Seakeeper FAQ list

• What restrictions does the Seakeeper Demo have?
• Zero-speed motions of Catamarans
• How does the 12 parameter mapping function work?
• How does the roll decrement function work on Catamarans?
• How accurate does Seakeeper predict motions of catamarans?

The demonstration version of Seakeeper allows you to open only the Seakeeper Sample.msd file. Copy and save is not available  for seakeeping analysis results.

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Seakeeper may be able to give approximate results for a vessel at zero speed. However, Seakeeper is first order, linear strip theory for a monohull vessel, so it will not account for the following:

Hull interactions -- these can be significant for a catamaran at zero speed. Seakeeper can give reasonable results for a catamaran with demihulls that are symmetrical about their local centrelines operating with sufficient forward speed that the interactions are negligible. This also depends on the demihull separation. In the Seakeeper manual, there are some comparisons for Seakeeper and some tank-test results for catamarans.

Second order drift forces can be important at zero speed -- especially if the vessel is moored - this will not be accounted for by Seakeeper.

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Q) Could you please explain the 12 parameter mapping function. I understand the three-parameter standard Lewis stuff, but what is this (a Frank's close fit panel method). Can you suggest a reference?

A) The 12 parameter mapping is just like the 3 parameter mapping. Its still a conformal mapping, but uses more parameters. Its not a Frank Close fit.

With a 3 (or even 5) param mapping, you can calculate the mapping parameters directly (the 5 one can use the area centroids to make more equations to solve -- though we dont do this in Seakeeper) For the higher order ones, you need to use an iterative method to find the solution. There are quite a few papers. See Bishop and Price and also Westlake, Wilson "New conformal mapping techniques for ship sections" ISP number 449, vol 47, April 2000. (though this deals with asymmetrical mappings, but we just do symmetrical ones at the moment, same principle though).
 

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Q) What is the roll decrement calculation doing with cats? Having run the monohull and catamaran, the roll amplitudes are clearly different but it is difficult to see where the centre of roll is for the cat.

A) The only modification we do for cats is work out the metacentric height correctly -- i.e. taking into account the separation of the demihulls. You still only put in a monohull on the centreline, but if you say its a cat the transverse waterplane inertia takes into
account the separation that you have specified.

The problem for cats is that you get a blanketing effect of the weather demihull, and this is not taken into account. For the roll
decrement curve, there is no wave excitation, so you are probably not too far off provided you get the damping right -- the main reason for putting in the roll decrement simulation was so that you could get a feel for whether you had put in a sensible damping. We are working on a method for estimating the damping based on hull shape.

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Seakeeper uses strip theory to predict the coupled vertical plane motions of heave and pitch. The added mass, added inertia and damping terms are calculated from the 2D sectional added mass and damping which are computed using Lewis conformal mapping methods.

The way you model a catamaran in Seakeeper is by modelling one demihull on the centreline, like a monohull.

Dr Patrick Couser, Naval architect and Seakeeper senior software developer, has found in his model testing studies that you can get reasonable approximations to the heave and pitch motions of catamarans with symmetrical demihulls by simply modelling one demihull and assuming that there is no interaction between the hulls. In practice there is some interaction between the hulls but if you try to model this numerically in a 2D sense, the interactions are greatly overestimated and the results less accurate than if you modelled just one hull. Once the Fn starts to increase (around Fn=0.5 for a catamaran with a spacing/length ratio of about 0.3) the interactions are almost insignificant and disappear as the Fn increases further. I would recommend that you study the first part of Appendix B in the Seakeeper manual - this gives comparisons of predicted motions of a wide range of catamarans with experimental results.

For roll motions Seakeeper uses a simplified model of a damped, mass-spring system. It is able to treat catamarans by using a single demihull Maxsurf model and then specifying the demihull centreline spacing in a dialog in Seakeeper.

Tip: If you want the animation videos to show a catamaran hullshape:

Step 1: Define a copy of one demihull on the centre line, like monohull and make these surfaces "Hullsurfaces.Step

Step 2: Make the transparency of the hull surfaces 100%.

Step 3: Then complete the rest of the design with all the surfaces of type "Structure" -- you can include the demihulls in their proper positions including all the superstructure surfaces etcetera.

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