Counter rotating engines on catamarans

[Cadence]

Originally Posted by Lepke

Bigger boats/ships using counter rotating props steer a straight line better, besides the maneuvering issues. Modern transmissions do the reverse rotation. About 30 years ago it became common for most dual engines to have the same rotation. In merchant ships, the engine runs in either direction for fwd or rev.

Most transmissions do the job. I did have a pair of 671 mirrored twins. I have even seen a tug that the reverse required shutting down and manually switching cams with a lever. I believe that is what you are speaking of with engines running in reverse. Thank God they never put me on her.

PS a TD crash box will run in reverse forever.

[a64pilot]

Originally Posted by 45acp

Thanks for the answers. I guess "P" factor must not have much effect in water. With air plane props the effect can be quite dramatic. I know that a lot of things that seem good on paper, can be a waist of time and money in the real world, this seems to the case here . Having one spare prop would be a huge savings.

P factor is because the nose is pitched up. Not an issue on boats especially non planing as the induced flow angle of attack remains the same
https://en.m.wikipedia.org/wiki/P-factor

[StuM]

Originally Posted by Lanevdl

Hi,
You will probably find all cruising cats have counter rotating props. Achieved via the transmissions. I drive large catamarans for a living and prop walk is definitely present regardless of how far apart the hulls are. So basically you can steer the vessel to port or to starboard in astern by the choice of engine.

Not mine.

[Rapanui]

With a saildrive or outboard where the props are inline with the flow, propwalk is neglible. Walk increases with shaft angle. As others have said there is no noticeable walk on our 2 LH saildrives. Propwash over the angled rudders behind the props is more controllable and useful so that it helps reversing in crosswind.

[EWOL Props]

Based on our experience catamarans can have either same sense of rotation (e.g. both props left rotating) or counter-rotating, there is not a precise rule, it all depends on the manufacturing shipyard / project.
Certainly counter-rotating props would be preferrable to reduce prop-walk.

One more thing is that new engines, when with Sail-drive transmission can be switched from forward left to forward right (or vice-versa), it is for sure the case of Volvo Penta, but also Yanmar newest engines can do the same. So should you buy a catamaran with S-Drives and feel a too strong prop walk, you will just need to switch the cables of one of the engines (but please check on the specific engine if it has this feature). Clearly if you change the sense of rotation you will have to replace one of the propellers.

One more point....having starboard left rotation and port right rotation will have an effect on navigation which is different than starboard right rotation and port left rotation !

Originally Posted by 45acp

I am a newbie to sailing and catamarans, have been studying adds for catamarans for about a year now and I have yet to see any one mention the use of counter rotating engines. I have run across discussions on prop walk with strait shaft inboard engines. Could it be that they all have counter rotating engines so they do not even say any thing in the specs? Or would this not have any bearing on the handling of a catamaran?
I have tried searching for other posts to no avail, maybe I'm a dummy and it would not any effect on the handling of a twin hull.
Any answers or links would be appreciated, thanks.

[PhiSig1071]

Originally Posted by AndyBerry

Can you share your sideways technique?

My guess would be that because "prop walk" occurs irrespective of direction of thrust, if you counter the drives (one forward one reverse) the props will both be rotating in the same direction, causing the stern to walk, but because of counter thrust from the engines the bow will try to rotate in the same direction, causing the boat to sort of crab sideways.

[a64pilot]

Originally Posted by PhiSig1071

My guess would be that because "prop walk" occurs irrespective of direction of thrust, if you counter the drives (one forward one reverse) the props will both be rotating in the same direction, causing the stern to walk, but because of counter thrust from the engines the bow will try to rotate in the same direction, causing the boat to sort of crab sideways.

That is how it is done on a mono hull twin engine powerboat anyway. I used to enjoy stopping a few feet from the fuel dock and walking the boat over slowly with zero forward or aft movement and tieing off single handed.

[StuM]

Originally Posted by PhiSig1071

My guess would be that because "prop walk" occurs irrespective of direction of thrust, if you counter the drives (one forward one reverse) the props will both be rotating in the same direction, causing the stern to walk, but because of counter thrust from the engines the bow will try to rotate in the same direction, causing the boat to sort of crab sideways.

You don't need counter rotating props to do it.

Forward on the outside prop, reverse on the inside prop and helm over away from the dock so that the props turn the bow in and prop wash over the rudders (primarily the outside rudder) pushes the stern in. It takes a bit of practice and balancing of the throttles to avoid moving forward or astern and with too much power you will just spin the bow in.

The bigger the rudders, the better and it doesn't work so well with cats where the prop is behind the rudder such as some Lagoons and FPs.