Moth 11 home build shroud placement

I have built a Moth 11 stitch and glue and would like advice on shroud placement. The boat has a cat rig. The distance between mast step and chainplates is 9" back and 19" to the side. It is a Dwyer aluminum mast with an oval cross section 1.75" x 2.75" and a total height of 16'10". There is no backstay. The sail has a luff of 15'. I am using a Laser boom. If I stick to the rule that the side view angle can be no greater than 5 degrees then my shrouds would attach to the mast 103" off the deck. That seems a bit low for a mast that is 202"and I was wondering if there are other considerations. Do I add spreaders?
(The actual mast step is not attached in the picture below)

 

Have you done the calculations on the mast? Maybe you don't need shrouds. Reinforce the step and the deck thwart, maybe even build in a sleeve that reaches from keel to just below the gooseneck and you may have better boom swing range.

However, I think the moths are not intended to sail down wind, so maybe that's not a preference.

-Will

 

It seems to be similar to some aspects of an old style International Moth and as such,I suggest you take a look at this site to see the sort of proportions that work. International Moth Lowriders UK – Dedicated to non-foiling International Moths https://intmothlowriders.org/ . Alternatively look for Moth designs of the Shelley,Skol and Duflos era when they were using 15 foot luff lengths.I don't think you should get too bogged down in theories that are generally applied to heavy displacement keelboats when there have ben quite a few examples of similar rigs that worked.The mast section seems quite large fore and aft and probably won't need a prodder but spreaders might help if the sailor is anything other than a lightweight,which really means anybody over 155 lbs.

 

From the Duckworks website



Along with the given dims there and the OP's post I get this, drawing could be improved if I knew length on deck and mast position on deck (there seems to be a 3 11/16 inch discrepancy on mast position):



It does look a little short in the stays. Is the forestay going to be masthead or fractional?

PS If you can make it, it would be fun to see the boat at the Lake Pepin Messabout, first weekend in June in Lake City, Minnesota.
Pepin Messabout https://sites.google.com/site/pepinmessabout

 

The intersection of that 180 dimension with the mast is the sort of region where I would expect the shrouds to be located.Certainly no higher than the insignia on the sail.A masthead forestay would be unlikely on any dinghy as they depend n the topmast flexing to de-power in stronger breezes.

 

@wet feet the 180" is from the 15 foot luff comment by the OP, the 108.x" is a Laser boom I found online.
So one might do better to move the side stays up and the forestay down?

Here is the last little stayed boat we had in the yard, an Omega 14 (sailboatdata.com):

 

I may not have conveyed my point too well,the region where the 180 dimension intersects the mast in the 3D view is the region where I would expect the shrouds to attach to the mast.I would also expect the forestay to be close to the same point.I will repeat that a masthead forestay is never,ever seen on a small sailing dinghy with a Bermuda rig.I am curious about where the "rule" about a 5 degree angle came from.

 

One measurement I forgot: the distance from mast to forestay tang is 23". And yes the mast is a bit beefy, but it was donated by White Bear Boatworks. I started with the plans that Duckworth sells from the Italian guy. We had Kinkos print the CAD files. The panels came together, but the bulkheads did not work so we have been ad libbing it ever since. The class members have been very helpful and one of them could probably just tell me where to put the shrouds, but I thought I would try to understand conceptually.

 

Added 23" dimension mast to lower forestay tang, moved upper tangs to about 124" (sort of across from the 180" dimensions). Purple section of mast is unstayed.

 

That looks a bit more normal,but I'd still estimate that the attachment point needs to be raised a bit as the unsupported topmast length looks excessive.I would suggest asking other sailors in the class for the correct location.Small boat masts work in the same way as any other mast,but they are normally intended to flex in stronger winds.The shrouds and forestay should be at the same height and if the lower section of the mast is flexing too much,a set of spreaders can be used to control the bend both fore and aft and sideways.With the hounds too low you would increase the leverage of the topmast.There are several good books about setting up dinghies for racing and you may find useful guidance there.

The point about the bulkheads not fitting the hull correctly raises alarm bells in my mind as some stitch and glue hulls need to have the bulkheads push the panels to their intended shape.I hope any adjustments were very minor and little displacement has been sacrificed.