quick catamaran design question

[bob fisher]

hi can anyone tell me what would be a reasonable lead length for a 8.5 metre performance racing catamaran?? at the moment i've got 2.5% of the lwl

[Crag Cay]

Generally the lead length for a performance cat is taken as zero. The lateral displacement of the CLR under the leeward hull compensates for any lack of lead. Think of the forces acting trough the CE and CLR in 3D, not just on a plane piece of paper.

[Raggi_Thor]

I suppose for wide cats with low rigs you could use negative lead length also.

[DSmith]

For the uneducated, what is the lead length?

In most rule of thumb methods for establishing the position of the rig(CE) relative to the keel/board(CLR) an amount of "lead" is usually specified as a percentage of the LWL ("CE" in front of the "CLR") . You can use one of those methods on a multi if you reduce the "lead length" to something around zero or even slightly negative. Eric Sponberg just recently answered a question on this forum about multihull balance but I don't remember the thread.
A couple of proven methods are in Skenes and in Larsson and Eliasson's books .
----------------------
Here is the thread where Eric answewrs this same question:
Catamaran C/board Position?(also by Bob Fisher) - Boat Design Forums
Address:Catamaran C/board Position?

[navarch_hish]

Hello there, i am a final year naval Architecture student doing my engineering.
I am planning to do a 700 passenger catamaran, i suppose a speed of 40 to 50knots. I have to start from Zero, so if anyone could help me or give me some lead so that i can proceed from it, my project duration is 5 months, starting now.
I will be really greatful if someone could send me few drawing of passenger catamaran, so that i can get some lead from it.

[SailDesign]

Hish,
If you are a Naval Architectural student, you should be designing this yourself.
You can't expect us to do your homework for you, you know...
Steve

[Raggi_Thor]

I also wonder wghat you mean "get some lead".
Shouldn't you start by doing your own spec?

[navarch_hish]

Whats the criteria to go for a chine bilge, are all power cats to have chine bilge??
Is G.A or passengers the first step in design of catamarans?

[Raggi_Thor]

You probably have some books on this :-)
Some things to consider is number of passengers, cargo weight, distance to travel, speed, sea conditions, economy.
Many books describe different kinds of "design spirals".

[navarch_hish]

Sir,

Thank you for taking time to reply my thread. Sir i am pretty much familiar with cargo ships and its general arrangment. I am not so familiar with passenger ships and their arrangment. I would be reallly greatful to you if u could suggest some books which are concentrate more on catamaran ferry design, starting right from guidlines for G.A to hull design and resistance calculation.

Sir which method is the most commonly used method for resistance calculation(manual method), i have seen Holtrop and Mennon method in some of the projects earlier carried out. Do you think its a very good method, are there any other manual method so that i can recheck my resistance value(for assurance purpose).

Is dead visual range the criterion for selecting the number of decks and hence the wheel house height?? Does the same 1.25L hold for cats as well?

Can u suggest me some sites where i can get important data regarding the specification of cats in the range of 700 passenger.
Sir should all cats above 35knots have chine bilge??

I am sorry if i have taken a lot of your, but i hope you can find time to reply to my mail.

[Raggi_Thor]

Now you are asking more than I can answer :-)
Are your sure your project has to be that detailed, I mean can you for example use some simple methods to estimate resistance?

Now it seems like you have made some work already.
For example you have probably made some sketches and some estimates for the main dimensions based on the number of passengers?

Most catamarans a see around here has hard chines.
I suppose longer hulls can be efficient without.

Here are some Norwegian designs, most of them probably smaller than yours:

http://www.batservice.no/showArticle.asp?News=1&ID=18
http://www.braa.no/english/Rygerfjord%20english.htm
http://www.abry.no/popups/popup11/slider.htm
http://www.multi-maritime.no/go.cfm?...27i51173i51438

[rkpshenoy]

Hey to make matters more complicated even I am stuck at one stage of my project.(Design of passenger catamarans too)
After fixing the main dimensions of the cat i got a body plan which was geometrically similar my cat. Its CB is given as 0.695
But after i have modelled the demihull using maxsurf i found that its CB is 0.37 as i expected with the hull form not very full and V.
Some one help me. this is my autocad design of bodyplan.

The body plan appeared in a paper "Experimental Study on the influence of hull spacing of hard chine catamaran motions"

[navarch_hish]

Hi there man,

I had a project on Passenger Catamaran. now i am over with it, i am working now.. well what i can suggest is that the CB of the vessel is not a big issue bec's if u refer to SDC by Lamb, it clearly indicates that the vessels resistance depends on L/B and Mass distribution factor rather than on CB and CP as the speed goes high..

Regarding your CB i think if ur going for a hard chine as can be seen from ur body plan, u will generally have CB in the range of .35 to .42 at the max. Depending on ur speed Maxsurf must have generated the Hard Chine model.

Well if ur looking for a CB of 0.69 then i think u shud go for round bilge semi displacement type of craft. These crafts have a straight line buttock and v shaped for. these vessels operate in a semi displacement speed. where there is sufficient lift due to the straight line buttock aft to prevent the sinkage due to low pressure created at the buttock in the case of convex buttocks as for the general conventional ships.

YOu will have to decide ur draft depending on ur hull form becs u need ur waterjet to be properly placed if u want to obtain the required thrust. if the too much immersed or too much above the water line, the desired thrust cant be achieved.
Another issue is designing waterjets. From what i know it would be very difficult to design a waterjet from scrap like the propeller. better to choose ur engine and choose a waterjet from standard waterjet makers that match ur engine power rating.

For resistance check out Papers in RINA TRANSACTIONS 1992 and above.. there are excellent papers by Insel and Mollard of Southhampton University.


Best of luck man, i shall help u as much i can. but i am a fresher as well ... but i can tell u what all things u need to take care ... check out my orkut profile to see the profile of my ship and ship model..

http://www.orkut.com/AlbumZoom.aspx?...74411793&pid=7

[rkpshenoy]

yeah but my displacemnt based on weight estimate has come out to be 260 tons so after i have fixed my main dimensions as below i am forced to go with CB above 0.68 or so. and moreover i checked out the RINA 1992 transactions by Insel and Molland but my Prof says the vessel must essentially be a planing cat not a displacement one. so i had to go with this hard chine having put down the round bilge form.

LBP = 40 m
B = 11.7 m
T = 1.56 m
b(demihull) = 3.3 m
V =45 knots
Passenger capacity = 500

Suggest me what i should do. I am getting into desperation