Diving & Sail Carrying
The main choices remaining are the Maximum Beam, Transom Beam and Freeboards and the determining factor here is diving. This is the bane of Footy sailing when, inevitably at some stage, the boat is caught out in a race with a higher wind speed than expected and hence too much sail. Any boat will dive in gusts with too much sail up and the small Footy size and unlimited sail area make it more prone to do this than most model yacht classes.
HullForm gives the Moment to change trim (MCT) in kg m/cm as part of its output and although this is a static value it will still be there when the boat is in motion. Obviously the larger the beam, and hence waterplane area, the more change in immersed volume there is fore and aft due to a change of trim and the more righting moment there is to counteract diving. MCT is proportional to beam so for maximum righting moment you need maximum beam. The Iambus2 therefore has the maximum beam that will fit in the measuring box. This feature also has the benefit of maximising the static righting moment in heel which is roughly proportional to Beam^3 and so gives the maximum sail carrying ability for the displacement.
The ratio of Transom Beam/ Maximum Beam or Waterline Beam does not appear in any of the Delft residuary drag correlations. The only guidance is current practice where Transom Beams have increased over the years and now seem to reach almost 100% on new racing yachts. There are obvious advantages in a high value. It enables the designer to get a high Prismatic Coefficient with a fairly flat and straight run of the rocker line to the stern. The large area near the stern also gives a high moment to change trim, as a change in trim lifts a lot of area in or out of the water a long way from the midship station. This can help reduce diving and also pitchwise oscillations in waves. So, the Iambus2 has a Transom Beam/Maximum Beam of 80%. The earlier Iambus had only 70% which was not enough as the resulting Cp was too low and the hull was excessively prone to diving.
An even higher Transom Beam ratio may be desirable but as theory does not really cover it there is no way of estimating this other than building a complete boat with a higher value and trying it. One qualitative disadvantage of too high a value is that the change in immersed hull shape with heel increases with Transom Beam so the boat may be unbalanced over a range of heel angles and difficult to sail. Full size yachts tend to have smaller heel angles than Footys so may get away with wider transoms.
HullForm gives the Moment to change trim (MCT) in kg m/cm as part of its output and although this is a static value it will still be there when the boat is in motion. Obviously the larger the beam, and hence waterplane area, the more change in immersed volume there is fore and aft due to a change of trim and the more righting moment there is to counteract diving. MCT is proportional to beam so for maximum righting moment you need maximum beam. The Iambus2 therefore has the maximum beam that will fit in the measuring box. This feature also has the benefit of maximising the static righting moment in heel which is roughly proportional to Beam^3 and so gives the maximum sail carrying ability for the displacement.
The ratio of Transom Beam/ Maximum Beam or Waterline Beam does not appear in any of the Delft residuary drag correlations. The only guidance is current practice where Transom Beams have increased over the years and now seem to reach almost 100% on new racing yachts. There are obvious advantages in a high value. It enables the designer to get a high Prismatic Coefficient with a fairly flat and straight run of the rocker line to the stern. The large area near the stern also gives a high moment to change trim, as a change in trim lifts a lot of area in or out of the water a long way from the midship station. This can help reduce diving and also pitchwise oscillations in waves. So, the Iambus2 has a Transom Beam/Maximum Beam of 80%. The earlier Iambus had only 70% which was not enough as the resulting Cp was too low and the hull was excessively prone to diving.
An even higher Transom Beam ratio may be desirable but as theory does not really cover it there is no way of estimating this other than building a complete boat with a higher value and trying it. One qualitative disadvantage of too high a value is that the change in immersed hull shape with heel increases with Transom Beam so the boat may be unbalanced over a range of heel angles and difficult to sail. Full size yachts tend to have smaller heel angles than Footys so may get away with wider transoms.
| diving__sail_carrying.pdf |