rooster tail and rpms

[Ashton.r]

Ill see if i can do a side shot this weekend. The boats out of the water a good bit. Ive had my dad look while in his boat beside me, says only about 4 ish feet is in the water so its on pad. I can feel it "float". The thing that's bothering me is the rpms. If i dont trim enough im stuck at like 4000-5000 rpms. Hence why im thinking maybe higher motor still and use less trim.
I plan to inspect the poppet this weekend make sure its clean, possibly put a new impeller in as well to make sure water is good. However if im getting 14 psi at 5000, shouldnt i get 15 past that since watwr pressure increase with rpms?

 

[jimmbo]

Have you checked your tach for accuracy?

 

[Ashton.r]

Had the tach on another merc it read right on that, check it over for propper setting before i tossed it on this 150, she idles at 750-800. Its a new tach too so it should be good. I dont think its the tach but how can i test the tach? The motor doesnt sound like its screaming 5500. I used to have a different prop that had slip when id try to get out of the hole rpms would shoot way way up and the tach read 5900. Im not to fimiliar with the calculations but how could i use a slip calculater and work back too see if the rpms match the speed that way i know im actually at 5000rpm. Im getting 51.3gps at 5000.

 

[jimmbo]

Which Merc 150 do you have. Different models have different gear ratios; 2:1 or 1.87:1

 

[Ashton.r]

Its an 1984 black max 150, blue banner. Im not sure the gear ratio, i looked into it a while ago and believe i found 1.87:1. Heres the number i pulled from a freeze plug i was directed too 6221597

 

[Ashton.r]

http://www.maxrules.com/models/Merc_model_files/115-150.html

Seems like its an 83 not 84.

 

[Ashton.r]

Slip calculater points that i should have a 1.87 gear ratio not the 2:1. 12% slip seems realistic i think.

Have you checked your tach for accuracy?

I ran calculations through a prop slip calculator. For 51.3gps with 23p and 12% slip the cal shows 5005 rpms so i think the tach checks out. Correct me if im wrong about using this as a way to verify tach.

 

[jimmbo]

The 1983 150 has a 2:1 gear ratio
I calculate 51.3mph at 5000 rpm to be approx. 6.5% slip, which BTW quite low.
If the motor is properly tuned and putting out all it can, I would recommend dropping a couple of inches in pitch. The 150 does its best power around 5400 - 5500
If you raise the engine any higher you will starve the engine for water, you are probably on the edge right now, unless you have blocked off the upper inlet holes. High engine heights can create a condition on the merc V6s where the where the engine is showing normal or even higher than normal water pressure but is in reality starving. Water flashing to steam on the upper cylinders is providing the higher than real pressure readings

 

[Ashton.r]

Hmmm how did you figure out the gear ratio? I tried researching but i came up short. I assumed through calc that slip lower that 10% would be unrealistic. I agree it makes sense to drop a pitch tbh i want to avoid that of i can.

Didnt know about the false pressure reading thats good to know! Appreciate that man.

I plan tomorrow to check out my poppet and buzzer make sure thats all good.

 

[jimmbo]

Sales brochure

 

[Ashton.r]

So which black maxs have a 1.87?

 

[jimmbo]

In 83 the 175 and 200. In 1998 the carbed 175 and 200, the fuel injected 150, 175, 200, and the 150hp optimax. I am not going to go thru 40 yrs of brochures.
It is easy for you to check the gear ratio just by rotating the flywheel either 2 turns or slightly less than 2 turns and the prop will only turn once

 

[Ashton.r]

Okay im stumped even more. Today i hooked up a over heat light because there was buzzer but the switch was there. Also verified it was a 2:1 and wasnt a swapped out foot. I then decided to toss on the stainless 24p prop that came with the motor when i bought it. The reason i did is because i recall getting good rpms with this. Wot i was able to push up 6000 rpms if i wanted to (i didnt). I achieved 55gps at 5500 funny enough not a bit of rooster tail. Oddly enough water psi was even better. Holeshot sucks in comparison. However motor sound like it was being launched. The other prop it holeshoted good but punching it the motor didnt jump in rpms. Now it jumps in rpms but holeshot is worse. Hence why i chose to take this off originially plus being ss and running rocky lakes is a turn off. I calculated for slip and got 15%... Think thats a healthy amount of slip. I did all this with the same setting on jackplate as the turning point. So whats the deal with the turning point prop? All i know about the ss is its a 24p...i can take a pic maybe if that may somehow help?

 

[jimmbo]

Pictures of all your props can shed light. Rear and side shots