Merc 115 Pro XS WOT - Fuel Pressure

[Capt Jim]

I have a 2020 Hell's Bay Marquesa with a Merc 115 Pro XS on it. Most of them come with the Spitfire X7 19" prop. Mine can't turn it up above 5000 RPM. Running an 18" Vengeance and can't get that above 5700 RPM. Guys with the Yammie 115 SHO are easily running this prop and getting 6000 rpm and over. I'm thinking a possible fuel line restriction/bad anti siphon valve. I'd have to drain the tank and pull it forward to get to it and the pick up. I was going to run it off of a 5 gallon tank first; but, if there is fuel line restriction, would it affect the fuel pressure at WOT or not. Also, what should WOT fuel pressure be?

[LFBB]

Bottom paint vs no bottom paint, same hull, different hull?

[Capt Jim]

No bottom paint. Same hull, meaning all are HB Marquesas.

[Parker Yacht]

Most fuel pressure's on Four Strokes these days stays constant. Meaning turn the key, pump comes on and 43#'s =/- is what you see. Idle or WOT.
Now, things may change from idle to WOT on how long the pump comes on, but pressure should remain constant.
I there is a restriction and the lift pump cannot supply the VST/FSM then it will fall on it's face, not stop at 5000 or 5700 rpm.
Is your engine height the same?
Jack plate?

[Capt Jim]

No jackplate; but, I'm going to have to confirm on the engine height. The Yammies I'm comparing it to are mounted in the second hole as mine. The other Merc boat I'd have to confirm.

If I understand you correctly, a sticky anti siphon valve or other type of fuel line restriction, would not lower the fuel pressure. The lift pump or VST/FSM would supply the proper fuel pressure until it couldn't and then fall down. Not limit RPM, correct?

[Parker Yacht]

No jackplate; but, I'm going to have to confirm on the engine height. The Yammies I'm comparing it to are mounted in the second hole as mine. The other Merc boat I'd have to confirm.

If I understand you correctly, a sticky anti siphon valve or other type of fuel line restriction, would not lower the fuel pressure. The lift pump or VST/FSM would supply the proper fuel pressure until it couldn't and then fall down. Not limit RPM, correct?

Correct.

[Capt Jim]

Here's the Yamaha performance bulletin: https://yamahaoutboards.com/en-us/ho...115_3-7-18_flt Same hull as mine, engine is mounted in hole #2 and that motor is spinning a 20" prop to 6000 RPM while I can barely turn up an 18" to 5700. I have to think something is off here.

[Parker Yacht]

Here's the Yamaha performance bulletin: https://yamahaoutboards.com/en-us/ho...115_3-7-18_flt Same hull as mine, engine is mounted in hole #2 and that motor is spinning a 20" prop to 6000 RPM while I can barely turn up an 18" to 5700. I have to think something is off here.

I would check the gearing of the two lower units.
The person to reach out to is Ken at Prop Gods.
I'll move the thread.

[Capt Jim]

My Merc has the 2.07 lower unit while the Yamaha has the 2.15 lower. I can't believe that would make that big of difference. I've spoken to Ken already; but, he really didn't have an answer and is also out of stock on props.

[ken2]

Speed will be the question.
The yamaha ran 50 MPH. 5 gal of fuel, they don't mention Trolling motor, and lithium ion batteries on a cool day with nothing in the boat.
what speed are you getting with your Mercury?
how are you loaded for your boat?

[Capt Jim]

Ken, I'm definitely heavier. More fuel, 3 AGM batteries and a second power pole. I'm getting low 40's with mine. Since you know props, how does that Yamaha Talon compare to PT's SCD series?

[ken2]

The SCD3 is closer to the Yamaha painted stainless, less bowlifting, easier to turn.
Depending on how the boat runs, you could go either way.
if the boat wants to porpoise easily and doesn't like a lot of trim, I'd go SCD3
if it likes bowlift, I'd go PTR3. (closer to the Talon)
with either, I think 16 pitch would be a better option to get your RPM up, and low 40's is probably all its going to get with the extra batteries and powerpoles, and trolling motor.
but we could try a 17 and see if you can turn it.

[Capt Jim]

Hell's Bay is supposed to be getting me a Spitfire in 17". Let me pull some numbers with that and we'll see if Power Tech's SCD is a better fit. Thanks for your help.

[Capt Jim]

Ken, a litle update here. I had the chance to try an SCD4 18" Up to about 4500 RPM's, it was faster than the 18" Vengeance. Above that, the Vengeance pulled away. The bad part is, the SCD porpoised twice as bad as the Vengeance! I believe there's more speed; but, putting the tabs down to control porpoising was like hitting the brakes. I thought the SCD was supposed to help alleviate the porpoising. Also, couldn't get the SCD4-18 over 5700 RPM's

Secondly, spoke to another Marquesa owner with almost the same setup. Merc 115 Pro XS, 3 AGM batteries and twin power poles. Runs either a 3 blade 19" Laser ll or the 4 blade 19" Fury. Spins both up to 6000 RPM, with the Laser ll pulling a little more and a little more speed.

Thoughts?

[ken2]

There are certain boats that actually need bowlift, so a sternlifting prop may not be what it likes.
the Laser 2 is definitely a bowlifter.
sounds like the other guy might be running a Command thrust gearcase though if he's turning a Fury 4, as that is a full size prop.