Prop talk.

[BarryH]

I've touched on it before I know. Looking at replacing the prop on the boat. Stainless steel seems to be the way to go, its all spangley in shiney, and don't get all chipped and split at the first sign a crap and flotsam. Phoned Steel developments about solas prop. Had quiet a good chat with one of the chaps there. Couple of things that flumoxed me tho. I know all about pitch and stuff. What surprised me was that he reckoned going the stainless route that I would need to DROP an inch of pitch! Something to do with 'cupping' of the trailing edge. Cant quiet grasp that one. So a few questions for the anoraks out there, Cupping, please explain and no not the womens bra thing! Long hub and rake, whats that please. And wash cone, he totally lost me at that point, got any ideas?

 

[MapisM]

If fast planing boat with outdrives or outboards is what you're talinking about, and assuming to compare an alu with a SS prop perfectly identical in shape (which is theory - they normally aren't) then YES, with the SS prop you should drop an inch of pitch.
That should lead to the same maximum RPM and to a slightly higher speed.
It might seem a nonsense, but that's because alu props are more flexible, and this reduces their efficiency at high rotation.
Edge cupping goes on top of that, but it has nothing to see with the pure raw material (alu vs. SS) comparison.

At least, that's the explanation an experienced guy once gave me.
Never made any test myself , though.

 

[BarryH]

oopps yep, sorry should have said. Drive is a volvo 280 on a planing hull. The cost seemed fairly good against new ali prop. Ali prop is £185'ish plus vat and a new stainless is £250 plus vat.

They said blade shape was better compared to the Ali one. So what exactly dues the cupping bit on the trailing edge do exactly then. This is the bit where the guy lost me a bit, well a lot actually!

 

[Col]

Volvo props are Long or short hub-exactly that-The length of the hub.
Rake is the "skew" of the blade (ie) blade curved, like a stumpy knife blade would be. So not symetrical.
Cupped- Blade is concave as opposed to flat.
Have you looked at the <A target="_blank" HREF=http://www.solas.com.au/propellerguide.htm> Solas</A> web site?

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[BarryH]

No, I've only talked to the guy at Steel Developments. I've used them for years to repair my ali props. Always a good service and quiet helpful in the past with re pitching props of mine.

This has come about as the boat has always easily hit max rev's with still a bit of throttle to go before wide open. Top rev's being 5200 and the thing will easily rev out to 5500 no probs. I can hit 30 knots at 5100'ish but dont want to kill the engine. I think thats why it ate a valve earlier in the year.

They were saying that with the right sized stainless prop the boat should easily hit 35/38 knots top whack and cruise at 28/29 knot mark. Currently I cruise the thing at about 22/24 knots. That sort of speed gives me a fair range on a tank of petrol. Buring in the region of about 6 or 7 gallons an hour.
The hub thing is easy to get round according to them, with a change of nose cone, but will have to keep the old nose cone for going back to the ali prop.

The other thing was weather to go three blade route, as it is now, or four blade. I thought four blade was for better acceleration and 3 blade for speed. Its a bit of a mine field this changing prop malarky so thought I'd throw it over to peeps with a bit more experiance than me.

 

[DepSol]

leave these guys to make the decisions they really know their stuff

Dom

I am boating again ;-)

 

[MapisM]

The difference is low, no doubt. Well, it's the price of the alu prop which seems rather high IMHO.
Maybe comparing an original alu Volvo vs. a non-original stainless steel?
Not that I'm suggesting to take the original - I wouldn't spend a penny for their name on the box, actually...
Re. cupping, as far as I remember one effect is to get the boat on plane faster, because the blades have a better "grip" on the water. But don't take my word for it; with a google search you'll surely find lots of tech details.
Also, http://www.boatprops4sale.com is worth a look.

 

[Col]

Did you click the link ?? loads of stuff on their site.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[BarryH]

Yep, clicked the link. Its quiet an informative site. I'm off up to town again today, so I'll take my prop in to SD. They also wanted waterline lenght weight of the boat etc. I'll see what they come up with as regards size and blade configuration. I think its the Solas props that they supply. The £250 price tag was not as bad as I thought it would be.

 

[Canboria]

Steel developments are correct re dropping prop size, more cupping will get the boat on the plane quicker ideal for water skiing, less cupping more top end, trial and error will find the balance, when I was racing I always took my props to Poole Propulsion, stainless props are obviously heavier than standard props, I always had them lightened and balanced, for racing we nearly always had a solid hub, we never ever put the engine in gear with the engine running always had to start in gear, to much torque from the engine coupled with a solid hub.

Poole Propulsion kept a 3 D computer image of my racing props so that if damaged they could rebuild to the exact dimensions.

Try and borrow as many different props to try, you will be surprised at the difference between manufactures.

Derek

 

[David2452]

There is no doubt that S/S will out perform Ali due to improved rigidity and thinner cross section, BUT as with all things there is a trade off, keep an extra eye on your annodes when used with outdrives as they increase galvanic action.

David