Ply Bottom Question

[baliano]

I'm building a 13' Utility OB called the Buddy from a Science and Mechanics article. The plans call for 1/4" ply sides and bottom. I'm thinking about going to 3/8" for the bottom to make it stiffer for trailoring and give more puncture protection for grounding etc.

Question: Should I bevel the bottom planks down to 1/4" at the stem to reduce the amount I rabbet the stem or just do a deeper rabbet? I'm worried about getting too thin a cross section from the heel down along the bottom of the stem. The stem is 2" thick so the deeper rabbets would leave approximately 1" of virgin wood where the chines meet the stem and taper out to about 1 1/4" thick virgin wood at the rear of the stem.

[Steve Lansdowne]

What do you know about designing boats that the designer of this boat does not? I imagine if he/she felt that 3/8" was needed it would have been so specified. If you want to increase abrasion resistance how about adding some Dynel and epoxy to the bottom?

[baliano]

Steve, I had hoped for a more constructive response. I don't know how much experience you have with 1940s-1950s magazine article boat designs but I can assure you there is not the level of design integrity you imply. I have already had to interpolate several measurements just to get the plan to make sense so adding 1/8" to the bottom for improved trailoring strength is no big deal. The only change will be a slight increase in hull weight, form remains as specified.

In addition, I said I wanted to improve puncture resistance not abrasion resistance. The addition of resin and cloth would add as much weight as the thicker plywood, maybe more, and do little for puncture resistance so why do you think that's such a good option.

[Steve Lansdowne]

Sorry if I came off as hostile. I didn't mean to. Guess I was reacting to the fact that in my experience of several years reading the Forum regularly and having built two small boats and taken a few WB classes, I've seen that many folks try to "improve" boat designs when they have little reason to or valid basis for doing so.

My thought is that you'd get a tougher surface all around by adding some type of cloth and epoxy on the bottom than you would by increasing the plywood size. There are various possibilities, such as fiberglass, Dynel, and Xynole, which you could research by using the search function on the Forum. I'm certainly no expert on this, which is why you'd be cautious about taking advice from anyone who posts here. The more you read here, the more you'll get a sense of who the relative experts are. I'm not one, for sure.

Bottoms and sides stiffen up considerably when they are all connected as part of a boat, and if there is a keel on your planned boat, that will also stiffen it. You can also add replaceable white oak skid rails of perhaps 1" by 1" on the bottom which are parallel to the centerline and would servive to protect the bottom when grounding as well as stiffen the bottom somewhat.

Good luck!

[Bert Langley]

I looked at the plans for "Buddy" available from Svenson's. If that is the boat you are building then I don;t think I would go to 3/8" ply. It is real hard for me to find 3/8" ply that is any good at all. Most I find only has 3 plys. On the other hand finding marine grade 1/4" with 5 plys is doable. Given that, I would stick with the 1/4. The boat you are building, has a full length keel and a number of frames. I think the 1/4 would be fine. However, that is really not the question you asked. In response to your actual question, I would likely just cut the rabbet deeper rather than trying to thin down the 3/8 to 1/4.

[baliano]

Thanks to both of you for the responses.

The boat is a semi-vee and has a fair amount of rocker as well, pretty typical displacement hull. There are enough frames to make it trailorable on longitudinal bunks if the hull stiffness is high. The sides and frames make a pretty stiff box but the bottom and 1 1/2" thick keel could oilcan on a trailor and eventually gap the seams. I thought the thicker ply might reduce the oilcanning. Alternatively, I could go to custom fit 6" wide transverse bunks directly under frames. Maybe that's the best solution after all.

The skid rails crossed my mind also but I couldn't predict the affect on handling so I dropped the idea. In addition, this boat should get to hull speed (about 4.5 kts at 600 lbs displacement) with a very small motor so I don't want to add any more drag.

[Mike Vogdes]

If you stick with good quality marine ply you shouldn't worry, many of those designs are actual APBA class boats and are built as light as possible. Maybe you could laminate some ply doublers in key places on the inside skin of the hull if you feel it needs additional heft.

You could also spend some time and custom fit the trailer bunks and keel rollers so the boats weight is properly distributed.

[Tom Lathrop]

In getting back to your question of thickness, it probably makes more sense to fit the trailer to the boat rather than the other way round. Trailer bunks or cross supports should arranged and fitted so that they are located under rigid areas of the hull like chines, keels, transom or others areas that don't flex. After trailing racing sailboats with 1/4" bottoms for many years and many miles, this is the way I do it. Placing the bunks under broad unsupported areas is a good way to permanently deform the bottom even if it is fairly thick.

Sorry, Mike. I did not notice your second paragraph but it is worth restating anway.

[ 01-04-2005, 09:13 AM: Message edited by: Tom Lathrop ]

[RodB]

I think it should be mentioned that a true increase of the scantlings is achieved by adding thickness to the plywood, not just adding more epoxy and fiberglass. Withstanding pressure at narrow points such as bunks will be enhanced significantly by increasing the hull thickness. If the boat will be trailered and sitting on bunks most of the time, I think adding some thickness to the hull scantlings would help the hull keep its shape over the long haul simply by just being stiffer and stronger. Would the additional weight really be a negative factor when all is considered?

I had the option of leaving my 18' hull 3/8" or adding an additional layer of wood, which I did (1/4" ply) ...ending up with a 5/8" hull. In addition to that I sheathed my hull with Xynole thus getting a really tough, stiff, and abrasion resistant hull. I figure the added weight for the additional layer will not matter on my trailered hull.

On a boat the size you specify I can't imagine a piece of 3/8" ply would be that much heavier than a sheet of 1/4" to make a real difference in the performance of the boat, plus the gain in strength, stiffness and rigidity would be significant.

If weight is a factor in the type of performance you require then you most likely will have to stay with the original design and just be sure to place all bunks under areas of the hull with interior support. From what you have said, I would definitely increase the bottom panel thickness to 3/8". World panel in Miami offers great quality 3/8" Okoumi or Merranti just to mention one source of good quality marine plywood.

RB

[ 01-05-2005, 01:30 AM: Message edited by: RodB ]

[baliano]

Thanks Rod. You have mirrored my thoughts exactly. The slight increase in weight won't matter to performance since it's a displacement hull. I'll simply give up a slight amount of capacity to maintain a pre-determined safe waterline. I've already decided it's a two person boat and that gives me quite a bit of extra capacity. The plans indicate it's designed for three but I think that's a stretch.

The posts about fitting the trailer to the boat make sense also. Since the rocker makes it difficult to design longitudinal bunks that would evenly carry the weight, I'll design cross bunks. I don't think rollers are needed for this size boat.

[boatbuilder.org]

I would not hesitate to go to 3/8" plywood, 1/4" seems a bit skinny to me for a 13' boat. I certainly agree with Mike: GOOD PLYWOOD, GOOD PLYWOOD, GOOD PLYWOOD. 1/4" of bs 1088 is better than 3/8" of crap fir.
---Joel---