Picking the Perfect Pontoon Boat Propeller
Picking the Right Propeller
Choosing the best propeller for your pontoon boat is not an exact science. If you don’t have the luxury of field-testing a variety of propellers, you will probably want to stick with the manufacturer’s original propeller style. Here is a list of the factors with which you can choose the best propeller for your pontoon-style boat that will help you do what you want to do with it.
Manufacturers label propellers by the diameter and then the pitch of the prop. These are the only standard marks provided on all propellers; other labeling is determined by the manufacturer. A “13 x 16” propeller has a 13-inch diameter and 16-inch pitch, Your engine's manual will provide you with prop recommendations for your boat. Tweaking your boat’s propeller should begin from the manufacturer’s standard recommended prop.
Aluminum propellers are the most common type used for pontoon boats. Stainless steel propellers are stronger, stiffer, and more efficient, and they may give you more satisfaction, but they do cost more. Older composite or plastic blades are useful as replacement props in an emergency, but I don't recommend them for everyday use. Now some of the newer composite props from manufacturers like Pirana have begun to show performance numbers and durability equal to or better than their metal cousins. Prices vary, but the prop's construction material is the primary factor that determines the cost of all boat propellers.
Check your manual and find out what the recommended RPM (rotation per minute) range of your engine is at full throttle. Usually, this is between 800 and 1000 RPM’s. When you select a prop, check first to see that the propeller operates within its rated RPMs with the throttle fully open in a straight line. If the prop makes the boat go slower or faster than this range, it isn’t right and you need to adjust one of the factors below.
The diameter or width of the blade is a critical factor. An overly large blade can overtax a small engine and make it run slower, while a blade that's too small can cause the engine to spin too fast and damage it. The weight, size, and the normal cruising speed of your boat determines the diameter range that is ideal for your boat.
The pitch is the angle of the blades. The number assigned to the pitch refers to the theoretical distance a specifically angled propeller will push forward through the water per rotation. A 24-pitch prop will ideally push 24 inches through the water in one revolution of the blade. In practice, slippage makes it a bit less than that. Lowering the pitch of the prop gives the boat more power and better acceleration. A higher pitch makes the boat go faster at the top end, but needs more power to run within a safe RPM operating range. If your full throttle test shows the RPMs are too high, then you need more pitch. Therefore, if your RPMs are too low at full throttle, it means you need less pitch. Experiment by trying the next larger or smaller pitch and testing again. A one inch pitch decrease will generally increase RPMs by 250 turns a minute. A one inch pitch increase will decrease RPMs by 250 RPMs.
Number of Blades
For practical purposes, three- and four-blade props can be used interchangeably with most boats. It's a matter or preference more than anything. The four-blade propeller, however, gives slightly more control at low speeds than the three-blade does at the same pitch. This is useful for steering a pontoon boat, which has a large profile. Surface winds can blow the boat around when docking or negotiating narrow spaces. A four-bladed prop helps provide you the control you need to safely dock in a breeze. You can also get the same level of control with a larger diameter three-bladed prop at lower pitch, but you’ll lose some speed on your top end.
Individual blade design determines how thick the blades are. The rule of thumb is this: the thinner the blade, the less drag and the more efficient the blade. However, if you will be traveling in water with obstructions and obstacles, you may want to choose a prop with thicker blades for added strength.
Cupping is the curl at the trailing edge of the propeller used to give the prop a better grip on the water. Remember that if you increase or decrease the pitch of the prop you select, then you should also consider whether the cupping changes as well. More cupping increases the propeller’s grab and increases stress on the engine, reducing RPMs. Less cupping will increase RPMs.
Slip is the difference between the theoretical movement of the prop through the water at the rated pitch and the actual performance. Typical propellers have a rated slippage of 10 to 30 percent. Slip numbers will impact pitch numbers. A 24-pitch prop with a 30 percent slip rating equals an actual performance of 16.8 inches per revolution. A 22-pitch prop with only 10 percent slip numbers equals a 19.8, making the 22 pitch prop faster than the 24-pitch.
Rake is the angle by which the propeller blade leans aft from the hub. Higher rake angles help high-performance boats reduce the loss of power due to air being pulled down from the surface between the turning blades. Lower-speed boats like pontoons use a lower rake-angle blade for better bite.
Questions & Answers
I have an older 24ft. pontoon boat with a 70 h Mercury Force. What size propeller would you recommend?
Like the article says, it depends on what you want to do with it. You can go with slow and more powerful, or fast and more easily bogged down (if you are towing a skier for instance. The article explains how to determine what sized propeller you need. The easiest way is to talk to the guy at the marina. If he has any technical experience he can walk you through the options.
I have a 115 e-tech on a 20 ft Sylvan pontoon. What propeller do I need?
It depends whether you're going for power or speed. Decide that first and then follow the instructions laid out in the article to determine the pitch of your prop. Your local marina guys can probably help you or the boat dealer you buy your props from. Do the tests recommended above so that you'll go in with a better understanding of what you want.
I have a 21-foot Pontoon boat rated for twelve people, and powered by a 1997 Johnson 50HP 2 Stroke motor. What is the best aluminum propr for this boat?
It depends on what you want to do with it; speed or power? That's your choice. Follow the suggestions for testing in the article to fine tune it. It's all a matter of whether you are getting the performance you want out of the prop. If you find yourself bogging down, you need to go for a prop with more power, but you sacrifice speed. If you want to go faster and don't load your boat down with a lot of people very often, you can go for a higher speed prop as described above. Every boat has it's own characteristics depending on hull shape, engines and the load it's carrying. You do have to do the testing bit. There's no off-the-top-of-your-head answer. It depends on your boat and what you want to do with it. Boats are like wives. You have to learn their idiosyncrasies if you want to get along with them.
I have a 21 Foot Pontoon boat, rated for 12 people. It is powered by a Johson 50 hp 2 stroke motor (1997). What would be the best prop?
I can't say. The article tells you how to figure that out. Once you know whether you want speed or power or some balance between the two, then you can talk with your marina guy, and he can help hook you up.