Simple (perhaps stupid?) rigging question

[bobnlesley]

My wife's just made what perhaps should be an easily answered question and I've prevaricated so far, but will shortly have to admit to not having a clue unless you can help:
Most monohulls carry masthead rigged foresails, but the overwhelming majority of catamarans are fractionally rigged; why?

[robert sailor]

Many modern mono hulls are fractional rigs as well. Mast head rigs typically share close to equal sail areas while fractional rigs feature large mains and smaller headsails. Cats have no backstay which means they can fly a much larger main with a huge roach area making it a very powerful sail. Sometimes the interior boat design on cruising boats will dictate the rig.

[catsketcher]

It's not a dumb question at all, but there are a few reasons.

Many early catamarans were masthead rigged. Here in Australia Crowther's early cats - the Spindrifts - were masthead. Crowther then changed his rigs to more fractional ones.

A few reasons for this -

One - cats have only a few places to step a mast. It really should go on the main beam in front of the cabin. (It has been done elsewhere but this is easiest). If you start increasing the cabin length and push the main beam forward the mast gets pushed forward. A jib doesn't work as well if its head angle gets too small so you pull the hounds down to match.

Two - Its more efficient to have a large main. Racing multis have a large main and small jib. Jibs, even though they don't have a mast in front of them are less efficient than headsails. (Planform doesn't help). In any fast racing boat where overall sail area counts the main is usually about 70% of the total area. On some boats - C class cats etc, the jib is totally removed.

Three - It's hard to keep the forestay tight on a masthead. This is a real killer on cats. It is possible to use two backstays to pull on the forestay but with some twisting it gets trickier to keep a mast in column on a boat that is not rock solid.

Four - It is easy to keep the forestay tight on a fractional rig. Wind on the mainsheet and tighten the forestay. Often the lee stay flaps around in breeze. You get better leverage if the forestay is down from the masthead and the jib is small. Makes the jib more efficient.

Five - the planform of the main gets more efficient. A fully battened roachy main is a better planform and faster than a triangular main that has to fit inside a backstay.

There are some nice masthead cats being built. The Easy range uses masthead rigs. Also if someone puts up their screecher, it is probably close to the top of the mast so many cats are tending back to almost a masthead for downwind with the screecher and fractional upwind. Best of both worlds.

cheers

Phil

[bobnlesley]

Thanks for that, I'll go back out now and pretend that I knew all along.

[roverhi]

It's actually harder or takes more wire/hardware to get headstay tension on a fractional rig with a typically rigged boat, otherwise, fore, back stays, lower and cap shrouds. The forestay is not triangulated and wants to pull the mast out of column. The main will have some beneficial effect on keeping the mast in common but not enough for my peace of mind. Boats with this type rig will either have running backstays or diamond stays to keep the mast in column. Other types of rigs like the no backstay type are usually set up so they do triangulate the forestay on a fractional rig.

[44'cruisingcat]

Originally Posted by roverhi

It's actually harder or takes more wire/hardware to get headstay tension on a fractional rig with a typically rigged boat, otherwise, fore, back stays, lower and cap shrouds. The forestay is not triangulated and wants to pull the mast out of column. The main will have some beneficial effect on keeping the mast in common but not enough for my peace of mind. Boats with this type rig will either have running backstays or diamond stays to keep the mast in column. Other types of rigs like the no backstay type are usually set up so they do triangulate the forestay on a fractional rig.

Not so much with Cat's. The forestay and shrouds usually meet at the same height, or very close to it. There's usually a few feet of unsupported mast above that.

[catsketcher]

As 44 says runners are rarely seen on cats - I can't remember any. I think Newick put some on some of his tris.

On your normal 3/4 rig mono the mast will be flexible and you use the backstay to induce mast bend. This is great for depowering in bigger winds. You use runners to keep the forestay tight. Not so on a cat. Most masts are absolute tree trunks and the masts are kept in column with diamonds. (My rig is different but it is 7/8ths).

Wind on the mainsheet and the forestay gets tighter. There is no backstay to pull on anyway and the effect of the main and diamonds keeps everything nice and in column. The sheet load is well distributed along the luff and so you don't get the backstay type tip aft flex.

When reefed the mast may invert its bend. This makes the main fuller which is a bummer. Often you will see masts with a fair bit of prebend in them, maybe double diamonds and even some lowers. This resists inverting and keeps the main shape nice and flat. Lock the mast in tight and don't let it move - very different to a Soling, Etchell or IOR fractional rig. Runners! RUNNERS!

cheers

Phil

[Bleemus]

The Newick tri I competed in the 94 Twostar didnt have a backstay just runners to the wing mast.

[Cadence]

Originally Posted by bobnlesley

My wife's just made what perhaps should be an easily answered question and I've prevaricated so far, but will shortly have to admit to not having a clue unless you can help:
Most monohulls carry masthead rigged foresails, but the overwhelming majority of catamarans are fractionally rigged; why?

There are far more stupid answers than questions in general.

[snort]

...and then there are the Prouts. That's a bit of a contrary philosophy regarding sail plan.

[jacktheflyer]

Thanks for the thread. I have always wondered this myself and till now haven't had a satisfactory explanation.

Another unanswered question I have regards large jibs and small mains. I had imagined this was because the jib is easier to deploy than the main and so becomes the primary sail. Any other explanations?

Scrubby

[jacktheflyer]

Originally Posted by catsketcher

It's not a dumb question at all, but there are a few reasons.



Two - Its more efficient to have a large main. Racing multis have a large main and small jib. Jibs, even though they don't have a mast in front of them are less efficient than headsails. (Planform doesn't help). In any fast racing boat where overall sail area counts the main is usually about 70% of the total area. On some boats - C class cats etc, the jib is totally removed.

What is the difference between the Jib and a Headsail per above?

[Bleemus]

Originally Posted by jacktheflyer

What is the difference between the Jib and a Headsail per above?

He is confused. Any sail flying off the forestay is a jib. A "working jib" is usually 100-110% of J dimension. A "yankee" is a higher clewed version that is typically less than 100%. A genoa is a jib that overlaps and is typically 110% to 150%. I have seen 170% genoas but they are rare.

There are no hard and fast rules so you can fudge the above and call it whatever you want.

[donradcliffe]

For cruising purposes, a larger roller furled jib gives you better performance, as it gives you a bigger range of easily and quickly adjusted sail area. You need to reef the main down for the highest expected gusts for safety, especially at night, and a small jib doesn't leave much to drive the boat.