"Glenn Ashmore" wrote in message
news:HwDDb.5375$JD6.5160@lakeread04...


garry crothers wrote:


I am talking about a sailboat., 18,000lbs, 33ft , double ended, with a
barn
door for an external rudder. (Notice I didnt use the word transom, I
thought I was simplfying things by describing a transom hung rudder, but
obviously not)

So what I was envisaging was two rams mounted either side of the rudder,
mounted one end on the hull, running aft, to a fixing bracket on either
side
of the rudder.

That is not a very efficient arrangement. Have you seen such a system
in use? In a parallel setup the force arm is the length of the bracket
and the load arm is the distance from the pivot point to the center of
pressure on the rudder. For example, if the bracket is 6" long and the
center of pressure is 18" aft of the pintle the mechanical advantage is
cut to 1:3. Every Kg of turning force will require 3 Kg of ram force.
If the ram extends 3" the rudder moves 9". That could make steering
extremely sensitive. You would have to make the brackets rather long and
set the cylinders away from the rudder to get a reasonable turning force.

Perhaps I am missing something in my calculation, taken from Vetus calalogue
I had contemplated using approx 20cm brackets set back from the pintels by
same 20cm distance.
(using Vetus MTC72 ram with a stroke of 225mmm I would need 196mm lever to
give me 2 x 35 degree rotation of rudder)

Area of rudder 0.69 m2
Max Speed 16 Kmh
Force on rudder = 23.3 X 0.69 x (16 x 16)
approx 4000N

Torque = Force on Rudder x Lever.
4000 x 0.2
800Nm


This figure is just outside of the rated capacity of the MTC72, thats why I
was asking if I used two Rams , one on each side of the rudder, can I divide
the torque required by 2.





The reason I am contemplating this hydraulic solution, is that the wheel
steering solution using cables that I currently have is rather
in-elegant
:)

I don't know that all that hydraulic equipment will be much more graceful


It cetainly can be any worse than what I have already.



garry