What propeller is right for my boat?
Every boat is unique. Every combination of engine, reduction (or lack thereof), hull size, cage size, and other special equipment is a specific set of trade-offs. Even if two boats were identical, they would differ in the way their owners used them. We ask detailed questions about the equipment on a given boat and, more importantly, what the owner wants from the boat - before making a recommendation.
First on the list is whether the boat is being built, or if the airboater is looking for a prop for a boat thats already built. From there, we proceed to engine size and type, horsepower rating, reduction unit & ratio, and optimum desired propeller RPM level that is, whether youre looking for "snap" or a wider range of operating RPM. Following are some basic guidelines:
·More "wing area" gives you more lifting surface
The more lifting surface available, the more "lift" (thrust) per revolution. There are several ways to increase this:
1. increase the diameter
2. use wider blades
3. add another blade
More wing area is generally better, if you have the horsepower (depending on the torque-curve of a given engine).
·Lower RPM means less noise
The faster the tips of a propeller have to spin to generate thrust, the more noise will be generated. If a propeller doesnt have to "hit" the air as often to create lift, then there is less shock. The tips of the propeller dont need to approach the Sound Barrier. The engine wont need to work as hard, either.
·More "wing area" also means more mass to spin up
·Less blades require more RPM for same lift
·The optimal propeller will achieve your goals for your boat.
The appropriate reduction unit will make any engine and propeller combination work together in perfect harmony. With a carefully thought-out combination of engine, reduction unit and propeller, any boat can achieve a very high degree of performance and efficiency. See the section "What pitch setting should I use?" on this page
What is the pitch of a Water Walker propeller?
Its important to look at the concept of "pitch," and how pitch is measured. The airboat industry typically refers to pitch in terms of degrees, when it is more accurately measured in inches. The pitch of a propeller is defined as follows: "The distance that a propeller would travel in an ideal medium during one complete revolution, measured parallel to the shaft of the propeller.
The pitch distribution (airfoil selection, and their orientation in space) of a Water Walker propeller is sufficiently different from any other propeller that a direct comparison is impossible. Furthermore, the pitch setting of a Water Walker propeller can be adjusted, providing a means to "fine-tune" the propeller to a given boat.
At the Reference Pitch if the Pitch Reference Mark is aligned with the parting plane of the hub - Signature Series blades have a 36" pitch, and Falcon Series blades have 56" pitch.
The combination of advanced composite materials and high-performance blade design have achieved as many as 4 pounds of thrust per available horsepower. The owner and designer, David Wine, went to great lengths to consider the myriad factors involved in transferring power from the engine of an airboat to the air. The resulting propeller, by design, remains the best long-term value per pound of thrust per horsepower.
What pitch setting should I use?
Every boat is different. Some trial-and-error will be necessary. However, between the torque-curve of a given engine, operating profile of a given boat, reduction ratio, available wing area, it will be possible to "back into" the right propeller combination and pitch setting. Each blade style has been optimized for a given set of operating parameters. Given the right propeller and reduction unit combination, the optimal pitch setting will top-out the engine at reference propeller RPM for that blade style. The Falcon Series performs best near 2,450 RPM. For Signature Series blades, optimum performance occurs at 2,800 RPM.
Water Walker uses a series of interchangeable components to arrive at an optimal propeller for the specifications of a given boat. Within increments defined by those combinations, the ground-adjustable pitch allows a propeller to be fine-tuned to the boat. The effective range of adjustment occurs within 4º of the reference pitch. Given the geometry of the blade shank, each 1/32" of shank circumference = 1º of rotation. Therefore, the Pitch Reference Mark can travel up to 1/8" forward or backward from the Reference Pitch (aligned with the parting plane of the hub) while remaining within the useful range of a given propeller combination. Above this range, air is literally thrown sideways, creating excess swirl, and "torque lifting" of the boat. Below this range, the tips begin to "go negative," or "flat," and produce decreasing thrust. Continued movement in the negative direction causes reverse thrust at the tips. The aerodynamic effect of "lift" is broken, and thrust is lost. Beyond ± 1/8" from reference pitch, a different combination is recommended.
What is the optimum RPM for your propeller?
Cypress Series: 2900 RPM
Signature Series: 2800 RPM
Falcon Series: 2450 RPM
Sirius Series: 2100 RPM
Maximus Series: 2200 RPM
What are the torque settings for my prop?
Refer to the Installation Instructions shipped with your propeller. (For earlier hubs, tapered counter-sunk allen bolts (inside) set to 35 foot-pounds.)
What can I do if I notice a vibration?
Before taking the trouble to send the propeller back for inspection, there are a few things you can do to make sure all blades are set to the same pitch. Any method to ensure balance must include identical relationships to a common reference point. The following are some suggestions:
1. The Pitch Reference Mark is consistent on all blades out of the same mold. As long as the Pitch Reference Marks of all blades in a propeller are set identically in relation to the parting plane of the hub, then the propeller will be in perfect balance. Smooth as glass
2. Angle finders can be found at any local hardware store. Take readings from the same place on all blades. If the angles read the same, then the propeller is in balance.
3. Tracking by something clamped to the cage. If the tips are the same in relation to the fixture on your cage, then the propeller should be in balance.
4. Lugs: Make sure all lugs are the same length (7/16"). Also, be sure that the lugs are set solidly, with no "play" or "travel".
5. Harmonic inherent in Cadillac Engines: On certain Direct-Drive applications with Cadillac engines, a harmonic vibration has been known to occur. To counter-act this, rotate the hub 60º in either direction and retry. Theres no way to know in advance where this harmonic will happen. If it does, rotate your hub, and try again.
If these methods do not eliminate the vibration, check the tip weights. Each tip should weigh within one gram of the others. If you find a difference in weight, let us know. The propeller might need to be sent in for balancing.
What if water gets inside a blade?
Water Walker Props, Inc. strongly recommends *AGAINST* drilling holes in the ends of our propeller blades. The problem is that, once holes are drilled, blockage of these holes, of any kind, has the potential to trap a *different* amount of air (or water, or various types of debris) in either blade. Air has weight. At RPM, under the influence of forces generated by the rotational motion of the propeller, any difference between the blades that might be introduced through such openings will be magnified, causing vibration, and possible damage to the propeller and/or boat (including operator, passengers, or any items carried in the boat).
To this effect, any physical alteration of the blades, particularly for purposes of racing, VOIDS THE WARRANTY. Racing, of any kind, voids the warranty.
Other methods exist for keeping water from the inside of the blades. Primarily, we recommend sealing the openings between the hub and blades with silicone sealant (as described in the Installation Instructions, shipped with EVERY propeller). Should water, despite these precautions, accumulate inside a blade, we recommend the following: Stand the blade upright on the shank-end (rounded area having 3 concentric ridges, where blade fits into hub) on a flat surface, preferably in direct, bright sunshine. Thermal expansion and contraction, especially through multiple cycles of light and dark, can be expected to drain any water from the blade(s). Once the water has drained, re-install the propeller, making sure to seal any openings through which water might penetrate between the halves of the hub and access the interior of any blade.
Why cut off my lugs?
Water Walker hubs are milled from forged ingots of 6061-T651 Aluminum. This material is inherently stronger than wood. Therefore, the lugs do not need to protrude as far into the mass of the hub to fasten the propeller to the engine drive plate. Placing the propeller assembly closer to the engine also reduces the over-center moment, which can make an airboat more stable.
If you want to swap back & forth, we suggest keeping a longer set of lugs specifically for the wooden propeller.
How can I sell my previous propeller?
If you are interested in moving up to a more modern propeller, but are limited by the need to sell your previous propeller beforehand, there are several options. In addition to your buddies, or the classified section of your local newspaper, try the following:
1. Dealers: any of Water Walkers dealers would be a good place to start.
2. Airboat Trader: Water Walker has operated a free listing of airboats and related accessories, as a service to the airboat world. For details, click here .
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