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prop diameter vs. pitch

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baddog1016

Regular Contributor
What is the trade off between prop diameter and pitch? I am looking for a different prop for my boat because the 14.25" x 21 prop that is currently on it limits it to 5100 rpm at WOT. I used the Mercury prop calculator and there are a lot of choices but two of them are 15.5 X 18 and 14.5 X 19. What's the difference? Why give up diameter for pitch? Thanks!
 
As an aluminum/stainless welder and prop repairman, I personally like more pitch and less diameter. I think that shorter blades can sometimes show less damage when struck by a rock. I have no suggestions performance wise, however. I do feel that the slower the propeller speed, the more blades the better. You certainly notice that on the bigger boats and ships. Economically on high speed propellers I prefer 2 or 3 blades.
 
As boat speed goes up the diameter and blade surface area comes down.------Water behaves different as speeds go up.
 
Of course a boat isn't a car, but just for the sake of understanding, think of it as one anyway. To pull a load, you use a lower gear. That would be a larger diameter prop (more blade area and lower pitch). For driving down the interstate at 70 MPH, use a higher gear (more pitch) A lot of blade area is counter productive at high speed because it only creates more drag as it slices through the water. Unfortunately, you can't shift gears with a boat, so you have to pick the best compromise. This help???
 
Thanks for the answers everyone, I'm learning. I have a lot of car experience and have been sailing for a long, long time. Powerboating is different.
Boat in question is a Dusky 203 with an Evinrude 225. Everything I'm reading makes me think I want diameter more than pitch and maybe a four bladed prop. I am not that interested in going 50 mph (I've seen 43mph at 5100 RPM against current) midrange causing and ability to stay on plane at lower speeds are what I'm interested in. I hear complaints from the front of the boat when going over wakes at high speed. With the engine trimmed down the boat does handle chop quite well
 
Another blunt statement.------" The prop attaches to the motor , but fits the boat and load."-----So selecting a prop has a lot to do with preventing damage to the motor !!
 
Like this discussion. Great info from guys that have knowledge. The big ships of course have variable pitch props which turn at less than 300 rpm. The 8000 hp Pielsticks used on many vessels on the great lakes here have engine rpm governed at 600. Then gear reduction to 150 rpm on the prop. I think modern outboards run prop rpm at about 900.
 
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Your motor is a 1.86 ratio and on that hull you need to stay in the 14+ diameter range...14.5 x19 will be real close
 
In my reasoning the factors determining plane/step or not, are boat loading, hull shape, angle of attack, and perhaps even wind. Since salt water is heavier than fresh water, then that will slightly lower planing speed. If these remain constant, then the speed of plane is a "given". So that "speed" will not change unless the aforementioned factors change. If a prop with 4 blades can change one of those factors......and it certainly can't, then I would say NO. This may not be the correct answer but it's MY answer.
 
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So much to learn. I understand hull shape and all of that for planing and I've figured out engine trim vs. smooth ride or speed. I know that what limits a boats speed, non planing, is that it is stuck between the bow and stern waves and it needs enough power/speed to get it on top of the bow wave to go faster. I know that once a boat is up on plane the throttle can be backed off of significantly while still maintaining plane. Believing this my thought process is that a prop that is better able to transfer power to the water will be able to hold the boat on plane at a lower speed. Maybe I'm confusing boat speed with engine RPM.
 
Right, engine rpm has to be kept in the "sweet spot". For instance, even in today's automobile engines we are shooting for horsepower with lighter rotating assemblies which most often means higher rpms.....shorter strokes.....and narrower power bands with less "over the counter" torque. Of course this dictates the need for more responsive transmissions with more gearing (speeds). Often times even CVT's using variable clutching like a snowmobile. Kevlar is utilized to extend belt life and special synthetic ATF's with careful filtering is critical. The old days of poor economy, and 2, 3, and 4 speed transmissions along with broad, high engine torque, with flatter horsepower profiles.......those days are history. The 4 stroke outboards have flattened torque and horsepower curves and of course cleaned up the emissions, but when dealing with getting a heavy load on step....without ample power and torque, the prop selection, load distribution, and trim control.....all have to be properly applied together.
 
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All you want is to transfer as much torque to water as possible. A set of tabs will let you slow crusie and stay on plane.
 
so I've settled on a 4.25X19 propeller and am looking at a Turning Point Hustler 21501911. Anyone have any comments on this prop or something different? I considered a stainless prop but I am still a novice with power boats and don't think I want to put any more money than necessary into my current boat and motor combination. (1999 Dusky 203 with 2000 Evinrude 225)
 
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