MPH / RPM / Prop

[tmt]

190 CCF Wellcraft dry 2350lb, fue-gear-people 2700lb.
Fresh 1997 Johnson 115 SPL, .030 over on all pistons.
15 pich 3 blade 14" diam.
At WOT RPMs 5200, GPS MPH 38
With 10% slip, 15p and 38mph, gears at 2/1 I think my rpms should be around 5900 I could be wrong.
I have ran another tach and they both read the same.
A timming light with a tach on it reads the same rpms as tach on dash.
Correct rpms??? I don't know if this is the correct prop.

 

[Philster]

Re: MPH / RPM / Prop

You have real-world WOT and RPM test data.

Forget calculations for a moment. What is the WOT operating RPM range for that engine?

Also, your weight estimates for fuel/people/gear seems low. 350 pound above dry weight? That's two guys at 175 lbs each and NO fuel or gear of any type!

120 lbs for gas
700 lbs for people
250 for gear
2350 dry weight of boat

= 3420 lbs

If you tested w/ yourself and light gas, then it makes sense.

 

[steelespike]

Re: MPH / RPM / Prop

I think the speed is optimistic even if you were at 5,600. 2700lbs is a bit of a load
and did you include extra batteries or accessories you may have added; bow rail,bumpers,anchor,swim platform,Heck most couples I know weigh in close to 300lbs. I'm inclined to think the speed should be closer to 36 or less.Perhaps another GPS to compare.Is your gps set on metric?

 

[tmt]

Re: MPH / RPM / Prop

WOT range is 5500 to 5700
25 gl. fuel 175 lbs?
people 375 lbs
gear 210 lbs
760+2350=3110lbs

 

[QC]

Re: MPH / RPM / Prop

15 pich 3 blade 14" diam. gears at 2/1
At WOT RPMs 5200, GPS MPH 38

3% negative slip, so one of the above numbers is probably wrong. Seems like you have nailed the tach number down, so I'm thinking gear ratio or prop pitch are different than stated . . . I trust your GPS. Any current involved?

Also, I am not a two cycle/OB guy, but I know that tach readings are from different pulses than I/O, 4 cycle etc. Verify that the tachs and timing light are set right for the application.

 

[tmt]

Re: MPH / RPM / Prop

The GPS is a good unit sonar gps combo $700, sonar setting has a speed wheel and it reads about the same as the gps.
I belive my tach is wrong. But I don't think the tach is bad. I think it is in the pluse signal or wiring.

Does the little Digital Laser Photo Non Contact Tachometer RPM Tester from ebay for less than $10 give me the correct rpm reading?
Or what can I do to know what rpms the motor is running.

 

[junior1113]

Re: MPH / RPM / Prop

2:1 , 2.25:1 if X long shaft. I'd work on setup first. where is cavitation plate in relation with bottom of transom. no hydra foil? speed if not an issue, getting wot rpm to 5500-5900 is. is this 5200 with motor out far as possible? i really doubt this combo could do 38

 

[QC]

Re: MPH / RPM / Prop

Photo tach will work if you can get a reading off of the flywheel.

 

[tmt]

Re: MPH / RPM / Prop

The motor is up highest hole.
Has hidrofoiles, when running WOT and trimed up I can see the top of the hidrofoils.
Motor is short shaft 2:1 gears

 

[junior1113]

Re: MPH / RPM / Prop

id do away with hyrafoils. they overstress transom. and create drag , not to mention turning issues. my 2 cents. is the prop ss or all.

 

[tmt]

Re: MPH / RPM / Prop

The hydrofoils are a must, they keep the boat from porpusing.
And when I trim up I can fill the bite of them cut loose.
Yes ss prop I think its the tach reading.

 

[Dhadley]

Re: MPH / RPM / Prop

Is the tach an OMC or BRP tach and set on position 6?

 

[tmt]

Re: MPH / RPM / Prop

Not omc is set on 6p