Inca's Balsa Log Raft
The Norwegian raft case
When thing goes wrong - what is the reason ?
The story - short:
Finally they were forced to abandon their rafts in open sea and call for pick-up.
- the problem is either the hull, the sail or missing seamanship.
What went wrong for the Norwegians?
That can't be their sail and rigging. They had mounted a typical Norwegian mono-masted square sail, and that rigging is the most Norwegian rig as is possible to imagine. They have used that rigging the last 1000 years - and they master it to excellence.
It can neither be their crews. The rafts had all experienced captains and around the half of their crew ware full-bodied sailors trained on square sails - the other half less.
They knew from the old Spanish chroniclers, that the chosen months would be the worst to sail to Easter Island. Too they knew, that the year was expected to be the worst El Niño year as affect the global climate with atypical and unpredictable weather - and they felt themselves strong and prepared for such conditions at sea - and they were!
The last suspected is the hull of their rafts.
The testimonies from early explorers all have told us, that the South American rafts without visible difficulty could beat against the wind steered by their Guara system. The common shape was reported as a long trunk in the middle and some smaller along the sides - as the shape of an outstretched hand.
What caused this final result was, that the rafters wanted to build "a fast sailing balsa raft".
"Building and testing raft models - How should one best build a balsa raft? Pointy front? Shaped logs? Does it make a difference?
Crew member Ola Borgfjord has built several models along with his father, Einar Borgfjord.
The results indicated, that a curved front and tapered logs in the back will be a good choice for the rafts."
The group carried therefor out comparative hydraulic tests in a water tank, measuring water resistance in relation to sail-speed. Subsequent both rafts were remodeled and constructed in accordance with the scientific result, as the tank test had indicated. They amputated the prow from the classic balsa raft and build a pair of tween rafts for their raid with square-off bow: 'stub-nosed and sloped'. They maybe could reach up to the double speed with this new hull shape, they expected.
Such a tank test probably is sufficient for a motor powered sailing, but we are not informed if they had carried out any test of their models for things as 'influence of wind abeam', 'directional stability' - etc. but the result was fatal.
As told, they build and sailed directly off without test nor trial of their new hull shape, and a month into their sailing, the rafters informed us, that they wasn't able to beat higher to the wind than 100 degrees - regardless where they plunged in their Guaras. (We have still not received their final report).
100 degrees to wind is in no way against.
The raft shown on the photo holds a rather rectangular shape, with a l/w ratio around 2½:1 and no pointed prow - she seems to use her starboard (lee) corner as prow. The position of yard indicate wind into aft port side = broad reach. The lifted front-end of port trunk together with the dipped starboard trunk-head indicate a good wind. The wake drawn from port side bow and too after the raft both indicate a deviation = a leeway of around 20 degrees
This photo gave us impulse to investigate the general theories around hull shapes, as described at page #7
As we now evaluate the case, the performed tank-tests demonstrated, that a rectangular hull could be pushed or towed rather well ahead as announced - but more tank tests, sailing along the diagonal with a front corner as prow would have shown nearly same low resistance.
A throughout investigation for sail directions all around the compass would probably have shown something like this oval vector field.
It seems as they have created a vessel with twin-prow.
The diagram indicate, that sailing forward in any direction between the centerline and a diagonal - or rather forward between both diagonals - doesn't offer much difference to stabilize anything, and with a wind directly abaft my experience says, that the craft will slalom ahead - first one bow then the other. Bistable we could name her.
Sailing with the wind more abeam the sideward forces will flip the hull over to diagonal sailing, where it will stay - and the raft will use the corner - the lee - as prow.
A little mathematic exercise:
A square-rigged boat has a rather large 'no-go' zone, but we know that a normal square-rigged craft at least will be able to go 80 degrees to the wind. In this Norwegian case the diagonal hold 20 degrees to the centerline of the hull, so if we subtract or add 20 degrees to the normal 80 degrees we get respectively 60 degrees and 100 degrees. 60 degrees is not possible for a square-rigger but 100 degrees was what we got - and that is what we can read from the photo.
Special theories around the limitation of square sail on square hull, can be found last at page #7.
That was perhaps a silly fault to change a well-proved hull shape, but their hazard arised in the moment they in their eager to start their Heyerdahl-adventure, against all their Norwegian tradition had pushed aside every final trial of their new craft before sailing out. The well established "prøveseglingsprosdyre" = 'procedure of test sailing' described by Jon Godal, they left out.
The fault was not to make an erroneous design.
The reckless error was to sail out on a waste ocean without any previous test of the new vessel.
- so stop that -
Beforehand we had felt much doubt around the archaeological purpose and value of this Kontiki-raid, but here came the great surprise showed up in that they verified - certainly unintended and by a self-sacrificial action - that the rafts in South America by the many hundred of years development had been so well optimized, that it had given us a useful hull shape for oceangoing balsa rafts, as not easily can be done better: The pointed prow, the long sideline - and a steering controlled by Guaras /daggerboards.
What could they have done
to escape the sad ending ?
- perhaps NEXT Norwegian raid will show more loyalty to the authentic South American culture
- and less freebooter -
They knew it, calculated the risk and took the chance.
But on their way to Easter Island they learned the fatal tacking capasity of their rafts, and therefor they could have waited for better wind in the harbour of Hangaroa - waite patiently, just as the old sailors did.
Or they could have changed the shape of their rafts (and tested the changes) before sailing out on next leg - thus eliminating the main reason for the missing ability for beating.
What they would have needed, was only a good sav, for cutting off the timbers and reshape a prow.
Cutting in Inca-maner: stepping down the trunks - or cutting slanted to shape a prov for the raft
Thankyou for your deed!We want here to express our deep gratefulness to the two authors and raft-skippers Thor Heyerdahl (Kontiki-1947) and John Haslett (Illa Tiki + Manteño Huancavilsa rafts 1994-98) for their carefully written accounts of their raft-raids in the Pacific.
- even if we still are waiting for the rest of the twenty raft-sailings, we have to recognize, that a great part of those accounts probably never will be written -