Propeller question

[gregorysmit]

My engine at WOT is about 300 rpm lower than recommended. If I drop any more pitch my scruising speed is affected. How much rpm could I gain by reducing the propeller diameter by 1 inch? Thanks.<br /><br />Greg

 

[mjrhoads]

Re: Propeller question

Normally it's 300-400RPM per 1" of pitch, so you'd want to go down by one (or maybe just have your prop re-pitched at a local prop shop)<br /><br />So you can't trim your outdrive under power to obtain your max RPM's at WOT?

 

[magster65]

Re: Propeller question

This change will affect your rpm vs. speed however... because your engine will not be working as hard (higher vacuum readings) with the lesser pitch you maybe pleasantly surprised at your new fuel economy. It's also possible your engine will rev a bit higher and you won't lose any speed either.

 

[Boatist]

Re: Propeller question

Gregorysmit<br />Are you 300 RPM's lower than the top RPM range or 300 RPM's lower than the lower range? Better yet what is you motors WOT RPM range and your WOT RPM's. <br /><br />I can not tell you how much 1 inch change in Diameter will make but one inch change in Pitch will make a difference of about 200 RPM's. My motors WOT RPM range is 4200-4600RPM's. I have two Stainless steel props. A 19 pitch at 4200 RPM's and 38 MPH and a 17 pitch at 4600 and 35 MPH. The 19 pitch also gets better fuel mileage but the hole shot not near as good. I use the 19 pitch for ocean water where we cruise farther and usually 15 to 25 mile per hour. I use the 17 pitch for lakes and rivers where many more marinea to slow down for or other boats fishing or skiing and do not go that far.

 

[gregorysmit]

Re: Propeller question

MY recommended WOT is 4600-500, I'm turning about 4300. If I drop an inch pitch I lose my cruising speed. Therefore, I was thinking of dropping about 1" diameter in order to get into the desired range. Perhaps with lesser torque than I had on my former 7.4 L the bigger wheel is just keeping my rpm down?<br /><br />Gregp

 

[Bondo]

Re: Propeller question

I'd Leave the Dia. Alone..... If your That Concerned... Have your prop Flattened an inch... Don't Mess with the dia....<br />No Matter What you do, The Entire RPM range Will be Afected....<br />4300rpms is not That out of line... I'd Leave it Alone...<br />You state what you Had(7.4l), What do you Have Now???...<br />If your now running a small block, the range is 4200rpm/ 4600rpm, So you Are in the range...

 

[gregorysmit]

Re: Propeller question

Recommended WOT for a 350 Mag MPI is 4600-5000.<br /><br />Greg

 

[popazog]

Re: Propeller question

gregorysmit... I too have had some difficulty with my 350 mag MPI. I run about 4400 at WOT with a 17" Mirage Plus. I would love to get my RPM up into the recomended range, but I am already using a lower pitched prop than I want to. My boat is only a year old and the dealership hasn't helped much regarding a solution. What pitch are you runing now? I have thought about adding a hydrofoil to the leg to see if that helps lessen the load and alow the engine to rev up a bit... anyone have any experience with that?

 

[Boatist]

Re: Propeller question

Gregorysmit<br />I think you could go down 2 inches in pitch and and get to 4700 RPMs or maybe 3 inches in pitch and get to 4900 RPMS. In general bigger heavier boats use a larger diameter prop and smaller lighter boats use a smaller diameter prop. Did you change from a 7.4 L to a 5.7 L? If so and you are useing the same outdrive may need a different gear ratio.<br /><br />Popazog<br />You should be able to go to a 15" pitch and get to 4800 rpms. Would give you a better hole shot but may not add more speed. The Hydrofoil will help you get on plane quicker by lifting the stern but probably actualy loose a little top end. When lift the stern the bow go down so have more weted surface or slower speed.<br /><br />Since both you guy have a simular problem and same engine could be a EFI software problem like a engine hitting the rev limiter. Never hurts to ask dealer if any service bulletins dealing with your problems.<br /><br />One more thing each of you may want to do is take all the extra weight out of the boat and with a light fuel load take it out for a run and see what it does.

 

[gregorysmit]

Re: Propeller question

To Boatist,<br /><br />I'm running a 16.75 X 19 wheel to realize about 4300-4400 rpm, just a little short of the recommended 4600-5000. That's why I thought dropping just a little diameter. I tried an 18 X 17 wheel but my cruising speed at 3600 rpm dropped considerably. The fustrating thing is that Bayliner indicates 25-26 mph cruising at 3400 with the Bravo III. Maybe that's the way to go.<br /><br />Greg

 

[gregorysmit]

Re: Propeller question

Popazog<br /><br />What boat are you running?<br /><br />Greg

 

[popazog]

Re: Propeller question

gregorysmit... I have a 2002 Campion Chase 700i (24' sport cuddy) Boatist... Thanks for your advice on going to a 15' pitch prop, but wouldn't that dramaticaly affect my top end even though I would get a higher RPM at WOT?

 

[Boatist]

Re: Propeller question

Gregorysmit<br />Your right, if you turn the same rpm with a lower pitch prop you will slow down. It just like running 2000 rpms in your car and taking it our of overdrive. However your WOT speed may be close to the same because will be turning more RPM's. Boat speed at a given RPM is dependent on many things, condition of the hull, and weight too biggest factors.<br /><br />Popazog<br />Yes it will make a difference, I would not expect more than two or three MPH. In your case would probably be a little slower. On my boat about 3 miles per hour faster with the higher pitch.