To correctly size a propeller for a sailboat, the boat, motor and how you use the boat must be considered.

Motor considerations
    The horsepower, rpm, and gear ratio of the motor are used to calculate the diameter of the propeller needed to transmit the hp and how far the boat must travel with each revolution to attain the desired boat speed. The motor rotation direction and the motor mounting style determine the direction of rotation of the propeller. V drive transmissions generally reverse the direction of rotation of the motor and Sail drive transmissions require special hub borings.
    Motors are usually specified as supplying horse power at a specific rpm. However the advertised horsepower is usually peak horsepower, which is rarely used in propeller selection. Buried within motor manufacturers specifications are the manufacturer recommendations for HP at RPM to be used for propeller matching. This can be listed in many forms: continuous operation, and propeller matching, are a few of the listings.  Some manufacturers will not have an obvious rating, in those cases 80% of rated HP and corresponding rpm is used.
Most of the motor information can be found on the motor specification chart supplied by the manufacturer, however the gear ratio must be taken from the gear box since many of the motors have optional gear boxes.

Boat considerations    
    Length at the water line (lwl) determines the maximum speed of the boat while the displacement determines the power required to attain that speed.
    Propeller mounting will determine the maximum diameter of propeller that can be used and shaft size. If your prop is in open water or in an aperature can limit propeller size and selection. see information chart below. The maximum propeller diameter that can fit on a boat may not be the correct propeller diameter. The motor may be more efficient or quieter with a larger or smaller diameter propeller.  Boats with old hull designs often have mismatched propellers!  Some have been repowered, others repropped, and in a few cases the factory supplied propeller was not the best match for the boat and motor but the only one that would fit.


How you use the boat
    The amount of gear aboard and the weight of the boat can be considerably different for the same make and model of boat depending on how you use the boat.
    The racer lightens the boat as much as possible and carries minimal supplies and gear. Only what is needed for the race is aboard.
    The day sailor may not bother with the water tanks and only has a quarter tank of gas because he only needs to get out of the marina and occasionally needs to motor home when the wind dies. The supplies are a six pack and a bag of chips. If the gear is not required for sailing or by law it is not aboard.
    The weekend sailor will have supplies aboard needed to get through weekend. The water and gas tanks are usually topped off in the spring and for most of the year they are only about half full. There is some extra gear aboard to make life easy but a lot of gear and spare parts are stocked at the local marine store until it is needed.
    The live aboard cruiser has everything they own aboard and all tanks are kept topped off. The ballast is complemented with the spare parts, extra gear, and a months food and extra bottled water, because you never know what will be available at the next port.
    So how does that affect the propeller? One gallon of water weighs around 8 lbs. and fuel is about 7 lbs so a full 50 gallon tank of each will add about 750 pounds to the boat. Adding in spare parts, an the extra anchor, a tool box, a change of clothes, couple of weeks of groceries, a dingy and its outboard etc. means that a crusing boat can easily weigh 2000 lbs or more than the same boat set up for racing. That extra weight can change the pitch or diameter of the prop because of the increased HP needed to get and keep the extra weight up to speed.  

To start the selection process manufacturers have information sheets to fill out that contain the information needed to select a propeller. one sample is below.

Propeller SIZING QUESTIONNAIRE
Name:____________________________ Street______________________________
City______________________________ State_____ Zip_________________
Phone# _______________   Fax# _____________ E-mail_______________________

TELL US ABOUT YOUR BOAT:
Make: ________________ Model:___________ length ____
length at water line:______  displacement:______
Tell us the following dimensions for your boat:
If your prop is in an aperture                          If your prop is in open water  

                

diameter of prop_____________  D____              diameter of prop_____ D_____
prop to rudder_______________ A ____             Prop to rudder_______ A____
top of prop to hull_____________C ____              top of prop to hull______C____
bottom of Prop to fairing: _______C ____              shaft diameter______
back of Prop to cutlas bearing____E____
shaft diameter__________

TELL US ABOUT YOUR CURRENT MOTOR:
Make:_______________________Model: _________________________
Max. Continuous Horsepower: ______ Max. Continuous RPM: __________
Gear Reduction______
Does your motor emit visible smoke with your current prop? ____________
MOTOR NOTES:


TELL US ABOUT YOUR CURRENT PROP:
Diameter:______ Pitch: ______ Shaft dia: ______         
# of Blades:______    ____FIXED ____FOLDING ____FEATHERING
Maximum Boat Speed @ what RPM: _____________________________
PROP NOTES:


Note if you have have a recent survey of your boat most of the information will the in the survey. However the dimensions around the prop will need to be measured.  Most propeller manufacturers keep lists of what size propellers were originally on the more popular boats and which of their props will work on the unmodified boat.  However many boats have had model changes or have been modified over the years and the dimensions are used to verify the proper fit of the new propeller.
Once filled out the processes of selecting a propeller for your boat can begin.
See sailboat propeller selection: calculation and clearences