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View PDF prop chart below.
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RC boating is a hobby where model boats, sometimes scaled down from their life size counterparts, are run (and often raced) via radio control (hence, "RC"). There are many forms of RC boating, including sail, electric, steam, gasoline and nitro powered. IMPBA, as a sanctioning body, primarily focuses on racing the nitro, gasoline and electric varieties. There is also a segment of the hobby that focuses on building 1/8th scale, detailed replicas of actual Unlimited Hydroplane boats. This is known as the Scale Unlimited class, and they participate in concours judging, as well as racing. The following gives some brief information on types of powering, hull classifications, and size categorizing.
Sail - A sailboat is powered primarily, as one might guess, by the wind. Radio control is used to move the sails and rudder to the positions needed to catch the wind and propel the boat in the intended direction.
Electric - Electric boats come in a wide variety of types, including submarines, tugboats, boats for show, and boats for racing. These boats are powered by 2- to 10-cell LiPo battery packs. Electric boats are quieter and cleaner than the nitro and gasoline types, and therefore more easily accepted on small, urban, running sites. This sector of the hobby continues to gain speed and enthusiastic support from hobbyists.
Nitro - Nitro boats use Nitro-Methane as their power source. Whereas batteries for an electric boat must be recharged after one run, a nitro boat can simply be refilled with fuel and sent out for another run. Each run typically lasts 5-10 minutes. Fuel costs approximately $30 per gallon. The fuel tank typically holds 6 to 12 ounces per run. The cost increases if you like the higher nitro (50% and up) for your boats. These were the first RC powered boats.
Gasoline - Typically large boats, these are powered by 2-stroke marine gasoline engines which run a gasoline/oil mixture. Run times exceed those of nitro boats, typically 10 to 20 minutes depending upon the size of the fuel tank. Of course, the fuel is less expensive. Gasoline boats gained in popularity in the late 90s and continue to grow.
There are two major classifications of hulls which are addressed in the IMPBA guidelines: mono hulls and hydro hulls. Within the hydro hull segment, there are eight approved classifications, although four are typically noted: outriggers, catamarans, tunnel hulls, and sport hydro hulls.
Mono Hulls - This type of hull has a continuous wetted surface when operating at racing speed. Their shape is in the form of a V. They are noted for their ability to slice through rough, choppy water. No point on the hull cross section can be deeper in the water than the center keel. The hull can be lap straked (at least one strake on each side of keel), but the strakes must not be as low as keel.
Outriggers & Sport Hydros (hydro hull) - The 3-point suspension, 4-point suspension, and multi-suspension hulls, as classified by IMPBA fall into this category. Along either side of the main portion of the boat are sponsons which do not run the length of the boat. These sponsons may be in the front, at the rear, or both. When racing, the sponsons and the propeller are typically the only parts of the boat touching the water.
Catamarans (hydro hull) - As with the tunnel hull, this inboard motor type has full side sponsons, running the full length of the boat. With an extended free board, they resemble full-sized offshore racing hulls.
Tunnel Hull (hydro) – These outboard motor boats are a variation of the 3-point design, but the sponsons run the full length of the boat. This creates a tunnel or cushion of air that is trapped along the length of the boat, helping it move faster.
IMPBA has distinct classes that separate nitro and gasoline boats by the Cubic Inch Displacement (CID) of their engine (or combination of engines).
Nitro classes: Class A (up through .129 CID), Class B (.130 through .219 CID), Class C (.220 through .300 CID), Class D (.301 through .458 CID), Class E (.459 through .670 CID), and Class F (.671 through 1.830 CID).
Gas classes: LSG 27 (up to 27 cc) LSG 36 (up to 36 cc) and twins (up to 27cc per engine for a total of 54 cc).
Electric classes: The voltage determines the class. FE Classes are N (3.7 to 8.46 volts), P (11.1 to 16.92 volts), Q (18.5 to 25.38 volts), S (25.9 to 33.84 volts), and T (33.3 to 42.3 volts)
For steam, there is a single class H (up through 1.8308 CID or up to 30 CC).
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