Jet Boat RPM's

[hjerk24]

Hey guys/gals , I finally got a jet boat.  Susquehanna...LOOK OUT!    It's a 16'-1999 Lowe, with a Mercury 60/40 jet.  I topped it out at about 25 mph which was a good speed for me, but...  The problem, and may be it isn't...is that I was looking at 6000 RPM's. My question:  What kind of RPM's should I be looking at when running, and what is too much?  I see a lot of conflicting RPM #'s online.  If you could shed any light on this, I'd appreciate it.  I am NOT looking to go, say 40 MPH, just want to run the boat at a safe RPM.  I never used to take my Bass boat over 5000, so I was a bit taken back at the Tack on the first running of the Jet.  Thanks for any help...you may have for me.

Jamie

[mikesjet]

The jet units are sized to load the engines down to their max rpm. I also have a 60/40 jet (merc) and I see 5100-200 rpm at WOT. GPS speed is 25 normally and 30 if light and water is glass.

Running 6000 sounds a little high. It sounds like you need to do a little maintenance on the impeller and sleeve. Here is a small toturial directly from the manufactuer. I did this to mine when I bought it (used) and it was like night and day. Also the the clearance between the sleeve and impeller needs to be tight, really close to 1/32". There are shim washers under the nut and under the impeller. Remove shim from under the impeller and it will sit deeper into the sleeve reducing clearance. Shims that were removed from under the impeller can be placed under the nut. If you have never had it apart, buy a new nut, as it is brass. Anything else please feel free to PM me.

http://www.outboardjets.com/faqs.php look at number 19 and follow the instructions.

[outlaw]

5500 is what outboard jets run at biggest problem is cavitation if you cavitate you may have to drop your motor a hole the bottom side of your intake foot should be even with the bottom of the boat also if the bottom of your boat is dented that will have effect on rpms been there done that dents in the bottom are a bad thing with a jetdrive

[jeepcrazy]

I'm having a similar problem with my 60/40.  I'm running 6200 or so at WoT and really not running all that fast.  I've experimented with moving the motor up and down a hole and it hasn't made much of a difference.

Something is most definitely not quite right and I'm not sure what I'm doing wrong. 

If anyone knows jets and would be willing to help me understand what I don't know, I'd be extremely grateful!  Aside from being really slow getting on plane (very...I wouldnt dare try shallow water I'd need 100 yards to get on plane!) it acts like something is dragging.  I looked into the impeller and the blades aren't nicked up and nothing is wound up in there. 

I'd love to see some thoughts too!  My RPM's have struck me as a little high for a while.

-JC

[Fishbone]

Jamie, is your Lowe a tunnel? I have been considering putting a jet on my G3 (no tunnel).

[jeepcrazy]

Mikesjet was dead-on!  My impeller had never been shimmed.  Lots of space in there.

I haven't gotten out to run it yet but I'm looking forward to seeing the difference.

I found this other website that has a really nice breakdown of what the inside of the jet looks like and how to do the shim procedure.

http://www.scottandjenn.com/etomite/index.php?id=22