Best prop? 1978 Marlin, 140 Merc I/O

[imported_BSB]

Hi,
I have a 1978 Marlin Venus, Tri Hull with a 140 Mercruiser I/O. I have run both a 15 X 17 and a 15 X 19 alum prop on this boat. I want to upgrade to stainles, is there any performance change from alum to SS?
17P, great hole shot for skiing, 33/34 mph, 5600 rpm
19P, decent hole shot, 36/37 mph 5600 rpm

Based on those numbers with alum, any suggestions for a SS?

Reading the forum this morning, I see WOT in reference to speed/rpm's What does that mean?

Thanks for any input or suggestions,
Bart

 

[tommays]

Re: Best prop? 1978 Marlin, 140 Merc I/O

WOT " Wide Open Throttle"

I dont know if your tack is correct BUT you should not be going over 4800 RPM WOT with that motor

The RPM and speed do not match up at all so i would guess your tack is off

Are you geting speed from a Speedo OR GPS ?

Tommays

 

[05GlastronSX]

Re: Best prop? 1978 Marlin, 140 Merc I/O

stainless steel will not flex unlike aluminum and therefore, you will get better bite and efficiency from a SS prop. SS is stronger than aluminum and because of this, prop manufacturers can build SS props to tighter specifications by making the blades thinner then their aluminum counterparts, thus making them more effiectient through the water.

The downside to SS is that if you hit an underwater object, the prop wont give, or break off a blade since it is stronger than aluminum. Normally, if you hit an underwater object hard enough with an SS prop, you will have a much greater chance of damaging your lower unit because it will not give.

Like tommays said, 5600 RPM is wayyy to high for any conventional sterndrive but since your getting the same RPM with 2 completely different props, im going to agree that your tach is off. It is crucial to know your WOT RPM's especially when your testing different props because you dont want to over-rev or lug the engine which can cause serious internal damage.