2 or 3 blade, Brass or aluminum??

[Mrmopar22]

I have started a project on my 1950's ski boat that I am hoping to finish so I can run it this summer. What is the performance difference between 2 or 3 blade propellers with the same diameter and pitch? I have heard that 3 blades will get you a better bite, but 2 blades have less resistance and you'll get more speed. Is there a performance difference between brass and aluminum? Most all of the 2 blade props are brass and the 3 blades seem to be aluminum, is it just coincidence or is there something to it?

What are your thoughts?

 

[Frank Acampora]

Bronze! Brass is a mixture of copper and zinc. Zinc acts like an anode and will dissociated from the alloy. Thus Brass is not used for marine props. Bronze is basically a mixture of copper and tin with some naval alloys using monel metal.

In the 1950 and very early 60s 2 blades were the way to go. With that technology the bronze 2 blade props were rugged and more efficient than 3 blade or aluminum. Nowadays, if you were using a modern prop it would be a wash and some 3 blade aluminum props would outperform older Bronze. Of course, stainless would be the best performance wise.

HOWEVER: Since you are restoring a 1950s ski boat I would go for a period correct 2 blade bronze prop polished to a mirror finish. Spend the extra bucks and spring for a custom prop. Nothing, not even stainless, beats the look of them glinting in the sun.

 

[Frank Acampora]

No Title

Here are a couple of my vintage 2 and 3 blade bronze props.

 

[Mrmopar22]

Bronze! Brass is a mixture of copper and zinc. Zinc acts like an anode and will dissociated from the alloy. Thus Brass is not used for marine props. Bronze is basically a mixture of copper and tin with some naval alloys using monel metal.

In the 1950 and very early 60s 2 blades were the way to go. With that technology the bronze 2 blade props were rugged and more efficient than 3 blade or aluminum. Nowadays, if you were using a modern prop it would be a wash and some 3 blade aluminum props would outperform older Bronze. Of course, stainless would be the best performance wise.

HOWEVER: Since you are restoring a 1950s ski boat I would go for a period correct 2 blade bronze prop polished to a mirror finish. Spend the extra bucks and spring for a custom prop. Nothing, not even stainless, beats the look of them glinting in the sun.

Bronze, thank you for the correction. So if I could find a bronze 3 blade would that be better over a 2 blade bronze for speed?

Who does custom pin drive props? I can buy 2 blade bronze for less than $50. How much can you expect to gain with a custom prop with 35 hp?

 

[Frank Acampora]

Back in the day, there was a myth that since less prop blades are turning in the wake of the others, less blades were more efficient. People making rubber powered model airplanes went so far as to make their own counterbalanced single blade propellers.

Now, it is true that less blades are more efficient PER BLADE but OVERALL efficiency tends to rise with more blades. Of course, with modern props surface running it really makes no difference because only one blade is in the water at a time.

All this to say: With twin stock Evinrude 35s, it probably won't matter whether or not you use aluminum, bronze, 2 blade, or three. As an example: The little blue boat in the photos was set up with the bronze 15 pitch 2 blade and does 45 MPH at 5500 RPM. At one point I was using a Michigan aluminum three blade 13 1/2 pitch and speed was---45 MPH at 5500 RPM. The negative was that the three blade was getting cavitation burn on the blades. Besides, the bronze simply looks better.

Don't expect to get a lot more speed with twin 35s. Even though you are doubling the horsepower, you are increasing the weight and drag. Besides, the rule of thumb is that it takes 4 times the horsepower to double speed. These were workhorse engines not built for racing BUT twin engines singing on a small hull sound so pretty. Depending upon the size of your hull. those engines could probably use between 12 1/2 pitch and 13 1/2 but you will never know without trying. Buy a cheap tachometer to monitor engine RPM . Unless you know exactly what the engine is turning you are working blind.

As far as custom pin drive props, you simply need to scour the internet. They are probably no longer made and you simply need to luck into an old prop. My OJ prop was an ebay find and I needed to modify it to fit my engine. You might get lucky by joining Scream and Fly, a hydro racing forum.

You can still get pin drive Michigan props on ebay for your engines. To get on the water, it might possibly be best to buy a pair of aluminum 3 blade props while you search for better choices.

Remember: Not to discourage you but you are getting into a very long range project. I have had that little blue boat for over ten years and I still experiment with transom height, props, weight distribution and engine modifications. If you want to see it run google Frank Acampora Mini Boats on youtube.

 

[Texasmark]

Bronze, thank you for the correction. So if I could find a bronze 3 blade would that be better over a 2 blade bronze for speed?

Who does custom pin drive props? I can buy 2 blade bronze for less than $50. How much can you expect to gain with a custom prop with 35 hp?

What engine? If a white Merc then 2 blade. If an OMC 3 blade. That's the way they were back then. Merc got it's go with high rpms and the 2 blade was pretty well suited for that and the most efficient, but most vibration....other than a single blade...Ha! OMC got it's from cubes and did it at lower rpms so the thrust of the 3rd blade came into play. SS didn't come out until about 1973 in production quantities....OMC SST....Stainless Steel with a Black Teflon coating (prevented rusting of the iron in the SS casting formula....I put one of the first available on my 1972 18 Caravelle tri hull bow rider with a 1972 Johnson 125. Finally quit bending blades EVERY time I went out. Cost was $185 and a lot of $$$ back then.

OMC also had a bronze pressed on deflector ring to keep the gasses out of the prop so you wouldn't have any prop related ventilation. Course OMC went in with their deflection because Merc had been flaring their bronze with their thru hub exhaust. They may have had the design patented and OMC couldn't copy it or they just did it to be different....spite Merc. Who knows. OMC still had the snauze sticking out the rear of the AV plate and it wasn't until around 1968 when they introduced the 3 cyl 60 horse looper with the streamlined, Merc almost copy thru hub exhaust lower unit. I think Johnson V4's came out with the thru hub in 1971 in their dark green and white bonnet color scheme....I think the Rude Triumph 55 came out in '68 and Johnson didn't get into the looping and thru hub exhaust until the '71 green and white year......but don't quote me on the who did what to what when other than the Triump 55 looper in 1968 and the date of the SST...that I know.

 

[fhhuber]

Back in the day, there was a myth that since less prop blades are turning in the wake of the others, less blades were more efficient. People making rubber powered model airplanes went so far as to make their own counterbalanced single blade propellers.

Now, it is true that less blades are more efficient PER BLADE but OVERALL efficiency tends to rise with more blades. Of course, with modern props surface running it really makes no difference because only one blade is in the water at a time.

Fact ... for AIRCRAFT propellers, less blades is more efficient, up to the point of introducing Mach effects. But, the dynamic imbalance is brutal on the drive line when you go to a single blade, reducing gear drive life by over 60% on a plane the size of a Piper Cub. (And I have the magazine article where they were testing the 1 blade props on a full scale Piper J-3 Cub laying around here somewhere... from appx 1939.)
THAT is why you don't see modern full scale aircraft with single blade props.
And you do still see some Free Flight competition model aircraft with single blade props.

But... what works for aircraft props often has no bearing on what works for boat props.

and for a boat, you really don't want half of the prop out of the water. That's really bad even for the extreme racing boats.

 

[Frank Acampora]

In extreme racing, top speed takes precedence over "really bad." That is why all prop driven drag boats operate with only one blade in the water. That is also why you see the really big "rooster tail." The boat rides "loosely" on the front sponsons and the prop hub for less drag and a lot of air under them. If you are lucky they stay dry side up. You also need to research "flats boats." These hulls operate in literally inches of water with the engine jacked all the way up, thus: one blade in the water.

Go over to Scream and fly and tell those fellows that it is very bad for their engines to have one prop blade in the water. I'll hear the laughter all the way over here.

 

[fhhuber]

If running with half the prop out of the water was a good idea... the anti ventilation plate would be in line with the prop shaft instead of above the prop.

 

[Frank Acampora]

Not going to get into an argument. However you are comparing apples and oranges. Submerged props will lose thrust if they entrain air. This is a fact. Because the blade nearest the surface has a negative pressure behind it, that blade will tend to suck in air.This is the reason for an anti-ventilation plate. But you already know that.

HOWEVER Surface running props are designed to run with only one blade in the water and in fact are very inefficient when run submerged. Additionally, since the blade(s) in the air are already ventilated 100% there is no need for an anti-ventilation plate. Indeed, because almost all (for example) outboards come with anti vent plates, if they are used in a surface running application why bother to cut it off? It is not in the water or on the surface of the water causing drag. I have yet to see a vee drive or straight drive drag boat with an anti-ventilation plate.

You can argue all day if you want but I certainly will not do so anymore.

Before you spout off, know what you are talking about.

 

[fhhuber]

I know that the drive line doesn't lake the asymetric load...

 

[Texasmark]

Back to the 2 or 3, back in the '60's or late '59, when Merc decided that Phantom Black was a better paint than White or Merc creme color, their sales brochures showed 3 bladed props even though the engines were still running about a hp per cu in at high rpms.

I don't know it it's still alive, but there used to be a site named "old-omc.ge", or whatever acronym the www has for something originating in Germany...remove the ge and plug in that acronym. The site owner had a multitude of sales manuals from the era, back into the '40's OMCs and are well worth your time to browse. While browsing, you won't find a 3 blade anywhere but you will find a lot of engines mounted low in the water (with today's streamlined gear case) pushing big loads for their size, which works for big waves and rather slow moving boats....boats under 35 or so mph (opinion).

 

[82rude]

Texasmark. I had a 68 55 triple johnson so they at least started there.It had loop charging and through hub exhaust plus was electric shift ,good motor.

 

[Texasmark]

Texasmark. I had a 68 55 triple johnson so they at least started there.It had loop charging and through hub exhaust plus was electric shift ,good motor.

That's why they called it the Triumph. Loop charging marine 2 strokers WAS a revolution in fuel economy and a Triumph...course it wasn't just the looping process it was also the CDI fast rise high voltage ignitions, and the exhaust horn tuning to go along with their new streamlined lower unit with thru hub exhaust, higher gear ratio so they could swing a bigger prop at slower speeds and get more bite and all that.

 

[82rude]

Nowhere on the Johnson did it say triumph.My brothers 55 e rude 55 did say triumph on the cowl though.http://www.bing.com/images/search?q=...ao0&ajaxhist=0 keeping with the op post I sure like the bronze props for looks especially on those old motors.

 

[Texasmark]

Nowhere on the Johnson did it say triumph.My brothers 55 e rude 55 did say triumph on the cowl though.http://www.bing.com/images/search?q=...ao0&ajaxhist=0 keeping with the op post I sure like the bronze props for looks especially on those old motors.

I thought we were talking about a Silver and White with Red decal Evinrude 55 hp 3 cylinder looper, a "Triumph", built for the 1968 Evinrude division of Outboard Marine Corporation Market. I have no recollection of a Johnson triple in that market, but that's not to say that they did or didn't have an entry.

I had a 1972 Johnson 125 hp V4 cylinder in white and sea moss green that had benefitted from OMC's looper production back in '68 and it had all the amenities except it was still cross flow combustion. Reason being was the looping process on 2 strokers followed the 120 degree rule and 4 cylinders, at 90 degree combustion didn't sync up the kick back from the exhaust tuning that was necessary to get the loopers to function properly. Mine had much better performance and fuel economy than previous OMC 4's but not as good as the looper!

 

[82rude]

https://www.flickr.com/photos/henrik_abel/2734448539/ it,s not much Texasamark,but heres a short blurb about the 55 triples below the picture.I believe there was also a 70 triple evinrude that was called a triumph.Johnsons were the same but just said seahorse on the cowls instead.I remember my 55 as a good running motor that punched above its weight class and being a triple was as smooth as glass.For the original poster if he,s dealing with or planning on using a 50,s era motor from omc a prop like I have would be the cats meow for setting up .Years ago I bought a 56 30 hp Johnson and it had a al. manually adjustable prop that covered a wide range of pitches.It was real handy for getting the correct pitch for different boats and yes different omc motors!Still have it I believe.