So I'm working away, getting the bugs out of my power vs. speed calculator, when I decide to enter three different prop pitches, and compare the predictions to some actual test data that I collected on my boat.

First I entered my test data for 25" Mirage props (80 @ 5300) to establish the hull curve. Then I entered the dyno numbers for my engines. Then I entered 27" and 29" as the pitch. The program predicts 76.7 @ 4615 for the 27 and 71.9 @ 4037 for the 29. But in real life the 27 actually gave me 80 @ 4800, and the 29 actually gave me 78 @ 4300! I have the dyno sheets for my engines so I know there is a lot less HP at the lower RPM, so what's going on?

Comparing the 25 and 27, they both gave me 80 mph, so the hull was getting the exactly the same thrust from the props, even though there is less propshaft HP delivered to the props at 4800 than 5300. The missing piece here is propeller efficiency. This factor is not included in the Mercury equation I used and its not the part of drivetrain losses. So its not included in the program.

Mercury says that, within a family of propellers, maximum efficiency will occur at a certain angle of attack, and the value of this maximum efficiency will decrease as pitch decreases. They say that 80% is the about the best efficiency you can get if you get everything right, and the efficiency range is narrow. It looks like it would be very easy to reduce efficiency to 70 % or even less.

This is a big difference. I think it explains why the 27 performs a little
better than the program predicts and the 29 performs a lot better. We often hear that spinning a bigger pitch at slower RPM is usually the best top speed in real life. When guys talk about propping the boat according to the torque curve, I think this is just another way of saying use the biggest pitch you can live with. You need a good torque curve to live with it.

The key thing here is: when you prop for top speed, you are maximizing thrust coming out of the prop. This does not necessarily occur at the RPM with the maximum propshaft HP entering the prop. Since maximizing prop thrust is what Mercury was doing when they did the tests that their formula is based on, the program does a good job of predicting top speed, but when you deliberately put more pitch in than the program says is optimum, the program will underestimate the speed achieved in real life.

I would like to get some info on propeller efficiency for Mirage, Bravo and Hydromotive props so I can include some kind of adjustment in the program. If that's not possible, I could use some actual test data from members who have tried a range of pitch in a given propeller family. This would allow me to estimate typical changes in efficiency if you can also give me your dyno numbers. Then if a user deliberately enters more pitch than the program says is optimum, an adjustment could be made for efficiency.

This may not work at all, it depends on what your data reveals, but I'll give it a try if you can send me some numbers.

Thanks in advance,

Tom

[ 01-29-2002: Message edited by: tomcat ]

Tomcat this is great stuff. Right at the time I repowered and soon to be reproping. One question that I don't know the answer to but may figure into your calculations. What if any effect is there on bowrise/sternlift between different pitches of the same prop. I would think that a higher pitched prop will product more lift, but does it equal itself back out with lower rpms. I think the only way to test this would be to measure running angle vs. trim vs. rpm. Any thoughts, keep up the good work. This is going to be cool when your done.

I don't know if pitch affects bow lift within a family of propellers. I hope it doesn't. But switching between "families", e.g. from Bravo four blades to Hydromotive four blades, definitely does. This is accounted for by entering new test data based on the different family of props.

Check your Torque numbers at those RPM's you are probably making more Torque @4800 RPM

Also I have found time and time again that speed is usually greater spinning a larger pitch prop slower then spinning a smaller pitch prop faster.

[ 01-29-2002: Message edited by: Lee ]

Tomcat,
Great start to a seemingly endless debate.

The biggest complaint I have, overall, is why I can’t find a good article on prop design in any type of boating magazine. One can basically summarize virtually every “Prop Tech” article by saying:

“Pitch is the theoretical amount of water that will be displaced in one revolution of the prop.”

Then the article generally goes starts praising the “prop advertiser of the month”.

Where is the data on cup, diameter, surface area, progressive pitch, etc…? I would think that all of these areas greatly affect the operating efficiency. Last year, I switched to a set of Spinelli 31 pitch props from a set of Bravo 28’s (both four blade). While I expected a significant speed increase (+150 hp engine upgrades) the Spinelli’s were a whole different monster. It seemed that they weren’t as efficient as the old Bravo’s. Getting on plane with the 31’s is very difficult and the mid range speed is less than I would have expected. Top speed is close to what I had planned on, but I’m wondering if a Bravo 30 or possibly 32 would be the ticket. Don’t get me wrong, Mike has been a great supplier, but I just doubt that it’s the best prop for the boat (32 Sunsation).

This past December I got bored and took a bunch of pictures of the Spinelli’s next to the Bravos. Surprisingly, the “pitch” angle of the Spinelli doesn’t look much (if any) more aggressive than the Bravo. However, the Spinelli does have quite a bit more “cup”. Also, both the blade surface area and overall diameter of the Spinelli is smaller than the Bravo.

See for yourself: Spinelli and Bravo Comparisons / Differences

My experience has found 25 inch pitch to be the most efficient on a mirage. But of course, you have to change gear ratios to get to your optimum engine rpm. Cant do it backwards to determine prop efficiency. What I mean is you cant just change prop pitches without changing the gear ratio. If you just change prop pitch you change engine rpm which brings in another variable. I think every hull reacts different too. Center of gravity, center of balance, v, steps, cat, fat wife, skinny girlfriend, you know. It all matters.

It doesn't matter if my wife was an extra 100 pound heavier... I'm still not "allowed" to go faster than 25 mph with her in the boat!

I checked the torque figures. 503 @ 5300, 513 @ 4800, 504 @ 4300. This is a very flat torque curve and doesn't explain the results. Besides, the prop efficiency equation that Mercury has in their book and the prop efficiency equation that I found on the web, do not have torque in the equation. Power in and power out is all. A big diesel will make a ton of torque, but after you gear it to spin the prop for the speed you want, its just power @ WOT that does the job.

Check out these dyno HP figures, modified by a guess at prop efficiency:

29" @ 4300 RPM - 412 HP X .80 = 330 HP
27" @ 4800 RPM - 469 HP X .75 = 351 HP
25" @ 5300 RPM - 507 HP X .70 = 355 HP

These figures would explain my results.
The lower RPM will also reduce drivetrain losses, which will close the gap between the final prop thrust even more. That doesn't mean you can actually live with the 29". Running 52 mph @ 2900 it was too sluggish.

I agree with you about articles on props, I want more detail too. Nice pictures. How did Spinelli know how much blade area to give you?

I too have been told that the 25 Mirage is a very good prop. The 27 and 29 props that I used were Mirage plus, and the 29" Mirage plus comes with a 15" diameter, not 14.5". Maybe these things made some difference, I don't know.

Tomcat....to muddy your equation a little more. I have found that there can be noticable variance in pitch and rake between identical Mirage props. I took one right out of the box and had it checked at my prop shop. They found the rake to be way off on one of the blades and the pitch varied by 2 inches at some places on the blade surface(from one blade to another) we sent it back and got another. This one ran about 150 RPM more. Sure enough it measured out with almost an inch less average pitch and the rake was different. Just more variables!

Bob

Hey Tomcat, Interesting that your motor mskes the same TQ at 4300 and 5300rpm. We discussed this before and I still think TQ is what you want to chart. TQ is what is measured and is the measurement of physics. Hp is just a calculation. TQ is what moves things. Maybe the problem is from the start in Mercury formulas using hp instead of Tq. So sith your test results, at the same torque in the 25" prop was more efficient in converting it to forward thrust 80MPH vs. the 29" and 78MPH. I have to think about this somemore and do some calculations. Marty.