Deck ply;   you will find that 6 mm thickness of the proper stuff,  i.e. marine ply  –  which I appreciate you can’t afford  –  is far heavier than the boat normally needs,  and would make the boat unduly heavy.    The minimum is 4 mm,  but for heavy duty use (including training,  and cruising)  many people prefer 5 mm.     6 mm would be somewhat over the top.

Since you are having to use an intrinsically less strong type of plywood,  you may in fact need the extra thickness,  but that is something for you to assess in the light of this information.

A few other points about exterior grade non-marine ply:

I used it once for a very temporary and experimental rudder blade on the big boat,   when I wanted to try out a new profile before making a good one with the proper materials.    I was horrified to find that top veneer of the brand new ply was starting to delaminate even in my workshop,  before being exposed to any weather,  as soon as I started to cut it,  and I also found breaks in the interior laminations.    So if you have to use exterior grade,  treat it with caution,  be very selective in whom you buy it from,  and also be very selective in checking the individual sheets before purchase.

<span style=”line-height: 1.5em;”>When I made my experimental rudder blade I coated the finished blade with epoxy,  and then three layers of woven glass mat.   The result was that it was a little too good  –  so it stayed in service longer than I originally intended;    but it was nonetheless still not good enough,  and snapped off in moderate swell off the Farne Islands,  with around 20 miles still to go to make port at Amble.   So be warned …</span>

I think that if I were doing the job in non -marine ply I would at the very least coat both sides of the ply in epoxy,  and all the edges.     Some of that is best done during construction,  rather than beforehand;  mark out where the joints are going to go,  and leave those uncoated,  so that when you assemble and glue up you can do so with wet epoxy direct onto bare wood.    If you coat everything first,  you will find considerable difficulty in getting neat joints because of the thickness of cured epoxy getting in the way of the joint you are trying to make.

Finally,  given that you are thinking of a greater thickness than you would need for the proper material,  and the possibly greater expenditure on epoxy,  it might be worth doing two costings before you start;   one using 5 mm or 6 mm exterior ply,  and one using 4 mm marine ply.   Make sure that you include <b>all </b>the materials in the costing,  not just the wood,  and the marine ply doesn’t have to be the very swish stripey sapele;    basic marine ply may well be a less expensive choice and should be perfectly adequate.   Only when you have the total costings for both routes can you decide whether the cheese-paring approach is justified;   if the saving is going to be very modest and the end result is likely to be much inferior it may well be a false economy.

That is before you even start to consider the value of your time …

Hope this helps,

Oliver

A handful of other details:

It seems that you are intending to remove the rubbing strakes,  and replace them once the work on the decks is finished.   Good.   In that case,  cut the outside edge of your new decks to slightly proud of the hull,  and plane / surform / sand it flush once they are fitted and glued down.   Then of course coat the newly exposed edges with epoxy,  preferably doing this as you glue the rubbing strakes back on.

Epoxy the underside of the ply as well as the top,  particularly if you are using non-marine ply.    If you use genuine marine ply you would get away with not epoxying the underside,  although best practice for the best long term job would still be to epoxy both sides.

5 coats of varnish is rather pushing your luck;  that is just about the absolute minimum.   I would normally expect to be just starting to get an acceptable finish,  and a coating that will last reasonably well,  at six coats,  and I normally aim for about nine.    That is not of course nine times the thickness of a single coat,  because I sand down alternate coats moderately heavily;    it is possibly – and I am guessing  –  about six times the thickness of a single coat once all the sanding is taken into account.

Oliver

If you are joining pieces of ply for the side decks a scarf joint (a.k.a. scarph joint) is neater than just a butt joint.    I appreciate your comment that you are no joiner,  but I wonder whether you have a contact who could make that joint for you?    It is not technically difficult,  but it does require a modicum of skill,  and the use of a workbench (or other good sturdy flat surface) as well as a plane and other basic hand tools.

Oliver

I don’t know whether they are still doing them,  but Fyne Boats used to sell kits of parts for the GP14,  and have been known in the past to supply individual panels on request.    http://www.fyneboatkits.co.uk

Local wood joiners,  or even boatbuilders (if any in your area) for the limited assistance you suggest would also be worth considering.

Where are you based?

Oliver

Over to some of our Scottish members to now pick this up;    do you know of any local firms who may be able to help?

Oliver

Re Oliver’s comment on scarph joint, you need a solid bench (or table) and a sharp plane.  The operation is not too difficult if you line up one piece of ply with the table edge (or a straight-edged sacrificial sheet). good side up.  Lay the next piece on top, wrong side up  and set back ‘x’ times the ply thickness, where ‘x’ is the ratio of the scarph. (I see Oliver suggests 12 in another mail, but anything over 6 is probably OK).  Lay a third piece of “spare” ply with a straight edge on top, again set back by ‘x’ times the thickness.  Rest the plane across the three edges and plane gently until the bottom sheet meets the table surface and there is no step between the two work pieces.  When gluing, ensure the pieces cannot slip before tightening any clamps.

Re plywood supplies, I have a friend in Ayr who is having the same problem and resigned to ordering from southern England.  You might co-operate with an order. Contact me on 01292 570520 before Wed 18th if interested.

Good luck with the project

Norman

I have just come back to this,  and checked a bit more on Robbins’ prices.   As you rightly say,  a 2500 x 1220 sheet of 4 mm (3-ply) bog standard Elite marine ply is £32.24 plus VAT.   The equivalent in 5 mm (5-ply) is £46.90 plus VAT.   The difference is not large,  and pales into insignificance by comparison with the total cost of the project,  and by comparison with teh value of your time;   the important thing is to make the right decision.

It would be well worth considering carefully which of the two to go for;  if your the expected use of the boat the 5 mm is preferable it would then be a pity to spoil the ship for a ha’p’worth of tar.   The class rules specify 4 mm minimum,  but a great many boats are built with 5 mm decks.    I think that the “right” thickness depends on how you expect the boat to be used.

I understand  –  very much at secondhand!)  –  that top racers are likely to prefer 4mm because it is lighter.    However their boats are lovingly caressed,  except when  driving hard during a race,  and in terms of ensuring that the boats never suffer abuse they probably have an easy life.

For a boat used for cruising,  or training,  there may well be more risk of higher localised loads on the deck (e.g. someone standing on it,  rather than sitting,  or carelessly plonking something heavy down on the deck).   For a boat that may suffer that sot of treatment from time to time,  even if only occasionally,  it is probably worth going for 5 mm.    When I had a new series 2 boat professionally built for me in 2005/6 specifically as a cruising boat the builder (Tim Harper) opted for 5 mm as standard,  and the boat still came out marginally under weight,  so carries corrector weights;   and I have absolutely no worries about her strength.

Oliver