New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole shot

[simon_canada]

Hi Guys,

Not sure that this is the perfect sub-forum for this, but it seems most logical for the moment.

We had the unfortunate luck a couple of weeks back of finding a rock shoal in our lake that we didn't know about before.

As a result the lower unit and propeller were replaced. The prop we destroyed was a 19 pitch, and the new prop is at 17.

We thought that the decrease in pitch would give us a loss on the top speed, but a better hole shot, which is what we were looking for for water sports.

The boat is a 17' with a 3.0L Merc Alpha One (gen II).

The marina replaced the lower unit, and advised that these boats does not appear to have different gear ratios in the lower unit, only the upper.


We are now seeing a max rpm at WOT of 3900 rpm. Top speed is down from 45mph to 41mph (as you'd almost expect).

Hole shot is terrible. doesn't bog, and is smooth, but it's like we are playing with a 65HP engine, and struggles to get someone out on two skis, when it was ripping skiers out on one before.

The only other change the marina said they made was adjusting the throttle linkage between the throttle body and handle so as to make it easier for us to find neutral (which we haven't seen).


What we can't figure out is why, if we lost the top end speed as we expected, why didn't we get the boost at the bottom (and in fact a loss?). Something else had to have changed, though the variables we know were changed are the lower unit and the throttle linkage (which we can see does in fact open properly at WOT)....


We've dropped it back off to the Marina, but I'm at a loss as to what could be up here...

Any thoughts on why a boat with a lower pitch prop would loose so much in the way of RPM, and hole shot?


Thanks in advance, folks!

 

[Texasmark]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

If the props were of the same design/mfgr then you may have a different gear ratio in your LU. Let's face it, you changed the LU and the prop and now things are tanking. I am not an I/O guy any more so I don't know where they change the gear ratios for the Alpha LU's. I do know that they have several to offer.

Go post this in the Mercruiser or whatever brand of outdrive you have section and you will probably get your answer.

Mark

 

[oldjeep]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

There are 2 ratios available in an Alpha one Gen 2 lower unit. 2.00:1 and 1.65:1 The 3.0L uses a 2.00:1

 

[Texasmark]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

There are 2 ratios available in an Alpha one Gen 2 lower unit. 2.00:1 and 1.65:1 The 3.0L uses a 2.00:1

Looks like possibly your smoking gun. It's not dooms day, just requires tweaking the prop to fit the different ratio.

Mark

 

[oldjeep]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Looks like possibly your smoking gun. It's not dooms day, just requires tweaking the prop to fit the different ratio.

Mark

No, it requires swapping on the correct lower unit

 

[Texasmark]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

No, it requires swapping on the correct lower unit

Hows that?

 

[oldjeep]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Hows that?

Since he paid someone who supposedly knew what they were doing, they should install the correct lower unit for his boat.

The 3.0L is supposed to have a 2.00:1 lower unit. What prop size do you suggest to compensate for a 1.65:1 lower unit, and do you think that compensating with a prop is going to work through the torque curve? If what you are saying made sense then they would not mfg a 2.00:1 lower unit specifically for the smaller engines

 

[GoFastr]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Go back top the mech and begin inquiring about the lower unit he/they installed.

What is the factory rpm range for your motor/boat and the actual U had wide open before the rock, and what is it doing now?

Another consideration, especially if it happened to be a stainless prop, is damage to the upper end unless they popped the cap off and did an inspection. But start with the mech and and have them reassure thru intelligent explanation the unit is right. Then go from there.

 

[Texasmark]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Since he paid someone who supposedly knew what they were doing, they should install the correct lower unit for his boat.

The 3.0L is supposed to have a 2.00:1 lower unit. What prop size do you suggest to compensate for a 1.65:1 lower unit, and do you think that compensating with a prop is going to work through the torque curve? If what you are saying made sense then they would not mfg a 2.00:1 lower unit specifically for the smaller engines

Interesting question about torque curve. I'm not a marine propulsion engineer so I don't know the equations. I have always wondered about the Merc 100 up running a 2:1 gearbox whereas the 90 runs a 2.3. I mean you are talking about roughly 10% which could easily be picked up in a 10% change in prop pitch. Course they market the 90 as a workhorse.....so.... Probably if you take it to the limit, there are some installations, like pushing a barge, where the difference could matter in trying to get a prop with a small enough pitch to do the job.

Mark

 

[90stingray]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

There are 2 ratios available in an Alpha one Gen 2 lower unit. 2.00:1 and 1.65:1 The 3.0L uses a 2.00:1

The 3.0 does use the 2.00 ratio. But there is a 1.84 Ratio available in the alpha as well. as the others mentioned above. OP, did the marina just change the lower half of the drive? If that is the case... your gears will be the same since the ratio is determined by the upper only. But, if the entire unit was changed... why dont you look at the drive and see what ratio is stamped in the housing? Like others have said, you need the right gears to make it work. They already tried to make up the difference with a prop... and you already know that doesnt work. The people that did the work should make this right.

 

[oldjeep]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

The 3.0 does use the 2.00 ratio. But there is a 1.84 Ratio available in the alpha as well. as the others mentioned above. OP, did the marina just change the lower half of the drive? If that is the case... your gears will be the same since the ratio is determined by the upper only. But, if the entire unit was changed... why dont you look at the drive and see what ratio is stamped in the housing? Like others have said, you need the right gears to make it work. They already tried to make up the difference with a prop... and you already know that doesnt work. The people that did the work should make this right.

Um- no the upper has a ratio and so does the bottom. The lower unit comes in a 2.0:1 and 1.65:1

 

[90stingray]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Um- no the upper has a ratio and so does the bottom. The lower unit comes in a 2.0:1 and 1.65:1

Well some beech... i have never heard of any such a thing. Everything i have read makes it sound like they are all the same. How would one find out what lower ratio one has... other than counting the gears?

 

[oldjeep]

Re: New prop after rock strike (17) previously 19, now lower rpm at WOT, weaker hole

Rotate input shaft, watch output shaft - same way you check the gear ratio on a chevy

Or if it is still relatively new, then look at the sticker that has the gear ratio on it.