Plywood planked dory

Hello,
I'm new here and in need of steering. I would like to build a small lapstrake dory which has a specified wood planking thickness of 1/2" with a bottom thickness of 7/8". If I were to build as a glued lapstrake plywood boat instead and have determined scantlings as required for the size and material from Gerr's "Elements", how concerned need I be that planking thickness has changed in regards to the lofting? The boat is only 13'+ in length and needs to be as light as possible due to space and transport requirements.
I know that hull displacement will change some due to the lighter material substitution, but if I were to add to the frames to get to the outside of the plank as originally designed could I get by with just fairing in by eye the required frame bevels in a plywood hull made of such skinny stuff? The frames will be slightly beefier than required if I use 3/8" ply and glass tape them in so I feel I could almost shift them around fore and aft and epoxy fillet them in without bothering to bevel anything but I feel it prudent to seek advice here. Is my thinking lazy or efficient? I am aware the "proper" way is to recalculate the frames using the bevels obtained from the lofting but I am also aware that people used to build by eye alone and that I'm really just making a stitch and glue boat now anyway. Thoughts? Discouragements?

 

Welcome to the Forum SO (I guess it is a bit difficult to get sun-burnt in Kentucky at this time of year?).
Who is the designer of the 13' lapstrake dory that you intend to build?

I am wondering if you are 'overthinking' everything here - for a simple dory in this size range, the easiest one to build might be the Gloucester Light Dory designed by Phil Bolger?
Gloucester Light Dory – 15′-6″ x 4′-0″ – H.H. Payson & Company https://www.instantboats.com/product/gloucester-light-dory-15-6-x-4-0/

 

I would not have a lot of confidence in the precision of Gerr's scantling formulas applied to 13 foot open boats.

If the lines are to the inside of planking just build the boat based on the lines. The 1/8" difference in outside surface location will not make any perceptible difference.

As bajansailor asked what is the design? Who is the designer? Gardner, Chapelle, ?????

 

Is plywood widely used as clinker planking ? Seems a lesser alternative to boards.

 

Yes. I am sure I am overthinking it a bit, but since it’s my first attempt at building a sea beast I was inclined to be cautious. The design is from Gardner, and happens to fit just right in my garage, in my shop and on top of my car if I can keep the weight down to manageable limits.
I would love to be able to build it as designed and the whole bit, but the reality is it will spend more time on the way to and from the water than in it here in Kentucky. Most people here only get wet when they spill their bourbon it would seem.
DCockey, as far as confidence in the scantlings derived via Gerr, would you feel that they are so off as to be unfounded? I’ve been lurking around the forums a bit and it seemed like they were in line with what I had seen others using for similar construction techniques.
I realize that things can get out of hand quickly when trying to improve things that don’t need improving but I like to tinker I guess and I hadn’t found the just right design that fit my space and use requirements till I stumbled upon this one. I had looked a while and even purchased plans for a small deadrise skiff but eventually decided it was within my talents to go ahead and build a lapstrake boat for the hell of it. Anyway, it’s lofted and I feel good about everything except for the planking thickness question which was never really an issue until it was. And now I guess it’s less so, thanks to the both of you for that.

 

Its very common and has been since the 1950's.Works extremely well and the planks don't split along the line of the fastenings,particularly if there aren't any fastenings-as with glued laps.Half inch is certainly adequate thickness and I would question how relevant Dave Gerr's scantlings are when applied to a small boat.I knew a fellow who built a plywood dory in the 1980's and he was delighted with it.It helped that his ply came free as it was the offcuts from a business that put ply liners in metal trailer bodies and they were very happy to lose a pile of the offcuts that accumulated in the factory.I can't ask him how long it lasted as he died last December.

Obviously,like all dories you will need a reasonably sturdy gunwhale/cap connected to a few frames.

 

Lesser? I feel as if there are quite a few examples out there but I guess it depends upon your perspective. I’m afraid that in my situation it’s the only alternative to boards.
Since moving to Kentucky I’ve been surprised to see that the quality of lumber available here doesn’t quite match what one can get with one days drive east. Couple that with the fact that the boat will live in a garage except for weekends and plywood looks quite logical to me. In a perfect world we would all have a wood lot out back and a salt marsh creek to float in but the older I get the more I realize the world's not perfect. How I do wish it was, but you know the thing about wishes...

 

After reading the responses so far I want to be clear as to the scantlings so that everyone is talking about the same boat. It’s 13’4” LOA and 4’1” wide. As designed by Gardner it requires 1/2” planking on 7/8” thick oak frames and a bottom of 7/8” white pine. The boat has four interior frames in combination with three thwarts and a bench seat in the stern, along with breasthook, transom knee, etc.
My changes would be to plank with 1/4” ply over 3/8” plywood frames and 3/8” ply bottom, with dimensions worked out using Gerr’s scantling rules. The plywood will be BS 1088 Okoume and will be glasses inside and out with 6 oz. glass. In addition the frames will be filleted in and taped with 10.5 oz bi-axial, also per Gerr.
Stem is white oak or perhaps Osage Orange or Black Locust depending on availability. Transom is doubled 3/8” ply. Knees and breasthook and gunwhale/sheer clamp assemblies are all following Gardner with wood substitutions per my intuition. All are epoxied in and tabbed with bi-axial where prudent.
It was my thought until now that this boat would now be somewhat overbuilt what with the glassing schedule as described. In fact, prior to this my research had concluded that interior glassing was way overkill except for the tabbing.
Am I wrong?

 

Its just that ply planks have end grain galore, and water wicks in pretty easily, necessitating careful sealing.

 

Agreed. The hull will be glassed and painted inside and out.

 

Note t0 forum participants. The original poster is interested in a 13 foot dory with plans by John Gardner. That means it is a dory-skiff, not a slap sided "banks" dory (nor a Boston Whaler type of hull). An example of the this type of boat is the 13' 4" Dory-Skiff by Chamberlin show at the bottom of page 167 of Mystic Seaport Watercraft. Rowing Craft – Flat Bottomed https://research.mysticseaport.org/item/l036560/l036560-c006/21/

I would compeletly ignore Gerr's scantling rules. They are based on larger boats. Trying to use those rules on a 13' open boat would be extrapolating outside the range of applicability. Also on smaller boats local loads can be more important than global loads. Instead look at how other similar size boats which have been successful are built.

If you use plywood and glue the laps then only a few frames, if any, will be needed, particularly if a stiff cap rail is used. 1/4" / 6 mm ply is probably sufficient but 3/8" / 9 mm would be at less risk of damage due to a concentrated load such as the corner of a dock or being run into by another boat. If weight is a concern consider only glass the bottom and a several inches around the chine for abrasion resistance.

I don't know the date of the plans you are looking at. Beginning sometime in the 1960's John Gardner suggested using plywood as a substitute for conventional lumber for planking dories. The plans for a 14-foot dory skiff in Chapter 27 of The Dory Book calls for 3/8" plywood planking. Note that Gardner was assuming mechanically fastened, not glued laps. With glued laps the planking can be thinner.

 

You will be using adequately strong material.Particularly if you intend to glass inside and out.I must admit I would go with 3/8 ply and omit the glassing, but I really dislike the prospect of glassing and unless you use douglas fir ply there shouldn't be too much risk of checking.Presumably there will be a decent chine stringer to give a good amount of landing.

 

Ok, now I feel better. The 13’4” boat by Chamberlain via Gardner is the boat I have lofted. From all the similar sized designs I have looked at it seemed as if my material choices were adequate for the construction methods I was planning. My thought was to build the boat on temporary frames that are covered in polyethylene plastic so the epoxy won’t stick. Then I can add chine stringers and use frames only where I want to add thwarts and flotation. As a curiosity could a reliable substitute for chine stringers be a strong fillet covered in biaxial glass tape? The building frames would be used as templates for the actual frames after the hull is popped off of the form. This has been the plan anyway. My only real concern was whether or not I could just build the boat as lofted to the inside of plank and disregard the difference in plank thickness from the original design?
Thanks for all the replies so far.

 

You want the chines in place before installing the bottoms or garboards, or use a "stitch and glue" type of chine. (See response to next question.)

Yes, if you first drill a series of holes along the edges of the bottom and sides, and use wire or zip-ties though the holes to hold the bottom and sides in position while you tape the seams inside and out. In other words use stitch and glue on the seams.

1/8" or 1/4" in the location of the outside surface will have negligable effect. Or pad the molds out by the 1/8" or 1/4". (Technically you can get the exact amount to pad the molds/frames out from the lofting but the difference will be small enough not to matter, particularly if you check the fairness of the molds/frames with battens before planking an adjust as needed.

 

If you glass,a chine stringer is pretty much redundant.You can ignore the difference a skin thickness makes and press on.Alternatively,if you feel like a challenge you could build some female formers and build inside out.Jeremy Rogers used to build OK's that way in the early sixties and very rapidly.You may find determining panel shapes a bit of a performance if you try the experiment.