Changing gear ratios -vs- prop size

[bhanson]

I have a 5.0L Mercruiser Alpha 1 w/ Hirel TBI on my 19' Rinker runabout. Currently has a High Five 21P prop and the hole shot is weak, struggling to pull a slalom skier out of the hole.

Turning 4,800 RPM WOT and about 3,000 RPM hole shot while it sits there pulling the skier out of the water. Local prop shop is scratching their heads; High Five should give great hole shot, RPMs seem right so reluctant to go lower pitch.

But then everyone says "yeah, but also depends on the gear ratio in the outdrive." I always thought the gear ratios and prop sizes were somewhat complementary and that one could compensate for the other. First of all, how do I determine the gear ratio in my outdrive, and second, is that a potential source of my hole shot issue? If so, how involved is it to change the gearing on an Alpha 1 drive?

 

[MikDee]

Re: Changing gear ratios -vs- prop size

Gear ratio should be fine for this engine, if it's stock? and it sounds right with that prop. Something else seems wrong, is your prop hub, or your engine coupling slipping some?

 

[Chris1956]

Re: Changing gear ratios -vs- prop size

Bhanson, If the WOT RPM is good (it is), you have some other issue. Gear ratio is fixe, and if you can get to 4800, it is correct for that application.

Some wild thoughts. Is the TBI working properly? Does it richen the mixture when you want to accelerate? Check to see if any plugs are fouled at low speed by water or fuel. Does the boat have a bunch of extra water weight? Are you trimming the motor in for takeoff?
you can try a 3 blade prop of the same pitch and see if things improve.

 

[bhanson]

Re: Changing gear ratios -vs- prop size

Guys, first of all, thank you for the replies. You have been a tremendous help before and already great thoughts on this one as well.

Everything stock (as far as I know), no extra water weight (been there before and that sucks !!!), and actually changed out to a 3-blade 21P this weekend and saw no difference.

I got the 3-blader from a REAL prop shop and they changed out the hub for me, so assuming everything is OK there. Will check plugs this evening. How would I check for "engine coupling slippaging"? Same for the TBI; how do I test it?

 

[Haut Medoc]

Re: Changing gear ratios -vs- prop size

21P sounds like too much pitch to me for a 5.0.......
What diameter?.....
You need to go to a lower pitch & suffer some top end loss if you want to ski......
Lugging your engine is about the worst thing for it......​

 

[bhanson]

Re: Changing gear ratios -vs- prop size

The High Five is 13.25" diameter, and the 3-blade I tried this weekend was a 15" diameter. Wondering as I go lower pitch what will happen to WOT RPMs since I am already at 4,800 with both those props.

Lugging the motor? I am getting 3,000 RPM during hole shot. How many RPMs should I be getting out of the hole?

 

[wire2]

Re: Changing gear ratios -vs- prop size

I have to side with Haut;
my 19 foot Glastron bowrider came from the dealer with a 21" but I have a 5.7 260 hp. It seemed just right all around for 1-2 seasons until I upped it to 300+ hp but that's another story.
A .7L difference will be noticeable, try a 19" for slalom skiing, put the 21 back on for cruising.

 

[tommays]

Re: Changing gear ratios -vs- prop size

You can install a test prop and hit 4800 tied up at the dock



A high 5 may fully load the motor at 4800 RPM when it is making its full power but still be to much prop at lower RPM like 3000 were the HP is much less



Tommays

 

[Bondo]

Re: Changing gear ratios -vs- prop size

Ayuh,........

Sounds to Me, Tommays on the Right track,.......

You're trying to Ski with a prop pitched for Cruising........

Use the 3blade Prop for Cruising,+ Dump the 5 blade for 1 with a 19" pitch for Skiing.........

 

[bhanson]

Re: Changing gear ratios -vs- prop size

Alright, thanks. Per recommendations will check plugs and change prop, then report back.

 

[MikDee]

Re: Changing gear ratios -vs- prop size

Guys, first of all, thank you for the replies. You have been a tremendous help before and already great thoughts on this one as well.

Everything stock (as far as I know), no extra water weight (been there before and that sucks !!!), and actually changed out to a 3-blade 21P this weekend and saw no difference.

I got the 3-blader from a REAL prop shop and they changed out the hub for me, so assuming everything is OK there. Will check plugs this evening. How would I check for "engine coupling slippaging"? Same for the TBI; how do I test it?

You have a point bhanson, if your prop was too aggresive with pitch to begin with, and you were lugging it to much (extra weight, & pulling up a heavy skier slalom) you just might have overloaded the coupling, and it's on the way out! a good indication was getting the same result with the 3 blade prop. You didn't mention your top end speed, but at 4800rpm (which sounds a bit high) with a 21 pitch prop, you should be seeing over 54mph, or it's slipping!. Another thing, if your engine is revving to 3000rpm as you begin to come out of the hole, it don't sound right? It should take a dive some, then rev up gradually as you increase speed. I'd suspect that coupling, it is the weakest link right now.

 

[QC]

Re: Changing gear ratios -vs- prop size

You didn't mention your top end speed, but at 4800rpm (which sounds a bit high) with a 21 pitch prop, you should be seeing over 54mph,

Whoaaaaaah, the propeller HAS to slip. I get 53 MPH with a 1.62 ratio and 10% slip, which would be great. Let's not get ahead here . . . That coupling is the LAST place I'd be looking at this point . . . As you said, we don't have the speed number.

bhanson,

Before you do anything, we need a GPS speed in no current . . . Year model and serial number would be nice too if we are depending on the ratio. Bondo and tommays are on this . . . There is no problem mechanically at this point unless it is running like carp. Again, we need a speed number

 

[MikDee]

Re: Changing gear ratios -vs- prop size

Whoaaaaaah, the propeller HAS to slip. I get 53 MPH with a 1.62 ratio and 10% slip, which would be great. Let's not get ahead here . . . That coupling is the LAST place I'd be looking at this point . . . As you said, we don't have the speed number.

bhanson,

Before you do anything, we need a GPS speed in no current . . . Year model and serial number would be nice too if we are depending on the ratio. Bondo and tommays are on this . . . There is no problem mechanically at this point unless it is running like carp. Again, we need a speed number

That is with prop slippage, I'm figuring standard 1.5 gear ratio as is used on all the Mercruiser V6's, & V8's, Whatever,,, I don't think the 5.0 in that boat can go that fast, maybe a 5.7? but, I think he's overrevving at WOT too. I was trying to prove a point here, another thing, he says he's revving to 3K rpm coming outta the hole! go figure, something ain't right? I still say it''s the coupling.

 

[QC]

Re: Changing gear ratios -vs- prop size

5.0s are 1.62 . . .

 

[MikDee]

Re: Changing gear ratios -vs- prop size

5.0s are 1.62 . . .

Ok, maybe so, on the Alpha? it wouldn't make that much difference anyway. I'm more familiar with the Type 1 outdrive, I remember reading it in the service manual on those, I had 5 of them, never made it to an Alpha,,,lol,,, But, think about it, I've had couplings go out on me, I ask you this, when does your prop spool up to 3k rpm as your coming out of the hole? especially a high five,,, Maybe your shaft is, Yes,,, but not the prop, unless you're cavitating?,,, but, that would also explain a 5.0 pulling 4800rpm with a 21" prop at WOT, but not matching speed. I'd like to see a spooling prop at 3k rpm grab suddenly,,, it would be like being shot out of a canon,,,lol,,, sorry, had to go there for a giggle.

 

[MikDee]

Re: Changing gear ratios -vs- prop size

By the way, we all know alot of people that put an overpitched 4, or 5 blade prop, (for max speed) on an overloaded boat, then pull overloaded tubes, or ski's way to fast, with a fairly powerful I/O motor, then wonder why either the prop hub gives out, or the coupling gives out, ask me how I know,,,lol,,, Yeah, I've done it, but under different dumbass circumstances.
Either that, or they're wondering why their 3.0 motors blew?,,,lol

 

[QC]

Re: Changing gear ratios -vs- prop size

Ok, maybe so, on the Alpha? it wouldn't make that much difference anyway. I'm more familiar with the Type 1 outdrive, I remember reading it in the service manual on those, I had 5 of them, never made it to an Alpha,,,lol,,, But, think about it, I've had couplings go out on me, I ask you this, when does your prop spool up to 3k rpm as your coming out of the hole? especially a high five,,, Maybe your shaft is, Yes,,, but not the prop, unless you're cavitating?,,, but, that would also explain a 5.0 pulling 4800rpm with a 21" prop at WOT, but not matching speed. I'd like to see a spooling prop at 3k rpm grab suddenly,,, it would be like being shot out of a canon,,,lol,,, sorry, had to go there for a giggle.

I agree, could be ventilating, but I just think we are ahead of the game here without a verified speed. Heck we don't have verified tach readings . . . I am always suspect when the numbers don't add up. The 1.62 made me feel a little better as I did have a 1.5 Alpha and 5.7 EFI that would run 57 MPH with a 21 High 5 at 5000 ish . . .

 

[bhanson]

Re: Changing gear ratios -vs- prop size

You guys provided tons of great info and I am just now getting back with some answers. I do not have a GPS to confirm top end speed; however, I had a speedo then installed a Perfect Pass and speed was consistent, as is the RPMs between PP and my tach.

Plugs were fine, hub is fine, and shop says coupling is fine. Next step was prop, so recently switched from a 21P High Five to a 19P High Five. HOLY COW !!! Cannot believe the difference that 2" of pitch made. Pulling my arms off now.

The difference seems much more profound than just changing 2" of pitch, so almost like the 21P had crossed some negative performance threshold and the 19P brought be back into the right power band.

All good now. Thanks a ton.

 

[Frank Acampora]

Re: Changing gear ratios -vs- prop size

Not sure if it would apply to your outdrive but what the H--gear ratios and pitch are gear ratios and pitch.

I have found on my 4 cylinder Force outboards that the lower units are 1.78 to 1 ratio. NOW: if I change to a 2.0 to 1 ratio, and increase prop pitch by 2, (for example, 17 to 19) the theoretical top speed is almost exactly the same. However, the actual top speed is about 5 MPH higher, engine rpm is almost the same (less than 100 different) and the hole shot is slightly better.. Not sure why it works, only that it does. I can only assume that the higher pitch prop turning slower through the water has less resistance and/or less slip.

So, for example, my Chrysler 140 on a 21 foot cuddy topped out at 33 MPH at about 4500 RPM with a 1.78 to 1 lower and a 17 pitch prop. Changing to a 2.0 to 1 lower and a 19 pitch prop gave me about 4500 RPM at 38 MPH.

BTW: 4500 is at the lower end of 4500-5500 running range but I will accept it.

 

[Distinctive636]

Re: Changing gear ratios -vs- prop size

i have a 89 19' baja open bow with a 4.3 mercruiser, and i have two different outdrives for it. the motor just got rebuit and i stuck the non-orginal outdrive in it. My stock alpha one outdrive gear ratio is 1.84 and the non orginal that is in the boat has a 1.98 gear ratio now what im asking is that is there a big difference between the outdrives? is it going to effect my performance? right now there is a 22 pitch 3 blade prop on it i dont know the rpms or speed cause its still in the shop and he put a new waterpump and changed the oil in the non orginal so im really hoping that it will work thanks