Effciency

captainlarry84

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Here is the formula for figuring out your boats efficiency:

RPMs divided by Reduction Gear Ratio X=s Pitch

X=s 60 (minutes) divided by 12 (inches per foot) divided by

5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots

Example my 31 JC 370 HP [email protected] 3300 RPMs 2:0 gear with a 23 X 21 propeller a WSOTY speed of 22.5 knots.

3300 (RPMs) divided by 2 = 1650 X's 21 (pitch) = 34650.00 X's 60 = 2079000.00 divided by 12 ("s per foot) = 173250.00 divided by 5280 (feet in a mile) = 32.81 (miles per hour) divided by 1.15 =28.53 knots @ 100 % efficiency

22.5 knots (Boats actual speed divided by 28.53 knots ='s .79% efficiency

Therefore the only things you need to get your boats efficiency is the follow:
speed at wot @ RPMs
reduction gear
pitch of your propeller
 

gregs

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It would seem that this may work at speeds other than wot - say at cruise speed?
Probably does not sound like a question but it is.
 

kmac

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It would seem that this may work at speeds other than wot - say at cruise speed?
Probably does not sound like a question but it is.

Yes it does and will vary at different speeds. It should be between 80 and 90% efficiency. If less you would generally go up in diameter.
 

petrel

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How do you figure it for a twin screw boat?
 

OLD BAY

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Fishinengineer

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On ships there is a theoretical distance that the ship would move based on the RPMs and the pitch of the propeller. Anything less is considered slip, if you move further than this theoretical distance it would be considered negative slip. This does happen on occasion and companies go to great lengths to find currents and weather patterns to reduce slip. Slip = fuel. Typical slip for a modern container ship under normal conditions would be about 3-6%. Bucking the gulfstream or a good head wind would see 10 or more percent slip. These are full displacement hulls, for our purposes a semi displacement or planing hull would expect a higher percentage of slip or lower hull efficiency.
 

Keelboater

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Just note that this is a method for checking the efficiency of the hull and prop package and really has nothing to do with efficiency of the motor and gear. But it all ties together. An educated guess would be that motor efficiency x gear efficiency x hull efficiency = a somewhat overall estimate of boat efficiency. The more you look at efficiency under a microscope, the lower the overall number gets. Funny how that works.
 

steveinak

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Here is the formula for figuring out your boats efficiency:

RPMs divided by Reduction Gear Ratio X=s Pitch

X=s 60 (minutes) divided by 12 (inches per foot) divided by

5280 (feet in a mile) ==s MPH Divided by 1.15 =s Knots

Example my 31 JC 370 HP [email protected] 3300 RPMs 2:0 gear with a 23 X 21 propeller a WSOTY speed of 22.5 knots.

3300 (RPMs) divided by 2 = 1650 X's 21 (pitch) = 34650.00 X's 60 = 2079000.00 divided by 12 ("s per foot) = 173250.00 divided by 5280 (feet in a mile) = 32.81 (miles per hour) divided by 1.15 =28.53 knots @ 100 % efficiency

22.5 knots (Boats actual speed divided by 28.53 knots ='s .79% efficiency

Therefore the only things you need to get your boats efficiency is the follow:
speed at wot @ RPMs
reduction gear
pitch of your propeller

22.5 knots (Boats actual speed divided by 28.53 knots ='s .79% efficiency
is the 22.5 knots the boats actual wot speed ???
 

captainlarry84

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How do you figure it for a twin screw boat?

You use the same formula, however you will notice that the efficiency will be much great with twin screw boats & wheels that do not have a keel blocking the water flow.

Let just do a quick re-cap on diameter & pitch:

Diameter: This is the overall width size of the propeller or as stated the diameter. The diameter is measure from one outer tip to the other and directly through the center. On all propellers the diameter and pitch is always stamped either between the blades on the hub or on the face of the hub. The first number is always the diameter. Therefore a 24 X 22 stamped propeller means that the diameter is 24 inches. Even we as layman can measure the diameter. Diameter of the propeller plays a big roll in overall performance. On most vessels when wheeled correctly they have the maximum diameter allowable to get the largest blade surface. Large blade surface is important in order for the propeller to catch enough water and allow the pitch (twist) of the propeller to do it job and push the vessel forward. When purchasing a vessel you only pay for diameter, as the larger the diameter the greater the amount of material which equals cost.

Pitch: Pitch is what pushes you forward. In a perfect world with zero slippage for each inch of pitch and every RPM your vessel’s shaft turns the boat should move forward 1". So a 1 shaft RPM with 22" pitch and 100% efficiency in theory you move forward 22 inches. But as we all know there are many variables which get in the way and reduce this number. Things such as keels, struts propeller pockets and just a few that lower our 100% efficiency number. Pitch is the second number on the stamp of a propeller; so on the same 24 X 22 propeller, the second number of 22" indicates 22" of pitch or twist in the blade. Unlikely diameter pitch cannot be measured by a simple ruler, as pitch is the gradual swipe or twist of the blade. On a flat plan and inch of pitch would be about 1/8" of rise at the outer edge. The rise of pitch however changes through the blade because the blade must end at the root of the hub of the propeller.
Pitch must be measured by a propeller shop. The wheel is place on a jig and a dial indicator is dropped down the blade to measure the pitch. Pitch is usually what we need to adjust when reconing or resizing to gain or lose load on the blades.
On many of the newer boats due to high horsepower and diameter clearance problems pitch may exceed diameter by as mush as 12" or more. So a 24" X 36" propeller is not uncommon on some of our high performance Sportfishing Boats to swing. Propellers such as these are not commonly seen on keeled bottom or displacement bottoms, as these types of hull bottoms could very provide enough water flow to feel propellers with such massive pitch.
 
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gregs

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So the propeller efficiency is higher on twins. Has anyone done comparison of the twin screw efficiency and how that relates to fuel burn?
 

WoundUpMarine

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Brooksie

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Interesting CL, but solving for slip is not the same as solving for efficiency. Slip is a factor in efficiency but it is not a measure of efficiency in itself and implying that the boat with the least slip is the most efficient is not actually correct.
 

kmac

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Propeller efficiency is only part of the overall efficiency of a boat but in a lot of cases the only one that can be improved economically. For example it would be very expensive to switch from gas to diesel, or change the shaft angle. The prop can be changed at a modest cost and make huge differences. Slight improvements to efficiency can really add up for the commercial fisherman especially with the rising cost of fuel.

In addition to diameter and pitch, E.A.R. (expanded area ratio) should also be considered when selecting a prop. For a given diameter prop, one with a higher E.A.R. will have more bite or surface area to push with. A boat that already has the maximum diameter wheel possible but with excessive slip will benefit with a prop with a higher E.A.R. This is a very important parameter to consider especially with skeg or keel boats.
 

osprey47

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Very useful thread. So what is the optimal efficiency range? Using the formula I get 80% for a BHM 36' with a Yanmar 350 where the transmission and prop have been tuned for fuel efficiency -- cruising 14kts, max speed 20kts.
 

captainlarry84

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Very useful thread. So what is the optimal efficiency range? Using the formula I get 80% for a BHM 36' with a Yanmar 350 where the transmission and prop have been tuned for fuel efficiency -- cruising 14kts, max speed 20kts.

80% STOP you are there it is perfect.
 

F/V First Team

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speaking of all this prop talk, how much pitch can be added to a wheel? I can get a 26x28 a friend has laying around, could two inches of pitch be added to make it a 26x30?

4 either way on a virgin wheel

Some of us though, crank 7 or 8 in there

You just need an awesome prop shop to do it
 

WoundUpMarine

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captainlarry84

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Propeller efficiency is only part of the overall efficiency of a boat but in a lot of cases the only one that can be improved economically. For example it would be very expensive to switch from gas to diesel, or change the shaft angle. The prop can be changed at a modest cost and make huge differences. Slight improvements to efficiency can really add up for the commercial fisherman especially with the rising cost of fuel.

In addition to diameter and pitch, E.A.R. (expanded area ratio) should also be considered when selecting a prop. For a given diameter prop, one with a higher E.A.R. will have more bite or surface area to push with. A boat that already has the maximum diameter wheel possible but with excessive slip will benefit with a prop with a higher E.A.R. This is a very important parameter to consider especially with skeg or keel boats.

This very true. If you are swing a 24 X 24 Dyna Quad with a E.A.R of .69 & decide to a DQX with an E.A.R of .735 you need the reduce the wheel size to a 24 X 23. As you can you cannot rob Peter to pay Paul. It is one or the other.
For DE boats I prefer the Quad & on twin screw the DQX. Also keep in mine that both wheels cost the same because they use the same amount of material to make them. In doing so the DQX blades are thinner & not as strong as the Dyna Quad.
Wheel with expanded blade are great when wheel clearance is limited. On Twin screw boats it is not uncommon for the to wheels way out of square like a 26 x 34.
 

steveinak

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using the formula i come out with 81% ?? I don't know my speed in knots so i left out the 1.15 in the calculation, so is that correct ??
 
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