Aluminium Catamaran 5.5M advice

[thephotoguru]

Hi All,

I am trying to design a catamaran primarily for fishing in North Queensland.

5.5 meters aluminium with 2 x 70hp outboard motors.

Trips to the reef at about 50Nm each way hopefully in some comfort.

Top speed of maybe 40 Nmph cruise at 25Nmph.

I have attached free ship file and Jpg for reference.

Having no boat design experience I have looked aound and made some "guesses" as to what will work.

your input on any aspect of my design would be appreciated.

Thank you Eamon

[Ad Hoc]

First thoughts..

boat is too small for the range and when caught out, shall slam bad with such a low raft clearance.

Why have you gone for such a complicated hull shape?

[baeckmo]

Sorry, but crazy proportions! To carry two 70 hp engines, she will lie on her belly, inviting for heavy slamming! With these tunnel sides, I would say that you need a tunnel roof clearance of ~5 % of WL length fully laden. Shape of forward parts of hulls and tunnel roof are lacking reserve buoyancy and dynamic lift capacity; she will dive deeply into wave slopes in following seas! You must be capable to produce a realistic weight estimate before you enter a catamaran project, in particular if you are going to use her in open sea!

When you go from monohull design to catamarans, the number of parameters to consider are multiplied, and so are chances to get it all wrong: A good catamaran is better than most for your purpose, but a "half-good" cat can be really bad, if not outright dangerous!

I fully concur with Ad Hoc, 50 Nm t/r trip in open seas requires more of a boat, and I recommend you to start with a monohull or a [b]proven[b] cat design, there is more in cat design than you realize!

[TeddyDiver]

Smth more realistic would be 5,5m monohull with one 70hp engine...

[thephotoguru]

Thanks for the input.

Adhoc I should have clarified the conditions that it will be used in.

It will not really be used in open seas as I will always be inside the great barrier reef, so you don't get big sea swells, but some short sharp chop type swells.

It is common for boats around 5 meters to do these trips.

I didn't realise it was complicated.

Baeckmo, tunnel height is 480mm from lowest part of hull, according free ships it will displade 1500 kgs at 270mm waterline.

I will need to look into reserve buoyancy and dynamic lift....

My estimate of weight is 1.5 tonne which I have estimated just by seeing what similar size boats are advertised as weighing.

Would anyone have a freeship file of a 'proven' catamaran design in about the size I am lookng at.

I have thought about a Mono of about 5.5 meters with 140Hp and been in a couple but have been less than impressed with the ride.

Any info on where to start to inprove my design?

Thank you Eamon

[Ad Hoc]

Eamon

I have done the trip to the reef myself. No real ocean swells, that i recall. But but sea sure blew up quick and a very confused short created sea. I was on a 45foot catamaran, most people were sick. The sea state was not slight either...so in a 5m mono, you are really pushing your luck.

But if you wish to risk your personnel safety that is your choice.

In addition to baeckmo's 5%, you should aim for 10% at the bow, to prevent a nose dive.

As for improving your design, if the above by all doesn't convince you to change your MO, then at least make it a very simple hull form...symmetrical and the hull created from single surface bottom and single surface side, ie 2 flat surfaces. Easy to fabricate then too.

[Willallison]

Eamon,
There are three main reasons why people choose to build a boat.
1. Because they want to
2. Because there's nothing out there that will fit their needs
3. Because they think they can do it cheaper than buying one.

On the last two counts, I can assure you that 2. there is and 3. you can't.
There are zillions of new and 2nd hand boats out there that will do the job you have described. And (I mean no offense by this) they will almost all do it better than anything that you can design and build yourself.
If your main reason for doing this is #1 - because you want to - then do yourself a favour (and save a lot of heartache & $) and go buy a set pf plans from a reputable designer. Again, there are plenty who specialise in the sort of boat you are after.

[sabahcat]

http://www.websterstwinfisher.com.au...display=125703

[baeckmo]

The "essence" of catamaran design is low weight and slenderness. !.5 tonnes on a WL length of abt 5 m is neither nor, it is a hog! There must be a reason to go for cat hulls, and you must know what you are doing.

Look at it this way: for your design, you have used the quantity of aluminium surface that would do for a 7.5 m monohull with really fine seakeeping qualities! Why spend it on a shape that will give you nothing but trouble and high fuel costs?

Sorry to ruin your dreams, but I speak from experience of designing and building both mono and cat workboats in alu to very stringent specs. My piece of advice is given "in good spirit", not to keep you off boatbuilding. Go for Willallison's advice!

[Guest625101138]

Originally Posted by thephotoguru

Hi All,

I am trying to design a catamaran primarily for fishing in North Queensland.

5.5 meters aluminium with 2 x 70hp outboard motors.

Trips to the reef at about 50Nm each way hopefully in some comfort.

Top speed of maybe 40 Nmph cruise at 25Nmph.

I have attached free ship file and Jpg for reference.

Having no boat design experience I have looked aound and made some "guesses" as to what will work.

your input on any aspect of my design would be appreciated.

Thank you Eamon

Eamon
Have you looked at Plate Alloy boats:
http://www.platealloy.com/cats.htm
I do not have any first hand experience with the boats but I stumbled across the factory when chasing some welding consumables a while back. At that time they had a boat under construction in the factory and it was going together well. They were only proving the concept then - about four years ago now, maybe more.

There have been some nice boats designed from scratch as you are aiming to do but it is a steep learning curve to get it close to right. I imagine you are an experienced aluminium fabricator as it takes skill to put a boat like to this together. Quite a lot more effort to design.

Plate Alloy may be able to give you a few references of their boats that have been built. You would want to make certain the design works well in your conditions.

Have you tried a shark cat up there:
http://www.boatpoint.com.au/boats-fo...aspx?R=7027752
http://www.boatpoint.com.au/boats-fo...aspx?R=7143892
http://www.boatpoint.com.au/boats-fo...aspx?R=7754752
They are competent boats in chop. The bigger ones are used as rescue vessels so capable in heavy weather.

Rick W

[thephotoguru]

Thank you for your input.

Maybe I am being a little ambitious with the Cat....

Might have to go back to me original plan of building the Boden Alloy Sea Boat at 6 meters.

Although I would like to build a Barcrusher at about 5.8 meters.

Back to the drawingboard....

Cheers Eamon

[raw]

Hi Eamon,

I have some other options that might suit your requirements.

Send me a private message if you like.

Also Queensland based.

raw