Boxy Fisher Catamaran idea

Fanie

Now that I see some drawings and understand what you are trying to do, I think that your idea has merit. I would be surprised if a similar boat has not been designed already.

Http://holiday23.com

Just for interest this is the locally built Holiday 23. It is not what you are looking for but it will give an idea of what you can get into 7m long by 2.5m wide monohull. The deck is probably not suitable for fishing. A 9 or 10 meter version is not impossible.

 

Fanie

I think you are underestimating the difficulty in the structural engineering and design, but then again I do not know you or your abilities.

You should consider buying plans for a similar boat and then make modifications from that. That way you will be sure you will have a safe platform that is suitably designed. There is a resonable chance that you may make a mistake in your first design project and you could loose alot of money or somebody may get hurt, or worse. If you consider the time and money you will invest to build this boat, a set of plans is a cost effective way of reducing the risk of something going wrong. It is the sensible thing to do.

These boats are not exactly what you want, but they are close and you could modify.

Radical Bay 8 http://www.schionningdesigns.com.au/www/welcome.cfm
Aquilon 265 http://www.2hulls.com/gallery/catamaran_model_99/Aquilon_Catamaran_Model.html
Aquilon 265 http://www.boats.com/news-reviews/articledetail.html?lid=13783
Aventura 28 http://www.go-catamaran.com/avent28.php

As long as you keep the structural considerations in mind and you dont add too much weight, you would end up with a very sucessfull boat.

Please dont take offence at what I am saying. I dont know your background or abilities, you may be an engineer for all I know and this may be a simple project for you. I like your philosiphy of you can do anything you put your mind to, but you may find it quicker, simpler and cheaper to use a designers experience and you will have the added bonus of less risk of something going wrong.

 

Fanie

Here's some impartial advice from a 58 year old fellow South African... no fool like an old fool!

Never, EVER become discouraged by other peoples negative criticism.

You have the dream - just go achieve it boet!

Since 1996 when I first started designing yachts, I've been told I can't do it - and you know what? - I listened!! Entirely my own choice you see...

Well two weeks ago I said "stuff that!" and started building a 12 meter cruising proa of my own design. Things were definitely messy at first - especially the epoxy! But you want to see my filleting now.

It's MY dream - I'll achieve it. Fok die res!

Yacht design is an open canvas - but some basic rules must be followed - for safety's sake. I've read (I think) all of your threads but I'm not sure where you are from a designers perspective. So I'm sure you'll forgive me when I ask if you've read the basics such as Skenes "Elements of yacht design" or Larsson and Elliasson's "Principles of yacht Design" A great catamaran book is Chris White's "The cruising multihull"

Point is Fanie, a grounding in the basics is imperative. [again. forgive me if I have underestimated or missunderstood your abilities] Also, don't overlook the power and input of this forum. There's a lot of advice out there - some encouraging and constructive, and others from fuddy-duddy fundamentalists who refuse to depart from what their grandfathers taught them. Look at everything objectively and chuck out that which disagrees with your ideals.

You do it, lad. Bokke Bo

 

Thanks Rick, your input is appreciated.

I've done a rough calculation on the cabin, 5 layers 300g fibreglass, it works out at about 75kg, assuming some foam and more glass to make it more sturdy it shouldn't weigh more than 100kg. Worst case 130kg. If wood is used it will be much heavier, but I'm sure foam should fo it.

I've already downloaded Delftship, it looks and works the same as Freeship, but I assume I can use both.

I was thinking of making protrusions on the hull's insides like a lip where the cabin could sit on, 100mm wide should do it. This could be covered with a polly foam strip to eliminate squeeking and protect the painted areas.

On the hull tops one could do the same with a lip protruding upwards in the shape of the cabin sides so that water won't flow from the walkway to the tabletop. I'll have to work out a way the cabin sides and the cabin can interface to keep water out.

The hulls could do with some curvature I agree. It will look better and improve drainage as you suggested.

The beams connecting the two hulls would do all the work rigiting the assembly. It seems most catamarans use three beams as a minimum, but I'm sure a fourth would improve load , and yes, the forces could be quite big so I assumed one wouldn't be skimpy reinforcing these.

I was thinking maybe a beam inside a beam like the aeroplanes use and make use of the full width of the hulls for bolting down. I'm not sure how other trailable boats gets assembled, but four 16mm to 20mm stainless bolts on each beam end should make it.

I have considered making a model, I usually do, to the fascination of friend and foe Easiest and fastest would probably be a 1:1 scale and get it over with

I'll get some stuff and put it together sometime.

I'm not ready to tackle the mast yet, but I keep it in mind.

 

Fanie,

Four guys can fit on a monohull that can be trailered, if it's long enough!

I do not know if you have seen either Glen-L designs or Bateau,com, but here is one possibility. http://glen-l.com/designs/hankinson/coastalcruiser.html

The vessel can be lengthened.http://glen-l.com/designs/hankinson/coastalcruiser-pho-2.html

Here are other sites.

http://www.bateau.com/index.php

http://www.duckworksmagazine.com/r/plansindex.htm

Hope you find something you like,

Pericles

 

Sorry Tuks, Trev and Pericles, missed your post as I' as per usual am doing three things at a time here.

I'll have a look at any and all your links, thanks. There really are some very beutiful boats out there. Real masterpieces.

Trev my man, what took you so long to start on the boat ? You know, it was my mothers birthday last Friday, one hundred and seventy one years old and she carries on like it's going to be forever ! Besides, who says you're not just barely half-way yet ?

Better start fishing, fishermen don't die, they just smell worse (or is it better ?)

I haven't started anything other than planning everything at first. Amazing how fast you can learn if there's some motivation eh !

Thanks for the encouragement.

 

Have you thought of using aluminium frames bolted to aluminium bulkhead frames. If I remember Sailcraft had this arrangement on their Iroquois but of course theirs was bolted permanently.

 

I have received a sample piece of rigid closed call foam that is used for boyancy in boats. The foam can be ordered in strips, sheets, blocks. Weight is 35kg/m3 which is very light. Considering how easy it is to sand/cut/grind/saw/file the foam it may well be worth using. Good for insulation and boyancy.

I've also widened the cabin and decided to split it in four. The cat looks better in proportion this way and I like it more. It is now 6m wide. The bottom two halves which would sit on the beams can be mounted first and would be easier to handle.

The four pieces would fit on a trailer.

I have a question regarding the front deck. There was a reason they use a 'trampoline' instead of a solid surface, but it wasn't clear from what I've read. The height of the front deck to the waterline is 1m500 minimum. Does anyone foresee a problem here ?

 

interesting folding mechanism

Fanie

Here is a folding mechanism, you might be interested in. I apologise for the number of posts I have made, your idea got me excited.

Savannah 26





You could design something similar so that there is access to the hulls from inside the cabin, it might make your assembly much simpler.




It is sold locally as the Elf 26, the website gives speeds you can expect from different motors

Elf 26 second hand R170000

 

Fanie,

Some catamarans have asymmetric hulls, which if pushed together, would look like a monohull. Scroll to bottom of link. http://www.alnmaritec.co.uk/downloads/Wave Supporter.pdf

Nearly all the boats produced at http://www.alnmaritec.co.uk/downloads/Wave Supporter.pdf have this configuration.

With sliding and interlocking beams attached to each hull and a separate cabin, mounted after assembly of the twin hulls, you could virtually have the vessel as wide as you want.

Pericles

 

i believe richard woods has a boat hes building at the moment something similar to this
sean

 

Fanie
A few issues covered here:

I did a bit more development of your original concept to try to show how you could transition from the hulls to the bridge.

The features of this that are only roughly developed are:
1. The bridge can be low where it meets the lower part of the hull and give wave clearance in the middle. So you can incorporate steps in the bridge up out of the hull that do not impinge on hull room like an a ladder would.
2. With hulls 4ft wide you can start the bridge in the hull - also the sides of the bridge can be just part of the hull, not separate pieces. Reduces points that require to be sealed.
3. You only need about a 1ft wide deck to actually walk from bow to stern.
4. The actual submersed section of the hulls do not need to be 4ft wide. You want a walkway maybe only 15 inches wide and then some more for shoulder room.
5. You do not need standing headroom in the ends of the hull. There is a lot of opportunity to be much smarter with the hull-bridge transition to reduce the height of the hulls.

The boat is still very boxy but this could be tidied up with further development. You really want to avoid slabby surfaces because these have higher wind loads.

I would be surprised if you could make a structurally strong bridge weighing 75kg. I would think a minium of 200kg for something of this size. It has an area of 77sq.m as I have shown it and this is smaller than your original drawing. Also this is the skin only. This could have load bearing capability if designed for it so the beams would be used to just hold the hulls while it is assembled.

The mesh deck at the bow on smallish cats reduces the impact of a large wave breaking on the bow. The loads resulting from a large volume of green water momentarily inundating a solid fordeck causes high structural loads that could break things.

I have included the FreeShip file. The underwater hull shape was originally generated by Godzilla as optimised for 8kts but I have moved it around with little care so it is now very rough.

Like I have said it is worth the effort playing with a bit of cardboard to get an appreciation of the thing in 3D. I played around with heaps of models trying to get a self-righting cat I liked. Most were 1m long. But I also built a 3m one I could sail. It breaks into three pieces much as you want but I just made fully sealed sections for the model. I have included a real boat photo just to make the point that in a time BC I did real sailing. I actually won a bottle of Bundy Rum on the particular day and those who know the Whitsinday's will know the sites are something to behold on the one particular day of the year the photo was taken.

You need to take care with foam selection. I think you are talking about polyurethane foam. It is more a filler than something you would use in a structural composite. There is heaps of information around on structural composites that you should delve into if this is your selected approach.

The Elf 26 looks clever and much faster to assemble than having to bring separate bits together and align them.

Rick W.

 

Hello Rick.

I've considered your suggestions, thanks. The steps in the bridge up out of the hull is something I consider. I do however attempt to keep the hull shapes as simple as possible to make the construction easier. If extending too far in the bridge one could have a step to worry about not stepping into or spring an ancle. Since the hulls are wide-ish there should be ample space for a step ladder or steps. I'll work out some details later.

The sides can be part of the hull and would be two loose pieces you don't have to transport separate. The only concern is the final hight all this would end up as, measured it comes to 3m250, which I don't feel too comfortable with, but I'll keep it in mind. Not a bad suggestion though. The hull's alone is around 2m high already.

The sides would serve not only to walk on, but to fish from as well. It would be nice if one can use any area to fish from.

The hulls at the bottom doesn't need to be as wide, but it may have a few advantages. The material saved if any won't be a big saving. The draft would be shallower because of more floatation. When assembling at a jetty you'd want the hulls as shallow as possible to do the assembly instead of wading through some water for this. It may get more weight as low as possible in the hulls like batteries etc if there's space to walk around it.

The head room in the hulls is not required, true, but the reason for the hight of the front is to get the deck up. I can just imagine how much fun a big wave on deck could be. I could shorten the deck some which I think I should do in any case. The hulls could of course be lowered, and the deck could be elevated.

There are comparason tables - I got one somewhere - that compares structural strength and thicknesses between wood and foam as well as other matrials. Since it's the hulls that has to keep your butt out of sharks way I'd say they're pretty important do do really well. I may still go for plywood if I can find some decent quality and if it's affordable.

The so called 'marine ply' we get here is of very poor quality last time I used some. I had to replace it within six or so months if I remember right. Someone else told me that the right quality imported is really expensive.

I can probably drop the cabin some and it will make a lot of things easier. I am however concerned about the cabin height above the water. Everyone sends pictures of very nice sleek crafts on flat water, but none of them are on knobby seas with swells on, so it's difficult to find out what clearance is acceptable. Last time we were at the coast there was a lot of swells and watching a particular ski boat I could tell it wasn't fun aboard. I don't think SA have such nice calm waters as you guys have.

If 700mm won't make any difference as to 1m I'd gladly drop the hull's height, it would also better windage on such high hulls as well as be less work to make.

The cabin area would be where one would be social, I wouldn't like a small crampy space. I also don't want to have to go outside the cabin to be able to go below into the hulls, so it's easier to just widen the cabin to span between the hulls. I could shorten the cabin some.

Still playing around with different things and ideas. The outcome may be very different yet.

 

I have made a few changes. The material for a model ordered.

 

I womder if one could add a rudder for steering the cat to an outboard motor. You'd use one steering system only for both when under sail or when using the outboards