What Difference in Pitch when going to Stainless Steel?

Simon B · 08 August 2008, 15:04

We may be going to a stainless prop, I think the one we have is a 4 blade Sola 13.5" x 15"P is there an improvement factorif going to stainless in that you can go up an inch in pitch?

Or is this an old wives tale?

jyasaki · 12 August 2008, 16:58

Stainless is stiffer and heavier than aluminum, so an otherwise identical prop will drop your rpm by some amount. Generally considered to be equal to an inch of pitch, but that assumes identical props of differing materials, which isn't going to happen (blade shape differences, diameter, cupping, etc.)

Short version is make a rough guess and try it.

jky

Dirk Diggler · 12 August 2008, 18:26

Quote:

Originally Posted by jyasaki

identical prop will drop your rpm by some amount.

jky

Cobblers, you ain't gonna find an identical prop in stainless, so I'd try the same pitch size at first!

Bigmuz7 · 12 August 2008, 18:36

Quote:

Originally Posted by jyasaki

Stainless is stiffer and heavier than aluminum, so an otherwise identical prop will drop your rpm by some amount. jky

But stainless blades are thinner .. no ? so I thought your revs would increase slightly if the pitch is the same

But a good question, as I have been considering this same issue

Turbodiesel · 12 August 2008, 22:03

Go up one pitch if your putting a stainless on.

Hightower · 13 August 2008, 00:20

It's all a black art and can cost a fortune to experiment with different pitches and types. But if you want to experiment then taking a few readings with the current prop is a good Idea and will give you a clue if you are propped right.

One place to start is to map out different speeds against RPM with the prop you have now. Compare the WOT (wide open throttle) RPM against the recommended WOT for the outboard (this should be a range of RPM). If the max RPM is outside this range then you're probably not getting maximum performance/economy out of the engine with the given prop. The other RPM range to take note of is the cruising speed, say at 3500 RPM for example. These figues will help you if you decide to try different props.

I experimented at some length with my previous RIB and chose an Alloy prop over any of the Stainless ones I tried, it didn't give me the fastest top speed but gave me the fastest cruising speed at a give RPM. So going stainless isn't the be all end all.

Hightower · 13 August 2008, 00:30

Quote:

Originally Posted by Turbodiesel

Go up one pitch if your putting a stainless on.

Shouldn't that be go down a pitch size?

Assuming you're in the recommended WOT range. Stainless props are stiffer and have thinner blade thickness so cause less slip, it actually requires the engine to work harder, thus reducing the Revs.

Bigmuz7 · 12 September 2008, 08:38

Went from a 13x19 ally on a Honda 90 giving 40 mph on the gps @ WOT of 5400 c/w anchors 100 litres go go juice and 2 up to a 13x21 stainless.

..Then got 3500 rpm tops

it totally killed it.

So .. changed to a 13x19 stainless and now Revs to 5400 and 40mph on gps as before.. better hole shot with stainless, no real detectable difference otherwise .. but test conditions were very flat on our Clyde run so havent had the chance to try in the rough stuff

The rev range is 6000 RPM for this engine and the leg seems an odd ratio of 2.33 :1 any thoughts gents ? .. is it worth trying a 17 ?

Havnt measured consumption on the Stainless 19 but I was getting 10lph on the ally 19.

Nos4r2 · 12 September 2008, 09:45

Quote:

Originally Posted by Turbodiesel

Go up one pitch if your putting a stainless on.

Just a personal guesstimate, but I reckon Turbodiesel is right if you're going from a 4 blade ally to a 3 blade stainless. That or the same pitch. Every stainless prop I've looked at carefully (admittedly it's not been many) has had visibly more agressive cupping than ally-which would (in my head) be offset by the extra blade on the ally prop.

Course, as Dirk said....

And as Hightower said too, I tried stainless but I actually prefer ally for the same reasons-but I don't use my boat loaded with divers.

Simon B · 12 September 2008, 14:45

The number of blades relate to the props ability to transmit "torque" for the same pitch (distance travelled)

"Cupping" I think helps reduce ventilation and may reduce cavitation, also heard that it can be used to increase the theoretical pitch.

Props are complicated things! but look beguilingly simple.

Nos4r2 · 12 September 2008, 16:46

Quote:

Originally Posted by Simon B

The number of blades relate to the props ability to transmit "torque" for the same pitch (distance travelled)

Also means the engine can't turn it as fast if you have more blades.

Simon B · 12 September 2008, 19:11

This would manifest itself as the engine needing more torque to turn it at the same rpm, the engine may indeed have this torque so you would not notice it, however, if your first prop was correct for WOT then a prop with more blades may reduce your WOT.

Bigmuz7 · 12 September 2008, 19:54

K.. ta boys I think Andy has a point too with the 'black art' comment, as with some of the new 4 strokes, the power band seems different ?. So some of the traditional rules perhaps are somewhat diluted now. The Honda hub is interchangeable with the merc, so that opens up the range a bit.

behavin · 14 September 2008, 21:51

can only comment on my own experiance:

had a twin delta with dt 65's and changed from the standard suzuki alloy props which were i seem to remember 16" pitch to two solas stainless props sent from steel development as a direct replacement.
The engines overrevved massively 6200 at wot compared to 5800 on the alloys (measured on rpm gauges) so i sent them back and they cupped them up a bit.
They were then at the top of the rev range when trimmed right out but were ok because as steel dev' said once you have small weed growth or are loaded then they will perform.

I found the top end only 2-3 knots higher on SS and also the standing start was simular...however the mid range punch was a lot more ,much more grip and she would leap from 15-30 knots much faster.

when i bought them i was understood to believe that a SS prop increases the revs by upto 200rpm as the prop is more effecient in the wtaer and also has more mass to spin so will spin faster

My current boat is under pitched slightly as it was set up for carrying load so i could go to 23" and maybe just nudge 50 knots but i run the prop that came with it 21.5 and hit 47 on my own but can still do 45 with 6 people in
good luck

jyasaki · 15 September 2008, 08:53

Quote:

Originally Posted by behavin

when i bought them i was understood to believe that a SS prop increases the revs by upto 200rpm as the prop is more effecient in the wtaer and also has more mass to spin so will spin faster

More mass takes more energy to spin, so will spin slower for a given power input.

jky