Plywood or Douglas Fir supports?

I'm rebuilding the cockpit sole and the original support framing was 2x4s. I'm considering laminating some marine plywood to make the supports which were 8' 2x4s. I was planning on using clear Douglas Fir but for about the same price I can make from 3/4" MDO. Of course it would all be epoxied. Would there be any disadvantages to using the MDO for making the 2x4s?

 

I think the clear douglass fir will be strionger and stiffer because the grain is oriented in one direction.

 

Plywood wouldn't be the best choice in this application. Plywood has many good things about it, but your sole support structure needs longitudinal strength in the members. Solid wood (stick built) performs this job much better then plywood and there's no goo and mess from laminating pieces of ply.

Toss out those metal brackets and build the supports into the carlins, stringers and floors. The typical arrangement for this is to screw or thru bolt the athwartship pieces to the cleat or stringer running down each side of the hull (at the deck edge) They go all the way across the hull, breaking only for openings like hatches, where a doubler will be installed (a carlin) for the hatch to rest on and to provide a landing for, fore and aft pieces that will boarder the opening, also some will provide additional stiffining. These fore and aft pieces are usually screwed to the athwartship supports, sometimes with a shiplap joint or preferably with a sloping (tapered to let water run out of the joint) pocket type joint. If you expect to have to remove sections of this support system at a later date (tanks, engine, trans, etc.) then don't glue the joints in these areas (I always do anyway) glue and fasten all the pieces in. It would be a good idea to coat each predrilled (for fasteners) piece with epoxy (two or more coats, CPES is better) let dry then assemble and paint.

 

PAR,

Thanks so much for the advise. I tried asking these questions many ways and finally you understand what I am doing and asking. Maybe the picture helped. But, "carlin" had me stumped. I checked several references I have on hand, Gougeon Brothers Boat Const. and Ship to Shore dictionary and neither had any reference to carlin. So, after reading your explanation several times, I think I may have a better understanding. I wish I could find an illustration so I were sure. All the wood support you see in the picture has been removed and I was already planing not to reuse the aluminum angles but wasn't sure how to connect the supports and still have the strength. There is one main hatch in the center for access to the gen set. I would like to add two small hatches to each side of the main and aft just a little to have access to an area for battery storage and also access to exhaust and other components. You explained well but I'm still trying to visualize the joints for the supports.


But thanks again for the reply!

 

Drs3317, running down each side of the boat (port and starboard) you'll have a nailer, just at the level of the deck. This is a stringer. At the foreword and aft most ends of this deck area there will be another nailer (or should be) these may be cleats nailed to a cabin bulkhead, a carlin or just another athwartship support. Carlins are a nailer that doubles the support pieces in areas where there are openings, just like is done in a house on land (like around a window or doorway). The joints for these and other support pieces should be designed to shed water if it gets inside the joint. This is done by making a sloped joint rather then using a notch or ship lap (both the notch and ship lap can trap water in the flat part of the joint) I suppose you could get away with any joint you feel comfortable with, but it's typically done (as are most joints found on boats) this way to permit the water to run out of the joint if it gets in. In other words avoid a dead horizontal surface joint where water can sit and start rot. Also try to picture the joint if it was loosely assembled and some water spilled on top, running into the joint. Are there places inside the joint that water could be trapped and not permitted to run down into the bilge.

Some builders just butt the pieces in and either use pocket holes (pigeon toed) for the screws or they stagger the pieces a little to drive screws straight in (that way sucks, because the fastener is holding end grain which is weak)

When you lay out your new supports, space them on as even a center to center measurement as possible. This keeps the plywood (decking) loading equal and all the support system can share the work. I lay out my athwartship pieces first (they run from one side to the other), figuring out where I need the fore and aft end of hatches and other holes. Just drop them down on the stringers and mark their positions. Then work out where you want the holes (hatches, etc.) these areas will need a carlin (a doubler) which serves to reinforce the area (there's a big hole there remember) that isn't helped out by the decking. On a vessel of your size, you'll likely have some fore and aft pieces mixed into the support system. This is because there is a couple of dimensions that were designed into the boat (fore and aft centers and athwartship centers) the sole support structure needs, to provide the strength to hold up your well fed butt (just kidding) You see, everything works in concert with everything else. The plywood needs to be attached to the support system with fasteners spaced close enough together that it becomes a single (solid) unit. If the support structure is substantially changed, the plywood may not have enough support, which will cause it to sag under load (well fed butt thing again) which cause the fasteners to pull, water gets in, rot starts, etc.

The joint I use for these little fore and aft pieces (athwartship pieces should be continuous where not broken by a hatch or other opening if possible) are kind of difficult to describe. Try to picture this. I place a small scrap of the stock I'll use in the location it needs to be on the athwartship member and draw a vertical line down each side, which marks the outside dimensions. I then use a square to bring these lines across the top (where the deck will get fastened) Since you're using 2 by stock, I'd then draw a line parallel to the athwartship piece, in from each fore and aft edge a 1/4" in-between the two lines I drew earlier. This is the depth of the top of the sloped notch (the bottom is flush with the outer edge). Then I cut on the line down to the 1/4" mark, but make it hit flush with the bottom of the member. This makes a sloped, shallow notch that will receive a fore and aft piece (which has a similar slope) This serves three purposes. One is the fore and aft piece can't fall through if the fasteners fail, the taper of the cut prevents this. Second is it will shed water into the bilge without hanging on anything. Lastly it captures the fore and aft piece with little tapered shoulders, preventing side to side movement. It's actually a butt joint, except it doesn't have 90 degree cuts. Cutting into the top a 1/4" doesn't weaken it very much and the benefits of additional strength and stability is great, especially if well glued.

If I have a bunch to do, I'll make a jig for a router to plow them out with a flat bottomed bit on the slope they all need to be. If it's not that many I'll cut the two vertical lines with a hand saw and knock out the middle with a chisel.

Hope this helps. If you drop me an email, I can send you some pictures of a structure I did recently, similar to yours.

 

PAR, that's a very good explanation
Thanks from all of us.

 

PAR,

I can't thank you enough. That is a great detailed explanation. I have read many of your posts and always very informative. Thanks for taking your time sharing your knowledge. Maybe you should consider writing a book! I'll drop you an email becasue I would surely enjoy seeing the pictures.
Thanks again,
Danny

 

PAR,

What size fastners do you use in this joint? And do you also epoxy it into place. I am using several coats of CPES on all stock but think it would also be a benift to epoxy all the joints. Would this be correct? Also I have been told not to screw the CEPS treated MDO deck to the supports but rather bond it into place. From your explanation, it sounds like you recommend screwing it down?
Thanks,
Danny

 

What boat is this, type of construction, year model, etc. It's nice to start on a level playing field.

When the choice of epoxy is decided on, with wood, you're options become limited. By this I mean, epoxy only works well (CPES is epoxy) if everything is coated. Every cutout, every fastener hole, every edge, etc. If this isn't done, moisture gets in and can't get out fast enough to prevent rot.

I like to get at things without ripping stuff up, so I'd screw or bolt the sole in place on the support structure, but it could be bonded ('glass hull only, not a wooden one) and this is what the manufactures commonly do. The problem with bonding is you can't inspect anything, which is why I don't.

Fastener size will vary on the wood species, bore depth, strength required, etc. Generally you'll want as much fastener in the joined piece as the is in the mating piece, maybe a little more. Pre-drill proper pilot and clearance holes for a snug fit and wet these holes out with epoxy. A better method would be to bond the fastener holes, to insure moisture can't get to the wood. Log onto www.WestSystem.com and look up the methods and techniques guides which will explain fastener bonding.

 

The boat is a 1980 Sea Ray 31' Sedan. Fiberglass of course, except the wood substructure and floors. I know this the the wooden boat forum but the knowledge I needed was on the wood parts of the boat. The cockpit sole had been removed when I got her. I assume the sole was 3/4" covered with a layer of glass. But first I need to rebuild the support system. I have it mostly planed, materials and design. I just want to make sure I only do this once, I want to do it right. It appears the original support system was made without thought of lasting. I would like to be able to unscrew the sole for access below, but I know it would not be possible to keep it watertight and prevent rotting if I use removable screws. So, I will have to settle for access hatches.

 

I was thinking wooden construction during most of the previous posts, Drs3317.

'Glass construction is handled differently (it figures, right) Typically the sole is supported with plywood set on edge and tabbed to the hull. Sometimes they'll toss is a 2x4 or three, but these are tabbed in also (and a weak point) Tabbing is a strip of 'glass shaped into a "L" creating flange that bonds to both the support and the hull. This works pretty well (until things get wet, which they eventually will) except with larger pieces of lumber, like 2 by stock. The 2 by stock is just to big to bond this way and make it stay stuck. Any moisture content changes in the wood will cause the bond with the tab to "sheer" or break away. The thickest material you can tab is 1", preferably less.

Another issue with 'glassed boats constructed like yours is the lack of "weep holes", which are semi-circular cutouts in the bottom of the supports, where they bond to the hull. These permit any water that does get in (it always does) to drain to the lowest point of the hull, where a bilge pump or drain can get it out.

Design your support system to allow water to drain, without pooling in places. Tab everything in sight to the hull and glue and screw the decking down. Access to the bilge is important, but mostly forgotten in 'glass hull construction. Seal the decks to the hull with more tabbing and build in access hatches, so you can get at stuff. The sealed deck will shed water into the bilge, where the pump(s) can take care of it.

Epoxy will stick better to the wood, but the manufacture of your boat used polyester. Epoxy sticks well to polyester, but not the other way around. If you use epoxy, use all epoxy. If you use polyester, you can use epoxy in areas that you might want additional strength.

The trick is to create a support system that drains water, holds up the deck boards, provides access and sheds water into places that it can be handled. The decking keeps the bilge as dry as possible (it's relative, depending on openings and stuff) provides a stable platform to bolt and screw things down on, isn't slippery when wet and doesn't cause tripping hazards (lifted seams and such)

These are pretty tough duties to call for from these two, related systems, but should be kept in mind when doing a new one.