Hi all,

I will be doing a fairly urgent re-beam on the front of #1769 in the next week, due to a crack through both sides at the dolphin striker strap bolt. Following the advice of several experienced campaigners at the Meningie Nats, I added more tension to my beam by using washers as spacers between the mast post and strap. I think a tensioning bolt would be essential, and I'll try to incorporate one into the new setup. With our state titles in 3 weeks, I need to get this done fairly quickly.
Have read all info pertaining to rounding the beam arris, but is there a regulation/recommendation regarding the angle to cut the beam ends??

Page 22

BUILDING INSTRUCTIONS FOR THE MOSQUITO CATAMARAN
(rewritten November 1995)

Hi Hack,
Thanks to Kingy for posting the suggested design from the Bldg Inst.
The cut-off angle suggested is the 25mm off-set. There is no regulation. This angle is for aesthetics. Any greater off-set may interfere with the location of the outer beam bolt.
The tension bolt (15 x 8 ss) is required as it removes the load from the inwale, it will be compressed if you don't have it.
Study the lower diagram; it illustrates fitting 5/4.8mm(3/16") monel or SS rivets. These rivets will reduce the sheer loads from the strap-to-beam attachment. Do not rely on just the tension and beam bolts to resist this sheer force.
The over-all length may vary but measure your existing beam for an indication. 2147mm was lifted off #1765 which has moderate hull width, wider than some, narrower than others. It has a maximum overall width of 2185mm.
I assume you will recycle major components.
Contact me direct through the MCASA if you need more info as the diagram is my production.
Phillip P

yeah sorry about that bit being in the shade of my hands and iPhone but it wasn't what I was after for hack at the time

if you click on the pic it should take you straight to photobucket for a larger version

Cheers Kingy and Phil,

I had a feeling the answer would be the one you provided, but I thought I should check.
Thanks for the useful diagram.

If I feature in this weeks race report, then you'll know the job was successful on Saturday.

Cheerio

Some more questions on beams.

What are the pros and cons of painting vs anodising vs powder coating?

Does anyone have any experience with non-anodised aluminium coated with Nyalic?
http://www.thesailboatshop.com.au/category-s/103.htm

I think the main benefit of anodising is that the whole beam (including the inside) is protected, whereas if you were to paint the beam, only the outside is. Anodising technically shouldn't change the weight of the beam compared with painting if you were really going for the nth percent on weight reduction?

as said..... anodising is full coverage protection whereas paint doesn't give that level of protection.

Another big disadvantage is that the paint chips quite easily, right in the area where the mast lays while you are attaching the shrouds prior to standing the mast or de-rigging the boat.

and one I hadn't figure on, the painted beam is slippery as heck when wet and that makes it a lot harder to clamber back on deck after a capsize..... ofcourse this isn't an issue if you don't get the boat wet or capsize it

Thanks for the responses.

Is it true that anodised is slightly 'weaker'?

no not at all... quite the opposite actually

Anodising is (simplest explanation) an electrical coating of the alloy much the same as chrome plating is, in the case of alloy, the anodising process forms a layer of aluminium oxide - Al203 (corundum), which is very hard, relatively inert, electrically insulating (which is why its suitable for salt water use ), and can absorb dyes to colour the alloy.

there are 3 main types of anodising.....

Sulphuric Acid Anodising -> used mainly for hard working areas where heavy use and wear n tear would normally cut down the protective surface in a short period of time, EG outdoor furniture, window frames etc etc

Hard Anodising -> significantly harder, thicker, denser films with a higher resistance to wear, corrosion, temperature effects etc.... your cars engine-bay will have components treated at this level

Chromic Acid Anodising -> These coatings are thin, and relatively soft and generally only used in specialised applications, the name plate on the trophies in your cabinet are chromic acid anodised.... good for looks and not much else

for beams on a boat, then the SULPHURIC ACID ANODISING is the best, its also the most commonly used form of anodising



https://www.youtube.com/watch?v=O0GQPHh3Szk

Thanks Kingy.

anodising is the best, powder coating is good also, as said paint and powder coating adds a tiny bit of weight.

Key is the prep work to get a good finish. Sand with the grain (yes alloy has a grain) with 240 grit and sand with different grades of scour pads.

Matt