Single engine operation of dual engines

[mattsmall1972]

My brother has twin 1995 Black Max 200hp outboards on his boat. We had a problem with the port engine on a trip once, so figured we would just use the starboard engine. What we noticed was that the starboard engine would not get to wot rpm on it's own. We've noticed that the same thing occurs with the port engine (after it was fixed) Is rev limiting and/or matching a built-in feature of the engine pair?

 

[JB]

Re: Single engine operation of dual engines

Seems to me that your engines are prop idealized for dual operation, Matt. That probably left either one on its own with too much prop.

 

[mattsmall1972]

Re: Single engine operation of dual engines

That means that the props would need to be changed to allow a single engine to carry the load?

 

[JB]

Re: Single engine operation of dual engines

If you intend a lot of single engine operation I would reevaluate your prop choice. Single engines prop optimized for single operation would not do well in dual engine operation.

I will move this to the Prop Forum for input by real prop experts.

 

[mattsmall1972]

Re: Single engine operation of dual engines

It's more about understanding the operation of the dual engines. I've always thought that the purpose behind dual engines (in most cases) was the safety feature of having a backup in case one of them has a problem. When I'm 50 miles offshore, I want to be able to come back in a decent time rather than have it take all day. Otherwise I don't know why I would really want dual outboards as it would be just as (or maybe more) effective to have a big outboard and a 10-20hp kicker.

When we took it out a couple of days ago, it seemed as though the RPMs stayed in sync with each other on each engine. We were convinced that there were some electronics inside the outboards that made these engines sync with each other. To that end, I was wondering if there was a way to disable this feature for full WOT when one of the engines is disabled.

Does any of this make sense?

 

[mpsyamaha]

Re: Single engine operation of dual engines

its an illusion... one engine is dragging the other one to similar rpm, even if the throttle is slightly more open on one than the other.

Very few twin outboard boats will plane with one engine. And, if you prop it to do so, then you will severely over-rev at wot with both running. hypothetical example: twin 200s run the correct wot rpm with 21p props, a single 200 would require something like a 13p to get the correct rpms on the same boat (and obviously be much much slower). and if you put 13p props on both engines you would have the same very slow top speed and the motors would probly barely be at half throttle to get wot rpms. hopefully that makes sense to you

you have to keep in mind that there is no transmission to take advantage of the power like a car has, its only one gear with one gear ratio. and, there is most definitely no connection between the two motors... even a 2013 four stroke outboard does not communicate with its buddy like you are thinking. your brothers old black maxes are very simple carbeurated motors, they will only spin what the prop lets them.

There are other advantages to twin engines besides getting back to port safely. If you use them properly, maneuverability around the dock is much improved (aka driving with the throttles, like larger vessels do). Also, twin outboards are often simply a necessity due to boat size.... what would you rather have, a 400hp single outboard?

another thing to think about, using only one engine while the other is fully trimmed out of the water places serious force on the steering tie bar (it binds up). I have personally seen them break from attempting this.

 

[mattsmall1972]

Re: Single engine operation of dual engines

Wow, thank you for the good information! I never expected a single engine of the pair to make us go full speed, but I didn't expect it to be that sluggish either.

Theoretical question: say we were out 50 miles and a single engine were to give out. If we were to somehow replace the prop on the working engine with one that is propped for single engine use, would that make it a better scenario? Do they have easy-removal props for this reason?

Thank you!

 

[mpsyamaha]

Re: Single engine operation of dual engines

Outboard props are easy to change as long as you have the tools on board. Going swimming while you do it might be necessary though. For your scenario to work, you would have to carry a spare left and right prop with lower pitches in the boat all the time. I think most people are happy limping back to shore with one motor, its a lot better than when your single outboard boat loses a motor!