Semi-Displacement?

I take it then you haven't looked at the pdf file with skecthed in my post #14?



But since this thread is about SD boats, one would not really be debating low Fn's anyway...SD boats are higher Fn's.

 

It's a perfectly acceptable way of looking at it. I prefer to think in terms of the hydrostatic and hydrodynamic forces and moments because I'm interested in actually calculating squat, and there's no advantage for me in using a buoyancy viewpoint.

Sure, but amongst all that you made statements about "all boats" and blocks of wood.

Do you have a preference for a rough minimum design Froude number for a semi-disp. boat?

All the best,
Leo.

 

I just had another look. It seems I misunderstood it on first glance. My apologies.

 

Noted.
But the causes of trim-squat is because of the wave profile along the hull and the result of the excess or reduction in buoyancy, locally along the hull, as there is no longer an equilibrium with the static LCG, thus the LCB and LCG difference must be addressed. Hence the vessel takes up the equilibrium position of LCB and LCG to be in line, which causes the hull to trim accordingly, albeit in a transient manner. Since with increasing speed the wave profile changes and thus the temp location of ‘dynamic’ LCB locally also changes, but the LCG does not. Ergo, trim.

I use terms which are generally accepted by most of the reading audience for understanding. If I explained in the same way I lecture to the post graduate I occasionally do, I would lose about 99.9% of the readers (this is not meant to be patronising, just noting not everyone is a NA on this forum); thus, too technical and too many perquisites required to fully engage. (Unless requested to do so). Thus, any message must be presented to the audience, not to me.

No, no ‘rough minimum’, since I wouldn’t design just to a Fn for an SD boat, or any boat. I would design to the SOR, and the Fn is what it is after the design has been done. And then others can pick apart if they consider the design to be SD or satisfying the SOR or not.

 

No prob's

 

The trouble with using this point of view is that it is sometimes hard to determine what the hull wave profile actually is. If all you had was a side-on photo of the hull it might be difficult to discriminate between the hull wave and a thinner sheet of water that clings to the hull but provides little or no buoyancy.

I agree completely with your statements concerning the variation of the LCB. In my computer code it is sometimes necessary to iterate up to 15 times to find the equilibrium position, i.e. a balance between hydrodynamic and hydrostatic forces and moments. That number is usually less at lower Froude numbers, typically below about F=0.5.

Squat is tough enough to calculate using thin-ship theory. Spare a thought for the CFD community. Moving the hull into position, and re-meshing at each step could be a very tedious process. I've seen CFD codes that take 7 days on a bank of work stations to calculate drag at one Froude number. Iterating the squat a few times would make it almost impractical. Maybe that's why we see very few squat calculations in the open literature.

Leo.

 

Hi everyone.

It seems to me that in an effort to define semi-planing you are first attempting to define planing. Mostly you are relating planing to a Froude number or wave pattern (which is the same thing).

I don't see any physical reason why planing has to start at speeds above 'hull speed'. Given a boat light enough and flat enough planing there is no reason why planing could not occur well before a Fn of 0.4. It is little more than co-incidence that, for typical boat shapes and sizes, planing happens at Fn above 0.4.

For my money semi-displacement is the region where the boat is still squatting, so displacement is actually higher than when stationary. When the planing forces exceed the suction forces (so that displacement is less than stationary displacement), I would say the boat is now fully planing.

Feel free to disagree though!

 

Err... well.....
Unless there's some other force acting downwards, then displacement cannot increase, unless the mass due to gravity does....
True, parts of the boat will be more deeply immersed, but displacement remains the same.
I do, however agree that planing speed is greatly influenced by weight - and particularly by the relationships between the area of the waterplane and the weight.

 

But there is a hydrodynamic force acting downwards at low Froude numbers.
Imagine the hull as an airfoil. Then the flow under the hull "pulls" it downwards in a similar fashion to the lift on an airfoil. I refer to the force (acting at the LCB) as the "sinkage" force. This force can reverse direction at higher speeds, and then the displacement decreases (relative to the undisturbed free-surface) and we have something like planing.

Cheers,
Leo.

 

Sorry, no.

The distinction is, ..is the boat being supported by hydrostatic forces or by hydrodynamic.

At low Fn’s, all the forces are considered as hydrostatic, because the volume/displacement of the hull is still immersed in the fluid, and the waterline at rest is the same as at low Fn’s, or very close to it. Ergo, still hydrostatic.

If waterline on the hull when moving is no longer, by an obvious difference of say 50% or so, coincident or close to the original datum at rest static waterline, then clearly hydrodynamic forces are contributing to ‘supporting’ the hull.

Thus when at low Fn it is always hydrostatic forces, nothing to do with an airfoil. The hull doesn’t act as an airfoil as you suggest. Because the medium is different on each surface, and each surface profile is different too. The hull side is smooth, the upper side, has cabins, deckhouses etc, and is thus cannot be related to an airfoil.

 

Sorry to contradict, but again no.

As I started out in my posts, the is not an easy question to answer and requires and understanding of what causes trim and squat and how that is related to the hull form etc.

Since here are fine examples of hulls which operate in the 0.8~1.2Fn range which are not planning and not even semi-displacement., These are all displacement hull forms.





Thus, understanding the mechanisms that create trim and squat is the key to understanding what is a semi-displacement hull. No decrease in displacement too..the boat weighs what it weighs, whether in air, water, oil...etc

 

I'm with Leo on this. The bottom of a convex hull form does act like an aerofoil - the top (above water bit) might be doing its own thing, but flow over a convex shape will preduce low pressure and hence a force acting downward.
The planing force is the vertical component of the force created from water hitting the hull - which requires a stagnation point.
If the sinkage force is greater than the planing force then the boat will displace more than it did at rest. If the planing force is greater, it will displace less. If they are equal the boat will sit on its static waterline.
Displacement is a measure of submerged volume, not boat mass.

 

In the point of view I prefer, there is a change in effective displacement, i.e the effective displacement is the weight of the vessel + sinkage force.
Of course, the weight of the vessel remains constant.

Leo.

 

I agree.

This report shows how to calculate squat for thin ships.
http://www.cyberiad.net/library/pdf/tsl01a.pdf

You can also consult:
Ronald W. Yeung,
"Sinkage and Trim in First-Order Thin-Ship Theory",
J. Ship Research, Mar. 1972, pp 47--59.

For the way to include the vertical component of the propulsion force and the effect of stabilisers, see:

Doctors, Lawrence J. and Day, Alexander H.,
"Nonlinear Free-surface effects on the resistance and squat of high-speed vessels with a transom stern",
24th Symp. on Naval Hydrodynamics, Fukuoka, Japan, July 8--13, 2002.

Regards,
Leo.

 

The last two figures in Ad Hoc's attachement in post 14 (Figs 29d and 29c) show that the boat has bodily sunk at Fn about 0.3. If it has sunk it must have higher displacement. Displacement is not just caused by self weight, but the sum of all vertical forces.