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Old 09-03-2010, 02:52   #1
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Plywood Use

Hi All,

Have to build a new rudder for my 22 foot yachtette and was planning on using epoxy / cloth sheather marine ply laminate ( 4 X 12mm sheets to be precise ). OK so far but have heard that ply can suffer from 'Rolling shear' particularly as half the plys are in line with the plane of most stress.

Just wondered if cutting the ply at 45 degrees so the grain of all plys is at 45 degrees to the vertical would prevent this or at least improve it?

I can see that doing this would mean ALL plys will be contributing to the strength instead of 50% but of course they will be contributing LESS strength as the strain will not be in line with the grain. I imagine that assuming an equal number of plys this would give the same strength ( 100% strength for half the plys = 50% for all ) but obviously that is VERY simplified, but if true just wondered if the rolling shear resistance would be higher and therefore worth wasting some board.

Comments anyone? ( or does anyone have any experience of this? )

thanx
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Old 09-03-2010, 05:10   #2
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How to Build Rudder Blades & Centerboards
by J.R. Watson (Gougeon Bros, WEST Systems)
Here ➥ http://www.mothboat.com/CMBA/Building/foils.htm

In the above tutorial, J.R. Watson makes a good case that:

“... plywood is not a good choice for cantilevered structures such as rudder blades and centerboards. This is because plywood is susceptible to rolling shear, shearing forces that roll the structural fibers across the grain.. Plywood's unidirectional wood fibers are laid in alternating layers, approximately half of them are oriented 90 degrees to the axis of the loads. Like a bundle of soda straws, which resist bending moments quite well one way, they simply lack cross-grain strength laterally and can roll against one another and fail under relatively low stress, especially in a cyclic environment. Therefore, when anticipated loads are primarily unidirectional, it is ideal to use a material with good unidirectional strength. Since only half of plywood's wood fiber is used to advantage, a plywood rudder blade or centerboard going from tack to tack (reverse axial loads) will fatigue much more rapidly than one built as described in this article...

... Western red cedar and redwood are good choices of wood to use for rudder blades and centerboards for boats up to 25 feet. Both of these woods bond very well, are generally clear and straight grained, have good dimensional stability, are easily worked and affordable. Cedar is just a little heavier than the foams used for rudders, is much stiffer and has far greater shear strength values. On larger craft, a higher-density material like African mahogany is a better choice. Oak is not a good choice.

Buy flat-grained 2'x6s or 2'x8s, and then rip them to the designed board thickness. Turn every other ripping end-for-end to neutralize the effects of any grain that does not run exactly parallel to the blank, and to reduce tendencies to warp or twist ..”
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Old 09-03-2010, 07:57   #3
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Hi Gord and thanks for the reply.

Spookily enough that is exactly the article that prompted my original post! I don't doubt at all that cedar or redwood would be better / best, ( If I can get such things ) I just wondered if the 'bundle of straws' would not move so readily if the strain were at 45 degrees to the line of grain and hence the risk of rolling shear reduced. The original rudder was ply but was protected by a skeg that is now AWOL having been 'tested for bouyancy' one dark and stormy night taking half the rudder with it. I just wondered as I don't have long to fashion the replacement......
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Old 09-03-2010, 08:01   #4
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Sorry to throw a spanner in the works, but have you considered the effect on stability of losing the skeg?
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Old 09-03-2010, 08:10   #5
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Hi Nemesis,

Yes, had considered that. I know of at least two other Invaders that sail without skegs. ( Not sure they ever had them to be honest ) I don't doubt it will perhaps not track as well and the steering will be a bit 'faster' so to speak but the other boats don't seem to have problems. The Invader is a twin keeler but the keels are quite long for a vessel of that type and are more or less vertical. Out of the water she sits quite happily on her keels with no tendancy to tip in the pitch plane either way to give you a clue to their length.
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Old 09-03-2010, 08:12   #6
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Oh and all spanners are welcome! would rather they are thrown now than find out I've made a boo boo when out on the water!
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Old 09-03-2010, 09:30   #7
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➥ Let me google that for you
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