T20 -- New development class

The Grainger series ( ST rings a bell ) with semi folding ( not on the water ) is a cool in between design but has stays. Any way a 15ft beam shouldn't need folders ( read extra weight ) but simple demountable beams.

Don't knock the A class boats, at 4k you get the right light weight Ama's, 14sq m of sail on a wing section carbon mast ( its probably the equivelent to 15 - 16 sqm of F18 sail ) and all the right rudders and boards + good quality fittings and blocks ( the A class boys don't tend to scrimp on fittings ). The beams would make perfect foward beams leaving only cheap rear ali beams to find. It may seem sacrilege to savage an A class but these older boats are now out classed by the curved board modern designs and will be increasingly at the back of the fleet or sold to good causes.

Extra sail areas could be found from the F18 jibs and spinnys which are quite available at relatively low cost from the top F18 crews. Fit a furliable screacher ( the centre hull would take the load ) and you have a very powered up craft with CE very low down on the mast.

Having been a life long tri fan I certainly have looked and thought along these lines and a class like this would be pretty good fun. Bieker did a centre hull design for Tornados, perhaps his centre hull may give a clue if scaled down a bit. The guy who designed the Weta ( forgotten his name ) I think actually has a 20ft Tri design on his web pages, looked a pretty good machine when I was looking a couple of years ago.

 

The A class donor boat does seem viable searching up parts it looks like their available for less than I initially thought for the early timber boyer hulls. Jibs, screechers and assys could set quite well off the reasonably tall mast.

I've never looked at one carefully so am not sure how much extra righting moment the very light mast section could handle I think a replacement mast section might be needed particularly if you were sailing the T20 two up.

Paul Biekers trinado is interesting but the main hull looks a bit heavy they ended up making new floats for the boat so there wasnt much tornado left in it. I suppose it depends on how racy you want the boat to be I'd get rid of any cruising pretensions personally and make it a simple raceboat. If owners want space there are plenty of cruiser/racer designs around.

You would end up with a fairly heavy boat or a very light boat depending on float and rig choice I like the idea of being able to drop back to a 150sqft main in heavy conditions it would allow you to stay sailing and powered up well after the A class cat boys would have had to pull the plug.

 

I'm not sure that the A class leave the scene quite so early as they used to, we seem to be sailing now with the soft top masts and loads of downhaul on, to middle 20's, it all gets a bit survival mode above that.

Just with the stick up only and coming back into shore with the sail down, in the 20's, you still are doing 6 - 8 knots, it always fascinates me just how little energy is needed to push a well designed light weight hull along. I'm thinking of cutting down an older sail just for those "big" days.

The older carbon masts were much stiffer than todays and I'm pretty sure that they wouldn't need much fettling to stiffen them up sufficient to take the additional loads, they would certainly be a good starting point, far more so than a heavy Ali non wing section.

Don't confuse theretical requirements to actual, under way the hulls tend to try and raise to the surface ( don't want to mention planing ) you want to only maintain stability when toodling about at the start line etc. Yes a bad skipper will turtle it if they get it wrong by poor seamanship with limited flotation Ama's but a good skipper will go even faster with smaller Amas.

 

The thread "Hybrid 6.3 metre cruising trimaran " details the suitability of an A class doner boat.

What about 20ft options what is avail?

Also strikes me that some people want an alternative to a beach cat with appropriate lightness & speed, some people want an alternative to an F22-24 with appropriate strength & offshore capability.

I would like to see this type of boat capable of sailing in up to 30knots and a decent sea.

Alex

 

Here is the link to that thread: http://www.boatdesign.net/forums/multihulls/hybrid-6-3-metre-cruising-trimaran-32810.html


Richard Woods Strike 18 trimaran: http://www.sailingcatamarans.com/Strike 18.htm

-------------------
Alex has a good point. The discussion really breaks down to whether you want a mini cruising boat like the Strike 18, or a sportboat like the Weta.
Idkfa's earlier A Class tri(A Class ama's-post 34) has an approx. total weight including two 165lb/75kg crew of 660lb(300kg). According to Richard Woods figures the nominal total buoyancy of an A Class hull is 672lb(305kg). So total ama buoyancy as a percentage of total weight is slightly over 100%.
The Weta weighs 220lb/100kg(all up) +165lb/75kg(same single crew weight Idkfa used)= 385lb/175kg and the total ama buoyancy is 340lb/154.5kg or 89%.
--
As long as the A class hulls were used for a light weight sportboat they will be fine. In post 35 I showed that if the A Class amas were sailed to their normal immersion as a cat( with the windward hull just flying +1" Max before depowering= 330lb/150kg +56lb/25.5kg(est. Lb per in. immersion)= 386lb/175.5kg= nominal max immersion of the A Class ama before depowering) that the max RM would be within the design parameters of an F18 rig. I used Idkfas proposed numbers for the main hull and crew. I don't think the A class hulls could be used reliably with much more weight than Idkfa proposes at 660lb/300kg.(all up sailing weight). So you would have a larger version of a Weta-a daysailer not a "cruiser".

 

Doug

Do you have the Max design windpressure (prior to depowering) of the A mast and sail combo and also the CE height ?

 

--------------------
No, but I'll try to find it. Most beach cats are in the vicinity of 1.8-2lb per sq.ft. design pressure with the crew on trapeze and the windward hull just flying. The max RM is easy: 1/2 the overall beam minus 1/2 the hull beam times the hull weight=Hull contribution to RM(for utmost accuracy you'd have to subtract a bit for the leeward movement of the mast cg). Then for the crew contribution to RM: 1/2 the overall beam minus 1/2 the hull beam + 1/2 the overall beam+ 3'(crew on trapeze) X crew weight.
Then you just need the CE. That can be estimated by: mast height X .4 .
-----
For max windpressure: Total RM divided by CLR-CE, with the result divided by the SA in sq.ft.
So very approximately: 165 X 3.5= boat contribution to RM= 577.5ft.lb.
crew contribution: 3.5+4+3= 10.5ft; 10.5 X 165= 1732.5ft.lb
Total estimated RM: 2310ft.lb.
Estimated wind pressure: 2310/12= 192.5 ; 192.5/ 150= 1.28lb.per sq.ft
That should be in the ballpark but seems low-I'll get the actual boat info later and re do. It probably is in the ballpark because I used 8' for overall beam and 12' CE(based on a 30' mast).
=========
UPDATE-10/10/11: This is close to being accurate. According to Erwan in the A Class thread the mast length is 9 meters-30.32'-I used 30'. Beam is 7.5' instead of the 8' I used but all in all I think the wind pressure(before depowering) is close enough for jazz-and well below an F18.

 

I suppose the thing is we are awash with cruiser/racer style trimarans F22 is going into production and the base kit price is proposed to be somewhere in the vicinity of $25,000. If you wanted a very basic cruiser/racer trimaran in Australia early trailertri's and Bucc24's are now available in the $10,000 - $11,000 range and may be a better fit for your needs. You can still race these boats at a club level but you wont be at the front of the fleet more towards the back as they say, compromises.

A really light homebuilt sportboat trimaran in the 20' range has not been tried as far as I know in modern times (the bunyip ply daysailor/racer trimaran by Lock Crowther was a good success lots of plans were sold and boats built in the late 60's). I like the thought of a lightweight trimaran that sits somewhere between an A class catamaran and an F18 catamaran in performance I still think you could have a very basic lightweight cuddy on this boat too and it would be quite seaworthy.

Three metre waves should be no problem I've sailed those on a 16' beachcat it wasnt comfortable but it was safe idkfa also doesnt mention if their short or long ocean waves. If they are longer duration ocean waves no problems at all short chop that is steep is more uncomfortable and dangerous.

Also keep in mind that if you sit two bodies on the windward side of the boat it will decrease the ama immersion on the lee considerably crew weight is very important on a sportboat of this size. Hiking makes it a different boat to the more cruising oriented daysailor concept though.

 

There are indeed a myriad of 22 - 24 ft designs available, mostly with cabins of some kind for a good reason. You need to sell 20K of costs to the wife by saying whilst not racing we can cruise with the family

 

====================
Olden times:this is a 20 fter I designed and built in 1971 with a planing main hull and amas similar to Hobie 16 hulls except symetrical. About 450lb and 250 sq.ft. SA. 16'beam. Had a retractable centerboard and small ama fins. It was a lot of fun back then. I also did a few other experimental tri's including a 80lb 14 fter with an unstayed 80 sq.ft. rig and wee small amas( 1 cubic ft buoyancy) ,8' beam.

 

Sorry Doug, I did not word that very well not suggesting your a fossil or anything. The thing is though as you know so many materials have become available for our use since that time the whole platform can be so much lighter, stiffer and stronger so good ideas are worth revisiting.

 

T20

-----
I got it, Corley- not a problem- though I think "antique" would be better than "fossil". I know some fossil's but, of course, I'm not one.... Just used it as an excuse to post those pictures. Funny to see the old boat and compare it to my "new" thinking....

 

Its all about the initial selling job isnt it? Most of our club boats have no real interior all the loose bits, mattresses and seats have been removed. It's a gradual job "hon we wont be needing this bit for a while, how about I leave it off the boat in the garage at home so it doesnt deteriorate in the UV and sea air"

 

I'm bidding on an auction site for an early boyer A class. If I get the boat at a reasonable price I'll build a prototype of my vision for a light division 1 T20 ply trimaran using donor hulls.

 

T-20, Div. 1, minor details

-----------
How long before you know? My boat, with the gantry included, would be a bit over 20'. On many dinghies the gantry isn't included. It's a minor detail but a Div.1 boat 20' long might well want to add a gantry for increased pitch stability.
Whats the concensus here on that? And are scratch built and designed boats-no donor hulls for amas "legal"? My boat has a "donor" main hull...?


"donor" main hull: (unmodified)