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

Eric Johnson (ej@tx3.com)
Sat, 30 Jan 1999 08:43:44 -0800


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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I'm a little slow getting caught up in the potter email, so here's a few
thoughts:

> 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.

Agreed. On my P19 I built a new lower rudder half and am using bronze
bushings and they work well. At least it has stopped enlarging of the hole
in the wood. The fiberglass cheek plates are another matter.

> 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.

In a P15, yes. The P19 rudder pieces don't overlap - they are butted up
against eachother, so the pivot assembly takes a lot of torque (and has a
lot of slop) from side the side. The P15 assmebly seems to make a lot more
sense to me, but I don't know if it will scale upwards very well.

>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.

On the P19 the cheekplate dig deep grooves into the lower rudder. On my new
lower rudder I laid strips of aluminum and stainless along all the bearing
surfaces to prevent this.

>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.

I'm inclined to agree. With the P19 kickup design, you get either an
easy-to-lift but sloppy rudder feel, or you tighten the bolts way down and
have a more solid unit (though not REALLY solids because the fiberglass
cheeks just can't hold it tight - maybe the aluminum would be better) that
you can't kick up. Plus those darned cheeks gotta cause a fain amount of
drag.

I think a strong, lightweight, high-aspect glass rudder (with neutral
buoyancy below the waterline) would work well - you could just pop the whole
thing out and easily stow it. I've sketched out a few designs. If I ever
build one of 'em, I am also going to put the tiller pivot point on a bit of
a cantilever hanging off the rear of the blade, in such a way that for
stowage, i can take the rudder from its normal sailing horizontal position,
and flip it up, over, and back 270 degrees so it folds back along the blade
to make it easier to fit in the truck.