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Old 12-28-2012, 06:55 AM   #41
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If cost were no issue the ultimate would be to have a variable pitch propeller so you could feather your throttle and your prop to get the perfect match. You don't see that much on the typical 40' trawler.
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Old 12-28-2012, 10:41 AM   #42
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where with a better prop choice (and some knowledge and an EGT) an almost silent 1300-1500 cruise would be far more enjoyable and the more heavily loaded engine would be more efficient. and longer lasting.
This is exactly what I was getting at. I'm not concerned with what the boat can do at settings I don't run. The noise difference between 1400 and 1800 is not even close to a resultant speed increase. And I like a peaceful cruise, I was also surprised at the noise difference on the flybridge with the bimini down and not reflecting the wake and exhaust noises back down onto me.
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Old 12-28-2012, 11:16 AM   #43
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WOW ..... Valhalla where have you been?

Judging from your bow color you've been cruising some distance .. probably distances.

I like your post #39 a lot and agree w most everything except I think you may have meant cruising at 75% load instead of 75% rpm and all the Monk boats I've seen are semi-disp. Did your 75%/75% trickle down from Steve DeAntonio of PMM? I cruise at 75% rpm but it's only about 55% load.

I think you'll be considerably overpropped going up 3" in pitch and w a big increase in blade area. Before you spend any time and money on that one I'd do more research. Could be fine if you are trading a big wide 3 blade for a 4 blade w smaller and higher aspect ratio blades. But if ther'e both typical props ..........

Stern thruster is good.

Where have you done all that cruising?

Don't slow down your posting. We need guys like you.
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Old 12-28-2012, 11:49 AM   #44
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I have read that a one blade propeller is most efficent many times, what I find interesting is that the trend has been 4,5 even 6&7 blade props on everything from 150mph offshore racers to 8kt trawlers I wonder why that is ?
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Old 12-28-2012, 12:16 PM   #45
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I have read that a one blade propeller is most efficent many times, what I find interesting is that the trend has been 4,5 even 6&7 blade props on everything from 150mph offshore racers to 8kt trawlers I wonder why that is ?
One reason is space available for the prop relative to HP and thrust. Put another way: picture a classic muscle car with a 450 hp engine. It comes from the factory of tires with a 7" wide tread pattern and every time somebody wants to "run'em" it makes lots of noise and smoke but not much go for that much oomph. He can't go much taller on the tires due to fender wells, so he goes wider getting more bite on the blacktop. This is a gross over generalization, so don't start picking it apart with physics. Up to a point more blades is like more tread. It may not be the most efficient but its the only way to be able to put the power to use effectively.

Ever wanna see something that flies in the face of convention check out an Arneson surface drive. Theoretically it should produce horrible vibes and cavitation. Its the speed at which its designed to run that makes it work.
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Old 12-28-2012, 12:48 PM   #46
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Motion 30,

Mostly I think it's to be able to use narrow high aspect ratio blades. They are very much more efficient.

Short stubby winged aircraft are terribly inefficient and all sail planes (soaring) have long skinny wings. A light compressible fluid is different than a heavy non-compresible fluid but strong similarities remain or exist. Helicopters and Gyrocopters won't fly on low aspect ratio wings.

The Scimitar prop was (I believe) developed for ultra quiet operation. I saw and heard (barely) a 7 or 9 blade propped stealth airplane fly that had a big exhaust muffler on it's side and it was unbelievable how quiet it was.

The efficiency of these many blade high aspect ratio propellers whether in water or in the air comes to pass as a result of vastly reduced tip losses. That's one of the big things making the noise. The air (or water) rushing around the tips of the blades to slam down on the back side of the propeller ruining much of the lift or thrust. A propeller that has a very wide tip blades w a lot of the blade at full prop diameter will spill lots of fluid around it's tips robbing huge amounts of thrust. The higher the propeller loading the higher the losses. But w too little blade loading the friction of the water moving across the blades becomes the greater loss. You'll notice the very efficient blades are very pointed w very small tips to minimize the tip losses and keep the air/water separated from each side of the blade.

For our trawlers we're more interested in smoothness and utilizing the restricted space given for the propeller. We're more concerned about strength, repairability, economy and user friendliness than ultra efficiency. Also water being not compressible dosn't require as much diameter as the aircraft. My little prop for Willy was most of $1000 for a basic 3 blade in bronze. And significant chunks of wood can pass through those 3 blades with out damage. I like that.
And there are many other reasons why we don't have extreme propellers.
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Old 12-28-2012, 02:05 PM   #47
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Eric / Manyboats
I have only cruised in my own boat from Oregon to better than half way up Canada but most of my cruising has been Puget Sound and the San Juan Islands. As for the color on the bow, don't know.

I agree that many of the Monk designs are semi-displacement, but mine is full displacement. My hull was bought by my father from Ron Rawson in 1972 in Redmond, WA. They dropped a brand new 3160 in it and trucked it home to Olympia where he and I spent 2-1/2 years building the rest of the boat from the hull up (I was a teenager at the time). I purchased the boat from my father several years back. And yes, at 79 years old he's still boating, has two boats that he uses for salmon fishing, one for the bay and one for the ocean.

Given that I have a full displacement hull and can't really go much past hull speed and given that the loading on a boat engine is like driving a loaded truck uphill all the time I did mean 75% rpm. And no, my info initially came from a guy who started as a diesel truck mechanic and ended up managing a huge national trucking company (a high school buddy of my father's who still, at 80 years old, helps my father tinker on his boats (one diesel I/O and one gas OB), but I do agree with what Steve De Antonio of PMM says.

As to my prop pitch, I'm a smidge on the light side with 24x16 and probably need 24x17 right now. Given that I'm extending the hull under the swim step I'll pick up a little speed and thus should be able to get by with a little stiffer prop.

As for cost, I try to avoid buying new/retail. I bought the 24"x19"x2" LH 4-blade Nibral prop in mint condition (supposedly carried as a spare because it was not quite the right pitch for the guy who had it) at a swap meet for $250.

As for thrusters, I'm adding a bow and stern thruster, dual prop 12 volt units that I might convert to hydraulic someday. I hope to move aboard when done with all of this and live out my days on the water while chasing the sunshine.
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Old 12-28-2012, 03:33 PM   #48
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What I'm trying to say is that on low speed boats, while important its not as critical to have the "perfect" prop. I would much rather my trawler speed boat be at its Tq peak or PDC at the top of my cruising speeds. I.E. 1600 RPM at 8.5-9 kts. I don't care if it will turn 2800 with that prop/gearing configuration, its not where I operate my boat.

Let the flaming begin.....its my boat and my opinion based on my experience, not some industry concocted blanket rule.
your comment reminded me something i used to do with IO powered boats which i used to work on. I found that slightly over proping a boat would result in better speed and fuel economy at mid rpm than following the manufacturers recomendation for prop selection.
But this was with IO powered gas engine boats not displacement diesel powered vessels
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Old 12-28-2012, 04:14 PM   #49
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Same w gas, IO, OB and my ultralight aircraft. Propping to rated engine speed is just as correct in all these places. My ultralight and OB would be dogs if I over propped them. There is only something to gain if your vehicle is overpowered. And even then the price is too much to pay.

71,
I'm not say'in it's critical Or that you "need" to have the perfect prop ...... Only that that's the only way to do it correctly. I've often gone outside "perfect" practices and been better off but on this one I've got my goat feet firmly planted on the ground. And I'm not being more critical of you other than to say what I think.
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Old 12-28-2012, 07:56 PM   #50
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A propeller that has a very wide tip blades w a lot of the blade at full prop diameter will spill lots of fluid around it's tips robbing huge amounts of thrust.
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.

You hardly ever see them anymore (Kenmore Air Harbor used to run them in the 70s) because the tips ran just under the speed of sound at takeoff and the props were absolutely deafening. I've flown a Beaver with one of these props and the yowl was ear splitting.

They were far better for takeoff than the shorter, narrower, less efficient three bladed props used on Beavers today and operators hated to give them up. But noise complaints have killed them off except for very remote areas where some operators still use them.

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Old 12-28-2012, 08:25 PM   #51
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Marin the old bell 212/412s also had pretty blunt blade ends.

I don't know the designations to look them up but those "black" choppers that are so quiet, Do they have different blade shapes or are they just quieted by shrouding the tail rotor?
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Old 12-28-2012, 08:27 PM   #52
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Yes it is Marin. There are other elements that enter into it on a regular basis like blade area w an acceptable diameter. Wide tip props are frequently a trade off. But as far as efficiency goes a higher aspect ratio and rather pointed tips is almost always better a better option. I'll bet those props on the transport were absolutely limited in diameter and had no choice but to go to more blades (and the prop tips were probably too close together for more blades) so a wider tip was needed to absorb the power and those big turbo's have a lot of power. It's a world of compromise.
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Old 12-28-2012, 08:48 PM   #53
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I don't know the designations to look them up but those "black" choppers that are so quiet, Do they have different blade shapes or are they just quieted by shrouding the tail rotor?
I don't know. I do know that the Hughes (now Boeing) NOTAR (no tail rotor) helicopters are extremely quiet.
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Old 12-28-2012, 09:02 PM   #54
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Wide tip props are frequently a trade off. But as far as efficiency goes a higher aspect ratio and rather pointed tips is almost always better a better option.
I don't know. All I know is that a Beaver with that long, wide, squared-off tip "water" prop got off the water and climbed like a rocket even with a full load compared to a Beaver with the shorter, narrower, round tip, three-bladed prop they all use today. And the plane was faster (but not by much: we are talking a float-equipped Beaver here ) than it is with the three-blade prop. As I say, operators hated to give up that big two-bladed water prop.

The three-blade prop offers only two advantages, neither one of them related to performance or efficiency. The shorter blades are farther off the water and so are not as susceptible to erosion from spray on takeoff, and since the individual blades are shorter their tips are going slower at takeoff rpm and so are much quieter.

But for a given distance a two-bladed Beaver goes faster and burns less fuel than a three-bladed Beaver.

One can talk about theoretical formulas all one likes, but at the end of the day how well the plane (or boat) gets the job done is all that matters.
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Old 12-28-2012, 09:44 PM   #55
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Marin I'm not talking about "theoretical formulas".

I'm talking about standard run of the mill aerodynamics. There are no exceptions to physics as applied to aerodynamics.
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Old 12-28-2012, 10:21 PM   #56
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Prop tips are a trade-off in power versus tip speed. With a fast enough head speed and relative speed, helos start to suffer from tip-burble... small sonic booms that happen when the tips exceed supersonic speeds. They have helped this a great deal by raking, draping and tapering the tips.

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Old 12-28-2012, 11:08 PM   #57
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SS,
Wow that looks like an outrageous bird.
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Old 12-29-2012, 12:23 AM   #58
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Marin I'm not talking about "theoretical formulas".

I'm talking about standard run of the mill aerodynamics. There are no exceptions to physics as applied to aerodynamics.
Believe what you want, but in the case I just cited your propeller "theory" doesn't hold any water.
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Old 12-29-2012, 06:32 AM   #59
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The larger diameter prop requires more depth to the vessel.

This added draft might easily cause more groundings or limit the cruising areas that are easily transited.

In FL many folks wander over to the Bahamas , loads easier to visit with 3 ft draft than 6 ft.
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Old 12-29-2012, 06:42 AM   #60
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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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