how would you rig this?

hi all.
i'm designing my own catamaran, basically a tornado cruiser.

full length bridge deck, 35 by 16 ft, 85 cm clearance, 190 headroom in hulls and on deck,
all other specs yet to be finalised,
but roughly;
strip plank hulls, deck, and cabin 15 to 20 mm kiri, 700 uni, 300 triax epoxy either side of the sandwich. displacement fully loaded min 2000Kg, max 2500 Kg
The goal is to have a cruiser stiff and still good after 30 years of sailing.




What I need are some rough rigging guidlines, to estimate my costs, chainplate positions etc etc.


So who thinks what?

 

Hi DungBeetle (nice name), I've read a lot of bad things about full length stern to bow bridgedecks. They are apparently dangerous in large seas. A good cat is designed to keep the weight off the ends of the hulls. My own cat design was weighty on the ends so I lengthened the hulls but kept the bridgedeck centralised, and hopefully it isn't too bad now.
Going by your proportions, I would just shorten the bridgedeck.
As for the rig.....this is where I have come to a halt with my design. It becomes complicated to work out all the different forces. Boat designers use real life models and complicated VPP software to choose the best sail plan and best position to find the right balance. I don't think it's impossible for you to make a good choice, but as for me....I'm just leaving that part to the professionals.
If you go with a fractional rig you can't go too wrong.....you'll have the best all round performance, provided your design can stay upright with a tall mainsail. How tall? How easily does your boat tip over?

I hope others can give you better advice than this! I'll be reading if they do

- Richard

 

true,
we'll have to go to a pro rigger eventually, but surely we can iron out some design bugs and get some good ideas from people who have been there, done that before us.

others will chip in as we go



how about i copy these guys rigging? Renolds 33 http://www.r33.com/en/rs33/design.asp
because they say
"Safety is our number one concern"

so proportionaly, a 55 ft tall mast, simple 3 stay, one spreader rig, 1000 sq ft of sail.?
(since safety is high on my list also)





i'm thinking plain vanilla full sloop (looks better than fractional
3 stays, (simplest, least work
one spreader, (max value from one
10 m tall mast right in the middle, (as the minimum, reynolds 33 proportional = 55 ft as the maximum
3mm thick 20 cm round

a 110% genoa, (110% sounds better than 100%
13.5 degrees to the chain plates (about average
10 mm spectra rope chain plates (cheap n easy as
50 m2 sail area all up, center of effort 3.7m (25m2 front, 25m2 main; you gotta start somewhere

so somewhere between 6 mil to 10 mil 1x19 stainless for this rig (light V's 500% saftey margin

reef at half one hull weight (1250 Kg per hull, 50m2 sail, so just over 20 knots (F=.6V2



who's happy with what?


i'm thinking spectra shrouds, but how long will they last?


cheers all,
mal.

 

richard, i also saw your cat design,
and,
i think you should let me design it for you (as per your kind offer in post #12)
i'd go with a solid floor, the reynolds 33 seating, and put the same kind of solid shelter over the hulls (in case the weather turns COLD!)





re full bridgedeck and hull slamming;
all true, but gentlemen don't sail to weather.



also;
the reason i'm asking here about rigging is that i've read several scientific methods for calculating rigging specs, and all are drivel, giving wildly different results, based on different starting assumptions. and the necessary guess factors far outweigh any true calculations.

if these methods came to within 5% of each other i'd call the process scientific and believable.
but they give 300% difference.


one method i did like was based on sail load (max sail load plus safety factor below 20% heel angle, and move the mast foreward 3 feet to avoid straight 90 degree heeling ) but then some divide the max sail load by three for a 3 stay mast, others include mast base and boom sheet, others don't etc etc etc.


so i'm after real experience rather than engineers.

what works well. who out there is happy with what.

 

Well, the rig calculation have to be based on maximum righting moment of the craft. Sail load is secondary, it can not be bigger than the craft is able to carry.

What scientific methods do you mean?

 

if you do a google you can find many methods, formulas etc

all start somewhere, and then make assumptions

some are dynamic models (very complicated), some empirical, some static



to quote this guy

http://www.boatdesign.net/forums/showthread.php?t=5242
This really is not trivial static engineering, without a thorough course in Dynamics and some knowledge of systems higher order differential equations you havn't a hope in predicting the rigging loads in anything other than a flat calm. If you come up with static figures then what factor of safety do you apply to account for the dynamic loads? Suddenly you have a fiddle factor and you are straight back to a rule of thumb.

well stated i reckon.



so a much easier way is just to ask people

what advice you got for me terhohalme?

 

I have used two rigging scantlings:

Nordic Boat Standard, look at Larsson, Eliasson: Principles of Yacht Design, Rig Construction and

Bureau Veritas, look at Claughton, Wellicome & Shenoi: Sailing Yacht Design Theory, Mast and rigging design

Also standards ISO 12215-7 (Multihulls) ja 12215-9 (Appendages and rig attachement) contain valuable information of rigging loads, safety factors and righting moment.