do i have the right prop?

[ebrich2]

ok guys here it goes,
1988 ranger 374v
1988 mercury 150xr4
25p high five stainless prop
engine rpm range is 5400
i have no problem getting to 5400 rpm but i don't feel like i'm going the speed i should be getting.
i clocked it at 57mph on the river going down wind at about 5400 to 5600 rpm.
and i might add i had it trimmed up pretty high also.
does this sound normal or do i need less pitch to get the mph faster at the same rpm?
Thanks,
Ben

 

[Silvertip]

Re: do i have the right prop?

What are you checking speed with. If you are using the boat speedo and you are running DOWNSTREAM on a river a pitot speedometer will need to have the river current speed ADDED to the indicated speed. If going up river you subtract the river speed. Of course the best speed measurement is with a GPS. Running your numbers through the prop calculator shows 16% slip (assuming the engine has a 1.87:1 gearset). That number is high so perhaps you need to work on setup (engine mounting height -- not trim) and by all means use a GPS for speed measurement.

 

[ebrich2]

Re: do i have the right prop?

ok thanks silvertip. it was the gps on my cell phone i was using. so you saying my motor may be to high or low?

 

[steelespike]

Re: do i have the right prop?

ok thanks silvertip. it was the gps on my cell phone i was using. so you saying my motor may be to high or low?

The antivent plate(just above the prop) should be even with the bottom to about an inch above.
a 1200lb(dry) 18 footer going almost 60 seems pretty good to me.but with 16% slip its likely you could get more Does the boat respond well to up trim?Bottom clean and smooth?
motor in excellent tune? I don't know the high 5s technical features but I don't think a 5 blade is the best selection for all out speed. It would surely allow a really high motor height and lots of trim
and pretty amazing low to midrange performance.
But 5 blades seems like a lot to turn when looking for speed.I'm sure someone will be along with more specific info.

 

[cwilt]

Re: do i have the right prop?

Rich,
Rangers are heavy boats. i have one. ditch the hi-5 and get a three blade prop. the merc tempest plus is a good prop it has lots of cupping. trim it way up and keep an eye on water pressure. with a stock 150 the low 60's is about all that you are going to get. do you have a jack plate???

 

[ebrich2]

Re: do i have the right prop?

yeah i have a jack plate but one of the previous owners evidently didn't feel the need of repairing it when it quit working and wedged a piece of metal thru it so it won't go up or down. i have just finished going through the motor replacing all kinds of things( stator, trigger, starter, and two new power packs) so the motor is running top notch. also rebuilt all three carbs, replaced a bent reed valve.

 

[ebrich2]

Re: do i have the right prop?

ok i just checked the motor hight by the cav plate, it is a good 2 inches above the very bottom of the hull.

 

[steelespike]

Re: do i have the right prop?

The jack plate allows more height.and the high 5 can improve on that.If you have no venting/cavitation issues at this point you may be able to go higher.
When going close to 60 less motor in the water could help speed alot.Just be sure it pumps water.