steering puzzle

[wfleenor]

Embarrassed to admit I have 3 engineering degrees and have been around diesel engines for over 60 years, as well as boats of all sizes for nearly 50 years. I have a 48ft custom power catamaran with the props 16ft apart. I rarely use the rudders for anything under cruising speed. Before leaving the dock I always physically check that rudders are dead straight. This week I motored out of dock area with engines in and out of idle with no issue and when I got out into the channel I added starboard throttle to turn to port. The boat turned to starboard. I dropped back to idle, and in forward and reverse and all was normal. Added starboard throttle again and the boat turned to starboard. In trying the port throttle, the boat turned to starboard as expected. Short of dragging an anchor off the starboard aft corner, I am at a loss to even understand how this can occur. Any thoughts? Thanks.

 

[BruceK]

Starboard engine in reverse, not fwd?

 

[toocoys]

How bad was your current?

When entering our channel and going against a full speed tide movement, my boat would do all sorts of weird things. When we would leave from our marina basin, and enter the channel, on more than one occasion I would have to put starboard in forward, and port in reverse because the water was coming in perpendicular to the boat hitting it on the port beam. Unless you stood up on the throttle, it would push your bow in any direction it felt like.

 

[psneeld]

Rudders straight?

This is a common issue with twin outboards or outdrives if they are turned when people are just using throttle to turn but the units are turned the opposite way.

It usually isnt a problem with trwin inboards but on a wide cat maybe? I have no experience with them.

Other possibility...what is the drive train? Conventional shafts and props? If the prop where power was applied started to slip when throttling up ( for some reason tranny or prop)....you might have a thrust situation to turn the other way.

 

[twistedtree]

I’d take a close look at you gear/throttle controls. Cable ends come loose all the time causing shift problem. As BruceK said, it would explain everything if your stbd engine was actually in rev rather than fed.

 

[Sailor of Fortune]

Index the rudders first. Verify that the rudders are amidship by looking at the rudder posts in each hull. On a conventional mono hull, there would be a drag link connecting both tiller arms together. Check each of yours separately. Once you verify its not a rudder/steering issue, check connections at gear box and under control stations. I doubt its a current or wind issue. Its aboard the boat

 

[DDW]

Often a cat will have hydraulic steering, and it's easy for the rudders to get out of alignment. That wold be the first thing I would check.

 

[Shrew]

Often a cat will have hydraulic steering, and it's easy for the rudders to get out of alignment. That wold be the first thing I would check.

This was my thought as well. Since there is no linkage between rudders, if one were to introduce air into the system, the rudders could be out of alignment. if the Stbd rudder is turned to stbd then stbd throttle would turn the boat (slowly) to stbd.

I'd try to turn the helm to Port and then put the stbd engine fwd to see if I'm still turning to stbd. This might give you a feel for how pervasive the problem is. In reverse you'd never notice the misaligned rudders.

In fact, I'd probably turn the rudder to Stbd, then dive on it. Then turn to port and dive on it again.

 

[wfleenor]

I apparently wasn't clear. I always check the rudders (physically) because a few years ago I did have a sync problem in heavy seas. They were both straight. Motored out using port and starboard engines in idle forward and reverse. All operated normally. With both in forward idle, I added starboard forward throttle and the boat veered to starboard.

 

[BruceK]

I apparently wasn't clear. I always check the rudders (physically) because a few years ago I did have a sync problem in .....

You were clear, but are you infallible? Go out and see if you can reproduce it.

 

[Shrew]

I apparently wasn't clear. I always check the rudders (physically) because a few years ago I did have a sync problem in heavy seas. They were both straight. Motored out using port and starboard engines in idle forward and reverse. All operated normally. With both in forward idle, I added starboard forward throttle and the boat veered to starboard.

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.

Stbd engine (only) turning the boat stbd can only be one of three things:

1) helm hard over to Stbd
2) Stbd rudder not properly indexed
2) Prop turning in the wrong direction.

 

[O C Diver]

You were clear, but are you infallible? Go out and see if you can reproduce it.

I agree with the above.
Secondly, I've seen people push the wrong throttle and swear they were pushing the correct one.
Third, does your boat have an electronic engine synchronizer?

Ted

 

[BruceK]

wfleenor,you have to resolve this issue. In accident investigation terms,on your account, your boat made an "uncommanded turn". Next time there could be a kayak, a dock,another boat, a rock, etc, where it turns.

 

[TG]

Make sure the autopilot is not engaged.

 

[wfleenor]

Thanks BruceK, for your response. I was really hoping someone like you would simply say I cannot be correct about what I was stating. It simply cannot happen that way. However, I verified afterward that the props were synchronized and straight ahead by diving on them. The throttle sync was not engaged. Starboard prop was turning in the correct direction at idle. I could not confirm that it was at advanced throttle because I didn't have enough qualified assistance.

But the only logical solution I can produce is that the advanced starboard throttle actually advances the port prop even though it shows on the instruments as the starboard side.

As we always say in academia, more research is needed.

Bill

 

[BruceK]

But the only logical solution I can produce is that the advanced starboard throttle actually advances the port prop even though it shows on the instruments as the starboard side.

As we always say in academia, more research is needed.

Bill

I see the logic of that, but how? Could the throttle cables somehow interact or bind on each other, as they leave the helm? Your boat is a cat, I`m assuming one engine in each hull, well and truly separated.
Good luck with it. Maybe a one off that will never recur.

 

[O C Diver]

Thanks BruceK, for your response. I was really hoping someone like you would simply say I cannot be correct about what I was stating. It simply cannot happen that way. However, I verified afterward that the props were synchronized and straight ahead by diving on them. The throttle sync was not engaged. Starboard prop was turning in the correct direction at idle. I could not confirm that it was at advanced throttle because I didn't have enough qualified assistance.

But the only logical solution I can produce is that the advanced starboard throttle actually advances the port prop even though it shows on the instruments as the starboard side.

As we always say in academia, more research is needed.

Bill

I have pondered the possibilities of errant throttle advances with electronic throttle controls tied to a computer or synchronizer. The concept of several outboards computer linked to generate dynamic positioning stabilization, has me thinking, "What could possibly go wrong".

Ted

 

[Brooksie]

All good suggestions above. I would also consider, if this only happened in the channel the one time, that one hull was "feeling the bottom" and being drawn to that edge of the channel. More power would make it "feel" more.

 

[ProMaritime]

I have no engineering degrees and still can't come up with a solution to the problem. Just kidding, you've got some good ideas to check out from some of the other posts.

 

[CapnCrunch]

How bout this: Your starboard flex-plate (or clutch) or transmission is slipping. Hence, when you apply more throttle, you actually get fewer rotations out of that prop which allows the port engine to overpower the starboard and push the boat in that direction.

This is similar to what happens when you "spin the hub" of an outboard prop - the boat will move normally at idle speed but the more gas you give it, the slower it goes.

 

[Konfla]

I would guess that because your hulls are asymmetrical, the boat could be pushed to starboard at lower speeds by water flowing faster on one side.Possibly combined with wind or current. To test try increase throttle and see if the power overcomes.