Plywood Bottom Thickness

[Dale Genther]

I'm working on putting a new plywood bottom on a power boat and would like some opinions on how thick the plywood bottom needs to be. Specs are:

-Length:22 ft.
-Beam: 7'3"
-Displacement: approx. 2700 Lb.
-Engine: 270 HP V8 inboard.
-V bottom forward becomming more flat towards the stern ( fairly typical planning powerboat bottom)
-Expected top end: Approx. 38 knots
-Mahogany frames from keel to chine are approx. 1 1/2 inches wide X 4 inch high. Frames are on approx. 24 inch centers.

I'm planning on using three layers of 5 mm Okume plywood which will equal approx 5/8 inch thick. First layer will be attached to the frames with adhesive and screws. Next two layers will be 6 in wide strips layed diagonaly, in one direction than in the other direction, and glued with West Epoxy.

Thick enough? What say you?

[Hwyl]

What was the original bottom?

My gut feeling says add another layer (maybe fore and aft)

[RodB]

I vote for thicker.... at least one more layer.

My 18 foot skiff with a 90hp outboard has a 5/8" bottom with Xynole sheathing...very tough and strong... Your boat has lots of motor and hp....

RB

[Tom Lathrop]

My thought is that heavy frames on 24 inch centers and a plywood hull is not the best ballance of scantlings. Strong frames but too far apart. 5/8 bottom should be adequate but not with that framing system. Even a thicker bottom with that spacing is probably not optimum. I'd think that fore and aft stringers would be a more efficient use of more weight and material. Actually, I'd prefer full height fore and aft plywood bulkheads on no more than 10 inch centers bonded to both hull bottom and sole. This is both stronger, stiffer and lighter than any frame or stringer system and, in some similar form, is used by most manufacturers today.

Also, what is the reason for the last two ply layers to be in narrow strips? If the first layer can be a ply sheet, then following layers can be the same. Unless the shape of the hull dictates otherwise, why not just use plywood of the full thickness to begin with. If the hUll is "developable" you will gain nothing other than more work, material and cost by making your own plywood. Of course, on the bow sections, some laminating may be required to meet the bending necessary.

If this a reconstruction, then I agree that 5/8 plywood on 24 inch spacing is probably marginal for such a powerboat making 38 knots in rough water.

[Dale Genther]

The original bottom was 3/16" mahogany layed diagonally on the inside. On the outside the was 7/16" mahogany planks laid fore and aft. Cotton set in bedding compound between these layers. The shape of the bottom does not allow for full thickness plywood, therefore the diagonal strips to take the compound curves. My choice of materials and construction technique was based upon the experience of somebody who has rebuilt many boats such as the one I'm working on, but I want to check it out anywhy. In the area of the hull where the bottom would be "taking" the chop the frame spacing in more like 16 inchs. There is a total of 15 frames.

[ 01-01-2005, 10:05 PM: Message edited by: Dale Genther ]

[mmd]

A couple of questions, then a comment:

Are you going to sheath the reconstructed bottom panel in ‘glass; and if so, what thickness and type of ‘glass?

What is the depth of immersion of the hull at the transom? That is, what is the distance from the waterline to the deepest part of the vee-bottom at the transom?

Are there any longitudinal structural members on the hull bottom, or just transverse frames?

I don’t quite follow the logic in laying plywood strips at a diagonal relative to the keel, if the ply strips are narrow. Wood is quite strong in tension along the grain and quite weak in tension across the grain. To overcome this, plywood is constructed of plies laid at ninety degrees to each other so that one layer’s strength compensates for the adjacent layer’s weakness. If you cut the ply into narrow strips you will negate the strength of the short-axis plies and effectively turn them into non-structural filler similar to veneers sandwiching a foam core. By cutting the ply into discrete strips you are creating wood planks that are 33% weaker than a solid plank of similar dimensions. The epoxy that the planks are laid in adds some strength, I suppose, but not enough to compensate for the severing of the alternating plies. To retain the strength of the ply panel, it must be as wide as possible, Lay the ply on an angle, but keep the panels as wide as the curvature of the hull will permit.

Is 5/8” thick enough? It depends. At first blush it seems barely so, but it depends on the sheathing (if any) thickness and underlying structure arrangement. Tell me about sheathing, depth of immersion, and longitudinal structure (pictures would be great) and I’ll try to give a better answer.

[BRobinson]

Dale,

There are several inherent problems with installing plywood bottoms on old runabouts.
1.) The boat was not designed for this type of bottom.
2.) It is diverting from originality in a big way. i.e. if the boat is ever judged at a boat show this is a large deduction.
3.) Because the plywood cannot move, the bottom will eventually split at the keel and/or chines, especially if these pieces are original wood.
4.) If the rest of the boat is not "glued up" more flexing will be transmitted to the topsides and deck.
5.) Plywood bottoms are the hardest riding of the current bottom replacement choices.
6.) I like having 3/8" to 1/2" of solid wood on the outside as original in case of trailer dings or any other mishaps.

[Dale Genther]

mmd - As far as glass sheathing is concerned, I was going to lay on a layer of maybe 8 oz. glass, just for abrasion resistance.
The are stringers which also serve as the engine beds which run almost the full lenght of the boats waterline. These do not contact the actual bottom, but they are tied into each frame. They are about 20 inches apart of 2 X 6 construction. Double that in the engine bed area.
The immersed depth at the transom is about 8 inches.
I see your point about the narrow strips and will modify my plans accordingly.

BRobinson- I don't ever intend to show the boat, it is just a boat that we want to use and to look reasonably good. It will be the replacement for a 22 ft. FRP Wellcraft that we are selling because we don't enjoy doing the fiberglass thing. We have shown and won first place and best in class with our 41 ft. wood sloop. I really don't want to become that obcessed with this boat. I've repaired/replaced all frames and floors. All of which I made to much greater scantlings then the origianl. All fasteners have also been replaced. I totally rebuilt the transom framing and added additional structural member beyond the original.

[Tom Lathrop]

Originally posted by Dale Genther:
mmd - As far as glass sheathing is concerned, I was going to lay on a layer of maybe 8 oz. glass, just for abrasion resistance..If you want some abrasion resistance, 8 oz glass will not give you very much. a single layer of Xynole in epoxy is many times more resistant than that.

I see what you are doing better now. The closer spacing of the frames at the area pounding is likely to occur is good but I still like longitudinal stringers and think that MMD will agree with that.

I often observe the building of Carolina Sportsfishermen along the Outer Banks of NC. They have mostly settled on the same technique of cross laminating 6mm plywood in building boats up to and beyond 60 feet that daily challenge some of the roughest water on the east coast. They take very rough treatment running into waves at high speed. It does make sense to use the widest strips that will fit the local curvature. They draw up the laminates with screws and fender washers.

Some of these boats run into the millions so this is not shade tree stuff. At last count there were over 40 boatyards building in laminated plywood/epoxy along the Outer Banks.

[paul oman]

The "damage" I often see is a result of 20 or so years of sitting on the contact points of a trailer, slowly bending, warping, etc. the bottom of the hull.......

paul
progressive epoxy polymers

[WOODZOWL]

Dale.
All the skiffs that I build have composite bottoms; ie, multi layer plywood and fiberglass cloth bedded in epoxy. A 16' skiff, would typically have a 3/8" + 1/4" ply covered w/ two layers of 6 oz. cloth. 18' and larger have a mininum of 1/2" + 1/4" plus double glass. One of my skiff designs, was finished using, 1/2" + 1/2" ply; which is an option in the plans. Had a chance to give this skiff a river test; and found that it may be worth the extra effort. This was an 18' boat! Jerry

[L.W. Baxter]

On the other hand... I'm building a 26' Bartender, displacement 4350 lbs, and the bottom is two layers of ply for a total thickness of 3/4". Sheathing with glass and epoxy is optional. Frames, sided 7/8" and molded 3", are spaced 16" o.c., with a keelson and chines but no stringers. I'm fairly confident that this bottom is sufficient, as I'm not the first to build this model (he says, dryly). Any more thickness "to be on the safe side" would be a waste.

Why would you change scantlings when you reframed the bottom? Was the original framing insufficient?

I find it hard to believe that plywood laminated as you state will not be at least as stiff and durable as the original planking. Why increase thickness (weight) when you don't have to?

Also, have you really tested the curvature of the bottom, to see if you can't use thicker plywood and plank only twice, rather than three times? If you're going to rip your ply into bendable strips...it seems like it could be as thick as the original planking, no?

--Lee

[Dale Genther]

The choice of three layers of 5 to 6 mm each was based on my conversations with several people who restore these type of boats for a living. I increased the scantlings mostly to give me a wider screwing surface to attach the new bottom to. Plus at the time it was easly to do. I also have a (sometimes bad) habit of making everything I rebuild stronger than it was originally. Judging from the weight of the magogany I cut up to do this I estimate the additional weight is only 30 to 40 lbs. over the original. By the end of today I'll know how the forward planking (where the bottom developes a compound curve) lays against the frames, chine and keel. Since I lived only a short distance from Harbor Sales, I only bought enough plywood for the first layer. The next layers or layer will be purchased based on my experience with the first layer and any conclusions from reached from this forum.