The ideal yacht will be different for every sailor, since everyone has their own style of sailing. To decide which would be the most suitable yacht for you, here are some considerations on hull types, with their advantages and disadvantages:
| Performances | performance cruiser | fin keel yacht | long keeled yacht | V-shaped hull | centreboard | |
|
Sailing
|
light air running light air tacking strong wind running strong wind tacking |
very good very good very good very good |
good good good very good |
moderate-poor poor moderate-good moderate |
good moderate-poor good moderate |
good moderate good moderate |
| Steering |
hand steered
reaction speed selfsteer mechanic. autopilot |
sensitive & light
very fast good till hull speed good |
sensitive
fast good moderate-good |
steady & heavy
very slow good-very good moderate |
easy & light
slow very good moderate-good |
easy
slow very good moderate |
|
Sea behaviour
|
movement
wet deck |
short & vigorous
little |
lively
moderate-little |
easy
much |
very easy
moderate |
easy
moderate |
|
tank capacity
|
small |
moderate-big
|
very big
|
big
|
big | |
|
Influence of
increased load
|
speed
forces on rig |
high
high |
moderate
moderate |
slow
little |
moderate-slow
little |
moderate-slow
little |
|
Susceptible
to damage
|
keel & rudder
propeller hull |
high
moderate moderate |
moderate
moderate small |
little
small small |
little
little modrate-little |
small
small moderate-little |
|
Beaching
|
for maintenance
in general |
big tide
not suitable |
medium tide
possible along quay |
medium tide
possible along quay
|
medium-small tide
possible, big heel |
small tide possible |
The information in this table is meant as guideline, because it also depends on the building method and the choice of materials.
A performance cruiser is fast as long as she is lightly built and a long keeled yacht is wet, when she is heavily built.
A glass/epoxy V-shaped hull with a lot of ballast tacks better than a V-shaped hull made of steel with the same amount of ballast.
To choose a certain type of yacht is partly a logical choice. What would you like to do with it? What are you going to take with you? What sort of equipment will you take and what will be the weight? In the list above, you will see a number of considerations for each type of hull. These in combination with budget, weight, length, and draught will limit your choices. Furthermore, there are a great number of other components that affect the sea kindness of a yacht.
Resistance
Every ship is dealing with the same principles: a balance between resistance and driving force.
Basically, resistance consists of two components: frictional resistance and wave making resistance.
When the speed of the boat is doubled, the friction resistance will be 4 times bigger, the wave making resistance 8 times.
At low speed, the wave making resistance is relatively small.
As long as you don't make big waves, the friction resistance will be the bigger component of the two.
Wave making resistance is also dependant on the waterline length. The longer the waterline length, the smaller the wave making resistance for a given speed.
Most cruising boats have a limited speed, the so-called hull speed. This can be calculated by: 2,43 * waterline length (m) ^0.5 (for lwl 10 this is 7,7 kn). In this situation, there is only a bow and stern wave, and these are opposite. The bow wave pushes the boat back, the stern wave will push it forward. This limits the wave resistance. If the boat goes at a higher speed the stern wave will be aft of the boat and will give no compensation to the bow wave.
Friction resistance is depending on the wetted surface, i.e. the surface you put antifouling on. Heavy boats do not necessarily have much more wetted surface. If you see the boat as a bottom and sides, the bottom stays the same and only the side area doubles when ship weight doubles.
When a boat increases in speed the water underneath the bottom gives lift. The bottom starts to act as a foil and the boat partly comes out of the water. A light boat can reach this situation in a smooth sea, a heavy boat needs the help of a wave to get her at the minimum speed to get enough lift.
|
Length of waterline
|
A greater length gives a bigger maximum speed.
|
|
Beam on waterline
|
A wider beam gives more stability and more wetted surface.
|
|
Weight
|
More weight can give more stability. More wetted surface, more wave making resistance.
|
|
Sail area
|
More sail area gives more driving force as long as the maximum heel angle is not reached.
|
|
Draught
|
More draft gives more efficient side forces, more stability, but might give more wetted surface.
|
If it is all about maximum speed, there are only two options:
A long waterline
A light planing boat
For average speed, also in a light breeze, I would prefer the long waterline.
For maximum speed, I would prefer the light planing boat.
For cruising sailors who will take extra load, the planing boat will be too heavy and consequently won't plane anymore. However, the planing boat might be an interesting option for the cruising sailor who is prepared to sacrifice extra load for speed.
A long distance cruiser will not reach planing mode easily and can be dangerous under the control of an autopilot or an inexperienced helmsman. The speed, and with this the energy in the boat, is big enough to knock down the boat when running down wind, out of control.
Mass inertia
Mass inertia means the distribution of the weights in the boat, with regards to the centre of gravity.
When you turn a boat without speed, the weight in the middle will stay in the same place, while the weight fore and aft will move. The further from the middle that the weight is, the more energy is needed to turn the boat. Under sail this means more energy to steer the boat, and also in a seaway also more water on the foredeck.
The reactions of the boat are slower.
With waves on the beam and with only a little sail hoisted, the transverse mass inertia can give a damping effect.
In waves, you will need more buoyancy in the ends to keep a dry fore and aft deck.
In addition to its own mass, when tacking, a deep forefoot will take more water, so more mass will be added.
Especially when short tacking, heavy ends and a deep hull will give extra-added resistance at every tack and will result in the loss of speed through the tack.
However, when passage making on longer tracks, you will have an easier motion and the shipwill keep a straighter course.