What Difference in Pitch when going to Stainless Steel?

[Simon B]

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]

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]

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]

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]

Go up one pitch if your putting a stainless on.

[Hightower]

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]

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]

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]

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]

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.