catamaran on hullspeed

[german]

hi all
is there any way to accurately measure a catamaran model (class 1 like offshore hull) on hullspeed, if so, which metods and tips you may have...
thanks
cheers
german

[Tim B]

if it's a racing class-1 type cat It's planing, and it's an awfully long way above hull-speed. You really need to do a planing regime power prediction on it. A cross between Stavitsky's work and AF Molland's might work, but it's an area where not much work has been done because of the high speeds. Otherwise it's time to crack open the CFD.

Tim B.

[dimitarp]

hi all
is there any way to accurately measure a catamaran model (class 1 like offshore hull) on hullspeed, if so, which metods and tips you may have...
thanks
cheers
german


For hullspeed you can use a computer simulating programm. Results from that programs are very closer to real speed. But in planning regime the results is not corect. If you have a catamaran model in igs. format, you can send me and I will make this prediction.
partenov@yahoo.com

[german]

thank you all for your responses. it is what i was afraid of, as the hull is a planning hull. the thing is that im about to start with this hull on maxsurf and i had the doubt about using hullspeed for the calculations, dimitarp, as soon as i finish the hull model we will be in touch.
but now, does anyone of you have the Savitsky & Brown, "Procedures for Hydrodynamic Evaluation of Planing Hulls in Smooth and Rough Water" paper?
cheers
german

[dimitarp]

Here is an usefull link
http://www.lesliefield.com/other_history/speed_boat_developments_from_the_past_into_the_future.htm#[18]

[german]

thanks dimitarp..... it will be usefull!!!
cheers
german

[Andrew Mason]

German

You can use the Svitsky or Latiharju algorithms in Hullspeed to get resistance for a single hull, but there is no measure of the interaction between hulls.

The Hullspeed slender body method is for displacement hulls only.

Andrew

[burakreis]

for a class 1 like offshore hull, simply no way.

for a simpler prediction in that case (much faster than 100 knots) you will need real life measurment data of similar boats.

as a harder way, you have to use some cfd. (tank testing is not possible for such case, wind tunnel testing may help)
but getting an accurate result with cfd (i assume you have a large parallel computer) will be exteremly hard; full 6 dof solid body, two phase flow modeling (air and water) with free surface and mixture flow (air lubrication) behind the steps plus ground effect all together is a real challange todays cfd technology.

i can say you have to focus on aerodynamics than the hydrodynamics for a class 1 like boat. the air flow between the hull and the water surface is one of the most critical part of these designs. for a better understanding the ground effect phenomena you can examine specially the old soviet ekranoplan technology...

regards,

[yipster]

jimboats sotware at http://www.aeromarineresearch.com/tbdp6.html can be helpfull

[Tim B]

It is a soluble problem in CFD. However, it would take someone with a great deal of time, knowledge and experience to solve it. It would be easier to use fixed geometry, but the number of runs you'd have to do would be considerable (probably 50+) and would likely take months to run. To do a full NS case you're only looking at sensible times with 50 odd processors.

If I were you I'd ask people in the business what they think works. They might tell you, failing that, do what sums you can and then go out and test. Class One offshore racing is not an area that new designers instantly go into for exactly that reason.

Tim B.

[german]

thank you all!!!

andrew, i will use savitzky and Latiharju just to have some reference.

burakreis, real life measurement will be almost impossible for me, unless there is something on the internet....we have a towing tank here but due to the speeds, the scale of the model will be really small (meaning useless). i do have a few articles about ekranoplans so i'll be giving them a try.
cfd will be an option but it would take way too long for me, as i do not have the resources for that kind of testing....let me tell you all that this is like a hobby for me as i do work full time on a shipyard and class-1 boats are a passion i have since i was at the university......
i do have some background on aerodynamics and flow theory so i think it might be a very good way to start, and also i do have some good hull pictures to start with the hull shape.

tim, i have tried to contact people on the "class1 world" but i havent had any luck on design aproaches or design tips for this kind of boats....

yipster, have you used that program? if so, any comments?

cheers
German

[Davor]

Try Series 64. It is good for narrow hulls. I have it programmed and we succesfully applied it for one catamaran.

see my demo program www.sea-power.net

rgds

Davor


hi all
is there any way to accurately measure a catamaran model (class 1 like offshore hull) on hullspeed, if so, which metods and tips you may have...
thanks
cheers
german

[Erwan]

According to basic calculations for a A Cat (18 feet single handed Cat)
it appears that as soon as you fly a hull full trapeze windward = maximum righting moment, with only 5 knts actual wind and about 7.5 apparent wind hydrodynamic drag account for 90 Newtons with 160 Newtons global drag.
At low wind speed it is the induced drag which is the biggest component of aero drag, just because at these wind speeds you need a big lift coefficient and therefore a big induced drag coef which moves according to square lift coef.
With more wind you flatten and twist the sail reducing lift coef then other component of aero drag take the lead.
Ther is probably the same issue with all light displacement boats, where the hydro drag is not the biggest issue.

Hope it will help

[Erwan]

Sorry my former post is irrelevant , I though it was sailing cat