On Mon, 15 Dec 2003 12:54:24 -0000, "garry crothers"
wrote:

I am do the sums to calculate the required size of hydraulic ram need to
drive a transom hung rudder.

I read somewhere that it is preferable to use two rams (one on each side )
to minimise the amount of twisting forces on a rudder.

If I decide to go for this method, is it acceptable to divide the torque
requirement by two.

I.E
If I need a 90KgM torque to drive the rudder, can I size the rams for 2 x
45, or should I use 2 x 90 ?

garry

If you are visualizing what I am visualizing, then two single acting
rams can substiture for one double acting ram.
Each s.a. ram needs to develop the full torque: one to starboard, the
other to port. There is no difference on the torque experienced by
the rudder post, as far as I can see.

Brian W

Brian Whatcott wrote:

If you are visualizing what I am visualizing, then two single acting
rams can substiture for one double acting ram.
Each s.a. ram needs to develop the full torque: one to starboard, the
other to port. There is no difference on the torque experienced by
the rudder post, as far as I can see.

Brian W


Actually, there is a difference and two pushing cylinders make it worse.
Think force vectors for a second. Lets assume we have a 72º hard over
to hard over rudder angle and with the rudder amidships the ram
intersects the centerline of the rudder at 45º. When the cylinder
extends the ram's force is opposed by two forces. One is the water
pushing sideways and the other is the rudder itself pulling against the
pintle. The sum of the two vectors is in line with the ram. As the
rudder turns away from the cylinder that angle gets narrower and the
force opposing the cylinder starts to line up with the rudder creating
more stress on the pintle than usefull turning force.

If the cylinders pull just the oposite happens. As the rudder goes hard
over towards the cylinder the turning force vector increases while the
stress on the pintle decreases.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Mon, 15 Dec 2003 14:19:01 -0500, Glenn Ashmore
wrote:



Brian Whatcott wrote:

If you are visualizing what I am visualizing, then two single acting
rams can substiture for one double acting ram. /snip/

Actually, there is a difference and two pushing cylinders make it worse.
Think force vectors for a second. Lets assume we have a 72º hard over
to hard over rudder angle and with the rudder amidships the ram
intersects the centerline of the rudder at 45º. /snip/

I am evidently not visualizing what you are visualizing, Glenn.
Why would a ram be set at 45 deg to the long axis of the hull?

If I visualize an aft rudder, with a forward facing tiller. I can fit
either one double acting ram acting from side to side.
Or two single acting rams.
I could place these two rams transversely (like the double acting
ram) but I *could* consider an arrangement that places each ram
connecting to the tiller at 36 degrees aft of the transverse
direction.
Then, at either hard over position of the tiller, the ram is acting at
90 degrees to the tiller, the most advatageous position.
But with moderate hydrodynamic balancing of the rudder surface to
moderate the steering forces is this such a problem?

Brian W

I am not visualizing what you are either. In your original post you
were talking about a transom mounted rudder and hydraulic steering. To
me that usually means a power boat with the hydraulics mounted on the
transom. Maybe you should explain exactly what you have and what you
are trying to do. If this is a sailboat with a tiller why are you going
through the hassle of hydraulic steering?

Brian Whatcott wrote:

On Mon, 15 Dec 2003 14:19:01 -0500, Glenn Ashmore
wrote:



Brian Whatcott wrote:

If you are visualizing what I am visualizing, then two single acting

rams can substiture for one double acting ram. /snip/

Actually, there is a difference and two pushing cylinders make it worse.
Think force vectors for a second. Lets assume we have a 72º hard over
to hard over rudder angle and with the rudder amidships the ram
intersects the centerline of the rudder at 45º. /snip/


I am evidently not visualizing what you are visualizing, Glenn.
Why would a ram be set at 45 deg to the long axis of the hull?

If I visualize an aft rudder, with a forward facing tiller. I can fit
either one double acting ram acting from side to side.
Or two single acting rams.
I could place these two rams transversely (like the double acting
ram) but I *could* consider an arrangement that places each ram
connecting to the tiller at 36 degrees aft of the transverse
direction.
Then, at either hard over position of the tiller, the ram is acting at
90 degrees to the tiller, the most advatageous position.
But with moderate hydrodynamic balancing of the rudder surface to
moderate the steering forces is this such a problem?

Brian W

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Glenn Ashmore" wrote in message
news:vItDb.5348$JD6.3954@lakeread04...
I am not visualizing what you are either. In your original post you
were talking about a transom mounted rudder and hydraulic steering. To
me that usually means a power boat with the hydraulics mounted on the
transom. Maybe you should explain exactly what you have and what you
are trying to do. If this is a sailboat with a tiller why are you going
through the hassle of hydraulic steering?


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.

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 do have an emergency tiller that fits on the top of the rudder stock, but
for long journeys, or heavy weather the tiller is way too much work.

regards
garry

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.

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.
--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"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