Engine maxing out at 4600

Brunedaprune · 06 August 2023, 12:25

Hi there,

I have a Yamaha 150 on a 5.8 rib quest that currently does 42 knots. It maxes out at 4600 but the official Yamaha range is 4500 to 5500. Does anyone have any advice on how to squeeze the last little bit out?

Jon Tallis · 06 August 2023, 12:37

Are you running fully trimmed out? What prop are you running? I assume you're after max boat speed, not just to hit max engine speed?
Clearly you need to get your trim spot on for max boat speed, not just engine trim angle but also boat loading.
You could try a shorter pitch prop but you might not actually achieve more boat speed as you've only got about 12% of your rev range left, and going down a pitch will typically gear you down about 10-11% so any gain would be marginal. You would of course get slightly better acceleration

Brunedaprune · 06 August 2023, 13:13

Boat trim is pretty spot on I think. It gets really wobbly and sensitive to any steering change. I’m after max boat speed. Apparently it once had a bigger prop on that took it up to 49 knots. Will maybe just have to experiment with a bigger prop.

Jon Tallis · 06 August 2023, 20:40

By bigger, I presume you mean longer pitch. That's not going to raise your engine speed. Given your engine isn't reaching max revs with the now shorter prop it's going to drop even further with a longer one. Dependant on the power curve of your engine you might get a few more kts but given your current wide-open engine speed I'd say that was unlikely....but there's only one way to be sure....

Pikey Dave · 06 August 2023, 21:52

Quote:

Originally Posted by Brunedaprune

Boat trim is pretty spot on I think. It gets really wobbly and sensitive to any steering change. I’m after max boat speed. Apparently it once had a bigger prop on that took it up to 49 knots. Will maybe just have to experiment with a bigger prop.

If the boat is getting twitchy at your current speed, increasing top end probably won’t end well. You’re at the bottom end of the recommended rev range, so you’re wandering into overpropping territory by going higher pitch. You’ll sacrifice holeshot for more top end, IMHO hole shot (I.e. throttle response) is more important on a balanced outfit than top speed.
Overpropping risks premature engine failure.
Ultimately it’s your call.

neil.mccrirrick · 07 August 2023, 07:39

How clean is your hull? Easy to lose revs and speed if your dragging a sea garden around

Jon Tallis · 07 August 2023, 07:42

Quote:

Originally Posted by Pikey Dave

If the boat is getting twitchy at your current speed, increasing top end probably won’t end well. You’re at the bottom end of the recommended rev range, so you’re wandering into overpropping territory by going higher pitch. You’ll sacrifice holeshot for more top end, IMHO hole shot (I.e. throttle response) is more important on a balanced outfit than top speed.
Overpropping risks premature engine failure.
Ultimately it’s your call.

Good advice as usual from PD [emoji106][emoji106]

jonp · 09 August 2023, 12:36

I would be using a smaller pitch prop to get the rpm closer to the recommended max rpm range to reduce the engine labouring.

Pikey Dave · 09 August 2023, 14:40

Quote:

Originally Posted by jonp

I would be using a smaller pitch prop to get the rpm closer to the recommended max rpm range to reduce the engine labouring.

Agreed, but I don’t think that’s what the OP wants to hear

willk · 09 August 2023, 19:22

Quote:

Originally Posted by Brunedaprune

Does anyone have any advice on how to squeeze the last little bit out?

Leave the prop alone. Lighten the boat - be ruthless, eat less pies and find a skinny partner (you'll thank me in the end). Polish the hull and loose the flags. You'll be pleasantly surprised...