Prop diameter vs pitch question

[CharlestonMako]

I have a 1988 140 Johnson on a 20ft Mako. I just got it out for the first time a couple of days ago but am concerned about the WOT RPMs. I have a 15" 17 pitch prop and only gets about 4700 RPMs at WOT. I know this is low so my questions are:

what will it do to my motor ro run at this RPM?

And, will switching to a 14" 19 pitch prop help (i know it SHOULD bring the RPMs even lower, but the smaller diameter may play a role?)? I have a spare of this size.

FYI, I just hooked the tach up... It is my first boat, and I am by no means an expert... can I assume the tach is accurate (I did set it for hte number of poles on my stator... somthing about a "6")?

 

[Silvertip]

Re: Prop diameter vs pitch question

First, you don't need to bring RPMS lower. They need to be higher at wide open throttle with an average load. Before you begin spending money on props, make sure the engine is tuned properly, that has proper compression, that the throttle plates are opening fully (perfectly horizontal in the carb bore). Lastly, work with setup (engine mounting height). Changing diameter by an inch has an effect on performance since you would be working with more blade area. More pitch (higher number) lowers engine rpm so going from a 17P to a 19P would make your situation worse than it already is. If you do all the other checks, and rpm doesn't improve, then and only then should you switch props and the switch should be from a 17P to a 15P. My suggestion is to check with some local dealers to see if they have a used prop you can try. They may want you to leave yours there during the trial or to leave a deposit. But when you find what works, be a sport and buy it from them. Otherwise, buy your prop right here on iBoats.

 

[steelespike]

Re: Prop diameter vs pitch question

4,700 is too low as I understand it it can cause coking ? don't know exactly what it is but I think it relates to the carbon getting cooked. You should also supply your max speed by GPS.Helps determine if something odd is going on.Is the prop stainless or aluminum.I think your close to the max on your pitch.Prop diameter is a product of prop design and shouldn't usually be considered as a solution to performance problems.In this case it wouldn't work anyway.
I think before making any prop changes you should fine tune your setup.
Be sure the motor is in good tune and a check of compression.Be sure throttle is opening all the way.Be sure the prop is in good shape.Adjust the verticle height starting with the antivent plate(just above the prop)approximately even with the bottom of the boat and raise the motor a hole at a time until just before ventilation becomes a problem.Be sure it pumps water.You'll probably have to still change the prop but you will be able to tell better where you are at.

 

[CharlestonMako]

Re: Prop diameter vs pitch question

I?m sorry, I meant the RPMs should be increased at WOT. Secondly, I think the compression is good (121, 119, 120, 119). Also, I have checked the throttle plates, and they are all the way open (perfectly horizontal). I have not changed the motor height. Will raising, or lowering the motor raise RPMs?

One thing I have noticed is that the control lever has to be pretty far forward before the throttle starts to open, and raise RPMs. It is probably half way down (45degrees form the vertical neutral) before it has shifted all the way into gear and can then accelerate/increase RPMs. Is it possible there is a loose linkage somewhere, and I am just unable to push the control lever forward enough to get max RPMs? But then again, I guess that would result in the throttle plates not being fully opened.

I will definitely try the loner prop idea? thanks for the input. Also, will running the boat under these conditions hurt anything, or just decrease performance? Thanks.

 

[CharlestonMako]

Re: Prop diameter vs pitch question

Thanks for the reply steelespike. I was writing mine as you posted and missed it. I do not have a GPS yet. My ol? petot tube speedometer was saying we were toping out about 30mph, but I don?t know if I can trust that either. It is a Stainless prop.

The motor antivent plate is currently about and inch or two below the keel? maybe too low? Also, I was hoping to run a can of seafoam through it today or tomorrow, take it out to really open it up again, and see if that helps. I obviously don?t want to do that though if there is a serious possibility of damage.

Thanks

 

[steelespike]

Re: Prop diameter vs pitch question

Raising the motor will increase rpm and speed.If the antivent plate is 2 inches below the keel it is too low.It is possible you could raise the motor 3 inches
or more. It depends on the reaction before venting becomes a problem.Your stainless prop will make it possible to get the best of your height.It's possible for as little as an inch to result in 2 mph or more in speed and of course more rpm.Speedo is not too accurate but I would say 4,700 and 30 mph is in the ball park.Other than testing I don't think I would run it too much until you get her set up.By the way this will also improve your MPG.

 

[CharlestonMako]

Re: Prop diameter vs pitch question

Well, it was good advice to suspect the prop last. I checked the spark, and the motor had only been running on three cylinders. I tried the coil... it was good. Then replaced the power pack (I have a spare parts motor), and walla... all four started sparking with a strong blue spark. ran a can of seafoam deepcreep through it, put it in the water, and it ran like a top. i got 10 more mph out of it, and the RPMs topped off at about 5600. Thanks to everyone for your advice.