Loads on Yacht Rudder Steering Query

[savageseadog]

Anyone know what loads one might expect on the stock and hence other parts of a yacht steering system? I am considering a modern 38ft racing boat with wheel steering and was trying to work out the forces on the turning blocks. I've felt the load on the wheel but couldn't easily quantify it as the wheel is very large.

I assume the load on the stock would be expressed as a torque. The load is transferred via the cable and 90 degrees round the blocks to the wheel gear. Any input welcome please.

 

[scottie]

There is a formula to work out rudder loads which I no longer have but which should be able to be googled.
The area of the rudder and speed are as I remember the critical input speed being squared.

 

[westernman]

There is a formula to work out rudder loads which I no longer have but which should be able to be googled.
The area of the rudder and speed are as I remember the critical input speed being squared.

I suspect the heaviest load occurs when beating to windward (it certainly is in my boat) and much more related to the role it plays in preventing leeway. Of course a badly balanced sail plan can increase that further.

The load on a sailing boat due to its speed i would expect to be small for the kind of boats most of us are sailing.

 

[savageseadog]

I would expect the heaviest load to be when a yacht is about to broach?

 

[scottie]

When the torque necessary to manoeuvre a boat is not specified by naval architect nor
by the shipbuilders yard, it can be calculated as follows:-
It must be known that the torque necessary to manoeuvre a boat depends on:
• the speed of the water flowing on the surface of the rudder at a certain angle;
• the rudder size;
• the total sweep of rudder (& a part of the boat) when rudder shaft is not perpendicular;
• the compensating surface;
Torque calculation for a maximum speed about 25 knots:
T = S x [(0.4a) -b ] x V
2
x k
T = Torque in kgm
S = Total surface space of the rudder in m
2
(h x a)
h = Height of the rudder in m
a = Width of rudder in m
b = The value of the compensation in m
V = Speed of the boat in knots
k = Coefficient according to the total angle of rudder
- Port to starboard 50° k = 10.84
- Port to starboard 60° k = 13.01
- Port to starboard 70° k = 15.89
- Port to starboard 80° k = 17.80
- Port to starboard 90° k = 19.52
Straightening according type of boats:
- For boats equipped with jet engine T x 1.3
- For power boats with two engines and one rudder T x 0.5
- For sailboats T x 0.5
- For twin rudders T x 2
Example: Assume a rudder with a height of 0.5m(h), an overall width of 0.35m(a) and a
balance of 0.10(b). Maximum boat speed = 22 Knots. Port to starboard = 70°.
The maximum torque on the rudder blade is given by:
T = S x (0.4a-b) x V
2
x 15.89
T = (0.5 x 0.35) x ((0.4 x 0.35) - 0.10) x 22
2
x 15.89
T = 0.175 x 0.04 x 484 x 15.89
T = 53.9 Kgm Assuming twin rudders maximum steering torque = 107.8Kg

 

[LittleSister]

I can't remember the details, but some years ago there was a series of articles in one of the yacht mags (Yachting Monthly?) by someone in the trade, who was arguing that conventional formulae for rudder loads accounted for normal loads but overlooked the much higher loads imposed in some circumstances (maybe surfing down a wave, broaching or the boat being thrown bodily backwards in big seas, I can't remember). I seem to vaguely recall this was in response to a series of rudder failures in one or two makes of boat (Moodys, perhaps?).

 

[Clyde_Wanderer]

Anyone know what loads one might expect on the stock and hence other parts of a yacht steering system? I am considering a modern 38ft racing boat with wheel steering and was trying to work out the forces on the turning blocks. I've felt the load on the wheel but couldn't easily quantify it as the wheel is very large.

I assume the load on the stock would be expressed as a torque. The load is transferred via the cable and 90 degrees round the blocks to the wheel gear. Any input welcome please.

Dont know about a wheel steering, but on my boat its, One man with his ears cocked back and pulling like a donky, per 700mm of tiller.
C_W

 

[Plomong]

Anyone know what loads one might expect on the stock and hence other parts of a yacht steering system? I am considering a modern 38ft racing boat with wheel steering and was trying to work out the forces on the turning blocks. I've felt the load on the wheel but couldn't easily quantify it as the wheel is very large.

I assume the load on the stock would be expressed as a torque. The load is transferred via the cable and 90 degrees round the blocks to the wheel gear. Any input welcome please.

Ignore losses, for the moment, and assume there is no force multiplier in the mechanism between wheel and rudder quadrant (single whip or gearing mechanism).

Torque = pull on wheel circumference x radius of wheel.

A pull of 25 kg = approx 250 Newtons, is a very hefty pull. (Think: lift 25 kg of potatoes in a sack ????)

Pull on rudder cable = Torque / radius of cable roller at wheel.

Then that is the force in the cable at any turning block in the mechanism, ignoring friction and losses.

Torque at rudder quadrant is cable force x quadrant radius,

and

Net rotational component of force on rudder surface (hydrodynamic in origin) is:

Rudder Torque / perpendicular distance of net force application point to axis of rudder stock.

Note: I said "Rotational component of force on rudder stock", because other components of the total force on the rudder will not be transferred to the cable, but to rudder stock supporting structure.

Anyone see any errors or omissions in this ???

Plomong

 

[snowleopard]

Coming at it from a different direction, I have done a few miles on a classic 1960s 40 footer, ex-Admirals cup. It has a 4 ft dia wheel and 3/4 turn lock-to-lock. Broad reaching in a heavy breeze made it very hard work requiring a load I would estimate at 40kg. If you assume 10:1 difference between wheel rim and cog, that means around 400kg in the wire. I would at least double that for the about-to-broach scenario and think in terms of a SWL of at least 1 tonne and a braking load of 5 tonnes.

 

[snooks]

Looking at this from a totally different angle.....

A Garmin auto pilot for a 13,000kg yacht (which should be the yachts displacement +20%) has a maximum thrust of 400kg.

So find the right auto pilot, find out it's thrust, and that should be good enough.

No fancy formula I'm afraid, I like to think that Garmin would have done their sums

 

[savageseadog]

I can't remember the details, but some years ago there was a series of articles in one of the yacht mags (Yachting Monthly?) by someone in the trade, who was arguing that conventional formulae for rudder loads accounted for normal loads but overlooked the much higher loads imposed in some circumstances (maybe surfing down a wave, broaching or the boat being thrown bodily backwards in big seas, I can't remember). I seem to vaguely recall this was in response to a series of rudder failures in one or two makes of boat (Moodys, perhaps?).

Indeed , normal loads I would expect to be calculable to some degree of accuracy but the non normal loads less so. I would expect some kind of empirical formula from experience?

 

[charles_reed]

I would expect the heaviest load to be when a yacht is about to broach?

Agreed - that's when mine broke.

I suspect a tendency amongst naval architects to underestimate the loads on rudders - I've heard of a number of occasions when people have lost rudder, etc.

 

[maxi77]

A long time ago, some 30+ years I used to design steering gear for commercial vessels. In most cases the heaviest loads on the system are going astern, just try steering a tiller steered vessels astern and you will see why. Back then we used formulae based on Froude which were accepted by Lloyds without further explanation.

I am afraid I have long forgotten the formulae but they should be relatively easy to find. I would expect that it would be relatively easy to adapt them to give a figure for a broach.

For the OP if you calculate the torque on the rudder shaft then divide by the tillr arm length yiou will end up with the foce you need to apply to the tiller, which with a sensible safety factor will give you your answer

 

[Talbot]

Whilst I recognise that the loads on a rudder when the boat is broaching are indeed high, they can not compare with the loads experienced when going astern.

Actually often during a broach, the rudder has stalled, whch indicates little load. Also when sailing under normal conditions, the load on the rudder has as much to do with how well balanced is the sailplan.

 

[jimbaerselman]

Torque is one, bending is another

There are two forces to consider on the rudder stock. The first is torque, the second is bending moment.

With tillers, torque is obvious. Pull force at the hand multiplied by distance from the stock is the torque.

Torque at a steering wheel is related to torque at the rudder stock by a gearing. With a wire driving system, the gearing is the radius of the wheel driving cog compared to the radius the force is applied to the rudder stock. This may be 3:1, or whatever. In which case, stock torque is three times wheel torque.

In the simple steering case, torque is limited by what a helmsman can apply - about 25 newtons (think Kg, or 50lb give or take an inch). So wheel diameter multiplied by gearing multiplied by force gives the torque at the rudder stock.

However, reverse forces, where the rudder drives the steering wheel, and movement is constrained, are likely to create much bigger torques. Hence a need for rudder stops to prevent deflection beyond a certain angle - not always considered! These forces apply when launching stern first down a fast slip, or in limited circumstances in heavy seaways.

Now for bending moments. Bending moments apply to spade or short keg rudders where the centre of pressure on the rudder is well below the lowest support. A sort of springboard effect. The force depends on the square of water speed and the deflection of the rudder. With a balanced rudder (area in front of the rotational axis) it can be easy to apply a large angle at a high speed. But cruising sail boats aren't good at doing high speeds, unless they're surfing down a very big wave.

More critical is the case when flow is reversed. This may force the rudder to its stops - if it has them. You then have extreme deflection.

 

[ffiill]

Just suprised anyone bothers with links and cables-in the motoring world cables and solid links went out in the 1930s to be replaced by hydraulics-Spitfires were controled by hydraulics so what is wrong with a hydraulic system on a boat.
To put it in perspective my 80/20 sail/power 10 ton motor sailer steers with one finger and one quickly gets used to no rudder feed back as it has non return valves.You can set it on course and it quite happily sails a straight line hands off without its old Sharps hydraulic autopilot.
The hydraulics of the vetus pump are so simple as to be almost failsafe and in an emergency you just attach an emergency tiller to the rudder stock.

 

[HinewaisMan]

It's about feedback

Just suprised anyone bothers with links and cables-in the motoring world cables and solid links went out in the 1930s to be replaced by hydraulics-Spitfires were controled by hydraulics so what is wrong with a hydraulic system on a boat.
To put it in perspective my 80/20 sail/power 10 ton motor sailer steers with one finger and one quickly gets used to no rudder feed back as it has non return valves.You can set it on course and it quite happily sails a straight line hands off without its old Sharps hydraulic autopilot.
The hydraulics of the vetus pump are so simple as to be almost failsafe and in an emergency you just attach an emergency tiller to the rudder stock.

Our old Roberts Mauritius ketch had the original small, “Oh my God where are you going boat as we surf down this wave” rudder.

Joys of steel – built a new bigger rudder and re-hung it straight – huge difference. Works great – cured that problem.

Two new problems though

Firstly, the steering became uncomfortably heavy for SWMBO to steer.

Then one time going astern coming out of the pen, the loads were so great it “popped” the steering linkages the wrong way – jammed the steering, hello sandbar. Thank Heavens for 4lb hammers.

So we went hydraulic.

AND I HATE IT. It’s done huge service on the trip so far, but there is absolutely no feedback when sailing – and I so miss that.

So one job, albeit way down, on the list is to work out a heavy duty mechanical system and fit that.

 

[KellysEye]

I don't know where you intend to sail but in the Atlantic it's not unusual to get a ten to twelve foot swell from behind, a three to foot swell from a north Atlantic gale and a three to four foot swell from a south Atlantic gale. The boat corkscrews. I think at that point the calculations go out of the window which is why spade and small skeg rudders break fairly often.