How much to tension the lower shrouds?

[svrevelations]

The naval architect who design the yacht have now replied to my questions and also emailed me a drawing (from which I made my own drawing to avoid copyright issues);

"The short answer, Sparcraft is correct.

The long answer,

This always happens as a rig on a catamaran is far from rigid, it can't be. as there is always a certain amount of twise between the hulls in a seaway which will vary the distances from the chainplates to the mast tangs.

The correct way to have a rig like this set on a catamaran is to spar straight sideways with tight (as tight as possible/practical) diamonds (inner rigging running over spreaders) Fore and aft the mast can be set up with some pre-bend to get the desirable curve to fit the mainsail shape (all depending how flat you like your main) Don't go to excessive on the pre-bend, no more than 1x the fore and aft measurment of the mast tube.

The forestay and cap should be set up do that the mast has the correct rake and stand up straight sideways. The tension put on the caps can be reasonable so that the rig is tight at rest without the boat bending excessively (not so that the doors don't close anymore inside)

The tension on the inner shrouds should also be tight but no more than to keep the mast straight (in column) sideways when sailing. (You can only check this when sailing upwind with some fair load in the main and jib). If it is too much the mast will start pulling to weather, if it is to loose it may kick out to lee or hang loose.

The mastbase: As the rigging stretches under load the mast always rocks a little fore and aft. As such it's physically impossible to keep contact all around the base of the mast when it is cut straight. So we suggest to cut the base at a slight curve so that the mast can freely rock fore and aft. Reality is that most sparmakers today don't do this and then we let the mast rest at the back as we also have more material there (mast track).

I made a detailed drawing for you showing how we used to do the radius. You can have this done if you like whenever the mast comes off the boat for servicing, absolutely not critical or urgent, 99% of our multihull designs have it cut straight."

[UNCIVILIZED]

I understand that he's the NA, but something still smells fishy to me. Especially having seen a plethora of rigs with pre-bend measured in feet, with zero gap at the butt. Mutli's & mono's.
It would be worth asking how to measure deflection in the boat due to normal mast loadings; fore & aft, transversely, & on the diagonal. And to ask what the "normal" changes to the boat's structures are (when measured) once the rig is properly tuned, vs. when there's no load on the rigging at all.
Most likely/preferably when she's in the water, & thus fully supported. This is simply a normal check on some boats, large racers in particular.

I doubt that you've broken the boat, or the rig. But since the above things are measureable, I can't see it hurting anything to check. Particularly given that she's "new". As well as going over all of her rigging bits in detail at the pending rig overhaul. Since the vast majority of these items are quite easy to throughly inspect. Which for me anyway, is a big benefit to my peace of mind. Especially if I think or know that anything has "stretched".

Which, the stretched bit is in quotes, as I'm wondering what exactly has stretched, or rather, what several rigging bits? And by how much?
It's late here, so maybe others can chime in on this. But in my mind, once a rig has settled in after it's initial tuning & testing, not much should be stretching. Since to me, stretching (metal) implies pushing (pulling) it hard enough that it's entering the plastic deformation range. Which is physically damaging. Such as the visible change you can see in a piece of wire or a coathanger when you bend it back & forth repeatedly. Is my logic off the mark here?


For the overhaul, dye penetrant, & a magnifier; both a magnifying glass, & pocket microscope (thank you Brion Toss) are quite helpful. As is Loctite, & Tefgel. Plus calipers, & a thread & pitch gauge.
Along with labeling everything when taking it off of the rig for inspection & refurbishment/swapping out. And when working on rigging & spars, I always heed my intuition, including when it tells me to let something percolate in my brain for a day or three, or to get a 2nd opinion on something. Both are usually invaluable, as well as free.

Oh, & take lots of before, during, & after pics of everything; meaning of each assembly. Ditto regarding notes about same, & or sketches with instructions. Along with meticulous notes detailing parts sizes, brands, & numbers... For the next time, or the next owner. They can be priceless.
Such as when you find a bit with an unusual thread pitch, or reversed threads, etc.


BTW, it would also be worth getting pics & spec's on the mast step design. Since it's kinda' the norm for the step to prevent movement in the mast butt. If by naught else than it's snug male to female fit inside the lower end of the spar, where the mast's seated on the step. Though again, ask around; the NA, Sparcraft, & other owners. Along with the drawings of course.
Comments?

[UNCIVILIZED]

PS: The NA who designed your boat is not the all knowing sum of knowledge when it comes to rig tuning, boat flex, etc. Thus on these subjects it pays to ask around, even when it comes to learned & degreed folks in the field. And honestly, some of them at quite uneducated on some things. But then boats are a big area of knowledge, which is why there are marine sub-specialties.

[44'cruisingcat]

Originally Posted by Revelations

The naval architect who design the yacht have now replied to my questions and also emailed me a drawing (from which I made my own drawing to avoid copyright issues);

"The short answer, Sparcraft is correct.

The long answer,

This always happens as a rig on a catamaran is far from rigid, it can't be. as there is always a certain amount of twise between the hulls in a seaway which will vary the distances from the chainplates to the mast tangs.

The correct way to have a rig like this set on a catamaran is to spar straight sideways with tight (as tight as possible/practical) diamonds (inner rigging running over spreaders) Fore and aft the mast can be set up with some pre-bend to get the desirable curve to fit the mainsail shape (all depending how flat you like your main) Don't go to excessive on the pre-bend, no more than 1x the fore and aft measurment of the mast tube.

The forestay and cap should be set up do that the mast has the correct rake and stand up straight sideways. The tension put on the caps can be reasonable so that the rig is tight at rest without the boat bending excessively (not so that the doors don't close anymore inside)

The tension on the inner shrouds should also be tight but no more than to keep the mast straight (in column) sideways when sailing. (You can only check this when sailing upwind with some fair load in the main and jib). If it is too much the mast will start pulling to weather, if it is to loose it may kick out to lee or hang loose.

The mastbase: As the rigging stretches under load the mast always rocks a little fore and aft. As such it's physically impossible to keep contact all around the base of the mast when it is cut straight. So we suggest to cut the base at a slight curve so that the mast can freely rock fore and aft. Reality is that most sparmakers today don't do this and then we let the mast rest at the back as we also have more material there (mast track).

I made a detailed drawing for you showing how we used to do the radius. You can have this done if you like whenever the mast comes off the boat for servicing, absolutely not critical or urgent, 99% of our multihull designs have it cut straight."

Oh well. I would have said they were wrong.

Tightening the lowers will tend to reduce the prebend, and pull the mast further back. Which will actually make the situation at the mast step worse.

If it were me, I'd loosen the top shrouds a bit and tighten the forestay. Pull the masthead forward a bit, which would improve the mast step alignment, and add tension to the lowers simultaneously.

Depending on how much it takes, you may need to loosen the lowers a bit too.

[svrevelations]

Originally Posted by UNCIVILIZED

PS: The NA who designed your boat is not the all knowing sum of knowledge when it comes to rig tuning, boat flex, etc. Thus on these subjects it pays to ask around, even when it comes to learned & degreed folks in the field. And honestly, some of them at quite uneducated on some things. But then boats are a big area of knowledge, which is why there are marine sub-specialties.

You are right and this is why I started this thread.

Originally Posted by 44'cruisingcat

Oh well. I would have said they were wrong.

Tightening the lowers will tend to reduce the prebend, and pull the mast further back. Which will actually make the situation at the mast step worse.

If it were me, I'd loosen the top shrouds a bit and tighten the forestay. Pull the masthead forward a bit, which would improve the mast step alignment, and add tension to the lowers simultaneously.

Depending on how much it takes, you may need to loosen the lowers a bit too.

This is also what I have in mind. But I will evaulate things again when I get to the yacht and I will also be mindful of all the other sound advice given.


Thank you all once again for taking the time to give advice and answers. Much appreciated!

[UNCIVILIZED]

If you genuinely think that certain bits of rigging (wire), or tangs, or fittings have stretched, then it's worth sending them out for metallurgical analysis to find out why. This along with a serious inspection of everything else so as to ensure that the materials that comprise them are sound. Since from time to time manufacturers can get a bad lot of metal, or rigging wire, so that the end product winds up being substandard. And that's something which needs determining before a problem crops up & creates an emergency at sea.

Which, legit experts are sometimes needed to determine this. Including sending out components for lab analysis, x-ray testing, etc. Though if they're truly that questionable, it's often best to simply replace them. Unless you're talking about something like an entire mast tube.

There can also be substandard construction in the structures which support the mast. Be it the step itself, or the structure of the boat. Some boats will bend or compress with time. Such as decks & compression posts on deck stepped monohulls. For example, Ranger 33's weren't built with any blocking between the butt of the compression post where it met the hull's inner liner, & the hull itself. So over time, the posts get pushed downwards slightly. Not critically so, but enough to notice, & eventually require a retrofitting with proper blocking under the post's butt.

And someone could have even goofed in the laminate in a structural area when building the boat, or the designer could have transposed numbers, etc. So if something seems wrong, investigate it. Straight edges, & especially laser levels make this easier than ever. Which is part of why I suggested asking the designer for the numbers on hull/structural flex with a loaded vs. unloaded rig. If it's a custom boat, odds are these numbers were run during the design phase.

[rognvald]

Excellent advice was given by UC especially . . ." And when working on rigging & spars, I always heed my intuition, including when it tells me to let something percolate in my brain for a day or three, or to get a 2nd opinion on something. Both are usually invaluable, as well as free." Something more is going on with your rigging. I don't buy the "rocking mast" concept. Good luck and safe sailing.