Boxy Fisher Catamaran idea

You know enoug already.

You cannot really build a GRP boat in a single skin glass and make it lightweight and strong unless you make the hull thick (heavy and expensive). This method is used in ski-boats.

If you take a single piece of paper it bends really easy. Then take a carboard piece of box, it is much stiffer. The reason is the distance between the two sides of outer paper. The same effect you get when you have 2 layers of glass both sides of the foam. To bend the one has to either bend around the other or the one has to crimp so the other can bend around it.

By glassing both sides you get a thicker stiffer construction material to work with. The same is used for trailers, two laminated pieces of flat bar is stiffer than a single piece of thicker flat bar.

Bouyancy, thermal insulation, sound insulation, your aditional leak barrier, easy to work with, lightweight.

It is a comforting thought knowing that nothing can sink even when badly holed.

 

I don't have an assembled model yet. Be patient, a lot of things has to come together still. Once I have the info together I will throw in a box, shake and see what comes out

I have just transferred the hull frame profiles to another program which took all evening so I can add a feature which seems difficult to do in Delftship. Once done I have to weigh the pro's and con's and see if it is worth doing. If, implement else dispose off...

 

OK thanks Fanie. Sounds like I'll be using good foam and fibreglass.
Sorry mate, just one more ******* question: how thick should I expect my hull walls to be?
I hope that question hasn't already been answered somewhere. Haven't had time to read all those links posted by Pericles.

Cheers

 

Done some reading....foam core is 3/4 inch, so I guess I should allow for about an inch in total? I promise to keep all future boring questions in my own thread.

 

Hi Richard, sorry I missed your post. The time showed on the forum is slightly out from our time here, last post was rather early this morning... four-ish or something... had to inch into bed else the wife craps me out for comming to bed so late

No such thing ! I often ask questions of which I even know the answer of, but it's just to get confirmation or maybe get another perspective on something.

You should decide how you are going to apply the foam. Ideally you get the big block and carve out what you don't want and the rest you glass over. Too expensive though but it would be the ideal solution. I saw a CNC add somewhere where they can mill the outside of your hull out... precisely as you drawed it !

You could use any thickness, taking in consideration the thicker you use the better (curvier) it drapes around a jig or form that hasn't got sharp edges. Thin foam tends to show cavities easier and has to be supported better to sustain the profile initially untill the glass has cured, but is cheaper and forms easier around sharper bends. Another compromise

I use 20mm foam, as I have found it nice for me to work with. I glue pieces together when I need more thickness and it's a nice thickness to add features or simple ribs with. I would recommend you get a few pieces of various thickness to play with and to get an idea of how it will work. Thick foam, ie 100mm is much harder than a thin piece. You'll see what I mean when you get some pieces.

Be sure to jump in the pool with it, the floatation of it is amazing, especially if you can find a largish sample piece. I'm sure a 40mm sheet could serve as a live raft. Amazing to think a 1m x 2m piece 500mm thick can float a toyota 1600, and it weighs only about 33 kg's.

I'm drawing my hulls over from scratch, (sigh) a few funny errors slipped in somewhere.

 

My turn, this is probably a stupid question, but I'm trying to form an idea of the power one can expect from sails.

I have just redrawed the hulls. To save the weight and the extra work of adding a floor in the hulls , the bottoms will be flat so you can walk on them. This has caused more hull drag required to achieve a certain speed, going up from 650N to 750N to reach 8kn and 900N is required to reach 14kn.

I know it depends on the sail size and wind speed and so on, what I want to know is, is this a lot of power that is required for sailing at those speeds or not ? 10m motorized sailer cat, draft 500mm. Sailing area 50m^2.

 

Hi Fanie. Thanks for all the info you've given me about boatbuilding. (thanks to Pericles too). I have a much better perspective now.

Regarding your boat's efficiency....I know that there are pro-built 'floating hotel' cats out there that will "hardly ever exceed 6 knots" according to their disappointed owners. I read about these boats when I first started getting interested in cats. There has been a steady decline in catamaran performance in recent years, to make way for huge accomodations and hull extrusions.
I think you just have to decide for yourself what you will be happy with. If you wish to compare your design with all the high performance cats out there, then you are probably looking at it the wrong way, cos your boat will have high windage, and poorer handling, unless you decide to lower the bridgedeck cabin, and shed some weight. I don't think you should, because you sound like you know what you like.
My boat will not perform as well as other cats when getting over the 8kn mark, but up to 8kn it will be fast, only needing a 5hp outboard to reach 8kn, in a flat sea (unless windage is higher than expected).
Do you intend to do a lot of beaching? If so, don't you think it would be an advantage to have a round bottom with floor?

 

Fanie, I had a look at a Bob Oram 44 sail cat being built near here. Google the designer & look at his designs. They are fast, so long as you don't add weight! and STRONG - the mad B is reported to have beached on a bit of coral, waited for the tide & bounced back off - no bruising or other detectable damage. So the story goes? The lightness & stiffness are strong points...

I was impressed with the simplicity and deep understanding of tieing the construction all together.... I am not a technical expert, just expressing a view and gut feeling from a general understanding of the engineering.

Now I am making a mess of saying "The boat looked good and sturdy and appeared to be elegantly engineered for strength & robustness retaining the objective of being light"... Will be launched in about 3 months - will try to get some pics of the event & post...

 

Hi Richard,

Did you get some foam samples ? I suggest you get some samples before committing... see what it's like.

The draft on my boat is only 500mm, I'm sure I could beach her on that, or do you think it's too much ? The beam on the submerged hulls are 0,9m and total submerged area is 10.6m^2

I have in fact widened the hulls aft which was what is pushing the performance down a bit. The hull's ratio is around 11 point something. Not expecting a racer, but it mustn't be an anchor either

The windyness - I could drop the hulls, but would they become crampy then, and I would have to sacrifice deck clearance also. Dropping the cabin would have the same negatives. Either could be worse than the windyness.
I will re-evaluate the cabin shape some time soon and see how I can cull the wind some. Keep in mind comfort is high on my list, as is deck clearance. Our seas are a bit rougher than yours (at times).

I'm glad I did the hulls over from scratch, they came out very well.

 

Hai Masalai,

I have LOTS of Bobs boats pictures here. Every time I look through them I get new ideas and then I want to build a larger boat that wouldn't fit in the yard. Right. The 10m Iwill have to be content with. It is the max size I can get away with here and the minimum size I would take to sea feeling secure in. Sucks having to compromize so much but no way around it yet.

 

PS.
My 'Sailing' folder sits on 830mB, 2828 files and 38 folders. I guess I need more stuff... could double it before the boat is done.

Would love to see some assembling hints and tips, as well as the launch... Shouldn't one offer to work there for free for what you can learn ?

 

Most builders I have found to be more than happy to share & teach/advise for free - same as here on the net - share what you know & see, with the feeling it may be relevant & useful....

The other thing is, being perceived as an interested person, you are likely to recommend them to others as being knowledgeable & competent. What goes around comes around

This approach helps to reduce the complexity of your filing system - such as, - - one posts a question, - & someone with better recall posts the site/thread with the answer!. Too easy. We all do what we can to help....... even if it is only throwing communist fish (red herrings)

 

Masalai, I agree. I have learnt a whole lot of stuff here on the net, which is especially relevant to my requirements. You can't ask a book questions!

Fanie, I am not playing around with foam. I just wanted to know a little about building techniques and materials to help me with design decisions. I still want to leave the final construction to someone else. My hulls are so simple that a computer is good enough at this stage. I may start playing with foam and stuff later if I have the time. A test model would be good. Or I might just let the final designer play with it.

As for your boat....
I can see a couple of advantages with all that rocker. When your boat is stationary at sea, it will be more stable, particularly if you centralise the weight as much as possible low in the hulls. I think this is a big bonus if your main goal is fishing. In this context I would agree with the 'ball with ballast' principle you and Pericles insisted on
Also, the large rocker is handy when beaching, because you can easily jack the hulls into level position, so you can take full advantage of your huge cabin when beached.
However, when your hulls are level on land, you will have a lot of weight pressing on one flat point of the hull, with no floor or reinforcement. Wouldn't like a rock to be under there!
You say that comfort is your priority. That's why I believe you should not worry too much about speed. The boat should be slow and economical to run, and as stable and strong as it can be. If you want to travel for miles and miles (and miles and miles) to get to a good fishing spot, then I guess you should compromise on comfort.
In my opinion, a well ballasted boat with strong canoe hulls would make a good fishing boat.

 

Fanie, if you limit the standing headroom to just the areas where the hulls join to the bridgedeck, then you could reduce a lot of windage. It just means that all the kitchen duties etc. would be done by standing in the hulls (a good place to put heavy appliances).
I don't think it is important to have standing headroom in the bridgedeck cabin. You just need to be comfortable when you are sitting.
You will have lower centre of gravity, less windage, nicer looking boat, and better rigging options.

 

The 'kitchen' is going to be in the cabin to be part of the social activities. This will be the main area, all activities will take place here except fishing and sleeping and of course private stuff like shower etc. (I'll not elaborate the etc.) I even want the steering, controls everything to be done from inside the cabin hence big hatches that can be opened. Just providing sitting space is going to have a few drawbacks. I also don't want to kinda squeeze past someone to get somewhere.

The hulls will be mostly for berthing and storage, but I also don't want a hole you crawl in to sleep in and when you want to dress you have to do it lying down or shower in the same position The wife will catch a fit just on that alone. Also, the beam of each hull is 1m200 max, which is not very wide compared to other boats.

All that said, the cabin floor is currently one meter above the waterline. What would be an acceptable clearance without running into troubles ?