prop results after testing

[Groupergulper]

1993 Robalo 2120 , 21' center console, T-Top, trim tabs 1999 Yamaha 225 OX66 Saltwater Series
AS TESTED:
3600 dry weight, Yamaha weighs like (550), 100 gallons of Gas (630) Coolers, gear, ect. (400), 3 people (550), Total: 5730
Motor trimmed to the same position for each prop, trim tabs in same position
1999 Yamaha 225 OX66 = Operating Range per manual is 4500-5500
Mph is from gps

First prop, Not sure of brand, Stainless steel in like new condition, ( made my lower unit noisy at idle and low rpm )
14.5x19 5000 WOT
5000 = 41 MPH
4000 = 29 MPH
3500 = 21 MPH

Second Prop, Solas Amita 3, Aluminum in like new condition
15.3x19 4800 WOT
4800 = 41 MPH
4000 = 34 MPH
3500 = 27 MPH

My goal is to get the best fuel economy/performance I can, the fuel economy doesn't seem that good, after testing the props yesterday we went fishing, I left the 15.3x19 prop on and continued for a total of 52 miles round trip, the mpg was 1.8, I cruise normally between 3400-4000 depending on seas. The 15.3x19 seems like its a good fit for my boat but i wonder if there is a better fit?

I dont see much discussion about prop diameters, mostly pitch. I would think for a heavy boat you would want more diameter or at least more blade surface area.
I dont fully understand the operating range? Who runs wide open all the time? Is operating range peak HP? Has to be sucking fuel at that rpm. Am I missing something here?

Is it odd that both props had the same max MPH at different WOT?

 

[Maclin]

The higher you can get into the operating range when at WOT the easier it is on the engine and also get the best of its' performance potential. One might think lower rpm = more economy but it is at the risk of lugging the engine, and the "savings" is minimal to no-existent in real-world usage.

 

[steelespike]

Keep in mind Maclin is referring to lowering rpm with a prop not reducing the throttle setting.
If that is carbed 225 it will use up to 27 gph (gallons per hour) at wot.Whether pushing a house boat or a runabout.
Obviously less throttle will use less fuel.
Your right a larger diameter/blade area may work better on a heavy boat. You'll note the Solas had a decided advantage over the ss prop
at reduced throttle.Your job is to figure the best speed for the least gph.
There are flow meters that calculate mpg in real time.
27 at 3500 seems promising.
Best cruising speed could be from just on plane to about 1000 rpm higher.

 

[Groupergulper]

Its EFI, a flow sensor is my next step, i think that 27mph at 3500 is pretty good, the flow sensor will tell the tale for fuel consumption.
So as far as operating range: whatever prop I end up with it should be able to reach at least the minimum Operating rpm at WOT?

 

[Maclin]

"...reach at least the minimum Operating rpm at WOT..."

Yes, to ensure you are not lugging the engine with too much load. Too much pitch to a boat engine is kind of like pedaling uphill in 10th gear on a 10 speed bicycle.

 

[Sea Rider]

Personally like going for the full max wot range while lightly loaded, once begin adding extra weight or passengers max rpm will drop accordingly, will add extra weight or number of passengers to a point to achieve no less than middle rpm when at wot. Like my engines to rev smooth without stressing them. Full throttle to plane faster, once on plane reduce throttle to maintain good planing speed with less fuel consumption as opposed running at higher revs and faster. It's not a matter on how fast you get there, it's a matter of enjoying the water ride and environment specially if boating with large HP engines.

Happy Boating

 

[Groupergulper]

Just to update, I continued to run the 15-3/10 x 19 aluminum prop. The saltwater seems to be very abrasive and the shallow muddy water takes its toll on the prop. I came across a great deal on a YAMAHA SS 2 15x19 So I tried it and here are the results.

4900 rpm - 41 mph (Wide Open Throttle)
4200 rpm - 35 mph
4000 rpm - 32 mph
3500 rpm - 27 mph

The SS Prop is cupped and seems to have much better throttle response and it jumps on plane easier. I think its good.

 

[Sea Rider]

What's the max wot rpm for that OB, 5500 ? 4900 seems bit low with current load..

Happy Boating

 

[Groupergulper]

The range is 4500-5500, So Im right in the middle.

 

[Sea Rider]

You're at middle wot rpm range, was that with current posted load or while lightly loaded and you alone ?

Happy Boating

 

[Groupergulper]

That was with the current posted load.
AS TESTED:
3600 dry weight, Yamaha weighs like (550), 100 gallons of Gas (630) Coolers, gear, ect. (400), 3 people (550), Total: 5730
Motor trimmed to the same position for each prop, trim tabs in same position
1999 Yamaha 225 OX66 = Operating Range per manual is 4500-5500
Mph is from gps

 

[Sea Rider]

How is hole shot with current prop and given load ? takes its time, jumps happily on plane ? You could go less in pitch to bump wot revs to max 5500 rpm, the only issue is that's impossible to know if will need going one or 2 less pitch to achieve that. + 200 gained revs per one less pitch as stated near rule of thumb not always works as you could achieve more.

Happy Boating