Prop pitch

[jam39vw995]

Here's another question of prop pitch.<br /><br />I've read through a number of posts after a search and found a lot of info. that helped me but I need to ask a couple questions.<br /><br />I pretty much know that a 1" change in pitch will equal roughly a 200 rpm increase or decrease.<br /><br />I have a 165 merc that runs about 34-3500 rpm at WOT which is way too low. It should be something like 4400-4900 right?<br /><br />Assuming there's a 19 on it now, I would have to go down to a 14 to see a 1000 rpm increase. Does that seem correct? Is there even such a thing?<br /><br />How can I tell what pitch the prop is now? I didn't see any numbers on it. I've seen numbers on other props but I have no idea what they mean. Anyone have a crash course on reading these **** things?<br /><br />I suspect maybe the timing is off as well, does someone have the timing spec. for this engine?<br /><br />And of course, any other hints or suggestions in trying to get this thing up into a more efficient RPM range.<br /><br />thanks

 

[Doug Durako]

Re: Prop pitch

If that is the straight six GM motor, it should dial in at 4200 rpm at wide open.<br /><br />What is the length, weight, hull design,year and make of your boat?<br /><br />Do the motor tune-up work first before messing with your prop. Most manuals will give you all the motor specs.

 

[jam39vw995]

Re: Prop pitch

Ya, it's the inline 6, ok, 4200.<br /><br />1971 21 ft. Sangster, deep V, pretty heavy boat probably 5000. This boat is almost identical, except for the V6<br /><br />K&C <br /><br />Tune up has been done, new plugs, cap, rotor, points, wires etc. new carb kit installed.<br /><br />It comes out of the water slow but planes ok and RPM's are 34-3500. fuel economy sucks.<br />I think it might be timing but haven't had time to check it.<br /><br />thanks ddboater

 

[Bondo]

Re: Prop pitch

If your motor Is up to snuff......<br />It Sure Sounds like you're Over-Proped.....<br />Prop Pitches are USUALLY stamped either on the side of the body, between to blades, <br />Or, More Likely, Under the 1 1/16th" nut that holds the prop on.....<br />I think you'll find that the Smallest pitched Merc props are 15"......<br />After that you'll have to decide if the outdrive Ratio has been Changed.....<br /><br />BTW, Is this a Dock boat, or Trailer boat ?????<br />A Dirty Bottom will Kill a little motor like yours......

 

[mercrewser]

Re: Prop pitch

If everything is legit with boat and motor, I would recommend the 16x16 (elephant ear) propeller, that is if you have a 14.5x19 on it now. Second # is the pitch, first is diameter. That 200rpm per inch rule in my book doesnt mean much because of all the different variables in play.

 

[mercrewser]

Re: Prop pitch

or maybe try a 14.5 by 17 if you do alot of cruising and you dont have to yank skiers.

 

[DJ]

Re: Prop pitch

Excellent prop suggestion from Mercrewser. (16" X 16" "elephant ear".<br /><br />I used to see alot of those 250 cid GM engines running that prop. 4200 is about as tight as you want to wind it.<br /><br />Your fuel economy will probably increase with a lower pitch prop. It won't be working so hard.

 

[jam39vw995]

Re: Prop pitch

Thanks for the info.<br /><br />Ya, I think you are right about the fuel economy thing. The engine is working pretty hard where it is now.<br />I'll check a few other things and try that prop or something similar.

 

[jam39vw995]

Re: Prop pitch

This is a dock boat but it doesn't sit. It's used every day to commute. The bottom was recently cleaned and painted with antifoul paint.<br /><br />thanks

 

[jam39vw995]

Re: Prop pitch

We had the boat out on Sunday to change the leg oil and the number on the prop ended in 15P.<br />So I'm assuming that's a 15 pitch so the rpm issue is not prop related.<br />My other post about no power, popping back through the carb etc. is more likely the problem.<br />Through a process of elimination it's probably the carb.