# dynamic lift calculation

Discussion in 'Boat Design' started by csarmvs, Aug 31, 2009.

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### csarmvsing. naval

Hi everyone, I'm wonder if there is a way to calculate the dynamic lift in a vessel in semidisplacemente regimen, this I need to know how the draught is changing vs speed
regards
cesar

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### csarmvsing. naval

nobody knows?

3. ### Guest625101138Previous Member

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what type of boat is it...and what Fn's?

What is the purpose for calculating the draft v speed?...since this is not a 'normal' calculation that is done during the design. What you are "looking at", is normally achieved by the rise/fall of VCG with speed...again rarely done done, too expensive and more often than not, unnecessary...or sinkage/trim measurements, which is done regularly, in tank testing.

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### csarmvsing. naval

Thanks Rick, I will check this links, right now I'm reading the Savitsky papers and the Hydrodinamics of High Speed Marine Vehicles
regards
cesar

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### csarmvsing. naval

Hi Ad-Hoc, I'm doing my degree thesis investigation about a 12m catamaran , and I'm trying to predict the draft at 0.75 Fn, and others speeds, the next month I will recieve the scale models to run in the towing tank, but I wanna know if there is a way to predict it.
for example you say that this is done in the design stage .. but how you do it? or the only way is the tank testing?
best regards

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### baeckmoHydrodynamics

"Series 64 Resistance Experiments on High Speed Displacement Forms" by H.Y.H. Yeh in Marine Technology July -65.

There you find vertical change measured for stem and transom at varying speeds, block coeff:s and slenderness ratios.

PS This article is not showing a calculation method, but is mentioned as an example of what these hulls actually do. Your speed range is slightly lower than the normal acceptance range of Savitsky's algorithms. Some versions of these do stretch the application into lower speeds by the introduction of a compensating factor for "wetted bow", meaning that the water surface is actually not cutting the straight keel line, which is necessary for draft calculation, so be careful there!

Btw, could you show us the hull shape you are considering?

Last edited: Sep 7, 2009
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cesar M

The series 64, as mentioned by baeckmo is an excellent paper.

But all these papers, are good for "their range of hull forms". As such, if you want to establish some kind of 'prediction', you need to find a paper that uses a hull form very very close to yours, which is why showing your hull shape helps. Then reading the paper, again as baeckmo noted, you can extract the data in the paper for your own purposes.

Since there is no exact scientific method for draft prediction that satisfies all hull forms at all speeds. This aspect is not an exact science...lots of trial and error and arriving at empirical data, just as Savitsky et al have done.

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### csarmvsing. naval

I'm adding a screenshot of the demihull, and sure I'm reading again the Series 64 and the "RESISTANCE EXPERIMENTS ON SYSTEMATIC SERIES OF HIGH SPEED DISPLACEMENT CATAMARAN FORMS: VARIATION OF LENGTH-DISPLACEMENT RATIO AND BREADTH-DRAUGHT RATIO" by A.F. Molland, J.F. Wellicome and P.R. Couser-1994 based in the retested NPL series
but .. like I said .. I was hoping to find a method to predict the draught at some speed . I was reading .. and maybe I can do it with the pressure analysis of Michlet and some interations
thanks for the your comments guys, I hope you can continue helping me
best regards

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10. Joined: Oct 2008
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With a hull form like that, you'll only get sinkage/trim, not any dynamic lift at all.

As I've have said above...you wont find any methods for predicting draft at speeds, just observations during tank testing, if at all, for certain hull forms...ie it is just empirical and then interpolation.

What is the aim of predicting the drafts at speeds?..this is not clear.

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### Leo LazauskasSenior Member

Don't waste your time! Michlet does not do "pressure analysis" - it only estimates the far-field component of the wave field. And it does so using spectral methods, so by-passing actual pressure calculations which are very sensitive to hull discretisation and representation. O

Other software I have can estimate the squat of thin hulls, but it is not something that you should be worried about at the preliminary design stage. Look for the SWPE reports (co-authored with E.O. Tuck and D.C. Scullen) on my home page if you want the equations for thin ships.

As mentioned by others, the Series 64 paper should give you an idea of how much sinkage and trim you are likely to get.
For example, the top plot of Figure 4 of Yeh's paper shows that at your Froude numbers the stern of the hulls will stay at a constant level for a considerable range, while at the same time the bow will rise a little. That means that the draft (as measured by the position of the stern) remains constant, but the "dynamic displacement" is slowly reducing. Therefore there appears to be a very small amount of dynamic lift.

In shallow water the effect is much more pronounced. Look for papers by E.O. Tuck and, more recently, by Dr. Tim Gourlay who is at Curtin University in Western Australia. In finite width waters (e.g. towing tanks), a bore can develop between the demihulls, and solitons can shoot ahead of the vessel. Fun and interesting stuff!

Tim's PhD thesis was concerned with the maximum sinkage (and draft) of ships in finite depth water. Predicting squat in deep water is, IMO, a little more difficult, but for thin ships it is not of great practical importance. (Well, it is important for elite level rowing, but that's another story!)

Leo.

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### csarmvsing. naval

I'm sorry for the late answer, My job was absorbent this days, about the aim
of my question .. in the deepest meaning .. is just a doubt, a mental exercise
just to see if there is a change in the draught when the ship is running

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### csarmvsing. naval

THANKS Leo, yes I studied your program a little .. is great but like you
said . don't resolves my problem.
Thanks for the advice in the literature, I already checked the SWPE report, like
a year ago I searched all your home page haha .. actually I have it all in my
computer, great stuff you have there

Best regards, and like always, yours answers are very useful

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