In many cases the design of the hull is controlled by use etc. A narrow boat is going to have a flat or shallow V bottom to accommodate shallow draught. A well-designed yacht hull will be designed to give symmetrical wetted hull areas when heeled. Any deficiency in the performance of these common designs resulting from their limitations have to be accepted.
Any hull can be put into one of two categories:-
DISPLACEMENT virtually everything bar power type boats and a few high performance sailing dinghies come into this category.
PLANING Power boats which "climb" up onto their own bow wave are planing craft.
In general terms, a well-designed displacement hull can be driven through the water with moderate power and with minimum wash.
When the speed increases the stern eventually falls into the trough of the bow wave, at this point any increase in power only makes the wash worse rather than increasing the speed. At attempted high speed these hulls create undue wash.
The maximum speed of these hulls can be calculated as shown later in the course.
These hulls are designed to run at high speed on the top of their own bow wave. When planing they make comparatively little wash for the speed they are doing.
At low speed these hulls usually make far more wash than a displacement hull.
The engine should be powerful enough to force the boat up onto its bow wave.
HULL TYPE RECOGNITION
Typical displacement hull, easily driven, minimum wash at low speeds.
SHALLOW V or HARD CHINE
Typical "Mass produced" GRP or "easy build" wooden hull, also steel hulls.
Hard Chine refers to the sharp angle where the bottom meets the sides. The chine (the angle) may well be rounded on GRP boats, especially those designed for displacement rather than planing use.
This hull type will plane, but the lack of SPRAY RAILS and an extended keel tend to suggest this one was designed as a displacement craft.
Likely to make more wash than a round bilge type, but the amount depends on the exact design.
DEEP V HULL
This hull is typical of the high speed planing types. The spray rails designed to "trap" the bow wave under the hull show this to be a high-speed type.
Tends to be very dirty at low speed and needs lots of power for high speed.
Most hulls appear to be easiest to steer when trimmed to be slightly bow up. Anything that alters this trim will have an adverse effect.
A deep V hull with an "inland" type engine is likely to trim bow down without extra ballast to compensate for the light engine. Putting a SMALL outboard on the back is even worse.
A steeply angled drive line will tend to "push" the stern up as power is applied, this also alters the trim.
MAXIMUM HULL SPEED
Most craft owned by our typical class members will have their maximum speed limited by wave making rather than friction.
A 72ft narrow boat MIGHT be limited by friction, but as canal use imposes hydraulic limitations on speed like the boat trying to push the water along the canal in front of it, the maximum speed is academic.
Any speed trials must be done in water of ample depth and width.
A small displacement craft can have its maximum speed calculated by the formula:-
Speed = square root of water line length x Constant
The constant should have been calculated by the designer, but it is usually about 1.3 to 1.35.
A 30 footer with a water line length of 25 feet would have a speed of:-
5 (Sq. Rt. of 25) x 1.3 = 6.5 knots
It is absolutely pointless trying to drive a displacement boat any faster, you will only spend fuel making larger waves.
There is also little point in installing an engine so large it is just off tickover at 6.5 knots (exceptions allowed for narrow boat type who like the noise they can pay for any problems created).
Again there is little point in messing about with propeller size to try and get a boat with an inadequate engine up to the design speed.
An engine running light and at low speed will not get hot enough to evaporate condensation from the oil, the cylinder bores might glaze (go shiny and let oil get burnt), and the alternator will not charge the battery. The coolant may not get up to temperature so more wear takes place and if a calorifier is fitted the domestic water will not get hot.
An engine running flat out will overheat its oil leading to wear, the high revs will cause wear and tear, and the fuel consumption would be bad. An engine trying to operate beyond its power (known as "over loading") will make black smoke. If the cooling system is inadequate, the engine will overheat.
Again assess the hull type for suitability of purpose. A deep V on a canal is not a good idea and will probably be difficult to control.
Assess the match of engine to hull and try to ensure the engine works moderately hard during normal use.