Propeller question

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Marin any Beaver that "climbs like a rocket" must have a good prop on it.
 
There were a few attempts in the 60's to get prop driven planes above supersonic. They were all abandoned because of the huge in inefficiencies they got when pushing them that fast.

I fly RC planes and some I've seen get into that situation and usually throw props at high speeds like that.

Boat props need to be able to get clean water on each bite. With a displacement boat like a trawler, you would have problems adding blades to get much smaller.

I have 4 bladed 26" x 18" props on mine and a spare set of 3 bladed 26" x 19" props as well.

Reducing the blades means trading diameter or pitch and you run out of pitch pretty quickly.
 

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I am most likely wrong but have been told in very basic layman's terms that the bottom third of a marine prop is the most efficient. Top 2 thirds not so much. Arneson surface drives where created to take advantage of that principal while reducing drag.


Can't wait to see Healhustler photo shop an Arneson drive onto Eric's Willard.
 
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CP,
Willy hits whales so I don't want to go any faster.

So Larry I'm look'in ...???

looks like ther'e there to pull in air.
 
Howard Arneson shop is 20 minutes from where we live. I pass by often. In past years I’ve had the pleasure of meeting the fellow as well as being inside his shop. We see Arneson test boats on SF Bay and the pretty semi-rig that trailers them from shop to launch on tests. Howard is a genius - pure and simple. As most inventive geniuses will say - 90% sweat / !0% brains! He always worked hard. :thumb:
 
I saw one in Alaska in 03 that had only one Arnason drive and went 70 knots.
 
I watched one of Howard's boats "fly" through Raccoon Strait on the north side of Angel Island a couple of years ago. My rough timing and calculation had him pushing about 95 knots suckin' air the whole way!
 
Larry M said:
Yes, for better bite? I don't know. Way faster than we are though. :)
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They are called vent tubes and are used to help get the boat on plane faster by ventilating the props so they work as designed rather than in "clean" water.
 
Due to jet engine propulsion and/or type of fuel usage, some of Howard’s speed boats were evidently registered with the FAA. I’ve heard stories there is basically no limit to the speed attainable so long as the boat can be kept on/in the water with the uniquely designed props still having water propulsion capabilities. Local rumor had it in the 90’s that 150 + mph was not unusual during some speed tests. :eek:

Personally, I like cruising in my pleasure boat at hull speed... etc, and, hanging on the hook to play with our tow behind runabout or go swimming deep inside SF Delta warm freshwater! :dance: :D
 
manyboats said:
Marin any Beaver that "climbs like a rocket" must have a good prop on it.
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Been on a few beavers in my day, not fussy when they move too fast. I like enjoying all the ride and oh ya the scenery!

Sent from my iPhone using Trawler
 
Such boats (not trawlers) make "rooster tails" at speed.

img_122873_0_33ca79bfeacb8e49ee36e645d592c7ec.jpg


Meanwhile, in an opposite view, Ray and I contemplate Sam's (in Tiburon, CA) menu:

img_122873_1_e353db19528f4815269cd7342dd20139.jpg
 
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I like your pictures Mark and the humor that frequently tags along.

I get just about as excited about rooster tail boats.

The original poster only made one post after #1 and never did proclaim how the 3 blade prop was loaded. He says it "turns 2400 at speed". Is that w a 2500 rpm engine or perhaps a 2800 rpm engine. To your specific question I'd say yes it should be close. Two inches in dia is a lot but an extra blade is a lot too. The Michigan MP "workhorse" propeller is basically a commercial propeller for fish boats and such but it's very heavy duty and has lots of blade area. A possibility.

But I suspect that most likely there is nothing wrong w your 3 blade prop. Try to find other sources for your "problem" (the chop chop noise) unless the 4 blade is handy (as in don't have to buy it to try it) then just try it.

Most of us (at least lots of us) are running 2.57-1 BWVD gears so your gear ratio is'nt so deep as to be unusual. So there is an excellent chance your props are not overloaded and the 3 blade/4 blade issue is of very little importance.

If you can put your head right down near the bottom of the boat while running and if your boat is MUCH more noisier than others your prop to hull clearance is probably too close.

Perhaps your boat would benefit from a nice big fat overlay of matt right over your propeller on the hull inside the boat.
 
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I cruise at 1400 rpm with our 2500rpm FL 120s. To load the engine at this slow turn we have always over-propped and never had a problem. We put many hundreds, closer to a thousand hours a year on our engines. Works for us.
 
manyboats said:
Most of us (at least lots of us) are running 2.57-1 BWVD gears ....
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The ratios of our Velvet Drives (and all the VDs used on GB36s) are 1.91:1 and 2.1:1.

And Eric, a piston Beaver with the so-called water prop on it does indeed get off the water much faster and climb like a rocket compared to the same plane with the three-baded prop most of them have today.

The Turbo Beaver, however, takes off and climbs like a rocket, period. Particularly Kenmore Air's two re-engined ones. Well, at least like a somewhat slow rocket. :)
 
Marin your'e thinking GB as I think the 2.57-1 ratio is much more common.

I remember when I was 19 I flew from Anchorage to the Bethel area on an F27 Fairchild. My first turbine flight and I was probably almost afraid it climbed so steep.

I had an ultralight that climbed at 1500fpm at 40 mph. It felt like about 45 degrees. I had to look between my legs to see the ground looking ahead. That bird used a high aspect ratio 4 blade prop. Interestingly it had a 2.57-1 gear ratio. Just number of teeth I guess.

I could probably use a lower gear (3-1 or more) and swing a considerably larger prop. Willard choose an 18" dia prop. Very deep ratios I think unload much quicker (that is w less rpm drop) than smaller and faster props. That's why OBs and especially small OBs have very small props. They don't know what boat the engine will be required to push so they must have a combination that won't allow the engine to over rev or over load too much. With a small engine in a heavy disp trawler w a deep gear ratio backing down while entering a slip basically will need almost full rpm to slow her down. With a 4-1 gear one would need to run at a higher rpm (all other things being equal) to load the engine the same as w a higher geared boat w a smaller faster prop. Summing up ... deep gearing is not very flexible. And basically you'll only get more efficiency at full load/speed.
 
manyboats said:
Marin your'e thinking GB as I think the 2.57-1 ratio is much more common.
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I don't know if Borg Warner made custom gear ratios for specific manufacturers or not. I know of other boats than GBs that use the same ratio transmissions as our boat. So I suspect the transmissions in our boat are off-the-shelf units offered by BW at the time.

Single engine GB36s used the 2.1:1 ratio transmission.
 
Mark, a little tail has never hurt anyone.:D

img_123116_0_3c9cb266c86a634f4d2808a68472fb67.jpg
 
markpierce said:
The hull-speed Coot's four-blade propeller seems well matched with the 2400 RPM 80-horsepower JD engine (don't recall the gear-reduction ratio). ...
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It's a 2:78:1 ratio ZF transmission
 
psneeld said:
I know on helos...the reason for more/shorter blades varies but at some point all aircraft propellers/rotors have to watch that the speed of the tips don't go supersonic as it totally messes up the aerodynamics.
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I don't know if they are getting supersonic but I've ran airboats that have had electronic ignitions so they could be easily and precisely governed because of prop tip speed issues. I do know that if they went higher than spec'ed the noise changed significantly. Made a sound like it was "beating" the air, if that makes sense.
 
twiisted71 said:
I don't know if they are getting supersonic but I've ran airboats that have had electronic ignitions so they could be easily and precisely governed because of prop tip speed issues. I do know that if they went higher than spec'ed the noise changed significantly. Made a sound like it was "beating" the air, if that makes sense.
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That's what I was taught in aerodynamics...

The whop whop of a Huey 2 blade is them approaching supersonic..that's why the faster and more maneuverable Cobras went to shorter 4 blades.

.here's another tidbit...

[FONT=arial,helvetica]NACA research during the mid-1940s did show that supersonic prop-driven aircraft were feasible, but the key was developing propellers that could operate at or near the speed of sound. NACA began a high-speed propeller development program that culminated in the supersonic propeller, but this kind of prop is substantially different from those used on the fastest propeller-driven aircraft of the day, like the P-51 Mustang. Beyond speeds of Mach 0.9, the prop blades must become significantly shorter and thinner and the blade angle (the angle of attack of the blade) must be decreased compared to blades used on previous aircraft. An example is the test propeller mounted to the nose of an XF-88 research aircraft. [/FONT]

[FONT=arial,helvetica]The reason a supersonic prop looks so different from standard propellers is because of shock waves that form on the propeller blades as the aircraft approaches Mach 1. Even before the plane reaches the speed of sound, portions of the blades do reach or exceed Mach 1 creating "bubbles" of supersonic flow. As we discussed in a previous question on supercritical airfoils, these supersonic regions are accompanied by shock waves. These shock waves not only greatly reduce the efficiency of the propeller, but create tremendous forces that can tear the prop apart. Making the blades shorter and thinner and reducing the blade angle effectively makes the blades supercritical meaning these regions of supersonic flow form at higher speeds than is possible with standard props. By delaying the onset of shock waves and reducing their strength once they do form, the supersonic prop can operate at higher speeds. [/FONT]
 
This is one of the cool things about this site. Loads of experience from widely differing backgrounds.

Ii just wish some were more open-minded, and would acknowledge that everything they did isn't necessarily the only way things can be done (and while not widely practiced sometimes neither is wrong).
 
My back ground is with offshore powerboats As the speeds have increased with some boats over 200mph and prop blade pitch upwards of 40'' The cast blades have become unreliable, slinging blade tips and complete blades off at high speed . A few small shops now make forged props with cnc finishing. Of course this has nothing to do with out 10kt trawlers Thankfully we do not have to spend $15000 for a set of props
 
Marin said:
That may be true for water propellers, I don't know. It is not true for air propellers.

I should add that the 2-bladed prop used on float-equipped Beavers is even wider and more square at the tip than the one pictured. And possibly longer-- I can't tell that from the photo.

Dont know about planes but your post reminded me of a conversation i had with a mercrusier engineer in thier prop department. I asked almost the same question in regards to props, 4/ 3 blade and blade width. His answer was basically the higher the operating rpm the narrower the blades for best efficiency. Function of total prop load, rpm, design speed.
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