# Methods for estimating resistance of existing multi-hull design

Discussion in 'Hydrodynamics and Aerodynamics' started by ryanonthebeach, Sep 19, 2017.

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### ryanonthebeachJunior Member

What are the best methods (or software) for estimating combined hydro+aero resistance for an ocean going multi-hull (~65')
That gives an output something like a polar diagram for resistance, for different sea states?
Cruising level of accuracy/effort (not AC racing level)

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### tspeerSenior Member

I assume you're not talking about a professional CFD code like Star CCM+, which can do everything you're asking for. I suspect you're going to need to combine the results from several methods to create a velocity prediction program (VPP). Michlet will calculate multihull wave drag and has simple methods for adding the viscous and form drag components. However, it does not handle leeway or the drag due to lift on the hulls or appendages. AVL will calculate the characteristics of lifting appendages, to which you can add the profile drag from XFOIL. The aerodynamics of the topsides will be pretty much all bluff-body separated flow, so empirical handbook methods may be best here - see Hoerner's Fluid Dynamic Drag. Then you have the aerodynamics of the rig, which can be handled using AVL and XFOIL, although the latter isn't great for thin shapes like jibs, and it can't handle massive amounts of separation like on downwind sails. All that is for flat water. I'm not sure what you'd use for added resistance in waves. SMP doesn't handle multihulls, and I don't know of any low-cost multihull seakeeping code. Excel's Solver can find the equilibrium solution, once a spreadsheet is loaded with tables of data from the other methods.

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### Jean Marc DelaplaceJunior Member

I just signed in for this purpose. I have a trimaran house boat for inland waterways with an electric propulsion. I designed everything except the hulls, that a shipyard suggested to me.
The three hulls are merely cylinders with a kind of bow made by pinching the cylinder at the front.
I am not satisfied with the speed I get, or rather, with the power it takes to ride, and I would like to study whether it is possible to improve it by a relatively simple change.