Catamaran Calculations

In the process of designing a 74' Cat that my wife and I intend to build. All my calculations are based on monohulls. Anyone know where I can get Catamaran calculations?

 

Welcome to the forum, ScubaPro.

Assuming you are interested in a sailing catamaran, you might find the spreadsheet referenced in this thread helpful.

http://www.boatdesign.net/forums/multihulls/sailing-catamaran-parameters-22063.html

You may need to wander down through the thread a ways to find the most current document. That being said, 74' is a HUGE boat! Good luck!

 

I hope you have the necessary knowledge and skills to both design and build a catamaran of that size. And I hope you do realize how big (no, huge) is the task you are about to undertake, how huge will be the building costs, how much time and space and resources will the build require, that the vessel will have to comply to a series of regulations and laws and will be subject to inspections and conformity assessment by your local authorities and/or by the classification society...?

 

Thats a big boat, and a huge expense.......be careful .

You would be wise to immediately contact a Naval Architect, run you preliminary design by him and seek comment.

Classification society approval is critical to the future resale value of your boat.

Naval Architects are cheap..the cheapest system in your whole project.

 

At 74' to maintain any resale value at all, or likely to be insurance it must have a NA sign off on the plans. This is a huge boat with enormous loads, and very expensive systems. I would put down the pen and immediately call someone with big multihull design experience. Spending millions to build a boat without taking the few thousand dollars for competent plans is a huge mistake.

And crating the plans before speaking to a NA is a false economy. They will still have to redo all of your work in order to check it for sutability. Plus you loose out on all the experience they have while working on the plan.

 

Thanks All

Yes its a hugh boat. My time line is 5 to 7 years and we are assembing all the requirments to build it right now. I have no problem reducing the size down so long as i can get the amenities on board. This will be our retirment home. Im a master carpenter and am not worried about the skill level. Budget and where to assemble the boat tops the list right now, and running the final plan past an architech is my intention. I have 3000 hours into the design so far, so there is a lot of detail. This form has saved me more than once already. Any input is welcome. The spreedsheet looks good, but it will take me a while to plug the variables in.
Thanks to all

 

If I may give you a wholehearted advice - get in contact with a Naval Architect (NA) as soon as possible. You are already too much into the design, and there will almost certainly be some things to modify in order to meet the regulations and to make the vessel seaworthy, efficient and built according to sound engineering principles.

As Stumble have observed, the NA will have to go through all the calculations from the bottom up, as if he had to start it all from the scratch, so it is very important to sort all the eventual design issues with him with no further delay, for the sake of your wallet, of your and your wife's safety. It should have been done right from the initial stage of feasibility study and preliminary general arrangement, but now at least try to avoid getting to the point when it will be too late to change anything without really starting it all over from the scratch...

Cheers

 

Scuba,

No matter how skilled you are as a craftsman a boat of this size is well beyond the capability of a single person, the amount of labor involved is just too high, and the number of different skills required is incredible.

Taking a look at the Sunreef 70 with a displacement of 34 tons, a good boat builder can build about 7lbs of boat an hour, you are looking at about 10,000man hours of work. Now this is assuming you don't account for the time spend running to the store, shopping, dealing with vendors ect. To build this boat, and be launched in 7 years you would need to start working on it full time today. 40 hours a week, 50 weeks a year, and you just might make in realistically.

Not to mention it is almost never cheaper to home build a boat than to buy from the market new, the numbers look even worse if you are willing to buy a boat even 5 years old.

 

I would agree with all the comments so far. A 10,000 hour build time would be the minimum. And that's for a foam sandwich boat, I wouldn't build one that size in wood.

My first question when someone asks me for a new design is always "what will you do with the boat". If it is to be a retirement home for a couple then you don't need a boat anything like that big. Most people who have boats that size use them as a business and almost certainly have a full time crew.

Finding somewhere to build a 40ft wide boat might be the hardest problem to solve. Maybe Jaynes marine in the Chesapeake? And you'd need to think about liftout facilities for later maintenance. I think there are only three places on the whole of the east coast USA where you can lift such a wide boat.

Don't forget that annual running costs are about 10% of the boats value

I suggest you look at this forum

http://cruising.stuffiminto.com/boat-building-design/1163-building-kurt-hughes-bridgedeck-45-a.html

and read about Sully's experiences with building his 45ft catamaran, which is about as long as the width of the boat you propose.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

OK Enough

I worked in a boat yard for 14 years, i know whats involved. This is my third design and im not reinventing the wheel. Im using what has worked before and am modifiying it. I have no intention of doing all the work myself and already have most of the material. I spoke with the USCG and a have applied for a manufactures certifcate. Building this vesssel is my full time job. I just needed some help with the multihull calculations....take some time and go read "Buehlers Backyard Boatbuilding" thank God its not my first boat, you guys could discouarge Edison from inventing the light bulb

 

What sort of help do you exactly need?

 

OK...but why not contact a NA ?

I cant imagine building a boat of that size without the experienced input of an Architect familiar with multi hull design and engineering .

 

Well said with the "you guys could discourage Edison from inventing the light bulb" with the exception of Richard who is held in high regard by all who deal with him, however I know of none of your fellow advisors design's builds and the question would be just how many builds and designs do your fellow advisers have to their credit? Richard is the only one to advise with question.... What are you going to do with the boat..... All the best with your undertaking, Talk to Richard off line perhaps, a wealth of information advise right there.

 

Help with Multihull Calculations

Most of the calculations on the hulls have been worked out from the perspective of a mono hull. Center of effort, that sort of thing cant be worked out the same way on a catamaran. My old boss looked over the drawings (he is a NA), but he dosen't do multihulls, and he has retired and isn't in a position to go research them. Also i need to convert rig data from aluminum to carbon fiber, just to get an accurate qoute. Untill all is worked out I cant really even qoute the boat. This probably will not be be sandwich construction. Im old school and like cold molded. The scantlings for cold molded are very light, because its generaly used for mono hulls with a heavy keel, and ofcourse I dont have ballasted keels.

 

Precisely.
Building a 74' cat with the help from internet forums is, well...

There are several members in this forum which have an extensive experience with catamaran designs, some are NA's with proven professional track record, like, for example, Ad Hoc (http://www.boatdesign.net/forums/profile/ad-hoc.html), some are not NA'a but again with proven professional track record, like Richard Woods.

Our new forum friend is simply advised, first, to get in contact with an experienced professional and, second, to do it as soon as possible - before his design gets to a stage beyond which no simple eventual modifications will be possible.

Cheers