Hydraulic Steering in Multihulls

[44'cruisingcat]

I've been trying to pin down exactly what my requirements are for a hydraulic system. The steering manufacturers, IMHO, seem to be overspecifying by incredible amounts.

Hydrive reckon I will need a system capable of exerting 300kg/m of torque for my boat! Frankly I think this is ridiculous.

For one thing, tillers are an option on my boat (the tillers would be under 2m long) how on Earth would anyone exert 300kg/m of torque on tillers? I think a maximum of around 30kg/m is more realistic.

There's a similar size, weight and speed cat called "McMoggy" which is tiller steered, and reportedly the steering effort is very light, barely a kilo or two on the tillers, which are also less than 2m long.

I suspect the steering manufacturers are covering their backsides, and also think they are basing a lot of their calculations on monohulls, which do tend to have higher steering loads.

Any thoughts?

[mikereed100]

44,

I too will be looking at hydraulics soon so this interests me. If I remember from physics, torque = force x length of lever arm. If my math is correct, and I have no reason to believe that it is, then, using Hydrive's guidelines, a 2m tiller would be expected to exert 150kg of force. A 30cm tiller arm, as I have on my boat could expect 1000kg of force. I agree with you that that seems excessive. Your buddies boat, with 2kg on a 2m tiller would be equivalent to 13.3kg on a 30cm arm. Granted, there will be times when the boat is harder to steer but still...I will be interested to see what the more knowledgeable have to say. Good topic.

Mike

[drewke07]

It is when going backwards that the load grows quickly. The rudder is no longer balanced, or semi balanced. Once the hydraulic system is grossly overloaded it entirely fails and you are in trouble.

The other problem with hydraulic steering with cats is trying th keep the rudders lined up. Tiny amounts of fluid bypassing the seals in the rams will get the rudders out of line quickly.

[44'cruisingcat]

Really, hydraulic steering is just a different method of gaining mechanical advantage. It's just a different kind of reduction gear.

There's a mechanically steered Schionning, "Cheetah" which is very similar in spec to my boat, and he has just one turn from lock to lock. Steering effort is very acceptable according to the owner.

I see no reason why I couldn't have a hydraulic system with around two turns lock to lock. The system Hydrive are specifying, apart from weighing a ton, would give me between 5 and 7 turns L/L, which is far too slow.

Dave (Catmando) put me on to Ultraflex, who seem to be a bit more reasonable with their steering torque expectations. prices seem reasonable too.

[ssullivan]

Originally Posted by drewke07

It is when going backwards that the load grows quickly. The rudder is no longer balanced, or semi balanced. Once the hydraulic system is grossly overloaded it entirely fails and you are in trouble.

The other problem with hydraulic steering with cats is trying th keep the rudders lined up. Tiny amounts of fluid bypassing the seals in the rams will get the rudders out of line quickly.

I don't know about the rudders coming out of alignment...

I've put something like 2400 miles on my cat since I got her a few months back. Nothing like that has happened to my hydraulic steering.

Not saying it doesn't, but maybe it isn't a common problem - or I'm lucky....

PS: 44'Cat - I have 5 turns, lock to lock on my system, if that's of any help. Can get you some specifics to see what a smaller cruising cat has, if it helps.

[44'cruisingcat]

Originally Posted by drewke07

It is when going backwards that the load grows quickly. The rudder is no longer balanced, or semi balanced. Once the hydraulic system is grossly overloaded it entirely fails and you are in trouble.

The other problem with hydraulic steering with cats is trying th keep the rudders lined up. Tiny amounts of fluid bypassing the seals in the rams will get the rudders out of line quickly.

You can fit a bypass round one cylinder (or both) to re-align the rudders. The method is to motor along in a straight line, and just open the bypass valve. The rudder is straightened by the water flow. This shouldn't need to be done all that often.

I understand that loads could potentially be greater when going backwards, but will they be in reality? I mean I could very likely be seeing 20+ knots forwards, certainly high teens will be regular. I doubt if I will ever be going backwards at those kinds of speeds. (I hope) Also, most hydraulic steering pumps have a pressure relief valve fitted.

[44'cruisingcat]

Originally Posted by ssullivan

I don't know about the rudders coming out of alignment...

I've put something like 2400 miles on my cat since I got her a few months back. Nothing like that has happened to my hydraulic steering.

Not saying it doesn't, but maybe it isn't a common problem - or I'm lucky....

PS: 44'Cat - I have 5 turns, lock to lock on my system, if that's of any help. Can get you some specifics to see what a smaller cruising cat has, if it helps.

G'day Sean, thanks for the input. How do you find 5 turn L/L? I'm hoping to get somewhere between 2 and 3 turns. (preferably closer to 2) Your steering must be very light with that kind of reduction.

[ssullivan]

Originally Posted by 44'cruisingcat

G'day Sean, thanks for the input. How do you find 5 turn L/L? I'm hoping to get somewhere between 2 and 3 turns. (preferably closer to 2) Your steering must be very light with that kind of reduction.

Considering I displace just 6 tons (with no gear), the steering is extremely light. The only time it gets stiffer is when she's backing down in reverse and you turn the helm.

Still, very light - like a luxury car with power steering!

But, you can feel the rudders through even this system. You can feel if they are well centered or not by a very slight pressure back on your steering.

I would say there is nothing wrong with having an extra turn or two. Keeps fatigue down. This boat is a dream for making passages... very easy on you. Might be something to consider....

[44'cruisingcat]

Thanks Sean. Food for thought. It's good to know you still get some feel. I expect on passage the autopilot will be doing almost all the work, the hand steering will be mainly for when I'm "just out for a sail" or maybe if I enter the odd race. Which is why I'd like it to be responsive.

I'm probably going to fit a third helm station inside, which would be mostly for if the A/P has failed and the weather is crappy enough that you'd rather steer from inside. I could install a smaller volume pump there, for lighter/slower steering, to reduce fatigue, as you suggest.

[cat man do]

I also had a bit of mild panic when doing my steering, especially as it is truly a one off so no other to refer to.

In you're instance 44cc, what does Bob say? There would be others that have been done before.

I spoke to Rob New at home - marine castings & manufacturers as he has done shafts and head stock for mine and we compared my ruddershape to what he sells as stock and then to what he supplies for most of the other Brisbane cat manufacturers.

He felt it amusing when I told him what I had got as he thought it was overkill.


As far as extra load when backing down, well I always planned on centering the rudders and using the engines which are 22 ft apart.


Dave

[billyehh]

My experience with hydraulic steering is that it is not responsive. On my PDQ 32 and on various charters that had hydraulics there was no feedback of weather or lee helm. To my pleasant surprise, the mechanical cable steering in my Voyage 38 cat has almost the same feedback as a monohull.

[sailrjim]

Originally Posted by billyehh

My experience with hydraulic steering is that it is not responsive. On my PDQ 32 and on various charters that had hydraulics there was no feedback of weather or lee helm. To my pleasant surprise, the mechanical cable steering in my Voyage 38 cat has almost the same feedback as a monohull.

Billyehh,

What material is your steering cable? metal?, synthetic fibre?

44' cruisin cat,

As has been mentioned, the reason the rudders see greater load when backing is that the moment arm about the stock is much greater as the force balance is reversed.

As for the design of the steering system, would it not make sense to design for failure of the least expensive component? Which would you have break first, rudder/stock, or hydraulic system? And besides, why not design the hydraulics to incorporate a relief valve so the system never sees force near the breaking point of the rudders (or whatever component of the system you may choose)?

[Evan]

I disagree that the highest loads are seen in reverse. The forces are proportional to Velocity squared. Given that a typical cat might see speeds of 15 knots in the ahead condition, that's the sort of speed you have to design to.

Yes, in reverse, the Center of pressure might be twice as far back from the rudder stock, but the much lower astern speeds more than make up for it.

Yes, the suggested hydraulic system is over the top.

I have a 40' cat with both tillers and a rope drum / wheel setup that connects to the tillers. The tillers are 4' long or so, and I have steered with just tillers, beam reaching at 15 knots with full sail in 25 knots of wind, windward hull lifting up about 1/3 - 1/2. Very little helm loads where I would have expected tons on a monohull.

The rope drum/wheel gives an additional 3:1 or 4:1 mechanical advantage, and there is never any appreciable helm load on the wheel. About 3-1/2 turns.

Hydraulic systems on cats can get out of alignment, but if the seals are in good condition, it can take many months before re-alignment is required.

[fastcat435]

On our cats we advice to check the alignment once every month un less lots of sailing is done and realigning the rudders takes 5 seconds , we have made a open circuit valve and when moving forward at 8 knots plus under power the rudders align themselves when the valve is opened for a couple of seconds

Greetings
Gideon

[ssullivan]

Originally Posted by Evan

I disagree that the highest loads are seen in reverse. The forces are proportional to Velocity squared. Given that a typical cat might see speeds of 15 knots in the ahead condition, that's the sort of speed you have to design to.

Yes, in reverse, the Center of pressure might be twice as far back from the rudder stock, but the much lower astern speeds more than make up for it.

Try it out. Put her in reverse with the engines at 3/4 max rpm. Go hard over to port or starboard. Compare this pressure you feel at the helm to the pressure you feel moving forward. Should be greater...


but may depend on rudder configuration. Mine are "barn door" type hung from skegs like a cruising mono would have. Well, come to think of it, every mono I've owned with balanced spade rudders behaved the same way with any speed on in reverse, so not sure how your boat doesn't.