rudder alignment Q

[vas]

hello all,

Back on the boat, doing various small jobs getting ready for the delivery trip hopefully next w/e. Seriously amazed at the level of incompetence of the various ppl worked on her over the years.

Last time I noticed that the starboard rudder had a play of around 5degrees as the lever (dunno if it has a special name) that the hydraulic steering pump activates upon was not firmly secured (bolt was tight, but diameters didn't quite match). Someone had wedged a 5 3.5mm wood screws in between the lever and the axle of the rudder to secure it (unsucessfully...)
I noticed that next to this mess was a thin copper sleeve that these guys missed in assembly (as the port one had the sleeve and no play). Anyway, half an hour of swearing later, it was all nice and tight.
However, I noticed that the two rudders are not quite parallel to each other... I guess they are meant to be parallel it's not like car steering wheels and toe-in job, right? Easy to sort, just want to be sure!

Second question, on aligning the rudder pickup for the autopilot (that was around 15deg off...) I realised that I have a problem figuring out where is straight so I can align.
Rudders are around 5-6cm off centre to the shafts. Do I take the shafts direction and align, do I try to get them parallel to the keel (which disappears a few metres ahead of the stern and wont be exceptionally easy.

I know that I can wait and when on sea, get myself going straight and regulate on the spot but I thought I'd sort it now if at all possible.

cheers

V.

 

[westernman]

No idea for mobos. But the rudders on a Hobie Cat are adjusted to have a slight toe in.

 

[Cheekybrat]

parallel rudders

As far as I am aware rudders on twin engined shaft drive boats are slightly offset since normally one prop rotates one way and the other is opposite (not forced to be right though)
Nick Heath

 

[MapisM]

(dunno if it has a special name)

It's the tiller you're talking about, as I understand.

I noticed that the two rudders are not quite parallel to each other... I guess they are meant to be parallel it's not like car steering wheels and toe-in job, right?

Yep, they're meant to be parallel.
I've only heard of toe-in/out installations (depending on whether the props are spinning in/out) with high power blocks and single prop outdrives, where the props while spinning tend to twist the drives outward/inward, hence "straightening" them while running.
And even then, that doesn't apply if the outdrives have (as with most really fast boats) a strong external hydraulic steering and are connected with a tie-bar.
I can't see a reason to have toe-in/out rudders with shafts.

Do I take the shafts direction and align, do I try to get them parallel to the keel

Well, that should be the same, hopefully...!

 

[MapisM]

As far as I am aware rudders on twin engined shaft drive boats are slightly offset since normally one prop rotates one way and the other is opposite

Yep, but the offset is a different matter altogether, and doesn't imply that the rudders aren't parallel.
And it can't be changed of course, whilst it's easy to sort the angle, as virtuvas said.

 

[vas]

It's the tiller you're talking about, as I understand.

thanks Mapis, that's the word.

Yep, they're meant to be parallel.
I've only heard of toe-in/out installations (depending on whether the props are spinning in/out) with high power blocks and single prop outdrives, where the props while spinning tend to twist the drives outward/inward, hence "straightening" them while running.
And even then, that doesn't apply if the outdrives have (as with most really fast boats) a strong external hydraulic steering and are connected with a tie-bar.
I can't see a reason to have toe-in/out rudders with shafts.


Well, that should be the same, hopefully...!

Thanks for clarifying, that's what I thought, but it's not quite easy to do accurately as small axial offsets here and there mess the whole thing. Plus the boat is on the hard, with a irregular hill 10m high on it's stern so not enough space to mark points on the ground at a distance and measure as I'd like to do. Well, I'll find a way surely.

Next issue is that AutoPilot compass when tilted and aligned to the bow-stern axis as it should shows 15degrees more than the magnetic compass. I wonder if my laptop with OpenCPN will confuse the hell out of the AP. No, there wasn't a chart plotter originally on the boat. We shall see, first will try to solve a wee leak on the tiny return (I guess) pipe from the steering pump, then check the ap config settings.

cheers

V.

 

[jfm]

It's normal on fast planing motorboats for the rudders to have slight toe-in.

As for centering them with the a/pilot, just get it as close as you can by sight. Then you do the centering at sea, becuase it's a soft setting inside the a/pilot electronics.

 

[jfm]

AutoPilot compass when tilted and aligned to the bow-stern axis as it should shows 15degrees more than the magnetic compass.

You should set the control head's reading of the a/pilot compass by doing the set up routine as speciaifed by the a/pilot manufacturer. Normally involves turning the boat 360deg in the water, a few times. If the readings still differ, you have deviation on your helm compass. This might be partially curable with magnets but you will likely still need a deviation card

You refer to only the helm compass as "magnetic". Please note, BOTH compasses are magnetic. The electronic fluxgate compass for the a/pilot still works on magentism and senses the earth's magentic field. Therefore both compasses can therefore be affected by metal objects

 

[vas]

You should set the control head's reading of the a/pilot compass by doing the set up routine as speciaifed by the a/pilot manufacturer. Normally involves turning the boat 360deg in the water, a few times. If the readings still differ, you have deviation on your helm compass. This might be partially curable with magnets but you will likely still need a deviation card

You refer to only the helm compass as "magnetic". Please note, BOTH compasses are magnetic. The electronic fluxgate compass for the a/pilot still works on magentism and senses the earth's magentic field. Therefore both compasses can therefore be affected by metal objects

thanks jfm,

I understand that both are magnetic but I needed a way to differentiate hence the wrong term.
AP compass was literaly dumped in a self, next to lots of metal (as in the aircon unit), wasn't even bolted down securely. ATM and for testing I've brought it up to the helm (cable was long enough and no mods were needed) and I have it wedged with a pair of alloy binoculars on a spot infront of the navigation instruments. Will try to secure it better, but even holding it on the air far away from anything with my right hand (no watch involved) didn't change. Will try to compare my phone's gps (which I doubt is v.accurate anyway) and the f/b compass to this lot and if not, wait for the sea test.

thanks!

 

[Deleted User YDKXO]

thanks jfm,

I understand that both are magnetic but I needed a way to differentiate hence the wrong term.
AP compass was literaly dumped in a self, next to lots of metal (as in the aircon unit), wasn't even bolted down securely. ATM and for testing I've brought it up to the helm (cable was long enough and no mods were needed) and I have it wedged with a pair of alloy binoculars on a spot infront of the navigation instruments. Will try to secure it better, but even holding it on the air far away from anything with my right hand (no watch involved) didn't change. Will try to compare my phone's gps (which I doubt is v.accurate anyway) and the f/b compass to this lot and if not, wait for the sea test.

thanks!

The AP compass should be as far away from anything metal as possible (and that includes beer cans!). I'm not sure that wedging it in front of the nav instruments will help either. Generally the compass is located in the bilges or a locker well forward of the engine bay. As jfm says, the AP compass needs to be swung before you can know for sure that it is giving an accurate reading. Its quite an easy thing to do (just drive the boat in a slow circle) but you need to find the correct page in the AP menu first (which is the harder thing to do)

 

[MapisM]

It's normal on fast planing motorboats for the rudders to have slight toe-in.

Really? I've seen boats with surface transmissions, capable of 100+ kts, where I'm pretty sure that the rudders were perfectly parallel, fwiw...
What's the rationale behind that, anyway?

 

[jfm]

Really? I've seen boats with surface transmissions, capable of 100+ kts, where I'm pretty sure that the rudders were perfectly parallel, fwiw...
What's the rationale behind that, anyway?

Yup really. I have no idea what the rationale is tbh, but all the major builders talk about setting the rudders with a little bit of toe in. I don't even know how they measure it. On a 100mph boat I would however expect there to be zero or ~0.01degress of toe in

 

[jfm]

Have you completed the purchase of the boat V? In which case, congratualtions! Sorry if I missed the thread on this - the last I read (grey tanks), you had not completed yet.

 

[Deleted User YDKXO]

Really? I've seen boats with surface transmissions, capable of 100+ kts, where I'm pretty sure that the rudders were perfectly parallel, fwiw...
What's the rationale behind that, anyway?

Yes I understand that rudders on planing boats are generally toed in slightly. AFAIK, it's so that the rudder is always loaded from one side avoiding the oscillation which might occur if the rudders were perfectly parallel and which could cause premature wear in the rudder linkages.

 

[spannerman]

Yout AP compass should be low down near the boats center of roll so its not affect by movement so much and definitely not near the instrument panel as there are magnetic fields from things like chartplotters, power supplies, wiper moters etc.

 

[Latestarter1]

Yup really. I have no idea what the rationale is tbh, but all the major builders talk about setting the rudders with a little bit of toe in. I don't even know how they measure it. On a 100mph boat I would however expect there to be zero or ~0.01degress of toe in

One builder I go out with swears by the 'let the boat tell you' method during sea trials.........Takes the pin out on one rudder clevis once under way. So boat was steering on one rudder and second rudder lined up based on hydro dynamic forces. While going straight ahead at desired cruise speed, adjusts the rudder cross tie link so pin can be dropped in. Locks everything down, job done. Just REMEMBER do not forget and go astern with that pin out. Getting rudder tangled in the prop on brand new boat can spoil your day!

When vessel is hauled dynamic method of rudder setting generally results in 1 inch to 1/2 inch of toe in. So called identical vessels can have different toe in charteristics

 

[MapisM]

One builder I go out with swears by the 'let the boat tell you' method during sea trials.........

Now, that makes much more sense than a generic "toe in" principle.
Though I suspect that it's just a method to fine tune the same effect that the offset is supposed to cure. I mean, if the offset would be perfectly calibrated for a specific hull/transmission/prop/optimal speed, why the result of the empirical test you're talking about shouldn't be that the rudder stays parallel to the shaft?

 

[Latestarter1]

Now, that makes much more sense than a generic "toe in" principle.
Though I suspect that it's just a method to fine tune the same effect that the offset is supposed to cure. I mean, if the offset would be perfectly calibrated for a specific hull/transmission/prop/optimal speed, why the result of the empirical test you're talking about shouldn't be that the rudder stays parallel to the shaft?

No reason at all....Seen all this before, can degenerate into which way water turns when it goes down plug hole in Northern Hemisphere aguement.

Alan Burnard ex Fairey Marine designer is a small amount of toe in man for two reasons.

#1 In a turn the outboard rudder scribes a larger turning radius, therefore vessel turns sharper.
#2 Mechanical steering systems with feedback boat boat tracks straighter with helm amidships.

Personally I am a 'let the boat decide' whatever it turns out to be. At least it should feel right.

By the way I KNOW that water goes down the plugole clockwise North of the Equator!

 

[Neil_Y]

One of our past suppliers (who had built many fast boats in his time) said toe in requirement was dependant on speed so a variable toe in linked to speed was the best solution. The faster it goes the more the rudders toe in.

In practice I'd go for parrallel and see how the boat behaves.

 

[jfm]

When vessel is hauled dynamic method of rudder setting generally results in 1 inch to 1/2 inch of toe in. So called identical vessels can have different toe in charteristics

Ah yes thanks Latestarter, you've reminded me. The water flow isn't purely axial along the hull, becuase the V of the hull splays the water out sideways a bit. Much more at 20kts than 100mph of course. So to have the rudders parallel with the water flow, ie minimum drag, you need toe in. On my stabs, we set them so they find their own centre just as you describe, then lock that position in the stab controller memory as "centre" , and when we crane the boat out there is very visible toe-in on the stabs