Semi-planing vs semi-displacement

[WayGray]

I have seen authors discuss semi-planing hull designs and semi-displacement hull designs as if they were two different concepts. What am I missing? To me, a hull designed to efficiently operate in the transitional speed range from displacement "hull speed" to achieving some sort of planing status would have a single set of characteristics no matter what it were called.

[gonzo]

Sometimes the arguments over terminology rage longer than about the technology itself. lol

[Peter_T]

http://powerboat.about.com/library/weekly/aa012403b.htm

Semi- prefix to planing mean the boat hull is not fully opened to planing at high speed. Planing boats use the gliding bow bottom surface on a chine boundary to ride up over the water and over the wave surface. The problem is when the boat is fully lifted at the high speed running condition over waves. The boat will follow the wave crest to trough to maintain planing. This create some sea sickness to the crew.

Semi-planing apply a lesser degree of bow lifting. Boats with chine steps provide a way to lift the boat at the forward quarter forward zone. Bow wave train being break off at the chine steps to help in reducing resistance. The boat maintain in displacement, with lesser pitch motion. The web link article explains.

Semi-displacement means the boat deploys a feature to elevate the boat somewhat. It would be difficult to draw the line, as to what is exacting class as this type of vessel. Boats that are lifted partially by means of cushions as for SES, should be called Semi-SES. Effort in lifting the boat partially with a foil system can be called Semi-displacement, as long as, the boat's hull is still linked to the entrant mass of the displaced waterplane. Some of these special designs are protected by patents.

Moving to the Semi-style of the bow lift planing hull or the whole body lift hydrofoils, is to leave the extreme vibrating ride comfort or dynamic ride comfort to the more comfortable ride condition when the hull is connected by the entrant mass of the displaced waterplane.

Peter

[gonzo]

Any hull moving through the water has a degree of bow lift. Therefore a strict distinction between displacement, semi-displacement, semi-planing and planing is moot. We can take arbitrary limits, say 1.3, 2.0 and 3.0 hull speed, to define the argument. However, it is misleading to represent them as completely different since one mode transitions into the other. There is no exact physical limit like for example the speed of sound.

[SuperPiper]

What is the Couch number? Does it not identify the planing/displacement threshold?

[gonzo]

Sure, but it is arbitrary or there would be no arguments about semi-displacement or maximum hull speed. For example, a very narrow semicircular section hull can do over 2.0 "hull speed" without planing. I suppose the only true displacement mode would be in a hull with a flat horizontal bottom and vertical sides; there would be no dynamic lift.