Main Sail Size/Aspect Ratio

[Snowgoose35]

So my at has a much shorter mast than most built more recently as Prout went with conservative rigs to counter the multihulls capsize prejudice in the 70s and 80s.

She's a 15m cat with an 8 wire rig and a 15m mast and 5m boom
The design was originally for a 35m2 mainsail and bigger jib - not to silly compared to Prout's tiny main and big jib ethos on the smaller boats with a back stepped mast - this at least has a centre stepped mast.

So having lost over a tonne in weight at the rebuilt and put a taller gib on the front (longer forestay now with a seagull striker rather than a wire bridle off the front beam that lost 1m in stay length) I have a quite quick boat but still under powered for the nice fine hulls.

I have a big roach laminate main (not square top) that I bought second hand to play with and now want to replace it with a new one. I cannot afford a new taller mast, which is what the boat really needs , as that's £30K at least but I have been offer a good second hand boom 1m longer. I've already decided to change the fixed backstays to running backstays so I can move the existing boom further for downwind sailing so maybe the longer boom is a good idea ?

This would up the sail area from about 43m2 now to around 50m2 but from what I have read a low aspect ratio sail can cause poor upwind sailing and this would make an already low aspect 3:1 sail into a lower still 2.5:1

Has anyone here experience of this kind of thing or a better grasp of sail performance to offer an opinion. Obviously I will talk to the sailmaker when the time comes but all have different opinions and its not the most commonplace question so I'd like to gather as much information as I can first TIA

[fxykty]

A 6m boom on a 15m mast (not sure how high your boom is from the base of the mast and that affects the actual luff length) isn’t outrageously lopsided and doesn’t approach the ratios of an Optimist dinghy (1:1) FWIW. So I think from a performance point of view you’d be OK. As an example, we have a 6.4m foot and 17.1m luff on our mainsail (also round top, not square).

But, can your shrouds, chainplates, hull, bulkheads, etc., handle increasing the size of your mainsail? Is the boat so structurally over-built that you can add more sail area?

[Snowgoose35]

Thanks. Mast is actually 16m so I meant 15 above the boom. Prout's were considered very overbuilt and when I refitted her I built bigger chainplates (though I may laminate in carbon ones this winter) and she has an 8 wire rig - it kept the mast up through Irma and Maria (both Cat5 hurricanes) so I'm pretty confident in the ability to cope with more power

[Tupaia]

Adding to the boom will move the CE aft where is this relative to the CR. How will this affect the balance? If it changes the CE / CR significantly you may end up with a steering / helm issue. Might be worth plotting these first to establish any possible short comings.

[catsketcher]

Good points from others here - especially re balance upwind. Weather helm can be a real pain.

Lowering aspect ratio is not a problem in downhill sails - this is why kites can be low aspect. It is at closer angles that aspect ratio is a good thing. You can see this in multi class racing - the low aspect cats like Mosquitoes can keep up with high aspect cats when everyone is forced to sail low (on a lake or river) but when the high aspect cats can sail higher they speed away.

That being said, I would be interested in the geometry of keeping your forestay tight when you get rid of the backstay - The Quasar (IIRC) has a back to front geometry - meaning that the mast to forestay base (J) is longer that the mast to backstay base. In many cats the J is really small compared to the boom length meaning that the mainsheet has lots of leverage to pull the forestay tight. You will have the opposite problem and getting rid of an effective forestay tightening system may compromise your upwind sailing.

On top of this, mains are not super good at sailing deep downwind, they work fine, but if you want more sail downwind, maybe you should think of putting a prodder on and getting the sailmaker to make you the biggest possible Code zero or assy. Then your cat will be pulled downwind by the nose, which cats like much better than being pushed in the bum.

cheers

Phil

[Snowgoose35]

Originally Posted by catsketcher

That being said, I would be interested in the geometry of keeping your forestay tight when you get rid of the backstay - The Quasar (IIRC) has a back to front geometry - meaning that the mast to forestay base (J) is longer that the mast to backstay base. In many cats the J is really small compared to the boom length meaning that the mainsheet has lots of leverage to pull the forestay tight. You will have the opposite problem and getting rid of an effective forestay tightening system may compromise your upwind sailing.

I am not getting rid of the backstays - just changing them to running back stays so one will always be tight but off the wind the other will slacked off to let the boom out further. The rest of the time both backstays will be on and tight as always. The Quasar mast is basically right in the middle of the bot and the forestay and backstays within a degree or two the same angle.

I have a reasonable sized assy but am getting an enormous one made - 205 sq m - which will of course help downwind hugely. Up wind the boat is pretty good - sails down to 35 degrees and with the battened foresail I have, sheeted in well she has done better than true wind speed down to about 7 knots. 14 knots boat speed in to wind is the most we've seen and frankly after 4 hours I'd had enough

The "problem " comes when the winds reach 15 knots or so and the kite has to go away (on a reach or run) we lose boat speed despite the higher winds. I'm really not sure whether the fact that the boat is (deliberately from the factory) under canvased or whether I just need to learn to sail better. I do worry that I'm not getting the best out of what I have so perhaps a day with a good multihull racer will do more than a bigger sail.

[Cavalier MK2]

It might be helpful to know how your deck is rigged. A barber hauler to pull the jib lead outboard is a great help off wind for example.

[catsketcher]

On my cat I have a few different modes downhill. First is the kite and then the Code zero. The zero is smaller than the kite but easier to get away when the wind blows up.

If the genoa is not terribly large then a zero on a prodder could be a good halfway point between the assy and the genoa. It does seem like a lot of money to plug a performance hole. You did write that you have a small assy. Could you maybe out that in a sock and use it when the wind blows up a little?

One thing I learnt after racing Tornadoes and skiffs was how to tack downwind. When we left for cruising I would tack downwind often with our cat. Now I don't. My 11.6m 4000kg daggerboard performance cat does not go fast enough to make up for the extra distance made tacking downwind. So in the end I sold the assy and use a reacher/code zero and a symmetrical. You really need to have a fast boat to make tacking downwind worth it.

I would like a new style assy that rolls its luff in without collapsing for broad reaches when we are laying the course, but until a cheap one comes up secondhand I will go without.

But one important thing to do on our cat is NOT to always sail the course. I often sail 15 degrees or so too high or low and just get the boat going. When daysailing, it is most important to get the boat sailing well with the reacher/kite drawing well and producing apparent, rather than aim straight at the destination. Often we were the only boat sailing as we headed a little off course, usually too high in lighter winds. Then as the wind came in, we could bear away and head for our destination. Make sure you work out how to get the boat drawing well in ligter winds, generate some apparent and then work out the course later. It is different from slow mono sailing (but fast monos do this too) but it is nice not to motor in calm and smooth conditions.

As Cav says - leading the genoa sheet out wide is also a real help. I often use both sheets to leeward - one to the barber hauler on the beam and the other on the coachroof - it allows me to dial up exactly where I want the clew, out faras possible whilst not luffing and having about 15% depth. Racing does help - a lot.

cheers

Phil