merc 165 il six reprop / slip. Confused !

[73 Dolphin]

I have a 1974 starcraft chieftain 21'6" aluminum center console conversion.

The previous owner swore up and down that it would do 40 mph. As there is no speedo on boat he must have used gps, as do I.

I've completely restored the boat. The original outdrive was toast. I replaced with (presumably) the correct outdrive. 1.65 gear ratio.

Firstly I know that I am overpropped.
I am running a 21 ss prop and max out at 3500rpm. It should be around 4100.

Working on the theory that each 2 inch of prop reduces rpm by around 400 I guess I need an 18" prop.......but........

.......working with prop the calculater, my 21 pitch ss ,3500 rpm with 1.65 ratio and 30mph top speed suggests a prop slip of 28%.

Seems very high ? No ?
To confuse matters if I change slip on calculator to 15% and change ratio to 1.84 I see the exact figures I'm actually getting in real life.

So the question is whether it is more likely
That I have 28% slip or an outdrive rebuilt with wrong ratio ?

Any advice much appreciated !

Weight loaded should be around 2800 lbs.

 

[73 Dolphin]

Re: merc 165 il six reprop / slip. Confused !

Ok, so I guess it comes down to prop slip more than anything. I'm not sure how that calculation works but assume as my 21 ss is only turning 3500 instead of 4100 then the 28% slip is correct.

I'll also presume the 1.65:1 ratio is good too. The 1.84:1 was just coincidence.

So the theory goes if I change to a 19 pitch prop and achieve, say 15% slip I should at right around 38 top speed. Thoughts ?

 

[steelespike]

Re: merc 165 il six reprop / slip. Confused !

I would be suspicious of the out drive until you prove to yourself it is 1.65.
unless the guy is a boldfaced liar it just seems unlikely it wouldn't even make its rpm range.
You could remove the plugs and mark the flywheel or crank pulley and the prop and carefully turn by hand count the times the motor turns for once for the prop.
Also make sure the motor is timed right and advancing as it should.
be sure the throttle opens all the way.
The prop doesn't seem excessively high on the list of props; one or two higher than the middle of the group.

 

[73 Dolphin]

Re: merc 165 il six reprop / slip. Confused !

Like I said the original drive was toast. I replaced with another marked as 165 so ratio should have been 1.65:1.

Tried another prop calculator 1.65, 21, 3500 should mean 42 mph. Getting 29.5
Suggests slip of 30%.

Will put a 19 pitch on this weekend to see if I gain 400 rpm. Also will check throttle and use cell phone gps alongside chartplotter gps.

Will post results incase its usefull
to others.