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