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Author Topic:   Props again
cc13 posted 05-09-2003 12:33 PM ET (US)   Profile for cc13   Send Email to cc13  
There has been a lot lately about props. Am I correct that, in general, the following would be true: 1) a lower pitch (15 or lower) provides a better hole shot, ski pulling, etc., 2) a higher pitch (19 or better) provides better gas mileage and range at cruising speed (3800-4000rpm). This is for a 17' pre smirk hull with 70 yam. Finally, to optimixe range and gas mileage on the above whaler, what prop would you recommend at cruising speed? I'm not trying to set the water on fire.
lhg posted 05-09-2003 03:09 PM ET (US)     Profile for lhg    
The general principle is that with a light load in the boat, as light as you will ever run it, you want the engine to turn up to it's full rated RPM, with enigne properly trimmed.

Mercury's Propeller manual indicates that in some instances, if you want hole shot pulling power, such as water skiing, you can drop down 2", but be careful not to overrev.

Conversely, if you want long distance cruising ecomony, at lower RPMS and engine noise, you can go up 2", but also be careful not to run the engine full throttle propped like this.

cc13 posted 05-09-2003 04:19 PM ET (US)     Profile for cc13  Send Email to cc13     
How does that square with the general notion that outboards use 10% of horsepower in gallons per hour at WOT and use less than half of that at cruise. In other words a 70 at WOT uses 7 gallons per hour. The same engine at cruise should use less than 3.5 g/hr. Seems to me lower RPMs will increase fuel economy. Why run at WOT unless you just want to go fast. That's why I don't use a speedometer, I just need to know whether I'm runnning "wide open" or not. Whatever that speed is is not important. What I'm trying to do is to increase fuel economy and range. What type prop works best for that purpose?
Bigshot posted 05-09-2003 04:44 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Simple CC....An outboard is designed to run in a 'range" at WOT say 5000-5500. You want to be as close to the 5500 without exceeding it with a light load. If you prop your boat to run at 4500WOT the engine is gonna lug like a car going uphill in 5th gear, not good for the engine. Also if your boat propped right does 30mph at 4k but overpropped it does 35 at 4k it does not mean you are getting beter mpg, that engine thinks it is doing 5k or at least burning the same amount of fuel. Efficiency is king here and stay within the range or you will eventually blow something.

To answer your question a 13x17 should be the perfect prop for you.

cc13 posted 05-09-2003 06:02 PM ET (US)     Profile for cc13  Send Email to cc13     
I think I'm getting it. A higher pitch ie 19 will produce better gas mileage than a lower pitch ie 15,at least at a cruising speed. Is it adisable to turn the motor at WOT = 5500+ all the time? Seems to me that gunning wot every time you get in the boat would not be good for the engine.
jimh posted 05-10-2003 10:46 AM ET (US)     Profile for jimh  Send Email to jimh     
cc13--Don't think that the engine controls the propeller. The propeller controls the engine. At a particular throttle setting, the engine speed is determined by the load it sees, and that is determined by the propeller.

If you change the propeller and reset to precisely the same throttle setting, the engine speed will be different (unless the engine has much, much more torque than is needed to turn the propeller, in other words, loads of reserve).

You might want to read the multiple-part article in the Reference section where this and much other information is presented in what I hope is a coherent and organized fashion.

Sal DiMercurio posted 05-10-2003 10:57 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
CC13, whoa just a second.
Your thinking in the wrong direction, more pitch dosen't give you better fuel economy, "UNLESS" the engine is capeable of turning that prop with more pitch to it's maximum rpms, like 5,500 rpms.
If it can only turn a 19 pitch prop to 4,800 rpms, your lugging that motor big time & using "FAR" more fuel, "THROUGH OUT" the whole rpm range, from 600 rpms to 4,800 rpms then if you had a 15 pitch that can turn that majic number of 5,600 rpms. at wot.
Don't get confused thinking the engine can twist any prop you feel like putting on, because it can't.
As Jim said, the engine dosen't control the prop, the prop controls the engine, & the wrong prop with more pitch then is needed, is going to cost you a lot of money in repairs, because you will lead that engine to an early grave by over proping it.
The first thing you do is get the correct prop for the boat for cruising, this prop "MUST" be able to allow the engine to reach it's maximum rpms at wot, which is 5,500........I agree with Bigshot that a 13 x 17 should be just about perfect for cruise & everything else, but if you decide to water ski & make deep water starts, & your buddies are all porkers at 200 lbs plus, you would be better off using a 13 x 15 "JUST FOR SKING", not cruising because the engine will now be able to spin up to 5,900 rpms, which is to high.
In my opinion, a 19 pitch for that boat is to much pitch & you will be trying to turn to a prop that is not going to give you economy, in fact it will use far more, so forget the more pitch = better milage, because it's totally wrong.
Keep in mind, you really don't have to drop down in pitch to pull a skier, if the boat pulls them up ok & dosen't strain doing it with a 17 pitch, leave it alone.
cc13 posted 05-11-2003 08:01 AM ET (US)     Profile for cc13  Send Email to cc13     
lhg, big,jim,sal
Thanks for the reply. Having read the forum for awhile you guys seem to be very knowledgeable and give good advice. What I was thinking about doing was going to a 19 for general use and a 15 for skiing. I currently run a 17 and 15. Thanks to you I'll stick with the 17 for general use and the 15 for skiing. I do find the 15 is best for pulling slaloms up quickly.

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