OK peoples, max radius is 2 to 3 mm and yes fit the beam out and trial fit it then strip it down then get it anodized. When you have an existing beam to copy off, no need to trial fit,

Matt

Make sure you measure the boat for maximum width before drilling holes. Don't drill any holes in the beam until you check out the hull where it will be mounted, you may prefer to plug the holes and put bolts in a new area

seeing I'm going to replace both the beams on Karp I may as well 'radius' the beams while I'm at it.....
YAY .... no more scun skin

How would a router go at making the radius edge over a flapper disc I have heaps of bits that would make a very nice curve and several that I'm not fussed in loosing ...

Other than that does it have to be a curve, could I for example simply bevel the edge with a plane ???

To support Peter’s suggestion and with no malice directed to Matt, if you decide to remove the corner/arris of the beams, BRR states a maximum radius of 2mm. BRR 6.2 The main and rear beam shall be; a rectangular hollow section 50 mm plus or minus 1 mm by 39 mm plus or minus 1 mm by minimum 3 mm wall. Beam edges may be rounded to a maximum of 2 mm radius. From the Building Instructions: A 2mm radius arris can be removed from the cross beams. This is a restricted measurement! Make a radius guide by drilling a 4mm hole through a piece of sheetmetal. Remove the corner along the tangent lines. See Diag. 8.2. Remove the beam arris. Plane off the bulk with a woodwork hand plane, a No.4 or No.5 is ideal. Draw file with a mill 2nd cut file to final shape. Smooth with emery cloth.
I have heard of using a router with a radius cutter. This would also work but ensure the correct size cutter is selected and it is tungsten tipped as a HSS will dull before completion. Removing too much material from the corners will compromise the strength of the beam and it is a fact that mozzie beams are notoriously under engineered.

I find this all very interesting but I am moved to ask, why put an external radius on the beams? Is it for appearance, comfort of the sailor(s) or weight reduction?

hi Greg,

The main reason is for the comfort of the sailor. The new beams have a sharp square edge and they can cut if you slide into them etc.

Yes that's right, I have cut myself lots on sharp beams

I see. Thanks Peter and Matt.

Whilst Karp's & Shy's front beams arent radiused I certainly intend to do the new beams on Karp prior to anodising, and Shy will get the router treatment after all the burrs have been sanded off

Shy's front beams edge is nasty, years & years of mast raising/lowering and other unknown gear being dragged over it have left a serated edge specifically around the mast base ......
The marks on the boom would suggest it was loosely tied there during transporting....

I think that the removal of the edge other for human skin would be to stop such flaring of the square edge......
I dont hold much hope but I'll try for a pic of the nasty edge on Shy.

Is there any advantage in cutting a radius on the rear beam ???
There's a great big traveler beam sitting on them !!!!!

When sloop the skipper sits on the rear beam and the crew is trapping with foot on rear beam.... Nasty when something goes wrong with sharp edges

So a quick update:

Got internal radius beam sections from Capral as per Matt's notes and used a Tungsten Carbide 2mm rad router bit on a hand router which worked a treat to produce a consistent 2mm external radius.

In disassembling the old front beam and jumper strap assembly I noticed a fair bit of corrosion on the dolphin striker tube module so will have to replace this also. It also didn't have a great amount of tension on it which is a bit of a concern which leads me to the question...

I am a little perplexed why Mozzie setups don't have any tension adjustment in the design of this part. On my previous cat (a Paper Tiger) it was essential to have a bit of pre-tension on the front beam assembly to ensure no sag of the beam when under full load, and when new it allowed the jumper strap to settle and then be rechecked/tensioned as required.

Is this something the Mozzie doesn't need or is not allowed in the plans? To me it would seem like a good idea with the high loads on the rig with spinakers/double traps etc??

I was thinking I might replace it with this type of assembly..
Dolphin Striker PT

Be interested in anyone's thoughts and latest setups.

Cheers
Doug

I've seen a few that have had a non-adjustable striker setup, they are however pre-loaded during the build..... these boats have been in the 1650 numbers and up

Both my mozzies do have an adjustable striker as does the plan set I have which specifies a 5/16th stainless bolt with locknut & adjuster nut being tig welded to the down tube under the mast step.

I recall breaking a bolt on my first mozzie (245) way back when....

With the non adjustable they were preloaded but the strap was also riveted so the strap didn't slop around and elongate the bolt holes thereby undoing the pretension, if the rivets are snapped off more than likely there's not much pretension. Bit hard to check as you need to take the main beam off. Can be scary to see how much corrosion there is between the stainless strap and aluminium beam

I'm currently replacing the beams on my boat and when i put the striker back on i will be adding a tensioning device to it. There was little to no pretension on it b4 which is probably why my beam had cracked.
When rigged (i run a pretty tight rig) my front beam usually sagged up to 5mm and probably more whilst sailing.
Also where the strap bolts to the beam add a square/rectangular stainless washer that spans the width of your strap so the strap won't stretch and possibly break at the bolt.
i hope that answers your question Doug

The strap is pre tention but after a while the beams settle in and the strap stretches. With no load on mast step the beam should have 3mm rise tension so when boat is loaded up the beam should be 1mm rise or straight. im adding a tension device to mine tho. cos I can

Is 3mm enough? most cats i have worked with require 10mm. 3mm would be pretty easy to compress under sailing loads, especially sloop rigged.

i stand to be corrected, i looked on the plans and the building rules and says nothing about tension, so personaly i would tension the beam so when the rig tension is on it has a positive bend of 1mm

does this help ?????


Scorpian's setup.....



TwiceShy's setup.....

old trick from former state tittle holders....



hang the boat from a solid beam / rafter etc so that only the transom is resting on the ground

that hanging point is the centre tube / front beam connection point, use the TUBE and NOT the beam as the tie point


adjust the striker tension until it is FIRM but not overly tight..... lockup the locking nut .... done



This is a BASELINE set point....
a starting point for the fine tuners to play from, add slightly more tension for heavier weather and slightly less tension for light weather sailors. The amount you'd adjust would be (pending on the thread's per inch of the adjuster bolt) would be at most 3/4 of a turn, fine threaded bolts would obviously require more.

Final check is to rig the boat fully and reef in the main (and jib if fitted) to a high pointing position, sighting along the beam the beam should be dead straight .... if its not you need to make further adjustments.

In the photos you have there, those bolts are a little small and have known to fail, I'm putting in a 1\2 inch Unc thread as the striker post has to 19mm, so I will lathe up a nylon bush to keep the adjuster center of the 19mm tube