20 foot 1997 Maxum 2050 bowrider propeller question

[Rick Stephens]

Need to think of this as if it is a car always going up hill and stuck in only one gear.. Higher pitch prop is like running in too high a gear, always.

 

[dingbat]

I know what you're trying to say lol... but I don't necessarily agree... if I was driving my boat to achieve good efficiency, I don't think I would cruise around at full throttle.. I would try to just get on plane but keep rpms low.. same reason cars have transmissions...
If your car had the equivalent of only 1st gear, then you'd accelerate fast but your long road trip would be pretty inefficient and take forever... but if you're car only had 5th gear, your mileage would be much better (if someone gave you a good push start) lol...
Again, not sure this theory works cuz boats and cars are definitely not the same...
Just still makes sense in my head that you would want a prop with higher pitch that would maximize your speed at minimum rpm

I run a fuel monitoring system. Water conditions (flat vs rough) has a bigger effect on fuel usage than anything else.

Reduced my fuel consumption by 10% going from a 14.25 x 17, 4 blade to a 15 x 15, 3 blade

 

[AShipShow]

I see what you're saying and it makes sense but I think that would be analogous to going full throttle in your boat and struggling to get on plane... but I think having a good balance between what most people would consider a good all around prop and what would be too high of a pitch would be a best option for long distance cruising... again, no science here, just still thinking out loud.

 

[nola mike]

If your car had the equivalent of only 1st gear, then you'd accelerate fast but your long road trip would be pretty inefficient and take forever...

You'd hit the rev limiter at 20mph, the engine would have much more power untapped.

but if you're car only had 5th gear, your mileage would be much better (if someone gave you a good push start) lol...

In 5th gear you're geared for wot. once you start going up a hill you don't keep it in 5th when the rpms drop. 1000 rpms with the pedal to the floor doesn't make sense, right? In a car your torque curve is also typically shifted towards higher rpms.

Again, not sure this theory works cuz boats and cars are definitely not the same...
Just still makes sense in my head that you would want a prop with higher pitch that would maximize your speed at minimum rpm

I see what you're saying, but I think the efficiency gains in a car are mostly due to part throttle conditions (also why the efficiency gains with FI vs. carb are so much greater in a car). In a boat under constant load at a given speed, your throttle is open wider for a higher pitch prop to get the same power (you need the same torque, so you need the same air flow/fuel input). I know @achris did some experiments with different props/speeds/rpms. @scott_danforth could probably add something to the discussion as well.

 

[AShipShow]

Turns out there's a lot of information on this here internets.... did some googlefoo and found an article where they tested a whole bunch of things and their affect on fuel efficiency in planing hulls including props...
Ya know... it takes a big man to admit when he is wrong...
Seems that the most fuel efficient props are ones that give you maximum specd rpm per your motor without overspeeding... if you go too high or low on pitch you decrease fuel efficiency...

 

[nola mike]

Link?

 

[dingbat]

Seems that the most fuel efficient props are ones that give you maximum specd rpm per your motor without overspeeding... if you go too high or low on pitch you decrease fuel efficiency...

It’s all about maximizing the efficiency of converting HP into thrust.

What is lost to most people is that a prop’s dimension is just a number. Two props of the same dimension, but of different design, can have very different performance characteristics.

If your really what to know how your prop is performing, note your rpm and speed at 500 to 1000 rpm intervals up to WOT. Run your numbers through a prop slip calculator and plot the results.
Can be a real eye opener. A change in prop design cut my slip (lost energy) almost in half.

 

[AShipShow]

Link?

How to Save Fuel Tips | Boating Mag

These green tactics will let you leave fuel docks in your wake.

www.boatingmag.com

 

[gica]

What would be an acceptable slip number.
With the 18 pitch I got 14.
If the numbers stay with a 17 I get 9. That would be pretty great