New deck for fishing boat

[zak]

Hi,

I have a complete new cockpit deck to lay for a friend of a friend on a Mitchell 31 fishing boat. I've some experience with wooden boat repairs but have not done a great deal with plywood , I wondered if anyone could share their experience and expertise please?

The 2x3 supporting structure looks sound and I thought ¾ inch marine ply would be the ticket sheathed with woven roving and epoxy? As the deck will be sheathed is it OK to butt joint the ply over beams or should it be scarfed? Any suggestions on how to approach access hatches for sea cocks etc and the engine box please, eg keeping them water tight and construction. Whats the way forward for the scuppers, maybe an epoxy mush fillet?

sorry for the barrage of questions!!

rgds

 

[fishermantwo]

Hi,

I have a complete new cockpit deck to lay for a friend of a friend on a Mitchell 31 fishing boat. I've some experience with wooden boat repairs but have not done a great deal with plywood , I wondered if anyone could share their experience and expertise please?

The 2x3 supporting structure looks sound and I thought ¾ inch marine ply would be the ticket sheathed with woven roving and epoxy? As the deck will be sheathed is it OK to butt joint the ply over beams or should it be scarfed? Any suggestions on how to approach access hatches for sea cocks etc and the engine box please, eg keeping them water tight and construction. Whats the way forward for the scuppers, maybe an epoxy mush fillet?

sorry for the barrage of questions!!

rgds

For my fishing vessel the best deck I found was treated pine decking laid with the grooves up. Then two layers of CSM and fibreglass resin. Finished off with oil based paving paint with builders sand sprinkled on while still wet.

Avoid ply if you can. Hard use will cause leaks and water migrating up the edge of the ply. With the decking its laid with butt joints and the resin will soak into the gaps. Much firmer to walk on compared to the drumming you get with ply and fibreglass.

 

[Gelmaster]

Marine ply is fine to use, its just a waterproof binding glue used to laminate the plys together!
not sure though if 18 mm will be too stiff to flex over the bow of the deck.
Might be better off using 12mm ply.
No need to scarf...but i would be tempted to GRP bond the joints from underneath as well.
How much bonding area have you got where the ply runs off against the toerails?
That is the area where you are likely to get future leaks!

Before you commence...decide how you want the hatches.
You can buy pre-made hatches which offer a good seal, either with or without glass or polycarbonate! can be a tad pricey though.

or you can DIY by cutting your apperture, then glue on approx 40mm flange of ply giving an upstand.
GRP over this upstand , then make your hatch lid also out of ply,again covered in GRP. Make sure you have a pencil sized radius on the corners of the ply though before glassing. Fibreglass will not form properly around sharp corners!!
Also, i strongly recommend sanding over the plywood with an 80 grit paper to give it a much better bond for the GRP.
Sometimes, the ply is left with a shiny, kind of waxy finish, which i have known to break bond with GRP.
Use silicone sealant on the hinge bolts when pushed home!
If you want a totally flat hatch lid, simply taylor your upstand to suit!
PS Polyester resin is just as good as epoxy for this project, cheaper too!

 

[pcatterall]

I made templates with 1/4 inch ply and took them home. I used exterior ply which is as good as marine but may have the odd small void. make sure you prime the ply all round with a thinned epoxy coat first especially the edges. Yes, fit the upstands for hatches then glass all over.

 

[zak]

Thanks for the replies.

Fishermantwo, sounds like a seriously hard wearing deck on your fishing vessel! The owner wants a ply deck though as thats what these boats usually have.

Gelmaster - the deck is virtually flat as its in the cockpit so I can get away with fairly heavy ply. The owner wanted 1 inch thick!! It all sits on a 2 x 3 subframe and a grp rail at the edges where it meets the hull, so I need a bit of a radius at that joint.
The access hatches are just for sea cocks etc, anyone know a good source of them?

 

[Gelmaster]

Thanks for the replies.

Fishermantwo, sounds like a seriously hard wearing deck on your fishing vessel! The owner wants a ply deck though as thats what these boats usually have.

Gelmaster - the deck is virtually flat as its in the cockpit so I can get away with fairly heavy ply. The owner wanted 1 inch thick!! It all sits on a 2 x 3 subframe and a grp rail at the edges where it meets the hull, so I need a bit of a radius at that joint.
The access hatches are just for sea cocks etc, anyone know a good source of them?

I wish i could see exactly what you are up to, as i have been a boatbuilder for decades. If it is the cockpit floor...there are self drainage issues around hatches to contend with.
I was visualising the top deck and hatches.

 

[zak]

Heres a couple of pics of the boat.

 

[Gelmaster]

Heres a couple of pics of the boat.

Aaah ok then!
There should already be a 1 degree water flow fall built into the 3 by 2 joists towards the drain off at the stern against the natural waterline!
There should also be a drain off outlet/s on the transom, (Usually a hole/s with a flap) !!
If not, you will have to put these drain off points in, as long as they are above the waterline!

You will need a 75mm (3") threshold (upstand) by the cabin door.
Laying the floor....18mm marine ply and 2 sheets of 2 ounce(600gm)using polyester resin.
Finish off with a 50/50 mix of white gelcoat and resin with 10% liquid wax solution. Also with added sand!

One more point....Make sure to key back at least 4" up into that blue flow-coat all the way around, for when the GRP is glassed to it.
I normally use a grinder for that purpose!!

Now the tricky bit........
the engine & inspection hatch
2 ways to do it.....
either build in approx an 8" upstand, or more around the apperture
of the hatch,creating a box, allowing enough room if poss for engine removal "sadly, not always possible".

Or the much trickier method, which allows for a flat, level cockpit floor.........

You will have to build in a drainage channel, approx 50mm deep around the hatch, which runs the water off to stern.
Build this out of timber, and simply glass over with the floor!
The hatch/Lid edges can be fashioned to drop into this channel, but allow at least a 10mm gap around the edges of the lid for the water to drain into this channel!!

I hope this info is of use...
If i can help out with further advice, ask away!!!!

 

[oldsaltoz]

You can use exterior builders ply, same waterproof glue but more knots in the ply, but the thickness you plan to use will eliminate any problems.

The main reason most ply decks in fishing boats fail is moisture from below the ply not above.

The ply MUST be protected on BOTH sides to avoid this problem.

Standard poly resin will not bond to timber long term even when heavily scored with a grinder, this practice may also weaken the ply if not oversized to start with.

I use an epoxy after all cutting and shaping has been completed and the ply dry fitted to ensure at least 2 mm clearance all round.

Then sand with an 40 grit disk and clean with Acetone, mix epoxy resin and hardener then add 25% by volume of Methylated Spirits, this will thin the epoxy and help penetration, note the edges will take a lot more resin due to the exposed end grain, keep coating wet on tacky till it forms a good surface layer. Let this cure, and wash from top first working down with a running hose and plastic scoch brite kitchen scourer till beads no longer form on the surface, this removes the residue left by by the curing process. Sand with a 250 grit to remove 30% of the high gloss finish.

Now apply more epoxy resin and hardener and a layer of 300 gram rovings over both sides and all edges of the ply, let this this cure then wash and sand as above.

Drop the floor into position after marking the stringers on the sides and ends of the hull. Mark the location of fixings to secure the ply to the stringers and drill a hole oversize for the screws, fill each hole with a mix or epoxy resin and Micro Fibres and let this cure then re drill the the size of screw shank; This prevents water getting into the stringers. secure the deck.

Now cut some strips of 300 gram bi directional cloth, enough to go right around the deck to hull joint trice, the first strip should be 75 mm wide and the second 100 mm wide.

Mix more epoxy resin and Micro fibres and spread this between the hull and heck joint using a small round object to form a neat rounded gusset. When this goes tacky apply the narrow layer of glass followed by the wider strip, roll out as much resin as you can, too much resin will weaken the job, a ratio of 1:1 is best, that's one part resin to one part cloth, not 3:1 as with poly resins.

If you want an invisible seam, coat the cured area with resin and Micro Balloons, sand smooth and you are ready to apply some paint.

Note, epoxy MUST be protected from UV rays, so try not to miss any exposed resin.

Hope this helps.

Good luck.