Boxy Fisher Catamaran idea

This is my idea of a way to get our butts to sea. It is a simple catamaran LOA 8m x 5m beam. Following is a very basic description of what I have in mind, but your critique would be valued.

Cryteria as follows -
Trailable
Min 2 person assembly
Entrance from inside the cabin into the hulls
Able to walk around for 360 deg fishing.
Safety
Comfort


The cat will consist of 6 main parts. Two hulls, one cabin, two cabin sides and one front deck. The idea is to make the cat trailable with two trailers. This is not a problem, since we're going to the coast with a minimum of two vehicles any way.

The two hulls would trail on one trailer side by side with the front deck on top of the cavities that provides entrance into the hulls while the cabin and two cabin sides rides on the other trailer.

The hulls would launch at a slipway as one would a normal boat, it would not be difficult to make it so you can winch the hulls off the back, or back up.
Once off on the water, the hulls are then drifted apart with one beam end bolted to each so they cannot flip over. If this could be too risky a guiding hole could be made through the front of the hulls and a pole stick trough it. Once the proper distance apart the ends of the beams are located and bolted down.

Next the cabin would be placed. Since it's a basic thing the cabin wouldn't have fixed stoves, fridges in it that would make it heavy. I have not worked out it's weight yet, but my female instinct tells me this may be the more difficult to do. The second trailer could of course have a simple hydraulic jack with a swing arm to lift the cabin up and swing it over to where it can be lowered in place.

The cabin would also serve as a place where all the loose stuff rides in, gass bottles, fridges, bedding, groceries etc etc the list seems never ending eh ! These would be unpacked before the cabin goes up.

When the cabin is bolted in place, the two cabin sides can be bolted in place.

One note on the 'bolting' - the nuts could be part of the item where something would get bolted to, so you won't fiddle with loose nuts or bolt-throughs. The bolts could have a short piece of stainless cable fastened to the item to be bolted - that would prevent a bolt from getting lost, so it's right where it would be used at any time.

I want a solid front deck where the anchor can be operated from and serve as a fishing from area. The front deck draft is around 1m600 minimum. so that goes up next. A gap could be left between the front deck and the cabin for water drainage if this can be a problem.

If the cabin sides are fitted as indicated the outside of the hulls become a walkway, while the insides (inside the cabin) serves as a table or work space for placing stuff like a gas stove or whatever else has to live there.

There would be two steering areas - one left and the other right, and each has a lower area (aft deck) or for better visibility you can get on the hull. I have not yet figured the details out, but the left (port) could be for doing the sailing from and the right (starboard) for the outboards.

Some steps has to be implemented so one can board from the waterline and would also be where you'd pick a fish up from. The outboards and rudders also has to be mounted there.

This is basically as far as I am now. The mast and position has to be determined. For now a single main sail is planned for simplicity, but I'm sure you guys could give a lot of hints and tips on this. The boom I would like to have 2m higher than the hulls so it won't smack you overboard with a nice lump as well. The mast could be allu sections with a sail you wound around the mast to trim it smaller.

I'm also not sure where the mast should be.

Please feel free to comment on any or all items. Thanks in advance

 

Hi Fanie

Again, let me start the ball rolling with a couple of observations...

Last things first - the mast should be so placed that the centre of effort of the sail plan is vertically in line with the centre of lateral resistance of the submerged part of the hull/keel. This is generally accepted for sailing cats although monohulls need the sail plan about 10 - 15% forward of that line.

Careful calculations will also be required for stability. A high centre of effort of the sails could reduce transverse stability.

Standing headroom is great but on a comparatively small boat (8 meters), the hull and superstructure will present quite a bit of windage.

Once again, it's over to my more experienced forumists.

Take care!

 

Thank you Trevlyns, windage is a problem at most times. The only way I figured one could compensate for this to some extent would be to either anchor down or trim the sail with an auto pilot to compensate for wind to some extent if this could be possible.

The high hulls is to get deck clearance as high as possible, but it should add to comfort in the hulls.

 

Fanie
It would probably help to know what sort of engineering skill you have so the responses can be directed at the right level.

I have not followed all your other threads but it would be nice to know how many people you want to accommodate.

Just some very priliminary comment.

I think getting the deck structure on to the hull will be more difficult than you envisage. If you do not have access to a wharf that can take the trailer and vehicle then you will need quite a reach on the lifting boom. This alone would weigh a tonne or so hence you are looking at a hoist truck not a trailer.

Have you looked around the net for something that might do the trick or even be close. For example:
http://www.members.optusnet.com.au/rhturner1/j7.html
http://www.youtube.com/watch?v=eQwoq5AlPcg
http://www.2hulls.com/gallery/catamaran_model_99/Aquilon_Catamaran_Model.html

The thing aboat these is that is gives ideas on shapes and the sort fo space you will have.

I think you could get smarter with the central portion so it has minihulls formed below and on either side of the deck structure. These could provide the place to locate steps down into the main hulls without impinging on space in the hull. These mini hulls would allow you to support the cabin on the trailer and also float it out into the water. The main hulls would have sliding bridging beams that close up for trailering and then slide apart just a bit wider than needed to float the cabin between them. You then use the sheet and halyard winches for a 4-point lift to raise the cabin between the hulls. That is one way! There are no doubt many more that avoid a hoist.

Designing it to be light and strong in the right places will require a competent designer. There are large forces in the rigging and mast when sailing and these have to be resolved properly.

I am assuming your hull shapes are rough. They will need good design to get acceptable performaance under sail.

Building simple cardboard models can help you envisage what you are trying to do and often leads to more elegant solutions because you can work in 3D.

If you have some CAD knowledge then you should have a go with Delftship. You can get a free download of this and it is one of the easiest to use CAD packages you could find for hull rendering. It would help getting more detail into your ideas.

Rick W.

 

Hi Fanie

Just a bit of 'back to basics' for me
What was your main reason for a catamaran configuration?

If it wasnt all that significant, my personal thinking/investigation process led me to observe that the simplest, roomiest, sailingest, trailingest way to get on the water is a monohull, much as I like multihulls.

The reasons are that a multihull has virtually 3 hulls to build (cabin and two hulls) , a larger hull surface area to develop,fair and finish, possible trouble of launching (try holding up 10 impatient other boats while you assemble your multi-hull on the ramp as I used to do with my trimaran as I couldnt afford a Farrier trimaran ) without all the extra interior low-down space in a monohull - I was left with an obvious decision.

However, as a catamaran idea, your 3 components concept is available from at least one designer I can think of straight away

http://www.kelsall.com/trailercat.html

hey - if you want to go really radical - how about a monohull that bolts bow and stern section together to form a REALLY roomy cruiser - a large verion of the Phil Bolger folding schooner.

Good luck with the project whatever way you go -

 

Fanie
Here are some basic renderings of your boxy boat. What I am trying to show is how you might transition from the hulls to the centre bridge. The red portion of the bridge in the perspective view would be the minihulls I noted in earlier email.

It is certainly a boxy looking boat.

I trimmed down the width of the hull below the waterline as this is what something designed for 8kts under sail would look like.

Rick W.

 

Rick, very nice but why is the deckhouse open sideways?

 

Thanks so far guys,

Wrt engineering skills I would consider myself ok. I have some boating experience although it is freshwater power boats for fishing of course, and I have rebuilt four boats completely with fibreglass.

The cat must be able to accommodate a max of four adults with gear for fishing for a week. We are usually three, with an occasional fourth person.

The reasons why I chose a catamaran is for the deck space and stability. The centre cabin would be a nice place for socializing while the hull's would give each a sleeping space and some privacy without being cramped up - nothing worse than that, especially when the weather is bad and rainy. I'm not fond of monohulls for attempting rough water. The reviews I read and one person I spoke to who owns a catamaran claims the type of boat behaviour I can live with. Calm waters is never a problem for any kind of craft, it's when it gets unpleasant that it begins to count.

Getting the cabin in place would be my only current concern, although in SA it should not really be a problem since there's always about a million Fridays that could assist one Robinson

I haven't had time to spend on the cabin, but I'll come up with an approximate weight for it. The beams could of course be separate items and the cabin shell only may be somewhat less hefty.

Some of the catamarans looks really attractive, as does the pricing of course. I just miss the centre cabin part. Not being able to socialize especially in bad weather would take a lot of fun out of it. I would consider making the cabin smaller if it is going to too heavy, or even split it to make it lighter, but I think having one a requirement.

The cat that seperates it's hulls with the scissors and winches seems a nice idea, those are big hulls. Won't mind saying they are mine.

The hull shape is a very rought indication and was drawed in Solid Edge. I've started drawing a hull in Freeship and it looks better than the picture I uploaded. I'll have a look at Deftship, thanks.

Could the hight of the hulls with the cabin make it top heavy ?

 

If I drop the cabin down I will be losing draft height. Thumb suck the water line currently would be around 300mm, and have a 1m draft on top of that.

 

Fanie;
I admire your enthusiasm and your capacity for thinking about new ways to do things. On the other hand, I believe that you're not only shooting yourself in the foot but shooting both feet.

A boat is used and enjoyed in inverse proportion to the PITA (pain in the ***) factor. I believe that you are seriously under estimating the time and difficulty in assembling and disassembling your boat. Whereas a regulation monohull will need but one trailer, go into and out of the water with little fuss and time, and send you fishing more often and more pleasureably. The inverse use rule is a time tested one that deserves strong consideration.

The catamaran is not necessarily the most stable platform either. The fine hulls tend to pitch more so than a mono. The cat will roll two times with every wave that is taken abeam. cats have an evil tendency to bury the lee bow when under sail in a breeze, an unnerving trait. In calm water, cats are a stable platform to be sure. That is well and good but when there is calm water ther will likely be little or no wind to sail. Are you anticipating calm conditions always?

Someone above mentioned Bolger. Check out the fairly recent "Blackliner"design he has done. Now there is a sensible boat designed for simplicity, trailering, economy, durability, and seaworthiness. The boat was intended for commercial fishing and other uses, in the US northwest and north to Alaska. It comes in a variety of lengths.

Blackliner or something like it can be launched from the trailer and be on its way in a half hour or so. Your cat will take the better part of a day, a bunch of skinned knuckles, generous applications of profanity, and ration of frustration.

 

Hi Messabout, youre not messing about with me on this, are you

I think you misunderstand. A trailable monohul can only be 2m500 wide. How are four guys going to live on it for a week !


Do I get the impression that MOST of you disagree with anyone attempting to design and build his own boat ?

 

Totally off the subject here - where the hell would I
"Someone above mentioned Bolger. Check out the fairly recent "Blackliner"design he has done. "
It doesnt seem to google.
The man doesnt have a web site, no email address I can find.
He seems to make it hard to get any design info.
has he made enough money already ?

 

I googled for both. Found some wooden dinghy's and some plans for them.

Messabout, I've done some other big projects before. Usually when other people are too lazy and sit and watch TV I spend time on my projects. Later I will hear how lucky I am for having what I have. I know what it takes, although this will be the largest boating project I ever took up.

I may not have all the answers, but I'm willing to learn with or without you guys on the forum's assistance - hopefully with.

There may be a solution for the assembling of the catamaran, I'm sure I'll find something sensable when I get there.

 

These are additional bits and I just did not want to spend the time to work out how the bits would mate.

Rick W.

 

Fanie
Freeship is just an earlier version of Delftship. Stick with Freeship.

I think you would be able to improve the lines by lowering the ends. You do not need full head room in the ends as this is usually where feet can be put.

The cabin sides could form the top of your hull as well. Flat on a boat is not "flat" on the water because it moves around even if it is a 250kt tanker. You also need some curve to get draining.

From an engineering perspective you are dealing with big twisting loads through the bridge from loading at the ends of a hulls in a quartering sea. You have high compression load under the mast and bending in the bridge dues to halyard loads. Like I said just making a cardboard model will help you appreciate these loads.

Rick W.