Prop efficiency; is bigger better?

[Mark42]

I was wondering why most props are about 12 - 13" diameter. Wouldn't a larger diameter prop grab more water and reduce prop slip? The only real drawback I can see is reduced clearance due to larger diameter and longer shaft lengths that may cause a bit more drag from the gear box.<br /><br />Another item of interest are those prop "cages", the 3" wide band that encircles the prop. They claim to improve performance as well as protect the prop from impact damage. If so, why isn't that design standard on outboards?<br /><br />Just scratching my head....<br /><br />Mark.

 

[Dhadley]

Re: Prop efficiency; is bigger better?

Yes, the larger diameter is, in most cases, a real help. The biggest advantage it has is that it allows for a higher X dimension. <br /><br />Some motors use a large diameter to help acceleration. Some of the 4 stroke motors use a combination of a really low gear ratio and large diameter prop to make up for a lack of bottom end power.<br /><br />Keep watching the prop companies like Turbo. You'll see some very large diameter props coming. In fact they're on the shelf now.

 

[wvit100]

Re: Prop efficiency; is bigger better?

You need a prop that puts the rpm's in the correct place on the guage a wide open throttle. You don't want to be able to over rev the engine and neither do you want it to lug and not be able to reach the correct rpm range.<br /><br />Your owners manual will give you the correct operating rpm range for wide open throttle. Your prop selection needs to keep the engine in that range.

 

[Mark42]

Re: Prop efficiency; is bigger better?

Wvot100,<br /><br />I was talking in theory. I already went through changing props to get the right setup. That was kind of fun. I think. <br /><br />So wouldn't a 24" prop give more effeciency? I could see that there would be more strain on the transom from a longer shaft length to get the whole thing underwater. But it strikes me that there is a whole lot of room for improvement when some boats only get the equivelent of 1 or 2 MPG out of 1, 2 or 3 litre motor. <br /><br />I know things are different between planing and displacement hulls. A 3000lb displacement hull can cruise at 8 mph all day with 25hp using only 6 gallons. Is the difference in friction that much between displacement and planing that the planing hulls get crappy fuel economy?

 

[salty87]

Re: Prop efficiency; is bigger better?

no, the planing hull sees less friction when it's on plane. it just takes, comparatively, an incredible amount of power to get on plane and stay there. that will cost you fuel.<br /><br />there is a point where you will have too much diameter and the engine can't turn the prop without ripping itself up. just like riding a bicycle in too high of a gear, it becomes more inefficient at a certain point for a given motor and prop set up.

 

[wvit100]

Re: Prop efficiency; is bigger better?

I doubt your 40 hp motor could turn a 24" prop at the proper rpm range unless the prop had very little pitch to it. The problem is that as you increase the pitch and size of the prop you will need more horsepower to turn it

 

[Boatist]

Re: Prop efficiency; is bigger better?

Mark<br />In general you want a bigger flatter for heavier slower boats. For lighter faster boats you want a smaller prop with more pitch. <br /><br />Prop slip is not really a bad thing. It is kind of like your clutch. When you are stopped and kick it into gear and the prop did not slip you would lug the engine very bad. As you applied power you could burn a whole in a piston. With no slip it would be like taking a car with a manual tranmission and with out useing the clutch kick it into high gear. That would be very hard on the power head, drive line, Gears, prop, transom and the boat. Also a bigger prop has more drag as it turn in the water.<br /><br />With prop slip when you kick it into gear the prop turn easy and as you apply power again the prop get up to speed very quickly. As the boat get up to speed you get less prop slip. <br /><br />You can see some of the effects of less prop slip with 4 blade props. Yes they have less slip and usually give you a better hole shot but less top end. With the proper gears you can turn a small prop at a higher speed and generate the same or more thrust so which is really more effecient. <br /><br />A TUG boat pushing tons of weight has a very large flat prop. Miss Budwiser has a small 2 blade high pitch prop.

 

[Tom2697]

Re: Prop efficiency; is bigger better?

Prop diameter, pitch, blade area, and blade rake are the general rpm limiting aspects of a prop. As one dimension is changed, another must change as well if you are to keep the prop spinning at the same rpm (engine hp is based on a given rpm). The problem with just spinning a big prop (aka - large diam) is that you will get to a point where the motor spends more of its power trying to turn the prop than in moving the boat...unless you drop down in pitch, remove blade area, or change the rake angle on the prop (this has the least affect). The problem here is that the relationships are not linear. By increasing the diameter 1/2 inch, you need to drop the pitch by 1 inch to maintain the same rpm. Eventually, you'll get to a point where you'll be spinning the prop at a high rpm but moving hardly any water past it. Think about the definition of pitch, how far the prop moves (barring slip) on one rotation. If you are turning a large dia, low pitch prop at the same speed as a small dia, high pitch prop, which will move farther? Add in more slip for the small diameter prop and this prop will still move farther and hence, faster.