Rasing Motor...RPM's

[bulldog44]

1998 Xpress H51 with 90hp Yamaha (operating range is 4500-5500 rpms per Yamaha)
17 pitch Turbo will not go over 5100 rpms. My stilotte will give me 5500 rpms...
Going to stick with he 15...

[Altro Cat]

Most guys running H51s tend to be at around 4 to 4 1/2 prop to pad. Most are running a plate though. 6 inches does seem low to me.

Im running 4 1/8 prop to pad with a 6 inch plate. Water pressure still good there.

If you dint have a water pressure guage it is recommended if you are going to start raising your motor.

[VANDERA]

I used to have a ‘98 90hp Yamaha on my H18. It went 35-37 loaded and best I saw was 40 by myself. I had a 15p Solas Stainless prop that I had worked to a 16p(definitely not the ideal prop but it’s what I had). I also had a 6” manual jackplate which I would recommend. I don’t remember exact height but I think it was around 4”. Keep in mind you might not be able to raise it as high without a jackplate. You definitely want to be running at least 5000 rpms WOT. 5300 to 5500 would be better I was running 5500 with my old setup. If your only seeing 4500 at WOT your over working that motor and would not be good long term.

[bulldog44]

In the short time that I had to test the new prop all I got it to was 38 mph @ 4900 rpms...Full tank and heavy chop kept me from wringing it all the way out.
This weekend looks good for some more testing...If I cannot get it over 5000 rpms, I'll raise the motor...most I can go without a jack plate is to 4-1/2" (approx.) prop to pad distance.
Thank you both for info concerning this, it gives me an idea what I need to do.

[rocket man]

When your trimmed out at WOT you want the cavitation plate skimming the top of the water. You need a buddy to look over the transom and check.

[bulldog44]

Update:
Tach is off...loose wires I guess. Tapped the face and it will jumped up rpms. Got it to settle down, but still acted up during the day.
Got run boat some, not the best conditions, but better than last time. Got it up to 42 mph @ 5200 rpms WOT once and 40 mph @ 5400 rpms WOT on the last run in to the boat ramp. I'm going to check for loose wires at the tach...if that does not fix it, I will replace it.
Overall on this test, cannot get splash line past my seat without porpoising. Maybe on calm water, and no wind I can.
I'm going to play with weight placment, take out a few items, move some around to see if that will help,

[VANDERA]

On the back of the tach is an adjustment. They get corroded over time. Note the position. Turn it each way a few times and then back to the original position. That would be the first place to look. Hopefully it solves your problem.

As for the porpoising, I never had issues on my boat but I’ve seen complaints on here about the H51 doing it. Maybe someone who’s went through it can chime in. Otherwise try a search of previous posts for some help. I would guess lightening the front of the boat and raising the motor should help the cause.

[bulldog44]

Thanks Vandera, That gives a starting point on the tach...hopefully that's the fix.

I will say I was trying to move he splash line past my seat, under heavy chop it was a wasted effort.. Stop trimming when it gets to my seat, boat runs good, but trying to go past that invites the dreaded hop! Reading posts on setups, I notice some get good results @ 4-1/2" prop to pad distance (with jack plate), I believe I can achieve that by raising motor to the bottom hole on my motor, but will be motor mounted to transom, no jack plate. Has anyone here done that, if so, results?

Rocket Man, I've not been able to convince my wife to craw back and peek over the transom while WOT to see if the cavitation plate is skimming the top of the water!

[shackwylde]

Thanks Vandera, That gives a starting point on the tach...hopefully that's the fix.

I will say I was trying to move he splash line past my seat, under heavy chop it was a wasted effort.. Stop trimming when it gets to my seat, boat runs good, but trying to go past that invites the dreaded hop! Reading posts on setups, I notice some get good results @ 4-1/2" prop to pad distance (with jack plate), I believe I can achieve that by raising motor to the bottom hole on my motor, but will be motor mounted to transom, no jack plate. Has anyone here done that, if so, results?

Rocket Man, I've not been able to convince my wife to craw back and peek over the transom while WOT to see if the cavitation plate is skimming the top of the water!

I tried for years to stop porpoising on my H51.I finally emailed Xpress and this is what they responded with. THis stopped porpoising completely!

[PsychoXP18CC]

Ideally, you'd want the prop shaft height as high as you can get while maintaining good water pressure readings. Then prop to that height. This will require a jack plate, almost certainly. Mine is the explorer hull, on 6" of setback my prop actually surfaces on plane, you can hear it. I was told by my setup guru that the anti-cav plate is immaterial. I did install a water pressure gauge on mine as well. My prop shaft was 9.75" below the hull from the factory, when i got done it's now 4", for a gain of almost 6" of draft. Now, mine will not porpoise, ever, the prop will blow out before it does. It is like running a completely different boat now, better in every aspect of maneuverability, performance, and handling.

Once the motor height was set, rpm with my factory prop was stupid high, on the rev limiter before I even started trimming on a light load. Increased pitch 2" on a prop with significant;y more cup and it brought the pm down, and it carries the bow much better as well. Had some cup added to my factory prop and it's now a great power prop for heavy loads, where I rarely run it WOT and max trim. I can put the spray about a foot in front of the stern on an XP18CC with a 70hp Yammy.

[Kerusm]

Research a hydrofoil. Put one on my first H51 and it helped a lot in every
aspect. Recently added one to my H51 now which has a 6" inch manual
JP and it helped also. 2006 H51, 90 hp, 17' power tech, 6" JP, 5" ptp,
SE 300 hydrofoil, 51 gps. Boat handles excellent.

[bulldog44]

UPDATE:
My boat mechanic raised motor to last hole, should be approx. 4-1/2" prop to pad.
I ran boat before I had him raise it, 41 mph @ 5500 rpms with my old prop 15 pitch...He got it up to 5800-6000 rpms, but I had removed the gps so all he said was it seemed to be faster than the first time he ran it. This is without a jack plate...water pressure was 15-16 psi...
I'm picking up boat tomorrow, will be testing Saturday...I will post results.

[bulldog44]

Boat ran much better with motor moved up to last holes, mounted to transom...Water pressure is good.
Debating next step to send off old prop (15 pitch) to clean up and add some cup.
I will test with the Turbo 1 17 pitch, but not sure if I can get rpms up to where I need them to be without a jackplate.
I cannot make up my mind on what setback I want to go with on a jackplate, 6" or 10"...maybe go 8" to split the difference.

[1Holeshot]

Bulldog44, be sure to check your steering cable to see if it will accommodate more than a 6" setback plate.

My H17 Xpress which is essentially the same hull as an H51 has a Vance 6" jackplate and has no extra length in the steering cable for a bigger jackplate.

[bulldog44]

Thanks!
Question, 6" jackplate is good per what I've read...but is going to anything larger (8"-10" for speed increase), will I see better performance going to a longer setback?

[ibbh]

With that setup and motor, 6" should be your max. Anything other than that wont help get what your after.

[1Holeshot]

With more set back you should be able to run engine height a little higher while maintaining water pressure and prop grip.
Maybe get another mile or two per hour.

[bulldog44]

Ran Turbo 1 17 pitch today...ran well, but for not being able to get but 5100 rpms...Gps said I can go 42 mph...hole shot good, handles well. I can cruise all day at 35-38 mph (low rpms)... I'm waiting for old 15 pitch Stiletto to get back with more cup to see results...But I'm figuring I'll need a jakeplate to improve performance any more. Water pressure is running 16-18 WOT, and without a jackplate I cannot raise motor any more.