28' sail cat

[jamez]

Originally Posted by 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

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]

Originally Posted by boat fan

.

Have a look at Masalai`s build here :My little piece of peace

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 !!!!

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://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]

Originally Posted by mihari

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 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.
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

[jamez]

Originally Posted by sabahcat

Have a look at KIRI 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.
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

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]

Originally Posted by 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

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]

Originally Posted by jamez

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. 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]

Originally Posted by mihari

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]

Originally Posted by mihari

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

Concave



Tongue and groove

[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.