T20 -- New development class

The A hulls are not much smaller in volume if you take the difference in weight, the A hulls are typically less than 20kgs and the F18's probably around 40kgs +. ( = 20 litres of hull flotation )

The other concern you have is that the F18 mast is quite short and you will never be able to get the high aspect sail area you want to use the additional RM created by going down the tri route. The mast and sail area is set up for max 1.25m RM of the cat, not the 3m you will achieve in the tri. Wider RM means smaller AMA's needed which means less drag whilst in the water.

There are a lot of Tornados sitting around at the moment + Nacra 20's all at bargain prices ( probably less than a F18 ), perhaps the answer is to use the Nacra 20 hulls as Ama's, a central smaller ( say 18ft ) pod and crew space and by running bigger jibs and genakers, fly the central hull as much as possible as per the Seacart.

 

"Are you not getting a bit over excited about the F18 donor boat"

Agree the f18HT wold be near perfect as a doner just very rare.
A class offers good value for older boats falling from grace. More variation in design though.

Carbon mast a starting point would be a lot easier to reinforce..

Looking at a working main only doner boat:

Hulls turned into amas could be made longer with stern extensions. Got to be the easiest way to add volume, wider tri for a need to add volume up front or shift CE back so not ideal

daggerboard placement moves forward as platform becomes wider. Never really got comftable with this logic but as a tri is wider does the mast location needs to shift back relative to daggerboard?

Be nice if this could be balanced by just adding a small jib,and keeping relative mast location. Anyone have a rule of thumb here?

This would make a very dive prone boat...

 

I think you have to be very careful using donor rigs. For instance, an F18 with two guys on the trapeze has a maximum righting moment(RM) of 5024ft.lb. where a Div. 1 (square w/two trapezing)) tri has 10185 ft.lb. of RM! Bradfield used a 20' cat rig
temporarily on his 18' tri with full knowledge that he can't max out the boat w/o the rig breaking. And his boat is capable of developing even more RM than a square tri with two on the trap.
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One more point, again: you may come up with a tri using 18 hulls but it probably won't be faster than a beach cat-in fact it will probably have relatively low performance-compared to what a tri 18-20' could have.
There are a lot of small tri's like that-thats where the reputation of a "beach
tri"as slower than a beach cat came from.
In my opinion, a square or over square tri could be designed to be sailed with your feet in a center cockpit where the furthest you have to move each tack is the other side of the cockpit. Not only that, by using foils to lift the main hull early the "firehose effect" (huge amounts of spray directed at the crew) would be eliminated. Something to consider for a boat that would be incredibly comfortable and faster than the local beach cat.

 

A Class,

weight 75kg + helm 75kg = 150kg
Tri version = add vaka 75kg + crew 75kg = 150kg
150/150 = 100% increase in ama disp.


F18,

weight 190kg + (helm&crew) 150kg = 340kg
Tri version = add vaka 75kg
75/340 = 22% increase in ama disp.


Multi 23 mast height 10.8m, main area (inc mast) 19.5sqm, dry Disp 300kg, beam 4.8
Mast dimensions (152mmX76mm, approx circum 350mm)

T20 mast height 9.15m, main area (inc mast) 17sqm, dry Disp 265kg(?), beam 4.6
F18 mast dimensions (circum 385mm)

The class could supply at cost carbon crossbeams that incorporate folding mechanism. That could bring weight down and speed-up construction.

 

T20

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So lets look at the A Class version(A Class amas only) which is 253 lb.(115kg) lighter with the same crew,same sail area as the F18 version based on your figures above.
Lets assume a 15' beam overall. That would make it about 7.17' to the center of buoyancy of the main hull from the immersed ama. The approximate CE of the F18 rig is 12' above the center of lateral resistance. So:
1) the ideal buoyancy to sail the A Class hull(ama) at is approx. 330lb-where the windward hull is just kissing-that is the design displacement-lets add 1" additional immersion as the MAX immersion, which adds 56lb so the total MAX immersion for this design would be 386lb.
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2) 7.17 X 386=2767.6ft.lb.= RM due to the buoyancy of the A class hull at optimum immersion,
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3) Lets say the 330lb crew is sitting right at the edge of the windward hull, which makes them about 7' from the CB of the main hull so:
a. 7 X 330= 2310ft.lb. RM due to the crew,
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4) Total RM =#2 +#3= 2767.6+2310= 5077.6ftlb RM
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5) Wind Pressure: 5077.6/ 12(height of CE)=423.13 . Divide that by the sail area(227.6sq.ft) and you have 1.86 lb/sq.ft pressure which just so happens to coincide closely with the Max design windpressure(prior to depowering) of an F18 cat.(1.8-2.0 depending on crew).
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So by using A Class hulls(amas only) you have a lighter boat (253lb lighter), using an F18 rig within its design parameters.

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This seems like the way to go because it keeps the rig within its design parameters and keeps the boat significantly lighter.
and would be designed NOT to fly the main hull.

edit--Note: A Class hulls"ama's only" was in this post from the start but I added it in a couple of other places for emphasis .

 

By my calculations the F18 version is 340kg + 75kg = 415kg. Compare that to the much larger M23 at 450Kgs, which is a purpose built tri and you will see that the F18 is probably goner be a heavy cart horse. Now being a cart horse is never going to win many events as a lines honour machine, but the handicap systems is so against light weight machines that on handicap terms it may work with a larger mast and more sail area.

I keep stressing though it maybe just simpler to take a F20 which is heavly over engineered, fit it with 10ft beams and a small crew pod and you may have your cheap fast fun racer.

 

Yeah but its not a very interesting development platform if the T20 is going to be raced against other boats built to the same platform rules then it doesnt matter whether it is faster than an equivalent cat. You will see in my early replies I concede that a custom built trimaran will be much faster (and much more expensive).

I think the only reason that the class could succesful is that it is something different from a beach cat and provides an interesting development challenge. If you make it a cat with a vestigal pod its not a trimaran anymore even though it might be faster.

 

T20

Why not go with the version using the A Class hulls for amas?
-- lighter
-- fewer modifications
-- works with F18 or similar rig(without reinforcement)
-- doesn't fly the main hull if depowered in time
-- "A class" ama sails on its "normal" heeled waterline or close thereto.

 

T20-d2

Doug, I spent 1/2 an hr at 20-22K on an F24 in quartering following open seas with winds over 18K. I was alone in the cockpit with the spinnaker cleated but my hand on it. The crew (owner) weight forward, was at the mast. My heart rate was say 90, low, cause I was surfing on the volume of the centre hull and it did not matter much if I stuffed the ama.

To me that's the value of having a centre hull, for its volume, when you stuff the ama, in waves, like off-shore and not on the beach with wave high at 30cm. I'm sure an over square cat with a pod (tri with fully flying vaka) will be faster on the beach, but so will your heart rate, waiting for the moment when you get flung past the mast!

I'm looking for the T20-d2 to do double digits upwind, unlike the F24. All that said, "the manshed" is working on a divison1, so there will be a showdown, so until then....

To me, at the moment A Class hulls are just too low volume.

Over square cats don't work (off-the-wind), AC45 and Marstrom32 are 1/2 square (o-t-w).

Tris, just flying the vaka work. just. having inertia helps too.

 

T20

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I wasn't suggesting A Class hulls for the main hull-only for the ama. Do you still think they are too low volume for that?

PS- the total volume of an A class hull would fly the main hull before it pitchpoled or capsized.....

 

The problem I see is cost you have to buy two donor boats unless you can get a seperate rig from the F18 or something similar most of the time owners dont seperate rigs from cats. The A class rig would be too small to drive the T20 platform. Even an old boyer A class would still sell round the 4k mark in Australia thats quite a bit of extra coin for some donor hulls.

I like the idea of a class association providing beams, hulls etc but at the moment there is no class just an idea until money is stumped up and some sort of class gets going no economies of scale exist to manufacture anything.

If you can get even a sub optimal platform started using an F18 donor hull and rig it can sometimes get things going. Theres umpteen designs drawn up that are a good idea and the best platform for their particular niche but they dont go beyond a prototype or do any meaningful racing. Look at the laser I dont think anyone thinks that a better design isnt possible but it has the one design fleet support to make it work so lots of people sail them anyway.

 

Folding system like an M23, I dont get that one?

 

quite true the m23 isnt a folder its just a demountable full width beam maybe he meant the F22?

 

Sorry about that, was this the first Multi23?

Part of the reason I suggested class built akas was for the folding system. Been giving it some thought, not working as yet.

 

Thats the exploder 25 if I recall correctly I like their folding system for a light boat with low freeboard it looks good (no waterstays).