Prop question

D

DaveG

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I looked but didn't see a prop specific section.

Going from a 14.5/19p aluminium to a 14.5/21 SS prop what kind of decrease in wot can I expect?
I know rpm drops 200 per inch of increased pitch but what effect does changing prop material have? My crs memory wants me to believe that an Al to SS swap is good for another 50 drop. Yes, no, maybe?
 
Not enough info...... cup? #blades?......

My question is why?
 
Not enough info...... cup? #blades?......

My question is why?
 
Really my question is just how much difference in wot between identical size/pitched props, one being aluminum and the other SS.
I realize now that I phrased it poorly.
Ok, so here's the full story.
20.5 ft bowrider,  5.0l, A1G2, 14.5"/19p SS prop. WOT in the area of 4600RPM rb2@gr5t 48-52MPH depending on water condition and load.
Got water in the outdrive which trashed the gears.Could not afford a new SEI drive so found a nice used drive.
Here's where it gets a bit odd. 
The old drive was a 1.65, the "new" one is a 1.81. After checking with several people,  including a tech at SEI, the concensus was the 1.81 would work fine with reproping.
The new drive came with a new 14.5", 19p Al prop. My test drive had me hitting 5000 rpm at WOT. I don't know the top speed as the speedo is dead at the moment.
My target rpm is 4400-4800. This means I'm underproped by about 2".
I have a 14.5 x 21p ventalated SS prop that was previously too much for the boat.
I'm thinking it may be perfect for this new drive.

Personally, I like the SS prop. Been running one for the last 12 years or so with no problems. I think it gives a better bite when pulling skiers and we have a really clean lake that seldom has anything to hit.

So, how is wot affected Al vs SS?
 
quote:
Originally posted by DaveG

Click to expand...


With all other aspects of the props identical (not just diameter & pitch), I believe the thinner, lower-drag-blade SS prop with less effective pitch loss from flex should allow a little increase, not drop, in WOT RPM.

I kind of recall the last time I made that switch long ago on a 225 O/B, attainable RPM increased in the neighborhood between 50-100 PM.
 
My long ago memory also seems to remember better performance when I went from aluminum to SS prop.

George
 
5000 w/ what load?

If 1/2 fuel, 1-2 pax, I'd leave it as is. Hit something w a stainless prop and you'll be looking for another O/D.
 
quote:
Originally posted by Sandy

quote:
Originally posted by DaveG

Click to expand...


With all other aspects of the props identical (not just diameter & pitch), I believe the thinner, lower-drag-blade SS prop with less effective pitch loss from flex should allow a little increase, not drop, in WOT RPM.

I kind of recall the last time I made that switch long ago on a 225 O/B, attainable RPM increased in the neighborhood between 50-100 PM.

Click to expand...


Thank you Sandy. I know I'm asking a simplistic question for a complicated system but your answer helped.
 
SS props cause a drop in RPM because aluminum props are intended to flatten at speed to increase RPM and the ss doesn't. Personally ,if you are in Sinclair I wouldn't do stainless. Aluminum props may have to be replaced occasionally but they are so less expensive than shafts, gears and cranks. They provide a safety for these parts that ss doesn't.
 
Sinclair is an extremely clean lake, especially in tha areas I frequent. I'm far more concerned with picking up fishing line than I am in hitting something. I've ran a Stainless prop for the last 12 years and never suffered any damage from striking anything. I have, however, picked up fishing line 3 times that ruined seals. This last time I just didn't catch it soon enough and water trashed the gears. I like how SS preforms when towing so I intend to keep with that, especially since I already own a 21" SS. I will consider myself duely warned though.

And as far as leaving the 19p Al prop on and running 200 rpm under proped with a medium load, that's not gonna happen! I worry far more about that affect on the engine than the possibility of hitting something, especially since it's not happened in the last 12 years.

Thanks for the responses though. I will post up what my results are after next weekend after I get a chance to swap props. Heck, since I've got the 19p SS prop from the old drive, that I've run for 12 years, I may just pop it on to see how it changes things before I try the 21p.
 
You should be near same WOT with the SS vs aluminum. The SS props are supposed to have thinner blades so less drag and higher RPM. You will probably get a slight decrease in WOT because of the increase in pitch, but not much.
 
quote:
Originally posted by Sandy

quote:
Originally posted by DaveG

Click to expand...


With all other aspects of the props identical (not just diameter & pitch), I believe the thinner, lower-drag-blade SS prop with less effective pitch loss from flex should allow a little increase, not drop, in WOT RPM.

I kind of recall the last time I made that switch long ago on a 225 O/B, attainable RPM increased in the neighborhood between 50-100 PM.

Click to expand...


Thank you Sandy. I know I'm asking a simplistic question for a complicated system but your answer helped.
 
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