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.
A narrow boat with a decent dead rise of say 18+º will have a softer ride than a wide boat with the same dead rise.
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.
Too compensate, more weight can be added ( heavier Outboards, Water ballasting, Lead ballast, etc.) and or Design changes can be made …narrow the Hull and increase Chine width; the use of reverse Chines which bring the Chines down to the water.
( Ribs and Rbbs use this principle) by using Pontoons that act as wide Chines which allow a narrower Hull, thus allowing a bigger dead rise, with out loosing stability….a very big dead rise is a round 28º used on some Ribs ( that in some instances of extreme use can cause them to capsize.)
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.
Designers can also use variable dead rise, which means that the dead rise at the Bow is considerable greater the dead rise at the Stern which gives you a finer enter, ( smoother ride) the down side is that it reduces stability.
The advantage that pontoons have apart from that mentioned above:
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 roll over; 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 centre 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 floatation in the floor of the boat is in the wrong place it should be in the sides of the boat. Pontoon boats have long proved that argument.
High lift Bow or Flat Bow:
A raised sheer to the bow is beneficial for offshore use. Obviously such a design affords the boat increased recovery in a following sea in as much as it will serve to help the bow avoid burying itself in the back of a wave. I believe that a raised sheer tends to give the crew a greater feeling of confidence.
The correct angle of attack too will be all the less critical both in head and following sea states with a rBB having a strong raised sheer to the bow.
Deep Vee Hull.
Not all RIBs feature a true deep vee hull but in general terms a deep vee possesses a minimum dead rise angle of 20° If you intend to use your boat for those offshore to the fishing grounds then a deep vee is essential in providing the degree of comfort required by those making longer trips. The degree of deep vee alone does not guarantee a soft riding hull. It is the the overall design that gives a soft ride, stability under way and at rest, that is essential for a well preforming boat. While some spray rail/chine arrangement are beneficial in assisting the hull's performance and providing a dry ride. Further consideration might include, how rapidly the hull flattens out towards its aft section, for though this will assist in getting the boat up into the plane more readily it could in some cases also provide a harder ride.
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.
Moderate Vee Hull.
This type of hull has a shallower vee than its deep vee cousin. A moderate vee is generally highly suited to sports performance, although it is harder riding, It is not as so sea-kindly as a deep vee in offshore conditions.