Designer's Notebook: ...of hydroplanes, cats, monohulls and such

There are many types of boat hull configurations and although this text deals with powerboats some of the nomenclature would also apply to sailboats. The following hull types have been selected since they represent the most common types: monohulls, hydroplanes, stepped hulls, three points, catamarans, tunnel hulls, and hydrofoils.

MONOHULLS

A monohull has one supporting surface and is common to most powerboats. The single planing area may be completely flat, vee'd, arced, or round in cross section. In addition, a hull can be classed as one with a soft or round chine (junction of the bottom and sides), or as a hard chine where the bottom/side join in an abrupt corner. As viewed in profile the hull lines may be severely arced or relatively straight, particularly aft. Straight running lines aft, parallel, or almost so to the waterline, are intended for planing speeds. A planing hull skims over the water with the hull more or less above the water surface. A hull that goes through the water is classed as displacement... it pushes through the water, rather than on it as with the planing hull. Obviously there are categories in-between such as semi displacement, semi planing, etc. A flat bottom boat (with no vee in section, at the transom) is faster but the ride is rough, to put it mildly. Deep vee bottoms, usually considered to have about a 20 degree vee or more, came into popularity a few years back but do require more power. In smaller boats they also have a tendency to tilt severely to one side; called chine walking. In addition the small deep vee boat at slow speeds or at rest tends to list from side to side as the passengers move about. Boat bottoms are compromises that veer toward one performance characteristic or another, through an infinite range from a flat bottom to deep vee.

HYDROPLANE

A boat that planes or skims on top of the water surface is classed as a hydroplane and may be a monohull, or multi-planed such as three point and stepped hulls. Some designers include catamarans and tunnel hulls in this category. A hydroplane is intended for speed and does not work well at displacement speeds. Since speed is a function of weight, best performance will be obtained with lightweight hull construction and minimal passenger loading. When powerboats were in their infancy, planning the hulls was virtually out of the question as the available engines were extremely heavy and produced low horsepower. Thus the boats were long and narrow to go thru the water easily. When lightweight and more powerful motors became available the hulls were widened and shortened to take advantage of the ability to plane.

THREE POINTS

In common usage, the word "hydroplane" usually refers to a three-point. A three point hydroplane has two planing surfaces forward and one aft. The forward points (sponsons) are lower than, and are outboard, of the main hull. The third point is the aft end of the boat bottom and is generally flat. However, at high speed the third point is virtually free of the water, the boat is riding on the prop. Three points have been built with one point forward and two aft but have not been successful to the best of our knowledge. The common three point was as we know it was developed or perhaps better described as popularized about sixty years ago. Today they hold most major speed records for straightaway and circle racing. They have also become increasingly dangerous as they are pushed to higher and higher speeds. The hulls are almost free from the water and the air rushing between thru the forward sponsons creates considerable unstable lift. These hulls are not intended for offshore or rough water conditions they are primarily racing craft. However, GLEN-L has several smaller three points for the young at heart. Again, as with all hydros, weight is important. Heavily loaded, so that the aft end cannot lift out of the water, they are not that fast. It is possible to compensate for heavier driver weights in small outboard models by adding a hydofoil to the motor.

STEPPED HULL

Steps are transverse breaks in the bottom plane, straight or fan shaped. They create a clean break in the planing surface for reduced skin friction. Like a transom, a step ends a planing surface and the theory is to cause the water to miss contact with forward portion of the following plane. Substantial reduction in wetted surface under relatively smooth water conditions can thus be achieved. There must be a clean break of the water as it leaves the step; with air behind it. If not the step will create drag and even suction. Various numbers of steps have been used over the years but their current use is not common. Stepped hulls in general, have a poor reputation in rough water. Stepped hulls generally run very flat but many, particularly with a single step tend to porpoise, although a mild porpoise usually increases speed.

CATAMARANS

The word catamaran itself is unusual, as every other letter is an "a". The word is derived from the Polynesian words "kattu" (to tie) and "maran" (tree). The earliest form was a logical evolution from available materials to provide a stable platform.

The modern catamaran, mostly referred to as a "cat", has twin hulls for planing, side by side, separated by a tunnel or open area. The original Polynesian hulls were symmetrical, in section and were not power boats. Modern power cats have become assymetrical in most cases. A cat powerboat is a wide stable platform at rest or underway. It turns well, but flat, as opposed to the bank common to vee bottom craft. They are very soft riding in a chop, however, in conditions where the seas can strike the flat area of the tunnel, pounding will occur. Cats are preferably powered with twin motors, one on each hull. Various pods, third hulls, and other appendages have been developed to make single motors more efficient, but they still work better with twin motors. Like deep vees, cats have more draft than flat bottom hulls.

TUNNEL HULLS

A tunnel hull could be designated as a cat, but there is a notable difference. A tunnel hull is similar to a single hull split down the longitudinal centerline and spread apart by a tunnel. The hulls are asymmetrical; the inside surfaces of the tunnels are straight longitudinally and typically vertical. These are primarily fast boats and are commonly used for offshore racing with a rather high tunnel so the rougher seas make less contact with water. These hulls are best for twin motors but have been developed to work well with a single motor with adjustable prop height. The water/air mixture rushing thru the tunnel creates considerable lift and higher speeds in windy conditions can be dangerous. Again although thought of as race boats GLEN-l has developed small tunnel hulls for fast, safe, enjoyment on protected waters. The turns are flat as opposed to banking but relatively sharp with the hulls seemingly glued to the water. Fun boats!
Note: There is another type of hull that is often called a tunnel hull. These are usually slower hulls, more properly called "tunnel stern" hulls. These hulls have a fore/aft depression or "scoop" in the bottom for the prop. These may be in various configurations, and are meant to allow the hull to run in shallow water.

SEA SLED

The inverted vee bottomed Sea Sled was developed and patented by Albert Hickman more than 50 years ago. The forward section of the hull looks much like a "W" with the center vee section going to a flat plane at the transom. High claims were made for these boats and though many were justified, it was considered a poor sea boat. The boat's appearance was unusual and combined with the closely held patent, the number built was minimal. The comments made are primarily for historical information as few have been built in recent years.

HYDROFOILS

Hydrofoils are external appendages that support the hull (type may vary) above the water. As with a three-point, the idea is to decrease wetted surface and in theory should be capable of the fastest speeds. The struts, located fore and aft, have fins attached to lift the hull clear of the water as speed increases. The fins are similar to an airplane foil section but modified for water, parallel or slightly canted to the longitudinal waterline. The hull lifts free of the water as speed is increased, and settles into the water as speed is decreased to operate like a conventional boat. These are novelty boats, there have been some that operate successfully, but the moving parts, the vulnerability of the appendages, critical foil shape and angularity, make them impractical for the average builder or boater.

So what's the next advancement in hull design? Sorry, but we don't have a crystal ball. However, be assured it probably won't really be new. Over the generations boatbuilders and designers have added many different appendages to their hull designs. With advertising copy that described their "new" hull as "revolutionary", "a breakthrough", "exclusive design pat pending", "the superior boat of all times has finally arrived". Eureka it's here? Probably not, more likely a revision or modification of an old idea taken out of mothballs, often incorporated more with an eye to marketing than performance. And the advertising copy for this revolutionary innovation... it was most likely written by someone who didn't know his port from his starboard.

Inboard Hardware: Cam-driven Water Pumps

Inboard Hardware section.

In the past the pumps used on marine installations were belt-driven. Although these are still common, cam-driven pumps have replaced many of them, especially in ski boats where the motors are in the open and moving parts represent a hazard to the passengers. The belt-driven pumps take up more room and require more moving parts. Cam-driven pumps are clean, have fewer moving parts to maintain, have no "slippage" and are therefore more efficient.

How a cam-diven pump works... An adapter fits onto the exposed end of the cam shaft. A mount or timing gear cover, with a depth that allows for proper alignment with adapter is installed on the motor. The pump fits into adapter and bolts to mount. Our mounts come with a cam adapter.

Magnaflow pump... This pump is made by Glenwood, designed to appeal to the ski boat or runabout enthusiast. The pump comes in gleaming chrome with built-in dividers to direct the water to each manifold.

The drawing below shows a typical raw-water plumbing set up. The water is pulled in through an intake on the bottom of the boat, through the pump to the manifolds where it is pre-heated before entering the block. The water enters the block at the lowest point and exits at the highest point, where it is dumped into the water-cooled elbows or risers. This is not the only way to plumb the motor, but is the method we most often use.

Photos of the Magnaflow pump and Timing Gear Cover.

The above drawing shows additional options: an oil cooler and a line to recirculate the water to control temperature. A more typical way to recirculate the water would be to use two water by-pass hoses that would re-introduce the water in the hoses leading to the manifolds.