Prop size and fuel economy

[snowbrd84]

I have an 18' bowrider with a Mercury 150 XR2 and a 20p aluminum 4-blade prop. I can't even get to WOT because at about 48mph if feels like the boat is gonna fly out of the water... I don't really need to be able to go 50mph, but it is nice to have that power and get going real fast sometimes... I am basically at 1/2-3/4 throttle at cruising speeds 25-30mph. All GPS speeds.

I would like to be getting maximum economy at cruising speed. So is it better to stick with this 20P prop and cruise at 1/2-3/4 throttle? Or would I get better economy at a higher RPM and a lower pitch prop? I would lose that 50mph top speed obviously, but at nearly $4/gallon, its just too darned expensive...

 

[Starcraft5834]

higher rpm's dont equal high fuel economy with your engine or any engine.. it's best to get a prop that performs in the recommended ranges for that outboard engine.. once there,, if you want best economy, keep your right hand from going down to much..

 

[steelespike]

Your best practical cruising speed will likely be somewhere from just on plane to about 1000 rpm higher.
Decreasing pitch will usually reduce speed and increase rpm it may get to rpm with less throttle but slower speed will probably wash the results.
If your serious about increasing mpg a flow meter is the only way to go some just give fuel flow others give real time mpg calculations.
A prop that grips well will help. As rpm decreases slip goes up so it would seem that a prop that produces good slip numbers through its rpm range
would help mpg..A prop that keeps the boat on plane with less throttle and less fidgeting.

 

[Chris1956]

A rule of thumb is to stay below 2/3 throttle, and trim max, for best fuel economy. As 'spike says, a fuel flow meter is necessary to improve on that general rule.

 

[Texasmark]

Pull up some performance data on new boats. Look at the performance graphs. Answer as the other guys stated is right there.

2 strokers like to run, not lug. With that said running at the top end of the recommended operating range, at WOT with your normal load, is where you should set your prop pitch. Since you don't run up there it's hard to tell for your rig.

A 4 bladed prop that may be shy on pitch somewhat will give you the best cruising at the operating point we have suggested. Reason why is that is is suited for the load and will help to prevent your being constantly on the throttle to keep the boat at a fixed speed. Too much pitch and you will be going too fast to suit you one minute and falling back under plane the next. It will also be a good hole shooter....usually.

SS props do tend to run more efficiently due to thinner blade designs. Just how much is up for grabs as I see it. Cupping (slight bending forward of the trailing edge of the prop blade) for where you want to run I don't see as a necessity, however the cup is there to improve grip which is mainly needed at high trim angles and high speeds. Rake I don't think is all that necessary for the cruising operating range. Rake that usually comes with SS props (blades bent back rather than emanating directly out from the hub) is primarily for fast boats that need to get the hull out of the water to reduce drag.....not going to happen just over planing speed.

HTH,
Mark

 

[midcarolina]

Install a set of smart tabs............. Then decide if you need to reprop

 

[HT32BSX115]

I have an 18' bowrider with a Mercury 150 XR2 and a 20p aluminum 4-blade prop. I can't even get to WOT because at about 48mph if feels like the boat is gonna fly out of the water... I don't really need to be able to go 50mph, but it is nice to have that power and get going real fast sometimes... I am basically at 1/2-3/4 throttle at cruising speeds 25-30mph. All GPS speeds.

I would like to be getting maximum economy at cruising speed. So is it better to stick with this 20P prop and cruise at 1/2-3/4 throttle? Or would I get better economy at a higher RPM and a lower pitch prop? I would lose that 50mph top speed obviously, but at nearly $4/gallon, its just too darned expensive...

Howdy,

I am not sure you can speak about fuel economy and boat in the same sentence!

Having said that, the only way to KNOW you have the correct pitch prop is to load your boat the way you'll likely always operate and run it up to WOT (on a nice smooth day so you don't scare yourself!!!)................ trim the engine for maximum speed, and note the speed (using GPS unless you have verfied your Pitot speedo) and note your RPM (if you have verified your tach). (you MUST know RPM accurately)

If maximum speed occurs near the top of the recommended RPM range for your boat, you are propped correctly (regardless of how many blades you have in the water)

If you want maximum speed reduce the number of blades (a 2-blade will be the fastest) If you want better "hole-shot" go to a 4 or 5 blade (if you have the adequate HP)

Stainless steel will usually be most efficient. A 3-blade is usually the best all-around prop.

If you want Tabs, get cockpit-adjustable types. (like Bennett) .......... Spring-loaded tabs will add drag all the time and possibly reduce mileage at cruising speeds. Any time you drag anything in the water, that additional drag requires more fuel to produce a given speed.

You're much better off trimming the engine to produce minimum fuel flows......

Also, if the boat feels sort of "out of control" at higher speeds (in smooth water) and you do not have adjustable engine trim (up/down, in/out) You might consider installing an adjustable mount for the engine. you will then be able to trim for best fuel flow and better yet, smoother and better ride.


Regards,


Rick

 

[midcarolina]

Howdy,



If you want Tabs, get cockpit-adjustable types. (like Bennett) .......... Spring-loaded tabs will add drag all the time and possibly reduce mileage at cruising speeds. Any time you drag anything in the water, that additional drag requires more fuel to produce a given speed.




Regards,


Rick

I agree with the entire post with the exception of this part.............. that is false and is mere opinion.

 

[alldodge]

I agree with HT32, spring loaded tabs is in the mix with the rest of snake oil (seafoam, lucas, etc) but this to is also just my opinion

 

[HT32BSX115]

I agree with HT32, spring loaded tabs is in the mix with the rest of snake oil (seafoam, lucas, etc) but this to is also just my opinion

Everyone of us has opinions It would seem! but it's not my "opinion" that pushing on the water with 2 plates adds drag. It's simple aerodynamics (or hydrodynamics as it were)

Being able to change the center of gravity (and center of pressure) by moving the thrust angle (position and angle of the propeller) without increasing the whetted area on the hull is somewhat more efficient than pushing on the water with flat plates at the back of the boat.....whether they're fixed (& spring-loaded) or hydraulic.

But this is a discussion about props.......and as usual.......I digress..........

 

[bruceb58]

I agree with Rick as well. Smart tabs will reduce the fuel economy.

The only perfect way to figure out your most economical cruise is with a flow meter. Before I installed a flow meter, I was running at a lot slower speed thinking I was at my best MPG speed. There is no set throttle position and RPM that will be the sweet spot. It varies from boat to boat as well as engine combos.

 

[midcarolina]

I agree with HT32, spring loaded tabs is in the mix with the rest of snake oil (seafoam, lucas, etc) but this to is also just my opinion

Wow.............. you are saying in your opinion smart tabs are a ripoff and do not do what the manufacturer say's they will do........:faint2:

 

[snowbrd84]

I actually have a pair of smart tabs that I bought last year and I will be installing them at some point. Just because something adds drag, it does not mean it hurts performance. Lets say w/o tabs you dont start planing until 15mph and w/ tabs you start planing at 10mph. The drag created by the tabs at 10mph is FAR less than the drag created by the stern of the boat plowing through the water. Even once you are planing, if the tabs help lift the stern in an optimal way as to decrease the drag from the hull by a greater amount, it can increase performance. Obviously, it can go the other way as well, it would be different for every boat/motor/weight/condition...

Back to what I was originally trying to figure out. In a case like mine where I have lots of extra power, the boat is not lugging at 2/3 throttle, if I give it more throttle, it just happily goes faster... So it sounds like the larger prop is better in that I can maintain cruising speeds at 1/2 to 2/3 throttle thus getting better economy than if I had a smaller prop that would be revving higher at those same speeds. It would seem bad to me to run a motor at its top end RPM without it being loaded accordingly, kind of like driving a truck in 4-lo at highway speeds.