planing theory

A general point of view

See the attached.
Is that cover what you asked?

 

Thanks you indeed for cooperation

 

If you've ever waterskied you'd know the difference. Plowing through the water vs. riding on top of the water. A 6hp outboard will generally not plane a 330 lb boat, 7.5 hp is generally adequate to plane it and run about 15 mph. There's a big diffrerence in feel. Before planning the drag is enormous, the boat plows through the water making a big wake with the bow at a large trim angle. After planing the bow drops and the wake (form drag) falls off.

This is an old post, and I see that Gonzo was in his usual 'enlightening' form.

 

Savitsky Theory

Hello everyone,

I'm kind of new around here,

I'm trying to implement this method in excel, similiar to what Dingo did, in order to fully understand this method and avoid the use of "black boxes". Somethings are not clear in my mind yet,
Can somebody explain to me the following ??

on Savitsky paper of '64, he states in eqn (18) that:

drag, D=disp*tg(trim) + Df/cos(trim)

on the other hand, the equilibrium equations of motion (eqn 29,30,31) state that the horizontal component of drag is Df*cos(trim) (eqn 29) since Df is by definition tangential to surface. Thus, the total DRAG force, D, should be equal to the sum of horizontal forces = horizontal Thrust, since:

Sum of horizontal forces = Sum Fh = N*sin(trim) +Df*cos(trim) =Horizontal Thrust force;

where N is the normal reaction force, Perp. to planing surface;

Moreover, Df = 0.5*ro*(Vm^2)*(b^2)*Cf , where Vm is the average bottom velocity, from my point of view Eqn(18) is true due to the fact that
Vm==V/cos(trim) but Savitsky no longer uses this trigonometric relationship, instead he uses an empirical formulation for Vm is used (eqn 24).

where V stands for undisturbed flow velocity,

what am I not seing clearly?

many thanks in advance

Best regards to you all

MacStevens

 

Air Traps

If the boat is fast enough, an air trap takes weight off the planing surfaces reducing hydrodynamic drag and creating a faster boat.

 

But those equations are equilibrium equations, shouldnt they hold even in that case? I mean, what I'm not getting is the corresponce between the 2 formulations. I think I'm missing some important detail about the viscous drag

PS.: Df= 0.5*rho*(Vm^2)*Surf*(b^2)*Cf

 

NACA and David Taylor Model Basion concluded that the low drag angle of attack for a hydro-ski is 5-7 degrees or more (friction drag degreases with increased angle of attack, while induced drag increases (for the same support of weight) – the sum of the two has a low point at about 7 degrees angle of attack. However, hydroplanes use much lower angles of attack on their planing surfaces because 7 degrees produces a violent response to any wave action.

 

Why don't you ask Dan Savitsky. I know him and I'm sure he would be happy to answer your question dsavitsk@stevens.edu

 

Thank you very much,

 

If you've ever come out of the water on skis, and think about it, then you'll know it's not a mere definition. Plowing through the drink with enormous stress on your arms is quite different than gliding on top of the water and being able to hold the ski rope with one hand.

 

Somehow I missed this thread and just now found it. Since I wrote that little essay on planing, I guess its up to me to defend it. Leo, you are being more than a little harsh and also wrong on several counts.

This explanation was not written to debunk Savitski or any rigorous mathmetical analysis because I would never try that. It was written to help the novice understand what is going on under a planing boat and I think it does that, as many have read it and said so. Many well qualified designers have read and discussed it with me and none have said that there is any fundamental error in my thinking. If you will take the time to read the whole thing, perhaps you will see that there is some merit to it.

There is little actual flow parallel to the hull (regardless of what the mathmetics of Savitski may imply) and the water molecules do indeed "bounce" off the advancing hull. Actually I say the moving plate hits the stationary water molecules, not the other way round, although the effect may be viewed as equivalent in most ways. To deny that is to deny the fundamental laws of physics as given to us by Newton. Neither Savitski or any of the others you mention is of much help to the layman who wants to know a bit about the physics of planing. I used the "sea of peas" analogy offered by Bolger only in passing since many have read it and not as a principle part of the explanation.

There are some who have questioned Lord's suction analysis but they are wrong. There is indeed negative pressure (suction) generated under the hull of most all planing boats, some of them quite a lot. The most often quoted rebutal comes from NA Dave Gerr, but his experiment and his interpretation of it is wrong too. Any engineer worth his salt, and even knowing nothing about boats, should be able to see that.

If you want to discuss or counter any particulars of my essay, please do so, but the blanket dismissal you offered is unworthy. A NA will not get any numbers or formulas from my offering from which to design a planing boat, but the layman (and most on this forum are laymen is that sense) should get a better understanding of what causes their planing boats to act as they do. That is, after all, why it was written.

The forum should always be both informative and helpful. Nature is what it is and personal conflicts only get in the way. Cheers.

 

Conflict is what we need!

Any claim must be supported by evidence, especially if going against the grain of current theory. This is the essence of scientific and engineering rigour. Any criticism must be countered by the person making such a new claim. There is no room for emotions or personalities, just cold hard facts. Since such cold hard facts must be, I say again, must be repeatable independently by others any where. If they are not, the claim has no merit.

If all your claims are subjective observations, ie no facts, you can not demonstrate to anyone else what you "think" you observe. It becomes a debate like religion, trust me believe, don't question me, rather than intellectual rigour and demonstrating that a claim/theory can defend itself.

I had a quick read of your article...I'm sorry but i gave up after several paragraphs...far too verbose, woolly and lacking facts. Whilst observations are the source of any new theory, it must also be recognised that observations, based upon personal interactions are highly subjective too. Simple observation experiments (which have been repeated by other independent researchers) show that 30% of people fail to see a man dressed as an ape walking past a group of people that were being "observed" to gauge their actions. In slow motion the ape walking past is clear..yet 30% observing the expt, missed it!!

That is why science and engineering requires facts which can be independently verified, not just personal observations.

If criticism or critiquing like this irks you, then you should reconsider entering into such a field.

 

Mr. Lathrop,

As someone with an engineering background but no formal marine design training or education

I appreciated your tutorial and post above.

Now I bid you farewell, with thanks, as the over zealous tear you to shreads.

-Tom

P.S. I see it has already begun.

 

Actually "it" started about 3000 years ago.

It is a brave thing to stick your head up above everyone else. Most are not cut out for critiquing when they do...yet, it is the only way to advance science and engineering.

 

d'Alembert proved that Newton's view was wrong.
Euler and Bernoulli got it right.
Sorry if that's harsh.