Propellers - Repair and Selection

By Greg MacKay
Hall & Stavert, Charlottetown, PEI, Canada

Repair

Some parts of a boat can fall into disrepair and have little or no effect on your day-to-day sailing. Maybe it is a chip in the gel-coat, or a small crack in the windshield, but these are of little consequence except they may not look very good ! On the other hand, damage even in small amounts to a propeller can cause problems and cost money ! Whether the damage is caused by cavitation, electrolysis, or by some mysterious rock jumping up and hitting your propeller (this apparently does happen quite often) the problem should be repaired as soon as possible.

Damage left unrepaired can cause vibrations that in turn damage other boat components like bearings and seals; cause even further propeller damage thru increased cavitation erosion; and can cost a lot of money over time in lost efficiency (you'll use more fuel to travel the same distance). Realistically, propellers cannot be repaired correctly without proper tools and knowledge. This is a bitter pill for most of us who like to at least try to repair almost any part of our boat! 

The best advice if you discover propeller damage is to take it to a shop you know and trust, and to do it as soon as you can. The second best advice is to always carry a spare propeller ! Summer is short, and no one should have to miss time on the water waiting for repairs, or trying to find a new replacement

Selection

Propellers are one of the most important, yet least understood, parts of a boat's drive-train. Therefore their proper selection is very important to your vessel's efficiency and performance. Most boat manufacturers do a reasonable job of selecting an appropriate propeller for new boats that they manufacture. However, sometimes they do make compromises between price and performance that may have a negative effect on your boat. Also, if you have an older boat, the original propeller may have worked correctly before when the engine was new and running at full power. But now time has taken it's toll, and it may be necessary to change the propeller specifications.

Should you want to get spares, replace a damaged prop, or just check to see if another design might work more efficiently, there are mathematical methods that can be used to specify the best propeller for your boat. Note that I said "best" and not "perfect". Propeller selection is always a series of compromises, because your boat and the conditions under which it operates, change almost daily. The weight of the boat varies with the amounts of fuel, water, people and equipment you carry, the engine(s) operate differently based on air and water temperature, air pressure, and etc. 

In a perfect world you would need a different propeller for each of these variations. Of course this is unrealistic, so the propeller chosen for your boat has to be a compromise of the "best fit" for all of the possible scenarios.

In theory, the horsepower of your engine(s) can move a hull of a specific shape and weight, at a certain maximum speed. Using this information, along with some other details about your drive-train, a propeller can be selected that will do the best job possible. When choosing a new prop, you would be well advised to purchase it from a business that can take your information and make a suggestion as to what you should use. The things that will be specified with respect to what propeller you need will be:

Some boat equipment can be purchased easily at a department store, or on-line. But given the importance of the propeller in the overall efficiency of your boat, you should research your needs carefully. It will certainly be time well spent, and hopefully save you some headaches later.