Should torque steer trim tabs be set on new TomCats?

[drjohn71a]

Hey there...

I noticed very strong torque steering when using only the port or starboard engine on my new TomCat up in Seattle. I do not have access to the boat, and it was a rainy mess most of the time I was up there, so I am just guessing that the trim tabs on the engine legs were left straight and not set to eliminate torquing pressure from the props.

I also noticed that Brent and Dixie cruised for periods on a single engine and wonder if you'd have to correct for torque steering in order to do that. I wondered if the engines were more efficient as a set, each torque offsetting the other, if they were not corrected for torque steering.

Any info would be appreciated.

John

[tpbrady]

John,

Are you sure it is torque steer or just the one engine being so far from the center line. I forget which way the torque is going to push you (probably to port on a clockwise rotation), meaning if you are running the starboard engine, the boat would definitely pull to port. If you were on the port engine only, the position of the engine would tend to counteract the torque. I may have the directions confused but the problem sounds more like a thrust vector than a torque issue.

[drjohn71a]

Well, Tom, maybe it is just "prop walk", but it is stronger than I have felt in dual inboard installations. Maybe you are right in thinking that the wide spread between the outboards is exhagerating the turning effect.

I am thinking of trolling with one engine, maybe even slow cruising that way some, and wondered if you'd just have to correct for the strong turning effect by steering the other direction.

The effect is so strong that you can actually drive the boat, with engines steered straight and at idle, by alternatively engaging one or the other in the minimum forward position.

Last nite I was awake thinking just what you are now saying.... oh well.

btw, engaging only the port engine moves the stern to port and engaging only the starboard engine moves the stern to starboard.

Thanks , John

[Captains Cat]

[Doryman]

[drjohn71a]

Yes, the TomCats are definitely counter-rotating. Les told me the same thing... too expensive to carry dual rotating engine sets of three different manufacturers.

Seeing what you are saying makes me think it is mainly prop-walk. Maybe it is because the TomCat is relatively lighter than the dual prop boats I've been on before. I just thought the small diameter Honda props would walk less than the big bronze props on inboards.

Thanks for all your input, John

[Captains Cat]

[Wefings]

We wouldnt consider selling a Tomcat without counterrotating motors, and we mount them backwards! [ Port on Stbd side and vice versa] We learned this trick from other catamarans we sell . The just are more efficient that way . We even trained the factory to mount them that way for us when they rig one for us and I think they do it on all the boats now. It does not work on a monohull.
The torque tabs on the lower units of the motors will not affect your issue .Keep them close to straight and symmetrical as they just create drag that the engine "feels" . They are not real noticable with the hydraulic steering and they will not steer your boat.
First of all you need to trim the unused motor out of the water cause it creates a huge drag .
The boat itself being pushed from only one side creates a good bit of drag itself .Kinda like side tying to another boat ,and running , if you have ever done that . Thats what you are feeling . At low speed its not too bad and gets worse the faster you go . I hope this helps .
Marc

[Captains Cat]

I may stand corrected.... If they really are on that boat.

I thought they were very rare.... But then I thougt virgins were too....

Hard for a Naval Officer (even a former one) to admit too!!

Charlie

[mrw90]

I would guess that prop walk is part of what you are experiencing. I would also guess that the wide engine spacing is the greater part of the equation.

On one engine, as stated above, you will see improvement by lifting one engine out of the water to reduce drag. Ultimately, to go straight on one engine (in the case of trolling for example) you can compensate by making a slight opposite adjustment with the steering wheel.

[drjohn71a]

Thanks Marc and all for the great feedback. I don't know why it did not occur to me that having one engine down and dead would act like dragging a paddle on a canoe! I was kind of thinking that activating those torque trim tabs might just create drag for no reason and your feedback seems to reinforce that.

Thanks, John

[Hoop]

Marc,

Does "backwards" mounting of counter-rotating propellers mean that the clockwise rotating prop is on the port side and the counter-clockwise rotating prop is on the starboard side (rotation viewed from the rear)? On what basis is it felt that they are more efficient in being mounted backwards.

Thanks for posting.

[drjohn71a]

If propwalk is a significant factor in the slow speed turning on one engine, then it seems to my senses, while on the boat, that the counter clockwise outboard is on the port on my TomCat. Is this the backwards? It kind of makes sense to me to put the counter rotating on the port, but I have not had time nor weather to correctly analyse this.

Now, if I'm supposed to truly mount them backwards, then C-Dory is gonna have to revise those Armstrong brackets and the steering and even the windshield wipers, unless I'm sposed to be running in reverse all the time... Although, come to think about it, that'd make it easier to troll whilst resting on the porcelain throne!

John

[thataway]

Unfortunately Brent and Dixie left our home before I had checked on the list--so I cannot ask them--but my experience is that there is definate pull from the engine--more than in inboard twin screw boats. As Marc says, the faster you run the boat on a single, the more the boat pulls to the side.

However I had the impression that the boats with the liquid tie bar were not susposed to be run with the engine all of the way up????

Yes, my boat has counter rotating engines--they are "backward" as Marc notes. The pull is not prop walk, but due to the asymetry of propulsion force and the hull configuration. There is some prop walk , but most noticable in reverse--as in most boats.

[drjohn71a]

Thanks Bob.... But, exactly what is "backward"? Which prop is where? Why is one way considered "backward"?
What is a "liquid tie bar"? Hydraulic steering?
John