Making very simple multihull VPP.

I haven't found any software that isn't very expensive and can calculate multihull polars. Maybe I could do a simple one myself.

My idea is to get hull forces by using interpolation of pre-created hydrostatic and hydrodynamic data from Freeship and Michlet and use some simple algorithm to get forces from sails and fins.

For sails I have found this page:
http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm#Results
I can pick the sail force algorithm from JavaScript but I wonder if there is anything better that understands big roaches, rotating masts and asymmetric Gennakers that are typical for multihulls.

I would also like to know if there is a simplified formula for centerboard and rudder forces.

The program doesn't need to be extremely accurate. Just enough to compare design choices.

 

It is a pity you have not got any response to your mail since this is surely a very interesting topic. You may want to check out also the program PCSAIL for inspiration although this is again only related to monos. Version 1.0 can be found on the net but there is apparently a better version 2.5 but I don't know how to acquire it.

While Michlet can help you on drag below the surface, how do you plan to quantify the air drag? This can be quite significant for Multihulls.

 

You seem to be on the right track.

Look on the same website for something called vortex lattice method. This could be used to build new lift and drag coefficients.

Yes, basic foil theory will do the trick.

 

I do something very similar for propeller driven boats. My process is as follows:

1. I run Michlet for the hull in the required speed range to get the speed-out-by-ship.mlt file.
2. I load the Michlet text file into Excel and use either 2, 3 or 4 order regression to model the speed v drag curve so I end up with a single formula. The complexity of the curve will depend on the hull. Note that this does not allow for any trim or roll. I also add other drag components for wind, rudder drag and any other appendages.
3. I use JavaFoil to produce a series of data for the chosen prop section over a range of Re# and AoA. I save this data as a text file and then load into an Excel spreadsheet so I can perform regressions for Cl and Cd based on RE# and AoA. This means that for any value of Re# and AoA I can determine the corrseponding Cl and Cd for the foil I am using based on a single formula.
4. I have a segmented prop model based on the foil data and gross effects for induced drag. This produces thrust and power demand for a given rpm and boat speed.
5. I use a goal seek function to adjust boat speed until the prop thrust and the hull drag ballance. My model allows me to set the prop shaft speed and I get the corresponding boat speed and power requirements. I also have gearbox losses etc.

In your case the foil stuff is possibly simpler unless you want to bring in factors like sail twist, varying sail chord length and vertical wind velocity profile. In this case the sail model would have similarity to my prop model. The hull stuff is more complex as you need to bring in things like leeway, roll and trim. These might involve doing a series of curves from Michlet and another set of regressions. I have not looked at how much roll alters drag.

I think leeway on the hull can be accommodated by working out the induced drag for the hull as a very low aspect foil. Most of the lift force is generated in the keel.

I did a VPP for a single condition on a sailing cat where the windward hull was just lifting. This makes life very simple because there is no hull interaction. I also used very simple sail model based on a fixed Cd. I also positioned the keel so there was no steering moment required under this condition.

Rick W.

 

I have now written some code for my multihull VPP. The polar curves it draws looks about right shape. The hull resistance formula is quite poor and the program works like boat has infinite stability; it doesn't heel or captize. So the program has much room for improvement. It would be nice to find polars of Tornado etc. to help calibrating the program.