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mercruiser 4.3L alpha one speed prop

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cdbrummond

Contributing Member
1989 Lund Baron with a 4.3L V6 Vortex engine. Has an aluminum 19p prop on it and does about 25-30mph@4000rpms depending on weather. I would like to get the speed up to 28-35mph@3500-4000rpms to take the kids tubing. What is my best option for a prop to achieve my goal? I don't want to spend $600 on some fancy stainless steel prop. my budget is $100-$200. thanks in advance
 
speed is based on propeller rpm . engine rpm is based on the prop size and pitch.
engine rpm limits are based on the prop, if your under the wide open throttle rpm, use a smaller prop, if your above the max WOT rpm use a bigger prop.
Assuming the engine is healthy it should turn the 19P close to max rpm. So what health is the engine in?
 
Bt Doctur said:
speed is based on propeller rpm . engine rpm is based on the prop size and pitch.
engine rpm limits are based on the prop, if your under the wide open throttle rpm, use a smaller prop, if your above the max WOT rpm use a bigger prop.
Assuming the engine is healthy it should turn the 19P close to max rpm. So what health is the engine in?
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The engine runs great. Never had any problems. Only 150hrs. I was told a 21pitch would get more speed but It can get very confusing when researching it online.
 
If you are only tubing and want the boat to plane off quicker and not trying for higher speeds then go to a 17 pitch.

Your boat at wide open throttle should be reaching ~ 4800 rpm.

If you go to a 21 pitch you will now reach ~ 4500 rpm

If you go to a 17 pitch you will reach ~ 5100 rpm.

So with a 21 p, it will take a bit longer to plane off but max MPH will increase slightly (2-5 mph) and that will be at wide open throttle (max rpm) (typically not what you want towing)

with a 17 p you will plane off quicker (thus the hole shot) but you could possibly be over the MAX rpm specification and you will loose some mph (2-5 mph) even though your rpm has increased.
 
cdbrummond said:
Not too concerned about hole shot. Just want to go faster with the kids on a tube.
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Ayuh,..... You'd better be concerned with holeshot, or it'll never get on plane, especially with tubes in-tow,....

Yer motor only makes so much horsepower,...
Ya gotta prop the drive, so the motor can pull 'bout 4800 rpms at Wot,...
If the speed is to low at that point, you've run outa horsepower,...
A bigger pitched prop, at that point will not go faster, but will go slower, luggin' the motor, causin' detonation, which will burn down the motor to scrap iron,....

If ya under prop the motor, you'll have more power, but must be careful not to over-rev the motor,....

Top speeds will be whatever it comes out too,....
 
Ayuh.

I'm with Bondo on this one. Better to be in a 'lower' than needed gear than too high a one. As for top speed, you can't have both....unless you carry a second prop with you and a prop wrench (which one should have onboard in any case).


Jeff
 
fastjeff said:
Ayuh.

I'm with Bondo on this one. Better to be in a 'lower' than needed gear than too high a one. As for top speed, you can't have both....unless you carry a second prop with you and a prop wrench (which one should have onboard in any case).


I think I am more confused now then when I started this topic. I am not concerned about hole shot. The boat plans out fine, it has plan fins in the back that are adjustable mounted on the bottom of the boat. I just want a little more speed to take my kids tubing. I currently have a 14 1/4 x 19p prop. Tops out at 30mph@4000rpms. Is there a prop that will give me more speed? 21p, 15"x19p etc. just concerned about speed here, not hole shot. I don't need to get to 30mph+ in 2 seconds. Just want more speed.
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Tops out at 30mph@4000rpms.
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Ayuh,.... At 4,000 rpms at Wot, yer Luggin' the motor,....

Either the motor has a problem, or the prop is to Big, at 19",....

Yer motor has to be able to run 'tween 4400 rpms, 'n 4800 rpms at Wot,....
Or it's Luggin', All the time,....
That'll lead to detonation, 'n a burnt up motor,...

Ya can't go faster than you've got the horsepower to go,...
 
bondo said:
Ayuh,.... At 4,000 rpms at Wot, yer Luggin' the motor,....

Either the motor has a problem, or the prop is to Big, at 19",....

Yer motor has to be able to run 'tween 4400 rpms, 'n 4800 rpms at Wot,....
Or it's Luggin', All the time,....
That'll lead to detonation, 'n a burnt up motor,...

Ya can't go faster than you've got the horsepower to go,...
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The rpms top out @ 4000. I suppose the plaining fins mounted on the bottom of the boat may be creating some drag thus not allowing the motor to reach 4400-4800 rpms. I will have to remove them and see if that gets my rpms up as well as speed.
 
If you are only reaching 4000 rpm then you do have an issue.

Double check your prop to be sure it is NOT a 23 pitch or so.

Most boats like yours are designed to run a 19 pitch prop and reach specified rpm in your case 4600 rpm.

Some require one pitch up or down based on many factors but typically most boats such as yours run 17-21 pitch.

if you are already in that range there may be something else going on...
 
cdbrummond said:
I think I am more confused now then when I started this topic. I am not concerned about hole shot. The boat plans out fine, it has plan fins in the back that are adjustable mounted on the bottom of the boat. I just want a little more speed to take my kids tubing. I currently have a 14 1/4 x 19p prop. Tops out at 30mph@4000rpms. Is there a prop that will give me more speed? 21p, 15"x19p etc. just concerned about speed here, not hole shot. I don't need to get to 30mph+ in 2 seconds. Just want more speed.
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So your boat has trim tabs? Are they power adjustable or do you put them at a fixed position? The vast majority of trim tabs are excellent for helping get out of the hole and have tiny, if any, impact on top speed.

KGhost nailed it. If you can only reach 4000RPM @ WOT then you are already over propped or something in your drivetrain isn't quite right. You should spec out a prop to achieve max RPM with a 'normal' load in the boat which in your case should be 4800 RPM.

If you put a 21P or a 23P prop on the boat your acceleration will suffer big time, your WOT RPM will go down, and you probably won't see much increase in speed.

KJ
 
Towing anything puts a bigger load on the engine. Asking for more speed AND more tow capacity from a single prop is not going to happen, especially if you are correctly propped now. Correctly propped is defined as: Engine revs up to within the specified Wide Open Throttle range (often printed on the decal on the top of the flame arrestor) with a "normal" load. What ever speed you get with that prop is all the speed you are ever going to get... unless you run with an almost empty tank, no gear on board and are alone and weigh less than 100 lbs...with a tail wind on flat calm water.
 
I'd say remove the unessentials from the boat - its the cheapest option to improve things - and get the correct size prop
 
just an update on the boat. I can see some morons showed up at this post.
thanks bondo, bt doctor and kj for giving me lots of good info. I took all the accessories off, plan fins and trolling plate, and it made a huge improvement. the boat topped out at 37mph. the rpms got up to 4325 on the dash gauge and 4250 on a hand held. I can see that I'm still not in the suggested range for rpms. the boat had 60 gallons of gas and 400lbs or gear and people. I don't mind sacrificing some speed to get the motor in the right range. any suggestions? should I drop down to a smaller pitch or diameter? thanks again
 
What length is this boat? 60 gallon tank would tell me it is 21 ft or bigger.
If this is the case then a V6 is a bit under powered.
That being said, please tell us what pitch prop you have now.

But i would say go down one pitch to start. That should get you ~200 rpm gain. Possible a bit more....but going down 2 pitch would most likely be best.
 
Thanks for the advice kg. It has a 19p on there now. I think if I get a 17p stainless I will fix my rpm problem and not sacrifice much speed. I would like to stay within the 4400-4800 range pulling the kids on a tube so if I can get it to 4600-4700 under normal use I should still stay in the 4400+ range pulling the kids.
 
"Pulling the kids" @ wide open throttle is a recipe for shortened engine life. Actually running the engine for any length of time at WOT shortens life drastically. Once you get the boat so that you can get the WOT RPM to be within the specified range, then for good engine life you should limit continuous operation to 80% of the actual max rpm you get. For even better life, 70 to 75%.
 
I am no prop expert but I think you cannot compare a 17 pitch stainless steel to a 17 pitch aluminum.

Hopefully someone with a better understanding will chime in.

I believe if you are running a 19 pitch aluminum now then go to a 19 stainless, that is the same as going to a 17 aluminum.

I could be backwards and it would be like going to a 21 aluminum, but I think you get the point.

So be sure before you purchase anything
 
kghost said:
I am no prop expert but I think you cannot compare a 17 pitch stainless steel to a 17 pitch aluminum.

Hopefully someone with a better understanding will chime in.

I believe if you are running a 19 pitch aluminum now then go to a 19 stainless, that is the same as going to a 17 aluminum.

I could be backwards and it would be like going to a 21 aluminum, but I think you get the point.

So be sure before you purchase anything
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Spend the extra few bucks and buy your prop from a good prop shop (rebuilder) and not from the internet.
 
Gonna try and rent a 17px14 1/4 stainless next week and see if it fixes my rpm problem. Will let you all know. Researching prices and makes of stainless props is even more confusing...lol. The local Marina told me to stick with a merc made product, mirage or enertia. I'm not gonna complicate things any more by researching all the other stainless props out there.
 
Just remember one thing.....aluminum will bend and break typically before the prop shaft and gears.....stainless steel does not....
 
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