What would be the difference between these two props?

[Converse48]

My boat was wintered next to a sistership that was one year older and had the prior version of my engines (not common rail, "480" HP versus my "500"). But the sistership has bigger props with lower pitch.
My props are 24 x 27, while the sistership is sporting 26 x 26 wheels. Both are factory props (meaning that neither of us has done any consumer-engineering on our own). So my question is what would be the expected difference between these two configurations, and how much thought does SR put into prop size selection? I'm assuming they at least give it some thought.

 

[osd9]

I'll take a stab....

I looked at the Cummins home page and the two motors are rated at different WOT...the 480 is 2500 WOT RPM and the 500 is 2600 WOT RPM. That tells me that they also have different HP/Torque curves which, may also mean that they have different transmission ratios....hence...the different dia/pitch props....

Remember...it was just a stab....

 

[Nehalennia]

I'll take a stab....

I looked at the Cummins home page and the two motors are rated at different WOT...the 480 is 2500 WOT RPM and the 500 is 2600 WOT RPM. That tells me that they also have different HP/Torque curves which, may also mean that they have different transmission ratios....hence...the different dia/pitch props....

Remember...it was just a stab....

I agree that with your larger engines that you have the extra torque to step up in pitch and run more efficiently.

 

[Sea Gull]

I used to think I knew a lot about prop pitch, but my most recent experience propping the 58 has illuminated my ignorance.

My boat was delivered with 28x37 props. From day one, the boat did not achieve MANs specification of 2470 RPM. I had the props re-pitched to 28x36.5 which was much closer, getting me to about 2440. Since the boat was new, the dealer was footing the bill for re-pitching two sets of props, but it was a long process of sea trials, haul-outs and re-propping.

Near the end of the process, when I had finally settled on an ideal pitch of 28x36.25 Sea Ray stepped in and told me (and the dealer) that they had changed the prop specification to 30x33. They sent two sets of dynamically balanced 30x33 props which we promptly put on the boat. What a difference!

The new props went on near the end of last season, so I don't have full performance data on them. What we did note immediately was that these larger diameter, smaller pitch props give the boat much better acceleration and give up nothing at the top end. The 28x37 props tended to cavitate a lot, but the 30x33 props seem to get a cleaner "bite".

I was laboring under the impression that I could tweak the 28" diameter props for ideal performance. My eyes were opened when SR steered me to the larger diameter wheels. My advice to you is to talk with SR customer service, they have a wealth of knowledge for you to tap.

 

[fc3]

I think the general rule is larger diameter and more blade area has less slip, but the larger blade area results in more friction. The balance is finding an acceptable trade-off between slip and drag. The transmission gear ratio may also be fiddled, too. Reducing prop RPM should reduce the energy lost due to friction while allowing a bigger wheel to be turned. You need to consider drive ratio as well as prop size, and the characteristics of the engine.

Best regards,
Frank C.

 

[Converse48]

With the QSCs I am able to achieve rated max RPM, actually a little over at 2670 max (not that I spend much time there), and the acceleration of the boat is very impressive for a boat of this size (see http://www.youtube.com/watch?v=fT8N3xYp9vM). Also, I am pleased with the cruising speed. I will call SR customer service and see what the have to say. This is more of a curiosity than an obsession...

 

[Four Suns]

You just want to out run me this year....

 

[brimanst]

I can't talk so much about props, but I am not convinced that Searay ever has a master plan about what they send out of the factory. I had a different but similar experience with my old 340. They cam with 3-blade props, but it did not get the a$$-end out of the water, and I had to have it over 3000RPM to maintain the boat on plane. I called SR whow told me whaterver it shipped with is the recomended prop. I looked at the newer models of the 340 with the same motors and noticed they shipped with 4-blade props. I duplicated this (at my expense) and felt like I was driving a new boat, it was awesome, I did not loose ant RPMs at top end, but I could cruise 1 knot faster, and stay on plane at slower RPMs. The point here is talk to prop guys, other boaters, and look at newer models. If the sistership has spares, see if you can borrow them for a weekend and do your sea trials. I logged every speed and RPM every 200 RPMs as well as placed cinder blocks around the boat to shift weight around.

briman

 

[Converse48]

You just want to out run me this year....

I believe I out ran you last year.:lol: