Motoring with 1 or both engines

[admiralslater]

IMO rudder strain is a none issue catamarans love to go in a straight line . When I run on one no perceptible difference once we get moving

[Barra]

On the rudder offset issue I think its a bit more than 2 or 3 degrees. For us at least going from 2 to one engine the course will change by more like 9 or 10 degrees ie need to adjust heading by that amount to maintain course.

I guess on a keeled cat thats not the case?

I find I can play around with the opposite daggerboard to the engine to offset the leeway rather than use the rudder. Usually I drop it around 1/3 or so. AP works less, rudder back to dead ahead and boat speed a little better than rudder alone by maybe 1/2 knot.

[StuM]

Originally Posted by Barra

On the rudder offset issue I think its a bit more than 2 or 3 degrees. For us at least going from 2 to one engine the course will change by more like 9 or 10 degrees ie need to adjust heading by that amount to maintain course.

Heading change and rudder angle are two different things. A couple of degrees of rudder applied to the hull opposite the running engine at a decent water speed will have quite a large counteraction to the asymmetric thrust.

[Barra]

yes i do get that. I dont know what my rudder angle vs heading is but Im guessing around 2 to 1 max if that on small course corrections.

What im saying though is the rudder and AP is working harder than just a few degrees on a daggerboard cat as theres no mini keel to push against.

Adjusting the opposite board can get the required leeway offset to remove the rudder offset needed. The trade off in board drag is less than the offset rudder drag by enough to make up to 0.5 knots difference for us.

So you wanted a conflicting opinion and here it is - on daggerboard cats 2 engines are better than one unless you use the opposite daggerboard

[belizesailor]

Originally Posted by Barra

On the rudder offset issue I think its a bit more than 2 or 3 degrees. For us at least going from 2 to one engine the course will change by more like 9 or 10 degrees ie need to adjust heading by that amount to maintain course.

I guess on a keeled cat thats not the case?

I find I can play around with the opposite daggerboard to the engine to offset the leeway rather than use the rudder. Usually I drop it around 1/3 or so. AP works less, rudder back to dead ahead and boat speed a little better than rudder alone by maybe 1/2 knot.

Yes, it will be diff on mini keel vs boards. Boards up you've got very little lateral resistance so the boat will be less directionally stable and thus more challenging to steer straight on 1 engine. Some board down, more lateral resistance, problem solved.

Mini keels...they just like to go straight...all you need is a bit of water flow due to headway and off you go.

To reduce drag, you might experiement with how little board you can have down to get better tracking.

[44'cruisingcat]

Originally Posted by Barra

On the rudder offset issue I think its a bit more than 2 or 3 degrees. For us at least going from 2 to one engine the course will change by more like 9 or 10 degrees ie need to adjust heading by that amount to maintain course.

I guess on a keeled cat thats not the case?

I find I can play around with the opposite daggerboard to the engine to offset the leeway rather than use the rudder. Usually I drop it around 1/3 or so. AP works less, rudder back to dead ahead and boat speed a little better than rudder alone by maybe 1/2 knot.

Course change is different from rudder angle. Put 1 degree of rudder on and eventually the boat should go through 360 degrees.... and still keep turning.

We have no minikeels, but motoring on one engine only requires about 1-2 degrees of rudder correction.

I can't see how using the opposite daggerboard would reduce the amount of rudder. You'd be adding drag to that hull, which if anything, would then need MORE rudder to compensate. I can't see any way that would increase boat speed.

[Barra]

Originally Posted by 44'cruisingcat

Course change is different from rudder angle. Put 1 degree of rudder on and eventually the boat should go through 360 degrees.... and still keep turning.

We have no minikeels, but motoring on one engine only requires about 1-2 degrees of rudder correction.

I can't see how using the opposite daggerboard would reduce the amount of rudder. You'd be adding drag to that hull, which if anything, would then need MORE rudder to compensate. I can't see any way that would increase boat speed.

Well if im motoring im not sailing so im bored and doing plenty of experimenting

Ive tried all different configurations of boards and rudder and revs and so i can say on my boat at least the fastest one engine configuration is a bit less than 1/3 opposite board down which sets the rudders back to dead ahead.

No board and the rudders are hard enough over as to create more drag than the 1/3 board (plus dead ahead rudders). So much more im talking up to 1/2 knot boat speed difference and when your bored thats alot!

As i increase revs i need more board down so theres a sweet spot that varies on seas state etc.

[tomfl]

Seems like the boat design is a significant factor in running one or two engines. I would also add that things like prop size, pitch, and number of blades should also be considered. I have two high thrust long shaft 9.9 Yahamas in wells well forward in my Seawind. Also have elephant ear 12" three blade props. Like others have posted there is somewhere around 1.5 to 2 knots extra speed running both engines at full speed. I normally run about 3/4 throttle with a single engine when I motor. But on more than one occasion I have had to use both engines going into the eye of the wind with a significant current. I notice that under harsh conditions there is not really much of an increase (if any) in speed between 3/4 and full throttle.

Seems like the best way to answer the question is to have experience with the boat under varying conditions. While most boats do well running under one engine there may be exceptions.