Propeller question

manyboats said:

I looked at several boats (lusted over) on the internet that had 4-53 engines.

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I hope your lusting was satisfied when you inked the papers on your Willard. I have lusted over them from the first pic I saw of one, and yours is the sharpest looking one I can remember. It also sounds like the builder actually sat down and figured out a proper drivetrain (like most sailboat Co.s do). And as a result you likely have one of the more efficient powerboats around.
Do they make a 4x'er. Does your boat have ballast? I love its profile and rounded chines. It seems like a relatively small set of flopper stoppers and paravanes would make it rock steady. Shrimping/oystering luggers and tugboats are the only boats with rounded chines that I have personal experience with, but my next boat will have a displacement hull. After seeing how a 42 KK and numerous sailboats react to the same wakes as our boat, I'm ready for a gentler ride

Marin said:

That's exactly what the prop shops said. But, they said, it would be a bitch to balance.

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There are so many variables though, you'd have to lock down all the parameters of the props operation to determine what the best design approach would be.

Odd-numbered, high pitched, scimitar blades are extremely efficient (and quiet).



Even bladed scimitar props have been used on aircraft as well. This is one off an E-2C variant. Lots of thrust, hugely efficient.



The problem is the medium as well. Air being compressible and water not. You can REALLY get some heated discussions going on props.

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). There's no audible or tactile vibration despite the thick keel, but there is a bit of visible shaft vibration between 1900 and 2100 RPM. Usually run at 1600-1800 (a knot below hull speed) or 2200 (hull speed) RPM.

71,
Thanks much for the compliments on Willy but I've seen other Willards w $1000 per foot paint jobs and SS window ports. Mine looks like an old fishboat by comparison but I'm actually a little more comfortable w a boat that's a little rough. And lots of upgrades is just for lack of money. We haven't sold our Alaska house yet so we're in a "heads above water" maintenance lifestyle at the moment.

If you were on the Willard web site you'd see an extensive conversation frequently very critical of Willard about how they mounted intermediate prop shaft brngs. And the W30 could be much more efficient. They have two tons of ballast and the squared and chopped off "deadwood aft that don't help efficiency at all. Could use a bigger prop (there's room), take out the ballast and streamline the end of the keel and the efficiency could double. With a 23 hp engine I would bet on 1/2 gal per hour at 6 knots. One could make it 12' wide w very very little fuel burn penalty.

But most (I suspect I'd be included) would rather have the W30 as she is except for the keels TE. And there are times I wish I had Paravanes but it's not that often. The ballast gives her a fairly gentle motion. Nothing like that fishboat I keep posting though. Oh most sailboats have a V drive and I don't consider that a proper drive at all. Actually two Willard models have a V drive. Not for me. I think a boat should be designed to carry it's weight amidships or aft but not both. And I think that's one of the wonderful things about most inboards .... they carry their weight centered in their hulls .. where it belongs in my opinion. You can design a boat to be basically balanced but it won't ride as well as a boat w her weight amidships. I digress.

manyboats said:

71, They have two tons of ballast and the squared and chopped off "deadwood aft that don't help efficiency at all. .

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Do you have a pic of its stern while on the hard? What do you call deadwood? What do you have as ballast?

I have been toying with the idea of streamlining my keel when she's hauled next time. I too have a rpm specific driveline vibration, and am thinking fairing the keel end might alleviate it. 1400 is a pretty happy zone for speed, momentum to deal with head-on seas, and smoothness. 1500-1700 gives a little shimmy and if taking a longer ride I like the speed/rpm/feel at 1700.

I know that lightness= better efficiency, but I like the feel of a heavy boat. Thankfully my Mainship has a fine entry which makes it handle headseas as if it had much more mass.

Don't know why you Willard fans keep pissing about with those little 30' boats the company turned out. )) Here is a REAL Willard, one you can really go places in. We know the previous owners of this 60' boat. They bought it a number of years ago to live on after selling all their land to buy it plus using their previous 45' boat as part of the payment. The Willard is surprisingly old-- IIRC it's a 1970s boat.

The Willard then proceeded to cost them an arm and a leg to fix leaking fuel tanks and structural hull problems. In the end they sold it to a couple who take it to Alaska every summer and it lives in Bellingham marina for the winter, which is where it is currently. I've been on it once and it's a beautiful boat. But it is definitely a hole into which one pours large amounts of money. I have no idea how many of these things Willard turned out but it's a monster.

Very Very few Marin. But several. All of us Willardites are aware of them but that's about it. The "real" Willards are 30', 36' and 40'. I'm guessing that about 80% of Willards are 30' boats. There are 5 different models in the 30' lineup. The odd one is a sailing version called the "Eight Ton" w an extended keel and a low fore and aft trunk cabin typical of other sailboats. Then there's the Voyager, the wide body that I wish I had. The Nomad that I do have. The Horizon that has low aspect ratio sails and sails downwind and a little more. Last and saltiest is the Searcher w it's small raised pilothouse and low aft cabin. They all have concrete and steel punchings for ballast coming in at 4000lbs.

In the first pic see the flat spot/plane just ahead of the prop as though the keel was cut off with a big saw. Much turbulence rolls of the sides of the keel and tumbles around the flat and very abrupt end of the keel. Very hydrodynamically dirty.

The second pic is of the whole hull from the side and it is very clear how the stern of a full displacement hull returns the water back to the surface. It just smoothly flows up the steeply raked stern section back to the surface. Stand on the stern underway and look at the wake. The water at the stern shows very little boiling action. It just flows up from below w/o trying to follow the boat. Square sterns w considerable submerged transoms have a wake whereas the water is dancing up and down dancing in a crazy way rolling out from the bottom that is trying to keep it down. All that action of the water is energy lost making a mess of the water and serving no useful purpose. Full displacement sterns gently guide the water back to where it came from w very little disturbance. Very little energy lost.

The third pic is showing the cross sectional shape. As you can see it's the traditional "wine glass" section. But to a greater extent than most DF hulls the W30 has a part of the bottom about half way between the sides and the keel that is close to flat. This resulting shape gives the W30 a degree of stiffness that most FD hulls lack. This element of the hull design and the ballast is probably the reason I'm not crazy to get Paravanes.

In General:

More blades = less required prop diameter; such as is used where high hp engines need more blade surface but larger diameter three blade just won’t fit. Lower number of blades = higher efficiency... if you can maintain enough prop diameter to keep ample bite. One blade is best (but, impossible to balance), two blades is the most efficient useful design but very difficult to keep from vibration in large diameter at high rpm, plus the diameter needed for two blades usually precludes installation due to limited under hull space in coordination with required shaft-to-hull angle. High hp engines now often employ 5 or 6 blade props due to 2,000 ++ hp in a 40 to 45' boat that can’t allow enough diameter on a three blade prop. Also, the more blades on a well balanced prop = less vibration encountered at higher blade rpm.

Due to a plethora of mathematical calculations required to match the coordinates of hull design, trany reductions, hp/torque features at various rpm of specific engines, shaft angles under hull, and other items it is required that experts be employed to calc the best prop size and design in every instance. Guesses, although possibly coming close, simply will not result on the absolute best available match-ups for prop, to reduction ratio, to hull design, to horsepower, to standard usage parameters.

Our 34’ classic 1977 Tollycraft tri cabin has good condition twin 255 hp gas engines, 1.5 reduction, and straight shaft... it twists well balanced and clean 17 x 16 three blade props (I call them my “little cutters”). As a clean bottom, hard chine planing hull with 33” draft, 11 degree deadrise, and loaded weight of 21,000 lbs she performs well in exact engine synchronization with no vibration at hull speed (7 to 7.58 knots), planing cruse speed (16 to 17 knots), and WOT (21 to 22 knots). These props were mathematically designed by Tollycraft Corporation engineers in coordination with outsource engineers to match this boat’s range of requirements. I am completely satisfied with performance rendered.

I Recommend Rereading 2nd Paragraph: That’s the best way to proceed, and minimal of cost with greatly reduced aggravation compared to repeat “try and fail” prop design attempts to find the correct propeller!

If you look at a modern-day nuclear submarine you'll see that they typically have 7 bladed props with a high EAR ratio and it's all about being smooth and quiet - and they don't have the same power issues because they have seemingly unlimited power due to the nuclear powerplant and they can simply put on a larger prop due to hull design - i.e. it's a submarine.

If you look at a 1970's to 1990's era unlimited hydroplane running the big Allison and/or Rolls Merlin engines they almost always ran a 2-bladed chopper prop as it was all about getting the power to the water and they could not care less about noise.

Other factors are not only the shape of the back of the keel (squared off or tapered/rounded) but also the distance from the prop to the keel as well as the distance from the prop to the bottom of the hull directly over the prop - all of which will add or decrease the harmonics or vibration.

I am rebuilding a 38' full displacement trawler with a 13'6" beam and 4' draft. I have a single 3160 Cat N/A 210hp on a 2:1 TwinDisc gear with a 2" shaft. The current prop is a 24"x16" LH 3-blade. As I am doing a 3'6" hull extension (underneath the swim step so most people won't even notice but I'll pick up 500 gallons of tankage as well and incorporating a stern thruster) I will be increasing the distance from my bow wave to the stern wave and should pick up a little speed and efficiency, thus I'm putting on a 24"x19" LH 4-bladed prop. I'm hoping I'll still be able to hit my max RPM of 2,800 (so I don't burn my valves) but I can have the prop re-pitched down to a 24"x17" (about 10% is all you can change pitch) if I need to in order to tune it to the new hull length, etc.

I do not agree with the statement above that you can over prop and don't need to be able to reach max RPM - and I know some very, very good diesel mechanics that agree as well as the area Cat dealers. Most will agree that you need to be able to hit the max RPM at full throttle and then that further you should run the diesel at or close to 75% speed (i.e. 2100 RPM) about 75% of the time. On my boat we tend to run between 1,800 RPM (which is what they run the same engine at when it's powering a generator) and up to 2,100 RPM. We'd frequently run the engine up to 2,200 to 2,400 RPM the last 15 minutes or so of any long runs we made just to "clean things out" a bit.

I do agree with reading up on everything you can get your hands on regarding the prop sizing and design issues and I also agree with calling and/or email Michigan Props and getting their expert input.

In my case I figure that no matter what and even if I don't like how the 4-blade works then I can always put my 3-blade back on and toss the 4-blade in the bilge as a backup/get-home prop (which is what I'll do with the 3-blade if the 4-blade does work out as I hope it will).

Just my two cents and a little food for thought.

Good luck.

Engineers say to prop for WOT at the rated rpm. Most good propeller people will say the same thing.

Mostly its the folks that mfg or convert the engine for marine use, as any operation below WOT results in the engine being loaded very lightly , so should last the warentee period.

You might want to curl up with a propeller curve.

The "prop" curve is a theoretical, math produced, ideal guesstimate at what a non exstant prop would absorb in power.

It is very useful for anyone that does look , as one can easily see the lack of thrust,(hp absorbed) at low cruise RPM.

Hence the need for a "cruising " prop, a CPP or a 2 speed tranny for genuine efficiency.

It is doubtful that even a 30% increase in efficiency would be worth the cost or effort for a 200 hour a year vessel.

So we get "trawlers" set with 2400rpm props that cruise at 1800pm ,

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.

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.

FF said:

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.

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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.

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.

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 ?

motion30 said:

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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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.

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.

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.

twiisted71 said:

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.

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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

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.

manyboats said:

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.

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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.

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?

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.

twiisted71 said:

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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I don't know. I do know that the Hughes (now Boeing) NOTAR (no tail rotor) helicopters are extremely quiet.

manyboats said:

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.

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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.

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.

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.

SS,
Wow that looks like an outrageous bird.

manyboats said:

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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Believe what you want, but in the case I just cited your propeller "theory" doesn't hold any water.

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.

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.