Planing v semidisplacement

[Mat-C]

I've never really understood the difference between a semi-displacement hull shape and a full planing one. I mean, obviously they operate at different speed/length ratios, but if one stands back and looks at a boat out of the water, how can you tell the difference?

[SAQuestor]

I've never really understood the difference between a semi-displacement hull shape and a full planing one. I mean, obviously they operate at different speed/length ratios, but if one stands back and looks at a boat out of the water, how can you tell the difference?

Mat,

A quick math addition suggests that there are (give or take a few) about 45,000 posts that have the potential to answer your question.

At the top of the page is a search feature that actually works rather well. I suggest this because we have had a running discussion now for a year (give or take) about semi-displacement and full planing boat designs.

There is so much information available here that a quick short answer would not do all of the several hundred individuals that contributed any justice. Someone would be short changed. I'm positive that you'll find the answer to this question - and dozens of others that will arise - when you you the search feature.

[edit] I just realized that this could come off as being a flip answer - it is not meant to be so. It's just that there is a huge amount of information available on this site and a couple hours of reading will probably anwer your question - and more. Good luck!

Best,

Leo

[Mat-C]

Leo
Thanks for the suggestion (didn't take it the wrong way btw!) I've read much of the information in the threads you've talked about. I may have missed something, but I'm still unsure as to the actual differences beween the two.
Ok - some semi-displacement boats have round chines, but not all. Similarly, some planing boats have (smallish) keels. Most semiplaning boats feature variable deadrise, with low deadrise aft - but then, so do plenty of planing hulls.
So - is there a difference - or a semiplaning boats simply underpowered planing hulls?

[FAST FRED]

Semi plaining will usually have a running angle that keeps most all the hull in contact with the water, at speed.

The plaining boat will need to lift the bow , to run on a small surface aft to decrease the resistance at higher speed.

FF

[tom28571]

If you want one thing that separates planing from semi planing, it is that a semi planing boat will have some rocker in the aft sections and a planing boat will have none.

There are exceptions to that rule also but if you think about it a bit, you will see the logic of it. You need to know a bit about hydrodynamics but that is necessary to understand how a boat works anyway. You don't need to know any math or even the details of the science. Just the old round side of a spoon stuck under the faucet stream trick will do. The spoon is the displacement boat, the flat of the knife is the planing boat and the semi is in between.

[Mat-C]

Thanks Tom, Fred..
Ok - I've seen quite a lot of semi-planing boats that have rocker - but then I've probably seen just as many with a straight run aft, or even hook in the aft sections.
I understand the result of all these shapes, but what I don't understand is why any boat that travels beyond its hullspeed would still be better off with rocker in its sections- at least not from an efficiency point of view. From the point of view of comfort in a seaway, I can see some advantages....

[tom28571]

I had that same issue when I first looked at the aft bottom of a Carolina offshore sportfisherman. The boat was about 45' long and had a small but visible rocker in the bottom. The builder just said "that is the way the buyer wants it."

These boats go out the the Gulf Stream in the inhospitable waters off the NC Capes and are expected to run through big waves going and coming at high speed. They have a sharp bow to ease entry into these waves which is understandable. So, I think the reason for the aft rocker is two fold. It keeps the bow high to avoid submarining into big waves and also to keep that sharp bow from taking over the steering when it hits an off angle wave.

They power these boats (as big as 65+ feet) with huge engines to get the necessary speed to do their daily job of getting paying sport fishermen out to fishing grounds, spend time there and get them back no matter the weather. So, they can afford to live with reduced efficiency of the rocker at planing speed to meet their other objectives.

As far as hook on a semi planing boat bottom, I'd guess that someone made a mistake.

[Willallison]

As far as hook on a semi planing boat bottom, I'd guess that someone made a mistake.

Interesting comment Tom. The problem with hook - as opposed to using trim tabs - is that you can't 'dial it out'. In combination with a deep, high deadrise forefoot it can make for a very diificult boat in a following sea. And whilst most semi-displacement boats feature some sort of keel, they are usually quite small. This, along with smallish rudders, compounds the problem.

But it's a valid question that Mat-C poses. I don't think I've ever read a text that describes the differences between semi-displacement and planing hulls. And these days it's the marketing department that choose what to call a boat, so that's certainly no guide!

[tom28571]

But it's a valid question that Mat-C poses. I don't think I've ever read a text that describes the differences between semi-displacement and planing hulls. And these days it's the marketing department that choose what to call a boat, so that's certainly no guide!

True enough Will. The reason that there is no solid definition between semi displacement and planing hulls is that there are an unlimited number of variations between the two. A hull with straight and parallel buttock lines, when powered adequately, is obviously a planing boat. As soon as ANY convexity is introduced to the aft bottom, it starts to move toward a semi planing hull. The case I gave is just a first move in that direction, although a small one and these sportfishermen are still clearly very fast planing boats. The upper end of speed range for a semi planing hull and a full planing hull will almost always overlap.

I know that some propose thresholds of speed/length ratio as the breaking points between displacement, semi-displacement (semi-planing), and full planing hulls. Since I know of boats that totally ignore such distinctions, I don't think such a definition that is acceptable all the time makes any sense. Becoming locked in to any such dogmatic definitions is a sure way to stifle creativity. In my opinion, any one wanting to be a good boat designer should be able to accept the subtleties and compromises that exist in the process. For what it's worth of course, the opinion of an amateur designer.

I do understand the value of speed/length ratio in describing these things to a layman (particularly a client), but not as a design tool.