Propeller question

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bfloyd4445 said:
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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Air propeller length is governed by the tip speed. Assuming conventional propeller design, the tip speed cannot exceed the speed of sound. If it does it introduces the potential for structural problems.

The slower the propeller turns the longer it can be as witness helicopter rotors. For prop planes, given the blade rpm limit the only variable left with regards to developing thrust is blade width and blade design. This is why planes with a lot of power, like the WWII Corsair, had big fat blades on them. The blades still had to turn at about the same speed as the blades on a Cessna, but they could be made way broad to take advantage of the tremendous horsepower available to turn them so they could develop a massive amount of thrust.

The deHavilland Otter was designed for an 800 hp engine that never materialized. So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp. This was the only way enough thrust could be developed to get the Otter off the ground and climbing with a full load.

How all this-- tip speed and blade width-- relates to marine propellers I don't know since the medium a marine prop is running in is not compressible where the medium an air propeller is running in is. So it's probably apples and oranges.
 
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Marin said:
Air propeller length is governed by the tip speed. Assuming conventional propeller design, the tip speed cannot exceed the speed of sound. If it does it introduces the potential for structural problems.

The slower the propeller turns the longer it can be as witness helicopter rotors. For prop planes, given the blade rpm limit the only variable left with regards to developing thrust is blade width and blade design. This is why planes with a lot of power, like the WWII Corsair, had big fat blades on them. The blades still had to turn at about the same speed as the blades on a Cessna, but they could be made way broad to take advantage of the tremendous horsepower available to turn them so they could develop a massive amount of thrust.

The deHavilland Otter was designed for an 800 hp engine that never materialized. So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp. This was the only way enough thrust could be developed to get the Otter off the ground and climbing with a full load.

How all this-- tip speed and blade width-- relates to marine propellers I don't know since the medium a marine prop is running in is not compressible where the medium an air propeller is running in is. So it's probably apples and oranges.
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I think the same laws of physics apply to both so explanation is valid. The mathematical calculations if one takes into consideration of all the variables will be the same.
 
bfloyd4445 said:
I think the same laws of physics apply to both so explanation is valid. The mathematical calculations if one takes into consideration of all the variables will be the same.
Click to expand...

I hope you aren't suggesting that aerodynamics and hydrodynamics use all of the same physical principles....if so...not even remotely close....

here's a website clip....

Many complex flow problems are common to aerodynamics and hydrodynamics as they relate to fundamental phenomena such as turbulence, separation, and wake vortices. The same digital models (Euler, Navier-Stokes, etc.) and testing resources (wind tunnels, hydrodynamic tunnels) are therefore often used to handle subsonic aerodynamic and hydrodynamic problems. However, aerodynamics and hydrodynamics differ in their specific effects.

The particularities of hydrodynamics:

  • high fluid mass (gravity and inertial interaction between the fluid and structures),
  • presence of a free surface (diffraction-radiation of the swell by obstacles),
  • existence of two phase mixing (cavitation phenomenon in particular).

In aerodynamics, particularities relate to high speeds:

  • effects of air compressibility (shock waves),
  • thermal effects,
  • physico-chemical effects at high temperatures (combustion, hypersonic flows) and high altitudes (rarefaction).
 
psneeld said:
I hope you aren't suggesting that aerodynamics and hydrodynamics use all of the same physical principles....if so...not even remotely close....

here's a website clip....

Many complex flow problems are common to aerodynamics and hydrodynamics as they relate to fundamental phenomena such as turbulence, separation, and wake vortices. The same digital models (Euler, Navier-Stokes, etc.) and testing resources (wind tunnels, hydrodynamic tunnels) are therefore often used to handle subsonic aerodynamic and hydrodynamic problems. However, aerodynamics and hydrodynamics differ in their specific effects.

The particularities of hydrodynamics:

  • high fluid mass (gravity and inertial interaction between the fluid and structures),
  • presence of a free surface (diffraction-radiation of the swell by obstacles),
  • existence of two phase mixing (cavitation phenomenon in particular).
In aerodynamics, particularities relate to high speeds:

  • effects of air compressibility (shock waves),
  • thermal effects,
  • physico-chemical effects at high temperatures (combustion, hypersonic flows) and high altitudes (rarefaction).
Click to expand...

What i meant was that the same physical laws of nature that apply to an airplane propeller also apply to a boat propellar. Guess thats why they are both called propellers?
 
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bfloyd4445 said:
What i meant was that the same physical laws of nature that apply to an airplane propeller also apply to a boat propellar. Guess thats why they are both called propellers?
Click to expand...

I guess the article pointing out that not ALL the same physical laws apply is slipping by you? :eek:
 
psneeld said:
I guess the article pointing out that not ALL the same physical laws apply is slipping by you? :eek:
Click to expand...

I'm sorry, i guess my comment that propellers all must conform to the same laws of science and physics passed by you?
Each propeller has different design criteria but all have to obey the same laws of science. Because of the viscosity of the operating medium some factors will have a lessor effect upon the airplane prop than a boat prop but they will still be there and will effect both props. Engineers often develop what they call approximations for specific applications to simplify calculations. What you are used to seeing are these approximations so you conclude that each are governed by different laws of science and that is not true. What is true is that some factors have more of an effect upon a boat prop than one for aircraft but they are still both props and must conform to the same laws of science.
 
Marin said:
So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp.
Click to expand...

A gearbox doesn't increase horsepower, it increases (or decreases) torque and rpm proportionally, the horsepower remains the same.


P&W made the 600 hp 1340 in geared and direct drive versions. The extra 50hp above the standard 550 hp rating came from an additional 50 rpm at take-off and a higher speed blower.
 
There are more similarities than dissimilarities re the two fluids .. air and water.

Re Ricks post I thought surely Marin couldn't have made that mistake. I read back and see that he actually did. Got him fair and square Rick. He said it but I'm sure he knows the difference ... pretty sure. Marin are you taking advantage of the new state law about smoking?
 
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The difference between a 5' diameter and a 25'' diameter prop are HUGE .a Trawler prop is mostly likey turning slower as well. No issue with blade tip speed
 
RickB said:
A gearbox doesn't increase horsepower, it increases (or decreases) torque and rpm proportionally, the horsepower remains the same.
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You're rignt, of course. The horsepower of the engine itself remained the same. But by increasing the torque available the gearbox made the powerplant seem like it had more horsepower than it did. The same propeller fitted to the geared engine could have been swung without a gearbox by a more powerful engine.
 
manyboats said:
Got him fair and square Rick.
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Whoa, wasn't trying to "get" anyone, just clarifying a point so that other readers would not get the idea that red gears increase power.
 
manyboats said:
Marin are you taking advantage of the new state law about smoking?
Click to expand...

What does the law say? I'm just finishing up my second week of E-Cigarettes. (So far...so good!)
 

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Washington, Colorado and one or two other states in this last election made marijuana possession and use legal. Not just medical use, recreational use. So a person can possess some ounce limit legally and smoke it subject to the cigarette smoking regulations in place now. There are also some new driving under the influence of marijuana rules for the police to enforce.

Of course the growing, sale, and possession of marijuana is still illegal from a federal aspect. So the whole thing has still got a long way to go to reach some sort of resolution.

The advice being given in this state is don't smoke it in a public place and don't smoke it on federal land (National Parks, etc.).
 
Marin,

My reference to your smoking was another failed attempt at humor. My humor is poor or much too subtle or perhaps you're a very serious person.

Sorry

Eric
 
Marin said:
Washington, Colorado and one or two other states in this last election made marijuana possession and use legal. Not just medical use, recreational use. So a person can possess some ounce limit legally and smoke it subject to the cigarette smoking regulations in place now. There are also some new driving under the influence of marijuana rules for the police to enforce.

Of course the growing, sale, and possession of marijuana is still illegal from a federal aspect. So the whole thing has still got a long way to go to reach some sort of resolution.

The advice being given in this state is don't smoke it in a public place and don't smoke it on federal land (National Parks, etc.).
Click to expand...

what does this have to do with prop selection??????:dance:.....isnt this called trolling?
 
No, it's the answer to the question Walt posed in post 102 which was in response to the comment by Eric in post 98. Pay attention.

Trolling is dragging one or more lures through the water in an effort to entice salmon to bite on them. The boats on this forum are not only not trawlers, they are not trollers, either, as I don't see trolling poles, gurdies, etc. on any of the boats in our avatars.:)
 
That's a fairly serious accusation Floyd. Marin's never been criticized before. I wonder how he will take that. Being so very serious an all. So you say propellers are the topic that's legal. 80 mph isn't legal on the freeway either. I really don't know what to make of the staying on topic movement. The movement part is another joke. Actually this post is about 98% a joke. But that's not on topic either.
 
manyboats said:
Actually this post is about 98% a joke. But that's not on topic either.
Click to expand...

Shame on you. Just for that you have to stay after class (a propeller class) and write 100 times on the blackboard:

"Thread creep is evil personified and I promise to never do it again unless it's more interesting to go off-topic instead of staying on it, in which case I WILL do it again."
 
Got lots of company Marin.

Going to go buy a small OB from a GBO guy (Richard) in Seattle tomorrow.
 
Art said:
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! ;)

Click to expand...
Art,
You said- "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 am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT. I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative. Greg
 
Thud said:
Art,

You said- "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 am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT. I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative. Greg
Click to expand...

Wow Greg - You pulled back a few years to get my post; i.e. 2 days after Christmas in 2012... good going!!

So... I want to be as clear as possible regarding twin engine RPM's [at different throttle settings], prop size, nmph speed and nmpg fuel usage.

RPM: I watch each engine's RPM on old-school tachometers [seeming to be the 1977 originals still working though!]. Between the four tachs [two in salon and two on bridge] there seems to be a not too accurate RPM correlation between the lot that varies from 100 to 300 RPM differences at different throttle settings. As well, I always set both engines cooperative RPM [at what ever throttle setting I'm running] on a light sensor synchronizer... as well as my ear synchronizer, which due to decades experience I feel is quite accurate. When the engines are synced into same RPM [singing a pulsating mechanical melody with one another] I feel they are then darn close to exact turns per minute compared to one another [give or take 25 to 75 RPM] Having said that - The tachs can still vary their RPM reading from 100 to 300 RPM difference; and I don't care! It's how good the well running engines feel while performing in synchronization that interests me most. Also, I do not push engines past their own running-comfort level... except... for momentary needs of testing purposes or a quickly needed must-do to get the boat away from immediate danger.

Prop Size: Prop calculations are one of the most vexing math-design problems ever encountered in power boat use as they need shape, weight, size, # of blades and shaft diameter comparisons/configurations that cooperate with a boat's hull shape, size, draft, keel, and rudder as well as a myriad of other complications regarding prop match to engine size/type/HP and reduction gear numbers utilized. In other words... IMHO... only experienced propeller engineers can come close to "THE" Correct Prop! Therefore, due to millions spent and years/decades of tests performed by good builders of boats [such as Tollycraft]... I follow the doctors orders and use the prop they recommend.

Here's a bit of info I've gleaned regarding props for 34' Tollycraft sedans and tri cabins. As you can see, there are more than one chef in the kitchen

"Mid-70's 34 Tollycraft, with 255 HP gasoline engines came with 17" x 15" props. Most latter 70’s 34 tri cabins came with props around 17x16 to 17x17. If you reduce the pitch down to 12 you will find that this will put you in the sweet spot of the performance curves. With this HP, gear reduction and 9k+ lbs of engines and trany. Any more pitch will result in poor mid range and top end performance.


1983 34 tri cabin with 270 Crusaders was originally propped 17X16. First owner reduced the pitch to 14 which let engines run free but required more RPM. We repitched to 17X15 which is much more responsive, especially while docking. We run displacement most of the time, 11 knots GPS at 22-2300 RPM, about 9 gals per hour. Will top out at 4200 WOT

Anyway - Although I've never needed our props tuned I am going by the 17 x 16 numbers the original owner told me are on our baby. His decades experienced marine technician that had cared for the Tolly for 10 years agreed.

Now, if you are still reading and have not fallen asleep... I'll answer your question and make a bit of comment. See bold inserts in your quote below.

"I am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT." - Seems pretty slow to me. How many hours on engines - you sure they are running at full power output? [B]Was bottom perfectly clean with new paint? What size props? Were all tanks full, 1/2 full or empty? Was boat loaded. partially loaded or un loaded of food, cloths, tools, spare parts? How many persons aboard? [/B]

"I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative."

1. Running on one engine at 4 to 4.5 knots we get right at 3 nmpg
2. Running on twins at 6.5 to 7 knots it's 2 nmpg [7.58 knots is calced hull speed]
3. Running on full plane at 16 to 17 knots she averages 1 nmpg
4. WOT at 21 to 22 knots I imgine [maybe] 1/2 nmpg. Never kept her at that speed but for a minute or two.

Speeds were taken at slack tide by GPS. Fuel usage was calced by full tanks that were refilled at end of run time... with appropriate math used to enter distances traveled in regard to gallons of fuel used.


Cheers and Good Luck!

Art
 
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Art said:
Wow Greg - You pulled back a few years to get my post; i.e. 2 days after Christmas in 2012... good going!!

So... I want to be as clear as possible regarding twin engine RPM's [at different throttle settings], prop size, nmph speed and nmpg fuel usage.

RPM: I watch each engine's RPM on old-school tachometers [seeming to be the 1977 originals still working though!]. Between the four tachs [two in salon and two on bridge] there seems to be a not too accurate RPM correlation between the lot that varies from 100 to 300 RPM differences at different throttle settings. As well, I always set both engines cooperative RPM [at what ever throttle setting I'm running] on a light sensor synchronizer... as well as my ear synchronizer, which due to decades experience I feel is quite accurate. When the engines are synced into same RPM [singing a pulsating mechanical melody with one another] I feel they are then darn close to exact turns per minute compared to one another [give or take 25 to 75 RPM] Having said that - The tachs can still vary their RPM reading from 100 to 300 RPM difference; and I don't care! It's how good the well running engines feel while performing in synchronization that interests me most. Also, I do not push engines past their own running-comfort level... except... for momentary needs of testing purposes or a quickly needed must-do to get the boat away from immediate danger.

Prop Size: Prop calculations are one of the most vexing math-design problems ever encountered in power boat use as they need shape, weight, size, # of blades and shaft diameter comparisons/configurations that cooperate with a boat's hull shape, size, draft, keel, and rudder as well as a myriad of other complications regarding prop match to engine size/type/HP and reduction gear numbers utilized. In other words... IMHO... only experienced propeller engineers can come close to "THE" Correct Prop! Therefore, due to millions spent and years/decades of tests performed by good builders of boats [such as Tollycraft]... I follow the doctors orders and use the prop they recommend.

Here's a bit of info I've gleaned regarding props for 34' Tollycraft sedans and tri cabins. As you can see, there are more than one chef in the kitchen

"Mid-70's 34 Tollycraft, with 255 HP gasoline engines came with 17" x 15" props. Most latter 70’s 34 tri cabins came with props around 17x16 to 17x17. If you reduce the pitch down to 12 you will find that this will put you in the sweet spot of the performance curves. With this HP, gear reduction and 9k+ lbs of engines and trany. Any more pitch will result in poor mid range and top end performance.


1983 34 tri cabin with 270 Crusaders was originally propped 17X16. First owner reduced the pitch to 14 which let engines run free but required more RPM. We repitched to 17X15 which is much more responsive, especially while docking. We run displacement most of the time, 11 knots GPS at 22-2300 RPM, about 9 gals per hour. Will top out at 4200 WOT

Anyway - Although I've never needed our props tuned I am going by the 17 x 16 numbers the original owner told me are on our baby. His decades experienced marine technician that had cared for the Tolly for 10 years agreed.

Now, if you are still reading and have not fallen asleep... I'll answer your question and make a bit of comment. See bold inserts in your quote below.

"I am purchasing a 1977 Tollycraft 34 Tri-cabin with twin 255 hp gas engines running well. The current owner reports a maximum speed of 16-17 knots WOT." - Seems pretty slow to me. How many hours on engines - you sure they are running at full power output? [B]Was bottom perfectly clean with new paint? What size props? Were all tanks full, 1/2 full or empty? Was boat loaded. partially loaded or un loaded of food, cloths, tools, spare parts? How many persons aboard? [/B]

"I am wondering what gas consumption you have and what RPMs you find at what speeds... If you can post those results I would be very appreciative."

1. Running on one engine at 4 to 4.5 knots we get right at 3 nmpg
2. Running on twins at 6.5 to 7 knots it's 2 nmpg [7.58 knots is calced hull speed]
3. Running on full plane at 16 to 17 knots she averages 1 nmpg
4. WOT at 21 to 22 knots I imgine [maybe] 1/2 nmpg. Never kept her at that speed but for a minute or two.

Speeds were taken at slack tide by GPS. Fuel usage was calced by full tanks that were refilled at end of run time... with appropriate math used to enter distances traveled in regard to gallons of fuel used.


Cheers and Good Luck!

Art
Click to expand...
Thank you very much. I am sure some other Tolley 34 Tri's owners besides myself will benefit from your thorough respone. BTW, I am under contract to buy this boat and will have my sea trial next week. My perfomance data came from the seller.
 
Thud said:
Thank you very much. I am sure some other Tolley 34 Tri's owners besides myself will benefit from your thorough respone. BTW, I am under contract to buy this boat and will have my sea trial next week. My perfomance data came from the seller.
Click to expand...

Greg

You going to have professional survey done? Here's a couple items to keep in mind.

1. - Wood along edge of fly bridge [take ice pick and prod around over both doors] that attaches the bridge sides to salon top [i.e. fly bridge floor] may have some [or maybe a lot] of rot. If not too much rot there are ways to "inject hardeners into the wood and also to prevent further rot increase. If too rotten the wood can be removed and replaced with new treated wood or other material types. Replacement is about a 1.5 to 2 day project. Not very expensive if done by owner... materials are minimal.

2. - Check for moisture [maybe rot] around port holes in forward head and over the 1/2 high closet. Also check around front hatch and master bedroom windows and rear small door.

3. - Check the tightness [ridged stability] of SS railings' bolted on torpedo base attachments everywhere. If there are some with slop the keepers underneath have rusted and/or the location that the base is mounted has had water ingress with fiberglass repair needed. A poor choice was made by Tolly manufacturing to use mild steel keepers as a widening base under the bolts' nut and lock washer. When installed the workers put gobs of calking over each torpedo's underneath nut, washer and keeper. After 44 years some of the calking let in salt corrosion. Especially check tightness of the torpedo's and stand pipes at entry door. Realize... when the torpedo's were installed the deck was separate off the hull. Sooo... accessing the areas needing attachment improvements can be a bear; doable with a few choice 4-letter words injected!

I mention the three items above for your own knowledge and to maybe give you some wiggle room with seller.

They're a lot of nuances to purchasing a boat. No matter how well built at origination; much of an older boat's condition has to do with amount of care that was provided by previous owner. And, were mechanical things [motors. transmissions, generator] run too hard without proper servicing??

Pretty much after that just do a generally thorough review of all items. PM me if you run into things that seem odd or for which you have a question.

Tollycraft boats were built like tanks... while still providing great comfort and decades of usability.

Get it on! - Art
 
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Art,
Again thank you. The present owner had replaced many of the stanchion bases with new ones (bought from the OEM source) and there enough new stanchion bases to redo the entire boat. He mentioned not being able to get access to all the stanchion base bolts underside. Not sure what I will end up doing for those. I want to use some new stanchions to extend the railing along the mid-ship area which is lacking any railings right now. I will pass on your “experience speaks” to my surveyor. I may come back (via PM) with another question or two. Greg
 
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