Re: UHMW - Ultra High Molecular Weight Polyethylene Rudder Stocks

There is a photo of Peter Lenihan's rudder assembly at the bottom of
this link.

http://www.duckworksmagazine.com/00/DM1999/articles/micro/index.htm

Stainless steel rudder post with the ss supports welded on and riveted
to the solid MDO shaped rudder blade - All barrier coated after sealing
with epoxy I believe.

Nels

--- Inbolger@yahoogroups.com, "Myles J. Swift" <mswift@...> wrote:
>
> I've had no rudder problems with my 20 year old Micro. The whole
rudder is
> glassed. I did have a problem with the lower mount failing. I used the
> pieces as forms and cast aluminum ones. I put several layers of glass
in the
> area contacted by the lower mount and that has help up.
>

Andrew,

> I am interested in this thread because I have just been trying to decide what size and material to make a rudder stock from. I have been doing some calculations for a rudder stock for my Chebacco 25. I was very surprised at the torque (twisting) loads that the rudder can potentially experience.

...snip...

You raise some interesting concerns that probably rule out the material as a stock for anything but a very small rudder. Mine will be very small, but I'm limited to about 1" diameter for the stock, which is probably still too small for UHMW in this application.

Another problem I noticed with UHMW when drilling it is that the bit really tries wander. Even with a drill press, it was hard to drill a 1/2" hole longitudinally straight through a 1.5" section of 1" rod. There are also thermal issues. I made oarlock sockets using UHMW in the winter, and the oarlocks slipped easily into the 1/2" holes inside my cold shop. When I took the boat out this summer, I really had to force the oarlocks into the sockets. The sockets work well, with no noise and a very smooth action, but are noticeably tight. If doing it again, I would try a 9/16" bit.

Jon

I am interested in this thread because I have just been trying to decide what size and material to make a rudder stock from. I have been doing some calculations for a rudder stock for my Chebacco 25. I was very surprised at the torque (twisting) loads that the rudder can potentially experience.

I calculated a maximum torque of 600Nm when the rudder is turned 35degrees at a speed of 6.6kn. That is about 40kg exerted on the end of a 1.5m tiller.

(see Boat Mechanical Systems Handbook: How to Design, Install, and Recognize ... By Dave Gerr pp.183 and 188 for the formulae - this page is available to read on Google Books)

So I have decided on a 32mm stainless steel tube, 1.6mm thick. It is only just strong enough. I will avoid welding it by drilling holes through it and inserting stainless rods. The rods will be held in place embedded in the rudder. I got this idea from option 4 on the Jefa website:

http://www.southernseasmarine.com/ssm/jefa/rudders/products/rudderstock/stocks.htm

Now, back on topic, UHMWPE has a much lower ultimate strength than Stainless (25MPa versus 500MPa+ so you would need to be thinking larger diameters and solid bar. Another issue with UHMPE would be creep. Unless you keep the stress quite low (I guess 5MPa or less) the plastic will slowly deform. If you have a constant load for an hour or two due to weather helm you might end up with a permanently twisted stock.

I suspect that PE would be as strong as wood but would be more susceptible to creep. I think it deserves doing the calculations on before testing on a boat.

Andrew

(yes - I did study engineering once - does it show?)

--- Inbolger@yahoogroups.com, "Susanne@..." <philbolger@...> wrote:
>
> Hello all,
> has anyone tried MICRO, CHEBACCO etc. with a UHMW rudder-stock with rudder-blade through-bolted. We had proposed this a while back to prevent wooden stock-splitting or metal-work. Any takers for a fast&dirty experiment ?
> Is this notion an obviously bad idea before any experiments ?
> Susanne Altenburger, PB&F
>

My thinking is galvanized pipe and sheet metal for the rudder and end plate. UHMW for the bearings. But that does involve some welding with a protective coating if in salt water.(My neighbor is a welder.)

Nels

--- Inbolger@yahoogroups.com, Bruce Hallman <hallman@...> wrote:
>
> On Wed, Jul 28, 2010 at 5:44 PM, Susanne@...
> <philbolger@...> wrote:
> >
> >
> >
> > Hello all,
> >      has anyone tried MICRO, CHEBACCO etc. with a UHMW rudder-stock with rudder-blade through-bolted.  We had proposed this a while back to prevent wooden stock-splitting or metal-work.  Any takers for a fast&dirty experiment ?
> > Is this notion an obviously bad idea before any experiments ?
> > Susanne Altenburger, PB&F
>
>
> I used UHMW for my Micro rudder post bearings. My rudder post was
> made from some salvaged mahogany (shaped into a 2" round rod). The
> rudder blades made per plans with 1/4" fir plywood sandwich, and with
> an added bottom plate. It all seems to work satisfactory, was
> economical, and feels plenty strong.
>

On Wed, Jul 28, 2010 at 5:44 PM,Susanne@...
<philbolger@...> wrote:

>
>
>
> Hello all,
>      has anyone tried MICRO, CHEBACCO etc. with a UHMW rudder-stock with rudder-blade through-bolted.  We had proposed this a while back to prevent wooden stock-splitting or metal-work.  Any takers for a fast&dirty experiment ?
> Is this notion an obviously bad idea before any experiments ?
> Susanne Altenburger, PB&F

I used UHMW for my Micro rudder post bearings. My rudder post was
made from some salvaged mahogany (shaped into a 2" round rod). The
rudder blades made per plans with 1/4" fir plywood sandwich, and with
an added bottom plate. It all seems to work satisfactory, was
economical, and feels plenty strong.

Hello Susanne,

> has anyone tried MICRO, CHEBACCO etc. with a UHMW rudder-stock with rudder-blade through-bolted. We had proposed this a while back to prevent wooden stock-splitting or metal-work. Any takers for a fast&dirty experiment ?
> Is this notion an obviously bad idea before any experiments ?

This sounds like a good candidate for the experimental project I'm working on now. It's a 33% stretch of Rubens Nymph with a flanged box keel of my own design. It's to have a small end-plated inboard rudder, and I've been pondering what to do for a rudder stock for some time, but hadn't thought of using this material. I even have some round UHMW on hand left over from making oarlock sockets/bushings that might be the right size, so I'll give it a try. It won't be quick, though, since I'm spending more time using my little fleet than building at this time of year, and I suspect you are looking for feedback rather soon. I'll report back anyway whenever it gets done. Thanks for the suggestion.

Jon