Thread: multihull cross beam strenght

Peter,

If you were going to New Zealand in it I wouldn't reply!

I can't help you with the calcs, and if I were you I wouldn't trust anything someone told me on a forum! It's just silly to expect to get sensible engineering advice from somone whose qualifications you don't know.

On the other hand, Radiata is probably about as bad as you can get for your purpose. It doesn't have a consistent grain, and you'd be overdesigning to billyoh to make it work. I don't understand enough of the box design to comment, but wouldn't you be better with ply and glass instead of Radiata?

Cheers,

P

Yes I know radiata is bad

Nevetheless it is cheap, easy to work and it has a grain, thus the stenght is where you want it. I may well laminate a layer of 4mm ply on top of top timber and bottom of bottom timber. Equivalent hoop pine would cost $30 per crossbeam... i guess that is an option...The trouble is the only place I know that sells is in in seaford.. miles away

The boat is not a catamaran, but a tacking outrigger. Kind of like a sailing outrigger canoe. The outrigger canoe people are fairly helpful. There is a boat in similar class called flaquita, that just uses simple laminated wooden beams, not box beams or truss or anthing like that.

I know some heavy people, thus if 2 of us stand on it that would be 180kg. The maximum load is 220kg, because if it was anymore teh outrigger would be underwater!!

In addition I have 2 crossbeams, not 1, so in theory max load should be less, but better safe than sorry. A little aside. I am converting a 18' x 4ft dory. If teh crossbeams and ama break off, I still have a seaworthy craft in terms of a dory. It needs low down ballast is true. Good idea in theory I know.... in practice.....

I think these things come down to judgement... my gut feeling tells me it will be fine, would just like a little physics to add to that. Maybe I can get a sack of bricks... that ought to add 50kg in order to increase loads in phsyical testing. I have no intentions of doing anything particularly reckless

n peter evans

You seem to have it pretty much sussed Peter!

We built the first of the PDRacer masts of radiata, admittedly it was second hand wall lining material, and structurally about as bad as one could get, 12mm wall thickness hollow.

We only broke one of them, and it was in only one place!

We knew the risks when we started, just as you do, and ended up building the replacements out of oregon from my now almost entirely depleted collection of old pergolas.

Don't forget, to post the pics!

What are you using for steering and a rig?
Cheers,

P

If you build it like a truss vvvvvv with spacers at 45 degrees to your beam it will be extremely strong for downward and upward force but not sideways unless you put your beams on the flat.
There must be something better avaliable closer like kapur reeded decking just plane or saw off the reeds it has a straighter grain and is usually fairly available and would be lot stronger and more durable

hi all

I have made my first crossbeam as I had earlier described. I do not know if it is strong enough but I do know that it is not stiff enough. When I stand in the center (me 74kg) with the ends supported teh crossbeam sags too much, maybe 6 inches or more.

Thus I will cover the sides of my crossbeam with marine plywood to make a box beam. I think I will attach the plywood with both epoxy and with brass tacks I have. I bought a box of many hundreds years ago and never got around to using them,

When this is finished will redo the physical test and gradually increase load. Hopefully they will support 200kg load in the center when supported from the ends. If it passes this test I will be happy. First crossbeam should be finished tomorrow and then wait a couple days for epoxy to gain some strenght before testing.

n peter evans

Originally Posted by peterAustralia

I have made my first crossbeam as I had earlier described. I do not know if it is strong enough but I do know that it is not stiff enough. When I stand in the center (me 74kg) with the ends supported teh crossbeam sags too much, maybe 6 inches or more.

It'll be interesting to see whether, once it becomes stiff enough, it's too heavy!

I gather that you used the Radiata?

Cheers,

P

The limiting factor is the amount of buoyancy in your outriggers.

So If you have the volume of one of your outriggers in litres - that gives you how many kilograms of buoyancy.

That gets split between the two crossbeams.

So if the deflection (flex) is OK at around double that load then it will be OK.

So that is flex. Generally with the materials you have been using the strength is easier to achieve than the stiffness. So if you get the stiffness right there will be a lot of strength in reserve.

This is assuming not too many flaws in the timber, that your gluing is up to spec and any joints are up to spec too.

Best wishes
Michael

hi all,

small update. I have mostly built first outrigger, glued up the frame and added plywood to one side. However I have way over engineered it. The spacer blocks were 5 inches deep, adding the width of wood gives 6 inch deep crossbeams.

They are now just so strong as to be ridiculous, the windage will be high and the appearance is not good. So for a laugh I put it on its side to see how it goes. I was able to jump up and down with no worries at midpoint with ends supported. And this is with only one face of ply on the top side and nothing on underside as yet. Plus this ply was not continuous as there are more than one bit of ply and I have not joined them with but joints as yet.

When I jumped up and down gently it was fine. When I jumped a bit higher I could start to hear a couple of minor squeeks. So I will a layer of ply onto open face, and add butt joints to the layer I already have. I will then mount this crossbeam on it's side, as this is seems a better way to go as opposed to building a new lighter, smaller, less strong beam.

I am confidant that when I complete this crossbeam it will hold 220kg midpoint whilst supported on it's ends and whilts it is placed on it's side.

So based on what I have done my second crossbeam will be smaller and lighter. Instead of 5 inch spacer blocks, I will use 3.5 inch spacer blocks. This one should be strong as the upper and lower faces will be strong 15mm pine rather than thin 4mm plywood.

A photo taken a few days ago when I had just assembled it roughly and before dissambly so that it could be glued and before adding ply can be seen here

http://www.geocities.com/peterevans_33/crossbeam_2.JPG

regards,
n peter evans

The spacer blocks don't have enough bonding area to the top and bottom planks.

I suspect the thin ply front and back and fewer spacers might be lighter.

Make the spacers narrower out toward the ends - that way you get the strength across the main hull and less windage out towards the amas. for appearance and clearance keep the bottom plank horizontal.

If there is no deflection - bend - with your required load it is probably WAY too thick and heavy.

Is there any rigging to hold up the mast being attached to the outriggers - thats where a lot of the load will come from - but still the windward shroud can only pull up with around half the weight of the whole kaboodle - hulls, sails, mast, crew, bits and bobs - for the main hull to lift out of the water.

hi

the first crossbeam is finished. The second is almost done. We tested the first crossbeam yesterday with 195kg at midpoint. The ends supported by some bricks. It seemed fine and there was no deflection. So I think it will be fine. I have given up on any more stress about this. as far as I am concerned, they are OK, now time for other things to worry about.

The spacer blocks were glued yes, but they were also attached with two strong coach screws each per joint. Thus each spacer block has 4 quarter inch diameter coach screws.

After making the pine base, I then added plywood to teh sides to aid in rigidity. I use 4mm marine ply with epoxy inside and out. I used many many copper tacks to increase bonding beyond pure epoxy itself.

I am now moving onto the rudder tube. This is a tube inside the rear compartment through which the rudder stock rotates. The transom on my dory vaka slopes so much that a normal rudder is just not feasable. I would prefer a kickup rudder but that looks like too much work. So will just go with a fixed rudder. It is a bit small at roughly 1 sqr feet but if it is too small, I guess I can make another.

I will take some more photos in next few days and start a new thread...and close this thread.

n peter evans