Might have to get a jack plate for set back

[OBT]

Ok. First thing, never had a jack plate. I know about them. Hang around the BBC long enough and you'll pick up some of the basics. We're having some problems dialing in the engine and prop combination and we're not hitting the RPMs the engine should be hitting. And a Jack plate may be in my future.

The new engine is a 2018 Mercury Pro XS 115 hp 4-Stroke. The gear ratio is 2.07 to 1. Its the standard and not the Command Thrust gear case.

The boat is a 2008 Triton 18 Explorer @ 1200lbs. Max RPM range for the engine is 5300 to 6300 rpm. I'm told that these Pro XS 4-Strokes are best run at the higher end of the rpm range. We're not getting that. (See test runs)

I've had the boat for the 10 years. Before the upgrade I had a Mercury 115 HP ELPTO 2-Stroke set in the third hole running a Laser II 48-16992 22P. Had a decent hole shot and WOT was 48.5 at 5650 rpm with my gear and me. (Probably to much gear) and that was fine with me. The engine was starting to show its age and parts weren't getting easier to find and it would get tougher as more time past and thats why I upgraded to the Pro XS.

So, enter the new motor and the old Laser II prop that was newly worked on by Marks Propellers. After some initial test runs, he re-worked the pitch and added cup. The new engine is mounted the same as the 2-Stroke, third hole. Currently, the Engine is set at 4.75 inches below pad.

Ran some on the water tests...Using my Vessel View...

On an empty live well

HOLE SHOT- Engine trimmed in, on plane in 6 seconds.

WOT - Engine trimmed up.
WOT RPM's - 5800-5810
WOT Speed - 49.5 mph

On a full Live well...
HOLE SHOT- Engine trimmed in, on plane in 10-11 seconds. NOTE: There was one run where it behaved like a slug. Nose up and a long time before it came on plane. (100 yards)

WOT - Engine trimmed up.
WOT RPM's - 5690-5700
WOT Speed - 47.6 - 47.8 mph

Except for the nose up thing with the full live well, both results are something I can live with. It could use some more rpm's, and I guess we'll have to keep messing with the current prop, or just go with a new one.

One thing though, I'm being told that based on all the info I've been giving, including the motor some times pulsating, or kicking in and out at WOT (Don't know how else to describe it) that I need some "set-back". That means a jack plate. Well several things come to mind. One, I could only go for 4" max because any more and the boat won't fit in the garage.

Next is cost. I can cut that by going with a manual jack plate which I'm fine with. The Jack Plate I'm looking at is the CMC, Two Piece Manual Jack Plate, 4" setback. Get it set and leave it to do it's job and don't worry about constantly tweaking it.

But wait, there's more. And by more, I mean money! Cost of the Jack plate and the labor to install it. I'm guessing that I'll also need a new Steering cable because the engine location has changed (I just put a new one on last April!) And I'll probably need a another SS Prop unless we can keep cutting away at the 22P. Dollar bills are taxiing towards the runway getting ready to fly away!

Finally, I won't even know for sure whether the damn thing will fit in the garage until after everything's all installed!

What I don't understand is how the boat performed just fine with my 115hp 2-stroke using that same Laser II for ten years. It provided the right rpms (5650) for that engine and 48.5 mph @ WOT with me and all the gear. No set back, no jack plates etc. Yet I get a new engine, which was supposed to be an easy engine swap, and find my self falling into a very deep hole. That wasn't in the plan!

Please let me know your thoughts and suggestions.

[Rip Bass]

I’m not sure but those engines likely have a different gear ratio,that would account for the performance difference. Even a 4” plate will allow you to raise the engine and get a few more rpm plus a little more prop leverage and still run the Laser. Unless your steering cable is maxed out lengthwise now,you should be able to use it,depending on the angle it leaves the transom grommet. A four inch plate doesn’t add four inches to the needed cable length,the engine is just four inches aft,so maybe two extra inches is needed. If you want a good,inexpensive plate,check out Vance plates in Tennessee. I’ve used them for years and they are good plates.

[OBT]

Thanks Rip Bass. I don't recall the gear ratio on the old engine. I'll have to look it up but it might explain a few things.

As for the the new motor. I forgot to include that info in the first post but I've added it now. The gear ratio is 2.07 to 1. Its the standard and not the Command Thrust gear case.

Thanks for the info. It makes more sense than you think and I will look into those Vance Plates.

[Fishead1167]

I would suggest you raise the motor up with the holes available before going the setback route. That is just a labor cost and no cash. See how it does, I wonder if the new 4 stroke weighs more that the old 2 stroke. I have been going thur the same issues after adding talons and it has been a year of trial and error and $.

[OBT]

I'm now looking at the Vance JPL4400 4'' Setback Adjustable Outboard Boat Jack Plate. A manual jackplate that is priced right and might work without major modifications.

[mdtritn21]

Gear ratio is the same on both years 2.07 but at 5700 or 5800 your way under top rpm's of 6300 for that motor. I'd try try a 21" tempest over any laser II even a 20" or a 19" tempest if needed to get the rpm's up to 6000 +.

[RangerRic]

Rapid Jack has a 4" plate as well. Soooo easy to adjust, even on the water.
I would test props and height first as others have said. I had a plate on mine and ended up
taking it off because of very little performance increase and I hated the back wash
when backing down. disclaimer: generally, I am not a big believer in jack plates in this hp range.

[jprovencher80]

I'm pretty sure the 4s is heavier. A jack plate will add more weight. Plate will certainly allow you to raise engine height to gain RPM at speed but holeshot might suffer. Hydraulic plate would let you put engine low for shot and raise for top end. Also need to watch water pressure when raising. Do you have a guage? If it were my boat I'd find a 19p or 20p prop. Each inch drop in pitch should get you 150ish RPM

[mdtritn21]

The new motor is supposed to be lighter and I forgot about the lower unit being a 4.25" case which means a tempest won't fit so you will have to go with a laser II or a trophy.