Re: Rudder Upgrades and Sailing skills (WAS Re: Cracked bo

DAVID_KAUTZ@HP-Sunnyvale-om5.om.hp.com
Tue, 26 Jan 1999 10:04:15 -0800


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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I'll play at being an engineer for a few moments. According to my aging copy of
_Machinery's Handbook_, bronze has a linear thermal expansion rate 5 times
higher than wood. Having said that, let me now say I don't think its important
here. My intuition tells me that expansion of bronze (or any metal for that
matter) will be driven by temperature while the expansion of wood will be driven
by moisture. I can hear you telling me already that it's been slathered with
epoxy and can't possibly be effected by moisture, but I'll bet that getting a
hermetic seal on every surface of every piece of that rudder assembly is not
possible. Nylon, likewise, is hygroscopic and will change size with the ambient
moisture level.

If you want to add a bearing surface to the rudder pivot, how about attaching it
with a material that will remain slightly pliable like "Life Caulk". Once
everything is assembled, the bushing can't fall out.

I, personally, think you are both barking up the wrong tree. The loads on the
pivot are very low when sailing because the blade is being pinched by the outer
assembly and there is no regular movement. When the rudder blade IS moved, the
abrasion is primarily on the SIDES of the blade and supporting assembly where it
is being intentionally pinched. If you want the loads to be carried by the
pivot, you will need to redesign the entire assembly to use a shear pin as a
lock. Then you could put some slippery material between the moving parts to
delay wear. And still, any bit of grit in there will still find it's way through
your protective coating with movement over time.

My old boat had a kick up rudder - fiberglass blade and aluminum pivot assembly.
Although it was not affected by water I did eventually have problems with wear
of the mechanism and it would kick up at inopportune times. It may be that to
have a really robust rudder, it will have to be a one piece affair.

Dave Kautz
P-15 #1632 "Tilly Lucy"

much of conversation between Tom G. and Judy B. clipped....

I don't know If I would bother bonding the brass bushing inside the hole to
the wood. I'm no materials engineer, but I'm guessing that the bond probably
wouldn't last through too many episodes of getting overheated n the car in
the summer and being dunked into cold water. (Come on you engineers out
there, what about an educated answer???)

I'd use epoxy resin to seal the wood inside the hole and and to provide water
barrier. Then I'd just slide in the bronze bushing and put the bolt thru it.
The bronze bushing sounds find to me, but I don't have a bronze one, so I'll
probably use nylon when I gratefully copy your idea.

The whole issue of differing coefficients of expansion doesn't sound like it
applies in this situation, since the bronze doesn't HAVE to stay bonded to the
epoxy. It would be an important consideration, or so I've read, in other
situations like sealing a plexiglass window to a fiberglass hatch -- you'd
have to use caulk that remained very flexible and could stretch and compress.

more clipping...

Judith Blumhorst, DC
HMS18/P19 Sail Captain, Potter's Yachters
WWP-19 #266 "Red Wing"
SF Bay, CA









>
> Regards
>
> Tom Grimes
> P-14 #363 Far Horizon
> Muncie, Indiana
>