# Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

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### Perm StressSenior Member

. This indeed is several times more. On the other side, planing boats are usually light, and with powerful engines, and it is not easy to notice, whether your engines are using 5% or 15% of their rated power.

In simple words, planing hull at displacement speed is "seen" by the water more like brick, not like boat... so resistance HAS to be several times more.

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### Perm StressSenior Member

It is not possible to generalize. All of it do some work.

The planing hull and the displacement hull are completely different animals.

"I know from experience that a small planing hull travelling slowly and having weight shifted forward so the transom is much higher than normal, will run noticeably more freely..." By trimming transom out of the water, you also change longitudinal volume distribution dramatically, so water "see" the boat as poorly designed displacement hull, instead of planing boat with brick-like stern. Hence the result -dramatically less resistance.

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### AlikSenior Member

20+ years of design experience. Is this argument good enough?

I believe You should show us the sample of Your statement, not me

Take boats of same weight, same length but different hull shapes - displacement and planing. Calculate the resistance and compare resistance curves. There is no sense to study effect of hull shape on resistance if DLR of compared options is different.

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If same length, and same weight (displacement), then the length displacement ratio is the same. Thus, I'm not sure what your point is??

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### AlikSenior Member

Yes! Provide resistance curves for two boats with same DLR, but one with displacement hull, another one with planing shapes. This will give You the difference in resistance.

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Hmmmm….

If you have a hull that has a block coefficient of 1.0, with a certain LD ratio, if you then say oh, now I have a hull with a block coefficient of say 0.5 with the same LD ratio, this has less resistance.??...er no!

Since to maintain the same LD ratio, as the block coeff of 1.0, the draft must be deeper otherwise the LD is not constant….ie the displacement is not the same, hence hull must sit deeper in the water at the same given length. This increase in draft is added WSA….which is added frictional resistance. Back to square 1.

LD ratio is the key to understanding everything about resistance/hydrodynamics, not hull shape. Hull shape plays such a minor roll it is not worth considering; except for those looking at endless decimal places in their computer software print outs!

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### michael pierzgaSenior Member

Gee guys, while you are thinking about it the whole world is sailing past you !!!

moving crew or ballast weight forward and to leeward decreases wetted surface on nearly every small boat Ive ever been on...even my tender..

Heres the deal...you sit aft in you tender, Ill sit forward in mine and we will race to the cafe for beer....last one in pays for the beer. I love beer..bring your wallet.

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As you can read here this is about vessels under their own power, power cats, not sailing vessels.

Therefore, with regards to the resistance of sailing vessels what does induced resistance, heeling resistance, centre of effort and centre of lateral resistance, not to mention the aerodynamics and wind direction etc have to do with power cats?

Make sure your wallet is a large one, as the power cat motors past your sailing tender!!

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### AlikSenior Member

To compare effect of hull shape, we should take same DLR for both compared options. I am well aware that DLR is major factor of resistance in interested range; so let's fix it and compare the rest.

I have a lot of such calculations done for few boats (from pre-design studies of semi-displacement craft); actually I was doing such comparison just 3 days ago for 60' powercat hulls.

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### michael pierzgaSenior Member

Hey !!!!!!!!!!!! Thats not fair...our race is in 2hp yacht tenders......and Im gonna win...VRRRRRRRRRRROOOOOOOOOOOOOOOOOOM.......your gonna be buried in a cloud of two stroke outboard exhaust....bring your VISA card to the bar

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We have done just that.
Been doing it for over 20 years…shape plays such a minor roll only button pushers with software debate the “differences”.

All variations of hull shape have very very minor effects and differing at different Froude numbers. The only surprise is the lack of any real difference. All hulls with the same LD ratio.. The crude hull, that was a block of wood, block coefficient 1.0, but with simple angle for a "bow shape" from a saw cut as a child would make. See even the worse of the worst shape, makes very little difference.

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### AlikSenior Member

You do not need to convince me; I am telling exactly the same!

No way the difference in resistance between shapes can be 2-3 times, for same DLR.

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### Perm StressSenior Member

To be accurate, 20 years of experience is not an argument. Statement like this could be used to support value of some real argument, but do not prove anything by itself.

Please see strictly practical, full size, real world measurements, as published by F. Betwaite, in "High Performance Sailing", page 408. Please note, that real hulls were towed, so there are no scale effects involved. Please note, that all resistance values are recalculated for the same displacement. Check the difference in resistance values in 3-4 knot range. At 4 knots, Skiff (line #3) has ~50-60% more resistance than Tasar (line #1).

The other graph show same boats, with same values of resistance, but instead of absolute speed in knots, Froude Number (FL=v/((g*LWL)^0,5)) is used. Note that at Fn=0.3 (that is comfortably below hull speed) Skiff (yellow line) has ~26lbs and Tasar (blue line) ~12lbs of resistance. Is it close enough to "2 times"?

Please note, that graph #3 is from sailboat, which, if highly optimized for planing, is still designed keeping in mind low speed performance too. If this hull was designed without any regard for low speed qualities, would its resistance be some 20-30 % more as it is now, when below hull speed? This would make resistance difference close to 3 times.

Please also note, that Tasar in itself is also not strictly optimized for displacement sailing. So, her resistance (per pound of displacement) below hull speed is also somewhat more as if it was designed for displacement sailing only.

The book with different calculation methods is now not in the same room as me, so I cannot present calculations at the moments notice. In a few days I can do it, if you still insist.

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14. Joined: Sep 2009
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### Perm StressSenior Member

It is indeed strange statement. If it is true, all the towing tanks, CFD researchers, and everyone working on hull shape development (most of naval architects who develop hull shapes, not General arrangements only) get their grants, payment for contracts and salaries for nothing...
Shipowners look really stupid than...

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### AlikSenior Member

Sometimes it is good argument, if combined with degree and research

To begin with, Betwaite is not a naval architect or hydrodynamics expert. So how they re-calculated and what they tested is BIG question. If this is published in serious research paper I would look at results; once it is published in amateurish book for sailors - no need to pay attention to that pseudo-science. Read something really valuable, starting from PNA or Savitsky/Mercier paper as recommended.

From first glance, on the graph, line 3 does not seem to match physics of phenomena at lower speeds: it appears linear at range of FnL=0...0.3 that can not happen in reality.

Another problem is that tested hulls do not have same DLR (or relative length l=LWL/(D^0.333)= 7.0, 8.8 and 7.8 respectively - huge difference!). It would have major effect on resistance as me and AdHoc stated already.

Then, looking at numbers 500/443 etc seems they re-calculated total resistance using weight. Meanwhile they should have re-calculated residual and frictional components of resistance separately.

There are other mistakes evident, but I do not want to discuss it as it is clearly not worth any discussion. Just forget about this graph, it shows nothing but misbelief of their author.

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