Hull Configurations

Hull Configurations

The type of hull determines what kind of propeller is required to move the boat through the water.

There are two basic hull types displacement and planning. Displacement boats are comparatively slow. They are supported by weight of the water they displace ( static water pressure ). They are generally designed with a somewhat pointed or rounded stern as well as a pointed or rounded bow.
They ride low in the water like large ocean-going ships. The propellers on these boats run totally submerged and will generally be of lower pitch, when powered by an outboard or stern drive.

Planing boats which include most recreational boats, operate like a displacement hull at low/speed (below10 mph). With sufficent power, these boats rise to the water's surface and operate in a skimming or planning mode, supported primarily by the water's velocity pressure.

Planing boats are generally faster and more efficent than displacement boats. Their design is distinguislhed from a design by having a broad transom that meets the bottom with a basically sharp corner. The propellers on these boats often are not fully submerged and thus need to provide holding ability as well as higher pitch and rake because of the higher top-end speeds.

Flat bottom boat

When a boat's bottom has no "V" but is simply flat from side to side, it is generally referred to as a flat bottom. Where the bottom meets the side is called the "chine". It may have squarish corners (called square, sharp or hard chines) or rounded corners ( called round or soft chines).

Vee bottom

This is presently the most common bottom design, offering good speed with a softer ride that depends on the angle of the vee (called deadrise) the radius or shape of the keep line and the use of strakes. To increase top speed with only a little loss of softness in the ride, some boats are made with a small flat at the very bottom called a "pad".


Each manufacturer choose how many and how far back (towards the transom) to bring each strake in order to achieve the desired performance characteristics. A greater use of strakes tends to make the boat run higher and flatter, but a little harder.

Tri hull or Cthedral Hull

These boats are vee bottom with some degree of added outside hull often most prodeminant near the bow. The benefit of Tri hull is more stabelness at rest. The negative is rough ride in choppy water.

Tunnel Bottom

This design is most common on race boats. It differs from the older catamaran bottom in that the inside corners ( between the bottom and the tunnel) are quiet sharp. This allows great sharp turns and incredibly sharp high-speed turns and very soft ride. Some of these hulls experience handling problems at low speeds.


Tunnel vee

This combines a shallow vee bottom with twin tunnels, one on either side of a center pad. Top end performance is usually superior to a true vee bottom, but this generally comes with a rougher ride in choppy water and does not carry a load as well as a conventional vee.

Catamaran

Catamaran is basically a type of boat or ship consisting of two hulls joined by a frame. Catamarans can be sail- or engine-powered.

 

If planing hulls are more efficient why don't we see more rowing boats that can plane? Less effort to go faster.

If planing hulls are more efficient why don't we use them for carrying cargo and save the fuel that is lost in using the current heavy displacement bulk carriers?

The only time planing is more efficient than displacement is if you apply a length constraint.

Even hydrofoils are more efficient than planing in most speeds of interest.

Rick W

 

A planing boat is inefficient compared to displacement boats when operating at displacement speeds. Please ignore my generalization; I know that at planing speeds a planing boat will also displace water, so there is no clearcut distinction.

If you look at the transom, a planing boat needs to separate abruptly from the water flow or you will get a negative lift. At displacement speed there will be lots of turbulence bhind the transom caused by the flow separation, reducing efficiency.

A displacement boat is on the other hand optimised to let go of the water as gently as possible to minimize turbulence. This design is meant to avoid flow separation and therefore makes planing impossible.

As I understand it, the requirements for an efficient displacement boat and for planing are contradictive. You have to choose, or compromise to maybe get both modes acceptable.

Please correct me if I'm wrong and there is a planing boat that is also efficient at displacement speeds (compared to true displacement boats). I would like to have such a boat.

Erik

 

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Please correct me if I'm wrong and there is a planing boat that is also efficient at displacement speeds (compared to true displacement boats). I would like to have such a boat.

Erik[/QUOTE]


I think it can be done. I am working with a development of the Atkin hull.
The customers wish was a very efficient boat with electric drive. When I tested the model above the required speed it looked very promising also at "planing" speed.

Or should we say speed where dynamic lift is the greater part.

js

 

Erik818; The water is full of displacement boats that can plane readily. Go to any small boat sailing regatta and observe the likes of the Thistle class, Windmills, 420s, 470s, Lasers, and many many more. It is characteristic of such dual regime boats to have long fairly flat runs that have a very small angle with respect to the resting waterline. Generally speaking the angle must be less than 4 degrees or else planing performance will need excessive power. Hot rod style sailboats have a quarter buttock angle of about 1.5 to 2 degrees. The ability to get the transom above the water at displacement speeds, while keeping the angle low, is largely dependant on total displacement of boat, crew, and gear. This is another reason for keeping boats light in weight.

 

I admit that there are good compromises to get planing performance, at least for planing a couple of times the "hull speed", from a boat that is reasonably efficient also at displacement speed. Even though it might only be of academic interest, I still don't understand how a boat can be optimised for displacement mode and planing mode at the same time. In the real world "good enough" is more realistic than "optimised". Weight i relation to planing area seems to be one of the keys to a compromise that is good enough.

js, box keel boats are fascinating and possibly a good hull shape for building a 10 m boat that is well behaved at 7 kn as well as at 15 kn. At your coordinates 15 kn should be an attractive speed as it will get you to many interesting places within a few hours. 30 - 40 kn might be fun for a while but too wasteful, at least for my economy. Maybe the increasing fuel prices will make the market change to slow planing boats.

Erik

 

Hi there... I think planing hulls are quite efficient at low speed as any boat. Its always easy to go slow and requires very little power... its just that your running a large diesel to go slow which is not efficient? There is a point at where a planing hull becomes draggy and that's about 10kts till it reaches a plane... lets say that's 17 -18 kts? So lets say you have a means to add lift at 10 kts a lot of lift like 55% of the boats gross weight. So lets go a bit deeper and make a few changes to the hull... and say build it thinner since now you don't need as much surface for the boat to get on a plane at a given speed. Oh this has to be a catamaran or a very long thin tri. Now you have 2 thin hulls with a beam to length ratio of 16:1 and you build the boat out of cored composite (light). There is so little wake associated with a 16:1 ratio this hull will be efficient at all speeds. Now we deal with the lift issue buy using a foil that runs from keel to keel in the tunnel just forward of the C/G. At low speed to foil generate minimal drag equal to a very thin daggerboard? But at 10kts it begin to generate lift lots of lift and at 13 kts the boat begins to plane but is still not very efficient till it is solidly on a plane 15kts which is a good cruise for most multihulls. Chew on that for discussion please?

 

Hydrofoil assisted catamaran

heres a idea to bat around