Could S.S.prop be to heavy????

[kmurray802]

Boat:16' Scout with F60 Yam. 4/St.
Suggested RPM @ WOT-6100RPM / 38MPH.

Boat came new with 10.5/8x12" alum. With this prop boat over-revs and stalls @ WOT.Top speed only 34 MPH.
Scout rep suggested I go to a 10.3/8x13".(Don't ask me why boat was not shipped with the prop they recommend.)Anyway,I want a S.S.prop so I purchased a 10.5x13" Rapture.
Top speed remains about the same(approx. 34MPH).However the RPM's where drastically reduced to 5100 @ WOT.Gas mileage went way down.
The only real difference I can see in the 2 props other then 1" of pitch is the weight.The S.S. Rapture is a lot heavier then the alum.
I guess my question is "Doe's this matter"?? I would like to run a S.S.but if I have to go back to alum. to achieve best performance I will.
Any suggestions would be appreciated.

 

[Bondo]

Re: Could S.S.prop be to heavy????

It's Not the Weight,......

SSteel Doesn't Flex like Aluminum,......

You need to Drop Down in Pitch to get your RPMs back........

 

[Texasmark]

Re: Could S.S.prop be to heavy????

Gonna stab at this. Water is more dense than air so the weight difference of water/air has a slight affect on equalizing the weight difference.

Obviously twisting a heavier object and placing it in "orbit" takes more power, but once spinning, probably not that much difference in keeping it spinning.

I would think that SS props are usually more sophisticated than alum and in that sophistication comes added performance that washes out any weight disadvantage.

Your Rapture is eating a lot more water (including the equivalent of 1" more pitch with it's cupping) and in my opinion that is part of the reason for the drop in rpm's. The rest, see below.

Having been there done that (BTDT) had your boat/engine been able to accept this higher performance, you would have increased speed and rpm's wouldn't have dropped as much. You're telling me that your engine is at it's power limit.

The boat (wake) in my Avatar is from a 17' boat with a 90 hp on it. As you can tell by the wake, there isn't much boat in the water to put drag on the engine's horsepower.

I changed props 3 times (from what was on it when I bought it) and as I would put on more performance/pitch in the prop, the rig was able to accept it and not only did speed go up, rpm's did not drop proportionally so I had to increase pitch a second time till I got the rpm's down to a reasonable number.

HTH.

But now you are overpropped (rpm's too low) so you will have to fix that. Why not try to trade your Rapture back to the dealer for a lower pitch that will put your WOT rpm's near the engine's upper limit.

Also, I think 34 on a 16' boat with 60 hp is a good number.

Mark

 

[kmurray802]

Re: Could S.S.prop be to heavy????

Thanks for taking the time to assist.I would agree 34 for a 16' with F60 is not bad.However both Scout & Yamaha produce performance charts on this exact set up.Both show the boat topping out @ 6100RPM/38 MPH.
I feel like I'm over-propped with the 13" and under-propped with the 12".
It seems like I need something in between the two.
Do you think a S.S.quality prop in the same 12" pitch as the cheap alum.one that came on boat would do the trick?
How does the diameter play into the equation?Could I either go up in diam.with a 12"pitch or down with a 13" to get my Rpm where needed?
I'm not really that concerned over top speed,I just would like my RPM's @ WOT to be in range.
Thanks again.

 

[Silvertip]

Re: Could S.S.prop be to heavy????

Generally, motors from about 40 HP and up do not come from the factory with propellers because boat type, boat size and anticipated load is not known (in this case by Yamaha). So its up to the dealer to fit a prop that would provide the performance YOU expect based on the information YOU provided him at the time of sale. If the prop is wrong, most dealers will work with you to get it right. Some on this forum (including myself) feel the Rapture does not have the performance characteristics of other props with more "sophisticated" (for lack of a better term) designs. Two props with the same specs from two different manufacturers rarely perform identically. And as Mark suggested, 34 MPH with a 60 4S on a 16 foot boat is in the ballpark. I happen to have laying on my desk, performance data for a 75 Yamaha 4S on an Alumacraft Navigator 165CS and its top speed is only 35.4 MPH @ 5800 RPM. Those are numbers from the Yamaha Performance Test web site by the way. Speed numbers should always be verified by GPS since speedometers in boats are notoriously inaccurate.