25931
Well-known member
Is there a reason (apart, perhaps, from price ) that stainless props are not the norm ?
Props. s/s or bronze
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Tranona
Well-known member
They are. Bronze is rarely used these days as it is less strong and more expensive. 316 stainless is the norm.
vyv_cox
Well-known member
I'm not about to say that you are incorrect but your statement intrigued me so much that I spent some time checking. A good source of info seems to be http://www.deepblueyachtsupply.co.uk/propellers where a very large range of props for sale is listed. I failed to find any stainless steel ones amongst the hundred or so manganese bronze and nickel-aluminium bronze ones. Darglow use stainless steel blades and I found several suppliers for outboard engines in this material but that's about it. If you know different, please can you point me to the information.
Tranona
Well-known member
Brain fade! Read it too quickly and had assumed incorrectly the reference was to shafts rather than props.
You are of course, correct, bronze of various types is the norm for props because it is easy to cast and strong enough for the job. The drawback (as I understand it) with 316 is that it is best investment cast which means a different set of moulds for each blade shape. With a bronze prop you can vary the moulds easily for different blades, or even change the pitch of an existing blade. The stainless blades of the Darglow prop are all the same shape, so only one mould per blade size and the pitch is set by the stops on assembly.
You are of course, correct, bronze of various types is the norm for props because it is easy to cast and strong enough for the job. The drawback (as I understand it) with 316 is that it is best investment cast which means a different set of moulds for each blade shape. With a bronze prop you can vary the moulds easily for different blades, or even change the pitch of an existing blade. The stainless blades of the Darglow prop are all the same shape, so only one mould per blade size and the pitch is set by the stops on assembly.
Boo2
Well-known member
At the risk of slightly drifting the thread, can you tell me which, if either, of those two types of bronzes is preferred ?
Boo2
vyv_cox
Well-known member
Nickel aluminium bronze, by a long way. Manganese bronze is really only brass, suffering from dezincification. NiAl bronze is used by most ships, highly corrosion and cavitation resistant.
Boo2
Well-known member
I see from a .pdf I have that manganese bronze is Lloyds class Cu1 whereas Nickel aluminium bronze is LLoyds class Cu3. Can you tell me whether copper manganese aluminium bronze which is Lloyds class Cu4 is better still ?
Thanks,
Boo2
William_H
Well-known member
Stainless steel props
Stainless steel appears to be the preferred material over cheaper Aluminium for outboard propellers particularly in the high powered O/Bs. Is this because they tend to spend most of their time out of the water on a trailer. Is SS more likely to attract growth or fail under water? Our new club RIB on 60HP has a beautiful shiny SS prop. olewill
Stainless steel appears to be the preferred material over cheaper Aluminium for outboard propellers particularly in the high powered O/Bs. Is this because they tend to spend most of their time out of the water on a trailer. Is SS more likely to attract growth or fail under water? Our new club RIB on 60HP has a beautiful shiny SS prop. olewill
35mm
New member
I might be wrong, but I think outboards and stern drives always seem to use SS or ally props, and I think that's probably mostly a speed thing. For a start I think SS and ally are lighter than bronze. It may also be the case that bronze doesn't have the right qualities for spinning fast as is typical for outboards and stern drives. It might be that bronze would be too brittle and possibly break with those sorts of g-forces. I can't imagine a bronze, high speed, surface piercing prop lasting too long. However, on a slower turning engine bronze may well outlast SS or ally. That's my take on it any way, and I might be completely wrong!
oldsaltoz
New member
We have stainless steel props on the cat VP's, they seem to get very little growth on them and are easy to clean when we lift out about every 2 years.
Most of the thru hull fitting are Admiralty bronz, not sure what in that though, but they have been in for many years and never a problem.
Good luck and fair winds.
Top reply again Vyv.
Most of the thru hull fitting are Admiralty bronz, not sure what in that though, but they have been in for many years and never a problem.
Good luck and fair winds.
Top reply again Vyv.
rogerthebodger
Well-known member
I have just had a new prop fabricated from 316 stainless steel. The hub was machined including the taper and key way. The blades were then laser cut from a template taken from my original oversize bronze prop. The angles of the pitiching were calculated for blade root mid dia and blade tip. The blade was tig welded on to the hub at the root angle. the twist was then put in the blade by clamping the prop down and twisting each blade with a very long lever to get the mid and tip angles to the calculated values.
When I first fitted the new prop and did some sea trials the pitch was still too great and we bent the blades a little more to reduce the pitch some more. All the bending was done cold.
Next time I an on the boat I will be doing some more sea trials and if the pitch needs changing further we will cut the blades and re weld the blades reducing the root angle some more until I get it right.
This kind of modifications is not as easy with bronze and the welding of bronze is IMHO not as easy as with stainless steel.
Don't have a pic at home so cannot post one until after I have returned from Durban next month.
When I first fitted the new prop and did some sea trials the pitch was still too great and we bent the blades a little more to reduce the pitch some more. All the bending was done cold.
Next time I an on the boat I will be doing some more sea trials and if the pitch needs changing further we will cut the blades and re weld the blades reducing the root angle some more until I get it right.
This kind of modifications is not as easy with bronze and the welding of bronze is IMHO not as easy as with stainless steel.
Don't have a pic at home so cannot post one until after I have returned from Durban next month.
vyv_cox
Well-known member
Sorry, I don't really know anything about it and the Internet doesn't come up with much in a quick search. It seems to have good corrosion resistance, as you might expect, but it is not especially strong and it strength cannot be improved by heat treatment. My reference book says it is useful as a resistance material operating at a moderately elevated temperature, which doesn't seem a useful property for a propeller!
Tranona
Well-known member
More to do with construction method and reducing cost. These types of prop have a rubber bush in the centre with a splined drive. So the props (which are produced in large numbers) are cast and then the bush pressed in. Most bronze props have a solid centre with a tapered bore machined in. Props for Saildrives are constructed in the same way as outboard props if they are fixed blade - that is aluminium with a rubber bush and splined sleeve pressed into the casting. However when props have moving parts (folding or feathering) they are bronze castings and stainless components because they need wear resistance on the pins etc. This construction does, however result in mixed metals and attendant galvanic problems. Most stick with the rubber bush and splined sleeve which also isolates the prop from the shaft and leg. Some newer designs are dispensing with the bush but it is not easy to machine the long spline in bronze.
25931
Well-known member
Please can you tell me where I can find NiAl props ?
vyv_cox
Well-known member
They call it Nibral for some reason. The link I gave earlier lists lots of them. Mobos use them widely, don't seem quite so popular for yachts.
Tranona
Well-known member
As I understand it the MOBO usage is because they are stronger and less prone to dezincification - more important on such applications because of the much larger masses of dissimilar metals involved in such installations.
The requirements of a small sailboat auxilliary are different and lower spec materials are used. Look round any boatyard now and you will see 000's of props, many of them with no anodes in perfectly good condition after many years use.
The problems in recent years have come about because of the increased use of props such as folders and featherers which in themselves have large masses of dissimilar metals. Such props are often made with higher spec material to reduce this problem. For straightforward, low powered fixed props the lesser materials are perfectly adequate, perhaps with the addition of a small anode.
vyv_cox
Well-known member
The dezincification issue is a factor but I suspect the cavitation resistance is more important for a mobo. Manganese bronze is well down the scale for this but Ni-Al bronze is at, or very near to, the top. Plenty of one-piece castings for mobo props use it, as does virtually every ship propeller made.
Also, reading a few threads on the mobo forum, many posts suggest that a shaft anode interferes with water flow over the prop, reducing performance. I know I dislike it when I can't get up to my customary 35 knots
charles_reed
Active member
Except Bruntons who use AlBronze for the hub, and NiBronze for the blades. Some evidence of dezincification on autoprop blades but none of the hub.
vyv_cox
Well-known member
Pretty astonishing for an alloy that contains no zinc.
