sweeping spreaders/moving chainplates

hi guys, i am adapting an old IOR 50' for shorthanded cruising. to stabilise the mast and reduce the need for runners i am intending to sweep the spreaders by about 20 degrees - this necessitates moving the shroud termination aft by about 19" and inboard about 3.3" while reusing the existing spreaders.

my question concerns how extensively i will need to provide extra reinforcement in either the hull or deck to accommodate the moved shroud termination. the accompanying pics provide some detail - i hope they come out, the final pic poorly illustrates the current 'D' rings (arrowed, aluminium construction) and the one of the through deck shows the same pieces either side along with a point 19" aft.

moving the shroud 19" aft places the new termination just to one side of these existing D rings. does all the force that is generated as the shroud comes under load translate into compression through the deck? ie can i simply provide in effect a further 'spreader' at deck level to accomodate the change? or to what extent should i replicate the existing strengthining but shifted 19" aft?

 

hi there I would like to help as alloy is my trade,
do you mean tension, there is no compression except on the mast
You pics are not so clear, if you email me some I will help , whatever you do it is going to need fabrication,
that boat is a not so very well built I can see small throat thickness on fillets, but she has held together!!
you see that big weldment on the boast bottom plates, well if you took that link from your new chainplate to that weldment as it is , how much angle will there be ?
good idea tho sweeping spreaders, runners are really a pain

cheers
Stu

 

stu, im not a welder so i will leave it to others to comment on the quality of the construction but she should be reasonably well built coming out of a yard that was producing americas cup boats back in the 80s. maybe the small throat thickness was to save money?

if i take the link in the weldment back about 13" it will then be above the last of the large ribs (arrowed in attached pick) and will be in line with the new swept apreaders which come back from the mast at 20 degrees.

the angle now will be approx 11 degrees from vertical, currently the rod coming down to the weldment sits at approx 15 degrees from vertical. the rod will of course now also be projecting away from the horizontal in line amidships axis by 20 degrees.

 

yes a lot of those racers were built save weight, shifting the link rod aft looks fine to me , even if the pull is not strictly in line,

Doing it that way was very good,
Sounds like you know zakkly what you a re doing, sound like you will circumnavigate?

 

stu, you are correct. i am an expert at making it up as i go along the boat is in the usa at the minute - hauled out for the winter. the plan is to fix her up and take her back home to nz, start a family and then teach my kids how to sail - once ive learnt myself!

as for these modifications, im only cribbing from other peoples ideas. i will likely put some extra strenghtening in the deck/coachroof to put my mind at ease. if the attachment to the weldment is forward of the last big rib (as arrowed above) do you think it will be strong enough or will i need to extend the weldment further aft (away from the heads)?

 

What design is she????

 

german frers from 1985, originaly built for the owners of amway.

 

my bus was in NZ building alloy yachts
pm me and we can talk, be neat
i just bought a 38 footer, in UK, Finot group, Frers is my fave designer, but i could never afford him

 

stu, im not sure i can afford him either!!

 

DrNick,

I'm not going to make comments about the boat structure.

But I would like to give you some advise regarding the mast.
If you use the same spreaders and change the angle you are going to have less CPW (chainplate width). Doing that you are going to have more mast compresion. And with the angle of the spreaders you can take off the runners but you are going to have a ''new moment'' on the mast. For these two things you problaby need more Inertia in your mast.

Ok, now the probably solution, make the mast SHORTER, make LONGER NEW SPREADERS?? or BOTHS??
Maybe you allready thought this mast issue?

Regards,

 

the distance from the mast centre to the chain plate is 4.55' or 9.1' between the two chain plates. sweeping the spreaders by 20 degrees should by my calculation - cosA . 4.55' = 4.28' reduce the CPW by 0.27' x 2 or aprox 0.55' which i calculate as about 6% of the existing CPW - is that significant??

 

reading some books here its probably a well wired slim light weight racing mast
diamonds and shrouding should not bend that mast onder compression or it may come down

 

the mast section is slim but the mast weight is high, the alloy extrusion is thick walled. under the IOR rating there was no penalty for the reduced mast diameter so they made them thin sectioned to reduce windage - but they made them thick walled and consequently heavy. also the rigging is navtec rod, not wire.