Heavy Weights vs Light Weights

Originally posted by JimC Where you are going wrong is............

Edited by Barty

Originally posted by Barty The author assumes and expects that 'armchair' dinghy designers will criticise, neh, mock these calulations

Your diagrams demonstrate that a boat which has vertical topsides above the static waterine will gain WSA much more slowly than one which is flared. This is of course correct.

Where you are going wrong is in not thinking about the actual immersed shape of whole of your boats. Whilst in light air a typical wedge shaped skiff may have the whole of the mid section immersed up to the chine, the aft sections will be much less immersed with bow down trim, thus greatly flared above the waterplane, and its the aft areas where the WSA will come from.

Hmmm, I think you made a mistake somewhere.

The draught of th box section with 160kg crew weight is 39mm (in fresh water), increasing to 51mm for the heavier crew.  These equate to WSA of 8.468m^2 and 8.612m^2 respectively, which is an increase 0.144m^2, or 1.7%.

I did a triangle rather than a circle (cos it was easier!) and the draught is 78mm increasing to 102.5mm.  WSA is 8.332m^2 increasing to 8.452m^2 - an increase of 0.12m^2, or 1.4%.  So the trianglular section is 'better' (from a WSA weight sensitivity perspective) than the box section, which is as you would expect (as the waterplane area increases with draught).  However increasing waterplane area (and waterline beam) introduces its own problems...

And, of course, you have to consider the juxtoposition of Jupiter with Venus.

Originally posted by JimC You're assuming that waterplane area doesn't change with the extra immersion. On the flatter boats it can increase spectacularly.

I thought about this and did a few rough and ready calc's to see if that true. 

Imagine 2 boats that are 4m long, 2m wide and 0.5m deep.  Both have constant section along there length (see assumptions/disclaimer at the end) to make the lunch time maths easier.  Both boats weigh 150kgs all up and the crew weigh 160kg all up.  One is box section the other is round bilged

The box sections draft would be 2.2cm in salt water and the round bilge would be 9.5cm, nothing revolutionary there.  The STATIC wetted surface area (WSA) is 8.04m2 for the box and 4.52m2 for the bilge

Now if both crew members ate the pie diet and put on 100kgs between them the draft would be as follows:

So for the box section the draft has increased by 127% (2.8cm) and the bilge has increased by 26% (2.5cm).  The interesting bit is WSA, the box has increased by 0.7% whilst the bilge by 12.6%.

So by eloquent and logical deduction, box type craft, i.e. little or no rocker, large flat planning surfaces can carry weight better as the WSA doesn't increase greatly as it is already high (hence high drag as sub-planning speeds).  Where as round bilge or tradional designs suffer higher WSA's as weight increases and hence more frictional resistance as weight increases.

Assumption/Disclaimer

• No account has been taken for waves, wind, atmospheric pressure, the seagull to sky ratio, the ambient temperature of the water, whether the tree surrounding the water have leafs on them, whether the sailors are carrying clean hankies or answer to the name of Susan
• These calculations took place during a lunchtime thought shower and as such should not be used in anyway other than those intended.  The author reserves the right to provide any user with a 5p per litre discount during the promotion of these calculation (terms and conditions apply) but can and will withdraw the offer without notice.
• The boats used in the calculations are not real and as such should not be taken as a "recognised design" any forum member who willing choses to build and race a craft of identical/similar proportions shall have no claim against the author following disappointing results, soiling or spoiling of any sailing gear or ridicule.
• The author assumes and expects that 'armchair' dinghy designers will criticise, neh, mock these calulations
• Finally the author denies any involvement with the above and is currently under going an interview for MI5 to go and play briefcases in Russia

The end

Edited by Barty

An aside, but I think being light for a boat often makes you a better sailor out of necessity. Better hiking style, flat wiring, faster out on the wire out of tacks etc.