Pair of Opti's, one is burning much more fuel

[Krabill]

Update. I went offshore fishing instead of getting the engines looked at again. BUT, standing there looking at my boat Sunday morning I had a thought. Over the winter I mounted a giant Airmar TM260 transducer, of course the only place it could go was pretty much in front of the starboard engine. So I kind of had a revelation, that transducer is causing ventaliation in front of my starboard engine and that is why I can overrev it at full throttle and it is not burning enough fuel. I will still check out both engines but this is really what I'm thinking now.

Make any sense? That transducer is pretty big, and sadly I cant put it any other place than where it is. So to test this theory I could drop the engine lower (which I dont think is a possibility with my current armstrong bracket) OR can I possibly help this by repropping the one engine?? I have a 17p prop now. Without dropping the engine or moving that transducer is changing the prop any kind of solution to ventalation?

Pic of fuel burn while chuggin 19 knots this weekend in rough seas with boat loaded full of fuel ice etc, fuel burn looks perfectly fine here
2nd pic is the giant TM260 transducer, it is about 2 inches inboard of being right in front of the starboard engine. It is also the only place I really can mount it. It does work great at all speeds and I can see down to 3000ft.

[Motv18]

I was going to ask if you were listing to port, but yea that transducer will do it.

[XstreamVking]

It would be a factor in prop cavitation. But I am not sure about the fuel burn difference. But, stranger things have happened.

[Fishinmymission]

Not sure how to opti looks at throttle position vs rpm and how long it opens injectors to the motor and timing. Your idea sounds very probable but without knowing the programming to the fuel injection you'll have to find a way to prove your theory.

Possible that it tries to reach a certain rpm per throttle position with minimum fuel. But wouldn't we expect this motor to be at some sort of rev limiter with the throttle all the way down?

[Fishinmymission]

I just reread your first post. As long as your other prop is hitting the rev limiter it is basically telling you that there is not nearly as much load on it probably due to your transducer as your suspected. Does the boat list?

[flabum1017]

It would be a factor in prop cavitation. But I am not sure about the fuel burn difference. But, stranger things have happened.

If he's aerating the starboard prop, then the port will be under more load..... more fuel burn.....

[flabum1017]

Is the hull cored? if not, you can call airmar and see if that transducer can shoot thru the hull if you glue it inside, either that or get a thru-hull puck transducer.....

[Krabill]

Hull is not cored, but switching transducers is really out of the option, these large ones are close to $1000 to replace I really cant move it or want to make that my last option. Drilled some nice holes in the transom and it is the only place I can really fit it with trimtabs, underwater lights and 3 other transducers. I also made a custom bracket that is made to fit in that location.

The boat does not list, has a fairly shallow deadrise so it rides pretty flat. Talked to mechanic last night, he still wants to hookup both engines at the same time just to make sure, but says it is possible im venting the prop.

I have a 17p Mirage prop now. if it is really venting that is the issue is there a prop size I should try to help reduce it? less/more pitch? Mechanic has some props available but we didnt talk about changing size and I have not called a prop shop to ask about this yet.

Thanks all, boat is stuck in the barn this week while my truck get repaird from a deer strike, back at it next week.

[XstreamVking]

If the prop is cavitating really bad, it will have burn marks on the inner part of the blades.

[Fishinmymission]

You want more pitch possibly larger diameter on the prop that is hitting the Rev limiter. You want thrust to be the same between both engine and right now it's not because one engine is spinning in air or some strange hydrodynamic stream. Might be an instance where running two different props works best but then again changing the pitch might not help but won't hurt, these hydrodynamic issues are complex.

[Krabill]

You want more pitch possibly larger diameter on the prop that is hitting the Rev limiter.

Thanks! I will also look for any burn marks tonight

[Fishinmymission]

Running two different pitches could possibly even out the engines at wot but might make you uneven and list at idle and mid range. Worth trying though let us know. I think trying an 18 would work. I use 16 17 and 18 on my heavy fishing boat and see maybe 150 rpm changes per one pitch change.