Critical speeds for Semi-Planing

Hi Leo,
for me Michlet works and does not for low L/B.
It depends on hull load. It works pretty well for light dinghy hulls, but fails completely eg. for el-boat hulls with heavy battery loads.
Question: Is this because Michlet does not take any negative lift forces? How about adding these (iteration process) to total hull load figures in Michlet?
Ref1: Leo's opening "Critical speeds for Semi-Planing" (negative lift)
Ref2: http://www.dynaflow-inc.com/Publication/pdf_documents/Choi_J-K_Hsiao_Chahine-drag-reduce.pdf
(here are negative lift figures given in tables)

For a heavy e-boat hull,
there is a lot of negative lift, that never turns to positive as did the hulls Leo's example? I would be very interested to learn, how to calculate that negative lift for different speeds? Does Flotilla the job?

Cheers,
Reino Urala

 

Howdy,

Leo's computation is quite interesting and reflects what I have been saying on another thread - on a purely observational level.

Planing and Displacing are two parts of the same physical equation - they get added together to equal the total static displacement.

If the lifting force is the same as 30 percent displacement then you have to say

the boat is 30% planing
and 70% displacing.

The big problem is terms such as planing, displacing, semiplaning ... whatever come out of history as defining different hull types. I don't think these categories are valid in the normal way they are used as being mutually exclusive in some sense ...

... physical equations and explanations have to hold for all hull types - if you change the explanation for multihulls as different from "planing" or "displacing" hulls you have failed to understand the physics.

I think it would only be a slight risk of sticking my neck out to say that most moving boats are both planing and displacing at the same time - from the instant that there is any relative motion.

the only boat is truly planing is one that is not requiring any support from the water at all. This does not mean hydrofoils either which both displace and create a wave train (as subtle as it may be)

I have gone into it further in this thread
http://boatdesign.net/forums/showthread.php?t=22315
but have failed to explain the part that relates wavemaking to resistance in a coherent way yet.

It is timely and interesting to see that you folks have been discussing it too.

There are several dead threads on the same subject which have come to no conclusion on the topic over the last few years - simply report the same old stuff that there are these mutually exclusive "states" of "planing" and "displacing" and that multihulls "don't plane" but have lower resistance "for some other reason" whether it is delineated or not.

Best wishes
Michael Storer

 

So to take Leo's result and question about when does semiplaning happen.

1/ my statement about a boat both displacing and planing from rest is wrong. There may be a speed at which nett positive lift forces start to be developed.

2/ Planing, semi planing, displacing are flawed concepts. It is clearly a continuum where lift forces and displacing forces add up to equal the static displacement of the vessel (or close to).

3/ Where a boat is being partially supported by dynamic forces you can see the evidence behind the hull of both multihulls and planing craft in exactly the same way. There is hollow behind their transoms that is around the same volume as the missing displacement.

See the other thread link in my post immediately above for a more detailed argument, but be aware that I didn't understand 1/ above when I wrote it.

Best wishes
Michael

 

Yes, we do seem to have this discussion periodically. And no, I don't think it's ever been resolved so far.

Trying to delineate boats into distinct categories of "planing", "displacement", etc. is, I think, useful- to a point. In describing a C stock outboard racer, for instance, it's obvious to everyone that it's a planing hull. Likewise, a double-ender car ferry is obviously a displacement hull. But then we see a two-level, 40' long, 16' wide dock queen with a 17-degree V hull, and twin 8.1L V8s. Is that a planing hull? It has some attributes of one, but its bottom loading is too high for it to ever really plane.

I would suggest that unless it is completely clear what the dominant mode of support is, we're better off comparing based on more specific attributes- actual hull shape, for example- than broad categories with sometimes ambiguous boundaries.

On a Michlet note, now...
Those who keep asking if Michlet can be extended to low L/B hull forms and other shapes for which it does not work well might be well advised to take a look at J.H. Michell's 1898 paper, "The Wave-Resistance of a Ship" (see attached). Even if you can't follow the differential equations, understanding the assumptions made in their derivation can give some insight into the limitations of this type of method (and the ingenuity it took to figure out, in 1898, a method that couldn't be practically implemented until the days of the modern computer).

 

Michlet does not calculate dynamic sinkage and trim (or "squat").
The equations for calculating squat are given in a series of Reports at:
http://www.cyberiad.net/wakepredict.htm

SWPE Report Part 5: Speed-up and Squat.
http://www.cyberiad.net/library/pdf/tsl01a.pdf

The equations for calculating near-field effects are in Part 3.

There are several reasons that you are getting poor results for heavily-laden boats. It could be because of negative dynamic forces increasing the displacement, or it could be because Michlet is a thin ship theory and the results are not consistent when the fundamental assumptions of the theory are violated.

I have no plans to include squat calculations in Michlet. I can calculate squat using several of my codes (e.g. Flotilla which I used to produce the graphs shown earlier in this thread) but I use them for my own contract work so I am not keen to release them into the public domain.

Good luck!
Leo.

 

Yes, but there is also a hollow behind transom sterns at low speed where, at he same time, displacement is greater than at rest.

Many people have tried to use the hollow behind transom sterns to improve estimates of wave resistance. My code "Michlet" incorporates one technique, but I'm not happy with it for several reasons. The first and foremost is that the hollow cannot sustain a pressure and so it cannot really be considered to be a wave-making component of the ship. Sure, adding the hollow increases the length (and therefore reduces the effective Froude number) but it's really just a fudge.

There are, however, good physical reasons for including the hydrostatic resistance due to a transom stern being dry, but even that is a difficult quantity to estimate accurately. (There are many recent papers on this if you are really keen: search for Maki or Doctors or Beck for some very recent work).

All the best,
Leo.

 

Thanks for this Marshmat.

My feeling is however that almost everyone tries to fit the explanations to the historical categories. And base their assumptions on what they think is happening in each category.

They are not able to consider that a multhihull has lift because "it is a non planing hullform".

But Leo shows that there is significant and measurable lift.

The physics tells Leo that the behaviour of the hull is a continuum with a change of the ratio of the displacement taken each by dynamic and static forces.

However he asks ... when does the boat start "truly planing" or is it "semi planing".

Clearly there is no pure state called "planing" as it requires the whole mass of the boat to be supported by dynamic forces.

It cannot be touching the water at all.

rest displacement - dynamic lift = residual displacement.

I don't know the right terms - but I know the thinking is right.

The "dynamic lift" is the planing component
The "residual displacement" is the displacing component.

You can obviously optimise a hull for different regimes but Leo's question about when a boat starts "truly planing" is a meaningless one in the physical sense, though a really good one in the sense of enquiry as it shows the categories beg the question.

Also to make the point ... I take a bit of an issue with the car ferry being "displacement". What is it about the hullform that makes it displacement? If we reduce the displacement to length but use the same hullform will we see a positive lift similar to the one Leo describes?

I don't know whether it will or not - and in any case it is immaterial. The thing is that deciding that it belongs to a particular category has prevented us from looking at the real situation.

Best wishes
Michael

 

Howdy Leo,

(great respect from me for the development of Michlet and other contributions and also making yourself so available in some public forums).

The energy system of the boat does not terminate with the stern.

My reasoning is based on observation of real boats. When aboard a heavy deep vee sportsfisherman at speed the water immediately behind the boat reflects the shape of the transom and then fill in some distance behind the vessel.

It is most observable with boats of this type but also is visible behind lighter boats and multihulls moving at speed.

Now my (limited) understanding of theory - it is a layman talking here - I am not talking down - just mustering the language I have! And I am ignoring some effects.
-------------------------------------------
At speeds when there is no separation at the transom.

The forward part of the boat has to move the water down and out. This raises the pressures in the fore part of the boat above static with the result that there is a upward lift in that area.

However by the time the water gets to the back of the boat the energy has to be recovered and the water restored to its rest state at the stern of the boat.

This means that over the aft part of the boat there will be forces equal and opposite to the ones at the front end with pressures below static and a downward lift.

So when the water gets back to the static position after the boats passage that shows that there is no nett lift at all - the forces are balanced within the length of the hull
----------------------------------------------------------------

However as the boat develops a nett lift that means that at the transom the water has not been returned properly to the static position. It has to do that after the boat has passed.

So there is a separation at the transom and the water must get back up to the static position some distance behind the boat.

(I am loath to use the word lift as some might assume that it is operating upwards when clearly it can be downwards too).

Looking at the real situation again... when there is a nett upward vertical force operating some of the volume of the hollow behind the boat reflects the lift forces the boat is experiencing.

I am not saying it is the only reason - but logically at least part of the volume must reflect the nett vertical force the hull is experiencing.

I guess that is true when the boat is developing a nett downward force too that there will be a hump behind the boat. Maybe.

Best wishes
Michael

 

A fun thought is under the standard definitions of "planing" and "displacing" the front part of the boat is planing and the back is displacing.

Oh but it cannot be said to be planing until it exceeds Froude's 1.34 (I must get the right language together)

So the front part of the boat will always be developing positive lift but is not planing until the 1.34 is exceeded.

This is written as a bit of a joke - but it does show how we generally fail to look at our assumptions and the arbitraryness of it all.

It is all so arbitrary. Boats passing Froude 1.34 are "planing" - but at the same time we know that we can take a "displacement hull" past that same limit. It is all so vague - and it is the categories that are at fault.

It makes perfect sense as an equation that takes both effects into consideration.

"planing" and "displacing" may be used to talk about optimised hullforms but perhaps it is a good idea keep them well away from the physical explanation.

Best wishes
Michael Storer

 

Leo - what if we conceptually extend the hull back to fill the hollow without disturbing its shape.

Does that change any of the effects we are talking about?

If it doesn't change any of the effects - then it is being treated differently when the "hull extension" is present or not.

I don't know - I am just asking the question.

Best wishes
Michael

 

You have just described what several people are still trying to do to improve their mathematical models.

However, if the fictitious extension of the hull is the same shape as the hollow behind the transom then it is, essentially, a streamline of the flow. Therefore it is not creating any additional forces on the water.
Therefore, IMO, it cannot be making any waves in addition to those that have already been created.

Of course there is still the additional drag due to wave-breaking that should be accounted for, but that's a very tough mathematical problem.

All the best,
Leo.

 

Thanks Leo,
that "negative dynamic forces increasing the displacement" is what would be useful to understand. These have nothing to do with those positive lift forces due to planing. Even Savitsky writes about this negative lift, that happens also in case of planing hulls before the planing progress takes over.

But tell me, what are the physics behind this negative lift = negative pressure effect = negative dynamic forces. Does the hull bottom act like an inverted airplane wing profile with increased speed water flow sucking the thing down?
Mathematics to calculate numbers?

These negative lift figures at different speeds are fundamental figures in case of a heavily loaded el-boat with batteries. Eg. up to 2000N at 10kn for a 7m long vessel. Very essential!

cheers,
reino urala

 

Michlet example

Electric mono as an example, using Michlet:
L = 7.5m
B = 0.86m
L/B = 8.7
M = 900kg
Wetted: 6.64m^2
Section max: 0.207

L/B = 8.7, but still the curve drag data may fail because of negative lift?

Negative lift data may be like this (not sure):

Max sect area
[m^2]
0,207
0,207
0,207
negative lift
[N]
331,6376
1637,716
2911,496
Froude n
[ ]
0,27
0,6
0,8
speed
[m/s]
2,088638
4,641418
6,188557
speed
[kn]
4,057797
9,017326
12,0231

reino urala

EDIT:
This Michlet is great! I added these negative lift data as sinkage percentages to Michlet input file, and got drag curves with negative lift included. Gray backgound curves are curves with sinkage.

 

I think this is *exactly* what happens in the tunnel-stern Seabright skiffs designed by William Atkin.

Robb White built one and said his boat ran so stable that it felt like it weighed 10,000 pounds -- but it only really weighed a few hundred pounds -- so *something* was holding that hull onto the water's surface and making it feel uncharacteristically stable ...

 

Many people, including some well known designers claim that these negative lift forces do not exist. I don't understand what is so hard to accept about this force whether it's called suction or whatever. I suspect that some negative lift may exist under all except, perhaps, the very best planing boats. And yes, it is the result of a convex surface somewhere that has a radius of curvature with a upward vector component. Your inverted airplane wing is a way to look at it. Some question that the Bernoulli principle even exists.