Hi,  

David Doran here.  Just to clarify a point,  I have not done that much sailing yet in my Wayfarer conversion, so my conversion maybe not the best example to use. 

 Also, my sail area is approx 10.5 SqM versus 13.5 SqM for the Bermudan rig so it it going to be less stressed anyway.   

I ended up with the mast in exactly the same position as the original Bermudan with a steel tabernacle fitting into the original GRP tabernacle and tapering outward above the deck to the size required for the 3" mast and extending above the deck for 0.5M.  I actually used a piece of box section steel which I tapered down to fit into the GRP tabernacle.   

The entire 5M Aluminium tube is above deck level.  The tabernacle is held in place by 3 short shrouds to the original forestay fitting and 2 U-bolts which I bolted through the sidedecks which I reinforced underneath. I built up in front of the mast foot using timber and used the original backstop pin in the original mast foot track.  

My current sail balance in front of the mast is 29% sail area and the split and I think that for more positive feathering, which is more essential in an unballasted dinghyy than a keelboat, I will build another jiblet panel to bring that down to 25%.

Though I have to say that when I did manage to keep the centreboard fully down when going upwind (it had a non-functioning friction brake), the boat balance was very good,  the tiller was only a couple of degrees off central and I could almost sail it hands-free.  

As soon as the board slipped back up a bit,  I needed much more tiller to keep it in a straight line,  so the hull balance/mast postion seems fine.  

I built the rig the size I did because I need a shorter mast to get under a bridge.

It's too early yet for me to say how truly successful my conversion is.  I've only sailed solo in it and need to do more sailing until I am fully satisfied that it is safe to carry a passenger with the setup I have.  Health and Safety and all that.  

Best of luck with the build.  Dave.   

Thanks for the response!

I know David Doran used 3" x 10swg for his Wayfarer dinghy without issues so far.

I would guess that a Wayfarer is similar enough to an OsloDinghy for our purposes to be a good model. 3 1/2" x 10swg would give a yield moment of around 3000Nm so that is probably the optimal tube so far. These tubes are not expensive so I can try it and see how it goes - I won't be loading it up much anyway.

Arne - the figures I've got are 260MPa for the breaking stress, 170MPa for the yield stress. I'm using yield stress in my calculations. Is that the right thing to do?

I'll post more as the design progresses.

Thanks again,

Martin

Remember when comparing mast weight you are comparing a free standing mast with no wire rigging to a bermudan mast with a lot of wires attached, they form part of the weight that you are comparing to. The wire can often almost double the total weight of the rigged mast compared to the bare spar.

All the best with the project, David.

I'm trying to get a handle on the feasibility of converting a Wayfarer dinghy to junk rig. I know this has been done before - thanks for the help everyone! - but I'd like to go through the full exercise anyway.

The profile seems to work very nicely with a copy-and-paste of Slieve's Poppy Split Junk rig. I haven't tweaked it in any way but as a starting point it looks good. I think it will be possible to mount the mast just aft of the bermudan mast so I don't need to make any irreversible changes to the hull.

The current sticking point is the mast section. People hike out on dinghies to keep them upright, and on the Wayfarer there could be heavy people on the rail. This is potentially a big righting moment for the mast to cope with.

To try to get a handle on this I've taken some cross-sectional drawings and played with them. Best guess is that with two people on the rail (150kg total) righting moment at 30 degrees is about 2500Nm. Since this doesn't take acceleration into account, nor buckling of the mast wall, the 2x safety factor suggested by Arne seems about the minimum possible. Thus the mast needs to resist a bending moment of over 5000Nm before yield.

These are some figures for Aluminium tube in 6082 T6 from Aluminium Warehouse:

4 1/2" x 10swg (114mm x 3.25mm) seems about the minimum required. This is also about right for the slenderness factor. Downside is that it will be much heavier than I'd like - maybe double the bermudan mast even without a wooden topmast.

Does this sound like I'm on the right track?

Thanks!