Proper prop sizing

[Admiral Bird]

There appears to be alot of knowledge out there on propping and I hope someone can help me.

I have a 17' lund which has now a 1997 40HP Force outboard. Total gear/people weight is normally around 500-700 pounds.

I took the boat our recently (at about 7600' altitude) and could only obtain a top GPS speed of 11 mph. At 11 mph my rpm was only 3500. The WOT RPM recommended is a min of 4750 and max of 5250.

I am using a 10.5 X 14P alum. prop which came with the motor. I have two other props (in need of repair) ;one a 15 pitch and the other a 13 pitch. Now I understand that by lowering the pitch by one number I should gain about 200-250 in RPM but I am not sure if this will give me more speed or less. I am waiting for good weather to get on the lake to find out what changes I can achieve.

Does higher RPM normally give more speed or less.

Is it possible that I have a jetting problem as well and if so how can I tell if the carb is jetted properly for that altitude?

Thanks in advance for any and all advice.

 

[MARV1]

Re: Proper prop sizing

Properly propped you will attain more speed. I would say with these smaller outboards and their lower gearing the RPM changes tend to be 3-400 difference each pitch change. So going down to 12p would jump you up and then gain more speed and RPM as you'd be on step, but try the 13 is useable...

 

[craze1cars]

Re: Proper prop sizing

I took the boat our recently (at about 7600' altitude) and could only obtain a top GPS speed of 11 mph. At 11 mph my rpm was only 3500. The WOT RPM recommended is a min of 4750 and max of 5250.

I am using a 10.5 X 14P alum. prop which came with the motor. I have two other props (in need of repair) ;one a 15 pitch and the other a 13 pitch.

Your rule of thumb about dropping pitch and gaining 200 rpm goes right out the window if your boat isn't planed first. You simply can't determine whether your prop will get you proper RPM if you're not planing out. 11 mph is definitely not planed. And the reason your RPMS are way below target is ALSO because you're not planed.

Yes at 7600 ft you have a jetting problem, your motor is running pig rich on fuel and is down on power. Also be aware that your 40 hp, IF JETTED PROPERLY, will only make 31 hp at 7600 ft running at its peak. That's asking a lot to plane a 17 foot boat, but it is possible. But if it's NOT jetted properly, your motor is probably making somewhere between 20 and 25 hp....and only at 5000-ish rpm. At the 3500 rpm you're achieving now, it's probably making 15 hp or even less? With this, you simply can't plane a 17 foot boat. This is your primary problem.

First, contact a Force dealer or find a service manual/high altitude jetting chart. Rejet the carbs with the appropriate smaller jets and make whatever other adjustments they recommend, then put the 13 pitch prop on it, and see if the boat will plane out. From there, you can determine whether you're reaching your proper RPM range and whether you need to get a different prop or not.

 

[Frank Acampora]

Re: Proper prop sizing

At over 7000 feet altitude, your engine is not getting all the air it needs to burn the fuel delivered by the carb. It is running WAY too rich. You need to
1. Re-jet (I think about two sizes smaller) However, let your Mercury dealer tell you the correct size jets to use.
2. Re-prop to the 13 or perhaps even a 12 pitch to account for the lower horsepower produced.

Expect a speed loss and even with the changes, the hull still may not plane.

 

[Admiral Bird]

Re: Proper prop sizing

At over 7000 feet altitude, your engine is not getting all the air it needs to burn the fuel delivered by the carb. It is running WAY too rich. You need to
1. Re-jet (I think about two sizes smaller) However, let your Mercury dealer tell you the correct size jets to use.
2. Re-prop to the 13 or perhaps even a 12 pitch to account for the lower horsepower produced.

Expect a speed loss and even with the changes, the hull still may not plane.

Thank you all for your advice. I put my 13 pitch on and went to altitude and actually got to 18 MPH (up from 11 MPH) but the boat took a couple minuted to plain. Also, I picked up about 400-500 RPM going to the 13P from a 14P. I have ordered a 12P prop and hope I will plain quicker. If that doesn't do the job then i will look into re-jetting. The motor may have already been jetted to altitude before I bought it.
You guys are awesome, thanks for your help.

 

[craze1cars]

Re: Proper prop sizing

I honestly think you're going about this backwards. Note that both Frank and I recommended rejetting first...then prop shopping. But it sounds like you're buying props first. It takes a phone call and 5 minutes of inspection to see whether the jets in the carb are what the dealer recommends.

A 12 WILL help it plane quicker, no doubt, and will get your RPMS up higher. It might also be a waste of money because it might be the wrong pitch and might over-rev when you get the motor tuned for the altitude you use it at. Then again it might not. It's a crap shoot.

I realize your smaller prop is damaged and you probably need one anyway. But what pitch? I REALLY think you need to at least confirm your jetting before buying any props. Having proper jetting will not only improve power output, it will also reduce fuel consumption, extend spark plug life, reduce smoke & pollution, etc...

Hopefully that 12 is exactly what you need.

 

[Admiral Bird]

Re: Proper prop sizing

I honestly think you're going about this backwards. Note that both Frank and I recommended rejetting first...then prop shopping. But it sounds like you're buying props first. It takes a phone call and 5 minutes of inspection to see whether the jets in the carb are what the dealer recommends.

A 12 WILL help it plane quicker, no doubt, and will get your RPMS up higher. It might also be a waste of money because it might be the wrong pitch and might over-rev when you get the motor tuned for the altitude you use it at. Then again it might not. It's a crap shoot.

I realize your smaller prop is damaged and you probably need one anyway. But what pitch? I REALLY think you need to at least confirm your jetting before buying any props. Having proper jetting will not only improve power output, it will also reduce fuel consumption, extend spark plug life, reduce smoke & pollution, etc...

Hopefully that 12 is exactly what you need.

I talked with the mechanic who installed my motor and he said that it would be about 2 hours of his time to research the proper jets and install them so the costs would be ~$250 plus parts. A new prop was a much cheaper alternative to start with. The prop will be here on Tuesday and we will try it out then. It does feel like a crap shoot and knowing what to try first is tough.

I am not sure who you mean to call and how to inspect the jet. Can you provide insight here?