prop slip does not necessarily correspond to fuel economy

[David]

Round 2 of my prop testing with my new Merc Monitor. It reads instantaneous fuel economy. I had posted some numbers earlier. Lately it occcured to me that I had not checked fuel economy with my Mazco RE4. This prop has pretty slow slip at all speeds. Near peak economy speeds, props with much more slip did as well for economy. I was sort of hoping the RE4 would be the economy champion.

This is all solo. I still think the RE4 might be my best for economical cruising with a loaded boat. The RE4 has more lift then I need solo. I've not tried it much with people yet.

[David - WI]

Prop slip is a meaningless number unless you know the actual pitch of your props.

[David]

With progressive pitch, what does the actual pitch mean? The manufacturer’s estimate?

[David - WI]

With progressive pitch, what does the actual pitch mean? The manufacturer’s estimate?

It is the average pitch across the face of the blade... but also, really, the average of the three (or four or five) blades; which usually aren't the same.

I would expect CNC blades to be much better, but the prop still might be marked "optimistically". If I mark my 26 pitch props as 25... then my customers will see low slip numbers on paper.

[Pulsare2400]

Prop slip is a meaningless number unless you know the actual pitch of your props.

That is certainly a good point. If you move from a 24P to a 28P you add about 17% more pitch (if those values were the actual pitch). If the slip increases as well but less than the 17% pitch increase, you could easily gain mpg at the same rpm.

[David - WI]

That is certainly a good point. If you move from a 24P to a 28P you add about 17% more pitch (if those values were the actual pitch). If the slip increases as well but less than the 17% pitch increase, you could easily gain mpg at the same rpm.

Possibly. Most engines have their best efficiency at the peak torque rpm but then drag on a boat goes up & down rather than increasing exponentially like a car. (I'll see if I can find a graph from the Tunnelboat Design Program). I haven't figured out how to reconcile those other than by trial & error (testing) or using the simulator.

[Pulsare2400]

Possibly. Most engines have their best efficiency at the peak torque rpm but then drag on a boat goes up & down rather than increasing exponentially like a car. (I'll see if I can find a graph from the Tunnel boat Simulator). I haven't figured out how to reconcile those other than by trial & error (testing) or using the sim program.

I was looking at the higher pitch prop (in one of the examples) turning more mph at similar rpm even with additional slip. If the rpm is similar or less and the mph is higher (no matter what the slip) I think the mpg would improve in most cases. We used the data recorder to draw us a slip graph of the torque convertor on our drag car from launch to the traps. Many people wanted a convertor with lower slip that was "locked up" earlier. That was not always the fastest accelerating convertor. As you pointed out, some engines had a power curve where a higher slip convertor (more rpm early on) put the engine into a better part of its powerband, even though the convertor slipped more. As you pointed out, you also have the hull drag but I would think at similar speeds it should be similar, but then some props lift more which will reduce the drag and maybe the slip. Almost too many variables to easily simulate.

[David - WI]

This is the required hp to accelerate a 24' Skater with two 250hp outboards (this is a "canned" model that comes with the simulator). It seems to me your mileage would be better at 65 mph than at 55 mph... which is sort of counter to most vehicles.

So, I would guess that if you propped this boat so it was running at it's peak torque rpm going 65 mph... that would probably be the best mileage combo?

[XstreamVking]

Could the dip in the graph merely be the hull reaching "flying" attitude? So less drag?

[engineermike]

I did a lot of testing several years ago on a 23' Eliminator with a 400R. You are correct that slip and mpg don't necessarily correlate. A buried Maximus for instance, has very low slip. But with the huge blades and buried LU, the overall efficiency is worse, at least in this case. You can see that 35-40 mph is a good cruising speed, but 60 is just as efficient once it starts to "pack air". It's hard to hold 60 though, because the boat wants to accelerate at steady throttle as the speed is inherently unstable.

[hellbents-10]

Could the dip in the graph merely be the hull reaching "flying" attitude? So less drag?

I think that could be part of it, my tuff 24 hits a very efficient spot at 65 when you feel it pop out of the water a bit.

[David]

I was wondering if the lift of the Allison hull as speeds increase would offset velocity squared. Or as noted, perhaps the engine would be more efficient in some range.

As it turns out, faster is worse for fuel economy for my boat.

[transomstand]

Prop slip is a meaningless number

End of story. It has NOTHING to do with propeller efficiency like everybody thinks. But god forbid you try and tell anybody that, so it's better just to let the ignorant continue their slip obsession.

[XstreamVking]

And god help anyone that tries to explain to someone claiming negative slip #'s for their boat that it is impossible.

[David - WI]

If you had accurate rpm, mph, and pitch data... then "prop slip" would be a useful number; but not many "consumer" props are accurately marked apparently.

I've seen tachs off by 200 rpm, speedos off by 10 mph, and props off by 3" of pitch; so like any "computer" if you put garbage into the slip calculator you get garbage out.