what size prop would be best for my 150 Johnson??

[SparkieBoat]

I have a 1995 Johnson 150 GL model which I was told should have 165 HP at the prop. I put it on a 17.5 Mckee Craft 1979 model. It is a CC. I do not know the weight of the boat.but My guess is it is slightly lighter than the average 17.5 foot boat.
I ran it out today with 3 of my kids with me total of about 225 lbs. plus my 210 lbs. and 20 gallons of fuel. it came out of the hole good, but it is a light boat, most of these I see have 115s or 90s on them and they go about as fast as I could get today with my 150. I topped out at 37 MPH at 4250 RPMs trimmed out.
I have a 14 1/4 X 19 Stiletto SS prop. 3 blade.
I was expecting to hit 45-50 MPH pretty easily. So it preformed a bit below my expectations.
I am open to prop suggestions, maybe a larger diameter and a little less pitch??? I am not sure what to do any suggestions would be helpful. If I left out any info please let me know, and I will try to answer as best I can.

Thanks in advance, Sparkie

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

I just thought of another question...how much of the foot should be below the bottom of the boat..I know that today when I trimmed the motor back down while running it was like putting on the brakes..lot of drag. it is a 20" shaft

 

[steelespike]

Re: what size prop would be best for my 150 Johnson??

I'm not an expert but::
Your speed and rpm are way low. I don't know your rpm range but surely the max would be about at least 5,500.That prop should allow the motor to overrev easily in my opinion. If you don't have any ventilation(cavitation) issues your motor may be too low.Antivent plate(just above prop)even with the bottom is considered a starting point.Most end up about an inch above.
Some things to check,throttle opening all the way,bottom clean,good compression,all cylinders firing.( not unusual to be missing one cylinder and not realize it),Prop in excellent condition.

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

thanks Steel, just went through this motor pretty thoroughly, I just purchased it, had to put a new fuel pump on it and replace the clutch dog. all cylinders have good compression and the fire is a thick blue streak with a 1/2 " gap on the tester. Just cleaned and checked all carbs. I may have an issue with the height but I would not think it would make that big of a difference. Here are some pics of the boat and motor height.

 

[BF]

Re: what size prop would be best for my 150 Johnson??

yeah... speed and rpm's definitely seem low. I have a 150 merc V6 on a fairly light 16' ski boat (glastron CVX)... with 19 pitch 3 bladed aluminum prop and trimmed up, it does low to mid 50's (speedo speed) and close to 6K rpm. It pushes as fast as the hull can handle before getting into bad chinewalking. Trimmed down at WOT it does about 46/47... I forget the rpms... (I never run it trimmed in all the way). I could probably put on a higher pitch, but the boat is used for watersports mainly and it pulls good with a 19" on it.

Are your speeds speedo or gps? I just got a GPS unit, so I'm looking forward to verifying my speedo readings... they can be off

BTW, the motor height looks to be a bit low, but maybe not too bad (?) BTW, I think the height of my motor is also border line low which I think contributes to the chinewalking at high speeds.

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

it is the speedo..I will take a gps out with me next time to confirm speed..but it sure seems about right to me..but sometimes its hard to tell..but I feel sure it is not more than a few MPH off 5 at the very most. But it may be right on
I am going to raise the motor up some, see if that helps..but I feel it should still spin that 19 prop over 5K RPMs

 

[steelespike]

Re: what size prop would be best for my 150 Johnson??

Right I think at least 5,000+.Is the bottom clean?smooth paint?I don't think the motor height will get you the rpm But raise it to a point just before venting becomes a problem.I would think that prop would allow pretty good height.I don't think you can make up almost 1,000 rpm but you should gain some.When you trim out does the hull respond normally?

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

the hull is not the smoothest, the previous owner scraped all of the barnacles off though, so it is not bad at all, I would not think that it would be enough to notice any difference.
i will be sanding and repainting the hull this winter. I was going to check the timing, but my timing light has stopped working for some reason..got to check it out. I think timing is ok though.
I raised the motor up as far as it could go (bottom holes) , I was going to test it today..but no time, will try tomorrow. I will take a GPS with me. here is a pic of the motor VS bottom of boat.

 

[hwsiii]

Re: what size prop would be best for my 150 Johnson??

Sparkie, I will tell you right now that your main problem is the barnacles on the bottom of the boat. Raising the motor up should gain you a few RPM and a little speed, but when you sand that bottom down you will be very happy with your speed and you should increase your speed to over 43 MPH.

Bottom of Boat

I can see barnacles everywhere on the part of the bottom I can see, and let me tell you they eat up RPM and speed BAD. And if you keep it on a trailer you probably don't need to paint the bottom if it is smooth enough.



H

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

thanks HW, most of the barnacles have been scraped off, the rear in the pic is probably the worst place. I will continue to scrape and clean it. I really expected 50+ MPH...I will keep you posted. Thanks for the input everyone, you have been a big help.

 

[hwsiii]

Re: what size prop would be best for my 150 Johnson??

Sparkie, not only does it increase drag tremendously, but it also aerates the water so the prop does not get clean water flow and it creates ventilation which further causes you a loss of speed.


H

 

[steelespike]

Re: what size prop would be best for my 150 Johnson??

There was a post here I forget the figures but performance was way off .
Basically just a detereiated paint job.Figures similar to yours.
After he sanded and painted it solved his problem.
I think you will be pleased with your results in the spring.

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

i checked and the only place that had any barnacles left was right there at the back..i scraped them..there are now no barnacles on the hull..but it still is not smooth by any means. If it will make that big of a difference I will sand and paint now instead of waiting

 

[hwsiii]

Re: what size prop would be best for my 150 Johnson??

can you take a few pictures of the bottom at the worst spots so we can see what it looks like, I would hate for you to have to sand the bottom if it will not help your speed.


H

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

I am going to sand it anyway..I found what may be my issue..The selector on the back of the RPM gauge may be not working properly..I played with it some and I may have had it set on 8 Cyl. which would answer a lot of my questions..it just does not sound like the motor is over revving to me though.I have run a lot of outboards and have some idea of what 5K RPMs sounds like. .when I adjusted the tach some it started reading over 6K at WOT..I am not sure so I am going to check it with another tach. If my ear and the tach are both wrong I just need a bigger prop..maybe a 21 or 22 if I am taching over 6k.
I will do some more checking tomorrow. thanks for all of the help guys.

 

[hwsiii]

Re: what size prop would be best for my 150 Johnson??

I would not buy any prop until you sanded the bottom and verified the RPM.


H

 

[SparkieBoat]

Re: what size prop would be best for my 150 Johnson??

roger that, I have a friend who may have several different props I could borrow and see what works the best. I am thinking a 22 or 23 maybe..i will try a 21 first and we will see.

 

[Hackamore]

Re: what size prop would be best for my 150 Johnson??

There is a delicate balance when proping a motor to a boat. If you are getting a real good hole shot and over reving like it sounds you are. I would think that the motor shouldn't turn much more than 5500 at max. A general rule of thumb is if you need to cut your rpm's by 6% then you take your pitch up by 6%. Also if you want a little more top end and not affect the hole shot you can have the prop cupped if it is not already. The cup on the back edge of the prop at high rpm's will give the boat more speed, but will not affect the hole shot at the lower rpm's. Also if you go for the top speed it might and most likely will make your hole shot worse. This is the delicate balance that I was talking about. If your pulling a lot of weight in the boat, you might want to stay with the prop you have and not just over rev it. Work on that balance of a pretty good hole shot and still maitain your top end, with out over reving the enging. Good luck.