S/T-3 Trimaran, Cross

CROSSS/T-3

Trimaran, Cross

50' x 27' x 16 Tons

28-Ft. Dia. Parachute Sea Anchor

Force 10 Conditions

 

File S/T-3, obtained from the owner of the boat, residing in Durban, South Africa - Vessel name withheld by request, hailing port Yarmouth, England, trimaran ketch, designed by Norman Cross, LOA 50' x Beam 27' x Draft 6' x 16 Tons - Sea anchor: 28-ft. Diameter C-9 military class parachute on 500' x 3/4" nylon braid tether and bridle arms of 80' each, with 5/8" galvanized swivel - Full trip line - Deployed in an Atlantic storm in deep water south of Tristan Da Cunha with sustained winds of 50 knots and seas of 30 ft. - Vessel's bow yawed 20° - Drift was estimated to be 12 n.m. during 36 hours at sea anchor.

 

South African safety expert, yachtmaster, instructor and Intec Maritime Academy principal Henton Jaaback had heard about the Casanovas' pioneering work. He acquired a military parachute, which he ended up loaning to the owner of this passage-bound trimaran. On her way to Rio from Cape Town the trimaran ran into a horrendous storm south of the island of Tristan da Cunha. Exhausted, the owner and his wife deployed the parachute according to the Casanovas' guidelines. It pulled the three bows into the huge seas and kept them there for 36 hours - through the worst part of the storm. After the 36 hours there was a jerk, the bows fell off the wind, and the big multihull lay beam to the seas, drifting downwind. It is the owner's opinion that the galvanized swivel in the system seized - failed to rotate, though the 3-strand rope used for the bridle arms is suspect as well (3-strand will torque under load). At any rate when he pulled the lines back on board, all that remained were the two bridle arms of about 60 ft. each, twisted around each other and their ends unraveled. The full trip line had snapped at the same time, so they lost everything, hardware, swivel, tether, parachute and all.

Galvanized swivels have always been a source of concern to your author, though one looks at the Casanovas' file and sees that they never had a problem with theirs in eighteen years of cruising and storm use. The problem with these swivels lies in the galvanic process, which results in an acid-etched coarse surface, liable to stick or "gall" under load. Even so, the swivel - if of good American or Japanese make - usually gets a chance to rotate during slack cycles, as born out by many other files in this database. Moreover there are excellent stainless steel swivels on the market today.

Why do parachutes rotate under load? They may do so because of inconsistencies in fabrication, or shroud lines that are not precisely equal in length. But mostly they rotate because of the ratchet effect produced by the overlapping of the panels. These panels, shaped like pie wedges, have to be sewn together to form the circular shape of the canopy. The edge of the first panel is laid over the edge of the second and sewn, the edge of the second panel is laid over the edge of the third, etc., the radial seams being over, over, over, and this is where the ratchet effect crops up. To do away with this built-in cause of rotation one has to stagger the fit of the seams. The edge of the first panel is laid over the second and sewn, but the edge of the second panel is laid under the third, etc. Over, under, over, under. The parachutes that are used to slow down supersonic aircraft on the runway are of staggered fabrication. You won't see them spin.

CANOPY
Canopy panels

The panels on Para-Tech sea anchors are now sewn in such a way as to be spin-neutral, although swivels are still a good idea. Here is a transcript of the DDDB feedback provided by the owner and his wife:

 

Swivel on bridle galvanized iron 16 mm - swivel on parachute 16 mm also. The bridle was attached to the swivel of the main line with two shackles. Main line 20 mm "multiplait." Bridle was 25 mm nylon 3-strand rope. After recovering the remains of the bridle we saw we had lost the two shackles and the swivel. The remains of the arms of the bridle were unraveled & twisted around each other - everything else was lost. We have been informed that galvanized swivels apparently lock under strain....

The trip line also snapped when we lost the para-anchor. This was at about 3 pm (we felt a jerk). We rushed into foul weather gear and on deck.... When we started up the engine and tried to find the "rig," the wind was so strong the boat could make no headway - also the seas were white, so the [white] buoy was impossible to see! A red buoy would perhaps show up better, even though we could not have motored to get it....

Some hours after we lost the para-anchor, after broaching dangerously south of Tristan da Cunha, we decided to use a drogue to slow the boat and eliminate broaching. We were bare-pole doing 5-6 knots and descending the slope of waves at 12-13 knots. The drogue consisted of 150 ft. of "multiplat" 20 mm. rope plus 33 ft. of 1/2" chain with 4 knots to make more vortex [turbulence]. All the above was attached to a bridle of 30 ft. [each arm].... We used the engine [in conjunction with the drogue] at the minimum, about 1000 revs, that gave us a speed of approx 4-5 knots and maximum speed in descending wave slope of 7-8 knots and no more broaching.

We needed the engine to keep enough steering power. We had a 3-blade fixed prop that spoiled the efficiency of the rudder; I think we lost about 50% efficiency! But we used the autopilot without any problem and we really appreciated the work of the drogue in straightening the boat each time at the beginning of a broach. The bridle was fixed to two sampson posts of 4" square oak, fixed to the keel of each float. The wind was then about 40 knots, with big breaking seas for about 24 hours.

We were very surprised by the efforts [loads] imposed by the drogue and also by the parachute anchor and we don't think that normal cleat-type fitting would have lasted under the strain....

We now have two para-anchors.... We honestly feel we would not sail without one now. Our experience around Tristan, and the knowledge that we were safe and could ride out a storm, has made this indispensable.

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