Hull types

[Javaid Hosany]

How do you differentiate between hull displacement, semi-hull displacement and planning hull?

[DavidJ]

This is a big question and it can be answered many different ways. This article provides a decent primer on the subject,there are two pages:
http://powerboat.about.com/od/hulls/a/displacement.htm

[northerncat]

ill add to this how does a sailing catamaran overcome its displacement speed say 8 knots to move at speeds of 20 knots or so?
sean

[BOATMIK]

Howdy,

The categories come to us from history.

There is no physical basis for dividing boats into those groups.

The physics will treat the "planing" hull and the catamaran in the same way if they can exceed "hull speed".

It will also explain why some boats cannot achieve these higher speeds.

If they require different explanations then the physics is not clearly understood.

Best wishes
Michael Storer
(hehehehehe)

[BOATMIK]

...

As a contribution I want to puncture the myth of "planing" and "displacing".

Every boat develops some lift over its surface. Generally in the front end where the water is being forced down to make way for the volume of the hull there will be a nett upward force trying to push the hull upwards

Towards the stern the hull pulls the water back up to the static water level so there will be a negative pressure will be reduced and the forces will be trying to pull the boat down into the water.

These pressures/forces operate on any boat when it is moving.

So if you add up all the pressures in the vertical direction over the whole surface of the boat you might find that the total pressure is slightly upwards.

That the boat is experiencing an nett upward force. Some boats can experience a downward force (I would guess).

So when a boat is "planing" some of the displacement of the boat is supported by those dynamic forces.

Now here is the point ...

So if the boat is experiencing a lifting force of 30% of its displacement ... what is supporting the other 70%????

Clearly the boat is
Planing - 30%
Displacing - 70%

So Planing and displacing are not different modes but two parts that balance the one energy equation.

I think it would be almost fair to say that every moving boat is displacing and planing at the same time!!!

The only boat that is fully planing is one whose volume is not supported by any displacement and thus CANNOT BE TOUCHING (or better, deforming) THE WATER SURFACE!!!

OK - I will get off my hobby horse now.

You can actually see this on boats that are travelling quickly. They leave a hole in the water behind the boat that comes back up to the static water surface some distance behind the boat. The volume of water that should be in that depression is closely related to the "lost" displacement from the planing forces.

When a boat is quite light the depression is quite small. This is a 16ft boat with a hullweight of 130lbs and me aboard.



A bigger motor boat

Cynics would say that the hole behind this boat is where you pour the gasoline

In my example above the water that would fill the depression would be about equal to 30% of the boats displacement.

Before someone says that multihulls don't plane ... have a look at this ...



...we have ALL seen the same depression in the water trailing a multi. But most don't think about what it means. I am not saying that this is the MECHANISM responsible for multihull speed - I am saying that those who say "Multis don't plane" are talking through their hats.

That hollow means that some of the displacement of the volume (hull) is supported by dynamic forces.

Now where my understanding falls down (that statement indicates that I think I am right about all of the above!!! - hope I am!!!!) is that I can't make a really clear argument about why some boats are faster than others.

It is something about the energy required to move the water to make space for the boat's passage. (neglecting skin friction for the moment).

And both a hull that is partially planing and multihulls have to "move the water apart" much less than something that does not fulfil those conditions.

And this in turn results in much less wavemaking drag - they expend less energy creating waves around the boat - but I can't put it together in a semi-coherent explanation like the above.

In other words a big fat heavy boat "moves the water apart" more so expends more energy in creating waves but I can't link the "moving the water apart" and then "it coming together" after the energy system of the boat has passed by with the wavemaking. How does moving the water apart create a wave train?

(I do hope I am right enough!)

Best wishes
Michael Storer

[BOATMIK]

Aha - it relates to the pressure system around the hull. The higher the pressure the higher the wave and the lower the pressure the lower the wave ....

The big fat boat creates higher pressures and thus higher waves.

... and then propogation and Froude. And for some boats Froude makes things worse putting the trough where most of the displacement of the boat is.

1/ Multis - However multis have a more "stretched out" displacement so their displacement is not undermined by their shallow wavetrain.
2/ "Planing" hulls have shallower wavetrains and usually more volume in their ends so the lifting forces from the hull kick in at the right moment and prevent it from settling in their somewhat shallower troughs.

But can someone do the words!!!!! I'll have a shot tonight if no-one else does.

MIK

[Javaid Hosany]

Woh, BOATMIK!

Very good explanation, thanks

[BOATMIK]

I was wrong about "any boat that is moving will develop a positive (ie upward) lift. Clearly some hulls and maybe all hulls in some conditions may develop a negative lift at times.

But the main point that there are no separate conditions of "planing" or "displacing" is true - that they are complimentary and add up to the total static displacement of the vessel.

MIK

This thread
http://boatdesign.net/forums/showthread.php?t=22315
is discussing more or less the same thing and there are some good people involved - so I am going to continue there.