Help with prop selection

[Ram58]

I have a Crestliner 1860 with a Mercury 90 ELPTO with a 21 pitch prop on it. I can only get 4800 rpm's out of it. The motor is rated for 5500 rpm's. Does that prop have to much pitch for that boat/motor combo? Thoughts please.

 

[Dhadley]

Re: Help with prop selection

If the motor is healthy and running correctly, yes - it could be too much pitch. But we really need more to go on since we can't "see" the rig. Is this a new-to-you deal or did it get more rpm at one time? Are you at a high altitude or near sea level?

 

[Ram58]

Re: Help with prop selection

I recently bought the boat. The motor runs very smooth, don't hear any missing or hesitations out of it. I checked compression on all cylinders before buying, had 125 psi on all three cylinders. I've read some about prop selection, that's what made me wonder if it was too much pitch. Any help would be greatly apprieciated.

 

[Randybeall]

Re: Help with prop selection

Based on what you describe as the boat I would expect a 19" or 17" pitch prop. What is the power rating on the boat, I expect higher than 90. If the engine is truly running correctly go looking for a 17 and give it a try. Iboat does offer an exchange new prop program if you get the wrong one you can exchange for just freight cost on some propeller models.

 

[Ram58]

Re: Help with prop selection

The rating on the boat is 90 HP.

 

[Dhadley]

Re: Help with prop selection

I'm not a fan of changing props simply to gain rpm before maxing out the set up with what's on the motor now. We'll assume there is no jackplate but if the motor is mounted all the way down now I'd suggest raising the motor a couple holes to test how the hull responds to less gearcase drag.

What brand & style prop is on the motor now?

 

[Ram58]

Re: Help with prop selection

There is a jack plate. The motor is mounted to where the cavitation plate would be appx. 1 inch below the boat all the way down to appx. 4 inches above the bottom of the boat all the way up. The prop is a ProTech 13.5 X 21P SS, cupped. I'm in southern IN., 500 ft. elevation.

 

[Silvertip]

Re: Help with prop selection

That engine should perform nicely on that boat with a 19P aluminum or 18P stainless. 17P in either one is likely to be too little pitch. That engine has a 2.33 gear case and with the proper setup should spin in 5400 - 5500 rpm range. Raising the engine in small increments keeping an eye on the tell tale so you don't go so high as to lose water pressure. When optimizing setup a water pressure guage is highly recommended.

 

[Dhadley]

Re: Help with prop selection

Ok, you didn't say but we'll assume that the plate is hydraulic and the 4800 you get is the max rpm with the plate 1/2 way up to full up. In that case you probably are looking at a pitch change to get the rpm up.

Again, assuming the motor is running OK and all the linkages are going to full open. And the tach is correct.

 

[Ram58]

Re: Help with prop selection

Actually when running it full throttle I had the jack plate about an inch from the bottom or where the cavitation plate would have been even with the bottom of the boat and the motor trimmed up. And yes, the jack plate is hydraulic.

 

[Dhadley]

Re: Help with prop selection

If you run the plate up a tad does the prop just start to vent?

 

[Ram58]

Re: Help with prop selection

I have ran it with the plate up high, but was just cruising at around 3400 rpm's. It did not cavitate if that's what you mean. I just thought you were supposed to run the cavitation plate close to the bottom of the hull. This jack plate business is new to me. I do appreciate everyones help.

 

[Dhadley]

Re: Help with prop selection

You're on the right track. When running to get max speed try running the plate up some. It's not unusual for good props to stay hooked up at extreme X dimensions. A real good prop on a responsive hull design will stay hooked up when the lower unit quits getting water.

On performance hulls with performance props it's not unusual to refer to the X dimension as where the centerline of the propshaft is in relation to the bottom rather than where the AV plate is.

 

[Ram58]

Re: Help with prop selection

Weather is supposed to be in the upper 50's in the middle of the week. I'm going to try to go out and play with the jack plate on the higher side and see what happens.

 

[Captain Neil]

Re: Help with prop selection

I agree with Silvertip on this one. An aluminum boat in your size range and style powered by a 90HP enigne would typically run a 17 to 19 inch pitch prop and reach speeds around the 40mph mark with a SS prop. There are lots of options in both aluminum and stainless right here on iboats. For example, you might try a Solas Titan 13 1/4 X 19 X 3 Stainless prop (PN1441-133-19) or a Solas aluminum Amita 3 PN1411-130-19 which has a slightly smaller 13 inch diamter.

 

[Ram58]

Re: Help with prop selection

I'm going out tomorrow and going to mess with the jack plate. I'm going to run from lower to higher positions and see what happens. Then I'll come back here and tell all you guru's what's happening. Thanks for all your help

 

[Ram58]

Re: Help with prop selection

Went back out. Tried the jack plate in mid to high range and kept the motor trimmed. The best I could get was 5ooo rpm, but was hearing a little cavitation at that point. When I got back home I decided to make sure the throttle was opening all the way. It is hitting the stop, but since it's adjustable I'm not sure if it's set right. I did notice on the top of the throttle plate, there's a linkage that attaches to an arm under the flywheel. I'm guessing that changes the timing according to throttle position. The plastic clip that attaches the linkage to the arm is broken, so I will have to get one of those. Would that have an effect on the rpms?

 

[Captain Neil]

Re: Help with prop selection

You are probably right about the link rod that you saw. It does adjust the ignition timing based on throttle opening. The plastic clip may be a retainer to keep the adjustment from moving, but I am no expert when it comes to Mercury 2-strokes. The slight increase in RPM that you observed was probably from the ventilation caused by the high engine height and motor trim. I still think your rig should be running a 17 or 19 inch prop based on dozens of performance tests and bulletins I've seen. It wouldn't hurt to have a dealer check your ignition timing to ensure that the broken plastic clip hasn't allowed the timing to slip.

 

[Ram58]

Re: Help with prop selection

Went back out this morning with my glasses on and discovered the clip was not broken, but the linkage had only became detached. I reattached it and am going to go back out tomorrow weather pemitting.

 

[Ram58]

Re: Help with prop selection

Went out yesterday to test boat. With jack plate between 1/2 to 3/4 up, motor trimmed, I got 5300 rpm and speedometer read 45. I guess the linkage did have an effect on it. Thanks to all for your help.