Catamaran beams New! Improved!

[bob the builder]

and before you say anything,
yes i sanded the b!
yes i used bog, spray putty, high build (1 full can of each)
in the end, i just gave in

lessons learned?
get it right before you start sanding.
and, more importantly,
get other people to do the sanding, backpackers, friends, relatives, etc.

vital lessons i feel.



















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[bob the builder]

[bob the builder]

Design at the moment.

(trying to save weight, comment if you can!

(all sections from this G J James marine extrusion catalog (they give section modulus in the catalog)
920.145 is an example part number

450cm wide
31 feet long
hulls 90cm wide at the waterline, flat, no rocker, 15cm deep
60cm bridge deck clearance

boom 3 feet from deck, 390cm feet long
mast 10m high
sail area 15m2 main, 15 jib (center of effort 4m above the water, quite low.

Front Box Beam
is still a problem. can't decide what to do. mast section, ply box beam, or complicated welded aluminium structure.
there's going to be wood all around that triangular shape anyway. . .

at present, sorta keen on a builders tressel plank extrusion. astoundingly cheap! ten times more inertia than a mast section
(typically 200 or 300 x 50 x 3mm T6, Ix above 20 000 000) then build a box beam around this. thats what all these pictures show,
the rectangular beam at the front, for the jib, box beam built around it.

just a thought.

please comment.




Main Bulkhead Cross Beams
aluminium square tube bolted to the tops of the main 20mm ply bulkheads, and epoxied to the cabin ceiling, and another the same under the deck.
931.148 76.2 x 76.2 x 3mm 2.313 Kg/m Ix 750513




Mast Cross Beam
sandwich of 931.127 1.211Kg/m Ix 208870

60 x 60 x 2mm T6 x 250cm
deck in between (12mm kiri plank, 600gsm glass epoxy both sides
60 x 60 x 2mm T6 x 300cm

unusual i know
didn't want the mast cross beam to spoil the deck so much, so put some underneath the deck and some on top. put some bolts through them.
mast rotating for the same reason, smaller to have the little cup than the typically large deck step plate.

rather than bolted to a molding on the inner hull (like the seawind) i'm going to weld the under deck beam to flat aluminium strips,
920.145 80 x 8 1.7 kg/m Ix 341333
or
980.055 99 x 5.35 1.7 kg/m Ix 607000

and then bolt 1.5m long strips to 100 x 25 kiri plank on the top of the hull sides where they meet the deck



Wing Seat cross beam
931.065 50 x 50 x 2 .994Kg/m Ix 137 094
(is this reasonable? or should i up it a bit?)


Truss Transom
(perhaps a bit heavy.)
50mm wide is so the outboard can just fit straight on, no messing around building special fittings.

page 19 of the catalog,
941.360 50 x 100 x 3 2.223 Kg/m Ix 1011942

webbing (45 degree truss members)
931.065 50 x 50 x 2 0.994 Kg/m Ix 137094

the only real question left is should i stiffen up the truss in the z plane?
(ie welding same section on horizontally all the way across the bottom)

should a plate go on as well? 30 cm deep 40 wide 1 cm thick? (for the outboard)

beamboy (for 3m long 3mm 200mm deep T6) gives 6561590 mm4 inertia.
(which is more than a normal mast section for same job, as square section provides a 70% increase in stiffnes for 27% increase in weight,
or, 34% increase for the same weight.)
and
115mm deflection cantilever, (3m long, 600Kg load on the end, equivalent to lifting the entire hull on the one beam)

truss is more than 16 times stiffer, so
1/621 deflection / length
thats 0.16% deflection lifting a hull, (on this single beam!) and i can weld crap all over it (which makes me extra happy)



why all the cross beams?
if i want that huge open cabin space, and the truss transom,
the deck and hull sides have to be kept rigid and non-buckling.

what about the torsion resistance reduction?
the idea is to make torsion affect the entire structure, NOT a single isolated beam

all these cross beams bolted to bulkheads should do it.



please comment on anything!,
cheers big ears,
mal.















from here
http://uk.geocities.com/fnsnclr@btin...reads/fred.htm

[bob the builder]

station 1
30cm back from bow edge
nose

station 2
90cm from front
box beam front

station 3
180cm
box beam rear

station 4
390
main bulkhead

station 5
420
cupboard

station 6
630
wing seat

station 7
840
transom

station 8
930
stern

Originally Posted by bob the builder

Design at the moment.

(trying to save weight, comment if you can!

Just one,
find a good NA, what you are doing is not designing, you play designing!

[Ad Hoc]

"...Just one,
find a good NA, what you are doing is not designing, you play designing!.."

First sensible comment

[waynemarlow]

You could also use the bimini rear top section as the upper part of the truss, mount the main sheet and traveller on it as well, just probably only 2 diagonal connecting braces needed to make the single rear lower beam and top beam a true load bearing truss.

Originally Posted by Ad Hoc

"...Just one,
find a good NA, what you are doing is not designing, you play designing!.."

First sensible comment

well, I have bright moments! Though with decreasing tendency.........

[bob the builder]

the model was to see if i liked it.
i like it.

so i'm going to build a strip plank model and i'm looking to finalise the design now.

(on the scale model i'll be moving the mast back or forth as the mast placement i think is the only radical idea in the whole boat. theoretically it should be 2 1/2 feet forward, but i've seen so may different mast placements that i'll really be surprised if it makes a whole lot of difference to the balance. (weather/lee helm going upwind) and there are books showing how to trim for auto sailing without autohelm or wind vane or rudders)


any ideas, let me know.


Why is it designed this way?
Why is it built this way?
the reasoning.


Simple full sloop rig. The mast goes behind the hard top. The main sheet track along the transom. Jib sheet to the rear cabin tops.

A forward of the mast hardtop, and high foot jib for great visibility.
Unusual yes. Built for the pacific. Lots of reefs. Instant visibility all round is important. Any person in bed is only 2 feet away from all instruments and all round visibility. And dry in all weather.


Large open areas instead of cabins? Lots of flat open areas? Huge open spaces, and huge! storage for toys?
i've lived on a yacht before. stinky. fumes, smells, airless. you sweat all day and all night. this is built for the tropics. the entire front opens up as well. great for when you park on a beach.
Standing headroom in the kitchen/toilet/shower area, and hard top cockpit. 4 1/2 in the main cabin, bedrooms.
(all hulls save the kitchen/toilet/shower area are closed over)
good to live long term on.
lots of flat open area to DO things on.
even the large professional cats i've recently seen are far more cramped than this design.
anything nice to live on is far too large for me to feel comfortable handling all by myself.
top of my list was good to actually LIVE on the thing. it's no fun if there isn't enough room.
this small boat has more flat usable room than all the large pro cats i've been on or looked at.

Huge windows on the front that open all the way (fold over backwards)
nice area. for chairs and stuff when parked. may even have a tent and poles over it.
the deck/walkways to the sides of the windows are to stiffen the top of the window frames.


Stronger than average? (lots of aluminium beams and wood.)
strong enough is easy, long term stiffness harder.
felt old FRP (aka expensive modern foam cats). feel like walking on jelly fish. they loose 10% of their stiffness in the first year etc.
been on modern foam cats. remind me of yachts. i don't like the tunel feeling.
weigh far too much to beach without stretching something (no foam panel can take 5 tons on a single point)
built to last 30 good years of cruising, and still be stiff.
beaching often



Why the wing seats?
Vital, thats why.



3/4 bridge deck gives more hull slam
Not if you don't sail against the weather.
how often do you intend sailing in storms? this is not built to win races in any weather, but as a camper for south pacific island hopping.

Single centerboard
will give indifferent to weather pointing.
true. having sailed against the weather before, and now having all the time in the world, i prefer to wait for wind and weather, or turn the engine on.
centerboard 7 feet from the transom?
gives surfing stability in the open ocean, rather than turning in a shorter distance. hopefully i'll never see a marina.


No rocker
it's not that daring, so don't get excited.
when your cat weighs 6 tons, then you need rocker to turn the thing.
least wetted surface area from rocker curve? true. but this also has the largest underwater cross sectional area pushing water ahead.
slower in calm winds, faster in average winds. there are online hull resistance calculators, and the small amount of less wetted surface area of a round/rocker hull is unimportant at any reasonable sailing speed, especially compared to pushing all that water out of the way.
should surf better than any cat. (i just like surfing boats. good fun)

Flat hulls give a rougher ride.
at what speed? 20+ knots? certainly not at ten.
mainly visiting atolls, reefs.
hull depth 15cm
beaching often, so weight spread over a larger area


Why is it so small? Why is she so light compared to others?
it is really only 21 feet long.
plus
1ft for cupboards in the middle bulkhead,
6ft on front for more forward floatation
3ft on the back sugar scoops to sit on while traveling
No interior furniture. Hull framing in kiri which is half the weight of ply.

A minimum of 35 feet to go cruising?
referring to either living space, making way, or load carrying ability
This boat is nothing special and displaces 200 Kg for each centimeter over the waterlines.
Because it's a small boat, flat hulls (in the fore-aft direction) equal much larger round hulls (rocker) in terms of good passage making, surfing etc.


Boom is very low.
so stability is higher.

the center of sail effort is only 4m above the water, and 2.25m from the side. the boat weight is 1500Kg, it takes 750 kg on the sails to lift a hull. on 30m2 of sail, this is 25Kg/m2 (wind around 35 knots)

nice feel. gives that "yes we are sailing" that you might not experience on larger cats when the boom is 7 feet off the deck. they are also much wider and heavier ( and more expensive) to compensate having such a high center of effort.

lowering the boom and lowering the mast hight is a cheap way of increasing stability.

high rigs give better performance?
bugger. i won't enter a race then.


Center of gravity
unusually for a catamaran, is below the bridge deck floor, as there are no walkways in the hulls. instead these areas are for stores, water, fuel, batteries, toys etc. there's only empty volume above the deck.

hinged storm board gate in the transom bulkheads to repel solid water but allow it free escape


To Reduce the build time as much as practical.

Straight sides and single level deck

Bulkheads and frames are included in the strongback - so when the boat is turned, it's almost finished.

Strip planked whole - can take a day to strip plank both hulls and deck. stronger, faster.

Lots of aluminium beams - good time saved, just bolt them on the bulkheads, all done in a day.

Have a think about the build time for this design after you turn the boat over. There is not much left to do.



Weight Calculations
i've used 12mm strip plank kiri, 400gsm glass/epoxy either side, 80m2 for the 416Kg

what else do you have on board?

Wood/glass/epoxy 416
Aluminium beams 70
Bulkheads 60
Hull framing 40
Paint 25

Fit out and Rigging: 60
Bollards on front
anchor runner
anchor
chain
rope
jib self furler
front stay
shrouds
cleats x 4 on transom

Mast 2.3 Kg/m 23
Pumps plumbing 22
Engine 15hp o.b. 55
Water 100
Fuel 50
People 240
Batteries 20
Solar Panels 11
Food 24

Toys 50
Odds n ends 100

Total 1370 Kg
Normal cruising average


note the minimum i'm considering is 10mm. as will be parked often. remember i'm a novice, so please give your opinion.
i'm not really convinced 400gsm is strong enough. may do 600 outside n 400 inside. maybe 600+400 on the hull bottom exterior.
yes? no?
after 30 years of life, it would be nice to have everything still stiff.

here's a pallet of scantlings for a south pacific island hopper
kiri plank..........................................glass/epoxy
.........................600gsm= +2.4kg....400gsm= +1.6kg...300gsm= +1.2kg
15mm= 4.5kg/m2.............= 6.9 ................ = 6.1................ = 5.7
12mm= 3.6kg/m2............ = 6 ...................= 5.2................ = 4.8
10mm= 3 kg/m2.............. = 5.4 .................= 4.6............... = 4.2

80m2 in total =
..........................552....................488 ................ 456

...........................480...................416..................384

............................432..................368..................336



Changes for the next version.

the hardtop is ill conceived and poorly integrated at present.

deck camber
to build camber on just the back deck would be a real pain in the arse. any ideas anyone?

Aramid? instead of a second layer of glass on the bottom of the hulls. (it would hold together when on a reef. . .

small curve on the bow, just enough to survive any floating debris such as logs, or large steel containers



i'll be away for a week,
but looking forward to reading all your thoughts when i return.

cheers all.
mal

[Guest62110524]

bob you are kidding yourself
get your self round to my house I,ll give you some software and hopefully some pointers

[bob the builder]

onya whoosh.

kidding myself?
design it, engineer it, repeat till ready.

?



but i'll take you up on the offer!

i'll drop in for a chat one day. soonish.

(i'll send you an email, and thanks for the offer!)

cheers big ears,
mal.

[bob the builder]

wayne suggested
"bimini rear top section as the upper part of the truss"

i pondered this (and i'm still pondering making it even more so the upper member of a truss)

good idea. thank you wayne!

have just rebuilt the model with aluminium framing across the top back.



(also;
about that um, sorta lip i gave you on that other thread, um,
you know,
about you bein a sorta father figure to me n all that. . .
um,
this here is sorta um,
an apology.

)

[bob the builder]

true planing sa/d higher than 37 (according to US sail

330 sqft an 1100 kg gives me 31.5

400 sq feet gives me 37.8
so i'm thinking of upping the sail by 33%

the reasons being;
i seem right on the edge of good planing




at present, (1400Kg) i've got
115 Kg/m2 weight/ water planing area

this is roughly the same ratio as a large surfboard (7 feet long 45cm wide)

(and thats the heavy version of the boat)

if i stick to pure displacement hulls, i'll never fly, never have blue sky performance, never have some open space above, never break free of the sticky 2 dimensional drag,




fiona sinclairs and michlet are in close agreement,
for a single (700Kg) hull
around 35 kg drag at 10 knots
70 kg drag at 15 knots

30m2 sail area gives driving force
1.7 kg/m @10 knots wind = 51 kg
4 kg/m @15 knots wind = 120 kg
7.5 kg/m @20 knots wind = 225 kg

see?



i'm so close to planing,
i've decided to busta nut trying to lighten the boat, (1100 is achievable)
and up the power/weight by increasing the sail area.




who's got two cents?


mal.






ps
(michlet for a single hull 700kg, 9.3m long, i keep getting 1.12 wide so i know i'm doing something wrong), total drag 5 Kg higher than the hull i optimised with fiona sinclairs applet

the weight/area means it will start to surf at 10 knots, so i am trying to optimise for 13 (no idea really, just anything over 11 knots for good surfing)

i'm more than happy to have Worse slow speed performance (if you've got light airs, you're going slow anyway), and get the occaisional brick in the face FUN in stronger winds

you can see from my hull shape that i've been trying to maximise dynamic lift rather than low speed displacement hull drag optimisation.


if i stick to displacement, a la michlet, i'll never get blue sky, some open end performance.





?less form drag, instead lift the nose more

?water tanks at the back (and empty them when running home as well)

[bob the builder]

finished the beams

going to stick it into delftship now, then polish it, then take it to a engineer or architect.



Rick
torsion, lots of good advice, etc

thanks rick.


Wayne
turn the hard top into a giant truss
brilliant idea. have redone the mast beam to integrate in as well. whole thing is now many multiples stronger.
great idea.


Richard Woods
bending loads, lots of good beam advice
thanks richard!


Whoosh
you can't ask for more help than to be invited over for coffee.
thanks whoosh.

and all the other people who chipped in. jaydh, Saylaman
thank you all,

i stuck the probable design in a zip if you're interest (beam specs etc)

mal.

[Saylaman]

Good luck. Let us know when the design is finished and you start building.