Planing hull vs. displacement hull

[AMC]

General question for you JM. I understand the functional difference between a planing hull and a displacement hull, but perhaps you could give some of us a little tutorial. What are the overall advantages and disadvantages of each, and why would I want to use one or the other. Obviously I am not going to go 65 knots in a displacement hull, but other then that... Maybe even even mention the design cues that makes them on or the other.

[jacquesmm]

I can't cover all the details but in a nutshell, a displacement hull moves through the water by separating the water (displacing the water) while a planing hull runs on top of the water.
To plane, you need a specific hull shape and the power to 'climb" on top of the water = lots of power and planing surface towards the stern. Without that surface, whatever the power, you will not lift the boat to plane: you can't get a old fashioned sailboat hull to plane.
On the other side, displacement hulls are designed to separate the water smoothly without pushing a big bow wave and even more important, allow the water to close smoothly behind the transom.
A displacement hull will reach it's maximum speed at a certain wavelength and reaches it easily if well designed.
With more HP, the stern will be sucked down and she will not go faster. To add HP at that point is a total ridiculous waste: she will not go faster.

There are boat shapes in between but they rarely perform well: they waste HP at slow speed and never go as fast as true planing hull.

There are some hull shapes that will transition smoothly from displacement to planing without climbing on their bow wave: our ST21, the Pangas and the cats but they have their own limitations. Most are sensitive to weight, their top speed is limited and at displacement speeds, they are not as efficient are a well designed displacement hull.

When choosing a hull type, you must accept those limits and drawbacks. You can ask about putting a 10 HP on a FL14 and I will always say that she will go at the same speed than with a 3HP. I can't change the laws of physics.
Only in sales brochures will you see boats that do all those phenomenal things at all speeds but not in the real world.

[AMC]

Thanks JM, I just wanted you to confirm what I thought I knew. The part I find interesting is that a displacement hull can be efficient, and at low speed, more efficient then a planing hull. It just seems counter intuitive. I don't think I will be interested in a displacement hull anytime soon, but I thank you for your knowledge.

And sales brochures that offer everything are not exclusive to the boating industry.

[jacquesmm]

Unless it is a very bad design, a displacement hull is always more efficient, always.
When looking at mpg, same boat weight (displacement), a displacement hull will be between 4 and 20 times more efficient.

Before anybody give some extreme examples, I am describing displacement hulls at moderate displacement speeds.
A very well designed and light planing hull can give you up to 2mpg but my old sailboat was doing 30 t0 40 mpg. The difference is that I was going 5 mph and the planing hull goes 45 mph.