reducing hull twist?
 

Trying to imagine the least weight way to reduce twist in a light weight
narrow rectangular punt (11'x2'x1') made of thin plywood under
construction. The bottom skids and gunwales are on. There are glued and
screwed chine battens at the intersection of bottom and sides. Three
rectangular building molds are still in. When I torque one end there is
too much twist (flexibility) in the hull for my liking.

I am thinking bulkheads, frames, partial decks, or some arrangement of
stringers. (I opted for the shoebox shape for ease of construction. I know
that curves would have been better.)

Any suggestions?



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William R. Watt wrote:
Trying to imagine the least weight way to reduce twist in a light weight
narrow rectangular punt (11'x2'x1') made of thin plywood under
construction. The bottom skids and gunwales are on. There are glued and
screwed chine battens at the intersection of bottom and sides. Three
rectangular building molds are still in. When I torque one end there is
too much twist (flexibility) in the hull for my liking.

I am thinking bulkheads, frames, partial decks, or some arrangement of
stringers. (I opted for the shoebox shape for ease of construction. I know
that curves would have been better.)

Any suggestions?

Are you going to fiberglass it?

William R. Watt wrote:
Trying to imagine the least weight way to reduce twist in a light
weight
narrow rectangular punt (11'x2'x1') made of thin plywood under
construction. The bottom skids and gunwales are on. There are glued
and
screwed chine battens at the intersection of bottom and sides. Three
rectangular building molds are still in. When I torque one end there
is
too much twist (flexibility) in the hull for my liking.

I am thinking bulkheads, frames, partial decks, or some arrangement
of
stringers. (I opted for the shoebox shape for ease of construction. I
know
that curves would have been better.)

Any suggestions?


After you have a few of these under your belt you will find that they
stiffen up quite nicely once the seats, quarterknees, etc are put in,
provided the boat is properly designed and planned out before
construction...

Scotty...


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Box section seats at each end and the middle. Stiffen + buoyancy + stop free
surface when swamped

"William R. Watt" wrote in message
...

Trying to imagine the least weight way to reduce twist in a light weight
narrow rectangular punt (11'x2'x1') made of thin plywood under
construction. The bottom skids and gunwales are on. There are glued and
screwed chine battens at the intersection of bottom and sides. Three
rectangular building molds are still in. When I torque one end there is
too much twist (flexibility) in the hull for my liking.

I am thinking bulkheads, frames, partial decks, or some arrangement of
stringers. (I opted for the shoebox shape for ease of construction. I know
that curves would have been better.)

Any suggestions?



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William R Watt National Capital FreeNet Ottawa's free community
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No fibreglass, just lots of paint.

I think I could do something lightweight in the way of plywood seats
stiffened with rigid foam of which I have plenty of scraps. I wasn't
thinking seats since it's to be paddled from a kneeling postion heeled
over, solo canoe style.

thanks for the help


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William R Watt National Capital FreeNet Ottawa's free community network
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On 17 Feb 2005 13:43:37 GMT, (William R.
Watt) wrote:


Trying to imagine the least weight way to reduce twist in a light weight
narrow rectangular punt (11'x2'x1') made of thin plywood under
construction. The bottom skids and gunwales are on. There are glued and
screwed chine battens at the intersection of bottom and sides. Three
rectangular building molds are still in. When I torque one end there is
too much twist (flexibility) in the hull for my liking.

I am thinking bulkheads, frames, partial decks, or some arrangement of
stringers. (I opted for the shoebox shape for ease of construction. I know
that curves would have been better.)

Any suggestions?

The lightest, and probably least practical solution, is to box in the
fourth face of the shoe box.
If I recall, that increases stiffness to twisting forces about X100

But how would you sit in or load up?

The next best structural approach is a zig zag of diagonals along the
top. Still not practical.

Getting a shade more practical: if you can introduce a box section
along the keel, or two box sections along each chine that will provide
adequate stiffness. If you could build in those two box sections
along the gunwhales, they would stiffen the hull in probably a most
practical way.

You will be aware that an approach which approximates this stiffening
approach is often used with open top glass hulls:
the gunwhale folds in, then down to make a channel.
Not as stiff as a box, but usable.

Brian Whatcott Altus OK

Brian Whatcott ) writes:

The next best structural approach is a zig zag of diagonals along the
top. Still not practical.

Thinking of Frank Lloyd Wright, and out of curiosity, I tried a tight rope
around the top of the molds before posting the query, crossing before and
aft of the paddling position to leave that space open. Rope too elastic.
Arrangement too restrictive.

Getting a shade more practical: if you can introduce a box section
along the keel, or two box sections along each chine that will provide
adequate stiffness. If you could build in those two box sections
along the gunwhales, they would stiffen the hull in probably a most
practical way.

The 1/4" thick skids dry fit before, were glued on last night. Gluing made
an improvement to stiffness this morning. Don't know why . I've cut out a
second 1/4" layer of skid thickness for the centre portion of the boat to
strengthen the bottom but it should also improve twisting rigidity.

You will be aware that an approach which approximates this stiffening
approach is often used with open top glass hulls:
the gunwhale folds in, then down to make a channel.
Not as stiff as a box, but usable.

Yes! Side decks. I'd forgotten about those. Probably not here due to weight
but it's certainly something to keep in mind. Also reminds me now that
canoes have those ladder gunwales for stiffness with lightness. A bit of
fiddly work with lots of small pieces but certainly another possibility.

Thanks for all the suggestions.

I can make the boat okay for paddling now. I'd like to put a sail on it
later and then I'll have to think more about reducing hull twist.

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