Main sheet arch

[Alexis]

Hello there,

I am currently refitting a 34 footer, medium displacement cruiser. For comfort, access, security and practical reasons, I am looking into a way of having my main sheet travaller on an arch.
On my boat, the boom ends one foot abaft the companion way. My plan is to build some sort of arch that would also be used as the rear support of a semi rigid spray hood as well as the front support of a bimini/cockpit tent arrangement.

Has anyone out there any experience in such a project ?

Many rhanks,
Alexis

 

[LORDNELSON]

I have the same problem on my boat (37 foot) and, so far, have yet to solve it with a satisfying and economical design. I hope your query brings in a lot of suggestions from which I can benefit. The most likely idea I have had is to have a new overlength boom built bringing the outboard end of the boom over the stern deck and thus being able to have a main sheet horse on the stern deck - but this is expensive!

 

[bedouin]

I'm no expert, but things to bear in mind.

(a) An arch strong enough to handle the loading of the mainsheet will have to be a pretty substantial affair.

(b) how much clearance do you have between the top of the sprayhood and the boom? Certainly on my boat there wouldn't be enough space for the blocks between a traveller at that height and the boom.

(c) Reducing the distance between the boom and the traveller will also impact on the interaction of mainsheet, traveller and kicker, with small changes in mainsheet having a more significant impact on sheeting angle, and therefore sail shape. You may end up either having to adjust the traveller more frequently, or relying more on the kicker to control the twist in the sail when close hauled.

I believe the usual solution to your problem is to mount the traveller just in front of the sprayhood - that has the disadvantage of increasing the load on the mainsheet, but solves many of the other problems.

 

[Jules]

Legend have been doing this for years, may be worth looking at one to see guage of S/S and mount points, etc. You'll probably have to extend the boom tho....

 

[Gunfleet]

Spar makers reckon you need to go up a size in boom if you take the mainsheet to the coachroof. Try breaking the end of a lolly stick/resin mixer stick then try breaking the middle to see why.
On the otherhand if you have an overhead traveller won't you be drowning in spare sheet?

 

[snowleopard]

diy solution

i fitted a goalpost to my cat which has turned out to be very strong. first i wrapped 2 inch domestic plastic waste pipe in glass fibre, then added flat sheets of glass moulded on a melamine-faced table to either side of a pair of tubes and finally wrapped the whole assembly with more glass making an oval cross-section. i did this for both uprights and crossbar, giving the latter a curve for aesthetic reasons. with a span of 4 metres i can walk across it and i weigh 18 stone. the whole thing weighs around 20kg

 

[bedouin]

Re: diy solution

But the loads on the mainsheet are considerably greater than the weight of an individual - I would guess they are frequently in the 0.5-1 ton range - as an approximation I would say the track attachment needs to be of comparable strength to the boom itself.

 

[jeanne]

Snowleopard, your solution sounds brilliant: how did you do the corners of the goalpost and the flanges to attach it to the deck?

 

[Oldhand]

Have a look at a Malo, their "Targa" arch is GRP encapsulated steel bolted by 2 large s/s bolts into reinforced combings. It needs a high boom to work.

 

[Jacket]

Re: diy solution

As a rough guide:

-The horizontal bar forming the top of the arch carries the same loads as a mainsheet traveller.

- Lets say the length of the bar across the top of the arch is about twice the unsupported length of a cockpit mounted traveller.

In this case, the arch would experience twice the bending moment of the traveller. Therefore the cross-section of the member forming the arch would need to have twice the width of the member forming the cockpit traveller, or have 1.25 times the depth. And the dimensions of a typical mainsheet traveller aren't all that huge.

This is only a very 'back of an envelope' calculation, but shows that the forces experienced by an arch aren't all that huge, and that a mainsheet arch doesn't have to be made out of particularly huge members.

 

[bedouin]

Re: diy solution

But the main sheet traveller is bolted to the deck every 100mm - at least mine is.

It is generally reckoned that an average crew can exert a force of about 50kg on a sheet. If the mainsheet has a 4:1 advantage (and many have 6:1 or 8:1) then that equates to 200kg+ - with the mainsheet attached to the end of the boom. Now move the attachment point 1/3rd of the way towards the gooseneck, that gives 300kg static load - transient loads in gusts/squalls, and when gybing can easily be double that. Those figures are fairly conservative, so an estimate of 500kg-1ton does not seem excessive. This is confirmed by the stats on the Harken travellers for boats 27'-34', which have an SWL of 1000kg

 

[Jacket]

Re: diy solution

>But the main sheet traveller is bolted to the deck every 100mm - at least mine is

I was thinking of the sort that run across a cockpit from seat to seat, and are only supported at the ends.

>Those figures are fairly conservative, so an estimate of 500kg-1ton does not seem excessive.

Sorry, I didn't really make it clear what I was trying to get at. I quite agree with the estimate of loadings. What I was trying to show was that loads of a tonne are relatively low loads in structural terms, and don't really need that large a section to carry them safely. With a bit of care it should be possible to build a relatively slim mainsheet arch thats quite capable of carrying the loads.

Yes, the arch needs to have a similar strength to the boom, but it need not have anything like as large a cross section. With a boom, a prime objective is to keep the weight down, to reduce the kinetic energy in a gybe. It is made light but strong by making it of a large diameter, thin walled tube.

The arch doesn't have to be so light, so can reasonably be made of smaller diameter, thicker walled tubes, which will have a similar strength to the boom, but at a slight weight penalty.

 

[kds]

I agree, the arch does need to be strong, but another thing to consider is the force caused by the leverage at the base/attachement when the mainsheet goes across on a Gybe. It is enormous. I would also like to see details of the GRP idea for my 29ft. sloop.
The idea is so sensible, though, so long as it also includes handholds for getting past it onto the deck. The worst thing about going forward on so many boats is getting past the sprayhood. Beats me why the frame is not put on the outside of the hood, with tape or lacings to support the canvas, giving loads of good hand-holds. Solar panel support as well ? If far enough back it could hold arials ? Larry Pardey swears by a massive boom horse to stand at while boat is self-steering.

Check out some pictures of my boat at;
[link]www.canongrange.co.uk/boat/[link]

 

[Alexis]

Re: diy solution

Your solution seems very interesting.
Would you be kind enough to send photos or a sketch illustrating the finished product as well as details such as the way you did the angles between the vertical posts and the horizontal beam ?

I have sketched my project on a word document, and I would like to submit it to this forum but don't know how to attach the document.
Does anyone know if this is possible?

Many thanks,
Alexis