it was in fairly good shape.

I decided to add two support beams to the foredeck as part of the restoration but still there are unsupported spans of deck ranging from 2' to 4' port to starboard. I'm amazed at how strong this deck is. I'm not an engineer but it seems to me that the varying grain orientation in the plywood and the camber of the deck provides the strength. I did a thorough job of epoxying the deck all 6 sides, multiple coats. The two plys were laminated together with epoxy and thououghly stapled with 1/4" crown staples 7/8" long to hold it all together while the epoxy cured. The staples were entombed in multiple coats of epoxy. Of course the deck is sheathed in 6oz cloth.

I'd believe that your two plys of 3/8 would be quite adequate since you have supporting structure 15" on center.

Regards,

MIA

 

Par, because of the camber over 11 feet, I was concerned about excess squeeze-out. Is this why the beaded ply? So, you orient the panels so that the beading is facing each other and 45° offset from each other and that gives you a diamond pattern of epoxy in there which will hold good but, to me might entertain voids. Do you thicken the epoxy? You can overcome the generally lower grade of ply (voids, knots, possibly not the strongest glue) in beaded stuff with the extra epoxy there? Thanks, Mark

 

Par how thick is the veneer, is this done under survey

 

Drewcathell26, fasteners are dependent on what you have in the rest of the structure. Use what you have already there, steel for steel, bronze for bronze, do not mix them.

The last bead board deck job was a 40' motoryacht. It's pilothouse and coach roofs needed replacement and they were 3/4" teak over 1.25" tongue and groove cedar (yea, I thought it was heavy too). The teak was worn out, in spots down to 3/8" and "washed out" from scrubbing. The cedar I reused on another project. The replacement was an inner layer of "bead board" available from a big box store, which was to be painted and had WBP adhesive, I don't remember the brand. It was available in 1/4", 3/8", 1/2" and 3/4". This (3/8") was bedded over the restored beams, with the beads facing down (to the interior of the cabin). Glued to this (with a 3/32" notched trowel, but depends on how rough the surface is) with thickened epoxy was 9 mm Meranti, with temporary screws into the beams. The holes and seams filled after the screws are removed and the surface sheathed with 2 layers of 8 ounce fabric. When cured, 3/8" teak (I consider this a minimum thickness for mechanically fastened veneer) was applied in a bed of 3M 5200 and fastened to the beams. The beaded ply just simulates the old tongue and groove the plywood replaces.

I'd like to say I invented this, but I admit to blatantly stealing it after reading about it many years ago. I don't remember where I read it, but I've seen it many times since, the most recent in Dave Geer's book ". . . Boat Strength".

The key to a watertight deck is the 'glass work and well bonded fasteners.

I'd never thought about the beads facing each other to let air escape Mark775, interesting idea. I usually just use a some wedges along the edges of the ply to keep it elevated until I mash it down and hold it with fasteners. If you're religious about notched trowel use, trapping air and ooze out aren't that big a deal. I typically use a 1/16" square notch trowel on parts that fit well to each other and taller, "V" notch trowels if I need account for some irregularities.