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Old 02-04-2003
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Choosing the Right Propeller

 
The difference between just any prop and the right prop can be dramatic in terms of engine performance, fuel consumption, and speed.
 
As sailors, we can spend hours tuning the rig, tweaking the sheet leads, and searching for that perfect cut of sail to maximize our performance. Once those sails are lowered, however, it often comes down to that tiny little wheel—the propeller—to deliver us safely back home.

The right propeller can determine whether we're back at the club, drinking rum toddies with the boys and girls, or still out there motoring to windward with our egg-beater of a prop churning away, but going nowhere fast. Just because your current prop came with your boat doesn't mean it's the right one for your boat, your engine, or the type of sailing you do. The difference between having just a prop, and having the right prop, can sometimes be dramatic.

After Sue and I installed Serengeti's new, we began the quest for that perfectly matched propeller. We discovered that finding the right prop is hardly an exact science. There are many factors to consider. What's the right size for our boat, our engine, and the way that we plan to use the boat? Do we go with a fixed, folding, or feathering propeller? Each offers a unique set of advantages and disadvantages.

Here's a rundown of the main areas of consideration to help determine if you and your prop are made for each other.

 
Modifying the hull to fit the prop is also an option.
 
Sizing Your Propeller  The numbers and letters stamped on a prop start out looking like Greek to most of us, but soon make sense. Propeller size is denoted by two consecutive numbers that represent its diameter and pitch. A 15/11 prop means that it's 15 inches in diameter and has 11 inches of pitch (the distance a propeller moves forward in one revolution). Right Hand (RH) or Left Hand (LH) is usually stamped on each propeller and tells you which direction the prop is designed to rotate in forward gear if you're viewing it from astern. The diameter of your shaft and its taper are other details you will need to confirm before buying a new propeller.

Sailors commonly assume that the propeller on their boat must be correctly sized, simply because it's there. This is frequently not true and may be detrimental to your boat's performance. Here's how you determine if your propeller is right for your boat.

Your engine is designed to run under a pre-determined load. A correctly sized propeller allows for clean, efficient acceleration, and yields a specific desired maximum engine rpm at full throttle. This is necessary for the longest possible engine life. The target rpm is specified by the engine manufacturer for each model. It may be stamped on the engine itself, and it is a number you should know.

 
Recording rpms at full throttle on smooth water will tell all when compared to manufacturer's specifications.
 
To test your own prop, take your boat out in smooth water and put the hammer down on the throttle. Yes, all the way down! Record the maximum rpm achieved in this full throttle position. Don't worry, diesel mechanics assure us you won't harm your engine. If your maximum rpms fall within 200 either side of the specified target number for your engine, your prop size and pitch are OK. If the recorded rpms are outside of this range, however, you need to either re-pitch your existing prop, or purchase a new prop of a different diameter, and maybe a new pitch too. Assuming your prop has never been re-pitched before, three degrees of pitch can either be added or subtracted to your existing prop. Roughly speaking, on a three blade propeller, each degree of pitch adds or subtracts 200 rpms.

 
Free clearance between tip to hull should be a minimum of 20 percent of the prop diameter.
 
In the event that you can't re-pitch your existing prop enough to obtain an acceptable rpm reading at max throttle, you'll need to try either a larger or smaller diameter wheel to accomplish the task. The size of a new propeller is sometimes dictated by underwater clearances in an aperture, or to the hull. To minimize vibration, the free clearance from the prop tip to the hull should ideally be a minimum of 20 percent of the diameter of the prop. Keep in mind as you work on sizing your new prop that, with displacement hull sailboats, a larger diameter prop with a lower pitch is usually going to perform better than a smaller prop with a greater pitch.

Types of Propellers  There are many different kinds of propellers on the market today. Let's take a look at the advantages and disadvantages of each when choosing the right one for your boat and the type of boating you do.

 Fixed Props  The most common propeller you see on sailboats is the fixed type, made of bronze. These are either two or three bladed. The fixed prop has the advantage of being the least expensive. It offers good forward thrust and has no complicated parts that require maintenance. It's easy to put on and take off the boat, especially if you have a prop puller.

In reverse, however, a fixed prop performs poorly. Since its blades are designed for forward thrust, as much as 50 percent of the thrust is lost when shifted into reverse due to the trailing edge now acting as the leading edge. Also, there is considerable drag from a fixed prop when sailing. On our last boat with a 21-inch, three-bladed prop, we lost one knot of speed when we stopped it from spinning.

 
Fixed props are a delicate balance of the best and worst of all worlds.
 
To lessen this dreadful drag, some sailors allow their prop to spin freely while under sail. This may or may not be a good idea. If your engine transmission is a cone clutch type, then it's OK to allow your prop to spin freely. Hydraulic clutch type transmissions, however, should only be allowed to free wheel for a short period of time without the engine running. Overheating is likely to occur since the transmission fluid is not being cooled. Keep in mind if you allow your prop to spin freely while you sail, that there will be additional wear and tear on your cutless bearing regardless of what's going on with your transmission.

Unlike some of the fancier props available, a fixed prop always carries the designation of RH (Right Hand) or LH (Left Hand). A right hand prop spins clockwise when looking forward from the stern, and a LH prop spins counter-clockwise looking forward from the stern. It's this spin that determines which way your transom will "kick," when you put the boat in reverse. A right hand prop kicks left in reverse, and LH kicks right. This "propwalk" has been the demise of many sailors' docking attempts over the years. Once you learn to use "propwalk" to your advantage though, you'll look like a real pro coming into the docks.

 

Folding Props  Those in search of a super-slippery underbody when sailing usually choose a folding prop. The folding prop provides the least amount of drag of all propellers. When the engine is stopped, the blades of a folding prop collapse upon themselves and become very streamlined, with the tips of the blades facing aft. They look like a squid when folded.

Folding props suffer from the same poor thrust in reverse that fixed props do and they are considerably more expensive. In addition, they sometimes fail to open or close properly.

 Feathering Props  For the sailor who wants to get where he's going quickly under both power and sail, a feathering prop is the obvious choice. A feathering propeller offers several advantages when compared to a fixed prop. When the engine is shut down, water pressure passing by the blades of a feathering prop causes them to turn and align with the flow of the water, thus reducing drag by up to 85 percent. Certainly this is an advantage to the racing sailor and also the cruising sailor who likes to sail fast on long passages. In reverse, the blades of a feathering prop rotate and provide equal thrust when going either backward or forward. This helps reduce the nasty effects of "propwalk." On our last boat with a feathering prop, Sue used to back into the tightest possible slips with extraordinary ease while I watched women on the dock pounding their husband's shoulder and exclaiming, "Why can't you do that?"

Some feathering propellers allow you to adjust the pitch of the blades externally while diving, and others require that you haul the boat to make this adjustment. In either event, there is some flexibility in the pitch of a feathering prop. A new type of feathering prop now available, the Autoprop, adjusts the pitch automatically while underway to maximize thrust and minimize fuel consumption.

Although more expensive than a fixed prop, a feathering prop costs no more than a new sail for most boats, and if properly used and well maintained, could last virtually forever.

Sue and I had planned to transfer our feathering prop to our new boat, but unfortunately we found that there was not enough space fore and aft for this type of prop in the enclosed aperture that surrounds the propeller area on Serengeti. This led us to concentrate on getting the perfect fixed prop matched to our boat, with the knowledge it will always be a good spare if we later find another feathering type that fits and will keep us sailing fast.

 
If you're not getting the performance you'd like, quality time with a prop puller may be in your future.
 
Finding the right fixed prop took a little while and a bit of work. With the new, larger horsepower engine we just installed, we knew a larger diameter prop would be needed to work efficiently with the engine. For a larger fixed prop to fit, we had to grind away part of our rudder and enlarge our aperture. To get the right size, we started with a recommendation from the people at Yanmar as to what they thought would match the engine, then tried a couple of borrowed used props of varying diameters and pitches to close in on the correct engine rpms at full throttle. This process allowed us to finally order a new prop of a specific size with confidence that it was the right one.

If you're not getting the performance out of your boat that you would like when you motor, it's time to do some tweaking on something other than those sails. Do the quick check of engine rpms at full throttle. Improved performance may be as simple as replacing your propeller with one that properly matches your boat and your engine. A new fixed prop may be in order, or maybe one of the folding or feathering varieties if you're looking for increased sailing performance. And, if you'd like to experience those joys of backing up with better control in tight situations, you might just want to consider that feathering type. 

Propeller Tips

  • Always check for lines hanging overboard before putting the engine into gear. If you do wrap a line, shut down the engine immediately. With your gear shifter in neutral, try spinning your shaft in the opposite direction while someone pulls on the line. If this doesn't work, you'll need to dive overboard with a knife.

  • Never put your boat into gear with swimmers in the water.

  • If you put your boat in gear and experience little or no thrust, you may have a severely fouled prop. You'll need to dive and clean your prop before you can proceed.

  • All under water metals, especially your propeller, should be protected from electrolysis. This is done through the use of a sacrificial zinc anode. Zincs are commonly found clamped to the shaft just ahead of the prop, or sometimes seen in the shape of a cone and attached to the aft edge of the hub (central part) of the prop. Zincs should be checked on a regular basis and replaced as necessary.

  • Excessive noise and/or vibration around the propeller area can be caused by several different things. It may be an engine alignment problem, an excessively worn cutless bearing, a bent shaft, or an out of balance propeller, often the result of hitting something. It's best to investigate before the problem worsens.

  • If traveling far from home, it's wise to carry a spare propeller, and spare parts, such as prop nuts, cotter pins, zinc anodes, and a cutless bearing. A prop puller is necessary to make the change.



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