Before you can buy your new propeller, though, you really have to understand how to determine the pitch of a boat propeller. The diameter of your propeller is the measurement of the distance across the circle that is made by the tips of the propeller blades. This circle can vary depending on the speed of the propeller. The best way to grasp this concept, then, is to think of it this way:.

Imagine a screw going through a piece of wood. The wood, of course, is solid, but the screw is still able to rotate completely through it. Every time the screw rotates completely, it impresses a certain distance into the wood. Still confused? In short, determining the pitch of a boat propeller can be pretty tricky and confusing.

This is because the practice itself is highly theoretical. Though in this scenario the water would be like the wood for the screw, water itself is amorphous and moveable, making the distance the propeller impresses on it hard to measure.

As stated, boat propellers do not move through wood, making the measurement a little hard to translate. Unfortunately, this is not the case due to boat propellers rotating in water.

The water element changes the distance the propeller actually travels and introduces a number of other factors as well: hull drag, unit drag, ventilation, cavitation, and aerodynamics, among others. To begin, think of the theoretical speed. The full calculation looks something like this:. Once you have this solution, you can use it to determine slip.

The calculation is:. These equations are useful if you suspect that you have the wrong propeller pitchwhich can cause many issues when out on the water. Another important thing to remember about pitch is that it measures the maximum distance a propeller travels in one rotation.

When thinking of boats, this means how far the propeller moves the vessel through water. Due to elements already mentioned, such as drag, aerodynamics, and fluid dynamicswe already know that there is a certain loss of power when the propeller moves through the water, meaning a less-than-accurate pitch determination.

The best way to do this is to remove the propeller from the shaft and have it on a table or other flat surface where you can make clean measurements. You can still do the measurements just with a little less accuracy. To begin measuring, look for the widest part of one propeller blade.

Once this has been identified, draw a line on the blade from one edge to another. From here, you will measure the distance from the front of the hub to where you line meets each edge of the propeller blade. This measurement is best taken when viewing the propeller from the side.

Once you have your measurements, take the smaller one and subtract it from the bigger one.A boat propeller's size is described by its diameter and its pitch. When you measure a boat propeller, you don't simply measure the length of one of the blades, you also have to measure and calculate the size of the circle made by the tip of a blade as it moves.

You also have to measure and calculate the distance--in inches--that each full turn of the propeller moves the boat forward, a measurement call the propeller's "pitch.

Measure from the center of the propeller's hub--the center of the propeller to which the blades are attached--to the edge of the hub with a tape measure. Record this measurement. Add the two recorded measurements together and multiply the result by 2 to determine the diameter of the propeller, which is the first element of the propeller's measurement.

This is the radius of the propeller. Multiply the radius by 2. Use the grease pencil to draw a line around the propeller hub. Set the base of the protractor against the blade of the propeller and note the angle on the protractor between the blade of the propeller and the line drawn around the hub. Use your calculator's "TAN" or tangent function to determine the tangent of this angle.

Multiply the tangent of the angle by 6. Multiply the result by the radius of the propeller. The result is an approximation of the pitch. Pitch is always a whole number; if you get a fractional answer, round the number off. For example, if the diameter of the propeller is 14 inches, the radius of the propeller is 7 inches. The angle of the blade is 20 degrees. The tangent of 20 is 0. Rounded off to the nearest whole number, this means that the pitch of the propeller--the second element of this propeller's measurement--is " Record your propeller's measurement by its diameter and its pitch.

In the example, the diameter is "14" and the pitch is " Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering.

He is also a certified marine technician and the author of a popular text on writing local history. Items you will need Tape measure Grease pencil Protractor Calculator. Measure from the base of one blade to the tip of that same blade. About the Author. Photo Credits.Discussion in ' Props ' started by takoateliMay 31, Log in or Sign up.

Boat Design Net. I'm making a long tail outboard motor using a diesel engine and I'm trying to determine roughly the thrust the prop will be generating. Any help would be greatly appreciated.

The prop is a 7. The engine is turns at rpm max. The boat it will be pushing will be a metal boat shaped like a dugout canoe and I'm guessing it will travel at mph. The reason this is important is I'm considering connecting the long drive shaft which will have the prop on the far end directly to the crankshaft of the motor which will impart the thrust generated by the propeller to the crankshaft of the motor.

The motor is rated to take That's the magic number. If my prop is going to generator less than I've seen photos of motors that are the same general design as what I'm building, they take any old motor and slap a drive shaft and prop on it, and it appears without any additional external thrust bearing. It appears they let the engine's crank shaft take the thrust. Any input you could give me would be greatly appreciated. I'd like to know how much thrust the prop would develop as well as anyone's thoughts about applying the thrust directly the engine's crankshaft is a good or bad idea.

Greg Ihnen.

The thrust developed against the shaft depends on the resistance of the hull. Accelerating fast hole shot is a function of the mass of the boat more than anything else. However, that would be of short duration. You are getting great advice from daiquiri. Propeller thrust I strongly recommend getting a copy of "The propeller handbook", it is a suprisingly interesting read even if you don't wan to do the maths.

A simple rule of thumb is a good prop will make 20 lbs of thrust per hp. Sure in theory it can be higherbut 20 works. Use your engine rated power10HP x 20 would be lbs thrust.

That work for you? You must log in or sign up to reply here. Show Ignored Content. Similar Threads. Hi Thrust prop for outboard twin Power Trimaran mike w.

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Replies: 15 Views: The thrust of a propeller is calculated as its pitch. In theory, the pitch is how many inches forward the boat should move for each full turn of the propeller. In practice, the thrust transferred to your boat may vary because of the size of your boat, the kind of hull you have--whether it sits in the water or rises up to plane on top of the water--and a factor called "propeller slip", which is the difference between how far the propeller should make the boat move and how far the boat actually moves.

## Boat Propeller

Set the bottom straight edge of a protractor at the widest part of one blade of the propeller, so that the straight edge forms a line between the front of the widest part of the blade and the back of the widest part of the blade. Note the angle between the edge of the propeller hub and the base of the protractor.

This is called the pitch angle. Use your calculator to determine the tangent of the pitch angle. For example, if the pitch angle is 23 degrees, then its tangent is 0.

Say our propeller has a inch diameter. Multiply 0. The pitch of a propeller is always stated in even numbers, so the result, Will Charpentier is a writer who specializes in boating and maritime subjects.

A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history. Related How to Measure a Boat Propeller. Items you will need Clear plastic protractor Calculator.

Step 1 Set the bottom straight edge of a protractor at the widest part of one blade of the propeller, so that the straight edge forms a line between the front of the widest part of the blade and the back of the widest part of the blade.

About the Author. Photo Credits.Boat performance is equal parts power and propeller. Note: Results will often yield multiple propellers. Please contact a dealer once you've completed this process. This Prop Selector will help you calculate the right prop for your boat type and how you use it. Our test data is based on these general boat types. Please choose the type of boat below that corresponds most closely with your boat type.

Selecting a performance goal is crucial to finding the right prop.

The Prop Selector will recommend a propeller based on the Performance Goal you select. Your specifications will help narrow the field. Each boat and its use is slightly different, and sometimes different propeller lines could provide similar benefits. The propellers identified below will allow you to make a final decision based on either further research on our Web site or on recommendations from your dealer.

Step 1 Step 2 Step 3 Step 4 Step 5. Step 1: Boat Type This Prop Selector will help you calculate the right prop for your boat type and how you use it. Step 2: Usage. Are You at a High Altitude? Step 3: Engine Info. Step 4: Family Results. Compare Selected Products. Step 5: Pitch. Current Boat Performance Options based on your current setup. All Rights Reserved. Reproduction in whole or in part without permission is prohibited. Mercury Marine is a division of Brunswick Corporation. Aluminum - Console.

Aluminum - Tiller. Inshore Fishing. Fish and Ski. Offshore Fishing. Rigid Inflatable Boat. Runabout - Cuddy.Picking a perfect prop for an individual boat depends on a large number of factors including hull design, weight, altitude, engine, and usage patterns.

It is impossible to pick a perfect prop for every hull and situation. Fine tuning a boat's performance involves a great deal of trial and error though the expert's at Mark's Marine and Precision propeller can get you close.

We also stock a large number of different sizes of Michigan Propellers in the Vortex aluminum and Apollo Stainless Steel serries. These props feature interchangeable Hub Kits allowing them to fit a wide variety of different motors at an affordable price point.

Mercury's Prop Selector Mercury's: prop selector is based off a complex formula to calculate the appropriate prop based on an aproximation of your hull's performance charecteristics. Propeller Pitch. Choosing a prop Picking a perfect prop for an individual boat depends on a large number of factors including hull design, weight, altitude, engine, and usage patterns.

**Propeller: It is All About the Pitch!**

Prop Calculators on the web Mercury's Prop Selector Mercury's: prop selector is based off a complex formula to calculate the appropriate prop based on an aproximation of your hull's performance charecteristics.Calculators Relating to Propeller Selection.

We currently have two prop size calculators belowalong with a collection of "conversion" style calculators to aid in determining various weights and measures relating to propeller selection. Before you start, we would like to point out that these calculators are based on empirical data. They are not intended to perform complex cavitation or efficiency calculations. The results provide "best size" propeller dimensions based on your input data.

There will always be a number of propeller size combinations that will work on your vessel, finding the best one is the trick.

### Prop Slip Calculator

These calculators will help guide you to determine various combinations of power, gear ratio, and propeller for your vessel. Due to the nature of these calculators, Vicprop will not accept any liability, financial or otherwise, for the calculations or results provided. To begin, please select one of the calculators:. Prop Size Calculators.

Displacement and Semi-displacement Hulls. Planing Hulls. Conversion Calculators. Liquid Volume to Weight Calculator. Speed Conversion. Calculators Relating to Propeller Selection We currently have two prop size calculators belowalong with a collection of "conversion" style calculators to aid in determining various weights and measures relating to propeller selection. To begin, please select one of the calculators: Prop Size Calculators.