28' sail cat

[mihari]

Hi all,
I have been reading this forum for about a month now. A few months ago I got this crazy idea of owning a cat. A cruise cat.
I am thinking of designing my own. Budget is of the utmost importance, so buying plans is something I would prefer to avoid.
I am pretty good with my hands. I have built a pirogue out of 6mm okoume ply and also have epoxy-glassed a mayflower. So any kind of manual labor does not scare me. I have an engineering background, so I can probably overcome a lot of the design / construction problems that might come up.
The time frame I give myself for this cat is about 5 years. One year to design it and four to build it. If situation stays as is today, I will be working most weekends and maybe some afternoons. If things go worse than they are now with the economy, I will have all the time to myself to work 24/7 :rolleyes: If business picks up, I will not have time to build it or even sail it.
Second pair of hands might be available on demand.
There are a lot of issues with the construction site that I will have to solve too, if I finally decide to build it.

Anyway, I am starting this thread as means of posting progress and attracting feedback positive or negative, all is welcome (preferably positive), as I have seen posts by people in here that know their stuff.

OK, wish me luck.

[mihari]

First great big discussion / decision:
FOAM vs PLY

I have not been able to get any prices of foam cored composites on the net, but I guess good stuff goes for steep prices. I am much more confident about working with wood, and I can get plywood in Greece. Either plywood or foam core would be both imported, as the only thing we actually produce in Greece is deficit... and dreams of sailing cats...

Performance is not the main thing about this cat, so if foam core was to cost me twice as much, then its out the window. If I was to finally use plywood I would go for okoume, as it is pretty cheap (of the marine plies), very light (450kg/m3 @ 10% moisture), and average to good mechanical properties. I have used it and I like this stuff.

[marshmat]

Hi mihari,

Let me start by pointing out that the cost of stock plans is a very tiny component of the final price of a boat. Like, one percent or so. If the cost of plans will break the bank, well, sorry, you can't afford a boat that big. On the other hand, designing a 28 foot cat from scratch could take hundreds of hours. And it is a well known constant that the first build- the prototype- of any new design is always more labour-intensive (thus more expensive) than those that follow; the bugs get worked out of the build process once it's been done once or twice.

If your goal is to test your own skills as a designer, then by all means, go this route. But if your goal is to build and sail, a proven set of stock plans will bring with it a community of other builders and the knowledge that the boat will work as intended.

As for materials- Fancy high-tech stuff isn't much use if you can't get it locally. Once you've narrowed down your hunt for plans to a few designs that seem to suit you, the issue of materials can be addressed in more detail.

Happy hunting.

[bill broome]

get some plans. wharram for romance, woods for practical are my favorites.

seriously, the time you save in not scratching your head will pay for the plans. and you can be sure it will sail to spec when you're done.

if you must have the ego hit of 'all yer own work', go for simple, then simplify and simplicate. the resulting boat wont be a symphony of intricate wordwork, but it will get done.

[jamez]

Suggest you check out Bernd Kohlers KD 860 for inspiration. Plans are reasonably priced too.
http://www.ikarus342000.com/P86page2.htm

[kim s]

I have just received a full sett of plans for my 28'tri by HARTLEY . worked out approx £50. so for the price of 1.5 sheets of ply is not even worth considering. I would think you will waiste more than that just by building a bit, having an Ouzo,returning and thinking "Yikkeeees thats horrible"Like every one else has stated if its to be "your own design" etc etc then fine.

Kim

[mihari]

Thank you all for the replies, I hope you all keep up and more of the forum members come in and pitch in.

Marshmat, I understand your concern. I think of this project as a combination of 2 major ones. The first one being the design part, I was hoping to turn this into a forum wide project, with everyone giving their two cents worth of advice. Eventually the plans would be available for everyone to use... since I am not into the ship building business.

bill broome, I am not the romantic type, so wharram is out. Out of the Woods plans, the saggita is the closest to my liking. When I first started dreaming about my cat, I had a shuttleworth 28' in mind, with the modification of an added saloon. The hull sections are also pretty similar.

jamez, kd 860 is not what I was looking for. Thanx for posting that.

kim s, that sounds like a bargain. Too bad hartley doesn't have any cats. The only inquiry I dared make was for a 45' cat and the plans were for AU$8000. That scared me away, and I have not asked for a price again.

[boat fan]

T
The only inquiry I dared make was for a 45' cat and the plans were for AU$8000. That scared me away, and I have not asked for a price again.


Do yourself a favor buy some plans !

A project like this is too big , time consuming and expensive to gamble
on. Not to mention your life and resale value... They don`t have to cost $ 8000.00

Really , do yourself a favor.......:)

[mihari]

So, going on from here, I also agree that plywood is the solution for me. Woods likes it and uses 9mm plywood sheets for most of his structural parts, only rarely 12mm, as opposed to shuttleworth that seems to prefer composite construction.

I am convinced of a circular section bottom hull (as opposed to flat bottom) mainly for performance reasons. It is a lot safer, as it can slide down a sloping wave a lot easier than a hard chined hull. It also yields better maneuverability.

For the same reasons keels are out of the question. Daggerboards and pick up rudders are essential to bringing it close to a sandy beach. But that is a discussion to follow.

Instead of strip planking with softwood, I was thinking to make strips out of plywood to accommodate the round hull shape. Plywood is much more predictable than stripped wood with properties that are homogeneous throughout. Other than that, the same construction method will be followed. Is there anyone opposing to that?

[mihari]

Do yourself a favor buy some plans !

A project like this is too big , time consuming and expensive to gamble
on. Not to mention your life and resale value... They don`t have to cost $ 8000.00

Really , do yourself a favor.......:)

OK, I get it. No one would dare building something that big without sharing the risk with the naval architect. But I do have some time to spare, and I would like to see this design project through. Even if it stays at that. In the beginning of this thread I said (I think) that I give myself 1 year to design this boat, and then I will decide if I will build it or not.

So think of this as a "What if" project. I think that the knowledge in this forum, if accumulated, is more than adequate to bring this boat to final drawings.

[jamez]

In your second post you said performance was not a main concern, then in post 9 that you would choose a round bilge design over a flat bottom for 'performance reasons'. What sort of performance differences do you think there would be?

I mentioned the KD86o because I think it gives a good compromise between simplicity, cost, accomodation and likely performance. An alternative would be the more proven Woods Gypsy. Another flat bottom design, although it does have an option to increase costs by building the bottoms round bilge in cedar strip. Dagger boards are another option too.

It is not my intent to sound patronising but from the content of your posts I can't help wonder where your information comes from. I would suggest you read widely on boat design, boatbuilding, look at a few sets of plans and get some sailing experience on multihulls before you attempt to design one, rather than expecting the members here to do it for you.

[mihari]

jamez, thanx for taking the time to respond to this thread.
Performance still remains of lesser concern, since it is going to be a family cruiser. I think, from what I' ve read so far (and I am still reading), that hard chined hulls are:
1. more difficult to turn
2. easier to flip over while sailing in severe weather

I am sorry if you think that I have taken lightly your suggestion for the kd860 but It seems to me that the layout is not for me. That cross section of the hulls does not convince me as being strucurally the strongest around. Just from the structural point of view, a straight section is expected to bend when a point load is applied to it, where as a semicircular load will just distribute the load. So in my humble and vague opinion, I think a round bilge is structurally more stable than its hard chined counterpart.

(Please excuse my poor use of marine and engineering english, and be forgiving. Feel free to laugh and correct me also. I am here to learn...)

I don't think that a round bilge would raise the cost of the boat that much. It would definitelly spread the time frame that it needs to be built.

I never said that I am an expert in anything. I am reading all the time (only that I have only recently started). I don't expect anyone to do anything for me that they don't want to. Please, anyone who reads this thread, feel free not to post any replies.

You are right on one thing: the only sailing experience I have had on a cat was a day cruise with a 35' cruiser around an island. I loved it. Also a couple of hours sailing a tornado.

[boat fan]

You are probably aware of the Richard woods article here :



http://wzus1.ask.reference.com/r?t=p&d=us&s=a&c=a&l=dir&o=10616&sv=0a5c4248&ip=79d6dd79&id=8BF095447E32072CD5937DC6D98E3030&q=round+of+bilge&p=1&qs=2891&ac=24&g=4077yaU2F2EpvC&vc=3&ocq=0&ocp=0&ocu=0&ocf=0&qa1=151&en=te&io=0&ep=&eo=&b=a007&bc=&br=&tp=d&ec=1&pt=hull%20shapes&ex=tsrc%3Dtled&url=&u=http://www.sailingcatamarans.com/hullshapes.htm


I would bet a large bucket of gold that the difference between a well designed multichine cat versus round bilge would not be apparent on a cruising cat given equal size , weight , displacement , sail area etc .

[mihari]

boat fan, thanks for posting that!!! I get marine smarter everyday, and that is also thanks to you.

No, I had not read that article before. According to that article, all hull shapes that are shown there (except the deep V hull) are of simillar behaviour. If that is so, then why does he use the knucled U shape hull only for his larger/newer designs? Is it a marketing thing? Is it because they are so hard to build that it is too much hustle for the e-z build gipsy that would otherwise take 1200 hours to build, with an implied U hull the build time would rise to 1800? Is it the expense factor (I don't think it would be much more expensive) that it makes it appropriate only for the longer hulls, as it would be a smaller percentage price rise compared to that of a chine hulled cat?

[boat fan]

........ why does he use the knucled U shape hull only for his larger/newer designs? .......


You can gain lots of extra internal volume for galleys double bunks etc....

On some boats you cannot get decent widths for double bunks ( take a look at a wharram for example they tend to have tight berths , even at 38 ft or longer...a direct consequence of hull shape..)

On the smaller hulls you cannot fit doubles anyway , best to stick to single berths or put doubles inboard on the bridgedeck


http://i709.photobucket.com/albums/ww97/victorvector/galleytopaaa.jpg



[quote]
Is it a marketing thing? Is it because they are so hard to build that it is too much hustle for the e-z build gipsy that would otherwise take 1200 hours to build, with an implied U hull the build time would rise to 1800?
[quote]


Yes that too. There is an awful lot of fairing to do on a strip planked cat ....More epoxy to glue strips together , again ...filling and FAIRING with torture boards ..most of that expensive epoxy ends on the floor , as dust , all that after after many , many hours of sanding. Don`t underestimate the work involved in fairing a large cat.....There are builders that will NEVER... EVER.... do it again ..:D .

Have a look at Masalai`s build here :http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962.html

That`s DUFLEX he`s building with...all chine hulls.

Don`t for a minute believe that his boat will not perform as well as a round bilge strip planked hull. If he keeps it light it will sail like a witch !!!!:D

[jamez]

jamez, thanx for taking the time to respond to this thread.
Performance still remains of lesser concern, since it is going to be a family cruiser. I think, from what I' ve read so far (and I am still reading), that hard chined hulls are:
1. more difficult to turn
2. easier to flip over while sailing in severe weather

A properly designed chine hull should be no more difficult to turn than a round bilge. I know some people talk of turbulence associated with chines, but I'm not sure any difference in performance has ever been quantified. There are many ply cats and tri's of various hull configurations sailing here and in Australia. Properly designed and constructed they seem to work just fine. As far as chine boats being more likely to flip in heavy weather, the huge number of successful trans ocean voyages by Pivers, Searunners, wharrams etc wouldn't seem to bear this out.

I am sorry if you think that I have taken lightly your suggestion for the kd860 but It seems to me that the layout is not for me. That cross section of the hulls does not convince me as being strucurally the strongest around. Just from the structural point of view, a straight section is expected to bend when a point load is applied to it, where as a semicircular load will just distribute the load. So in my humble and vague opinion, I think a round bilge is structurally more stable than its hard chined counterpart.

Not at all, the KD was purely an example of a lot packed into a small package. Ultimately I agree with you, eggshell and all that, however a properly engineered flat bottom or dory hull - particularly a narrow one such as a multihull would use should not give cause for concern IMO

(Please excuse my poor use of marine and engineering english, and be forgiving. Feel free to laugh and correct me also. I am here to learn...)

I don't think that a round bilge would raise the cost of the boat that much. It would definitelly spread the time frame that it needs to be built.

Its down to choice of materials a hull with its round section built in double diagonal ply would probably cost little more that doing an equivalent boat in sheet ply. Cedar strip composite however, is (in new zealand anyway, the situation might be different in Greece) more expensive per sq metre then ply glass. The material itself is more expensive and it requires a relatively heavy glass skin on each side, whereas the glass on the outside of a sheet ply boat is usually only there as a protective layer. One round bilge method which may approach the cost of ply/epoxy is foam using polyester resins, which i think is how RW built the first Gypsy (although flat paneled).

Before I started building my tri one of the many boats I costed was a 25'racing cat in cedar strip. The cedar (premachined) alone for both hulls was around 5k. The materials for the 25' ply tri I eventually chose came to 5.5k for all the ply, solid timber, epoxy and glass - quite a bit more surface area in the tri too. I'm convinced you get more multihull for your buck with ply than any other material. Particularly under say 30 - 35 feet loa where the basic structure forms a higher proportion of the overall cost. There are some downsides, for instance ply construction does not help re-sale value and it requires more on-going maintenance than a glass boat.

However, if budget is the main concern I'd suggest that building the smallest boat you can get away with out of sheet ply is one way of getting on the water soonest with relatively least expenditure. Just remember all boats are holes in the water you throw money into to a greater or lesser extent :)

I never said that I am an expert in anything. I am reading all the time (only that I have only recently started). I don't expect anyone to do anything for me that they don't want to. Please, anyone who reads this thread, feel free not to post any replies.

You are right on one thing: the only sailing experience I have had on a cat was a day cruise with a 35' cruiser around an island. I loved it. Also a couple of hours sailing a tornado.

You might like to check out the Kelsall website for info on foam/poly and his KSS construction system, Thomas Firth Jones for info on building economical safe ply multi's and a couple more examples of different styles of boat.
http://www.members.optusnet.com.au/rhturner1/cc29.html
http://www.wallerdesign.com.au/cc31.html



......

[mihari]

.

Have a look at Masalai`s build here :http://www.boatdesign.net/forums/boat-building/my-little-piece-peace-25962.html

That`s DUFLEX he`s building with...all chine hulls.

Don`t for a minute believe that his boat will not perform as well as a round bilge strip planked hull. If he keeps it light it will sail like a witch !!!!:D

Yes, I have been following the progress of masalai's build. He is doing great. I envy him. Anyway, I don't think that his hull section is far from being round. And I don't think that there will be no difference to performance between his hull section and this:
http://www.ikarus342000.com/P86page2.htm

[boat fan]

Not much difference between them mihari.

The Oram may have a little less wetted surface , (if it were of the same length , displacement waterline beam etc....) so may have a slight advantage in very light air , but that would depend on other factors like rig design and so on .....

Bernt Kohler`s designs are relatively low cost ,I like them .

I also like Mike Waller`s cc31 :http://i709.photobucket.com/albums/ww97/victorvector/cc31_.gif ;)

http://www.wallerdesign.com.au/Photos/cc31q.JPG

http://www.wallerdesign.com.au/Graphics/cc31_03.gif

[mihari]

James, thanx again for your thoughts.
After reading a little bit more (not too much), I have come to realize that more people feel the same way with you about chined sections. Actually some think that when a chine is at aft of the boat might actually compliment the handling of a boat... I must reconsider.

Is there any up to date information of double diagonal? It seems like a 60's way to go for non linear surfaces. Does anybody build in double diagonal plywood... today?
If it red cedar is expensive there, then here it will probably only be found in museums of natural history... So strip planking in red cedar is out. The fact that fairing it would be of such difficulty, and weight adding, makes it even clearer to me that it is not the way to go.

The jarcat is extremely interesting as far as layout goes that it has the mast splitting the cockpit from the saloon. That gives equally great area to both spaces. The downside is that the produced form seems really home made. That would be really bad for resale value. But still, I find that it has great advantages over conventional standard modern design. You have access to the mast and all the lines right from the cockpit. The resulting mainsail is a bit narrow because of the aft wards positioned mast, and the jib/genoa is shorter than usual (because it has to be above the saloon). Maybe the transoms need to be redesigned (look at the waves it is producing) http://www.members.optusnet.com.au/rhturner1/cc29.html

[sabahcat]

If it red cedar is expensive there, then here it will probably only be found in museums of natural history... So strip planking in red cedar is out.

Have a look at KIRI (http://www.boatdesign.net/forums/search.php?searchid=1556422) as a replacement to WRC.

Also I had and built one of these from WRC, but in kiri, she would be lighter again, Simpson Ground Effect Catamaran (http://www.cruisersforum.com/attachments/gallery/3/1/1/9/036_36.JPG).
I think I will always regret selling this vessel, she was one of the best boats I have ever sailed on in her size range, not the fastest or the most comfortable, but in her size, she stacked up very favourably against any competition.

The plans are available Here (http://www.duckflatwoodenboats.com/mainpages/gallery?KID=65)

[jamez]

Have a look at KIRI (http://www.boatdesign.net/forums/search.php?searchid=1556422) as a replacement to WRC.

Also I had and built one of these from WRC, but in kiri, she would be lighter again, Simpson Ground Effect Catamaran (http://www.cruisersforum.com/attachments/gallery/3/1/1/9/036_36.JPG).
I think I will always regret selling this vessel, she was one of the best boats I have ever sailed on in her size range, not the fastest or the most comfortable, but in her size, she stacked up very favourably against any competition.

The plans are available Here (http://www.duckflatwoodenboats.com/mainpages/gallery?KID=65)

Sabahcat that is one nice boat. I reckon Roger Simpson really nailed it with the Backslash and Ground Effect. More boat than I need but I've always liked the GE. I've never found much in the way of photos or info on them. Any more pics you can post? particularly interested in interior/cockpit photo's. Now where did I put those study plans....

[jamez]

James, thanx again for your thoughts.
After reading a little bit more (not too much), I have come to realize that more people feel the same way with you about chined sections. Actually some think that when a chine is at aft of the boat might actually compliment the handling of a boat... I must reconsider.

Is there any up to date information of double diagonal? It seems like a 60's way to go for non linear surfaces. Does anybody build in double diagonal plywood... today?
If it red cedar is expensive there, then here it will probably only be found in museums of natural history... So strip planking in red cedar is out. The fact that fairing it would be of such difficulty, and weight adding, makes it even clearer to me that it is not the way to go.

The jarcat is extremely interesting as far as layout goes that it has the mast splitting the cockpit from the saloon. That gives equally great area to both spaces. The downside is that the produced form seems really home made. That would be really bad for resale value. But still, I find that it has great advantages over conventional standard modern design. You have access to the mast and all the lines right from the cockpit. The resulting mainsail is a bit narrow because of the aft wards positioned mast, and the jib/genoa is shorter than usual (because it has to be above the saloon). Maybe the transoms need to be redesigned (look at the waves it is producing) http://www.members.optusnet.com.au/rhturner1/cc29.html

I remember lots of boats being built in DD back in the 1970's when I was a kid. It fell out of favour after the introduction of strip composite. But if you can get a bunch of cheap ply why not? Its still a perfectly valid way of producing a round bilge hull. A lot of the Tennant GBE's here were built in 2 skins of 4mm ply over a mould that produced a strong, nearly clean hull inside. Other designers suggest doing the bilges in DD or strip and the flat topsides in ply (Woods / Simpson).

There are lots of options and no one of them is 'right'.
Gougeons book describes most methods, but so does a little known book by aussie designer Roger Simpson (the designer of Sabacats ground Effect) 'Simpson on Boatbuilding'. Difference is all the examples in this book are multi's and it covers things like building cross-beams, bridgedeck structures etc. etc. Its available from Boatcraft pacific in Aus.

The Coral Coast (jarcat) 29 might look a bit caravanish but I think as a family cruising boat it has a lot going for it as you have identified. Its relatively light approx 1300kg with a 1 ton payload on top of that. There is a wing mast fractional rig option too. Seems like a lot of boat for the money, but I've never sailed on or seen one. There is a bit of wake in the photo but you'd expect some at 12 knots. Unlike many bridgedeck cats the skipper can actually see where they are going.

[sabahcat]

Sabahcat that is one nice boat. I reckon Roger Simpson really nailed it with the Backslash and Ground Effect. More boat than I need but I've always liked the GE. Thank you very much, we liked her a lot and had many years of enjoyment over many years and won many bottles of rum with her before selling her off to a New Caledonian owner.

We got to sail her across and have several weeks with her saying our goodbyes

I've never found much in the way of photos or info on them. Any more pics you can post? particularly interested in interior/cockpit photo's. Now where did I put those study plans....

Have a flick through these pictures here. (http://www.cruisersforum.com/gallery/browseimages.php?do=member&catid=member&imageuser=3119) There are several shots of her amongst the pics.

Heres one that was being built in England, he came out to see mine. http://www.themultihullzone.com/workshop.html

[mihari]

Those are beautiful boats, beautifully built. They have my favourite section in the hulls. That knuckle is so sexy!!! I first saw it (and adorred it) in Shuttleworths designs. But I have to admit that it looks like a great deal of labour.

Is kiri the same as Paulownia tomentosa? (Chinese - Japanese species)

Has anyone (alive) tried to build in double diagonal plywood? In my simple mind, that seems like a little less work. I am only isnsisting in this matter because construction method is going to be a decisive factor of the overall design. Ofcourse, 2 layers of 4mm ply + epoxy inbetween is more pricey than 9mm ply. (I have to work out how much more expensive) It also seems a bit difficult to design the individual pieces of the ply to accommodate the curves of the hull. So, any experience in building/designing in double plywood would be appreciated.

[Richard Woods]

Lots of people do still build in double diagonal ply.

As a catamaran has long thin hulls the ply strips are almost parallel sided so are quick to fit, especially when using epoxy. It is on wide monohulls that you need to "spile" (plane curves on) the strips

The big disadvantage over other systems is the space the stringers and frames take up inside - and they collect dirt and water etc.

Also resale value is probably less. Also check insurance availability.

Hope this helps

Richard Woods of Woods Designs

www.sailingcatamarans.com

[sabahcat]

Those are beautiful boats, beautifully built. Why thank you sir

But I have to admit that it looks like a great deal of labour.
Not realy, just gotta get the knuckle straight, spend a bit more time getting temporary frames (mould) right, which saves a lot of work later.

The big white hulls (50ft) in the pictures hasn't even had a longboard yet and probably wont
Is kiri the same as Paulownia tomentosa? (Chinese - Japanese species)
That it is.

[mihari]

Since Japanese market absorbs 90% of world wide production, I assume that it will be really difficult to get around here, and if I could get it it would cost me leg and an arm. It is really light (~300 kg/m3, where okume is around 450kg/m3) and it looks like great stuff to work with.

Now we have 2 alternatives for strip planking. Western red cedar and kiri. Are there any other alternatives to strip planking? I know that traditionally greek boatbuilders use pine. That will be readily available and very cheap. But I know that it is not the way to go...

[mihari]

And I have another question, what does a machined strip's section (for planking) look like?

[sabahcat]

And I have another question, what does a machined strip's section (for planking) look like?

Concave

http://www.boatdesign.net/forums/attachments/materials/195d1023493019-speed-strip-tongue-groove-strip-planking-ceader-strip.jpg

Tongue and groove
http://jb510.files.wordpress.com/2006/07/tongue_groove_big.jpg

[jamez]

Another alternative is to use horizontal foam strips although I think a better alternative for a round bilge hull in foam is to use vertical foam strip/panel such as the latest Farrier boats. Farrier's Study Site is well worth the 10 bucks to have a look around IMO. You can see a vertical strip build of a 28' racing cat here: http://www.bobfishermarine.com/page8.html

Richard Woods now has a lot of study plan info available on his website. If you check out the build photos on the Strider design info you can see the difference between the same hull in sheet ply/DD vs strip plank. The strip hull interior is somewhat less cluttered.

Full DD (from the sheer) can allow clean hulls inside, although there are some limitations on shape compared to strip. The attached photo shows the interior of a 28' Great Barrier Express hull. In the DD version the hulls are laminated over a mold in 2 or 3 skins and glassed on the outside. The only solid timber in the completed hulls are keel, gunwale stringers and a stem support. Frames to support crossbeams etc are fitted later. A number of Malcom Tennants designs have this construction method as an option. In the pic the lighter coloured planking in the bottom center right is a repair following a trip onto some rocks.

[Fanie]

Hello Mihari,

Congratulations on deciding to build your own boat. It's going to keep you busy :D Oh yeah, and could I say welcome to the world of lunatics, take fair warning that boat building is an incurable illness. Very few kick the habit after the first one... ;)

Hint from me - When you finally decide what it is you want, you don't start the boat. You start to query every decision, from size to color to materials to everything else.

Simple, once the build starts you want to be sure you are building the thing you REALLY want. Some ways in the build and you 'discover' something that you would rather prefer as to what you have, is going to be a cap on your enthusiasm. It's a long haul, you are going to need each bit you can gather.

I would also suggest you go out on a boat smaller to larger than your own... just get a feel for it. You may (again) discover some things you weren't aware of before.

Good luck. I will follow your build (if you don't come to your senses before you start :D)

[boat fan]

http://farm4.static.flickr.com/3455/3826020896_6881e47240_o.jpghttp://farm3.static.flickr.com/2652/3819003891_117bd6453f_o.jpghttp://farm3.static.flickr.com/2450/3819003885_37353b882e_o.jpgMark Prescott Catamarans :)

[mihari]

Sabahcat, thanx for the enlightening pics. The concave/convex section looks a lot more forgiving and more flexible than the tongue/groove one. But more wood wasted. I guess that the idea behind using machined timber is that you get greater area for the glue to bond and get a better fit than with the rectangular sectioned strips. (and maybe a fairer hull?)

Jamez, that is a nice looking interior... I hope mine will look like that one day. Is that thing on the left a daggerboard pocket?

Fanie, thanx for the inspirational post.

Boat fan, two beautiful boats... Would it be safe to say that Australians have their own "school" of multihull design? I can only wish that my cat would look like that...

[jamez]

Jamez, that is a nice looking interior... I hope mine will look like that one day. Is that thing on the left a daggerboard pocket?

..

Yes its the daggerboard case. On the Tennant boats the boards are on the 'inside' of the hulls. BTW Mark Pescott also has a nice 28' tubecat design.

[boat fan]

Boat fan, two beautiful boats... Would it be safe to say that Australians have their own "school" of multihull design? I can only wish that my cat would look like that...





I guess so mihari .......Australians really took a liking to catamarans some while ago. Many have , and are being built here still...I think we do have good designers here .Good designers elsewhere too though , as you have already discovered yourself.

I think you could save some time and money if you combined strip planking ( knuckle ) with developable flat sheet sheet panels.( Plywood , and cedar , or all foam maybe ).

Rob Denney uses Kiri on his innovative boats :

http://www.harryproa.com/Elementarry/BuildingPhotos_1.htm

http://www.google.com.au/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Fwww.harryproa.com%2F&ei=2qG1SuzwI43gsQOsir3RDA&usg=AFQjCNGhm2YRGiiH0YkyUdlXU5WhiSSUaA

http://www.harryproa.com/Elementarry.htm

My fav. Australian designers include

Peter Snell ( Easys )
Shionning
Bob Oram (Mango)
Prescott http://www.youtube.com/watch?v=BfOUDz-qb-0&feature=related
Crowther
Grainger
Lidgard
Tim Clissold ( NZ )http://www.tcdesign.co.nz/range.htm

http://www.seeker.co.nz/bimaran/jasmin.htm


International ,

Chris White
Nigel Irens
Richard Woods
John Shuttleworth

Soooo many boats .....not enough time or money ......:D

[Petros]

Milhari,

Have you considered using Dura-Core? It comes in either sheets or strips, it is made of balsa core with hard wood veneer skins. It comes in different thicknesses, and you glass it once you have it assembled. It is supposed to be stronger than foam core and much less costly, especially if you buy large bundles of the bead and cove strips for making round hull shapes. If you are designing from scratch it is not a bad way to go, it gives you more design options.

[sabahcat]

Milhari,

Have you considered using Dura-Core?

Does anyone use Duracore anymore?
I thought Duflex was the norm if going an ATL product?

Never liked duracore anyway as it was impossible to fair before glassing, meaning you end up with more filler than should be necessary.

At least with cedar or kiri you can plane and sand the surface getting everything perfectly fair before glassing, meaning absolute minimal bog.

To do the same with duracore maeans sanding planing through the hardwood veneer face, which is a reduction in strength.

[sabahcat]

Sabahcat, thanx for the enlightening pics. The concave/convex section looks a lot more forgiving and more flexible than the tongue/groove one. But more wood wasted.

Yep, plenty of wood wasted using both the concave convex and T&G.
I having used tongue and groove and square edge I could never see the advantage of using T&G over it due to cost and waste. T&G offcuts cant be used for stifenners/stringers for furniture stiffening as they have a T&G edge, so more waste.

I guess that the idea behind using machined timber is that you get greater area for the glue to bond and get a better fit than with the rectangular sectioned strips. (and maybe a fairer hull?)

On the tongue and groove boat it was glued together with Purbond (http://boatcraft.com.au/adhesives.html), not sure if I liked this over epoxy or not, tests done on planking showed it seemed as strong as cedar, so I suppose it's OK.

I never had a problem with the square edges, I used long plastic strips and screws or staples to line planks up.

Takes a bit more time, but saves many dollars in machining and wasted timber costs over T&G and concave.

If you look at my earlier link to the 50ft hulls you can see we left a paint scraper gap between planks and rolled resin in, letting capillary attraction pull resin through the join, that and a bit of squeegee action at times.

More Here (http://www.boatdesign.net/forums/materials/speed-strip-tongue-groove-strip-planking-624-2.html#post116927) and a couple more in same thread.

You can see the plastic strips used to line up planks in some of the pics

[mihari]

OK all, thanx for your input. I think I am leaning towards plywood hulls and maybe (just maybe) strips for knuckles. Round hull has to have a second layer of flooring (actual hull skin cannot be used comfortably for walking or standing in the galley). I don't think the extra weight, lost space, and extra effort is really worth it for my kind of scope for this cat.

Next big topic: "Waterline shape"
For most cats that I have seen, the waterline section (I would call it water line plan) of the hull is pretty similar, all just slightly varying in proportions. None will fit a NACA4. I was wondering if there is any general rule as to how the hulls are shaped. The rate of change from the tip of the bow to the mid hull and then back to the transom. I know that their overall proportions (length to width) pretty much categorize the boat as being a cruiser or racer (or a boat house). But what rules govern the rate of width change?

I am also convinced of using a knuckle at the outer side to gain enough room for double bed, so the actual waterline width of the hull is not so important to be wide (as long at it can accommodate two people crossing)

[jamez]

Just discovered this. http://thecoastalpassage.com.au/cheapcat.html
Nice looking boat and apparently simple construction method.

[bob the builder]

31' strip plank cat.

this is the best boat i've seen yet.

simple, built to last decades and still be stiff.

not only is it the best cat available, but the plans are free as well!


yes,
i've seen many cat plans but this one is the best by far.





http://www.boatdesign.net/forums/multihulls/general-comments-28658.html


http://www.boatdesign.net/forums/multihulls/catamaran-beams-new-improved-28317.html

[mihari]

Jamez, yes, I had seen that before. From what I gathered this guy worked with no plans. (so he says)
And I think that it is true, because look at the beam supporting the mast... It doesn look like a box beam. And it doesn't look like steel. And it doesn't look like it is going to hold on for too long... I hope it does, but I don't think it is like what I am used to seeing with the usual monocoque construction.
Other than that it is an overall nice looking boat, and not far from what I think mine will finally look like. At least as far as the hulls go.

With what I have in mind, access to the hulls will be through a salon, and the salon will be able to accommodate a few people standing and/or seated.

[mihari]

After searching a bit more, I have found the answer to the question "What waterline shape?"
I am sure many naval architects have been searching all their lives for the golden rule of hull shapes. And I have found the answer in just a few days... "there is no rule".

I have to build a shape that pleases the eye and accommodates the needed spaces, and then play around refining it to find the best form... time to dive deep into the world of hull software.

Just a question before taking that deep breath... Rocker or no rocker? Most cats have rocker bottoms. I never thought that mine might be flat bottomed. I know there is an issue of beaching (that you get less draft with a no rocker) but how about attitude? Any experience to compare the two?

Hope to post here soon...

[boat fan]

After searching a bit more, I have found the answer to the question "What waterline shape?"
I am sure many naval architects have been searching all their lives for the golden rule of hull shapes. And I have found the answer in just a few days... "there is no rule".

I have to build a shape that pleases the eye and accommodates the needed spaces, and then play around refining it to find the best form... time to dive deep into the world of hull software.

Just a question before taking that deep breath... Rocker or no rocker? Most cats have rocker bottoms. I never thought that mine might be flat bottomed. I know there is an issue of beaching (that you get less draft with a no rocker) but how about attitude? Any experience to compare the two?

Hope to post here soon...


More rocker....easier to turn the boat...within reason.
Too much rocker too deep extra surface area / drag.Not enough buoyancy in the ends...( hobbyhorsing ).

No rocker...fast . you still need to turn up the " run " at the stern to prevent dragging that stern around .Unless its a fast power boat .
These are gross generalizations.

Need to find that happy medium for your displacement slenderness ratio l/b ...looking at successful boats that are close to your size and displacement is a good place to start.


Fusion Catamarans say this :

""The trend toward flatter rocker in recent years has largely been induced by the desire to increase the prismatic coefficient (buoyancy in the ends) and thereby dampen pitching and optimise reaching performance. Designing rocker into a new hull is a delicate balancing act as it has to be considered in respect to the flatness of the underwater sections and the required buoyancy.

For example if the sections forward are fairly flat or rounded (as opposed to a 'V' section) then it is important to ensure there is adequate rocker forward to avoid hard landings while beating in a seaway. The run aft should remain reasonably flat especially if some planing lift is being sought downwind. An excessively flat rocker line might also require that lost buoyancy be compensated by the beam to length ratio.""

Shuttleworth :

""Many early multihulls were prone to hobbyhorsing, and pitching. This was caused by too much rocker on the hull profile, and fine V sections both fore and aft. As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem, because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull . This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind. At the same time windward performance is improved, because the apparent wind direction is more stable across the sails.""

Lightspeed say :

""Flat rocker with rounded sections forward transitioning into distinct U-shaped sections aft derives from tank testing. The round forward sections give a gentle motion. The flatter aft sections provide a planning surface and additional buoyancy where the crew weight is centered. Flat rocker provides for high top end speed.'''

This may be helpful too...http://www.google.com.au/url?sa=t&source=web&ct=res&cd=28&url=http%3A%2F%2Fwww.multihulldynamics.com%2Fnews_article.asp%3FarticleID%3D174&ei=Hga8SvPjNYeAsgP6mLS6BA&usg=AFQjCNHBmUnL8hv-3AJH1aPzj7djxHXUtg

This Cruiser discussion maybe helpful :http://www.google.com.au/url?sa=t&source=web&ct=res&cd=29&url=http%3A%2F%2Fwww.cruisersforum.com%2Fforums%2Ff48%2Funderwater-lines-for-a-performance-cruiser-13290-5.html&ei=Hga8SvPjNYeAsgP6mLS6BA&usg=AFQjCNH0f2ote_RKbnLa5gtDkX2KnZeknw


It`s both art and science.Good luck.

[sailsocal]

Shuttleworth :

""....However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull . This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind.

What does he mean, moving the center of buoyancy forward and aft?

[boat fan]

What does he mean, moving the center of buoyancy forward and aft?

Shuttleworth :

"the center of buoyancy ( is ) being moved forward in the immersed hull"

Hard to explain ( at least for me)....:D I will try :

As the windward hull attempts to lift , the leeward hull`s center of buoyancy moves forward because the leeward hull is simultaneously forced forward and down ward.The stern attempts to lift out of the water.The waterline gets shorter , and c of b moves forward.
The boat " pivots " at a diagonal axis to abeam of the boat.

Does that make any sense ? :D

[mihari]

Without actually doing any calculations, I think that the c of b is not moved any more than a few cm at normal conditions, and less than half a meter at extreme conditions. We are talking about a cruiser here, not a racing cat. The lighter/faster the boat the more extreme this momentary occurance is.

Thank you boat fan you have been of great help so far.

[mihari]

Hull width calculation.

OK, I made my wife stand in the kitchen as if she was working on the counter to measure how much space we need. As I thought, the absolute minimum (for our average weight) for 2 people to cross without touching is 60 cm at bum level. A 5cm narrower space on either side is needed at the feet. At 55 cm effort is needed not to touch. At 50 you can be sued for sexual harrassment.

I think you cannot go any more minimal than this... for a cruising (maybe liveaboard) cat.

[boat fan]

Without actually doing any calculations, I think that the c of b is not moved any more than a few cm at normal conditions, and less than half a meter at extreme conditions. We are talking about a cruiser here, not a racing cat. The lighter/faster the boat the more extreme this momentary occurance is.



Yes c of b does not appear to be moving much.

However , a 28 ft cat like you are proposing will essentially be relatively lightweight , even if cruiser.You will still have to watch your weight or the boat WILL be a "pig" , that is to say it will be sluggish and wet and plow through the water , instead of gliding
over it.A 28ft cat can be nice .If you weigh it down you kill some of the very attributes that make a cat so appealing in the first place..The first commandment of building a good cat should always be " KEEP IT LIGHT " cruiser or not.

A c of b shift of .5 m (or 19 inches ) on a ( more or less ) 28 ft waterline could well make the difference between a boat that " bucks " or "hobbyhorses " and one that behaves in a more sedate manner.


I think you really should have at least 600 mm width to move around. A little wider in the hulls on a 28 footer will not necessarily be bad . In fact , I believe that the gains far outweigh the extra wetted area of a slightly wider hull, at least at this size.
Just the gain in displacement will float her higher .That has got to be a good thing. Some people may be surprised to find that your boat may well be FASTER with slightly wider hulls , given equal loading / weight / displacement.

I had a conversation with Ann Snell ( Australian Easy Cats ) about this. Peter`s smaller 29 ft should not carry more than 1ton. If you want more they strongly advise to build the 32. Like Ann said , there is no difference in cost.

As you get bigger ( longer ) things get a little more forgiving as far as weight goes.

[mihari]

Of course, as I said, that is what I thought to be an absolute minimum. I never said I was going to use that. I will probably go to something more rummy. This is a rough sketch of what I am thinking of...
...how the heck can I attach an image?

[boat fan]

If the image is already on the web somewhere , right click with your mouse select properties and copy the url into your post here.

If the image is on your computer its a little more messing around :


You need to join an image hosting website ( free ).Here is one:

http://photobucket.com/


Join up and Upload your image , then cut and paste the link into your post here.



BTW there is nothing wrong with the 28 ft size cat.
It`s just that you really need to trim what you carry on board. The other thing is getting headroom in saloon is near impossible under 40 ft.
Just saying ........

[jamez]

Or you can just select 'go advanced', then 'manage attachments' and attach them to your post.

[boat fan]

Or you can just select 'go advanced', then 'manage attachments' and attach them to your post.


Just learned something ....thanks Jamez.:D

[sailsocal]

Shuttleworth :

"the center of buoyancy ( is ) being moved forward in the immersed hull"

Hard to explain ( at least for me)....:D I will try :

As the windward hull attempts to lift , the leeward hull`s center of buoyancy moves forward because the leeward hull is simultaneously forced forward and down ward.The stern attempts to lift out of the water.The waterline gets shorter , and c of b moves forward.
The boat " pivots " at a diagonal axis to abeam of the boat.

Does that make any sense ? :D

No it doesn't. How does the C of B "move aft in the raised hull" as Shuttleworth states?

[boat fan]

No it doesn't. How does the C of B "move aft in the raised hull" as Shuttleworth states?

Don`t know ......ask him .

[catsketcher]

Gday all

I use Shuttleworth type hulls in my little folders. They are tricky to design right. If you go slightly over weight the boat will be a pig. As to the CB question.

If you look carefully at Shuttleworth's hulls the flare is different fore and aft. This makes the CB move as the hulls are raised or immersed. The tricky thing to remember in cat design is that the hulls are always moving up and down - leeward always down and windward always up. You can use this to make the CB move where you want if you are clever.

You should not use low flare hulls on a boat with a high immersion rate. Go slightly wider than you think because every boat comes out heavier than you think and you will be dragging the flare around if you stuff up.

cheers

Phil

[mihari]

(I hadn't seen the little button "manage attachments" at the bottom)

So, here is a rough section of what I had in mind. You are right, it is tough to get standing headroom in the salon without making it look like a bus. I will make a small compromise and give it a shot.

[mihari]

Originally Posted by sailsocal View Post
No it doesn't. How does the C of B "move aft in the raised hull" as Shuttleworth states?

As the C of B moves forward when the bow dives in the water, when it comes back up it moves stern-wards. I guess this would only be a momentary effect.

[boat fan]

Gday all

If you look carefully at Shuttleworth's hulls the flare is different fore and aft. This makes the CB move as the hulls are raised or immersed. The tricky thing to remember in cat design is that the hulls are always moving up and down - leeward always down and windward always up. You can use this to make the CB move where you want if you are clever.


Phil

Do you have any hull sections I could look at catsketcher ?

You should not use low flare hulls on a boat with a high immersion rate. Go slightly wider than you think because every boat comes out heavier than you think and you will be dragging the flare around if you stuff up.



That is real good advice catsketcher . I agree 100 % :)

[boat fan]

(I hadn't seen the little button "manage attachments" at the bottom)

You are right, it is tough to get standing headroom in the salon without making it look like a bus. I will make a small compromise and give it a shot.



:D That " Art meets science " thing is coming into play now ....:D

You could devise a saloon " pop top " to reduce height.

Richard Woods novel approach :[/QUOTE]http://www.sailingcatamarans.com/studyplans%20website/Sango/popbottom.htm[/QUOTE]

One question , how did you arrive at the .34 m2 cross section for your displacement ?

[mihari]

:D That " Art meets science " thing is coming into play now ....:D

You could devise a saloon " pop top " to reduce height.

Richard Woods novel approach : http://www.sailingcatamarans.com/studyplans%20website/Sango/popbottom.htm

One question , how did you arrive at the .34 m2 cross section for your displacement ?

(can somebody tell me how the multiquote button works?)

The pop top sounds like a great idea. Its max height would still be at the boom, and the top wouldn't help as a structural element to tie the boat together. But, I think it is a great idea. (I was aware of that pop down bottom, I didn't know it was Woods'). I will look into that later on.

The .34m2 cross section is the actual cross section of this particular sketch if the hull is immersed 40 cm under surface. It is not calculated that this is the needed depth of immersion of this particular design. I have done a couple of other models though for the same hull length, and had found that the needed draft was a bit less than 40 cm. This being wider than the previous hulls will probably need less draft at mid ship. So, it is just an indication of where waterline might be (could be off by 15 cm). It is just that I know that I want bridge deck clearance to be at least 50 cm.

Also, both inner and outer flares are at the point where the hull meets the flat water if the boat is geometrically tilted around where the central axis of symmetry meets the water, to where the opposite hull is barely touching flat water surface. Maybe this is a bit difficult for me to explain in words, so I will try a sketch.

[boat fan]

[(can somebody tell me how the multiquote button works?)

If you look through the thread , you can hit the multiquote button , ( it will be highlighted now ),
go to another message in the thread and again hit multiquote etc then
press reply / go advanced /...and you will see the messages you selected
pasted in your reply window in the order you selected them.

To remove some or all messages you selected just hit the multiquote button and it will no longer be highlighted so will not appear in your reply window.

[boat fan]

The pop top sounds like a great idea. Its max height would still be at the boom, and the top wouldn't help as a structural element to tie the boat together.

No , true , it would not help to strengthen the bridgedeck structure.
However , if you engineer the boat and cross beam to function like any open bridgedeck cat it becomes a non issue.
You will find most saloon roofs do not do a lot for strength anyway.They need to be lightweight and are inherently relatively flat .
The main forward beam needs to do the work , in particular it has to handle the substantial downward thrust of the mast and rig trying to push its way through the bridgedeck.
( assuming a bermudan fractional sloop rig now ) that rig seems the simplest with relatively good performance ...that`s a whole other issue .......Making those assumptions ,
your saloon will be there just to keep the elements out.

Even a tent would do that if it doesn`t get blown away :D

Are you using freeship ?

[mihari]

Are you using freeship ?

Not atm, but will soon probably.

These are the results I got from that excellent dimensioning tool by terhohalme. Note that it is a bit longer than 28' (9.10m Lwl).

[boat fan]

I noticed on the spread sheet thread by terhohalme you had some questions.I`m not a designer , so I cannot give the answers to some of your questions without doing a lot of work beyond the scope of time available.

Some of the questions below area little easier to answer :


1 .
Height of lateral area: what is it? I saw in your "manual" that it is below waterline, but to where?

-------

CENTRAL LATERAL PLANE
The immersed longitudinal vertical middle plane of a vessel.

CENTER OF LATERAL RESISTANCE
The point through which a single force could act and produce an effort equal to the lateral resistance of the vessel. It is ordinarily assumed to coincide with the center of gravity of the immersed central longitudinal planes. But not always.


All hulls have an underwater profile. The area of that profile is calculated and its center is plotted .This is your CLR or CENTRE OF LATERAL RESISTANCE.It is below the waterline because only the underwater area is calculated.Sometimes expressed as a negative number , if the DATUM line is set at Zero , at the designed waterline (DWL)

A "rough and ready" way to determine CLR is to lift the shape off your profile drawing , cut it out on card stock and balance the shape on a knife edge. Mark the vertical line at which the profile balances .If you place a pin near the centre along this line you will locate the theoretical point at which the shape will balance.The rudder is usually included.(Some designers only include half the area of the rudder ?...)
You now have your CLR.

Software like freeship will do this better for you of course, but you get the idea......


-------



2 .Height of fore triangle - the height of center of effort from waterline?
Is it for fractional rigging?
Height of mainsail - the height of center of effort from waterline?

--------


Theory is similar to CLR that is , the concept of a centre of area, but
this time as applied to sail area.Applies to ALL sail plans , weather fractional sloop or gaff or lateen ...whatever ...


http://www.google.com.au/url?sa=t&source=web&ct=res&cd=2&url=http%3A%2F%2Fwww.duckworksmagazine.com%2F05%2Farticles%2Freplacing%2Ffree.cfm&ei=0s-_St2yHpSotgPl781O&usg=AFQjCNELPgWCrJsrYQJ--zuqDrFhTfQtUw&sig2=fkUGJeSiAxE14iI9tNrQ5A

http://www.google.com.au/url?sa=t&source=web&ct=res&cd=4&url=http%3A%2F%2Fwww.paddlin.com%2Ffivelakes%2Fcenter_of_effort.html&ei=0s-_St2yHpSotgPl781O&usg=AFQjCNGqX4ctalIanEhTWK7RS7MqI90jxQ&sig2=NgLa1nLCDkG1MTTvmzvg_w




The effect of the relationship between CLR and CE of the sails are briefly discussed here :



http://www.google.com.au/url?sa=t&source=web&ct=res&cd=13&url=http%3A%2F%2Fwww.2020site.org%2Fcanoe%2Fsailboat.html&ei=0s-_St2yHpSotgPl781O&usg=AFQjCNHM-q3p86FUJSQO9nzg-bwXyYMp1A&sig2=FXYYuB9cqAshjwb8g6m-gA


3...Design righting moment - how does this help me?

Essential design consideration.

There are a number of reasons for depressing the leeward bows of multihull craft. The ratio of the lateral stability against the longitudinal stability has an effect on the pressure that is applied to the leeward bow.

Boats with greater lateral stability will be able to sustain greater sail forces before tipping over sideways, therefore allowing more driving force (and counterbalacing resistance) to be transmitted into the boat and thus resulting in a greater pitching moment depressing the bow.


This is a VERY basic theoretical comment.


4. Limiting righting moment - how does this help me?

---------


This has far reaching effect on stability , motion , pitching , and more.

Limiting righting moment reduces the stress on your rig.


At a very basic level , it works like this :

For multihulls , the wider the boat , the more INITIAL static stability .
Or it has a large righting moment. Naturally if you make the boat wider , the more it will be resist lifting the windward hull' and capsize.

As a result the boat has a huge righting moment.Some times referred to as being a " STIFF " boat . ( Low righting moment referred to as TENDER.)

Wind pressure on the sails acts to try and tip the boat over , ( overcoming its righting moment ), and also driving the boat forward.

As a boat heels some of the forces generated by the wind acting on the rig are transferred to immerse the hull.This reduces the stress on the rig.

The overly stiff boat develops larger stresses on its rig because it remains upright and does not depress its leeward hull as much as a more tender boat.The rig has to withstand much more force to remain upright.

LARGER RIGHTING MOMENT REQUIRES A STRONGER RIG.

A modern multihull is subjected to large vector forces . These are complex and non static as the boat sails , particularly in variable wind / sea conditions.

Hope this helps:)

[catsketcher]

Gday all

A few quick notes

I got it wrong when I said the flared hull shape was hard to get right with a "high immersion rate" I should have said "low Immersion rate" A high immersion rate will sink slowly.

Pop tops - I have only seen them on one cat - the Crowther inspired (flop moulded and copied I think) Osprey 32. It was not a success. I tried a semi pop top (hinged at front and up and down at back) on my cat and hated it. I just put a new cabin on it. A 28 footer will not have standing headroom. Get clever - Stand up behind the cabin and put a roof over the cockpit. See if you can incorporate this for hull to hull travel. ie don't use a pop top but a removable roof covered by the cockpit dodger going forward over the cabin. You only need standing headroom in the middle. After a few head hits you will be fine stooping to enter and exit the hulls and all you need is one spot you can unwind.

Longitudinal stability and athwartships stability. Do not think that you gain more stability going wider only. The reason cats are not square is that if you make a rig that flies a hull - going to windward or on a reach- on a very wide cat it will nosedive very easily downwind. This is doubly bad because you can ease off sheets to windward or on a reach but doing so on a run is useless. You should have much more fore and aft stability than sideways but this is tricky. If you put a large fordeck on a bow it will trip the boat better than if it had a smaller foredeck (and less fore and aft stability)

cheers

Phil Thompson

[boat fan]

Gday all

A few quick notes



Longitudinal stability and athwartships stability. Do not think that you gain more stability going wider only. The reason cats are not square is that if you make a rig that flies a hull - going to windward or on a reach- on a very wide cat it will nosedive very easily downwind. This is doubly bad because you can ease off sheets to windward or on a reach but doing so on a run is useless. You should have much more fore and aft stability than sideways but this is tricky. If you put a large fordeck on a bow it will trip the boat better than if it had a smaller foredeck (and less fore and aft stability)

cheers

Phil Thompson


Agreed .

I woud be a little more conservative regarding the rig.No need to tempt fate and fly a hull on a cruising boat ....:D

If I was building a 28 ft cat , I would put 36 ft hulls on it anyway. Length is cheap . And fast.



As to the pop top on the saloon , I know the girls like that saloon ,I think it could be done , but I would opt for a hard top myself , with drop down clears. Less windage lighter and airy. Terhohalme`s boat comes close to what I would have.We are blessed with our weather here in OZ , this boat would suit my cruising grounds. ( far right ) . Nice .

[ImaginaryNumber]

You should have much more fore and aft stability than sideways but this is tricky. If you put a large fordeck on a bow it will trip the boat better than if it had a smaller foredeck (and less fore and aft stability)


Phil,
What do you mean by a "large" or a "small" foredeck? Would you help me understand how the size of a foredeck affects a cat's propensity to trip.
Thanks,
Imaginary Number

[boat fan]

Just for fun ...............:D




Safari


http://www.gunboat.com/safari.php


The Pugh was pointing higher though , and steered off.

[catsketcher]

A multihull pitchpoles (capsizes over its bows) when the force from the rig going forward is greater than the drag created by the hulls. Of course at a constant speed both drag and rig force are equal.

If you push the bow of a hull under water then the drag increases greatly usually at the same time as the rig force forwards increases too. This will produce a torque unless the hull is free to accelerate.

If you have a large flat wide fordeck under water then the boat can't accelerate (in fact it will decelerate) and you will have the recipe for a pitch pole. This is why small cats like the A class have very fine bow decks. Racing trimarans have floats that can go under water with little increase in drag.

I once asked Nigel Irens how much stability a multi should have fore and aft relative to sideways stability. He said because of the drag question it is hard to give a quick answer. I think the best way is to think- Will this bow ever go under water? If it will then make the deck rounded and small. If it won't then you can have flare and a flatter deck.

I was able to demonstrate this phenomena when sailing my 31 ft Crowther tri. I could stick the bow in a wave when sailing at 15 knots plus and feel no difference. Nice thin float with rounded deck.

cheers

Phil

[boat fan]

A multihull pitchpoles (capsizes over its bows) when the force from the rig going forward is greater than the drag created by the hulls. Of course at a constant speed both drag and rig force are equal.

If you push the bow of a hull under water then the drag increases greatly usually at the same time as the rig force forwards increases too. This will produce a torque unless the hull is free to accelerate.

If you have a large flat wide fordeck under water then the boat can't accelerate (in fact it will decelerate) and you will have the recipe for a pitch pole. This is why small cats like the A class have very fine bow decks. Racing trimarans have floats that can go under water with little increase in drag.

I once asked Nigel Irens how much stability a multi should have fore and aft relative to sideways stability. He said because of the drag question it is hard to give a quick answer. I think the best way is to think- Will this bow ever go under water? If it will then make the deck rounded and small. If it won't then you can have flare and a flatter deck.

I was able to demonstrate this phenomena when sailing my 31 ft Crowther tri. I could stick the bow in a wave when sailing at 15 knots plus and feel no difference. Nice thin float with rounded deck.

cheers

Phil



The Alinghi hulls look very fine; is there enough forward buoyancy?


Nigel Irens: They are fine and in multihulls it’s always been a bit of a cleft stick. The layman’s view is that you must have lots of buoyancy to stop the boat capsizing forward, but to have that buoyancy you have to make the waterline a bit blunt and the result is that you get more drag than you get lift, so you actually cause the boat to trip over. That was the lesson learned from the mistakes of the '70s and '80’s though, interestingly, the Tornado – a most successful boat - was designed in the '60s and the man who designed it, Rodney March, realised then that you don’t create the capsizing force if the bow is fine, so you don’t need the buoyancy.


The BOR boat seems to be a lot longer than it was and a lot longer than Alinghi. How much of an advantage is that going to be?

Nigel Irens: I can’t say how much, but - apart from the obvious advantage that, in basic terms, longer waterline means faster boat - there is some advantage in that a longer boat is less sensitive to longitudinal trim issues. Clearly, if the sail plan is trying to force the boat fast in one direction the limiting factor is when it wants to cartwheel forward and a boat that is longer is going to have a bit more resistance.


Oracle has now gone to wave piercing bows, can you explain the advantage?

Nigel Irens: Well, I think the bows it had before could be thought of as more suitable for offshore sailing, where there are big waves. As I’ve said, you must keep the bows very fine, in order to reduce the tripping forces, but having sufficient buoyant force there is important. So, in recent years, people have elected to keep the entry fine but make it taller, so that if you are tearing along in the ocean and go into the back of a wave, you haven’t slowed down too much but you have considerable correcting force. In flatter water that is not quite the same and, if you put a big area of hull forward, there is quite a lot of structure involved, so quite a lot of weight and the bigger it is the more of a target it is to waves coming from the side. So, by making it a wave piercing bow, you reduce its side profile quite a lot and therefore the windage and the structural loadings. I’d say they’ve changed from a classic offshore bow to something that is more associated with inshore sailing.

[boat fan]

Pitchpoling......

Common sense does a lot :

[QUOTE]_______________
If you don't surf down the front of waves, it's unlikely that you will ever reach speeds that would make a pitchpole happen. You need a lot of energy to get a boat to flip end over end. To get all that energy on board, you need to be moving fast, and then you need to drive the bows under the water. That's not so easy to do in a cruising catamaran. As long as you aren't surfing down the waves, you are fairly safe. For me the rule of thumb is simple: Sail slower than the waves so that they pass under my yacht, and I am usually fine. If I pile on sail and start surfing down the front of waves, then I am setting myself up to where a pitchpole could occur. I consider that to be pilot error on my part.

I have seen pictures of a pitchpole on a yacht in which they were flying a spinnaker out in front of the yacht, and the wind suddenly jumped to over 40 knots. The spinnaker didn't blow out, and it forced the yacht into a pitchpole.

I sail in a conservative manner, and I regard pitchpoles to be pilot error. Although it's exciting to surf down the front of waves at 18 to 20 knots, I don't like putting myself into a situation where a mistake by the helmsman or autopilot could result in a pitchpole.

I read about one backwards pitchpole on a catamaran lying to a parachute sea anchor. In this case, the parachute bridle was attached to cleats on the forward crossbeam. The yacht had a wave strike on the bow, and the impact ripped the crossbeam out of the boat, and pushed the boat backwards. The large sugar scoops on the stern dug into the water, and a backward pitchpole happened. To me, this was also pilot error. On most catamarans it's unsafe and unwise to lie to a parachute with parachute bridle attached to cleats on the forward crossbeam.


_[QUOTE]_________________
Dave
Exit Only_______________

[catsketcher]

Thanks for the Irens quotes - I am a huge fan. I once bought him a beer - big highlight in my life. I got to talk to him when writing an article on a big tri built in Gosford.

Pitchpoling can occur on flat water. Lots of small cats do this - tearing along and then the bow dips and over you go.

It can also occur on cruising cats. A Lightwave 10.5 pitchpoled on the Wide Bay bar a few years ago. It surfed a wave which is normal on a bar. In fact it usually is safer to surf a cat than to try and slow it down. The problems may be that the Lightwave has a very uneven bouyancy distribution - it has very fat sterns and little bows. In a wave these powerful sterns can overpower the less bouyant bows and dig the bows in

I prefer a boat to have more balanced bouyancy distribution but then I was not asked to draw a boat that had lots of room in the ends of the hulls like the designer was.

cheers

Phil

[boat fan]

Thanks for the Irens quotes - I am a huge fan. I once bought him a beer - big highlight in my life. I got to talk to him when writing an article on a big tri built in Gosford.

Pitchpoling can occur on flat water. Lots of small cats do this - tearing along and then the bow dips and over you go.

It can also occur on cruising cats. A Lightwave 10.5 pitchpoled on the Wide Bay bar a few years ago. It surfed a wave which is normal on a bar. In fact it usually is safer to surf a cat than to try and slow it down. The problems may be that the Lightwave has a very uneven bouyancy distribution - it has very fat sterns and little bows. In a wave these powerful sterns can overpower the less bouyant bows and dig the bows in

I prefer a boat to have more balanced bouyancy distribution but then I was not asked to draw a boat that had lots of room in the ends of the hulls like the designer was.

cheers

Phil


Yes I like Irens too...seems everything he does is quality.

I stuffed a Nacra 5.8 in relatively flat water years ago :D

If the seas are really short and steep , surfing is not that smart . If your boat accelerates too much ..........it all depends on the conditions and the boat really.

I agree with you on the bouyancy distribution thing too , that`s why I would put longer hulls on a " small " cat ...leave the ends empty....just water tight bulkheads.

Oram likes small cats on longer hulls for the same reason. I looked at the first Mango he drew years ago , only small boat, and he told me to go longer ( only ) on the hulls . All makes sense to me.

[mihari]

Sorry it took me so long to respond... just needed some time to digest all the information... and do a little of research.

Boat fan, you are coming through for me again. Thank you for helping an ignorant, boat designer wannabe, to become less ignorant. I received a lot of attitude by posting those questions in terhohalme's thread. I hadn't realized that my questions can be that offending. Wouldn't it have been easier for everybody not to have responded to my questions?... Well anyway. Retrieving back to my little cave of darkness...

Apparently I have a huge gap with marine design terminology. For example: the "Height of lateral area" seems very vague to me. Then "LATERAL PLANE" and finally "CENTER OF LATERAL RESISTANCE" are more self explanatory. Its just so hard to make the connection if you don't know the terminology. That is all I asked for in the first place. Thanx again boat fan.

Phil, thank you for posting that about hull deck form. A lot to think about.

Pitchpolling is a condition that I think I will never have to face. Mediterranean seas are a lot calmer than the oceans... though less predictable. Pitchpoling can occur only in quite extreme conditions and being a leisure sailor, and not a racer, I will probably never need to worry about that. However, a boat that can deal with pitchpoling better than another, has better overall stability in diverse conditions, thus making it a safer boat.

Moving along with the design, since only 2 doubles and a single bed is what I was aiming for, at the length of 28' (to 30' max), there is plenty of space for buoyancy/watertight compartments. According to terhohalme's spreadsheet, the full load displacement is calculated to 2634kg (I am fine with this), empty boat to 1846kg (that seems too heavy unless I am making a terrible mistake in my rough caclulations). With a rough estimate on all the weights apart from bare boat, I came up with 1300kg, and that includes dinghy, sailors, rigging, provisions, batteries, water... everything I could think of. So unless "empty boat" has some extra weight other than bare boat mass, I have to start looking at a different hull design. I was hoping that bare boat was about 1300 - 1450 kg.

[boat fan]

Hey ....we all have to start somewhere.

On the whole this group is quite accomodating and helpful.

Some are very experienced and knowledgable , but don`t bother helping any newbies. ( Pity )

Don`t worry too much about the attitude , don`t take anything too personal either.

Terhohalme is right about anyone starting out on designing though ...you have to do a lot of reading , the more the better. It takes time. :)


Some people here are quite rude .When you get responses expressed like THIS .... it really pis**s me off .....
" net etiquette " ...sadly lacking with some :mad:

A small minority here also think their " knowledge " is "precious " .....best to just ignore those .......:D


When you look at some designers offerings ( and these are just a few examples ) you realize that the cost of their plans are such a bargain.

Kohler ( K designs )

Richard Woods

Mike Waller

Bob Oram Etc......... Etc.......

These guys also SAIL their boats , lots of experience and if you build to their PROVEN designs , ...You get a good boat without the risk of building a boat with major flaws that you or I could have overlooked.

[boat fan]

Pitchpolling is a condition that I think I will never have to face. Mediterranean seas are a lot calmer than the oceans... though less predictable. Pitchpoling can occur only in quite extreme conditions and being a leisure sailor, and not a racer, I will probably never need to worry about that. However, a boat that can deal with pitchpoling better than another, has better overall stability in diverse conditions, thus making it a safer boat.




Be careful here mihari .

If you have a powerful boat ( meaning a rig that generates a lot of power in relation to the overall displacement of the boat ) it can catch you out.

In a previous post I mentioned " stuffing " my Nacra , (back when I raced that boat we referred to pitchpolling as "stuffing it " :D )

That can happen in relatively smooth flat water.We used to look for the " darkies " on the surface...you see darker patches on the water`s surface caused by small ripples ...(areas of high pressure air) .The idea being that you search out the " darkies " and sail from one to another and get more lift and acceleration and boat speed.( We found one alright ....:D )

Those high pressure areas can be difficult to see and you can inadvertently sail right into one , next thing you know you are screaming along with the leeward bow doing its best to bury itself as the boat accelerates. If the sheets are cleated .... over you go .This can happen weather you are on a race boat or not. If you are unlucky and have conditions of short steep seas ( not necessarily high ) the problem can compound. There are numerous other scenarios that can lead to pitchpoling.( Long hulls are NICE !!!!!!!!!)

Watch right near the end ::D



http://www.youtube.com/watch?v=BnTWclNSfKI&feature=related

Results of "Stuffing it " below....:D

[mihari]

Being a windsurfer at heart, I do appreciate speed and fighting with the wind. But I will leave that to windsurfing. I don't plan to take my cat anywhere near extreme conditions.