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

Motor Considerations
    The make, model, 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. The advertised horsepower, however, 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, except for the gear ratio which 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.
    The 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 aperture this 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 was the only one that would fit.


Boat Usage Considerations
    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 it is used.
    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 may only have 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 may consist of a six-pack and a bag of chips. If gear is not required for sailing it is not aboard.
    The weekend sailor will have supplies aboard needed to get through the 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 needed.
    The live-aboard cruiser has everything he owns aboard and all tanks are kept topped off. The ballast is complemented with the spare parts, extra gear, one 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' worth of groceries, a dingy and its outboard etc. means that a cruising 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 which contain the information needed to select a propeller.