PROA MODELS
Just a suggestion…....
Proanauts (nuts?) seem to make a lot of models in isolation of different sizes and materials which means that the collective knowledge gained amongst us all is limited and difficult to quantify or verify.
If we were to have an agreed standard reference model shape which we could all make for ourselves against which all others were tested, I am sure that we would all learn a lot more a lot faster…..and more accurately.
Dave Culp mentioned the balance beam approach where you tow 2 models at the same time and adjust the tow point between to get equilibrium. Frank Bethwaite did this for years in developing many of his dinghy designs and other ideas.
The actual standard shape doesn’t really matter, because all other work will relate to being x% more (or less) efficient. So make it a dead easy one to make…..How about a symetrical hull flat bottomed and straight sided: Say LOA: 200cm, BOA: 15cm, H (above waterline): 20cm side curve generated by arc.To which you fix below the waterline another same profiled flat shape of depth corresponding to your particular required displacement (and ballast it accordingly as necessary)????
Then make any other shaped models as you want of the same displacement and LOA, and tow them from a (say 200cm between towing points).beam and we all have something to more closely relate to and duplicate if we want.
Proanauts (nuts?) seem to make a lot of models in isolation of different sizes and materials which means that the collective knowledge gained amongst us all is limited and difficult to quantify or verify.
If we were to have an agreed standard reference model shape which we could all make for ourselves against which all others were tested, I am sure that we would all learn a lot more a lot faster…..and more accurately.
This is a great idea!
It would be a good start if people building scale models, documented their experiences in some way and posted them here and/or Vimeo/Youtube etc. This would be a good start. I have got a lot of ideas looking at various Youtube videos, and I hope my very amateurish crude videos can give something back.
Cheers,
Johannes
Some amongst us have VPP and yacht design programmes, so you could make a “virtual” standard model as well…...............
If we were to have an agreed standard reference model shape which we could all make for ourselves against which all others were tested, I am sure that we would all learn a lot more a lot faster…..and more accurately.
An excellent idea, Rob. To help keep things consistent, perhaps once a standardised design has been settled on, then vector graphics files (eg DXF or SVG) could be produced. That would then allow uniform templates to be printed out on laser cutting machines or on CNC machines.
Perhaps the hull could be one of a “family” of hulls: one of them would be perfectly symmetrical (as you’ve suggested), with others having differing degrees of asymmetry.
Cheers,
Paul
As it happens, there already is a pseudo standard hullform used as a reference for tank testing and now CFD work It’s called the Wigley hull and the shape is defined mathematically:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA224000
The waterlines and sections are defined based on parabolic curves and it is fully parametric so that beam, length and draft can all be varied. The interesting thing is that the hull is bit like a proa hull, being double ended.
While I think that a standard hull model for comparison is a nice idea, you will soon run into problems when trying to compare hulls with widely varying displacement to length ratios and beam to draft ratios, The comparisons may soon become meaningless if you are using a hull with fixed dimensions as a standard (that’s why the Wigley hull is fully parametric). You would need to try and agree on a standard displacement to length ratio, or range of ratios, in order to get some consistency.
It probably isn’t necessary to fix the length of the reference model. It would probably be good enough to simply scale a standard geometry to suit the model you were testing against. I, for instance, tend to build models limited to the length of a standard sheet of balsa, which is 90cm. A 200cm model would be a very large model for me and in fact I have never built one that size. Not even the actual tank test models I have built were as long as that!
Mal.
Mal,
You are absolutely right about all of this…,Scale-able basic design is the way to go.
I can see that even agreeing a standard design and base displacement ratios etc between us all will be a major achievement, never mind how many people end up using it.
Wigley’s model looks a bit dated and complicated for simple minds like me. I was hoping for a simple quick and dirty fix to get the ball rolling…...I eventually hope to do some systematic research on proa shapes, so if nothing transpires in the meantime, I will let you all know what my base design and stats end up being.
Rob
Are you guys familiar with the series of hulls proposed by Edmund Bruce (Design for Fast Sailing, by AYRS)? Keel lines are arc of circle, sections all semi-circular, displacement identical, hulls only differing in L/B ratios?
Bruce gathered and published quite a lot of info on these, from 4:1 up to 16:1—I wish he’d gone to 20, even 24, but it was apparent that drag was reaching an asymptote, so the data is perhaps more complete than it looks.
Bruces stance wasn’t that these were perfect solutions in any way, just that they *are* a reference series, easy to replicate, easy to achieve shapes anywhere in the world.
Would these be useful?
Dave
Are you guys familiar with the series of hulls proposed by Edmund Bruce (Design for Fast Sailing, by AYRS)?............Bruces stance wasn’t that these were perfect solutions in any way, just that they *are* a reference series, easy to replicate, easy to achieve shapes anywhere in the world.
Dave,
They are useful as far as they go, as you have indicated…...But I am suggesting just ONE reference shape. As you say, it doesn’t really matter what it is, so I suggested a symmetrical sharpie, which has to be the easiest and cheapest shape to build, and then everyone could go off and pursue proa design in whatever direction / field they wanted to, and any results presented to the forum then are referenced back to it, say as a percentage better (or worse) in terms of drag, speed whatever works out the most practical / convenient way of comparison. The model size I suggested was on the large side, aimed to come off one sheet of thin ply, The fixed larger size to help reduce the variables in scaling and comparison.
The problem is that agreement on the shape and size probably won’t happen except by default when someone eventually makes a pile of models and presents the results.
Or we could just choose one of Bruce’s shapes as being the most relevant for all round practical purposes and use that….What do you suggest???......The results could then be another “Bruce” Number?????
Rob