flat sterns cause squat ?

Leo
I am interested in taking you up on this offer but I do not have time to get offsets right now.

However doesn't the squat comparison between the Wigley hull and the NPL hull prove the proposition.

The other thing that seems apparent is that there is greater error with the NPL data and the modelling at higher Froude for the trim angle. I am interested in this aspect as well as it may be showing the limitation of the modelling as dynamic lift starts to take over.

The original test work I linked to was used to show trim and sinkage in figure 9 and pressure in figure 10:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8895319&article=7
This is a higher B/L hull than you are considering but the trim is similar shape but more pronounced.

It would be interesting to see your pressure plots for fore and aft positions.

Rick W

 

I was lucky to have Ernie Tuck as a colleague or I would never have made any progress on my own.

Estimating squat in deep water is a very difficult problem, even for thin-ships. There is quite a lot of work on shallow-water squat in the open literature, but very little on the deep-water problem. I doubt that there are more than 20 papers published during the last 20 years that address the theoretical aspects.

Leo.

 

Someone pm'ed me asking for some calculations of the sinkage force on a hull underway.

Attached is a short note I knocked out in the last couple of hours comparing the squat and drag of two thin hulls. Sorry, but I can't give the exact details of the two hulls.

Have fun!
Leo.

 

When you talk about 'deep water' / 'shallow water', are you simply referring to the water being deep enough such that the bottom has no effect on the vessels waves? I'm surprised nobody's done more work on that - I would have though that the vast majority of boats would operate in 'deep water'...

 

1. Yes, that's the distinction I was implying by the term "deep water".

2. Squat in shallow water is probably more important because grounding a large prop can be very expensive. It's also a lot easier to estimate than deep water squat.

3. I doubt that most naval architects are concerned with a few percent here or there when estimating total resistance, so it wouldn't surprise me if they ignore squat in deep water.

Regards,
Leo.

 

When looking at trade-offs between beam and draft, I often consider the semi-circle mid-ship section as a conceptual reference. I consider, am I adjusting toward the minimum surface-area shaped section, or away from it.

Leo, in your example it looks like Hull 2 is closer in shape to the semi-circle mid-ship section than is Hull 1. Do you think this is a general trend? Does the semi-circle mid-ship section also produces the least sinkage (less negative Fs value) ?

 

No, (I'm guessing) it is not necessarily a general trend.

Sections with some flare at the waterplane would tend to sink a little less, but again, it's often a delicate trade-off between waterplane area, location of LCB and LCF, and (longitudinal) moment of inertia.
Catamarans are a little more complicated because of interference effects between the hulls.

Good luck!
Leo.

 

Gentlemen you are making this more complicated than it is
I believe that we are talking about a planing hull, with a square transom,
So some parameters
1) transom is relatively perpendicular to the water, plus or minus 20 degrees
2) flat surfaces for the hull, ie not concave and not convex
3) assume, just to keep other issues out of the discussion, that the hull is constant deadrise over the last 2/3rds of the hull,
4) positive pressure is anything above atmospheric and negative is below
5) that we define planing as when you can look over the back of the transom and see the transom all the way down to the water, lets just say that this is 10 knots

Take a 12 foot constant deadrise boat with a square transom, it has been built so that the chine is exactly 12 inches above the water all the way around and it is floating.
Go to the bow and lift the bow up 3 inches, what happens, the stern squats because by lifting the bow up 3 inches, you have taken away some of the bouyancy at the front and the rest of the hull has to "squat" to continue to float.

so how does this relate to a power boat,
A displacement hull floats because it displaces it weight in water with its volume.
AND is constructed so that there is pressure recovery along the length to the end of the hull. Ie not a squared of transom. It moves up to its hull speed and is relatively limited by the equation
A Planing hull floats for the same reason but has a flat transom and is designed with enough horsepower so the pressure from the water that it is moving out of the hulls crosssectional way, supports the weight of the boat.

But a planing hull operates from zero to the 10 knot planing speed as a combination of planing and displacment. At say 1 knot, there is little dynamic lift and the hull floats due to mainly bouyancy. and there is water behind the transom.
At say 5 knots, some lift is generated by the moving water and but still there is water behind the transom, but getting a lot of turbulence. The water pressure on the transom is now getting less, heading toward atmospheric, but still provide some bouyant lift

At 10 knots all the lift if carried by the hydrodynamic forces, ie there is not any water against the transom, and the hull is planing.

So why the squat, (forgot to put in the parameters, that we will ignore the type of drive, ie parallel outboard thrust, down angled shaft drive, a florida styled airplane prop, or a low axial thrust jet,)

Assume a 30 foot boat, just because we need some numbers for length.

At 5 knots, there is a bit of bow rise, as my 12 foot constant deadrise boat above, The bow squats because at 5 knots we need a little extra bouyancy to carry the weight, at 10 knots, the hull is planing but still the bow is up. Why

Some earlier responder said that the max pressure point is at the start of the wetted planing surface, Not right. depending on deadrise and speed and attitude, the highest pressure point is anywhere from 1/4 to 1/3 back from the front of the wetted surface. But the most important point is that if you were to integrate all of the forces acting VERTICAL on the hull, you can replace them with a single force, and this is called the center of dynamic lift, So when the hull starts moving, using 5 knots, the bow rises and the center of dynamic lift is say 5 feet back from the bow, this causes the front to lift and the back to squat, at 10 knots, pure planing, ie no bouyant forces, the stern is still squating because the center of lift is now say 10 feet back from the bow and because all the weight is now carried by dynamic forces only, this number is larger than when bouyant forces were involved. As you move say toward 15 knots, you have more lift, due to the speed, so you do not need as much wetted surface and the center of lifts moves further back, the bow up righting moment gets less as the moment arm is shorter and the bow comes back down.

There is no suction on the bottom of the boat. When you drill a hole in the bottom of the boat, and water rushes out, it is because there is positive pressure. No suction, no suction, with flat, smooth hull shapes on the bottom.
Another responder said that there is a combination of forces and even though the pressure is less than atmospheric the water rushes in, Ie the low pressure suction overcomes atmospheric pressure, ouch

Someone asked about putting in an enclose manometer to measure pressure, This may not work due to the viscous shear that may occur the minute you do not let the water move into the hull.

Also, the equation for hull speed that everyone who has read an article on it, refers to displacement hulls.

Often people do not understand this concept.

Say you push a 12 inch piece of broom handle axially through the water but underwater, the front is flat and creates resistance, lots becuase is it cut square, this build pressure ahead of it, this higher pressure moves past the front of the handle and pushes on the side of the handle toward the back. At the back, which is also cut square, the transom so to speak, there is a big negative pressure area, the high pressure water, just goes past it and it is gone.

Now we take the same broom handle and leaving the head at its original diameter, and square to the length, we machine the handle so that it tapers evenly to the transom. So what happens now, is that the high pressure area that is generated at the front, pushes on the sides of the tapered handle and because there is a vector forward, helps push the broom handle along.

Not believing it
Look at a Boeing 747, fat at the front, tapering to the rear, or a dauphin, the mahi mahi type, fast as sin, same shape, most fast fish have long pressure recovery type sides to take advantage of it. This is called pressure recovery and it is a FACT

Of course if you want your broom handle to even be more hydrodynamic, you can taper the front as well, but in the real world, and other mechanisms, like the 747, you need a wide space for first class passengers, and fish need a place for eyes, mouth etc.;

Another example, anyone shoot rifles, most bullets are pointed in the front and were square in the back, in that past 40 years, they changed the shape of the back of the bullet, tapered perhaps that last 20% of it to reduce drag, increase speed, and, get this, it is called a BOAT TAIL (boattail) bullet.
Some pressure recover at over mach one speeds,