Semi-displacement boats don't generate lift?

I worked closely with David Giles for quite some time as the design of FastShip Atlantic evolved. Worked with Gabor on a number of projects too, before his retirement from Carderock. Of course we worked with Don Blount on a number of projects too.."Destriero" among them. Suits like David's very seldom succeed...the one lodged against Alan Soares/AMD a number of years ago being an exception.

 

Exactly. Gabor and I applied that principle to the design of a 63m fast missile craft, with good results for reducing resistance. The tabs are part of an active motion stabilization as well.

 

Check the Ama of the Romaric Neyhousser designed trimaran Arkema 4 (now Solidaires en Peloton):

Looks like it hooks with possibly a reverse kink at the rudder. This is not unlike the feature that I think was the topic of the Giles/Freedom Class lawsuit except on those vessels it occurs at the jet drive intakes.

Similar principle, BMcF?

 

I've seen roughly similar transitions to the stern surface on many many "high speed" vessels ( the Vosper-Thornycroft and Abeking-Rasmussen portfolios come readily to mind) but most of those were prop-driven, not waterjet as are the FastShip and Freedom LCS. Be interesting to learn what details of the Freedom hull David considers worth a lawsuit; as far as I was aware, the Freedom LCS hull was originally scaled/derived from Don Blount's "Destriero".

 

At https://ecf.cofc.uscourts.gov/cgi-bin/show_public_doc?2012cv0484-181-0 the paragraph spanning pages 32-33 reads:

"The government further claims that the specification does not sufficiently describe how to determine whether the inlets under the stern are within the “high pressure area,” as stated in the claims. Def.’s Post-Trial Br. at 43. However, the specification discloses that the high pressure area is below the stern. ‘032 patent, col. 4, lines 60-68 (explaining that the inlets should be located in a high pressure area “correspond[ing] to the stern area of the semi-planing hull where high pressure is generated to lift the hull”). This description is sufficiently enabling because persons of ordinary skill in the art would know how to configure inlets and waterjets within the high pressure area under the stern to maximize performance of the waterjets. See Tr. 1650:25 to 1651:9 (Blount) (explaining that naval architects measure hydrodynamic lift on bare hull models in the location of waterjet inlets “as a matter of routine”). Thus, undue experimentation to determine where to place the inlets is not required to practice the invention, and the government has failed to show by a preponderance of the evidence that the subject claims are not enabled by the specification of the ‘032 and ‘946 patents."​

 

Amazing. Classic example of what can happen when a patent is issued to cover details long known and practiced by others, but unknown to the patent examiners. Gets right to the heart of the old saw "you can patent a ham sandwich".
Also interesting how many of those folks named are people I know and have worked with over the last nearly 40 years.

 

Group project proposed at
Electric Launch Design optimized for semi-displacement speeds https://www.boatdesign.net/threads/electric-launch-design-optimized-for-semi-displacement-speeds.68648/

 

I think that this is nearly a ridiculous statement.

Yes, there is such a thing as "hull speed". It is the top speed a vessel can go with a modest amount of power for its weight. Above that, the power needs increase exponentially beyond such limited power. And hull shape that is designed for such limited power is only good at going at hull speed. This is especially true if the vessel is heavy for its length and beam. Of course, if you add a rediculous amount of power, even a shape like that will plane. But such would require much more power than needed to get a hull shape of the same weight per length and beam, designed to plane, to do so.

And a vessel, heavy for its length and beam, and designed to plane, suffers a low-speed drag penalty for not planing.

If a vessel is very light for its length and beam, its underwater shape matters much less. And it can handle planing or not planing with far less drag penalty. The Length/Displacement (D/L) ratio is the best indicator of such. One on either side of 100 fits this bill quite well. One over, say 200, and the above mentioned drag penalties start to come into play. But to get a D/L of around 100 and get a decent payload capacity requires a rather long boat. Such may not be practical for cost of docking reasons. And something like that is likely going to cost more to build per given payload capacity, due to the strong, light matterials required to build it. And its lighter weight and lower payload capacity limits the amount of fuel it can carry, and therefore limits its range.

So the concept of "Hull Speed" is still relevent today. And one had better know about it if one intends to design a vessel with long range in mind and needs it to travel at a certain speed. The Hull Speed for its waterline length will indicate just about the upper limit for such speed with range (the boat needs to be heavy for its length and beam to have th payload capacity to get long range).

 

What units are you referring to?
JS

 

While I agree with some of what Sharpii2 has has said, above, I understand the term "hull speed" as defining the upper limit of the displacement domain (or "mode" or "regime") as 1.34*DWL^0.5 or length Froude Number = 0.4
Since the topic of this thread is the semi-displacement domain I'll add that I define that as being above "hull speed" by definition and below 3.0*DWL^0.5 or length Froude Number = 0.9, with the caveat that the onset of planing is not strictly length dependent and is extensively discussed in another thread in these forums.

 

A year of discussion and 7 pages of opinions and no one mentioned the hump speed. For all the talk of hole shots and getting over the hump that I have heard it would appear that it was just talk, or you guys are missing something. I will have to assume the former since the latter would be a huge disappointment. So you probably do know what you're talking about, but I'm not convinced you've said it, or spelled it out well enough for the uninitiated to have spotted it.

Froude number seems to be the key, and there seems to be some agreement that there are domains characterized by Froude numbers that could be named displacement, semi-displacement, semi-planing and planing. Froude for Thought. Or, what the Froude is gong on here? Does the linked article make sense? So displacement is fairly obvious, and planing, like ****, is as hard to define as it is obvious. The numbers associated with each appear to be FL<.4, FL> 1 and "Oh, that's just 2 much" respectively, but not much respect for the latter. Semi-planing and semi-displacement are in between and the line between them is unclear. So displacement trails off about 0.4, hump speed is about 0.5 and planing starts around 1. I got lost somewhere in the middle of slenderness ratio. The slenderness ratio (length (L) divided by the cube root of the volume of displaced water) depends on the displaced water at rest, correct? I did say uninitiated.

It seems obvious that when planing, the cg is higher, but the ratio of volume displaced at rest to that at speed is not normally considered when defining planing, although I have seen it done. When you look at a boat on a plane, it is clearly much higher in the water than when at rest, and the wake is generally a lot smaller than it was before it was planing. Never saw that quantified very well. Weston Farmer wrote about a Schock design that was 25' by about 5' and ran easily to over 12.5 mph, FL 0.65 with the bow slightly raised. I have followed a 35' x 16' (as configured for water mode) vehicle with a 7' draft that rose about 5' at 25 kt, FL=1.33. It looked like it was planing. I have ridden on a donated tax deduction that was 25' by 12' with 3 cat diesels that did 25 mph without appreciably changing it's draft, FL=1.27. Not so much displacement as plowing, but not apparently planing whatever the FL might say. The latter two seemed to be displacing a similar amount of water at a similar speed. One threw it 60 feet in every direction, including forward and the other was trailing a tsunami. The donation might have been higher in the water than it looked, but the massive wake made it hard to tell, and 45 years does nothing for the memory. So there has to be something more than the FL, and the article linked above does little to explain how th slenderness ratio figured in.

 

It seems to me "hump speed" is exactly what we've been talking about. The resistance "hump" is very clear on the Nigel Irens graph I posted on page 5 of this thread. That graph also makes clear it's possible to design a boat that doesn't have one - but it is a tradeoff.

 

I think that this is what most people who have not made a detailed study of the problem miss. There are two curves on the powering graph. The first is the resistance curve based mostly* on the hull form. The second is the powering curve based mostly* on the propulsor type. Change propulsors, you change the maximum resistance of the hull at max speed...likewise; change the hull you change the maximum power output of the propulsor at max speed. So...what is that * for? Because there is an interaction between the propulsive method and the resistance; sometimes small, sometimes not. You can make a brick hydroplane or fly if you give it the right power source (c.f. a jon boat or the F4 Phantom II).

The only thing "ridiculous" here sharpii2 is you hanging on to outdated concepts. And that is not a "nearly a ridiculous statement", but an actual flaw in logical engineering evaluation. Even your attempt to paint a narrow justification for "hull speed" is beset with engineering errors. Some full displacement hull forms have no humps to overcome. As I said: "The concept of hull speed is an archaic holdover from early theorists best left behind in the hubris of history"... Yes, some hull types and certain corresponding propulsion methods are bounded by their interacting limitations, but not solely by the physics of the hull.

 

Jehardiman: I think you're conflating hull resistance and propulsive efficiency. Of course propulsive efficiency is a thing, nobody's denying that.

If you're saying you'd like to see Nigel Irens' graph in slightly different terms, I agree, and have said so. Your conflation doesn't help, though.

 

No, I am not conflating them. Most people never design a hull form to work in unison with the propulsion method, I have done a couple. I understand that a lot of people on cursory examination do not comprehend that hull resistance and propulsive efficiency are inseparably intertwined, especially for displacement hulls. Most designers don't understand the interaction between increase in hull resistance and the increase in propulsive efficiency. This goes back to my post #2 about how vessel work.