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  #46  
Old 12-12-2010, 11:44 PM
Mr Efficiency Mr Efficiency is offline
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Why is the planing cat curve starting at 18 knots ? Doesn't give any indication of the low speed end of things.
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  #47  
Old 12-13-2010, 12:00 AM
Ad Hoc Ad Hoc is offline
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Quote:
Originally Posted by DCockey View Post
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"?
1)Correct.
This is from “Mechanics of Marine Vehicles”, By Clayton & Bishop.

2) This is from PNA. “The curves were based upon data from a variety of sources, and result in two pairs of empirical curves which define two ‘design lanes’

3) These are from varying only the displacement and then again repeated varying the length with constant displacement (to validate). The effects of hull shape are, or rather lack of them, is shown in the first graph I posted.
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  #48  
Old 12-13-2010, 12:53 AM
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Originally Posted by Ad Hoc View Post
1)Correct.
This is from “Mechanics of Marine Vehicles”, By Clayton & Bishop.
Original graph is mean values of resistance for series 62 and 65 from Hubble.
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  #49  
Old 12-13-2010, 12:55 AM
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Originally Posted by Mr Efficiency View Post
Why is the planing cat curve starting at 18 knots ? Doesn't give any indication of the low speed end of things.
Because we desgined that boat for 25kts...
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  #50  
Old 12-13-2010, 01:23 AM
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Originally Posted by Alik View Post
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.
Could you also post some rough sketch of hull form variations?
othervise it is not possible to see, what is compared to what.
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  #51  
Old 12-13-2010, 01:25 AM
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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....

.....

Attachment 51153

please note, that in PDF file graph is for the same Froude Number = same S/L ratio
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  #52  
Old 12-13-2010, 01:55 AM
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Originally Posted by Alik View Post
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.
These boats (australian 18 ft skiffs) sail at 14 knots close hauled. Also his HSP boats are capable of similar performance.
Of course, there is no science and naval architecture involved...
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  #53  
Old 12-13-2010, 02:55 AM
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Originally Posted by Perm Stress View Post
These boats (australian 18 ft skiffs) sail at 14 knots close hauled. Also his HSP boats are capable of similar performance.
Of course, there is no science and naval architecture involved...
To me, it does not matter how fast they sail; these are just small boats where design by trial and error works. And thus You are right - no science and NA involved. The graph they show in the book is just nonsense because every naval architect knows how to recalculate resistance from one size of boat to another. With this graph, their intuition was wrong
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  #54  
Old 12-13-2010, 03:09 AM
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Originally Posted by Perm Stress View Post
Could you also post some rough sketch of hull form variations?
othervise it is not possible to see, what is compared to what.
I have the final shape completed only - planing shape.

But the purpose of study was: what is better round bilge displacement or chine planing for this speed and load? It is a bit tricky issue, especially to convince the customer. Round bilge was studied with Molland series, so the parent hull is from there (actually it is NPL-series hull).
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  #55  
Old 12-13-2010, 06:39 AM
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I see a lots of graphs here, but each one appears to be valid inside it's own fence.
The original question was about the behaviour of various hull forms in the low-speed range, so I don't think it's useful to show a curve for a planing cat for 18+ kts, compared to curves of displacement hulls at lower speeds.

There was a similar thread started by Easy Rider, where I had attached a paper by Van Oossanen there (he does know something about ship design) which shows a general resistance trend as a function of immersed transom area:
Planing Hull at Disp Speeds
It was much debated afterwards due to the lack of info on the method used by Van Oossanen et al. in that research. However, it does show a 100% (or 2 times) higher resistance of a hull with immersed transom (like a planing hull), compared to a dry-transomed hull.

So now I'll put some more fuel in this fire by showing another paper, by Blount and McGrath. Several hard-chine and round-bilge hulls resistance data have been scaled to 500 t, and the variation of both LWL and wetted surface has reportedly been accounted for. Again, a significant difference (of the order of 100%, or 2 times) between the two hull types is shown in the low speed range (figures 1 and 2). Also, the influence of L/D^0.33 (or simply LD) ratio is shown in the figure 3, for the two hull types at Fn=0.6 (semi-displacement), with R/W data points again in favor of round bilge hulls.

I'd like to hear your opinions about this paper, it's conclusions and eventual flaws.

Cheers
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  #56  
Old 12-13-2010, 07:06 AM
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Originally Posted by daiquiri View Post
I'd like to hear your opinions about this paper, it's conclusions and eventual flaws.
Good paper; all based on well-known series though.
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  #57  
Old 12-13-2010, 07:46 AM
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Quote:
Originally Posted by Alik View Post
Good paper; all based on well-known series though.
The figure n.8 (Design Guidance) is particularily interesting and valuable, imho. It shows where various hull features jump in as the Froude number is increased.

If you take into consideration both the Blount's paper seen before and Van Oossanen's (which I'm linking here again, for comodity: http://www.oossanen.nl/download/perr...orm_design.pdf) you get a much larger picture about the factors influencing low-speed resistance, which goes beyond just considering LD or hard-chine or round-bilge.

The way I read these results in the displacement and semi-displacement speed range is:
  1. the LD ratio is the single most important factor for Fn>0.5 and when LD is less than 7. For LD>7 the difference between hulls with different LD gets much smaller and less important.
  2. For displacement speeds (Fn<0.4), LD has a very little influence (NPL data).
  3. the transom immersion can double the resistance at displacement speeds (Van Oossanen).
  4. the LCB (or LCG) location is of great importance in the displacement and semi-displacement range, and can double the resistance if placed too-far fwd or too-far aft of the optimum location.
  5. the L/B factor is important for low LD's, and the resistance increse of a L/B=3 hull over an L/B=5 can be as much as by 40%-50% in the semi-disp. range. For LD>6 it becomes much less significant. (NPL data).
Comments, corrections?

Cheers
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  #58  
Old 12-13-2010, 07:57 AM
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Originally Posted by daiquiri View Post
It was much debated afterwards due to the lack of info on the method used by Van Oossanen et al. in that research. However, it does show a 100% (or 2 times) higher resistance of a hull with immersed transom (like a planing hull), compared to a dry-transomed hull.
OK, I have seen that paper, but we do not know what the parameters of tested hulls are. Use of Telfer coefficient makes me think that they could have DLR also.

Another concern there is use of CFD for transom hulls and how they model transom effects.
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  #59  
Old 12-13-2010, 08:09 AM
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Originally Posted by daiquiri View Post
The way I read these results in the displacement and semi-displacement speed range is:
  1. the LD ratio is the single most important factor for Fn>0.5 and when LD is less than 7. For L/D>7 the difference gets significantly smaller and less important.
  2. For displacement speeds (Fn<0.4), LD has a very little influence (NPL data).
  3. the transom immersion can double the resistance at displacement speeds (Van Oossanen).
  4. the LCB (or LCG) location is of great importance in the displacement and semi-displacement range, and can double the resistance if placed too-far fwd or too-far aft of the optimum location.

Generally seems correct.

These conclusions and numbers are different from series to series.

Transom submersion might influence the resistance, but I would say less than double in range of displacement speeds.

CP would have more effect on resistance compared to LCB.
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  #60  
Old 12-13-2010, 08:21 PM
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Un-scientific input...

My home-made 1/4" plywood rowboat with round chines and a submerged transom had an interesting characteristic. With some overpitched props on the small dc motors, she would only run about 4 MPH. If I moved my weight all the way up front and lifted the transome out of the water, the speed would go up and the motors would unload. Just an estimate but I would say at least a 2 MPH increase with the transom lifted out of the water. The bow has a fine entry so it didn't seem to mind plowing.

Shifting the weight forward never worked well with off the shelf trolling motors on this hull because they are very speed limited by the rpm and pitch of the props.

This hull is pictured in my profile with a bad attempt at a surface drive. I never got over 8 MPH (GPS) with the surface drive. The was a giant 'hole' behind the transom above 5 mph and I couldn't climb up on plane. I never got a chance to see if shifting weight forward would have provided the extra speed to plane her out. Termites took her over last summer but got a really light Al hull with a similiar shape to play with next year.
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