Sizing gear for Trimarans...

philw · Post by **philw** » Fri 13 Sep 13, 20:29

Hi all,

I'm thinking about a furling kit for the asymmetric and wondered which size to go for. Typically, the entry level kit is quoted as suitable for kites up to 50sqm. This is the size of the 800 basic sail.

My question is this - do the high speeds on a tri mean that the loads are higher that a monohull? Is kit recommended for a 50sqm sail on a monohull going to be a good for the DF800?

Thanks, Phil

Steve B. · Post by **Steve B.** » Sat 14 Sep 13, 4:30

I doubt the loads are higher.
Boat weighs less and has less wetted surface, thus faster.

philw · Post by **philw** » Sat 14 Sep 13, 10:06

Hi Steve,

Yes - I agree, The loads will not be higher due to the interia of the hulls etc, but does the extra speed mean the loads are just as high despite the boat being lighter. Once the boat is going, the drag increases with speed to match the drive force available. Also, lift on a wing/aerofoil/(sail?) is dependant on the square of the windwpeed....

I managed to think myself into a corner on this - probably over-thinking it! It's an engineer thing.

Phil

Steve B. · Post by **Steve B.** » Sat 14 Sep 13, 15:52

I've had both, a heavy displacement full keel mono (Lord Nelson 35) and a Dragonfly 1000.

We sailed from Washington State to Mexico, cruised there for a few years.
We've owned the Dragonfly since 2000.

If anything, the Dragonfly is easier to sail.
For a given wind condition, I'd say loads are similar, you just go faster.

philw · Post by **philw** » Sat 14 Sep 13, 16:31

Hi Steve,

Thanks - good to know! Sounds like an awesome trip..

My background is mostly dinghies so this is a bit of a step-up for me. Loving it!

Phil

EarthBM · Post by **EarthBM** » Sun 15 Sep 13, 18:09

Would say tri or any multi loads are higher. It has to do with higher apparent winds and with less heel. A gust would hit a mono sail and it would heel, spilling the highest load. A multi would stay upright and the sail would need to take the load (hopefully translating it into forward speed eventually).

My first reef block broke in a gust - aluminum just sheared off. Sized for same size monos. I'd say the right size load bearing gear for multihulls is what's recomended for 1.5x larger monos.

Double Horizon · Post by **Double Horizon** » Fri 20 Sep 13, 2:49

I agree that loads are higher, for the reason that the boat has more resistance to heeling. I have been told by a sail maker they are at least two to three times as high as a monohull

This is illustrated by the following graph from the john Shuttleworth site, showing capsize forces.

Page reference for entire context http://www.john-shuttleworth.com/Articl ... STalk.html

Stefan Kolmodin · Post by **Stefan Kolmodin** » Fri 20 Sep 13, 17:35

I think the higher loads are mainly due to the trimaran's higher righting moment sideways. If you talk forestay/furlers trimarans haven't got higher righting moment fore/aft rather less due to no lead/lower weight so on this stay the loads should be lower than on a lead sailer.

Double Horizon · Post by **Double Horizon** » Fri 20 Sep 13, 17:56

Stefan Kolmodin wrote:I think the higher loads are mainly due to the trimaran's higher righting moment sideways. If you talk forestay/furlers trimarans haven't got higher righting moment fore/aft rather less due to no lead/lower weight so on this stay the loads should be lower than on a lead sailer.

It seems to me (based on observation) that any side loads are shared between the forestay and windward shroud. If I am correct, the forestay would also be highly loaded and it would be a mistake to undersize it.

Stefan Kolmodin · Post by **Stefan Kolmodin** » Fri 20 Sep 13, 19:08

I disagree, a stay in line with the mast/boat center doesn't support much sideway load. Also most gear on cruising boats are substancially oversized.
If you want to be super safe - oversize everything. If you like to race/go fast with a light boat check what gear other boats of the same model are using without failure - and you'll be safe and fast.

Double Horizon · Post by **Double Horizon** » Sat 21 Sep 13, 10:49

You're saying that an increased load on the main and headsail, regardless of wind direction, doesn't increase load on the forestay? I might be convinced when running downwind, but not when reaching or beating.

philw · Post by **philw** » Sat 21 Sep 13, 10:55

Double Horizon wrote:
Stefan Kolmodin wrote:I think the higher loads are mainly due to the trimaran's higher righting moment sideways. If you talk forestay/furlers trimarans haven't got higher righting moment fore/aft rather less due to no lead/lower weight so on this stay the loads should be lower than on a lead sailer.
It seems to me (based on observation) that any side loads are shared between the forestay and windward shroud. If I am correct, the forestay would also be highly loaded and it would be a mistake to undersize it.

Dead right... Rigging can only support loads along it's length (obviously) so loads at any other angle have to be balanced between at least 2 stays (I include the forestay in this instance) Increasing side loads on the rig will certainly increase the load on the forestay.

philw · Post by **philw** » Sat 21 Sep 13, 11:02

Having thought about this a little more, I think that the loads on a sail (which is where we started

) are almost completely dependant on the cut of the sail and the apparent wind it sees. It is pretty unimportant what the sail is attached to - mono or multi.

As such, the same sail flown on a multi at 15kts will produce far higher loads that the same sail on a mono doing 6kts. I guess the sails would be cut differently, but that's a whole different discussion

Steve B. · Post by **Steve B.** » Sat 21 Sep 13, 15:43

Nobody has mentioned the angle at which a dragonfly cap shroud takes the loads vs. a monohull's upper shrouds.

A mono has to support a similar sized mast over a narrow main hull's beam where a Dragonfly can support the same sized mast over a wide angle to the amas.

Imho, I think the tensions are similar. Just look at the chain plate designs.

The narrow angle of the waterstays proves my point.
Narrow angle, BIG load/ wide angle, small(er) load.

Stefan Kolmodin · Post by **Stefan Kolmodin** » Sat 21 Sep 13, 18:10

Of course there's loads in the forestay but not necessarily higher than on a monohull. Every load has an antagonist holding it in place which on a tri is a without a lead keel on a momentum arm hence smaller righting moment fore/aft. Where the balance is is individual to every construction. My observation when looking at different tris is that their gear is lighter/smaller compared to a monohull and working fine. We were 29 tris (fantastic experience, mostly Dagonfly and Corsair) on the line in the last race so quite a large population to analyze. I'm sailing with 47 m2 upwind and adding a code 0 at 31 or a gennacker at 71 m2 downwind on 1171 kg boat. Big sails with small gear and working fine and fast since 1995.

DRAGONFLY Trimarans User Forum

Sizing gear for Trimarans...

Sizing gear for Trimarans...

Speed and Loads