To get most benefit from the ratchet you want it at your end of the purchase system, but you also want a decent 'wrap' on the block. Moving it to the deck doesn't get it any closer to you in the purchase system, but just mean less wrap, so I guess that's why. 

But, it doesn't look like they are taking the main off the boom in either photo or diagram. So they're not really 'clearing the cockpit'. 

I guess they've gone down this route because playing the main frequently is fatiguing, but adding another purchase would end up with an unworkable amount of rope in the boat, and a slower gust response (because you have to sheet more for the same sheeting effect).

The downside of doing a full wrap on the ratchet (or adding another ratchet as I've seen before) is that it chews rope, and can add too much friction when the wind drop (but they can be tuned off). Adding ratchets also doesn't decrease the load when sheeting in, so if you'r struggling to pull in at all you have no other option than to add purchase. 

The 'nova arrangement is quite different though, a centre mainsheet with the last turning block at around deck hight instead of on the floor/turret. The only reason I can think of is to get the mainsheet lower than off the boom. The extra wrap idea makes sense but as most ratchet blocks work fine with the sheet in contact for only around ¼ of the circumference I'm not convinced that is the reason.

Still thinking about DaveB 2:1 vs SS 3:1, I'm reckoning more like 2.5:1 at the boom end due to lost leverage by coming off halfway down the boom.

I love it,  that's exactly what I thought but decided to not complicate matters further, of course the x.? depends exactly where on the boom the ratchet is. On the Blaze it's around 2.3:1 with it coming off the kicker take off webbing :)

That's cool. I've only ever seen universal joint blocks used on the boom here. The pivoting block only moves in one direction, does that cover all your sheeting angles? It certainly gets the block closer to the boom, which gives more head room. 

Sorry, I thought you had mistaken what was being shown the pictures. 

Plenty of people sheet off the boom in lasers and other boats (I do it in the 200). I tend to do it in light winds downwind, to remove the extra friction, and allow me to sit further forward (as the lasers do). 

However, off the boom sheeting isn't the utopia you describe! And I don't think it's because people are 'against innovation' as you seem to imply, but because it's not always the best solution.

First of all, you change the direction the final purchase in the your system is pulling at as you move around the boat. (you have a 2:1 purchase at the rear bridle, and a fractional purchase coming off the boom). If you have a block on the deck / thwart it keeps that final purchase consistent. (i.e. main sheets pulls more down on the boom as the boom approaches the center, which helps create leech tension when you're hard on the wind). 

For instance in skiffs when the crew has the main, when they move extremely far forward this purchase (from the boom to them) stops pulling any leech tension, and instead they start pulling the boom in to the mast, actually bending the mast and opening up the leech.

Secondly, having off the boom sheeting really depends on the size of the boat and other people in it. Trapeze boats, or wide boats (i.e most fast boats) will need longer tiller extensions for their length. This means even if they have off the boom sheeting, they will struggle to get eh tiller through the tacks without hitting the crew (or mast in the case of the moth).  The answer to this problem is either to run twin tillers (like many skiffs). Or have a central sheeting system, with no rear bridle, and pass the tiller around the back. 

Anyway, my general point is that I don't think in many classes off the boom sheeting is the solution. And I don't think it's because they're Luddites that classes like Merins haven't tried it. 

Yes, a proper discussion beloved of proper sailing nerds this.....

Yup, and as somebody said earlier if the block is on the weight and the rope fixed on the ceiling lifting up on the fall gives a 2:1 purchase, leading this through another fixed pully on the ceiling and pulling down still leaves you with 2:1 (but an easier job pulling down on the fall would make it seem easier I suspect).

Erm... except that's not what the ratio refers to. The ratio is a movement ratio (velocity). I.e you pull 2m, it moves 1m. You still have to put the same energy in (by pulling more rope), but the loads are reduced. 

People often work out the ratio by adding up the lines coming out of the moving end... but that only works when you have one fixed point and one moving point, and you're working the rope from the fixed end...

edit: "Admiral's Cup 2:1"   http://www.harken.co.uk/content.aspx?id=3901" rel="nofollow - http://www.harken.co.uk/content.aspx?id=3901

You can add any number of block and they don't add purchase as long as those blocks don't move in relation to each other as the boom moves. In the linked diagram as you sheet in, block C at the goose-neck doesn't move in relation to A on the boom or D on the floor. It is therefore a fixed block and adds no purchase.. so the system stays 2:1, despite all the extra blocks. 

When the rope comes off the boom at the mid point, that is a moving block (it get closer to the deck block as you sheet in). It's just that block in the middle of boom doesn't move as much the ones at the end, so has less impact on the velocity ratio. And like I said if you're taking the rope directly from midway on the boom, as you move around the ratio will change. 

 

I have 6:1 on the Spice centre mainsheet, sheeted from about halfway along the boom. I end up pulling more string but can get the leech tensioned while out (often somewhat unsteadily) on the wire. It still feels heavier than the Blaze but, again, with a 12m+ FB mainsail and 6 blocks I guess that's also to be expected. I suppose I might try twin extensions, off the boom and a rear bridal there too one day (I don't suppose anybody will be protesting me as I hack around racing the Ents over the water)

I know of a couple of big two person trapeze boats that use a direct split tail main. In these cases the only job of the main is to open and close the sheeting angle, the leech tension is all kicker, in fact if you don't have enough kicker applied the main loads are pretty unmanageable. 

SS. You confirm the point by saying 2:1 boom end is much lighter than 4:1 mid boom. I suspect the boom end is applying less downward pressure than the mid boom setup.

Some classes like the last few inches of main sheet movement to open and close the leech and it is easier to play one control than trying to juggle the kicker or cunningham. (Spice?)