Surprising Prop Test Results - can you guess which prop worked best?

[500dollar744ti]

Was the first prop a Solas HR Titan 3-blade stainless? That's what I've got on my boat and it has outperformed any other I've tested.

 

[Sea Rider]

Is not the first prop, it's the current factory delivered prop, the gear reduction rate is 1:85. Just trying to know if prop calculators are specific to different engine brands or prop brands. Usually maximize prop performance playing with just pitches while maintaining same diam, prop type & prop brand, experimentalise with other brands or prop types could end in a costly experiment.

Happy Boating

 

[Fed]

The Mercury calculator shows approx 42 KPH theoretical speed Sea Rider.

 

[Sea Rider]

Just to know. Which Data was used on Merc Calculator to show 42 KPH theoretical speed ? Say total boat's weight/lenght/HP ? The formula will be ? What about going full wot 5800 max revs for that engine ?

Thanks,
Happy Boating

 

[Fed]

Weight, length & HP don't factor into theoretical speed.
Going up to 5800 RPM shows 27 KPH.
http://www.mercuryracing.com/prop-slip-calculator/

You should note that prop pitches are usually measured in Inches, the theoretical distance a boat would move forward with 1 revolution of the prop.
Cupping can add about 1 Inch not 1 pitch.

 

[Sea Rider]

Thanks,

Will check the prop-slip calculator. Something doesn't match or it's a typo mistake.

If theoretically achieving 42 KPH at 5700 wot rpm, how can the speed drop so dramatically to 27 KPH at 5800 wot revs, or it's 27 MPH ? We're talking about Nautical Miles, right ?

Happy Boating

 

[Sea Rider]

Thanks,

Will check the prop-slip calculator. Something doesn't match or it's a typo error.

If theoretically achieving 42 KPH at 5700 wot rpm, how can the speed drop so dramatically to 27 KPH at 5800 wot rpm, or it's 27 MPH ? We're talking about Nautical Miles per Hour, right ?

Happy Boating

 

[Fed]

I forgot to convert the MPH to KPH...

Try 43 KPH (rounded down)


The 27 I mistakenly put up as KPH was MPH, Nautical Miles per Hour are called Knots.

 

[Sea Rider]

Yep, that seemed to me. Haven't have a chance to have a look at the Prop Calculator. Made a new wot test. The max achieved speed going full 5700 revs was : 41.5 KPH or 22.41 NMPH (knots)



Prop calculators speeds seems are too positive, that speed was reached on a near lab water course, that's a 2 Km flat calm, no wind. no current professional rowing track and impossible to duplicate same speed on the best open water outing. What found interesting was engine achieved + 100 more wot revs and 1.5 + KPH with one less trim setting while AV plate and combo were riding parallel to water level. Was one hole under trimmed and engine only achieving 4600 wot rpm and max 40 KPH.

According to your Prop Calculator Theoretical Data :

22.68 NMPH (knots) = 42.0 KPH at 5700 wot revs
27.00 " " = 50.0 KPH at 5800 " "

You can't possibly achieve such a difference by increasing 100 rpm more to its max 5800 wot rpm, not even sitting that 18 on a bathtub.

Not a bad rev/speed runing a boat rated for 50 HP for up to 6 passengers with just an 18 HP and current factory delivered 9 pitch with 1 up. If boating with 2 up max wot revs will decrease to 5300 just 100 over the lugging side of 5200, so a 8.5 prop pitch its on its way to better both sides of the wot range .

Happy Boating

 

[Fed]

According to your Prop Calculator Theoretical Data :

22.68 NMPH (knots) = 42.0 KPH at 5700 wot revs
27.00 " " = 50.0 KPH at 5800 " "

You can't possibly achieve such a difference by increasing 100 rpm more to its max 5800 wot rpm, not even sitting that 18 on a bathtub.

HaHaHa!
Now you have changed the MPH to Knots.

 

[MARV1]

Going from aluminum to SS nets 3-400 rpm. Dropping 2" pitch nets 3-400 rpm depending on blade design. 4-blade = efficiency 3-blade = speed My Merc 150 4-stroke loves a 4-blade 20p, speed and efficiency. A 3-blade 20p has 3-4 more rpm but less speed for me. Maybe my slightly too high mounting has something to do with that. I hit 52-5ph

 

[MARV1]

Going from aluminum to SS nets 3-400 rpm. Dropping 2" pitch nets 3-400 rpm depending on blade design.
4-blade = efficiency
3-blade = speed

My Merc 150 4-stroke loves a 4-blade 20p, speed and efficiency. A 3-blade 20p has 3-4 more rpm but less speed for me. Maybe my slightly too high mounting has something to do with that.
I hit 52-55mph

 

[Roj115]

For the record, the GPS reading was in KPH which I converted to MPH for this discussion knowing that most of you are not as used to metric measurements as we Canadians. The speeds I was getting with the 17 pitch 3 blade were around 79 - 81 kph which is right around 50 mph. The boat speedo was reading something similar. My speed readings may have been a bit off since I had an older model Garmin and the speed readings were jumping around a bit on it as I was doing the test. The water was a bit bumpy when I was doing the test so I had to have both hands on the steering wheel most of the time. I only took one hand off the wheel to quickly pick the gps up off the passenger seat and give it a quick read as I was skipping along.

I tend to trust the tachometer readings as they are generally pretty accurate on those motors.

There could also be some "memory errors" too. SInce neither of my dogs can write, I had to memorize the GPS and Speedometer speeds and tach rpm readings as I was skipping along doing the test and couldn't write them down on a piece of paper until after I slowed the boat down and brought it to a stop.

There was a bit of a wind that day (not a lot but some( so I did the tests at least one time in the downwind and one time in the upwind directions and averaged the numbers out. Current is pretty negligible on that lake,

The point is, just dropping from the stock BRP 19 pitch Aluminum 3 blade to the 17 pitch stainless Solas 3 blade made a lot more difference in rpms than I ever would have guessed. Was not expecting about 800 rpms difference between those two props.

 

[Sea Rider]

Do you have to drop one pich if going, say from a 17 alum to a 17 SS or this "formula" is only applicable when going from 3 to 4 blade prop while maintaining same diam ?

-If so, has somebody confirmed that same rpm and probably speed was achieved going from a alum17 to 16 SS same diam props while playing with just pitch ?

-Have only seen + 150-200 wot revs per 1 pitch in reduction while maximizing alum props on real heavy combos, I'm talking about 10-12 ton fishing vessels powered by OB's. For recreational boating which is considered light compared to real heavy combos, that formula doesn't work as stated.

Roj, are you satified with last prop performance, is engine over revving while lightly loaded ? If so, will need to compensate adding more weight to combo or throttling less while keeping both eyes on tach ?

Happy Boating