flat sterns cause squat ?

Thanks Will... wisdom I have ony just obtained by PM, both from you and another member

 

It surely depends on how the fairing is done...
And btw, how do we define "sucton"? Are we talking about a local net force acting downwards, or about a local horizontal net force (acting backwards - which so becomes a component of resistance)?
Or are we just talking generally about the region where pressure is lower than some reference value? What reference value - atmospheric pressure?

 

The bailer on Easys' boat is a common fixture. It even works on sailboats at very low speed. Sunfish have them, among others. Way back in time Paul Elvstrom marketed a floor mounted bailer that consisted of a flat plate with enclosed sides, something like a sugar scoop. It could be opened or closed with its' simple lever. It would bail like mad even at very low speeds on the order of 3 or four knots. If the bailer was open when the boat went fast or approached planing speed the Elvstrom would shreik and was prone to suck brick bats, baby dolls, and puppy dogs down the opening. But of course that is not the topic at hand. Easy can almost surely attest to the ability of his bailer to swallow great lengths of fishing line.

I read Toms essay, in the Blue Jacket site, with interest. I must go back and read it a few more times. On first read I thought that some of the comments tended to ignore the incompressible nature of water. If we push ,or pull, or deflect a particle of water it has to go somewhere, and that means that it must push another particle or particles out of the way. Water apparently has a mind of its' own because it can see a disturbance coming. Want to prove it to yourself? Put a few inches of water in the bath tub. Sprinkle it with black pepper flakes or sawdust. No need to cover the whole surface, just a small amount of floating debris will do. Now place a stick or perhaps a model boat, or a rubber ducky in the water. Move the boat slowly through the water. Observe that the sawdust, well forward of the boat, is beginning to move aside. I know that this is not part of the "suction" subject but it does relate to the graphic in Toms essay that shows the water piling up before the stagnation point. If you do this experiment, I suggest that you do it while your signifigant other is away on a shopping trip.

 

Mat-C, pinning down the boat’s relative speed is important. Here, it seems you are concerned displacement mode, well below hull speed.

I agree that the wave-system is the predominant contributing factor affecting the vertical position of the stern relative to the horizontal plane of the still water at speeds below hull speed. I don’t think there is a controversy here. I thought the issue was regarding can suction contribute to effecting the vertical position of the stern.


Suction is expected to be a function of the boats speed (as well as hull geometry and other factors) and becomes more important as the speed increases from a still position.

 

Messabout, that is a nice simple demostration, I like that.

 

Daiquiri, you are right to ask for a definition of the terms. I've been a bit sloppy with its use. ... A low pressure relative to atmospheric pressure is definitely a suction. Also, there are regions of fluid pressure that is low relative to hydrostatic pressure, which may or may not be less than atmospheric. This might also be thought of as suction, since fluid may flow toward it. The hydrostatic pressure is a little more difficult to use as a reference because it varies with depth. I’m not certain what terms would be clear. Perhaps you can suggest more precise terms.

 

BTW, I don't want to take the position that suction makes a significant effect on the vertical position either.

I will take the position that the suction (upside down lift) does exists. The extent that it effects the vertical position of the hull or stern is an interesting question. However, I think to answer that question we would need to do a quantitative anlysis.

My interest in suction is really more about it's contribution to drag or resistance. I consider being able to quantify the amount of suction (upside down lift) a hull creates, and under what conditions, an important question.

 

See Page 5 and 6 of the attached report for a thin-ship formulation.
Maybe it will give some people insights into the interplay of the LCF, waterplane area, and the moment of inertia of the waterplane.

If someone wants to give me the offsets of a couple of different hulls (in Michlet or Delftship format) I am happy to do the squat calculations to help the debate.

Leo.

 

messabout,

If I put any hint of fluid compressibility in my material, it was by mistake. Will have to go back and read the article to see if its there.

 

This confuses me just a bit. If the suction exists and its vector is such as to exhibit negative lift, how can it not affect the vertical position of the hull? My assumption is that negative lift contributes to an increase in displacement. As such, it also contributes to drag/resistance which is, as you say, an important design consideration. Since I am working with relatively simple hull shapes and think I know what elements contribute to the suction, I just work to avoid these when possible.

Hull "squat" or stern sinkage is not necessarily always a bad or a bad thing. If I have a boat that is designed to run at a trim angle less than optimum for planing speed and cause the stern to sink by moving weight aft, the speed can increase with no increase in power input. We can choose to run at less than optimum trim angle and accept the loss in efficiency to obtain other benefits of the lower trim angle.

 

Perhaps Rick could tell us whether he was referring to a vessel with flat aft sections (low deadrise) or a flat (square) transom....
One would expect it to be the former: I think there's little doubt that we would all agree that a flat transom increases drag, but there's little to suggest that it would result in a loss of hydrostatic lift.
The venturi is a fine example of how irregularities in the surface - whether intended or not - can and do result in areas of lower pressure, even at quite low speeds. But, regardless of shape, a hull will experience the tendency to 'squat' as its speed increases. My understanding is that this has nothing to do with an 'active' negative (downwards) pressure (suction) being exerted on the hull - simply the result of a loss of available buoyancy to support the vessel. This in turn is predominantly as a result of the change in the LCB caused by the wave motions and to a lesser extent due to the thickening of the boundary layer as it "moves aft".
Changes in hull shape will alter the wave pattern, so if by incorporating flat sections into a given hull, one alters the wave pattern such that a greater proportion of hydrostatic lift is lost aft, then the vessel will squat more. Is that suction...?

Oh - and Leo - as always, I'm confounded by the complexity of the 'world in which you live'.

 

Here are a couple of observations that many of you have probably made, but perhaps have now forgotten:

1) The bailer on a laser works. Therefore, its low pressure there, even when planing!

2) The centerboard trunk on a windsurfer sucks water down when the bow wave is well forward, and water shoots up when the bow wave is at the mast step or further aft.

These two observations support the theory that there is high pressure where the hull first makes contact with the water, and therefore bow lift, and there is actually suction further aft, even well forward of the transom.

Both of these hulls have a slight amount of rocker: they are not monohedron s.

 

Yes, but the bailer (venturi) works by creating a low pressure area... it isn't something that is common to the whole of the bottom.
Same goes for the centreboard case - though in this case it is an unintended consequence

 

Tom,
when I said ...“I don't want to take the position that suction makes a significant effect on the vertical position either.” I wanted to emphasize the importance of knowing the relative magnitudes of the forces effecting vertical position, i.e. sinkage.

I completely agree that negative lift contributes to an increase in displacement. However, without knowing the magnitude, I don’t know how perceptible or dramatic a difference it makes. I would need to identify the amount of suction (negative pressure) and then translate to a sinkage.

 

The experimental curve in Fig. 1 of Leo’s Part 5 Report gives us real data on the Wigley hull regarding part of what we have been speculating about, the amount of sinkage. The data that sinkage is maximum at about Froude number = 0.5 is new and interesting information to me. Just based on wave and hull geometry, I expect the wave-systems effect on sinkage would be at a maximum near hull speed, which is Froude number ~= 0.3. These two ideas together gives some insight to the relative magnitude of the suction ( negative lift). If sinkage is increasing from Froude number 0.3 to 0.5, then I think we can say hydrodynamic pressure (suction or negative lift) is significant and contributing to sinkage in this range.

Comments?