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View Full Version : Any reason to strip plank vs. plywood for a hard chine design?



Jefe
07-31-2015, 01:26 PM
Talking about a planing runabout in the 17-23 foot range here, with epoxy/glass inside and out.

Peerie Maa
07-31-2015, 01:39 PM
The only difference is the amount of shape in the panels. You have no freedom with plywood as the surfaces must be developable, whereas with strip the world is the mollusc of your choice. That said a developable ply bottom does yield a fast comfortable ride.

TerryLL
07-31-2015, 01:41 PM
Not all hard chine designs can be planked in plywood.

Jefe
07-31-2015, 01:43 PM
Not all hard chine designs can be planked in plywood.

Physically because it cannot make the bends, or are you talking about relative strength/stiffness?

Peerie Maa
07-31-2015, 01:53 PM
Physically because it cannot make the bends, or are you talking about relative strength/stiffness?

Ply will only bend in one direction, that is what i meant by developable. The surface must be either part of as cone or part of a cylinder and you must be able to place a straight edge along the surface.
Have a look at this thread http://forum.woodenboat.com/showthread.php?14513-chine-design-developable-surfaces

Strength is not an issue as you can always glue on another layer.

TerryLL
07-31-2015, 02:27 PM
Physically because it cannot make the bends, or are you talking about relative strength/stiffness?

It's because panel stock cannot take a compound bend, and there's a good many hard chine designs that have compound curves in the bottom.

upchurchmr
07-31-2015, 02:31 PM
The story that you cannot "develop" plywood panels is false, if you can go with thin ply.
If you want thick ply it is effectively true.

Here is a reference to what it takes; http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf
Its a free download of the book that really started wood epoxy construction 30+ years ago.

If you use high quality plywood (expensive marine Okume for example) you might be able to build a strip planked boat for less.
There is a technique where you nail the strips to building forms, leaving a gap. Then you trowel in thickened epoxy to glue the strips together. I haven't see a reference to this method for a few years. I expect the strip planked to take longer no matter what kind of technique you use to speed it up.

htom
07-31-2015, 03:49 PM
That's really strip planking using plywood for strips, and the gaps show where the surface was not developable.

A surface is developable if [math deleted]; take a sheet of paper, hold it by the opposite corners; notice the different shapes you can make by bending the sheet without creasing it. All of those surfaces are developable. You can take a long skinny slice and get lots of twists and turns in it.

That's what strip planking is; each individual flat strip traces a developable surface. The joints between the strips (and scarfs) are where the non-developed parts are hidden.

upchurchmr
07-31-2015, 03:58 PM
No it wasn't plywood into strips.
It was cedar and it was done on a powerboat.
You certainly are right that each individual strip is not 3D.
But after you bond them together and sand out the flats to get a smooth surface it ends up as a "complex" surface - not 2d developed.

All the nit picking on definitions in the end doesn't matter. It is a smooth 3d curved surface.

Gib Etheridge
07-31-2015, 05:58 PM
A couple more reasons, assuming that you have the extra time and that you don't mind sanding.

You have a nice stash of clear flat grained 2x4 and wider cedar.

You prefer the looks of lumber to the looks of ply.

JimConlin
07-31-2015, 06:44 PM
If both sides of the panel will have a substantial structural glass sheathing, a strip material that's lighter than plywood could be viewed as the core of a sandwich panel. You could save some weight .

Jefe
07-31-2015, 07:21 PM
Let's assume a "developable" curve so either would be a viable option.
Boat would be epoxy glassed & painted so aesthetics are not a concern.
Long term durability would be prioritized over material cost.
Would probably target a given weight for the finished hull (e.g. 1000# for an 18' hull)

So, if I am understanding advantages correctly so far:

Plywood:
Quicker

Strip:
Maybe cheaper
(Possibly lighter core, allowing more sheathing possibly resulting in stronger/stiffer hull?)

JimConlin
07-31-2015, 09:26 PM
...
So, if I am understanding advantages correctly so far:

Plywood:
Quicker

Strip:
Maybe cheaper
(Possibly lighter core, allowing more sheathing possibly resulting in stronger/stiffer hull?)

You could also end up lighter (and faster).

wizbang 13
07-31-2015, 11:37 PM
planing runabout,17-23 feet.

planing , 20 or 50 mph?
25 hp or 250hp/

I cannot see the advantage of strip over ply here.
So many fine plywood OB boats are built. The engineering of our world leans far towards plywood for OB speedboats

wizbang 13
07-31-2015, 11:40 PM
developable curves... nobody cares about that with a planing motorboat. That stuff is for sailboats that are IN the water.

Breakaway
08-01-2015, 12:21 AM
Ross Lillistone might disagree with y'all regarding plywood's inability to bend in two planes. ( SEE THIS THREAD (http://forum.woodenboat.com/showthread.php?147936-Ross-Lillistone-s-14-Low-Horsepower-Planing-Boat-FLEET)) It appears that,while the ply may not like it, it can be made to bend in two directions.


http://i578.photobucket.com/albums/ss225/RossLillistone/Flint4019.jpg

Peerie Maa
08-01-2015, 02:28 AM
developable curves... nobody cares about that with a planing motorboat.

You do :D

http://farm2.staticflickr.com/1087/5149760224_c98b13d65b_z_d.jpg

Peerie Maa
08-01-2015, 02:34 AM
Ross Lillistone might disagree with y'all regarding plywood's inability to bend in two planes. ( SEE THIS THREAD (http://forum.woodenboat.com/showthread.php?147936-Ross-Lillistone-s-14-Low-Horsepower-Planing-Boat-FLEET)) It appears that,while the ply may not like it, it can be made to bend in two directions.


http://i578.photobucket.com/albums/ss225/RossLillistone/Flint4019.jpg

No he don't.

As for the sections forward, it is possible to get hollow sections in the body-plan view and still have them fully developable. Yes, there will always be a straight line somewhere, so as to make for conical or cylindrical sections, but the result is unmistakably hollow when viewed normally. This sort of complex panel development needs the magic of a computer to carry out the huge number of calculations. from http://forum.woodenboat.com/showthread.php?147936-Ross-Lillistone-s-14-Low-Horsepower-Planing-Boat-FLEET

epoxyboy
08-01-2015, 02:49 AM
Ross Lillistone might disagree with y'all regarding plywood's inability to bend in two planes. ( SEE THIS THREAD (http://forum.woodenboat.com/showthread.php?147936-Ross-Lillistone-s-14-Low-Horsepower-Planing-Boat-FLEET)) It appears that,while the ply may not like it, it can be made to bend in two directions.


http://i578.photobucket.com/albums/ss225/RossLillistone/Flint4019.jpg
Those photos are extremely deceptive - I have a very similar one of the lower strake of a Welsford Pathfinder, and a CLC stitch and glue sea kayak.
I can almost guarantee that you will be able to put a straight edge across that plank at an angle slanting back and down from the chine, and have it sit flat. It will be a series of conic sections - ie developable, with maybe a tiny dash of "tortured ply". With stock that thick (I'm guessing 9mm), its tolerance for tight radius compound curves is close to zero.
If you were to sight along the keel from the stern from slightly off centre, the keel line would also appear to bend off to one side - this whole thing is a very compelling optical illusion.

To quote Ross Lillistone from the post linked to above "This is not "tortured ply" but a fully developable panel"

Pete