What adverse effect are there when not using counter rotating engines on a cat?

[Fatenhappy]

Ok guys/girls ...

Background .... I have a reasonable back ground relating to boats and boating, having experience in sail, inboards and outboards but as far as piloting goes - the later two however only as single installations, never twins.

I also understand "Torque effect and P-factor" when it comes to counter rotating twin engine installations.

In the mean time I note that a good percentage of boaties don't run counter rotating installations.

My question then is simply ..... "If and what bad or and dangerous effects if any do you get from not having a counter rotating installation with twins O/b's"?

Cheers
Greg

[freddofrog]

I suspect most boats don't have counter rotating props because most people have small boats (70% less than 6m if you go by my "what size is your boat" poll) and even less will have a twin set up and not many manufacturers build counter rotating motors to suit these size boats anyway so they don't have a choice.

Mine aren't counter rotating and I barely notice it.

[Fish Guts]

counter rotating reduces transverse thrust. if you were idling in a cat with two props spinning the same direction (clockwise) the boat pulls to port.

[finding_time]

Also travelling with the swell comming over your starboard quater you do tend to notice this torque abit more!

Ian

[Grand_Marlin]

Also travelling with the swell comming over your starboard quater you do tend to notice this torque abit more!

Ian

This is about the only dangerous thing with non counter rotating props... come down off a big, steep swell and the cat can dip the sponson which could be dicey ... especially in smaller cats.

With your 23 you wont have too much to worry about, but the counter rotating props will certainly make a difference that is noticeable.

Cheers

Pete

[Noelm]

just a note on counter rotating, I was just a part of an "experiment" a 23' Cat was fitted with a pair of new Motors, they where fitted with the tips pf the props turning inwards at the top, then the motors where swapped so the "counter rotation" was the other way, there was absolutly no difference in performance or handling, although there MAY have been a slight difference in very low spped turning (in the Harbour) but I was not convinced that there was, sorry to half hijack this, but it is SORT of relevant! and in my experience, untill you go a bit over about 115HP or so, counter rotation is "nice" but hardly needed.

[Fatenhappy]

Hey noelm ...

just a note on counter rotating, I was just a part of an "experiment" a 23' Cat was fitted with a pair of new Motors, they where fitted with the tips pf the props turning inwards at the top, then the motors where swapped so the "counter rotation" was the other way, there was absolutly no difference in performance or handling, although there MAY have been a slight difference in very low spped turning (in the Harbour) but I was not convinced that there was, sorry to half hijack this, but it is SORT of relevant! and in my experience, untill you go a bit over about 115HP or so, counter rotation is "nice" but hardly needed.

So excuse my nievity ... whats so special above 115 or so ... I'm guessing that because of the increase in HP above this, this is where the Torque effect starts to kick in?

Cheers
Greg

[Grand_Marlin]

I think that is the theory, and C/R (as far as I know) still isnt available in lower hp motors.
Yamaha makes a C/R 115 4 stroke, and they were talking about the V4 130 / 115 having C/R but I am not sure if this ever happened?

Cheers

Pete

[Noelm]

that seems to be the case, of course it is always there, but with smaller Motors it does not seem to be a real problem, a bit of adjusting the little trim tab and your done, especially with Hydraulic steering. I have had twin 55's, 70's, 90's and 115's all non counter rotating and never had any drama, but when you start getting up a bit from there it can be a bit of a pain, I guess it is all to do with the props being so much bigger it exagerates the problem.

[Noelm]

as an example, my mate had a 23' Sharkcat with twin 225's non C/R and cable steering, holy crap, that was a handful, but strangely enough, at high speed it was a dream, not too sure why, but anything under about 4,500 RPM it was like a Bus with one flat tyre or something, what a nightmare.