Low W.O.T. Rpms

[m1rodrig]

Well after reading through several posts & learning quite a bit I did not know. I am stuck right now & am looking for a bit of guidance & advice. I have a 19'CC Cape Craft with a 2001 150 Johnson Ocean Pro. Model # J150zxsig with a 14 1/2 x 19.At WOT I am at 4700 Rpm's at 38 Mph per GPS & Speedometer.From what I've read since I purchased this boat 3 weeks ago I have realized I am way low on rpm's.The motor runs fine & does not hesitate picks up speed fine & in my opinion runs excellent.I checked the Tach to see if it was set to something other than 6 cyl. What it says is for use on 5/6 pul/rev altenator equipped outboards & it has to settings 5 & 6 it is currently on 6.Thanks for any help in advance.

 

[walleyehed]

Re: Low W.O.T. Rpms

To get the RPM in the correct range, you would need to drop to a 15" pitch, but lets look at a few other things first.<br />I realise you say the engine runs "great", but, to make an educated recommendation, I would first have a compression check done and determine we don't have a reason to believe we have a loss of power.<br />I have a friend with a 20ft and the same 150, turning a 17 pitch at 5100RPM and your hulls are simular in design, so It wouldn't appear as though we have an engine issue, but check the compression and lets go from there.<br />There are many things we can try, but no use doing that until we know the engine is for sure, healthy.

 

[m1rodrig]

Re: Low W.O.T. Rpms

Thanks for the quick response, I will get a compression gauge & check compression. The mechanic at the boat dealer told me the compression was at 95 on all cylinders but I will test for myself.

 

[walleyehed]

Re: Low W.O.T. Rpms

That sounds reasonable...and lets assume that's true (Test anyway)<br />While you're in the process of checking compression, take note of which engine mounting hole the top bolts are in, and if the engine is sitting directly "ON" the transom, or up slightly.<br />Keep us posted.

 

[m1rodrig]

Re: Low W.O.T. Rpms

Tested the compression with a gauge & I get good readings 97 98 101 99 102 99. The top bolts on the motor are on the 3rd hole from the top.Should I test with different prop pitches or lower the motor.

 

[walleyehed]

Re: Low W.O.T. Rpms

I like the sound of where the engine is mounted, don't lower it. The transom bracket should be about 2" above the actual transom then, correct???<br />Comp readings look good too.<br />My suggestion would be to try a 17" pitch. Most dealers will let you test if you don't ding or scratch-then ya buy...<br />If you can get your hands on one, I would test an SS in 17".<br />Stiletto, Turbo, even a Bombardier SSTII, would be my first choices in 3-blades, and a Renagade Offshore 4-blade in a 17 could very well suit your set-up.<br />Now that I've said all of this, make sure the cav plate on the lower unit is slightly above(1"-2") the hull bottom.

 

[m1rodrig]

Re: Low W.O.T. Rpms

Walleye that is correct the transom bracket is about 2" from the actual transom.The cavitation plate is 1 to 2" above the bottom of the hull.I have a few places I can borrow a prop from I will try a 17" and see how that works out.I sure am glad I found this site I would have been lugging my engine & decrease the amount of years I could enjoy it if not for all the great info & help on this board.Thanks for your help & I will let you know how it turns out.

 

[m1rodrig]

Re: Low W.O.T. Rpms

what diameter should i use with that 17" pitch. Thanks

 

[angus63]

Re: Low W.O.T. Rpms

15"φ x 17 pitch very common. Diameter does not affect WOT RPM nearly as much as pitch does.

 

[m1rodrig]

Re: Low W.O.T. Rpms

Thanks I will go ahead & get 15" diameter

 

[m1rodrig]

Re: Low W.O.T. Rpms

Well I finally was able to get the 15 x 17 Prop. I didn't have the money for the stainless so I got an aluminum Michigan prop that I bought through Iboats. I ran at 5500 RPM's & 39.3 MPH. So I am very happy happy & would like to say thanks Walleyehead & Angus.

 

[angus63]

Re: Low W.O.T. Rpms

Excellent!!!!! Thanks for the feedback.<br />Enjoy................

 

[walleyehed]

Re: Low W.O.T. Rpms

yes, thanks for reporting back! It's good to know how things turn out...

 

[chris in va]

Re: Low W.O.T. Rpms

And if you ever decide to get the SS prop, I found it raised my top end from 43 to 51 when I had a 302 I/O last year.

 

[moderator1]

Re: Low W.O.T. Rpms

8mph is a bit more than just a prop....2-3mph would be an expected number.....your previous prop must have been a real club...

 

[m1rodrig]

Re: Low W.O.T. Rpms

I did go from 38 to 39.3 with this setup I thought i was going to lose topend by reducing pitch but I actually gained 1.3 MPH. I will eventually get a SS prop when I have a few extra dollars but for right now I am happy with what I got. The engine is really purring nicely she sounds great.