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Is it possible to increase speed while decreasing pitch?

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Is it possible to increase speed while decreasing pitch?

Old 08-08-2018, 08:31 PM
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Originally Posted by Aliboy View Post
Have you done the prop slip calc for your current prop & setup? Couple of minutes on the calculator and you should have a decent idea about how that prop is performing. Reducing slip by changing prop diameter or design and make a big difference is you have a lot of slip with the current prop.
Rough figures - your current 16x21.5 running 52mph @ 6000rpm (is that all correct?) is running a bit over 10%. If you could reduce that to say a really good number like 4% (might be tough) then you get 56mph with the current pitch. That might involve a little cupping or a little more diameter/blade area etc as some prop expert may be able to advise. To crack 60mph you would need to be running a 23" pitch prop at 4% slip @ 6000rpm. That sounds like it might be an unrealistic goal unless you can get some extra hp or reduce drag. Just getting a better 21.5"prop that has less slip might get you to 55 or 56mph, but unless you can turn a good 23" pitch prop to 6000rpm I don't see 60mph happening with your current loading. would be interesting if you could borrow a 15.5x23 or 15.75x23 3 blade prop to see what they would do (i.e. more rpms but carry the bigger pitch). Trouble is that there is a chance that your slip numbers might get worse with the smaller diameter and you could go backwards. Hence I would only try that with a loaner prop.

Great info. Yes, the 16x21.5 @ 6000 and 52 mph is correct. I like your idea of trying the smaller diameter in the 23 pitch. I guess the perfect all around prop is tough to settle on So is the 10% slip reasonable? Too much? What’s good here?. Thanks for your input.
Old 08-08-2018, 09:01 PM
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At first when I read this, I really thought 52 was ok but now that I look at it further I think 60 should be reachable. My old Mariah with a 260 HP Mercuiser (Chevy) engine and outdrive can do 55. And is it heavier than your boat and shorter, both disadvantages for me. Something seems to be wrong. How are you measuring the speed? You're not reading in knots by any chance ;-)

Last edited by GaryDoug; 08-08-2018 at 09:08 PM.
Old 08-09-2018, 12:22 AM
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The other big variable often overlooked (and doesnt seem to have been mentioned in this thread yet) is engine height.

If his engine is set even 1 hole too deep, it will easily have an impact on speed (and economy) that the OP is reporting.

Motor too deep = extra drag from the leg plus poor thrust angle = slower.

When playing with props, rule #1 = check engine height first before doing anything else. Anti-vent plate should be skimming the surface, not buried deep in the flow, at a reasonable cruise speed trimmmed out to optimum.

Given what youve described, I bet you motor is too deep.



Last edited by Moonlighter475; 08-09-2018 at 04:03 AM.
Old 08-09-2018, 01:45 AM
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Originally Posted by Reel Screamer View Post



Great info. Yes, the 16x21.5 @ 6000 and 52 mph is correct. I like your idea of trying the smaller diameter in the 23 pitch. I guess the perfect all around prop is tough to settle on So is the 10% slip reasonable? Too much? What’s good here?. Thanks for your input.
I suspect a lot of boats run around 10% or so at WOT. More at lower speeds. Something like 5% slip at WOT might be achievable, but you might spend a lot of time playing with props to get there. I guess the question is could you run a 23" pitch prop at a lower diameter giving back some of the lost rpms from the higher pitch and get a better result. That's where you want a prop expert to advise you rather than throwing money at props (or find a friend who won't miss his 15.5 x 23 prop for a day while you try it.).
Another thought is that you might be able to lift the engine a hole and run a different prop to keep the grip and maintain good slip numbers.
Old 08-09-2018, 04:18 AM
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Originally Posted by Moonlighter475 View Post
The other big variable often overlooked (and doesnt seem to have been mentioned in this thread yet) is engine height.

If his engine is set even 1 hole too deep, it will easily have an impact on speed (and economy) that the OP is reporting.

Motor too deep = extra drag from the leg plus poor thrust angle = slower.

When playing with props, rule #1 = check engine height first before doing anything else. Anti-vent plate should be skimming the surface, not buried deep in the flow, at a reasonable cruise speed trimmmed out to optimum.

Given what youve described, I bet you motor is too deep

I failed to mention that motor is on a Seastar 10” jackplate and numbers were given based on motor raised for optimum bite. So no, motor is not too low.
Old 08-09-2018, 06:42 AM
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Call Marcus at Power Tech Props.

Certain Prop geometries for a jack Plate can allow you to run higher without slip which reduces drag and increases speed.
You can also get props to increase bow and stern lift to reduce wetted surface on plane. 52 seems slow for you set up.
Old 08-09-2018, 09:38 AM
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its hard to compare Yamaha to Suzuki speeds due to the gear ratio differences (1.75 to 2.08) that plays a factor in the performance of the prop. I would look into testing an enertia eco 21

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