Discussion in 'Boat Design' started by Manie B, May 5, 2008.

  1. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 117, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Gents - when a displacement hull in Delfship shows a resistance of 1 Kn at 5.6 knots and 2 Kn at 6.3 knots (Delft series '98 graph) what would that difference mean in horsepower / kilowatt. Also at a wild guess what would the sail area be to achive this force as a standard bermuda rig, no fancy expensive sails = main and jib.

    I am just looking for rough basic guidelines eg. 3 hp up to 8 hp and 20 sqM and 35 sqM. I am trying to get a better "feel" for all the ******** sales pitch thrown around on the internet nowadays.

    I am currently using Delftship ver. 3.2 and Turbo CAD 14 and when i draw what looks similar to the "brochures" i find that manufacturers and designers are overstating the speeds and carrying capacity of cats in the real world. I am reasonably sure that many cats are very comfortable at 5 to 8 knots but to drive them to 16 knots - oh no:D -oh no:D

    And when "overloaded" by 15% = 50mm to 100mm deeper waterline they dont move very well do they:p

    Just thinking out loud = brain fart = days and days drawing on the computer = sleeping in the dog box again:D
  2. Guest625101138

    Guest625101138 Previous Member

    Power is force times speed.

    1 watt = 1 newton x 1m/s

    So, for example with 3kW you could push a force of 1kN at 3m/s.

    Some other things on units:
    There are 746W in one HP

    1 knot is roughly 0.5m/s - exact conversion:
    m/s = knot x 6080 / 3.28 / 3600

    10N approx = 1kgf (exact is 9.8)

    2.2lbf in 1kgf

    Metric is a much simpler system to work with because the units work together so simply. The fact that the gravitational constant is very nearly 10 helps make life simple.

    By the way I would not take much heed of drag information produced from Delftship.

    Rick W.
  3. Manie B
    Joined: Sep 2006
    Posts: 2,041
    Likes: 117, Points: 63, Legacy Rep: 1818
    Location: Cape Town South Africa

    Manie B Senior Member

    Rick thanks, it helps to get the mind clear again.

    Are there any websites or downloads that would give sail design data.

    Once again i am looking for some simple rule of thumb.
    Main = 30 sqm Jib = 15 sqm at a wind speed of 10 knots would give say 2 newton down wind and 2.5 newton across the wind.

    In other words how do you get to the sail area for a 36 foot cat (main and jib) surely there has got to be some kind of a starting point? For this exercise lets assume the average dimensions of the popular cats out there today.

  4. Guest625101138

    Guest625101138 Previous Member

    You do not need to get too concerned how well the sails work unless you want to outpoint the next boat. This is also not a function of the sails alone. Keel, rudder, hull shape and weight distribution are some of the factors that come into play.

    Ultimately it gets down to a force balance. So how much force can the hull handle before it capsizes/knocked down or pitchpoles if going downwind. Point is that the sailing performance will depend on how fast the boat goes before it gets unstable.

    If you really want to get into the sail design stuff then JavaFoil is a good place to start. There is some good coverage of JavaFoil on this site.

    You could work on a Cl of say 0.6. Cd of 0.08 for a sail close hauled. Maybe Cl of 0.8, Cd of 0.15 for a full sail on a broad reach and Cd of 1.4 for a sail going down wind.

    The force on the sail can be determined by:
    perpendicular force = 0.5 X 1.2 x Wap^2 x area x Cl
    similar for in line drag only Cl replaced with Cd

    As you can see the apparent wind speed plays a huge role. Hence the area of the sails really depend on the expected sailing conditions. Little point in having a rig with ability to carry sails that are always reefed.

    I am sure to get debate on the coefficients but in the end they are not all that important - as I said initially it is a matter of force balance. How fast can the boat go before it tips too far. Or, alternatively, how fast do you want to go in a particular hull.

    Rick W.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.