A Rough Guide to Boat Design.
What every Boatie and Designer are chasing, is a stable and soft riding boat.
As a general rule, its true to say that the greater the dead raise the softer the ride the less stable the boat. (The difference in ride between a 14º dead rise and 18º is significant.) A 14º dead rise Aluminium boat is often derogatorily known as a slapper because of the banging of the Hull in the rough and the associated hard ride.
So boat design is a compromise….many designers of Aluminium plate boats are reluctant to increase the dead rise for fear that the stability will be compromised…and rightly so, there are numerous boats around that if you get three people leaning over one side, the boat will all but capsize …not what you are chasing in a family type boat.
The greater the dead rise for a given weight the higher the boat floats and the more the chines loose contact with the water, causing instability. To compensate, more weight can be added. e.g. heavier Outboards, Water ballasting, Lead ballast, etc. And/or design changes can be made by using reverse Chines so that they remain in contact with the water.
Ribs (Rigid inflatable boats) and Rbbs (Rigid buoyancy boats) aka aluminium ribs, use this principle by using pontoons / sponsons that act as reverse Chines allowing a bigger dead rise, with out loosing stability.
The advantage RBBs: (Rigid Buoyancy Boats) Flotation where it should be
When a conventional boat with a sealed deck is swamped, water can move freely about the floor of the deck, ( Free surface effect) that causes a transference of weight, which badly effects its stability, that under some circumstances will cause it to capsize, particularly in heavy seas. Having a sealed deck creates a void of trapped air under the floor that prevents the boat from sinking, but causes the weight of the swamped water above the void, to have a much higher center of gravity than if it were in the very bottom of the boat. When personnel or heavy seas cause that body of water to slosh to one side, the weight of that water combined with the buoyancy of the void, greatly increase the risk of capsize. It can be successfully argued that the flotation in the floor of the boat is in the wrong place it should be in the sides of the boat. Ribs have long proved that argument.
High lift Bow :
A raised bow sheer reduces the chance of the bow burying itself in the back of a wave, or short chop.
Hull design is an elusive art with dozens of contributing factors involved-hence the many alternatives and theories that exist. To fine tune your choice and determine which deign best suits your needs, isolate in broad terms the type of hull you require, (i.e. deep vee, moderate vee etc.) Stability at rest.
Generally for a soft ride in a Aluminium boat The closer the dead rise is to 20 degrees the better.
