Prop Pitch

[ronjon1944]

Lost here!, have a 1988 mercruiser 3.0 alpha1,I've read the post on "the basics of selecting a propeller" very good info. my boat is 17-1/2ft 2000lbs with 4people,the most I will pull is 1 tube or skier. prop is now a 15/19 but worn down some, looks like it came with the boat in 1988. Merc. shows 4200-4600rpm WOT so I'm shoting for 4600rpm. Right now with worn prop at best 4000rpm WOT and a max of 35 MPH/knots?<br />Here is where I'm lost I'm thinking about a Vortex 4blade 14.5/20, but if I (HA!HA!)understand, if I go higher on pitch I will lose RPM?<br />Any help on this? lost at sea!

 

[ronjon1944]

Re: Prop Pitch

to add, What I'm after is to get to 4600rpm wot, more speed (45mph) and to be able to pull 1 tube.

 

[TwoBallScrewBall]

Re: Prop Pitch

Yes, more pitch at the same diameter means less RPM. However, going from a 15 X 19 to a 14.5 X 20 might even out, as you're losing 1/2" of diameter. So you might stay where you are.

 

[Elk Chaser]

Re: Prop Pitch

Pitch Change Calculation<br /><br />There's an easy calculation to determine how much pitch change may be required. Just follow these steps: <br /><br />Check the specifications in your operator's manual for the recommended operating range at wide open throttle (WOT) for your engine. A tachometer is necessary to determine the WOT RPM. <br />Run the boat/motor combination at WOT to determine the maximum RPM. Adjust engine trim angle for optimum performance. <br />If the WOT RPM is below the recommended RPM range of the engine, note the reading of the tachometer. Take that reading and subtract it from the top end of the operating range.<br /><br />Example: Operating range = 5000-5600 RPM<br />Top End of operating range = 5600 RPM<br />Tachometer reading = 4800 RPM<br />Difference = 800 RPM <br />For every 1 inch of pitch change, the effect will be approximately 200 RPM. Knowing this, take the difference in the above example at 800 and divide it by 200. The result is 4. The next propeller to try will be 4 in. in pitch less than the propeller that was first used. You should now either have the right propeller or be only one size off.<br /><br /><br />Switching Among Propeller Types/Families<br /><br />When changing from 3- to 4-blade, are the pitches the same?<br /><br />If you're changing to propellers manufactured by Mercury, the answer is "yes!" For example, a 4-blade propeller is designed to run at elevated transom heights (normally 2 to 3 inches above the transom). The 4-blade's aggressive blade design and larger diameter help gain a performance advantage. When compared to a 3-blade propeller at the same transom height, the 4-blade will exhibit a lower engine RPM but, when the engine is raised to the proper operating height, the RPMs of both propellers are close. <br /> <br />When upgrading from an aluminum propeller to stainless steel . . .<br /><br />The most important concern to be aware of is the correct RPM range for your engine. The propeller pitch regulates the engine RPM. Each inch of pitch is worth about 200 RPM. Lowering the propeller pitch (going from a 23 pitch to a 21 pitch) will increase the engine RPM (about 400 RPM). Increasing the pitch (23 pitch to a 25 pitch) will decrease engine RPM. Most applications will match up pitch to pitch and this is the best starting point. <br />In some instances, Mercury offers propellers in even pitches (20, 22, 24, etc.). In these situations, go to the next higher pitch (24 pitch stainless to replace a 23 pitch aluminum). If your aluminum was already operating at the top of the RPM range, the higher pitch propeller will allow you to see where you fall in the RPM range with the new propeller without over-revving.<br /><br /> http://www.daveymarine.com/Prop_selection.htm

 

[Elk Chaser]

Re: Prop Pitch

With all that said there are some variables, with my 3.7L boat I ran a 19 pitch for ski and tube towing to get the boat and skiier out of the water faster. I would switch to a 21 pitch if I was looking for a higher top end speed.<br /><br />So you should also have a short range of props to work with, if the 19 is giving you 4000 then technically you need to go to a 16 pitch for maximum WOT. This will give you a great ski prop as it will get you out of the water faster but it may not give the 45 mph top end you are looking for, for that you may need to step up within your WOT range and select an 18 pitch.<br /><br />All this of course is trail and error, I tried a 23 pitch on my 3.7L just to see what it would do and I could not even get out of the water.

 

[f_inscreenname]

Re: Prop Pitch

If you are running 4000rpm’s WOT you are wicked over propped now. I think you need a 17. You may lose a couple mph but the boat will perform a lot better.

 

[Doug Durako]

Re: Prop Pitch

Above is all true---much of this is trial and error. You might have your current prop rebuilt to a 17 pitch and then if you like it, get a new satinless steel one and use the old one as a spare. Steel of the same size and pitch will gain about 3 to 5 mph over aluminum.