When designing rc yachts, namely IOM’s do you use same parameters, e.g 0.53 co-efficient, as you would in a full size design? I’ve read that .053 should give good all round performance.
Second question, is it important to get bow, mid hull, and stern co-efficents around the same numbers? It appears that when hulls are set up with all numbers close to being the same, the hull looks to be a full displacement type shape that will be a ‘dog’ down wind, probably drag its stern and never get on the plane.
Others will probably disagree (and I’m just a theoretician!) but I can’t get the prismatic coeff to “talk” to me. It is just a gross measure that doesn’t help me decide if the hull shape I have in mind will actually have higher or lower drag, at what kinds of wind speeds or heel, and whether she’ll plane or nose-dive…
… is it important to get bow, mid hull, and stern co-efficents around the same numbers?
What coefficients are you thinking about here? AFAIK, prismatic applies to the whole hull rather than to parts of it…
Jeez Lester that was quick I only posted this ten minutes ago.
I use a free software program, HULLFORM, it has the co-efficients showing as bow, mid section, stern, block, and waterplane, and a whole figure. It seems that to the eye that a hull that looks good can have an overall co-efficiency of .053. But the bow is often around .050 and the stern can be around .060. I’m not much into theory. As my knowledge is on a need to know basis. I just want to know is this about right?
As a slight aside I have admired Micheal Scharmers narrow IOM. I didn’t realise just how narrow it was until I saw it at the worlds last week. I recently built one with .175mm beam. It really suffers with stern drag down wind, in light to moderate breeze, fantasic in big air, both up and down wind, yet the numbers were, according to Hullform, okay. I realise that you can’t use this boat as a benchmark as it is quite extreme.
OK. Might need a trip to the Hullform manual or help, 'cos “block” and “waterplane” coeffs aren’t prismatic coeffs, they are, erm, “block coeff” and “waterplane coeff” in their own right. So I wonder what exact flavour the “bow”, “mid section”, and “stern” coeffs might be…
It seems that to the eye that a hull that looks good can have an overall co-efficiency of .053. But the bow is often around .050 and the stern can be around .060
Yes, if these are “partial” prismatic coeffs, then this is telling you that your bow is finer with softer bilges than your stern… But I’m sure you knew that anyway! (You can see why I can’t get on with Cp in general.)
As a slight aside I have admired Micheal Scharmers narrow IOM. I didn’t realise just how narrow it was until I saw it at the worlds last week. I recently built one with .175mm beam. It really suffers with stern drag down wind, in light to moderate breeze, fantasic in big air, both up and down wind, yet the numbers were, according to Hullform, okay
Michael’s boat is extraordinary… If you suffer with stern drag, did you mention this to Michael? He has flattened the aft rocker of his hull totally… And, it might be worth (a) exploring your static trim and maybe putting weight forward so the stern is lifted, and (b) maybe setting your bulb cant to a less aggressive angle…
You’ll find that most IOMs fall within a relatively narrow range of prismatic coefficient. My own boat comes out at .545. My waterline beam is 189mm. In simple terms, you could describe it as a measure of “fullness”. Essentially, if you have to be 1m long, and weigh 4kg - you are really just changing the way you spread the fullness around within those fixed parameters. You can be fine in the ends and full in the middle, or, say, full throughout the length of the boat.
If you take an extremely narrow IOM, such as Micheal Scharmer’s, the Cp will be about .56 (based on my mock ups). (I still don’t know how he gets that hull to sit no deeper than 60mm - not that I’m questioning the measurement in anyway - just expressing admiration). A wide, shallow design will have a lower Cp, maybe down to .52, and I think that’s about the IOM range.
I agree with Lester’s comments and actually wouldn’t worry too much about Cp. You pick your concept and design to that concept and within the rules, and Cp is just a function of that.
My latest 1s 182mm waterline beam and Pc of .528.
Muzza is right the Pc shows how you have distributed the displacment…so skinny IOMs will need larger Pc than wider ones since they all displace 4kg.
The full size books tell of optimum values…Im not so sure there right for us anyway.
I am using hullform as well Tony.I have a well over 50 IOM designs in my folder.
Hullform is a great way to analyse various hulls.I learnt so much on my last IOM design using hullform…Building it right now.
Thought I might attach a design in hullform…This is not the boat I am building but an attempt to draw Bantocks Topiko from photos on the web.
For my latest IOM design I drew (tried anyway) to model as many different known hulls as possible in Hullform and compare them.I then started drawing my own and comparing.I drew at least a dozen existing hulls and about 20 newones to compare.I learnt a lot along the way.
I use the free version of Hullform to create and save the designs.I then use the pro demo version of Hullform to compare the designs…it can plot various graphs and do drag predictions etc.
Evcentually I arrived at “my” solution which is the boat I am building.I can’t wait for it to see the water and see if the Hullform program has been helpful or not…this boat will then form a basis for more refinement.
I have a Bamforth designed Stealth which will be the Benchmark boat for my testing.
2nd edit. Attached Scharmers mk8 type hull . Pc is around .58 with 55mm hull draft.Shows good balance and low drag…but not much stability.
I feel I now have a bit more of a handle on the whole thing. I really appreciate the time and effort you guys have contributed. Interesting to hear your thinking on waterline beams. I asked either here or on another forum for input regards IOM hull shapes, - wide or narrow-, which is the way to go? I feel that IOM design has become a bit of a one design class. Almost all the current boats are around 230-250mm beam and very round stern sections. I’ve wondered about going even wider than, say TS2, something that may have a short waterline, (good in very light wind) but increases as the hull heels. What stuffs this theory is wetted area. Though is wetted area the problem? Very narrow hulls seem to have the same problem. Michael Scharmers IOM seems to suffer in light air. Is it because of wetted surface or is the hull shape?
I’m sure I could rave on forever about the merits of beam and hull shape. I suppose thats why we keep designing and building in the hope of making some magical breakthrough.
How did that really narrow design you did a year or so back go? You told me at the time that one had been bulit in France.
No real feedback on that design Tony…so who knows???
would need to build myself to test…so many boats so little time.
Hullform has really improved my hull design ability recently by alowing me to see so much more has the hull heels etc.
Its a great tool…Thanks to Bill and Rob from the US1m gang for putting me onto it and urging me to try it.Its the best software I have found for sailboat hulls and can tell you more than the real expensive high end stuff.
I use the free version of the software to create and save the designs.
I then import them into the professional version which you can download a demo version.You cannot save anything from the demo…but you can look at all the fancy drag curves graphs etc it has.
So I am using 2 versions at the same time if you get my drift.