Semi-displacement / semi-planing.
Above the Waterline.
Flat bottomsBroad flat bottoms are ideal for stability.
V bottomA deep V is another simple hull to build.
Multi ChinedThese hulls use more but narrower boards in an attempt to compromise between hard chine and rounded bottoms.
Rounded bottom hulls are usually displacement hulls.
While the round bilge is the most versatile hull shape it is also more difficult to build and consequently more expensive.
These hull shapes however tend to be more maneuverable at slow speeds.
Rounded hulls range from U or barrel shape to wine glass and Y shapes.
Without a keel and either a skeg or deadwood at the aft end this hull shape will roll and be subject to excessive leeway.
There are also many hull designs which are a compromise between the round and hard chined in an attempt to gain the best from both shapes.
Displacement hulls are those where the hull is supported exclusively or predominantly by buoyancy.
In other words the hull pushes through the water rather than skimming over the top as in a planing hull.
This means that their speed is dependent on the waterline length.
Hull speeds vary at around 1 to 1.5 times the square root of the waterline length.
Displacement is a measure of the amount of water displaced by the hull, this also determines the carrying capacity.
These hull combine speed with sea-worthiness.
While they depend on buoyancy at slow speeds they are capable of developing a moderate amount of dynamic lift at higher speed.
Normally they are a combination of a V shape at the bow which becomes a flatter bottom towards the stern.
The bilge chine is normally quite pronounced for stability and ease of building.
Planing hull shapes are designed to run on the surface the water at high speeds.
Dynamic lift generated at speed raises them, which reduces area of the wetted surface thus reducing drag.
The most common design will have at least one chine to help planing.
However, in gaining speed this hull shape will forfeit maneuverability at slow speed, will be vulnerable to leeway and pound on waves.
It will also require extra power to get it up onto the plane.
The hull shapes vary from flat bottomed, to V bottomed and sometimes even a rounded bilge.
One of the main advantages of the multi hull is its stability.
A sailing proa for instance would be completely unstable without its outrigger.
There are also performance advantages.
At slow speed a multi hull with the same total displacement as a mono-hull will be less efficient as there is a greater wetted surface area.
However, the multi-hull will be less prone to heel.
At higher speeds the narrowness of the hulls, compared to the beam, will allow for an increased hull speed.
The trimaran is often an attempt to combine stability with a more capacious main hull.
Increasing the distance between the hulls with a wider platform will increase stability and accommodation but brings with it the problem of structural strength.
And donâ€™t forget that the mooring charges for multi hulls are generally higher than foe mono hull shapes.
The shape of the hull above the waterline will also have an effect on how she handles.
But as with most things on a boat there is an amount of compromise to be made.
High freeboard will add windage and leeway, however, it will increase the size of the accommodation and load carrying ability.
Load carrying is important factor for cruising boats.
Flared topsides especially at the bow are not just for looks.
They also help to reduce pitching.
The extra buoyancy in the flare helps to stop the bow digging in as it cuts through the waves.
Another reason for having more beam and buoyancy aft is because often that is where much of the weight, such as engines and people in the cockpit, tends to be concentrated.