Changing the prop on your outboard may be the least expensive yet dramatic way to fine tune and customize your boat’s performance for your use. There are plenty of websites with prop calculators that will take the size and weight of your boat, along with your engine, current propeller, and current performance data, then recommend a replacement prop based on your desires for improvement in a particular area. Say you want better fuel efficiency, or better top-end speed; maybe you use the boat for fishing offshore in rough water or water skiing with the whole family onboard and want to customize it for that—all without buying a new engine or different boat. What’s behind their recommendations, and how does the calculator work? The answer, in a word, is compromise.
You can change the prop and get a better “hole shot” for water skiing, but it will more than likely require a compromise in top-end speed, for instance. Engine manufacturers have likely installed a general-purpose prop that meets the recommended operating range your outboard was designed for. The operator’s manual will express this as a certain horsepower at a certain rpm (revolutions per minute). But what they can’t know is how you typically load your boat, what you’ll primarily use it for, and the conditions you normally encounter. This is where changing the prop may fine-tune your boat to make a huge difference to you.
Maybe the boat’s prior owner had his own uses in mind when he swapped props, and these conflict with how you intend to use the boat. The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended rpm range.
Before rushing out to buy a new prop, establish a current performance baseline:
- Look up the engine’s operating range in the operator’s manual and make special note of the top end of that range.
- Take the boat out for a test ride the way you would normally use it (weight and trim) and note the rpm at wide-open throttle (WOT) and what optimal speed you attain (you may have to adjust engine angle trim). If you do exceed max rpm, throttle back to recommended limits.
- It will also be helpful to know how much fuel you burn per hour at your normal cruising speed.
- Now find out what prop you have, and what material it’s made of.
Props are usually stamped with their diameter and pitch on the hub. The diameter number comes first and the pitch next. Pitch is defined as the distance the propeller would move through a solid material in one complete revolution, with no allowance for slippage. In other words, a propeller with a 21-inch pitch would screw 21 inches into the material in one turn. (If there were zero slippage involved in the watery medium of boats, a boat would therefore move forward 21 inches, too. But of course there is slippage.)
Outboard props come in a variety of materials, from plastic on smaller engines to aluminum, composites, and stainless steel on higher-end props. You may be able to increase performance by simply getting a higher-end material like stainless that won’t flex as much. Also, because it’s stronger it may have thinner blades and be more efficient.
Number of Blades
Generally speaking, more blades are more efficient at moving water and give you more power, but will have more drag and be less fuel efficient at higher speeds. More blades will allow your boat to stay on plane at lower rpm.
Diameter and pitch are the main design characteristics, but there are many other subtle design elements such as cup and rake. Cup is the small radius of curvature located on the trailing edge of the blade. This curved lip on the propeller allows it to get a better bite on the water. This results in reduced ventilation, slipping, and allows for a better hole shot in many cases. Rake is the degree the blades slant forward or backward in relation to the hub. Rake can affect the flow of water through the propeller. Aft rake helps to trim the bow of the boat upward, which often results in less wetted surface area and therefore higher top-end speed. Forward, or negative rake, helps hold the bow of the boat down. For a more detailed description of design elements and propeller definitions click here.
Just looking at the prop won’t tell you the exact details of the subtler design elements, and to know these quantitatively you may have to have your prop analyzed by a pro like H&H Propeller with the right equipment and software to measure these subtleties.
If your test ride results in over-revving the engine, you’ll need to increase the pitch of the propeller. Increasing the pitch increment by one inch will result in approximately a 200-rpm drop. Keep going until you are within the rpm limits of the manufacturer. If your test run results in a lower-than-maximum rpm rating given in your engine’s operator manual, you may need to decrease pitch — decreasing pitch will increase your rpm.
For every inch of change in pitch, the effect will be approximately 200 rpm. If you are going to employ the subtler design elements such as switching from an uncupped to a cupped propeller, which will also reduce your rpm—I recommend you talk to a pro about these nuances. A cupped propeller of the same pitch and diameter will typically reduce your rpm by approximately 200.
If you use your boat for multiple activities like fishing, cruising, and skiing, one prop probably won’t do all these things equally well. Remember, the compromise was to choose the best prop for a given set of circumstances, and it may be that multiple priorities will require more than one propeller — one for each specialty and all staying within the rpm limits of the engine. Once you know about your engine’s limits, current boat characteristics, use priorities, and current prop, you’ll be able to choose your next prop wisely—even if it means a little compromise on the way.