Replaced outdrives, now I'm 8-10mph slower
- Thread starter SeaSkip
- Start date
SabrToothSqrl
Well-known member
What was engine RPM before and after? Was the gearing altered? Same props?
toofast
Active member
Well they could have put on the wrong props.....
They could have dinged them....
They could have put them on the wrong way....
Your RPM's now are WAY LOW, as you know....get some more info and we can help you out.
Remember, you can lift the drives and get your hands on them to feel if something feels wrong....assume no vibration though as you did not comment on it.
So my notes from the day the boat was delivered are as follows:
4950 RPM @ 48.5 MPH. Conditions on the lake were fairly clear by a bit windy too.
Since then, I have maxed speed out at around 51.5 on the lake and about 55.3 on Puget Sound with the same RPM.
I got a message back from Formula TAG stating that I should verify that the gear ratio is still 2.0:1 and verify , as you stated, that they didn't mess the props up.
I'm taking her out this weekend to verify my RPM on both sides and will have my service center take a look at it all.
SabrToothSqrl
Well-known member
Add some super chargers. That will bring the speed back up 
AllDodge
Well-known member
If they were complete new drives from Merc they should have the drive ratio stamped on the side, next to the serial numbers.
If ratio is not there, it can be determined.
Disable the coil by disconnecting the harness wires going to it. Also pull coil wire just incase
Mark the harmonic balancer and makes no difference where. Just need to be able to count the revolutions
Use a grease pencil and mark one blade of the prop
Get a remote push button starter switch. Connect the two leads on the starter, main post and small post on the right.
Shift into FWD or REV and crank the motor over 10 times by the balancer, have another count the prop rotations
If ratio is not there, it can be determined.
Disable the coil by disconnecting the harness wires going to it. Also pull coil wire just incase
Mark the harmonic balancer and makes no difference where. Just need to be able to count the revolutions
Use a grease pencil and mark one blade of the prop
Get a remote push button starter switch. Connect the two leads on the starter, main post and small post on the right.
Shift into FWD or REV and crank the motor over 10 times by the balancer, have another count the prop rotations
So, it was a beautiful calm day Sunday and I took the boat out. There were no other boats on the lake so I had glass to run over.
It was interesting that I got different results but here's what happened:
With the trim tabs fully down, the boat got to between 43-44mph
With trim tabs fully up, I get to 46mph.
So, I'm close. I don't know why it was so much different on my last run, but I'll keep an eye on it.
That said, the starboard and port have different max RPMs. At WOT with the trim all the way up, I get about 4850 on port, and 4740 on starboard.
I'm going to have her hauled out and have the bottom washed/waxed next month. So hopefully that will get me closer to my launching conditions. I'll inspect the props while I have her out. I'm not sure why the starboard is slower though. That may also make up for some of the difference in speed.
It was interesting that I got different results but here's what happened:
With the trim tabs fully down, the boat got to between 43-44mph
With trim tabs fully up, I get to 46mph.
So, I'm close. I don't know why it was so much different on my last run, but I'll keep an eye on it.
That said, the starboard and port have different max RPMs. At WOT with the trim all the way up, I get about 4850 on port, and 4740 on starboard.
I'm going to have her hauled out and have the bottom washed/waxed next month. So hopefully that will get me closer to my launching conditions. I'll inspect the props while I have her out. I'm not sure why the starboard is slower though. That may also make up for some of the difference in speed.
toofast
Active member
Ok.....so these engines should WOT around 4400-4800, at your launch you were slightly over revving (not in damage range of course), but that makes sense as the boat was empty and bottom was pristine and lightly loaded.
My 31 would WOT at 46 at roughly 4600 rpm with 26P props.....
So I think based on your latest test you are ok...and perhaps your 55 MPH you were slightly over revving and barely loaded...but 55 is super fast for a 31PC for sure.
55 is fast and I'm as surprised as you. I regularly could see 52-55 on the sound with and without people loaded on it. Having half of my water tank full or more actually seemed to help because I could keep the trim tabs fully up and the bow wouldn't porpoise as much due to the water load. Allowed me to see great speeds.
The 8.2L mercs really push this boat and it's something I'm very proud of. That's why every MPH loss is a concern to me. I'll have the bottom cleaned in a month and I should be in close proximity. But I definitely want to determine why one maxes out lower than the other. They are out of sync with the throttle as well. If I push them up evenly, they are out of sync. I have to offset by pushing starboard slightly forward to give it the same power.
It's actually pretty close to it's RPM when I bought the boat. It is slightly low, and I want to address that for sure, but I'm not as worried as I was a couple weeks ago when it was 8-10mph slower.
toofast
Active member
Ah yes 8.2 vs 8.1's