"new" bronze prop.

[Frank Acampora]

As mast of you know, I lost my favorite bronze prop in the river. Since it was an emotional thing, I kept searching on ebay for another. I found this one off an OMC 25-40 and it was close to what I had. SO: I bought it and re-hubbbed it. Then I pinned it so it would not "spin" the first time I used it on my 50 HP Chrysler go-fast. Now I can replace that 13 pitch three blade aluminum with this 15 pitch bronze. I am happy again.

 

[redstrat]

Re: "new" bronze prop.

Help me out here. I understand the difference between 13 and 15 pitch, but what about aluminum vs. bronze, and 3 vs. 2 blade. Sure looks pretty.

 

[Frank Acampora]

Re: "new" bronze prop.

Bronze two blade prop is OLD School racing. It is just an emotional thing with me. To me there is NOTHING better looking on a boat than a newly polished bronze prop.

Years ago, before the technology permitted casting of stainless props, Bronze was the material to use because it was so much stronger than aluminum yet easy to cast and machine. Indeed, because of its ease of casting and machining it is still used on larger inboard cruisers in the range of 16 inch diameter and up. It is also used on older inboard specialty ski boats in the 10-13 inch range but has mostly been supplanted in these small sizes by stainless. Because bronze is stronger than aluminum, the blades can be made thinner with less parasitic drag through the water and just like stainless, the blades flex less under running loades, resulting in increased efficiency over aluminum.

The theory of two blade versus three blade was that each blade of a two blade prop was turning in "cleaner" water, undisturbed by other blades and was thus more efficient. The actuality is that while each blade is more efficient, more blades does increase the total efficiency of the prop by a few percentage points.

However as I said before, this is old school design and thinking. With new prop design, cleaver blades, and surface running props used in racing, this is no longer true. On the real Go-Fast boats with esoteric drives, you will see 5 and 6 blade props while on high powered outboards you may see 4 and 5 blade props.

 

[redstrat]

Re: "new" bronze prop.

Thanks Frank, I always learn something from you. Let us know how she works out for you.

 

[Texasmark]

Re: "new" bronze prop.

As Frank said, back in the day, 2 blades were it. Look at Chinewalker's Avatar and you have an older Merc Mark 20E if I remember correctly with a Quicksilver racing lower unit and the little bitty 2 blade racing prop with lots of pitch.

In my time they ran these engines and their big brothers, the Merc. Mark 55 4 cyl (called the 4 cyl classic design today) at 40 hp on primarily 3 pt. hydroplanes. The little props weren't hole shooters so to get up the driver would squeeze his spring loaded throttle and the little prop would shoot a 30 or so foot rooster tail. The boat would move slightly. Release and goose again and a little more movement. After a few cycles of that the boat would finally get on plane and after that it was just a blurrrr. Great sport. Course I was a kid at the time and I could just go along for the ride and watch.

Mark

 

[Frank Acampora]

Re: "new" bronze prop.



THERE YA GO! Now the engine looks "right" again. If the weather keeps in the 50s, I just might take it down the river and videotape a run.

The stainless collar retains the thrust pin and the 1/2 inch nylock nut and washers are a safety so if the collar should somehow come loose I won't lose the prop. (I doubt it will loosen--those split collars grab really tight). This is the same sized collar that I use on my 140 to take the thrust of a Ballistic prop and leave room between the blades and snout. If it can take the loads the 140 puts on it, it 'aint gunna loosen on a 50.

 

[jimbo_jwc]

Re: "new" bronze prop.

My old 20Hp Chyrsler came with a two blade bronze prop that has a speckeled paint finish on it . I wondered if it was factory or somone modified it on purpose ?

 

[Frank Acampora]

Re: "new" bronze prop.

Bronze was always an extra add-on. The engine came from the factory with an aluminum 3 blade prop and you bought a bronze one if you wanted to spend the extra money.

Back in those days, all of the smaller engines came with a standard prop. If you had a larger or smaller hull than average, you needed to buy another prop.