Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...
If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.


Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For
example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.

I kinda thought that it would just be as simple as divide by 2...



thanks again
garry

Sizing Hydraulic Rams
 

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod. For
example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.

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



--
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

Sizing Hydraulic Rams
 

The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.

Keep in mind though that if you use two cylinders you need to double the
capacity of the pump to get the same number of hard over turns.

garry crothers wrote:

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.



Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For

example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.


I kinda thought that it would just be as simple as divide by 2...



thanks again
garry



--
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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...
If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod. For
example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.

Unless the rod comes out at both ends.....

Meindert

Sizing Hydraulic Rams
 

"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

Sizing Hydraulic Rams
 

"Meindert Sprang" wrote in message
...
"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple
on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry

Sizing Hydraulic Rams
 

Doing a really rough vector analysis it appears that with the rudder
within 15º of midships there is very little difference in the turning
force between a single and a double cylinder setup but because one
cylinder is pushing out while the other is pulling in, the pressure on
the pintle is somewhat less. Not sure it is enough to justify a second
cylinder with the extra cost, hose and fittings though.

Not much to be gained in redundancy either. The most likely point of
failure is not the cylinders but the pump, hose and fittings.

garry crothers wrote:

"Meindert Sprang" wrote in message
...

"garry crothers" wrote in message
...

Excuse my ignorance, but how can I tell if the cylinder is double action

or

single. I've just looked at the Vetus catalogue and cant see any

reference

to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple

on

each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert


I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry



--
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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:QgkDb.4899$JD6.4249@lakeread04...
Doing a really rough vector analysis it appears that with the rudder
within 15º of midships there is very little difference in the turning
force between a single and a double cylinder setup but because one
cylinder is pushing out while the other is pulling in, the pressure on
the pintle is somewhat less. Not sure it is enough to justify a second
cylinder with the extra cost, hose and fittings though.

Not much to be gained in redundancy either. The most likely point of
failure is not the cylinders but the pump, hose and fittings.



Well, it would certainly make life a lot simpler if you reckon I could get
away with it.
I dont know much about hydraulics, hence the question, I will speak to
some of the suppliers for further opinions.

thanks again for your input

garry

Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

On Mon, 15 Dec 2003 08:57:02 -0500, Glenn Ashmore
wrote:

The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.

No. But your previous answer was good, so I am puzzled!

Brian W

Keep in mind though that if you use two cylinders you need to double the
capacity of the pump to get the same number of hard over turns.

garry crothers wrote:

"Glenn Ashmore" wrote in message
news:rJiDb.4799$JD6.1854@lakeread04...

If the cylinders are single action each needs to take the full load. If
double acting it gets more complicated both in calculation and
installing. The pulling force is reduced by the area of the rod.



Excuse my ignorance, but how can I tell if the cylinder is double action or
single. I've just looked at the Vetus catalogue and cant see any reference
to single or double?


For

example a 2" cylinder with a 1" rod will have a pull force about 75% of
the push so you can't quite divide by 2.


I kinda thought that it would just be as simple as divide by 2...



thanks again
garry

Sizing Hydraulic Rams
 

On Mon, 15 Dec 2003 14:24:24 -0000, "garry crothers"
wrote:


"Meindert Sprang" wrote in message
...
"garry crothers" wrote in message
...
Excuse my ignorance, but how can I tell if the cylinder is double action
or
single. I've just looked at the Vetus catalogue and cant see any
reference
to single or double?

I think all Vetus cylinders are double action. If you see one hose nipple
on
each end of the cylinder, they definately are.

Besides, I fail to see how two rams can prevent any twist anywhere in the
rudder system.

Meindert

I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

But then again I am no expert,

garry

Quite! :-) (no offence)

Brian W

Sizing Hydraulic Rams
 

Brian Whatcott wrote:

On Mon, 15 Dec 2003 08:57:02 -0500, Glenn Ashmore
wrote:


The Vettus system is a double acting "balanced" cylinder. Balanced
cylinders have the rod going all the way through so the pressure is the
same in both directions. In that case you can divide by 2.


No. But your previous answer was good, so I am puzzled!

My original post assumed a standard double acting cylinder. These have
the rod attached to one side of the piston. In these, the displacement
on the rod side is reduced by the volume of the rod and the pressure
face of the piston is reduced by the cross sectional area of the piston.
That means you get less force on the pull stroke than on the push for
the same fluid pressure. Also you get more movement for the same volume
of fluid. That makes them hard to use for steering unless they are used
in opposing pairs like on heavy earth moving equipment.

A balanced or "steering" cylinder has the rod continue through the
piston and out the other end of the cylinder. This end of the rod
usually carries no force but makes the displacement and pressure face
the same on the two sides so the force is the same in both directions.

The down side is that you have two sets of seals that will leak someday
rather than just one.


--
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

Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

"garry crothers" wrote in message
...

I believe that the advantage with two rams is that, because one cylinder
pushes aft, while the other pulls forward with the same force, the only
force on the rudder stock is the desired twisting, and no side loading is
imposed, as the case would be with the use of a single cylinder.

Ok, you're right. But my gut feeling tells me that this side load is
relatively small, especially compared to the side load induced by the water
pressure against the rudder.

Meindert

Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

"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

Sizing Hydraulic Rams
 

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

Sizing Hydraulic Rams
 

"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

Sizing Hydraulic Rams
 

I was unwise to attempt to help on the basis of too little
information. I hope that Glenn can visualize this
arrangement better.
But better yet would be a diagram to put in front of a person
familar with hydraulic steering layouts, I'd think.

Brian W

On Tue, 16 Dec 2003 14:31:19 -0000, "garry crothers"
wrote:


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.
///
garry

Sizing Hydraulic Rams
 

At this point I am kind of lost myself. Steering rams are rated for
operating more or less perpindicular to a tiller arm off the pivot
point. Alternately they are mounted off the transom at an angle as with
an outdrive.

What I think Garry is talking about is mounting the rams off the stern
parallel to the 7.5 sq.ft. rudder to brackets extending about 8" off
either side of the rudder. To me that seems to be a very odd
configuration that can't be calculated using the normal vendor supplied
formulas.

Brian Whatcott wrote:
I was unwise to attempt to help on the basis of too little
information. I hope that Glenn can visualize this
arrangement better.
But better yet would be a diagram to put in front of a person
familar with hydraulic steering layouts, I'd think.

Brian W

On Tue, 16 Dec 2003 14:31:19 -0000, "garry crothers"
wrote:



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.

///

garry




--
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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:ufQDb.5545$JD6.4631@lakeread04...
At this point I am kind of lost myself. Steering rams are rated for
operating more or less perpindicular to a tiller arm off the pivot
point. Alternately they are mounted off the transom at an angle as with
an outdrive.

What I think Garry is talking about is mounting the rams off the stern
parallel to the 7.5 sq.ft. rudder to brackets extending about 8" off
either side of the rudder. To me that seems to be a very odd
configuration that can't be calculated using the normal vendor supplied
formulas.

Say you have a bracket extending 8" off either side, and this extension is
mounted, say 8" off from the pivot point of the rudder. Then the mounting
point of the ram om this bracket is at the same point as if you would have
two tillers of 11.3" (pythagoras), each at an angle of 45 degrees from the
rudder. If you mount the each ram perpendicular to this 'virtual' tiller,
wouldn't that create the same setup as one tiller and two rams parallel to
the 'transom' (which he doesn't have)?

Meindert

Sizing Hydraulic Rams
 

Meindert Sprang wrote:
"Glenn Ashmore" wrote in message
news:ufQDb.5545$JD6.4631@lakeread04...

At this point I am kind of lost myself. Steering rams are rated for
operating more or less perpindicular to a tiller arm off the pivot
point. Alternately they are mounted off the transom at an angle as with
an outdrive.

What I think Garry is talking about is mounting the rams off the stern
parallel to the 7.5 sq.ft. rudder to brackets extending about 8" off
either side of the rudder. To me that seems to be a very odd
configuration that can't be calculated using the normal vendor supplied
formulas.


Say you have a bracket extending 8" off either side, and this extension is
mounted, say 8" off from the pivot point of the rudder. Then the mounting
point of the ram om this bracket is at the same point as if you would have
two tillers of 11.3" (pythagoras), each at an angle of 45 degrees from the
rudder. If you mount the each ram perpendicular to this 'virtual' tiller,
wouldn't that create the same setup as one tiller and two rams parallel to
the 'transom' (which he doesn't have)?

Meindert



That makes sense but he wants to mount the rams parallel to the rudder
which, if the setup is roughly 200mm square will set them at about 45º
to the "virtual tiller" in the center position. That arangement will
reduce the effective ram force by about 1/3. To complicate matters, the
Vetus MTC72 is 487mm long in the center position so the ram pivot has to
be mounted 287mm forward of the pintle. The geometry becomes a trapezoid
rather than a parallelogram and puts the action out of balance. A quick
Autocad sketch indicates 99 mm out and 140 mm in. He would have to use
two rams or the rudder would turn one way faster than the other.

--
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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:isYDb.5581$JD6.2101@lakeread04...
That makes sense but he wants to mount the rams parallel to the rudder
which, if the setup is roughly 200mm square will set them at about 45º
to the "virtual tiller" in the center position. That arangement will
reduce the effective ram force by about 1/3. To complicate matters, the
Vetus MTC72 is 487mm long in the center position so the ram pivot has to
be mounted 287mm forward of the pintle. The geometry becomes a trapezoid
rather than a parallelogram and puts the action out of balance. A quick
Autocad sketch indicates 99 mm out and 140 mm in. He would have to use
two rams or the rudder would turn one way faster than the other.

Indeed!

Meindert

Sizing Hydraulic Rams
 

garry crothers wrote:


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.


Took me a little while to figure this out. Being over 50 I have trouble
working with all these newfangled French measurements. I work in feet
knots and pounds. To me KmH is that other dial on the speedometer and a
Newton is a cookie with figs in the middle. :-)

Other than the physical arangement of the rams I see a problem with your
figures.

To get the torque at the pintle you have to divide the total force on
the rudder by the distance from the pivot point to the center of force
on the rudder, not the length of the tiller arm. The ceenter of
pressure is usually about 1/3 to 2/5 of the width of the rudder.
Without knowing the shape of the rudder it is impossible to know what
that this is but it is bound to be more than 20cm.

Once you get the required torque you can divide by the tiller arm to get
the force required to turn the rudder hard over. Normally you don't
have to do this because the vendors give the turning force based on a
particular length of tiller but you are going outside the norm so you
need to carry it further.

The MTC72 delivers 434 ft. lb on a 7.72" tiller. That works out to
about 675 pounds of force. As Meindert pointed out with an 8" bracket
you will have a "virtual tiller" length of about 11.3". That will
require more stroke than the MTC72 has so you will need to shorten the
brackets to about 7" (180mm) to stay in the stroke range. That will
give you an effective tiller arm of about 9.9". (250mm) Applying 675
pounds force at 90º to a tiller .825 ft. long produces a torque of 556
ft.lb. (76KgM)

That is the SIMPLE part. Steering rams and tillers convert linear
motion to circular motion. Exccept when the ram is perpendicular to the
tiller some portion of the force goes into compression or tension on the
tiller arm itself. The amount of linear force converted to circular
force varies as the Sine of the angle between the tiller and the ram.
In a "normal" arangement with the ram perpindicular to the tiller in the
neutral position, 82% of the force goes to turning at 35º hard over.
You are starting out at 45º so the turning force is only 71%. At hard
over away from the ram the angle is about 15º and the turning force is
about 26%. Conversely hard over towards the ram the angle is about 105º
and the turning force is about 96%.

With two rams working oposite each other you will get about 60% of the
total force converted to torque. SOOOOOO, two rams developing 76KgM
each times 60% works out to about 91 KgM. If your rudder's center of
pressure actually is close to 20cm from the pintle you have lucked up.

OTOH, you might wonder where the other 40% of the force went. It is
pulling against your pintle so you might better give the whole idea a
bit more thought.
--
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

Sizing Hydraulic Rams
 

"Glenn Ashmore" wrote in message
news:Uu%Db.5870$JD6.4178@lakeread04...


garry crothers wrote:


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.


Took me a little while to figure this out. Being over 50 I have trouble
working with all these newfangled French measurements. I work in feet
knots and pounds. To me KmH is that other dial on the speedometer and a
Newton is a cookie with figs in the middle. :-)

Other than the physical arangement of the rams I see a problem with your
figures.

To get the torque at the pintle you have to divide the total force on
the rudder by the distance from the pivot point to the center of force
on the rudder, not the length of the tiller arm.



AAhhh!!!
I new I was missing something in that calculation

snip

OTOH, you might wonder where the other 40% of the force went. It is
pulling against your pintle so you might better give the whole idea a
bit more thought.
--
Glenn Ashmore



o.k
I'm convinced, I'll lose the 2nd ram idea, and go back to the drawing
board and redo the figures.

Thanks for all your help Glenn

garry