Resistance factors, planing hull at low speed

This would be interesting! Perm Stress, you are sparring with the two most notable and experienced ship/boat designer in this forum. Ad Hoc and Alik.

And what do you mean by "All the stresses in my designs are 95% of permissible"? Is it 95% of ultimate or 95% of the allowable?

 

So transom immersion doesn't matter? I'm skeptical.

 

"So how they re-calculated and what they tested is BIG question." They towed real hulls. As they are used for racing. Than recalculated with weight. Yes it is not scientifically accurate. But would proper recalculation procedure make a difference in end result more as a few %? Would it make resistance per ton of weight equal? I doubt it.

"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!)." This huge differences all occur in the same end of DLR spectrum -extremely light.

"To begin with, Betwaite is not a naval architect or hydrodynamics expert"
"Just forget about this graph, it shows nothing but misbelief of their author"

Yes of course, he is not an expert. He only created some boats, capable to sail at up to twice the wind speed...

He only did some research and took part in creation of centreboards and rudders for 18 feet skiffs, suitable for boatspeeds of up to 35 or so knots...

He only did some research, and together with colleges, found a way to increase mainsail maximum lift coefficient from 1.25 to 1.85...
And to improve an L/D ratio in the same time...

What an insignificance...

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However, I still do not find numbers in your post.

I have presented first set of mine.

It is your words:

"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. "

It should be not so difficult to post some results of these calculations. With sketches of general hull shape, only to have an idea of hullform variation involved, not to enable anyone to copy your designs.

 

I mean, that once allowable stress level is set, I fiddle with design, until all possible stresses are at 95% of allowable. Allowable stress for the same material is different for different structures, applications etc..

 

It is very easy to check.
All that is required is to take small planing powerboat which is also capable to be rowed.
Than it is necessary to row it with transom immersed and with bow down trim and transom out of the water. Difference is immediately obvious to hands of the person at oars.

 

It might be impossible to steer.

 

What Alik is saying (I think) is that drag is not a fixed amount per pound or kilo of boat weight.......the amount of drag changes with speed (S/L ratio).

This is why Bethwaite's comparison as posted above makes no sense....there is no accounting for the difference in length (ie difference in speed length ratio). I can imagine drag differences could well be significantly different at very low speed, but in the neighborhood of "Hull speed" differences become rather small.....see my comparison below....

Thus...Yes, the little powerboat with transom immersed is hard to row at S/L of 1.0 or less.......but as speed increases the drag difference becomes less, then crosses (they are equal at some speed) and the displacement hull drag continues straight up while the planing hull drag levels out or perhaps drops (hump speed).......

These three designs (graph below) are all 28' overall and waterline lengths are similar, there is no correction for weight (displacement)........note that the displacement and planing hull resistance curves cross (are equal) at about 7 knots......If extended the drag for Ironbark would carry straight up....

 

TAD's graph, if accurate, indicates that at slightly below hull speed, say 5-6 knots, resistance is about 50% more for the full planing hull versus the displacement hull of similar proportions. Obviously, any faster and the picture changes quickly. That is helpful. Clearly in the presence of headwinds that would be whittled down to much less than 50%, but it is a significant figure for consideration, if resistance is directly proportional to fuel consumption, which I assume it is, very nearly so anyhow.

 

Don't read too much into that graph.....it was not created for this specific argument......it is an illustration of general drag (resistance) trends......

While the three boats are of similar dimensions, displacement (and thus cost) and most importantly capability is vastly different......Ironbark weighs 16,250 pounds and has a D/L of 421, Douglass Fir weighs 11,600 pounds and has a D/L of 273, and Red Cedar weighs 7800 pounds and has a D/L of 201.......

See the whole paper here......http://www.tadroberts.ca/about/pdf/power-boat-design-form-and-function.pdf

 

I’m sorry to say but all this statement shows is your lack of understanding of hydrodynamics and indeed naval architecture. Any competent professionally trained naval architect can tell you shape plays such a minor role. Your desire, dream or belief to the contrary will not change the existing and known laws of hydrodynamics, despite your protestations.

Correct.

Incorrect. Being at one end of a spectrum makes no difference. The differences are between each parent hull form. There is a 27% difference between the 14footer and the 17footer. Don’t know about you, but I would notice a difference of 27% in whatever form factor/resistance you care to select.

Agreed, so, apart from our own published data, ….as you ask, here are a selection from more “established” publications:



Whatever end of the spectrum ones hull is at....LD ratio all plays a significant role in resistance.

 

Created some 18' boats? This is huge input in science of naval architecture! With their level of understanding of hydrodynamics deriving from their graph I can imagine what kind of research they did!

I would say there is a lot of amateur designers around boats, and some of them produce nice and good working designs. But then it comes to research credit should be given to professionals.

 

This is sample of design study made last week for selection of hull shape for power catamaran. Parameters: LWL=17.5m; BCB=4m; BC or BWL=1.6m; mLDC=25000kg, cruising speed should be 25kts. Red line refers to sharp chine shapes with submersed transom (about 85% of middle area); rest of lines refer to round bilge shapes and stern with slightly submersed transom.

Vertical axis is SHP in h.p.; horizontal axis is speed in kts. All compared options have same displacement and waterline length.

It should be noted that for lighter catamarans intersection point between red line and rest of lines (i.e. speed where planing shapes become justified) will move to higher speeds.

 

How was the data generated: tank test, CFD, software tools, ???

 

We used systematic series methods, also 'anchored' to sea trials data. We (and customers!) can't afford testing/CFD program for design study, for every 60-footer we draw

CFD is useless for cats unless is validated properly. Last time we outsourced CFD calculations and got the resistance predictions that were 20% lower than actual.

 

The first chart appears to be a plot of non-dimensionalized resistance vs speed from a "series" test of a set of parametric hull designs. Is that correct?

Is the second one from a "series" test?

Third chart - what varied for the various "Length Displacement Ratio"?