Johnson Cajun combo

Johnnykat

Cadet
Joined
Mar 14, 2012
Messages
6
Good day all,
New to IBoats and hope to get some help. I bought a 1992 Cajun 1750 fish and ski. Has a 120 Johnson and weighs about 1100 empty. Has a 13.25X17 aluminum prop and WOT no trim is just over 6 grand. I run it about 4600 rpm and have ordered a 13.25X21 stainless for it. I was told by a guy at work that it wasnt enough motor to twist it. I do not mind losing a bit of hole shot as the 17 would throw it up like it was on a hinge, but it topped out about 36 mph on my gps. Anyone have a clue what it may do with a 21? or should I cancell and change it to a lower pitch? I have had many boats and they all had 21 pitch on them but also had 150hp or larger.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Johnson Cajun combo

Prop calculator says that at 6000 with 6% slip and 2.00 ratio boat would go 45 mph.
I think your Tach is wrong. The 6% is a guess so the number could be more or less but not much.
36 mph indicates 4600 rpm.I'm also assuming the motor is not an extra long shaft.not likely on that boat.
There may be a switch on the back of the tach;check its position and operate it a few times to clean the contacts.
See if that helps.You may be able to borrow a shop tach or A Tinytach can be hand held for almost any motor.
They have a web site.It appears the 21 is too much prop.You may need to confirm the motor is in excellent tune.
I think 17 or 19 is more typical for similar setups.
 

cyclops2

Banned
Joined
Apr 19, 2011
Messages
1,237
Re: Johnson Cajun combo

John

Cancel the prop order.
Right now no one knows if your tach is accurate, or your GPS is accurate. Or both are not right. No new props untill you prove your GPS & someone elses GPS agree. Same for the tach accuracy.

Cancel the prop.
 

Johnnykat

Cadet
Joined
Mar 14, 2012
Messages
6
Re: Johnson Cajun combo

Prop calculator says that at 6000 with 6% slip and 2.00 ratio boat would go 45 mph.
I think your Tach is wrong. The 6% is a guess so the number could be more or less but not much.
36 mph indicates 4600 rpm.I'm also assuming the motor is not an extra long shaft.not likely on that boat.
There may be a switch on the back of the tach;check its position and operate it a few times to clean the contacts.
See if that helps.You may be able to borrow a shop tach or A Tinytach can be hand held for almost any motor.
They have a web site.It appears the 21 is too much prop.You may need to confirm the motor is in excellent tune.
I think 17 or 19 is more typical for similar setups.
Maybe I worded that wrong. I shut it down at just over 6 grand, i am not sure how high it would have gone and that was without the motor trimmed. With it slowed down to about 4600 to 4800 is when i was running 36mph. Havent checked tach but GPS is dead on with both my Harley and my pickup. As of yesterday, I cancelled the prop but took a friends 13.25 x 21 aluminum and put on it,(didnt know he had one or i would have done this to begin with) I am lucky and live about 150 yards from a boat ramp. We took it out and with it wide open and trimmed it was running just over 5400 rpm's. I was an idiot and forgot GPS but it did not seem to lose much of the hole shot even though i could tell a difference, but at least I could get on plane and slow down without it falling back in.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Johnson Cajun combo

Calculator says almost 45 mph at 5400. Assuming 2.0 ratio and 6% slip
 

Johnnykat

Cadet
Joined
Mar 14, 2012
Messages
6
Re: Johnson Cajun combo

Calculator says almost 45 mph at 5400. Assuming 2.0 ratio and 6% slip

Thanks for the assistance. going back out today to spend some time experimenting. Do you think there would be much difference if any between a stainless prop and an aluminum one? He also has a "raker"? whatever that is. he said ss 13X20 raker. any clue? maybe the brand name?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Johnson Cajun combo

Speaking very generally going to a ss in the same brand same pitch should give an overall improvement.How much would depend on how well the al prop is working.On rare occasions there can be a decrease in performance.
Once you have your 21 well tested with wot rpm and speed and any other info ie: hole shot,ability to stay on plane .
I assume you have the advantage of testing the ss prop before you buy.I believe the raker will improve bow lift but may slow hole shot.
 

cyclops2

Banned
Joined
Apr 19, 2011
Messages
1,237
Re: Johnson Cajun combo

SS props SHOULD be slightly more efficient than a Aluminum prop for one basic reason.

SS allows THINNER blades. That allows more USEFULL power.....IF.....the SS replacement prop is....CORRECT....
A Prop Shop near you can / might / should / loan you what would do a better job. Then buy the correct prop.
 
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