Mercury outboard prop

[Unknown]

I’ve got a 75hp 2 stroke mercury on a sr5.4. It’s 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?

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

[Pikey Dave]

Originally Posted by Unknown

I’ve got a 75hp 2 stroke mercury on a sr5.4. It’s 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?



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

You’re already over propped, going taller will kill any remaining hole shot, probably increase your fuel consumption & place undue mechanical stress on your engine, which I’m guessing ain’t no spring chicken. You can’t fit a quart into a pint pot.

[Locozodiac]

What are the min-max wot rpm factory stated for that motor ? You run a variable or fixed load ?

Happy Boating

[Unknown]

Originally Posted by Locozodiac

What are the min-max wot rpm factory stated for that motor ? You run a variable or fixed load ?

Happy Boating

I think it’s 5000/5500 of the top of my head. It’s a fixed load, may occasionally get lighter if I don’t load the wife’s arse in but never heavier

[Unknown]

Originally Posted by Pikey Dave

You’re already over propped, going taller will kill any remaining hole shot, probably increase your fuel consumption & place undue mechanical stress on your engine, which I’m guessing ain’t no spring chicken. You can’t fit a quart into a pint pot.

Hi, what makes you say it’s already over propped?

[beamishken]

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

[Pikey Dave]

Originally Posted by Unknown

Hi, what makes you say it’s already over propped?

I’d assumed that the WOT for that engine was closer to 6000rpm than 5000. 5250 sounds low for an old skool 2t

[Unknown]

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.

[Pikey Dave]

Originally Posted by Unknown

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.

There you go, I’m 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.

[Locozodiac]

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.

Happy Boating

[Searider]

28 knots for 75hp on a 5.4 sounds a little low.
Is all well with your engine?

[Unknown]

Originally Posted by Searider

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.

[Locozodiac]

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..

Happy Boating

[Searider]

Originally Posted by Unknown

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?

[Searider]

Originally Posted by Locozodiac

Just curious, what's the max HP rated for that size boat ? Probably under powered running a heavy combo ?


Happy Boating

Most were rated for 90hp.
Early (1970s) ones 115hp. Must have been a bit hairy. They chine walk above about 35knots!

[Unknown]

Originally Posted by Searider

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.

[Searider]

Originally Posted by Unknown

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?

[Unknown]

Originally Posted by Searider

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.