Alpha 1 gen 2 ratio change

[Gregory G]

Hello!, I have 99 Maxum cabin cruiser that comes stock with small block v-8 with 260hp i believe. The outdrive was a alpha 1 gen 2 with 1.47 ratio. I bought the boat about 3 months ago and came with all service done and maintenance paperwork. In 2008 it was re-powered with a 350 mag mpi. The outdrive was already in bad shape and went out after 4 or 5 trips. It had a 14.5 x 17 aluminum prop on it. At WOT i was at 4800 rpm doing about 34mph. The mechanic put on a new SEI 116 outdrive with a ratio of 1.62. I put on a 15x17 Black Max prop and went thru the break in procedure. Now i'm way over the max rpm range of 5000 at WOT. Actually cant even get to WOT. Any reason why he would changed ratio from 1.47 to 1.62? The mechanic wont get back to me and i changed marinas last week.Did he do because i have a 300hp engine now instead of the 260hp stock? Should i increase pitch and go with a 14.5 x19 prop to bring the rpm down. Thanks in advance for any input!

 

[harringtondav]

First, I'd consider moving this post to the Merecury Mercruiser Engines and Out Drive forum. There is a wealth of experience on this topic. I think you may get buy with a higher pitch prop, but that forum's pros know other issues about that ratio's ability to handle that set up. I'll ping achris AllDodge Scott Danforth for you.

 

[alldodge]

The mechanic didn't do you any favors going to that ratio and not changing the props, would have been better to change to higher pitch prop only. That said, at least you were on the low side of pitch so you have some room to increase pitch more.

Find a prop shop and buy a prop from them with the understanding they are willing to work with you to find the correct pitch. You could start with a 19 but I would suggest a 21

 

[Gregory G]

Thanks Harrington, I will repost

 

[alldodge]

Thanks Harrington, I will repost

Don't repost, the Mods will move this the thread

GA_Boater

 

[Gregory G]

Hello!, I have 99 Maxum cabin cruiser that comes stock with small block v-8 with 260hp i believe. The outdrive was a alpha 1 gen 2 with 1.47 ratio. I bought the boat about 3 months ago and came with all service done and maintenance paperwork. In 2008 it was re-powered with a 350 mag mpi. The outdrive was already in bad shape and went out after 4 or 5 trips. It had a 14.5 x 17 aluminum prop on it. At WOT i was at 4800 rpm doing about 34mph. The mechanic put on a new SEI 116 outdrive with a ratio of 1.62. I put on a 15x17 Black Max prop and went thru the break in procedure. Now i'm way over the max rpm range of 5000 at WOT. Actually cant even get to WOT. Any reason why he would changed ratio from 1.47 to 1.62? The mechanic wont get back to me and i changed marinas last week.Did he do because i have a 300hp engine now instead of the 260hp stock? Should i increase pitch and go with a 14.5 x19 prop to bring the rpm down. Thanks in advance for any input!

 

[WesNewell]

1.47gr 17p prop @ 4800 rpm = 53 mph with 0 slip. at 34 mph max, that 36% slip. That's no too good. Anyway, any time you raise gear ratio, you need to increase prop pitch to keep rpm's the same. And going from 17 to 19p should give you the same rpm as before.

 

[Gregory G]

Ok thanks guys,looks like i have to pitch up. Is there any logic as to why the mechanic would not replace with same ratio? I

 

[Maclin]

Don't know how big your boat is. It seems your engine was struggling before the repower, hence the 17p on there now. A larger boat will benefit from the 1.62, and get your pitch back up to the more optimum19-21 range. This may have been a factor in the drive ratio selected by the mechanic.

Or your previous engine was tired and the ratio change is unwarranted. Describe your boat a bit more in relation to weight, length, beam, usage, etc.

 

[Gregory G]

Hey Maclin, Thanks for the advice! Seems very plausible.Its a 2400 scr (24'), 5800lb without gear,persons,gas. Beam is around 8 foot and mostly used for family cruising and fishing. fuel tank is 70 gal.

 

[TyeeMan]

Just speculating here, it could be a patent infringement thing on the part of SEI. By that I mean SEI legally may not be able to make a 1.47 drive as it would be copying Mercs gear ratio.

About 8 years ago I had a boat with an SEI drive, the drive was new when I bought the boat. The drive always had a gear noise that I could hear at closed throttle position, in forward.
I called SEI as I was concerned it might be a gear failing. Nope. The person at SEI said it's how they cut the gears. SEI has to cut the gears differently than Merc due to patent issues. He mentioned something about the undercut of the gears.

True/false I don't know, but the drive continued to work flawlessly for the 5 years I owned the boat.

For what it's worth, SEI drives do not stand up to a Kenworth semi tractor. :joyous: Yep, that's how she met her demise. Oh, and the boat too. :blue:

 

[achris]

He's done you a favour (in changing the ratio). Yes change the prop to bring the revs back down....

Chris..

 

[jimmbo]

As Maclin pointed out the Mechanic may have done you a favour choosing a lower gear ratio. You say you're "Way over" the max 5000 rpm. I would verify that the Tach is accurate first. going to a 19' will drop the rpm approx 400. How was you speed determined, GPS or water pressure gauge? If GPS, the results can be inaccurate if there is any current, and readings in the opposite direction would be needed to get an averaged result

 

[Gregory G]

Thanks for the input guys! SEI does make a 1.42 ratio on there website. I will pitch up.

 

[Gregory G]

What i meant with "way over in rpm" is i stopped giving throttle after hitting 5000rpm with more room to go. Alarm was going off and I'm in the 5 to 10 hour break in period which is burst of WOT and scaling back to 75%. I did use GPS for speed and i will definitely look into a more accurate tach. I will do what ALLDODGE said and try to get a prop shop to work with me trying to find the right pitch

 

[achris]

1.62/1.47*17=18.73. So a 19" should be just about spot on...

 

[wahlejim]

There is a switch on the back of your tachometer. It allows you to chose how many cylinders you have. Make sure it is set to 8.

 

[jimmbo]

Why was the Alarm going off? Oil Pressure? Water Temp? I have not heard of an alarm for exceeding the WOT rpm range

24 feet is asking a lot of a SB engine. On a boat that size I would want a BB as it would provide much more torque than the SB.

 

[Maclin]

My 4.3MPI has an alarm when the revs go up to rev-limiter territory.

 

[achris]

Why was the Alarm going off? Oil Pressure? Water Temp? I have not heard of an alarm for exceeding the WOT rpm range

All MPI engines have over-speed alarms. For both in gear and out of gear. Exceed 3,000rpm in neutral and you'll get an alarm.

24 feet is asking a lot of a SB engine. On a boat that size I would want a BB as it would provide much more torque than the SB.

No, it's not. I put a 180hp 4.3 V6 (non-vortec) in a 23' boat (Caribbean Crusader hardtop) and it ran to 36 knots... Correctly propped, 300hp should be plenty for a 24' boat....

Chris........