help find my slip % please.

[fishfeatures]

hi,

am currently running 1990 johnson 60hp on a 13ft ski boat (avatar) weight inc me and gear is approx 300 -330 kgs.
using a 13 3/4 x 15 prop
fully trimmed out it's at 6000 prm
doing 42 mph (on speedo my gps is gone missing)

I don't know the gearbox ratio for this motor as I lost my manual when moving house months ago.
can someone tell me it?

also if I go up to a 17P can i drill relief holes behind the blades so the hole shot doesn't suffer as much?

I believe rm for this motor is 5000-6000 so I'd like it to run about 5700-5800

 

[steelespike]

Re: help find my slip % please.

Your ratio is 2.42. Your numbers don't add up. To do 42 with that prop and ratio the motor would have to turn about 7900 rpm.
I think you would realize that if it was.Max rated rpm is 6000 i don't think it would stay together at 7900.
At 6000 The speed would be about 31 mph guessing 10% slip.
You need to confirm some things Speed would be easiest to get using a gps,some cells have a gps app.
From the size of your boat I'm sure you could run a higher pitch and you may want to confirm the present pitch.
from the looks of your sporty little boat I suspect the gas and battery are in the stern and your passengers are
pretty much stuck in the one seat not ideal for helping with hole shot then you have a small footprint so it's harder to get it up
on the water.You can vent a prop but I would do a lot of research on how to. I know there are inserts you can get to adjust the size if necessary but you need to know the size the inserts fit.

 

[fishfeatures]

Re: help find my slip % please.

2.42!!

that def doesn't add up?
pitch is def 15P
rpm is 6000 at wot
hole shot is good even with a guest and with 2 it's way slower.
fuel (max 30L) is in the stern with one battery.

I'll try borrow a gps but can't see speedo being off by 10mph??

I'll be back , thanks for that Sspike.

 

[Texasmark]

Re: help find my slip % please.

www.gofast.com has a calculator. Fill in all the blanks except in your case slip. It will calculate it for you.

Mark

 

[fishfeatures]

Re: help find my slip % please.

thanks for that Mark.

 

[fishfeatures]

Re: help find my slip % please.

ok 5900 on tach
gps says 37

 

[steelespike]

Re: help find my slip % please.

still doesn't add up. Are you sure it is a 13 3/4 X 15 prop?
is it possibly a 17" ?Or perhaps a 12 1/4 X 15
A 17" gives a slip of about 5% low but possible?

 

[fishfeatures]

Re: help find my slip % please.

still doesn't add up. Are you sure it is a 13 3/4 X 15 prop?
is it possibly a 17" ?
A 17" gives a slip of about 5% low but possible?

I'll take it out again tomorrow, and gather all info again just to be sure. And yes it is a 15" pitch never re worked either.

 

[steelespike]

Re: help find my slip % please.

Are you sure it is a 13 3/4 X 15 not a 12 1/4 X 15.
The 13 3/4 Is an indication of a large gear case and the 2.42 ratio.
If we use a 2,00 ratio it gives a slip of about 11% high normal.
If we can't get it to work we may have to pull the plugs and mark the flywheel and prop,
count by hand the turns of the flywheel to one turn of the prop.
2.42 is a little less than 2 1/2 turns.
motor needs to be in gear.