Prop pitch versus gas consumption

[WakeKev]

Hello
Can anybody tell me that if you change the pitch of your prop, would it effect your gas consumption? I guess I'm trying to relate it to rear end differential gears in a truck. If your towing something with a higher gear, it gives you extra torque as well as increase your fuel mileage at lower speeds but you end up losing MPH. Where if you use a smaller gear you gain MPH but end up losing torque and use more gas during towing at the same speed. If that makes any since. Please any info would help.
Thank you

 

[danfrompgh]

Re: Prop pitch versus gas consumption

Someone may correct me on this but I was always told:
Lower pitch = higher torque = better holeshot but less speed
Higher pitch = lower torque = slower holeshot but higher top speed.
Most motors have a sweet spot for gph,mine is at 3000 rpm.
just my 2 cents worth

 

[jerryjerry05]

Re: Prop pitch versus gas consumption

What kind of boat,year,size of boat.Type motor/drive. How fast is your boat and how many rpm at WOT.What size is your prop?(pitch,cupped or not)
These variables will affect the performance. What do you want from your boat?Answer these and then I can help.Jerry

 

[Silvertip]

Re: Prop pitch versus gas consumption

Considering fuel economy first when selecting a prop is a bad decision. First, last, and foremost, every engine manufacturer specifies wide open throttle RPM. Your boat should be propped to get the rpm at that rpm with an average load. Fuel economy will then be the best it can be. If you overprop the engine to slow it down, you end up lugging the engine meaning you need more throttle at ANY speed than you needed before. You especially need more throttle to get on plane. The net result is that even though you slowed the engine down it is actually consuming more fuel than before. Even if you did manage to come up with some savings, it would not be enough for most boaters to offset the potential engine damage caused by over propping.

 

[steelespike]

Re: Prop pitch versus gas consumption

Marine engines gas cunsumption is rated in "gallons per hour"gph. at wot(wide open throttle)
Rule of thumb gph = 10% of rated hp. thus a 120 hp motor uses about 12gph.@ wot.doesn't matter if it is pushing a house boat or a speed boat.
So the idea is to get the most speed while staying within the motors rpm range.Of course roughly speaking a motor will use about 1/2 its gph at half throttle.Best mileage cruising speed can be as much as 2,000 rpm above planing speed.experience or a flow meter is about the only way to tell where your boat does best.

 

[WakeKev]

Re: Prop pitch versus gas consumption

Wow. Thank you everybody for all great information. The boat is a 99 Ebbtide 182 Campione with a 4.3L Mercruiser V6. We mainly use it for pulling friends and family on wakeboards or tubes. I usually keep the RPM at 3000 which will keep the boat around 22 to 23 mph. Now since changing the pitch of the prop to get a better bite on the water would help to get on plain faster and would raise my RPM at 22 to 23 mph, would this cause the motor to use up gas faster? The only time we are at full throttle is when we are going from one side of the lake to the other side.

 

[steelespike]

Re: Prop pitch versus gas consumption

Theoretically the prop will allow the motor to rev more easily(less throttle?)
There is no way of knowing if it could use less or more gas without running it.Differences in prop design (not pitch) can affect performance as well.As I said the best practical mileage can range over a couple of thousand rpm.above planing speed.The difference would most likely be difficult to tell without a flow meter.unless the setup was way out of specs.
Picking an arbitrary rpm to save gas is the wrong way.A flow meter is the best way but experiment can work over time to develop a best rpm range.

 

[Silvertip]

Re: Prop pitch versus gas consumption

Running at part throttle with an incorrect prop is no different than running wide open with an incorrect prop. As was described, the engine manufacturer specifies what the recommended wide open throttle rpm is and that's what the boat should be propped for. Fuel economy will be whatever you get with that "correct" prop. If you over prop to lower rpm you lug the engine to get on plane and it lugs all the way across the board. As a result the throttle needs to be open farther to maintain the same speed so fuel consumption goes up. If you lower pitch to get better hole shot, that increases the risk of over reving the engine at WOT and it means the engine is running faster than it needs to at the same speed as before, again using more fuel. The best way to reduce fuel consumption is slow down, lighten the load, and get on plane gradually rather than hammering the throttle. It doesn't matter how fast or slow you drive, if you have an incorrect prop you cannot expect optimum fuel economy.

 

[jerryjerry05]

Re: Prop pitch versus gas consumption

I have 21 cc with twin 85hp.It came stock with 17c's I thought maybe I'd try going up in pitch and get more speed. My boat went from 33-34mph to 38-39.
Low end performance was about the same.The gas consumption across the board went up 10-15%.A friend had the same boat and had it equipped with stainless 15p. He beat me out of the water everytime.
You didn't state if you have stainless or aluminum. Stainless has less flex than aluminum. So better accelleration with stainless.
If you have 19 alum then a 17 stainless would be what you would need.Jerry

 

[WakeKev]

Re: Prop pitch versus gas consumption

That's great to know. I'm not sure of the pitch because the stamped numbers have became so worn you can't ready them. I am using an aluminum prop and have hard good and bad things about switching to stainless. The good is of course performance the bad is that if I hit something with the prop, instead of eating up just the prop, now I'm starting to damage things in the out drive. I appreciate all the replies and will set off to buy a flow meter and different size prop to start experimenting. I will post what I have discovered. Thank you all again.