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Old 06 August 2023, 13:25   #1
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Engine maxing out at 4600

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?
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Old 06 August 2023, 13:37   #2
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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
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Old 06 August 2023, 14:13   #3
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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.
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Old 06 August 2023, 21:40   #4
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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....
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Old 06 August 2023, 22:52   #5
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Engine maxing out at 4600

Quote:
Originally Posted by Brunedaprune View Post
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.
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Old 07 August 2023, 08:39   #6
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How clean is your hull? Easy to lose revs and speed if your dragging a sea garden around
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Old 07 August 2023, 08:42   #7
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Quote:
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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]
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Old 09 August 2023, 13:36   #8
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I would be using a smaller pitch prop to get the rpm closer to the recommended max rpm range to reduce the engine labouring.
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Old 09 August 2023, 15:40   #9
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Quote:
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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
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Old 09 August 2023, 20:22   #10
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Originally Posted by Brunedaprune View Post
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...
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Old 12 August 2023, 07:57   #11
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42 knots on a 5.8 - how much faster do you want to go?
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Old 12 August 2023, 09:35   #12
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how much faster do you want to go?
R+25% tends to be the target, where R= a Reasonable Speed...
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Old 12 August 2023, 10:04   #13
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Eat less pies.
We all need to do this to achieve warp speed.
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Old 12 August 2023, 11:05   #14
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We all need to do this to achieve warp speed.
To avoid Pie R squared - where you just go around in circles...
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