flat sterns cause squat ?

Discussion in 'Boat Design' started by Mat-C, Feb 18, 2010.

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Mat-CSenior Member

This was posted in another thread and I wanted to question its validity... I always thought that the stern 'sinks' initially as a result of being in the tough of the hulls wave system. The flat aft sections are thus presented at an angle to the flow, creating lift...

2. Guest625101138Previous Member

The water just ahead of the transom of the hull is under pressure and is accelerated toward the low pressure area behind the hull. As the velocity increases the pressure is reduced. Hence the stern sinks or squats a little.

The high pressure area and most lift on the hull occurs where the water initially impacts the hull and is accelerated forward and downward. So as the boat passes the water is moved forward (slows relative to the hull) then it starts to accelerate in the opposite direction to the boat (speeds up relative to the hull) as it is forced to the low pressure area.

Off the centreline the flow has a side component as well as the water flows to the low pressure zone beyond the chine.

This paper has some useful data to show what is going on:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=7
Figure 15 and the subsequent discussion explains what is going on.

Rick W

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tom28571Senior Member

It is possible to get to the correct answer by more than one path and many people have their own way of looking at it. Rick's second paragraph more closely matches my own but in the real world, there is no water flow under the boat until the hull causes some. As Rick said, the hull impacts the water (which was just resting there) and the Newton reaction on the water is down and forward for a flat bottom. As the hull moves forward, it meets this water, which is now moving away from it. That means that as you look further aft on the bottom, the Newton lift reaction must be less as the water is moving further away from the hull the further aft you go. That can be the major cause of stern squat on a planing but your wave solution is the major cause of squat on a displacement hull. On very long hulls and slower speed, there can be a rebound reaction but no need to get that complicated.

Here is my own (no math) explanation of planing which has helped some understand the process.
http://www.bluejacketboats.com/planing_boat_theory1.htm

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gonzoSenior Member

Sterns sink or not depending on the centers of gravity and floatation of the hull.

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Joe PetrichDesigner

In a static condition this is true. In a dynamic condition with the boat moving, hull shape, speed/length ratio, and CG are other contributing factors to squat along with those mentioned in the last 2 posts.

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Easy RiderSenior Member

Mat,
I agree. " I always thought that the stern 'sinks' initially as a result of being in the tough of the hulls wave system. The flat aft sections are thus presented at an angle to the flow, creating lift...". I think that is the best general take on the question but I'm anxious to read Tom's stuff .. looks really good but people are coming at 10 and I'm not dressed yet. Like an optical illusion I think the stern "squatting" may be misleading sensation in that pitch sensation may be quite independent of rising or falling, lifting or sinking. The stern sinks, as a result of being in the trough because the water level is lower at that point. The entire hull assumes a rather steep angle of attack.The angle of attack causes lift and it will be noted that the stern increases it's angle of attack exactly as much as the bow. The sideways flow of the water under the bottom hasn't yet got up much velocity so more water stays under the hull fwd more so than aft so the bow lift is very apparent. When the stern comes along the water not only is racing aft but toward the chine as well but now the sideways movement of water is impeded by solid water at the sides of the boat slowing down the sideways flow and more or less trapping the water under the aft section. So as I see it it's possible that
A. Which end of the boat has the most lift is not a given
B. When a person in a boat experiences "squat" most of the sensation of the stern lowering itself is just the DIFFERENCE between how much the bow rises relative to the stern.
C. It's possible that the lift under the stern is greater than the lowering of the water level due to the trough effect and that the stern doesn't sink at all but rises a bit.
The stern squat is subject to lift or suction relative to the sideways flow and the chine shape. Soft chines will tend to pull the water up at the chine whereas the hard chine just spits it out sideways. The opposite reaction to lifting water up will be to pull the stern down.

Easy

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We are going full circle here. The "suction aft" hypothesis was the principal subject of an entire book from the past. See; Lindsay Lord, The Naval Architecture of Planing Hulls.

L. Lords premise was that the conventional warped plane hull was doing it wrong. He advocated continuous deadrise. Maybe his book had some influence on the boats we now call "deep Vee".

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Easy RiderSenior Member

Circles. Yea .. could be. But I'm quite sure a hull warped all the way aft will produce more lift in the stern and be a lot more efficient than a constant DR hull. Regarding Mr Lord, the common medium depth V bottomed boat w a warped or developed bottom is certainly not "doing it all wrong".

Easy

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tom28571Senior Member

Got to be careful of what the variables are. Can't just make blanket statements. I follow the testing programs that show greater lift for warped bottoms at low to moderate speed and superiority to the monohedron for high speed. My own tests show the warped bottom having more stability problems at high speed.

Lord certainly discusses aft suction on warped bottom forms at length but there is a ton more in his book. Tad may pop in and have something to say here.

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Easy RiderSenior Member

That's interesting tom.
"My own tests show the warped bottom having more stability problems at high speed." Over 20 knots I assume.
What is:
"aft suction on warped bottom". Excluding propellers and tunnels I don't see where suction would be an element on a hard chine boat.

Easy

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TollyWallySenior Member

I recently got my hands on a copy of Lord's book and have read it a few times now. It is a little hard for me to follow hence the multiple readings. It'll take another time or two no doubt. Dr. Lord does spend quite a bit of time discussing suction which I sometimes take to mean the opposite of lift. I see lift as forces pushing up on the hull and suction being similar to forces pulling down on the hull.

It seems to me like the stern is falling into a hole until there is sufficient lift generated to pull the stern up and out of the hole as the boat gets over the hump. But falling into a hole so to speak doesn't quite seem like being pulled down as I envision suction.

None of this is crystal clear in my mind and I look forward to others providing more concise examples.

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tom28571Senior Member

Wally, you have it dead right. Suction is, in the example we are looking at, the opposite of lift. It is negative lift. Any unfair surface on a hull moving through the water will produce forces normal (perpindicular) to the surface. These forces can only be positive or negative but may resolve into vectors in any direction from the hull. We are generally only concerned with vertical forces: positive lift or negative lift. As such, suction is a perfectly good word to describe negative lift. Any surface moving through the water that has convexity as part of it will generate suction. The main culprit is rocker under the stern but strut fairings, shafts, or even depth sounder fairings can do it.

All warps are not created equal. One may generate suction while another may not. It depends on what the water flow sees as it runs aft. If you start with a low deadrise monohedron aft bottom and warp it by lifting the keel area, it will generate suction in the keel area along with greater lift in the chine area. If it is warped by pulling down the chine areas, it may not generate any suction and will generate lift. The later is what my aft chine design is meant to do. You should look at the photo sequence of the Bluejacket 24 I linked above to see whether it is successful or not.

Dave Gerr devised a test that is claimed to show that there is no such thing as suction under the hull of a planing boat. That test is faulty and can be torn apart by any basic engineering student.

The instability I mention for the warped hull occurred at a full scale speed of about 30kts. This is highly dependent on the degree of warp, which was of the raised keel form. I still hope to complete these scale towing tests in more detail. The instability was in the form of yaw and generated oscillations so great as to turn the model completely around and tear up the test rig.

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Tom; I respect your input as always. I am having a mental snag here. If we lift the keel as opposed to lowering the chine, then that is essentially the same thing from a geometric standpoint. If you raise the keel in such a way as to create aft rocker, then the "suction" thing is intuitive (sort of). If however the keel runs straight then we are lowering the chines to get the warp. Is my premise O.K. so far?

While we are at the subject.... I have sailed many dinghys that could and did plane. Not at 30 knots to be sure, but plane they did. Almost all of them have some keel rise. The better ones seem to have a near straight run. Also the better ones seem to be chesty. I mean the lowest part of the keel is ahead of the middle of the boat. That makes it more nearly possible to get a straight run while getting the transom above the waterline. Gerr goes into some detail with the notion of quarter beam buttock angle and their affect on potential S/L ratio. Comments?

P.S. Lord is said to have used a bank of manometers in the afterplanes to prove his point.

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TollyWallySenior Member

Thanks Tom for your further explanations. I am having a little difficulty "seeing" what you describe as raising the keel area. I can "see" the lowered chines fairly well. I am assuming the suction caused by convexity is negative lift similar to an airplane wing turned upside down if that makes any sense. The suction or lift so to speak being normal to the surface of the curved structure.

While not the same as your towing tests I have experimented with different towpoints etc. when towing a skiff behind my boat. I can commiserate with "oscillations so great as to turn the model completely around and tear up the test rig." I am toying with the idea of trying something like the auxillary trailer tanks mentioned in Lord's book.

Best wishes for a speedy recovery from your recent operation.

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tom28571Senior Member

Well yes, it would seem so but all is not quite equal. If we take a monohedron hull and pull the keel up toward the stern, we cannot do that without introducing some longitudinal convexity in the area where we pulled the keel up. When the water flow encounters this convex surface some negative lift or suction occurs. Can’t be helped.

If we take the same hull and pull the chine areas down a bit, we introduce only some concavity, which means increased lift as the water flows past it.

You can make these two cases very close but they can never be geometrically equal. I pondered this a long time before convincing myself that this is a correct view. It looks to me as if you have reached essentially the same conclusion. All my basic design starts with visualizing what is happening to water under the hull. Not everyone will like that approach but I am basically an analog guy and that kind of engineering has worked well for me for a long time. Like everyone else, I have gotten caught in false assumptions, so it is always good to check everything with all your resources. The further toward basic principles you go, the less the chance of getting caught on limb.

You are correct about sailboats but we must remember that almost all sailboats are designed to run in the displacement mode most of the time. Many compromises, like you outline, are required from the ideal planing hullform and the foremost among these is that the stern must be kept from dragging low in the water. Newer and faster small boats are, as you say, getting flatter and flatter aft.

About 15 years ago I did a redesign of the Windmill one design sloop for the purpose of building with composite plywood/glass/epoxy in a female mold. While at that task I introduced my ideas on offsets within the Class tolerances. They are as you suggest. That is, chesty forward and flatter aft. The reasons relate to the fact that we can change the effective shape of the hull bottom by crew placement.

If Lord proved his thoughts on suction with manometers, he must have done it by making multiple measurements on the same hull with some difference in bottom shapes. I don’t think it could be done directly. The total force is the resultant of several and the suction would need to be isolated to measure the effect of it. Its the superposition theorem.

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