Testing 3 to 4 blade props

[Aqua Susurro]

Recently had the opportunity to test two props back to back in the same conditions.

Boat is a 74 Starcraft SS repowered with a 165 Mercruiser Inline 6.

The three blade is a 14 1/4 x 21 Solas
The four blade is a 14. x. 21. Mercury

I wanted to experiment with similar dia and pitch so only the number of blades and geometry would make the biggest difference. The 1/4” in dia is the only other difference and perhaps a big one.

The two props behaved very differently. IMHO


The three blade would get the boat on plane pretty good and topped out at aporox. 3700rpm and 60 km/hr tops. With the wind in flat conditions.

The test day we did 56km/hr at 3600 rpm.

The four blade makes the boat Launch! Gets up on plane fast and accellerates easilly up to 4300 rpm.

The test day we did 55km/hr at 4300 rpm.

Over all I think the different blade geometry makes the four blade a clear winner.

In my opinion it is the most exciting change Ive made on the boat. It really is that huge of a difference.

Happy Boating. T

 

[dingbat]

Blade geometry (cup, etc.) has a lot too do with how a prop performs.

The fact that you couldn't get a 3 blade up to WOT tells me the blade geometry is much different between the two props.

The 4 blade having 17% more slip (4300 vs. 3700 rpm) than the 3 blade at 56/56 km/hr confirms the point.

In theory, extrapolating rpm/speed data, running the same 4300 WOT, the 3 blade would run 70 km/hr, the 4 blade 55 km/hr. That's a huge loose of top end speed

 

[Aqua Susurro]

Thanks for the reply,

It would be if the motor had enough power to get the 3 blade up to 4300rpm. But it seems it was “over propped” The 4 blade puts the RPM exactly where it should be at WOT its running 4300RPM.

Im going boatin.

 

[Texasmark]

Got a picture of the two props side by side from the side and side by side from the rear?

 

[QBhoy]

Very unusual results there. Totally out of the norm. What type of props were they ? Something doesn’t seem right at all. Only thing that rings true to me is the 4 blade being quicker at accelerating

 

[Scott06]

Very unusual results there. Totally out of the norm. What type of props were they ? Something doesn’t seem right at all. Only thing that rings true to me is the 4 blade being quicker at accelerating

Agreed that four blade should be better accelerating, and agreed that four should be lower rpm and slip , but one thing to consider is that four blade may give better stern lift. This assumes tach is accurate .. but on my own boat I get best top speed and slip witha four blade (rev 4) vs three blade (enertia ) because it lifts the stern and hull out of the water better. Possible the OP isseeinthe same? I see 4 blades getting negative press often, but in I my experince they perform better In 90% of boating of my typical (loaded and skiing) situations, only losing in top speed.

 

[QBhoy]

Agreed that four blade should be better accelerating, and agreed that four should be lower rpm and slip , but one thing to consider is that four blade may give better stern lift. This assumes tach is accurate .. but on my own boat I get best top speed and slip witha four blade (rev 4) vs three blade (enertia ) because it lifts the stern and hull out of the water better. Possible the OP isseeinthe same? I see 4 blades getting negative press often, but in I my experince they perform better In 90% of boating of my typical (loaded and skiing) situations, only losing in top speed.

I’m guilty of not being a big fan of a 4 blade. But I totally get that they are ideal for some uses. Especially if the guy doesn’t care for top end too much. The rev4 is the best of them to be fair. Known to outperform some good 3 blades on occasion. I’m amazed it out performs an enertia though. Suppose it depends on the boat. Enertia is designed for high power to weight ratios. Needs to run high and plenty power to make it sing. I can see how a rev 4 might perform better on a heavier boat with lesser power on it. That said. Not that much less. The rev4 takes plenty power to swing those big blades.

 

[Aqua Susurro]

The three blade is a brand new Solas aluminium prop. Nothing special. It was simply the same dia and pitch as the mangled prop my drive came with. The four blade is a mercury nemesis also new.

I believe the results are simply because the 141/4-21 was to much prop for the engine as it would only rev to about 3700 RPM but would do 60km/hr GPS in flat water when light.

The 1/4” in dia difference is also a factor.
How much I cant say.

In my opinion, even though they are very close in dia and the same pitch the blade form is completely different. The three blade design
is early while the four is much later and has had alot more thought put into the blade geometry. It may be that a three with later geometry will outperform a four. Not sure...

Took the kids out on the tube today and love the acceleration. Yes I lost some top speed. Maybe try a 14-22. next year. For now Im very pleased with the results.