propeller doubt

ste80 · Apr 26, 2025

good afternoon guys; now that motor is completed and waiting for onboaring i'm working on boat restauration and other sides activities. one doubt that i've got is about propeller.
my old mercruiser 4.3 efi was using an alluminium 15 1/2 x 15 propeller, but i'm preatty sure it was not the right one for that motor
my boat is a searay seville 21 with this spec

my motor is a mercruiser 4.3 lx 200 hp with alpha one gen II
if i look onto sea ray site, for my model they suggest a propeller 14 x 17

at home i'vo got a stinless still 14 3/4 x 17
old owner of my motor was using a 21 pitch propeller (13-3/4 x 21) on other boat so it doesn't mean so much

what would you do? would you go for the 14 3/4 x 17 or should i buy a 14 x 19 as per manual? in your opinion with 14-3/4 x 17 what i would loose compared with the recomended one ( 14x19)
thanks

Pmt133 · Apr 26, 2025

I'd try what you have first then go from there personally.

Just to add, the diameter is relatively meaningless compared to pitch.

ste80 · Apr 26, 2025

Pmt133 said:
I'd try what you have first then go from there personally.

Just to add, the diameter is relatively meaningless compared to pitch.

Tks
Yes this is why i have a doubt if buy the 19 or not.
Theorically speacking 2 " in the pitch should be more or less 3 knot at same rpm i don't know whot this means in tearms of fuel compsution .
I mean, if i have the 19 theorically at 3600 rpm i shiuld be at 30.4 knot
With 17 @ 27.2

With the 17 to be at same speed i should increase my rmp @ at 4000
This mean 11% more

I don't think this mean 11% more petrol because motor will have less load .

Pmt133 · Apr 26, 2025

ste80 said:
Tks
Yes this is why i have a doubt if buy the 19 or not.
Theorically speacking 2 " in the pitch should be more or less 3 knot at same rpm i don't know whot this means in tearms of fuel compsution .
I mean, if i have the 19 theorically at 3600 rpm i shiuld be at 30.4 knot
With 17 @ 27.2

With the 17 to be at same speed i should increase my rmp @ at 4000
This mean 11% more

I don't think this mean 11% more petrol because motor will have less load .

The flip side of this is if you can achieve proper wide open throttle rpm. If you cannot, none of that matters. The 17 is a good starting point imo.

I'd rather be under propped than over to start.

Mc Tool · Apr 26, 2025

If you have a " ball park " stainless prop you should fit that and see how it goes . Save yourself some stress and forget the numbers for now .
Try out the prop you have and if your not happy with the numbers then maybe

you can put them against the cost of a new prop . Dunno bout you guys in the US but I wouldnt get any change out of 500 bucks for a new stainless prop and that buys a lot of extra fuel .....even @ $2.80 / litre for 95 oct.

ste80 · Apr 27, 2025

Pmt133 said:
The flip side of this is if you can achieve proper wide open throttle rpm. If you cannot, none of that matters. The 17 is a good starting point imo.

I'd rather be under propped than over to start.

thanks, i will try with this one then before doing any other activities

ste80 · Apr 27, 2025

Mc Tool said:
If you have a " ball park " stainless prop you should fit that and see how it goes . Save yourself some stress and forget the numbers for now .
Try out the prop you have and if your not happy with the numbers then maybe you can put them against the cost of a new prop . Dunno bout you guys in the US but I wouldnt get any change out of 500 bucks for a new stainless prop and that buys a lot of extra fuel .....even @ $2.80 / litre for 95 oct.

thanks, clear. i will try mine then to check how it is.
the cost is a matter of how much petrol you will save , because a prop II hand it will cost 300 $ here the petrol is 2€/lt i don't know it should be calculated. probabilly you are right it not worth it .

for sure the original one that i got 15 1/2 x 15 is wrong, but maybe the 17 is cloose to the suggested one

bajaman123 · Apr 27, 2025

What outdrive ratio? The Alpha One Gen II outdrive for the 4.3 LX was typically 1.81:1. You don't have a whole lot of ponies available so you need to get baseline numbers as many people have already said. You need to be able to hit your rev limiter/or max recommended rpm...assuming 4600 in your case. You also need to understand what the boat will be mainly used for...if for example you plan on having the whole clan on board while little Billy and Susie are tubing, you want to be on the under-propped side of things. On the other hand, if it's just you and the missus cruising the lake, opt for a higher pitch prop. IF you have a 1.81:1 outdrive and are putting out a healthy 200+ hp, you SHOULD be able to run a 17" pitch Mirage Plus prop, MAYBE a 19" if you don't do a lot of towing. A 17" Revolution 4 prop would probably be ideal. Anyway, get some numbers, make sure you can hit WOT and get back to us.

ste80 · Apr 27, 2025

bajaman123 said:
What outdrive ratio? The Alpha One Gen II outdrive for the 4.3 LX was typically 1.81:1. You don't have a whole lot of ponies available so you need to get baseline numbers as many people have already said. You need to be able to hit your rev limiter/or max recommended rpm...assuming 4600 in your case. You also need to understand what the boat will be mainly used for...if for example you plan on having the whole clan on board while little Billy and Susie are tubing, you want to be on the under-propped side of things. On the other hand, if it's just you and the missus cruising the lake, opt for a higher pitch prop. IF you have a 1.81:1 outdrive and are putting out a healthy 200+ hp, you SHOULD be able to run a 17" pitch Mirage Plus prop, MAYBE a 19" if you don't do a lot of towing. A 17" Revolution 4 prop would probably be ideal. Anyway, get some numbers, make sure you can hit WOT and get back to us.

thanks a lot. sure i will do some test and i'll be back.
The ratio is 1.81 ( or maybe origina was 1.85? anyway the original one yes)
boat normally is used for trip with family ( or around here or longher cruise) i don't care so much about full speed, but normally when i run the boat i run 3600 rpm at cruise speed .

motor is 200 hp preatty new ( just rebuilt with almost all components new, i didn't try yet but it sound good

my stainless still propeller is a SST propeller . i do not know the brand and how it is. do yu know this model?

tks

Scott Danforth · Apr 28, 2025

the motor cares that it can go to full RPM. Do not lug the motor, especially on a new rebuild. that will kill it.

ste80 said:
i don't care so much about full speed, but normally when i run the boat i run 3600 rpm at cruise speed .

what ever prop pushes the boat to WOT RPM of 4800-5200 RPM is the prop to use. on a boat that big with a 4.3, most likely a 13p or 15p.

the 17p prop you have is probably too much prop. especially on a new rebuild.

your boat should have had a V8.

ste80 · Apr 28, 2025

Scott Danforth said:
the motor cares that it can go to full RPM. Do not lug the motor, especially on a new rebuild. that will kill it.

what ever prop pushes the boat to WOT RPM of 4800-5200 RPM is the prop to use. on a boat that big with a 4.3, most likely a 13p or 15p.

the 17p prop you have is probably too much prop. especially on a new rebuild.

your boat should have had a V8.

Thanks.
Actually what i've got is:
1 prop that was installed in my old 4.3 efi (but it is damage a little bit so i would like not to use) 15 1/2 x 15 (alluminium)
One from original omc motor that i had 15x17 (alluminium)
One stainless still 14 3/4 x 17 ( i 'm not sure if it was from omc or efi because i've got in my garage from a wile but i never actually use)
I think that omc cobra and mercruiser have same shape of prop, i will check size and diameter)

cyclops222 · Monday at 5:04 PM

My 19 foot Chaparral bow rider with a perfect running merc cruiser 5.0L 220 hp. Would easily do the rev limiter at 50 mph. It weighed 3800 pounds on a scale. Prop Gods & Power Props Tech support got the boat with 4 220 pounders to plane in1/2 a second. No blow outs were possible. Trim control only affected WOT speed. WOT is 45 mph. The boat has no bow lift from a dead stop floored. IF a person want to stand on takeoff ? I can toss him off and he will clear the rear deck. The G Force is strong enough to cause some people to complain about their lungs being forced to exhale. No stress cracks yet.

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