Prop suggestion for a Mercruiser 4.3 sterndrive

[mia9516]

I am looking to replace the existing prop on my 4.3L Mercruiser. It is mated to an Alpha One outdrive. The current prop is 14.5 x 17p aluminum job that has seen better days. The WOT range for my boat is 4400-4800 rpm. Fully loaded (8 people plus 100 lbs of gear) she hardly gets into that range. Do I have to use a Quicksilver prop? Would I need to purchase a hub kit in order to change to prop out? Would it be safe to upgrade to a 16" or 15" without over-revving the engine? Also, would I be able to go with a 15" diameter or maybe a 4 blade prop? Thanks for all your help...~Milton

 

[steelespike]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

If you read the stickies at the top of the page it may help you understand prop selection better.
We need details boat length,dry weight,Deep V? Deadrise? Your present actual max rpm and speed light and loaded? horsepower?Dependng on your boat a 17" could be close to over reving with a light load now. You can use any prop designed for you application.Some types require a hub.

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

If you read the stickies at the top of the page it may help you understand prop selection better.
We need details boat length,dry weight,Deep V? Deadrise? Your present actual max rpm and speed light and loaded? horsepower?Dependng on your boat a 17" could be close to over reving with a light load now. You can use any prop designed for you application.Some types require a hub.

The boat length is 19'. Dry weight is 2500lbs. It is a v-hull boat. Not sure what the deadrise is or how to measure it. My max rpm loaded is 4000-4200. Not sure about the speed due to inacurracy of boat speedo. Horsepower is 190. Let me know if you have any more questions or as to how I can figure out the deadrise of this thing. Thanks again. ~Milt

 

[steelespike]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Unfortunately I'm not able to be too specific.But if you typically run with 8 people
you could drop your pitch. A same brand same model 15 would probably add 300 to 400 rpm but you would have to be very careful with a light load. A 4blade 15 would really improve your low to mid range performance.And would probably still over rev but to a lesser extent.

 

[180shabah]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

My max rpm loaded is 4000-4200.

That is very low and risking some pretty nasty engine damage. Dropping 2" of pitch on the same prop should bring you up around the 44-4600 range leaving you room when you are lightly loaded.

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

That is very low and risking some pretty nasty engine damage. Dropping 2" of pitch on the same prop should bring you up around the 44-4600 range leaving you room when you are lightly loaded.

Not sure what you mean about very low. Are you saying this because it sounds to you like the motor is really straining? To tell you the truth, I've never run my engine over 4k, loaded or unloaded, since I've owned it, which is about a month now. From what I understand, my max RPM range is 4400 to 4800 RPM's. I believe this RPM range is with either one or two people in the boat. Thanks again.~Milton

 

[180shabah]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

which is it? max rpm = 4000-4200 or I never run it above 4000????

If you want prop advice, we need to know the EXACT WOT performance with the current setup.

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Rog. I'll do some test runs this Sunday and get back to you. Thanks again.

 

[oops!]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

hey milt...

just shy of 5 grand on a 4.3 is really working the little 6. in fact flying apart at the seams is about right. check the mfgrs specs on wot for the motor.

i had that pkg and a 19p was about right for me with 19 feet. actual results may vary.....lol

cheers
oops

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

actual results may vary.....lol

I know that's right brotha! So many variables, so little time LOL. I hope I get the answers I need so I can get some good advice...

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

So, I took the boat out this weekend and this is what I came up with:

Boat Weight: 2600lbs (30 gallons of fuel) plus driver
Current prop: 14.5x17p
Trim: Set to all the way down(boat sitting even)
WOT 4400 RPM's
Speed: 32mph(boat speedo)
OTA: 85
Water Temp: 74
Light Chop

The performance issue(s) that i am trying to correct is loss of speed on turns when pulling a tube. The hole shot is pretty decent I think and I'd like to get better top end but not necessarily a must. After doing some research I am leaning towards a 4-blade 15x16 aluminum. Would this be a good prop choice? Thanks.~Milt

 

[180shabah]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Trim: Set to all the way down(boat sitting even)
WOT 4400 RPM's

Properly adjusting the trim should give a couple hundered rpm and some more speed, moving squarely into the middle of your WOT range. You can safely step back an inch in pitch when changing to a four blade. Just watch your diameter - large dia props are generally designed for big heavy boats and thus don't perform well on smaller faster boats.

FWIW - something just isn't right here. Your boat is 19' and 2500lbs, mine is a foot shorter, but weighs very close to the same. I am running a 175hp 4.3 and am getting 15mph more from it, spinning a 21" Hustler to 4800rpm.

How healthy is your 4.3? have you compression tested it? full tune up? Are you running a 1.84:1 drive?

 

[180shabah]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

One more question:
You don't have one of those wings on your drive do you?

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

FWIW - something just isn't right here. Your boat is 19' and 2500lbs, mine is a foot shorter, but weighs very close to the same. I am running a 175hp 4.3 and am getting 15mph more from it, spinning a 21" Hustler to 4800rpm.

How healthy is your 4.3? have you compression tested it? full tune up? Are you running a 1.84:1 drive?

You are right. Looking on the NADA site, the net weight of the boat is listed at 3200lbs. Don't know why I thought it was 2500. Like I said, I have a 4.3L Mercruiser non EFI. I am the second owner of the boat and from what I heard the previous owner didn't run it very hard. As far as the 1.84 drive goes, I am not sure. I believe that the Alpha One outdrive is stock and has not been regeared, if that's even possible. And if you are referring to a "Doal Fin", then yes I do have one. Should I take it off? Thanks again...

 

[Tail_Gunner]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Properly adjusting the trim should give a couple hundered rpm and some more speed, moving squarely into the middle of your WOT range. You can safely step back an inch in pitch when changing to a four blade. Just watch your diameter - large dia props are generally designed for big heavy boats and thus don't perform well on smaller faster boats.

FWIW - something just isn't right here. Your boat is 19' and 2500lbs, mine is a foot shorter, but weighs very close to the same. I am running a 175hp 4.3 and am getting 15mph more from it, spinning a 21" Hustler to 4800rpm.

How healthy is your 4.3? have you compression tested it? full tune up? Are you running a 1.84:1 drive?

as above your getting 4cyl results from a v6

1. verify speeds with gps
2. verify tach
3.If they are verified and stay the same you have a engine problem
4. what year is the boat and is the carb a 4 barrell

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

as above your getting 4cyl results from a v6

1. verify speeds with gps
2. verify tach
3.If they are verified and stay the same you have a engine problem
4. what year is the boat and is the carb a 4 barrell

It's an '88 with a 2 barrel carb. Can I use the speedo on my X50 DS Lowrance fish finder or should I use a Garmin hand-held unit? And how would I verify the tach?

 

[Tail_Gunner]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

It's an '88 with a 2 barrel carb. Can I use the speedo on my X50 DS Lowrance fish finder or should I use a Garmin hand-held unit? And how would I verify the tach?

88....165 horse...

hand held....napa should have a cheap tach or walmart..try to find your gearing....Side note that motor might be a bit tired...

 

[mia9516]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

88....165 horse...

hand held....napa should have a cheap tach or walmart..try to find your gearing....Side note that motor might be a bit tired...

Why the confusion? My boat has a 4.3L Mercruiser Alpha One with a 2-bbl carb. The manufacture specs say 190hp. I will try to figure out what gear ratio is in the outdrive. How much of a difference will that make? And what am I looking at, ball park, for a motor rebuild?

 

[180shabah]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Why the confusion?

Looking at page 1B-4 of manual #7 the 2bbl is rated at 185 and the 4bbl is rated at 205hp. These are both crank ratings, so subtract 30hp for the modern prop rating equivilent.

I will try to figure out what gear ratio is in the outdrive. How much of a difference will that make?

If you have the wrong gear ratio, then you won't be puting as much power TO the prop.

 

[Tail_Gunner]

Re: Prop suggestion for a Mercruiser 4.3 sterndrive

Were just looking for potential issues...Some things just dont add up giving your gear ratio is part of the process it will help determine many things.....After all your prop is going to be your final gear and they do cost money...

That figure i threw out of 165 hp was just a WAG....But a 165 hp motor should push a boat of that size to at least the mid 40's or low 50's even