# Wave resistance

Discussion in 'Multihulls' started by weys, Jan 15, 2007.

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### weysNaval architect student

As some of you know I am naval architect student , designing a cat ,

I need your help again .

I am trying the calculate the resistance ,

the resistance has been build up from -friction
-skin roughness
-Viscous pressure
-Wave
-Heel
-induced

For friction I used Rf=Cf*0.5*rho*V^2*Sw

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### weysNaval architect student

but how about the viscous pressure resistance how can you calculate that , or what is a reasonable % ?

For skinnroughness the permissible roughness in microns is H=100/V v in m/s

Wave resistance all i can find is the Delft serie's but they are not very accurrate for cat because I have an Lwl/Bwl of 11 and the delft go to 5 , so which ones can i use and where can i find them?

Heel resistance is very low in cat but again only have formula of the delft series !

lot of question hope you have some answers!

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### PI DesignSenior Member

Catamaran resitance is a pretty sticky subject, if you want to be accurate. You need to consider the effect of hull spacing on the wave making etc. It is really too complex to describe fully in a forum, your best bet is to ask your lecturer. Your best bet is to treat the resistance as you would for a monohull and add some fudge factors. Again, your lecturers may be able to help with chosing suitable ones.

If your ultimate aim is simply to estimate the top speed of your design, you can try this empical formula, specifically for catamarans:

v = (1.7*L^0.5*SA^0.353)/(D^0.253)

where
v is speed in knots (1kt = 0.5144m/s)
L is waterline length in feet (1ft =0.3048m)
SA is sail area in square feet
D is displacement in pounds (2.2lbs = 1kg)

I'm afraid I've lost the reference for this equation, but an internet search might find it for you, or someone here may know it.

Sorry I can't be more help.

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### yipsterdesigner

i'm studying Freeship and think Kaper -for small canoes- may be good comparing resistance, than export to Michlett

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### weysNaval architect student

I am studying michlett at the moment but not sure how to import a hull in the program do you need Freeship for it , or do you do it in text file in winedt
cause I designed my ship in Maxsurf ?

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### yipsterdesigner

in michlett you have to replace the "in.mlt" with your or one of the demo files, text input is also possible, better to read the manual
freeship input can be in text also but a hull is quikly drawn and tested in freeships kaper resistence and than exported to michlett
its easy ;-)

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### weysNaval architect student

haha big smile , yeah easy, but already designed the the hull maxsurf and looking if i can export something without drawing it again!! any ideas if you can import out of maxsurf or rhino?

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### weysNaval architect student

wow have found it export out of rhino vrml and then imported it

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### yipsterdesigner

wow indeed, so vrml into freeship works? vrml1 or 97? i try'd but without gettin a good looking mesh so far
had the limited maxsurf and forgot if hullspeed with savitski, holtrop and delft and what else was on the demo
here some kaper spreadsheets but dont know what happens with speed and resistance on bigger canoes

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### PI DesignSenior Member

Don't forget though, that a catamaran is more than just two independent canoes. There is interference between the hulls. Depending on the hull spacing this may not be insignificant.

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### yipsterdesigner

good pointing that out but understand michlett deals with that
lateral hull spacing (for various speeds) is important Leo pointed out and
the caravan wave canceling effect between the hulls not so much
in shallow water however i've been reading this wave cancelling may work

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### PI DesignSenior Member

Gosh, I really should have a look at Michlett. It sounds excellent.

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### weysNaval architect student

you are right after checking the mess it isn't perfect , but first i could do Kaper analyse in freeship but now i don't have any displacement

find out what happend

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### weysNaval architect student

Small question i have finally design freesurf , but wanna export to michlet , I have designed it with two hulls mirrored in freesurf but only get a flat line in michlet , Or do you design one hull in freesurf and then import and say it is catamaran and give spacing
??

15. Joined: Oct 2002
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### yipsterdesigner

in freesurf (haha) and michlett both options work
export a cat to michlett with spacing between hulls
if calculater in freeship stops you have something out of range
hope this helps, now back to my own study

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