Ideal Prop Pitch
 

Several variables are found when comparing similar boats.

One may have 7,500 RPM engines, 1.68 gears and a 30" prop pitch. Therefore,the prop turns at the rate of 17.8571" pitch/engine revolution. Or,at 7,500 RPM and "0" slip,top speed of 126.83 MPH.

The next boat may have 5,200 RPM engines,1.33 gears and a 34.2558" prop pitch. This boat also runs 126.83 MPH w/ "0" slip. However,the effective prop pitch rate is 25.7562"/engine revolution. This prop has 12.42 % more pitch and 30.67 % more effective pitch,due to gearing.

The question is,can the 30" prop be compared to a wood screw and the 34+" prop, compared to a machine screw?

What are the positive and negative benefits of the above examples?

I am not an expert by any means but, the higher the pitch the harder it is on lowers going into gear. Atleast in my HiPo outboard experience. I actually disconnect the Neutral safety switch so I can start my Outy in gear to save on the lower. On the other hand efficiency will most likely go up on the big end with higher pitched props and lower gears. The more rpm you have to spin a prop for a given velocity the more water has to pass over the blades. In the real world the difference of either may be nill.

My $.02

C can Do, I would think the 30's would be the machine screw and the 34's are the wood screw.

I think what matters is what engines you are going to be using? Your first example sounds like some race motors and the second some pleasure blower motors. I think the first example will really be faster. Just a long distance guess.

The 30 pitch blades are more perpendicular to the load. Therefore,one may assume the 30 blades have most bite. The 30 is also thought to turn easier (as stated by JRyder),go on plane better and accelerate harder. A wood screw has those same features. The wood screw pitch is more perpendicular to the force line and augers ahead.

The 34 blade is less perpendicular to the load. Therefore,one may assume as the load is being started, more slip/slap would occur. The slip and flatter blade slap must cause aeration while the blade is trying to get a bite,at least when going on plane. However,some of the same might occur during acceleration. So, wouldn't the 34 be more similar to the increased thread count/inch,short thread machine screw?

Screw types may seem a bit silly when comparing props. The screw type is used only here because it offers insight. We have all used different screws for different jobs and know how they perform. When I think of the prop, I construe it to be a screw going through water.

The next question is,which prop pitch has most deflection. Deflection or bending usually causes heat and work hardening. Like bending a wire,sooner or later the wire breaks. Or, which pitch is safer to lab finish? We know the lab finish blades are thinner.

I think you may be over illustrating the examples.

First, there are rarely similar boats. We have seen that over the last few years with the Doug Wright 32's. Even if the hulls are the same, engine placement side to side and up and down affect the pitch versus gear ratio efficiancy. In therory, directly rearward thrust will be the most efficiant. However, if you put ballast in the bow, slightly upward thrust may improve the numbers. More pitch more stern lift. Too many R's, have to go up in pitch. Still too many R's, have to change gear ratio. The list and variables go on on forever. That's why the team that tests the most in all conditions and course configurations and takes notes, is the toughest to beat.

Labbing a high pitch on a light load won't cause as much deflection because of the HP to weight ratio. Equally, Labbing a low pitch on a heavy load dosen't cause as much deflection because the angle of attack isn't as great.

With boats and water and waves and arreation and drive design and, and, and, and

You get the picture. All the therory and practical experience can go right out the window in the next boat you set up. But, use all that knowledge for a great starting point.

For you first question, no, mercman was right. The wood screw travels farther with each revolution. So does a 34 pitch propeller. The gearing equals the efficiancy in a perfect scenario like turning the props into the ground. Each boat will dictate its preference to g.r. vs. pitch.

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The examples were intended to be extreme, illustrating how combinations could result in similar top speeds.

However,planing and acceleration with one combination could result in a definite difference from the other combination.

With that said,if only one pitch was available,what pitch would it be ? Obviously,the other variables would require adjusting to fit that pitch.

lower gears and higher pitches.

2 times the diameter makes for a good pitch combination.

I can't give away all my secrets. LOL!