View Full Version : Ply thickness question
01-07-2002, 01:46 PM
I've drawn a 16.5' pulling boat to built in either stitch and glue or glued lapstrake plywood. It's an attractive shape (to me at least) both on paper and as a 3-d model made from poster board. But I am concerned about whether or not the garboard strake will accept the twist in real plywood. The garboard will have to twist from a deadrise of ~20 degrees amidships to nearly vertical at the stem and stern over a distance of about eight feet. The plank is only about 6" wide at the stem and 4" wide at the stern, but 10" wide amidships. I'm pretty confident that 4mm will make the twist, will 6mm"? It all pulls into shape well in poster board. I'm just concerned that there will be a lot of expensive cracking sounds if I go full size with the thicker ply.
My tentative plan on moving forward with this project is to model it in quarter scale with 1/8" lauan doorskin. A reasonable approach or does someboady have a better idea?
[This message has been edited by mqnada (edited 01-07-2002).]
01-07-2002, 02:15 PM
I never had trouble bending, twisting or torturing 1/4" (pretty near 6mm) ply - very good stuff form years back with 5 plys.
If you have trouble stock with voids, you may have a problem. But if you can twist it on and the CPES it after, it'll hold.
01-07-2002, 06:08 PM
My plywood lapstrake dinghy has about the same twist in the garboards. I used 6mm sapele, but I made patterns out of 3/16" "lauan". This stuff was the worst plywood I ever saw, but it still took the twist. The trick is to twist it slowly and clamp it every few inches. I had no problem at all twisting the sapele. I am sure BS1088 6mm okoume would have taken the twist as well.
01-07-2002, 08:05 PM
Don, if you don't mind my asking how big was your dinghy and how wide were the strakes?
Here is a picture of what I'm working on.
01-07-2002, 09:20 PM
I have no opinion on the plywood question, but are you sure that 4'11" is not too beamy for good rowing at that length and weight of boat? I would have thought that you could lose some 6" with advantage.
Did you have a special reason for that beam?
01-07-2002, 09:23 PM
I can't really offer any help as far as the ply wood is concerned, but the lines look nice!
01-07-2002, 09:33 PM
Actually the beam that one is 4'7 1/2" to the inside of the planking, what's listed is the beam over the outside of the wales. The picture shown is not the latest version, which is narrower at 4'3" to the inside of the planking. I'm also working on a smaller version at 16'x4', too. I tend to spend a lot of time tweaking a design with numerous variations and models before finally getting tired of the process and doing a construction plan. But what you see there is essentially the concept.
My original thinking on this boat was to try to swing 8' oars without outriggers. The other reason was that I was fooling around with have two rowing positions so my wife could have fun with me. The original concept was also nearly 18' long, too. It was shortened becuase I don't have room for another boat that big!
I do appreciate the constructive criticism, though. Really.
01-07-2002, 09:57 PM
If you haven't already considered it, have a look at the L.F.Herreshoff rowboat as drawn by John Gardner in "More Building Classic Small Craft." It is very similar to what you have drawn. 17' LOA X 42" or 48" beam (2 versions.) Gardner uses laminated bent Doug fir frames and 1/4" plywood planking. He shows 2 rowing stations.
01-07-2002, 11:36 PM
My boat is a Iain Oughtred designed Tammie Norrie, 13'-6" LOA, 13' LWL, 4'-6" beam, 8 strakes per side. Garboards are 8" wide at the widest point, 6 1/2" at the transom and about 4" at the stem. The garboard meets the transom at about a 45 degree angle, flattens out to 7 degrees at station 5, then meets the stem at 90 degrees. The narrowest strake is about 4 1/2" at the widest point.
Your boat is similar in dimensions to Iain Oughtred's Ness Boat design. It is a double ender, 16'-6" LOA 5'-2" beam, 4 strakes per side. He specifies 8 or 9 mm plywood.
01-08-2002, 05:19 PM
I wouldn't worry to much about the beam. I've been intrigued with the Penobscot 14 (arch Davis) which has a beam of 4' 2" and everyone says at first that 'it should be better for rowing than sailing.' Those who sail one say it does fine. Rows nice too.
01-09-2002, 09:09 PM
Thanks to all who replied, I'm a lot more comfortable moving forward with this project now.
01-10-2002, 01:04 AM
While I've not tested this, Sam Devlin says in his book "Devlin's Boat Building" that using mylar from a drafting store on a model helps simulate the problems of plywood (like ply, it only likes to bend in one direction.) Cardboard tends to bend equally in all directions. Its on pages 67 and 68 of his book.
01-10-2002, 09:44 AM
And, for further reassurance, Oughtred's MacGregor canoes are made with 6 mm ply, glued lapstrake. I had no trouble bending 1/4" fir plywood on the first one built. Had my head examined then built the second one with Okoume 1088, 6mm.
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