Hull design dictates propeller type
Displacement hulls are generally slow-moving boats. They include tug boats and ships. They sit deep in the water, and their propellers operate completely submerged under the water and generally have a very large diameter with a low pitch angle. The large diameter helps them to grip the water like wide tires add traction to your pick-up truck.
The second hull design, the planing boat, includes most recreational boats. When they are operated at idle speeds, they act much like a displacement boat, but as speed increases, the boat lifts to the water’s surface and skims on top of the water. The planing boat’s propeller is usually not fully submerged, and it may require the better holding power of a cupped blade along with a higher rake and higher pitch in order to achieve the speed capabilities of the boat it is powering. There are many variations of the planing boat bottom design, and each of these designs may require a different propeller to maximize the boat and engine’s capabilities.
The first of these bottom designs is probably the most common hull you will find in South Louisiana marshes — the flat-bottom boat. The bottom of this boat has no “V”; it is just flat from one side to the other. Flat bottoms are most commonly found on aluminum or fiberglass jonboats, but don’t be surprised if you also find it on sophisticated professional ski boats and fishing boats.
There are slight differences in the area where the sides of the boat meet the bottom. This area is called the “chine.” Most flat boats will have a hard chine, which is created when the bottom is bent sharply upward to form the sides of the boat.
Other flat bottoms such as the ones that are popular with duck hunters will have either a round chine or a tapered chine. The round chine is created by bending the bottom upward in a smooth radius curve (no sharp edge) in order to form the sides of the hull, and the tapered chine is created through the use of one or more small bends in the bottom. These tapered bends are usually 1 to 2 inches wide. The bottom is bent at a slight angle one, two or three times in order to form the side.
The hard-chine boat is usually a bit faster, but may have a tendency to slide when attempting a hard turning maneuver. The tapered chine boat will perform close to the hard chine, but because of the tapered chine there is very little problem with sliding in a turn, and the round chine boat turns like it was on a set of rails. Round and tapered chine boats are also less likely to get stuck in the mud when the tide falls out.
In general, flat-bottom boats are designed to achieve their peak performance by skimming across the surface of the water. Most of these boats have the propeller operating just under the surface of the water, so they do not need a very aggressive cup or high rake design propeller. The one exception to this rule is the air-cooled, surface-drive mud motors that are popular for duck hunting. These motors run with as much as 50 percent of their propeller out of the water, so their blades are heavily cupped and they usually have a much higher rake design than a stock outboard motor prop.
For those of you who prefer to venture out into big-water areas such as lakes and bays, you may prefer a V-bottom hull. The degree of V in the bottom is called “dead rise.” This can vary from as little as 5 degrees to more than 20 degrees. The extreme deep V is usually used for an offshore boat that may have to operate in heavy seas. The lesser degree of V is most popular for bay boats and other inland-water boats. The V hull will usually have several flat surfaces called “strakes” that give the boat lift and increased top end speed while the hull design offers a soft ride in rough waters.
The pad V bottom was made popular by the bass boat industry. These boats have the center section of the bottom flattened, usually about 12 inches wide. The boat offers much of the comfort of a V hull, while the pad gives the hull more lift in order to reduce drag and increase top-end speed. In order to maximize the performance of these boats, the engine usually has to be jacked up higher on the transom so that more of the propeller is at or above the surface of the water. This requires a cupped, high-rake propeller with a lot of pitch to the blades. Set up is critical. It can be time consuming and costly, but the rewards are great when you can out run your buddy.
A boat design you do not see very much is the cathedral, or tri-hull. To look at it from the front, it looks like three V bow boats hooked together. Usually the center V is slightly deeper than the two outside V’s. These boats offer a very stable platform when at rest. This can be a big advantage when the boat is anchored and you are trying to fish. The trade off is that it tends to ride rough in choppy seas.
Tunnel and tunnel V boats fall mostly into the high-performance race boat category. They do not lend themselves very well to the average boater or fisherman. Engine set up and propeller selections for these rigs can be extremely technical, time consuming and costly.
Propeller selection can be confusing. Choosing the right prop for your boat means finding that optimum compromise between load-carrying ability, top-end speed, maximum fuel economy and engine longevity. Check with your dealer, and ask his advice and assistance. Above all don’t guess. Experiment, make notes and compare.
If you have any questions about your boat, motor or trailer, you can contact me at firstname.lastname@example.org.
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