Discussion in 'Stability' started by terrnz, May 31, 2017.

  1. terrnz
    Joined: Mar 2014
    Posts: 67
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    thank you for your reply.
    1)the foil would indeed be close to Bmax, the hull is fairly slab sided with a soft chine at waterline
    2) the notes in the spreadsheet , unless I was mistaken which is often, addressed the issue of being close to the surface:
    The conditions when these expressions are valid:
    Near-PLANE (low deadrise), SEPARATE (or installed at bow; but not at stern since flow can be affected by bow foil), SHALLOWLY submerged (depth is less than span), NON-CAVITATING, STEADIALY moving foil in CALM seas.
    Hydrofoil is the profilled supporting surface on which lifting force appears at the movement of a ship.
    Lifting force is found by the expression (1), where 'rho' - liquid density, V - velocity, C - lifting force coefficient, S - hydrofoil area.
    z = ( rho / 2 )* V^2 * Ct * S


    The influence of free surface is taken into account by the function (4), c - the biggest thickness of the foil, h - depth of of foil immerssing.
    Ki = 1- (.5 + Cb * exp( -2 * h^.6))
    3) the incline I thought was dealt with by span*cos(heel), the taken as 50%depth foil tip

    I note the conditions are for calm seas so your comment about waves is certainly valid.

    however if the foil span were 2 m and heel 10 deg ( cos .9848) the horizontal component is only affected 2%
    given the foil is just below the waterline 0.1m the depth of the tip would be approx .45m.

  2. terrnz
    Joined: Mar 2014
    Posts: 67
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    inputting figures into the spread sheet using an emphirical treatment I have the following observations;
    increasing chord by 25% gives only a 10% increase in righting moment
    increasing span by just 10% gives a 30% increase in righting moment
    both the above are at a speed of just 4m/sec upwind
    the righting moment does not change much between 10 and 30 deg, perhaps the effectiveness due increase in depth and AR is greater than the loss due to effective area and arm.
    the lift/righting moment increases enormously with increased velocity.

    the increased stability, according to my rough calculations, would mean I could take 500kg of lead out of the bulb, and in power reaching condition add a huge increase in righting moment.

    It seems to me the DSS option is well worth further investigation.
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.