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Old 11 December 2019, 12:44   #1
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Mercury outboard prop

Iíve got a 75hp 2 stroke mercury on a sr5.4. Itís currently running a 13.25x17 prop. This sits me at 28knots WOT at 5250 rpm.

I would like a bigger prop so I can lower the rpm at slower speeds but get more top end for the occasional mad moment. What would people recommend?

Iím looking at either a 13x19 or 13x21, would I be better with a bigger or smaller pitch? 12.5 or 13.25 maybe?
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Old 11 December 2019, 13:57   #2
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Iíve got a 75hp 2 stroke mercury on a sr5.4. Itís currently running a 13.25x17 prop. This sits me at 28knots WOT at 5250 rpm.



I would like a bigger prop so I can lower the rpm at slower speeds but get more top end for the occasional mad moment. What would people recommend?



Iím looking at either a 13x19 or 13x21, would I be better with a bigger or smaller pitch? 12.5 or 13.25 maybe?


Youíre already over propped, going taller will kill any remaining hole shot, probably increase your fuel consumption & place undue mechanical stress on your engine, which Iím guessing ainít no spring chicken. You canít fit a quart into a pint pot.
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Old 11 December 2019, 17:35   #3
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What are the min-max wot rpm factory stated for that motor ? You run a variable or fixed load ?

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Old 11 December 2019, 18:58   #4
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What are the min-max wot rpm factory stated for that motor ? You run a variable or fixed load ?

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I think itís 5000/5500 of the top of my head. Itís a fixed load, may occasionally get lighter if I donít load the wifeís arse in but never heavier
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Old 11 December 2019, 19:01   #5
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Youíre already over propped, going taller will kill any remaining hole shot, probably increase your fuel consumption & place undue mechanical stress on your engine, which Iím guessing ainít no spring chicken. You canít fit a quart into a pint pot.
Hi, what makes you say itís already over propped?
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Old 12 December 2019, 03:53   #6
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If your operating range is 5000-5500 & your wot rpm is 5250 your bang in the middle if the operating range. You are probably unlikely to gain by going steeper pitch and may actually go slower if the pitch increase takes too many rpm off.
If you were making over the 5500 then you would have room to increase pitch but you dont.
Your engine is already giving its max you cant get more.
Think of it like going up a hill in 4th & you decide to change up to 5th but the car doesnt have the guts for 5th so it slows down, the same could happen switching to a steeper prop
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Old 12 December 2019, 04:17   #7
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Hi, what makes you say itís already over propped?


Iíd assumed that the WOT for that engine was closer to 6000rpm than 5000. 5250 sounds low for an old skool 2t
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Old 12 December 2019, 09:42   #8
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So I’ve just checked the manual and WOT is 4750-5250 which means I’m sat at the top.

Reading on the internet there is roughly 200rpm change per 1 of pitch. Do we think I’ll be ok with going from 17-19. It should drop my rpm to roughly 4850 at WOT.
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Old 12 December 2019, 09:52   #9
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So Iíve just checked the manual and WOT is 4750-5250 which means Iím sat at the top.

Reading on the internet there is roughly 200rpm change per 1 of pitch. Do we think Iíll be ok with going from 17-19. It should drop my rpm to roughly 4850 at WOT.


There you go, Iím wrong again
Yup, you should be ok going up an inch. There will be a corresponding drop in holeshot though. Getting on the plane may also be slower. Throttle response may also be slower. Personally I prefer to be at the underpropped end, 2 strokes like to rev, especially older tech ones.
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Old 12 December 2019, 12:13   #10
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That near cast on stone theory about gaining + 200 wot rpm per one pitch down is a tech fallacy, will depend entirely if running a light, medium light or a heavy combo, its hull shape, deck load, trim setting with any given motor...

As an example, my half ton 450 Rib has gained + 500 wot revs going from a 9 to a 8 pitch 3 blade prop with just a 2 stroke Tohatsu 18 HP motor. Now runs to its full wot rpm range of 5800 with a fixed load of 2 up. Has excellent hole shot and runs 40 KPH on flat calm water cond. Can't ask for a better water performance.

Stay with current prop as the motor is running at its max rpm range, throttle bit less when solo boating.

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Old 13 December 2019, 05:23   #11
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28 knots for 75hp on a 5.4 sounds a little low.
Is all well with your engine?
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Old 13 December 2019, 06:47   #12
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28 knots for 75hp on a 5.4 sounds a little low.
Is all well with your engine?

A few electrical problems in the way of old wiring but mechanically its sound yea.
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Old 13 December 2019, 08:00   #13
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Just curious, what's the max HP rated for that size boat ? Probably under powered running a heavy combo ?

If 5,250 revs is the max of the wot rpm range running a 17 pitch prop, could install a tad larger prop say a 18 for motor to run at middle of its wot rpm range to increase slightly the top end speed while losing top hole shot. But don't dream about having a speed monster, at the most 1-2 MPH increase on flat calm water cond..

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Old 13 December 2019, 10:35   #14
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A few electrical problems in the way of old wiring but mechanically its sound yea.
I only suggest that all might not be right as 37-40knots seems to be what you get with 90hp. I can't imagine you'd be 10 knots down with only 15hp less.

Is the boat new to you?
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Old 13 December 2019, 10:37   #15
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Just curious, what's the max HP rated for that size boat ? Probably under powered running a heavy combo ?


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Most were rated for 90hp.
Early (1970s) ones 115hp. Must have been a bit hairy. They chine walk above about 35knots!
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Old 13 December 2019, 10:56   #16
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I only suggest that all might not be right as 37-40knots seems to be what you get with 90hp. I can't imagine you'd be 10 knots down with only 15hp less.

Is the boat new to you?

The boat is new to me, its a sr5.4 marine spec but the engine isn't new to me. I've had it over a year now, was fitted to my speed boat originally but swapped it onto this when I got it.
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Old 13 December 2019, 11:01   #17
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The boat is new to me, its a sr5.4 marine spec but the engine isn't new to me. I've had it over a year now, was fitted to my speed boat originally but swapped it onto this when I got it.
Is the height correct? Cavitation plate level with or preferably a little higher than the bottom of the boat?
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Old 13 December 2019, 11:02   #18
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Is the height correct? Cavitation plate level with or preferably a little higher than the bottom of the boat?

I believe so but i will need to check tomorrow when it's light again.
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