Papa_Bear
Contributor
I think this bed is too hard for you! Try the baby bed!
Apollo Bio-Fin, Atomic Smoke or TUSA Zooms, which one is the best Split Fin?
- Thread starter BORG
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
What was your major there, my very uncivil friend? Did you graduate?.
I did not study hydrodynamics, but I do know that "screw" props are quite different from typical props. A cross section of a typical propeller blade element is an airfoil comparable to a cross section of an airplane wing. One surface of the blade is chambered or curved, similar to the upper surface of an airplane wing, while the other surface is flat like the bottom surface of a wing. As in a wing, the leading edge is the thick edge of the blade that meets the air as the propeller rotates.
Now, how does a ship moving WITHOUT turning it's screws have anything to do with this discussion.
Papa_Bear
Contributor
I have to apologize for a basic misunderstanding! I agree that a lever provides lift and the same lift that puts a hilo in the air! But for propulsion it is force of forward motion (lift) or leverage that displaces water and forces a boat forward! I was not using Lift as it was intended to be used! As in Aerodynamics of pressure lift! These principles although close work in very different mediums! Water being 800 times denser!
But to apply these principles to split fins is just as wrong!
But to apply these principles to split fins is just as wrong!
Was that directed at me, Reed?
Papa_Bear
Contributor
SIZING
Marine propeller sizes are always specified by diameter and pitch (diameter x pitch) with the diameter dimension specified first. For example: a 15 x 12 propeller has a 15" diameter and a 12" pitch. Normally, these are the only dimensions given in a prop size, which is unfortunate because there are other characteristics of the prop you need to consider when selecting the size. You should always start your search with the diameter and pitch of the prop first then work from there.
PITCH
Pitch is measured in inches and it is a theoretical measure of how far the propeller should move through the water in one revolution if there was no slippage at all. For example, an 18" pitch prop would move 18" forward in one revolution (provided there was no slippage).
The primary effect of pitch on your performance is the same as the gear-shift in your car. Higher pitch numbers are like the higher gears (4th and 5th) in your car, more speed but less acceleration. Lower pitches are like lower gears (2nd and 3rd): more acceleration but less top speed.
DIAMETER
Diameter is just what you think it is: the diameter of the spinning prop. You can quickly check the diameter by measuring from the center of your prop nut to the tip of one of your prop blades. Double the number you get to get your diameter. Diameter can influence the speed you get, but it has its greatest affect on your acceleration and thrust. Bigger diameter is like adding bigger tires to your car: more traction or more appropriately, less slip. Larger diameters put more load on your motor because they move more water though so don't over do it. A blade moving through water does experience drag . The less blade there is (less diameter), the less the drag will be. Provided your motor and prop produce enough thrust, when you decrease your prop diameter your speed will increase a bit because there is less drag.
NUMBER OF BLADES
AB 200 AB 300 AB 400 How many blades should your prop have? Three blade propellers are the most common, but 4 and even 5 blade props are available. The immediate benefit to increasing your propeller's blade count is increased thrust and a smoother ride (Much like adding more cylinders to your engine). Theoretically, the more blades the lower the efficiency of the propeller. However, increased blade count generally means that each blade no longer has to deal with as much horsepower and consequently the blades can be made a bit thinner which improves their individual efficiency. For example, on a three blade prop running on a 300HP motor, each blade has to handle a 100 hp each. On a 4 blade prop on the same motor, each blade only has to handle 75 HP! This allows the designer to build a prop with a thinner blade without sacrificing stiffness or strength. Unfortunately, the overall efficiency of a multi-bladed propeller is reduced when the blades are forced to run through confused waters - increasing drag. Equally, drag rises to the square (approximately) of speed. Also, a narrow blade, just like a thinner blade passes through the water with less drag. Ideally, therefore, we want a thin section, narrow twin blade, revolving as slowly as possible. Which is just not practical for most boating applications.
CUPPING
Cupping is added to most propeller blades to improve the propeller's bite on the water and decrease slippage. It is most commonly seen on the trailing edge of the blade. Usually the effect of cupping makes the prop perform like a higher pitch propeller, but it does enhance your thrust as well. In addition, cup reduces a prop's tendancy to ventilate or slip. Custom prop builders often use aggressive degrees of cupping to fix slippage problems on large diameter, low pitch props on certain boating applications.
Most props you will encounter are cupped. Most likely, you will only see an uncupped propeller in performance applications. On smaller outboards, which produce most of their power in the upper third of their RPM range, an uncupped prop with a semi-cleaver design often produces better performance. To get maximum performance, it is necessary to get the motor spinning up in this range as quickly as possible. An uncupped prop loads the motor a bit less, allowing the rpm to build more quickly. This enables you to operate your motor in the meat of its powerband where you'll get maximum power. You will notice faster hole-shot, quicker acceleration, and often times, higher top speeds with these semi-cleaver, uncupped style blades. If you are currently running a cupped style propeller and want to improve your overall performance consider trying a semi-cleaver design. Generally, when switching to an uncupped semi-cleaver style prop, you can up the pitch 2 inches and still have better hole-shot, quicker acceleration and often better top speed! Cup can also provide additional bow lift when utilized on the rake line of the prop. Applying cup to the trailing edge of the prop along the pitch line will increase the effective pitch of the propeller. A standard cup will typically result in a decrease of 200 to 400 rpm's. This usually means a decrease in pitch of 1 to 2 inches is required to run a cupped propeller in place of an uncupped wheel.
BLADE RAKE
Blade Rake represents the angle of attachment of the blade to the hub of the propeller. This is not to be confused with the pitch, which is a measure of the twist or screw progression. The amount of rake built into a propeller blade is not something many people consider when buying a new prop, but it can be just what the Dr. ordered in some cases. Rake angle isn't an immediately apparent thing to the untrained eye, but if the propeller blade is cut down the center, it is readily apparent. The biggest benefit of a high rake design is a greater resistance to ventilation. This allows you to trim the outdrive up toward the surface more without the prop ventilating as readily during tight manuevering. You should seriously consider using a high rake propeller if ventilation is a problem for you.
If you have a large boat like a house or deck boat a high rake prop may not be the best choice because it does not produce as much reverse thrust. This may reduce your ability to manuever your vessel at low speeds making your boat more difficult to dock.
Higher rake normally improves performance in ventilating or cavitating situations (high engine elevations and high trim angles). Additionally, higher rake can provide higher bow lift, which will frequently improve speed. Low rake blades are typically used on motors with propellers running fully submerged, typically carrying moderate to heavy loads. The rake angle can either be straight or the average angle of a parabolic curve.
EFFICIENCY
Not many manufacturers quote the efficiency of their propellers and even fewer customers ask. The efficiency of a propeller is defined as the power coming out of a prop divided by the power going in:
Power out
-------------- X 100 For a solar powered boat, efficiency is probably the most important statistic.
Power in
BLADE THICKNESS (thinner blades cut better)
Why is Stainless Steel the first choice for high performance props? A: It is much stronger and far stiffer than aluminum, but that's not the whole answer. The higher strength of stainless means that the blades can be made thinner and that is where the performance benefit comes from: thin blades slice through the water more efficiently than thicker blades. As a result stainless props produce a couple more mph over even the best aluminum designs.
This Design concept is part of the reason our 4-Blade propellers and our uncupped "XB" style 3-Blade propellers outperform our Standard 3-Blade Designs. By incorporating a thinner blade cross-section, those props cut through the water with less resistance. There is a compromise to be made though and that compromise is in durability. The thinner blades are faster, but they are not as durable as our thicker blade designs. If you run up and down a shallow river and hit things frequently, you may be happier sacrificing some performance to run the tougher blades. If you typically run in the ocean with little chance of a strike, you may prefer the added speed of our thinner blades.
Marine propeller sizes are always specified by diameter and pitch (diameter x pitch) with the diameter dimension specified first. For example: a 15 x 12 propeller has a 15" diameter and a 12" pitch. Normally, these are the only dimensions given in a prop size, which is unfortunate because there are other characteristics of the prop you need to consider when selecting the size. You should always start your search with the diameter and pitch of the prop first then work from there.
PITCH
Pitch is measured in inches and it is a theoretical measure of how far the propeller should move through the water in one revolution if there was no slippage at all. For example, an 18" pitch prop would move 18" forward in one revolution (provided there was no slippage).
The primary effect of pitch on your performance is the same as the gear-shift in your car. Higher pitch numbers are like the higher gears (4th and 5th) in your car, more speed but less acceleration. Lower pitches are like lower gears (2nd and 3rd): more acceleration but less top speed.
DIAMETER
Diameter is just what you think it is: the diameter of the spinning prop. You can quickly check the diameter by measuring from the center of your prop nut to the tip of one of your prop blades. Double the number you get to get your diameter. Diameter can influence the speed you get, but it has its greatest affect on your acceleration and thrust. Bigger diameter is like adding bigger tires to your car: more traction or more appropriately, less slip. Larger diameters put more load on your motor because they move more water though so don't over do it. A blade moving through water does experience drag . The less blade there is (less diameter), the less the drag will be. Provided your motor and prop produce enough thrust, when you decrease your prop diameter your speed will increase a bit because there is less drag.
NUMBER OF BLADES
AB 200 AB 300 AB 400 How many blades should your prop have? Three blade propellers are the most common, but 4 and even 5 blade props are available. The immediate benefit to increasing your propeller's blade count is increased thrust and a smoother ride (Much like adding more cylinders to your engine). Theoretically, the more blades the lower the efficiency of the propeller. However, increased blade count generally means that each blade no longer has to deal with as much horsepower and consequently the blades can be made a bit thinner which improves their individual efficiency. For example, on a three blade prop running on a 300HP motor, each blade has to handle a 100 hp each. On a 4 blade prop on the same motor, each blade only has to handle 75 HP! This allows the designer to build a prop with a thinner blade without sacrificing stiffness or strength. Unfortunately, the overall efficiency of a multi-bladed propeller is reduced when the blades are forced to run through confused waters - increasing drag. Equally, drag rises to the square (approximately) of speed. Also, a narrow blade, just like a thinner blade passes through the water with less drag. Ideally, therefore, we want a thin section, narrow twin blade, revolving as slowly as possible. Which is just not practical for most boating applications.
CUPPING
Cupping is added to most propeller blades to improve the propeller's bite on the water and decrease slippage. It is most commonly seen on the trailing edge of the blade. Usually the effect of cupping makes the prop perform like a higher pitch propeller, but it does enhance your thrust as well. In addition, cup reduces a prop's tendancy to ventilate or slip. Custom prop builders often use aggressive degrees of cupping to fix slippage problems on large diameter, low pitch props on certain boating applications.
Most props you will encounter are cupped. Most likely, you will only see an uncupped propeller in performance applications. On smaller outboards, which produce most of their power in the upper third of their RPM range, an uncupped prop with a semi-cleaver design often produces better performance. To get maximum performance, it is necessary to get the motor spinning up in this range as quickly as possible. An uncupped prop loads the motor a bit less, allowing the rpm to build more quickly. This enables you to operate your motor in the meat of its powerband where you'll get maximum power. You will notice faster hole-shot, quicker acceleration, and often times, higher top speeds with these semi-cleaver, uncupped style blades. If you are currently running a cupped style propeller and want to improve your overall performance consider trying a semi-cleaver design. Generally, when switching to an uncupped semi-cleaver style prop, you can up the pitch 2 inches and still have better hole-shot, quicker acceleration and often better top speed! Cup can also provide additional bow lift when utilized on the rake line of the prop. Applying cup to the trailing edge of the prop along the pitch line will increase the effective pitch of the propeller. A standard cup will typically result in a decrease of 200 to 400 rpm's. This usually means a decrease in pitch of 1 to 2 inches is required to run a cupped propeller in place of an uncupped wheel.
BLADE RAKE
Blade Rake represents the angle of attachment of the blade to the hub of the propeller. This is not to be confused with the pitch, which is a measure of the twist or screw progression. The amount of rake built into a propeller blade is not something many people consider when buying a new prop, but it can be just what the Dr. ordered in some cases. Rake angle isn't an immediately apparent thing to the untrained eye, but if the propeller blade is cut down the center, it is readily apparent. The biggest benefit of a high rake design is a greater resistance to ventilation. This allows you to trim the outdrive up toward the surface more without the prop ventilating as readily during tight manuevering. You should seriously consider using a high rake propeller if ventilation is a problem for you.
If you have a large boat like a house or deck boat a high rake prop may not be the best choice because it does not produce as much reverse thrust. This may reduce your ability to manuever your vessel at low speeds making your boat more difficult to dock.
Higher rake normally improves performance in ventilating or cavitating situations (high engine elevations and high trim angles). Additionally, higher rake can provide higher bow lift, which will frequently improve speed. Low rake blades are typically used on motors with propellers running fully submerged, typically carrying moderate to heavy loads. The rake angle can either be straight or the average angle of a parabolic curve.
EFFICIENCY
Not many manufacturers quote the efficiency of their propellers and even fewer customers ask. The efficiency of a propeller is defined as the power coming out of a prop divided by the power going in:
Power out
-------------- X 100 For a solar powered boat, efficiency is probably the most important statistic.
Power in
BLADE THICKNESS (thinner blades cut better)
Why is Stainless Steel the first choice for high performance props? A: It is much stronger and far stiffer than aluminum, but that's not the whole answer. The higher strength of stainless means that the blades can be made thinner and that is where the performance benefit comes from: thin blades slice through the water more efficiently than thicker blades. As a result stainless props produce a couple more mph over even the best aluminum designs.
This Design concept is part of the reason our 4-Blade propellers and our uncupped "XB" style 3-Blade propellers outperform our Standard 3-Blade Designs. By incorporating a thinner blade cross-section, those props cut through the water with less resistance. There is a compromise to be made though and that compromise is in durability. The thinner blades are faster, but they are not as durable as our thicker blade designs. If you run up and down a shallow river and hit things frequently, you may be happier sacrificing some performance to run the tougher blades. If you typically run in the ocean with little chance of a strike, you may prefer the added speed of our thinner blades.
I have to apologize for a basic misunderstanding! I agree that a lever provides lift and the same lift that puts a hilo in the air! But for propulsion it is force of forward motion (lift) or leverage that displaces water and forces a boat forward! I was not using Lift as it was intended to be used! As in Aerodynamics of pressure lift! These principles although close work in very different mediums! Water being 800 times denser!
But to apply these principles to split fins is just as wrong!
Does a boat prop "paddle" against the water? Might one not surmise that the rail of a split fin is simlar to the leading edge of a marine prop?
kidsdream
Contributor
Deleted - not worth the effort to respond to ???
I originally posted a technical response, but after looking at your website I decided it was a futile effort - nice photo credits though! But if you ever want to trade engineering pedigrees, have it sir.
I originally posted a technical response, but after looking at your website I decided it was a futile effort - nice photo credits though! But if you ever want to trade engineering pedigrees, have it sir.
laurelgsc
Contributor
Meanwhile back at the ranch............ I just bought a pair of Apollo Bio Fins with spring straps. I love them, but I guess my buddies were being nice and never said anything about me being slow, not being able to hover and constantly silting up the dive site. :mooner:
Papa_Bear
Contributor
Isn't that the point? It, lift, has nothing to do with split fins! I think I know a little about Boat props! Split fins and boat props aren't even in the same universe! My point on physics is fins fall under mechanical transfer of energy! Water because of it's density provides resistance, it is a body at rest, seeking its own level and resisting tidal pull always settling! Fins work because you push down, or down stroke, and the water tries to stay were it was! The energy exerted must be greater than your surface resistance and your initial inertia in order to move you forward! So your stroke must over come your mass! The faster you stroke the greater the water resistance to change! The same as jumping into the water from heights!
A slow powerful kick is better than a faster shallow kick! the ideal kick would be fast down and slow recovery! You are looking for resistance in order to move your mass forward! The weight of the water and your pushing against it is what gives you forward motion! The most efficient kick would be one as fast and large as possible given the size and length of your lag in relation to your torso! A whales tale is not split it is broad and powerful in order to move the whale forward! If it were split it would collapse and wouldn't be efficient! But some how we are rewriting the same laws of physics so that floppy is better!
How that ever got morphed into lift and levers I haven't got a clue, but then again thats the problem with the false logic of split fins!
A slow powerful kick is better than a faster shallow kick! the ideal kick would be fast down and slow recovery! You are looking for resistance in order to move your mass forward! The weight of the water and your pushing against it is what gives you forward motion! The most efficient kick would be one as fast and large as possible given the size and length of your lag in relation to your torso! A whales tale is not split it is broad and powerful in order to move the whale forward! If it were split it would collapse and wouldn't be efficient! But some how we are rewriting the same laws of physics so that floppy is better!
How that ever got morphed into lift and levers I haven't got a clue, but then again thats the problem with the false logic of split fins!
Similar threads
