Prop pitch and diameter for mercruiser 3.0 alpha w alpha one outdrive

Guymontag

Seaman Apprentice
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Oct 10, 2018
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48
. IveI'got a1994 scarab sprint 18. It's got a mercruiser 3.0 alpha with an alpha one outdrive. Not sure if it's the original or correct outdrive but the p.o mentioned he replaced it or rebuilt it.i just got the boat and it had no prop on it so I have no base line.i have 2 Quicksilver black diamond aluminium props.

A j 14 x23p wot 4800rpm I get 40 mph
​With that prop @3600 rpmrI get 29mph(gps). Kinda studders on the acceleration out of the hole for a second or 2 but gets on plane quick enough.

So I switched to a Quicksilver black diamond
J14.25x21p and I get wot 5200rpm 38mph 3600rpm and 25 mph. no hesitatiom out of the hole and up onn plane in about the same time maybe slightly quicker.

Im looking to find a prop for top end speed I don't do watersports or anything just run the boat straight 5 miles or so to diff restaurants bars. So I'd like to get the boat fastest top end with out killing the motor. Once I get the right diameter and pitch I will get a ss prop or something nice.

I don't know the correct diameter or pitch and the prop calculators i used I don't have all the info, and the selections are all over the place.
Maybe the boats just under powered and I'm gonna have to live with the 29mph? These boats came with the 4.3 or 3.0 iv read people saying the 3.0 should get to 50mph?
 

TyeeMan

Master Chief Petty Officer
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Feb 27, 2006
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849
Speed is pretty much irrelevant, you prop the boat so the engine runs at WOT in it's WOT RPM range. 5200 is too high ( I'm pretty sure) and 4800 should be right on, , on the high side.

So first off you need to know what the boats WOT range is. Should be something like 4400-4800 RPM. You want to run a prop that gets you into that RPM range when you have all the stuff you would have in the boat all the time, and people you would have in the boat some of the time.

In my opinion, if it's propped correctly, you should need to back off the throttle (otherwise you'll over rev) when your all by yourself, and with your buds and their stuff in the boat you should not be below 4400 or whatever the lowest RPM is of your WOT RPM range.

Prop diameter and pitch go hand in hand, you don't get to choose what diameter you want and what pitch you want.

If you are running an aluminum prop that gets you perfectly into your WOT RPM range, switching to a stainless prop will drop the rpms by 200 to maybe even 400 rpm. So it could be possible that your 14.25 X 21 which is now giving you 5200 RPM at WOT, if that were a stainless prop of the same diameter and pitch, that could bring you right into the proper RPM range.

That being said, prop style, and any cupping that has been added to the blade can make a big difference as well.

The 3.0L could possibly hit 50 mph but it would be in a very light boat. I had a 17 1/2 ft Lund aluminum fish and ski type boat with a 3.0L. Had it perfectly propped and I could get 42 mph GPS if everything was right.
 

QBhoy

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Mar 10, 2016
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Agree with bondo. If you are turning a 23” at 4800 you are propped perfectly. I’d only maybe say that without looking at the numbers on a prop slip basis...maybe you should be seeing more speed than that. Even with the lowest of ratios on the Alpha. I know guys who can only turn a 21” on a low ratio Alpha, 3.0 merc and they are seeing early 40’s at around 4600 rpm give or take. Nothing special hull wise. They are bayliner 175 and a maxum 1800
 

Ed Harris

Petty Officer 2nd Class
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Jan 7, 2016
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I am running a wellcraft 1985 170 American with an aluminum 14x19. Cruised at 3200 and 30mph gps. Full tank only me in the boat. Top end 4300 and 40 gps. Tank still full
 

QBhoy

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I am running a wellcraft 1985 170 American with an aluminum 14x19. Cruised at 3200 and 30mph gps. Full tank only me in the boat. Top end 4300 and 40 gps. Tank still full

More like it Ed. That’s numbers to be expected I’d think. Rpms a little low maybe ?
 

Guymontag

Seaman Apprentice
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Oct 10, 2018
Messages
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Hmm guess I'm right on then. maybe weight is 2k 16ft on the water line thought I'd be going alot faster. But Ed seems to have a similar boat and similar speed. I guess the old "what's the best prop for top end" question is one that'll get alot of responses. .but.. .. haha any recommendations? OrO maybe my motors notn wellw iI did a compression test it was good. Im Trygi to get every mph out of it. .
 

Scott06

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Apr 20, 2014
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I would verify the tach is accurate. 23" is awful big for a 3.0 to get to 4800. My 91 Sea Ray 170 with 3.0, could hit 5000 witha 17" , 4700 with a 19" . Only thing is if you have a high altitude 2.4:1 drive vs typical 1.98:1 , which is not likely
 

harringtondav

Commander
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May 26, 2018
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This is empirical - seat of pants reasoning. I think you're doing well with your 3.0. I don't know the hp of a '94, but I'd guess 160 tops. My '96 pre-vortec 4.3 is rated at 180 hp in a 2500# boat. Top speed is 52 mph speedo with my 14x23, but it is too much pitch for 3000 rpm cruising. The engine is working and heating up a bit more than I like. A 21P is great as long as I use the right hand rev limiter. 5200 rpm & 48 mph speedo.

Your engine is fit enough to deliver 4800-5200 rpm. That's all your props care about. Tweaking your engine may give you a better hole shot, but pitch x rpm x slippage = boat speed.
 

TyeeMan

Master Chief Petty Officer
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If your looking for a speed demon you have the wrong engine.
Do a full tune up, ensure timing is correct, prop it correctly and work with what you have.
Any other improvements like a stainless prop will be marginal at best.
 

QBhoy

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I always thought that 4800 was the max for the wee 3.0 like this ?
as mentioned earlier. Something doesn’t seem right to me. If you can turn a 23” at 4800 on that rig...even at the lowest of ratios...you should be flying. Would be extremely unusual to turn a 23” at the max 4800 rpm and only 40mph on your set up. As mentioned. Best I’ve heard similar doing is early 40’s to max 45 gps with a 21”. And that running light and going well.
 

Guymontag

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Oct 10, 2018
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I think max rpm is 4600 4800 . Guess that's all I'm gonna get. Small boat with the letters scarab that looks fast ,but guess I got the wrong motor. I'm still ok with it. I'm new to powerboats so let me ask this i read an article that said the "new" 4 blade props are just as fast top end? I think I'm gonna stay with the 21 pitch the motor feels better at thhelow end .plus I tested both props with 1/2 tank ,no gear, and flat calm. I assume when it gets choppy the 21 will push through waves better aswell? Using the 21 as a baseline if I were to upgrade to a ss prop or a 4 blade. What do you guys think of that worth it?
 

Guymontag

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Oct 10, 2018
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P.s compression tested good .I replaced the coil pack and ignition module it has an electronic ignition. And ran a new fuel line and fuel filters. So I ought to check the timing. Wonder if my carb should be tuned too.
 

jimmbo

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May 24, 2004
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Getting 40 mph on a 18 ft boat, with only a 3.0 litre is doing very good. Getting 50 out of a 3 litre? It might do it in a light weight racing hull. Anyone telling you should get speeds like that is; Pulling your leg, or talking/breathing through a hole in the top of their head
 

Vroom_vroom

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May 7, 2019
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Your doing well with the numbers you have given, had a beat up black max 21 on my maxum 1800 (3.0 as well) and I couldn’t get to 4K rpms with two guys @37
swapped to a 19 turning point and can wot at 4600 @37 as well. Sounds like your already ahead of most of us with the 3.0
 

Guymontag

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Oct 10, 2018
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Saw this online the 3.0 lx is rated at 135hp says a 13.5 23p will be fastest? Seem right given my numbers? Screenshot_20190506-210712_Chrome.jpg
 

Starcraft5834

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I had a 3.0, same engine on a heavier cuddy.. i got 30 and was thrilled, 40 is good for that 3.0.. your hitting your numbers, that's all your going to get out of it. 50? you'd get that dropping it off a cliff only :) enjoy
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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with a 3.0, your getting all she has. run it above 4800 RPM for 5 miles and you will learn to rebuild the motor (3.0's dont like to run higher RPM's and live)

if you want 50mph, you need to put it on a trailer and tow it at those speeds..... or get a bigger motor and different drive ratio
 
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