DANSK

Incas' Oceangoing Balsa Log Raft

The Power of Hull and Stem

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The Hull: LOW forward hydraulic resistance combined with a HIGH lateral ditto

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Rule for free floating bodies:

Anything as can float free, we can move-on and navigate by hands and feet, poles, paddles or oars - or too with a motor of some kind -
That statement is true; but if we want to use sail for more than downwind running, then we have to take care and develop the underwater hull of our vessel with a LOW forward resistance and a HIGH lateral ditto

Sail Ship development

A sail powered ship need a clear axis to keep a steady course when sailing ahead with wind across. 'Clear axis' is the one with obvious smallest water resistance compared with any other saildirection.
A 'clear axis' not necessarily means a symmetric hull, because a heeling sail-ship always has an asymmetric underwater hull shape - more heeling = more asymmetry - and despite their mounted "sidecar", neither the Pacific sail-powered out-rigger canoe nor the 'flying proa' from Marianas Islands show any sail-problem - on the contrary. The hull of a proa is normaly asymmetric along the axis, and is peculiar by always having same side - the outrigger side - to windward. She is shunting to reverse direction when tacking.
The elegant shaped Venetian gondola too is designed with an asymmetric hull shape, but that is to compensate the one-sided propulsion from her only oar.

The underwater shape of a hull should hold one (and only one) such sail direction, which is identified by the lowest Hydraulic Resistance - and that direction should be identical with the pointing. To take in account both wind from starboard as well as from port, it should be a good choice to have the centerline as this prefered saildirection - and not any diagonal or other.
- in all other directions a craft powered by sail need to have an increased Water Resistance, specially perpendicular to the sail direction - to make the pointing stable and keep leeway of minor size.
Whatever you use for propulsion, the lesson learned by our hundreds of years development indicates that a pointed prow and a smooth behind make sailing easier and speedier, but the years too indicate, that a hull shape with a high length /beam ratio - or a hull with straight sides will improve the stability on the pointed course. Later on we learned to talk about streamlined shape.

When we in pre-historic time tried to mount a sail on our "floating vessel" to catch the wind, we first drifted downwind, but nevertheless we had got a propulsion, as in some situations was supplementary to our paddeling. But not each time we hoisted our sail, the wind blew to where we wanted to go with our vessel, and we had therefor to get something else, as could push along and keep a pointed course - even with wind across.
And that could our rowing boats, so we didn't meet difficulties in that. But to use a sail as only propulsion is rather more problematic, because we have forces from the wind crossing our sail-direction. But soon we learned to manage the sail sailing along in directions other than the wind blew.
But the old steer-oar remained many centuries on its place aft starboard side, where it was landed as paddle from our canoes and rowing boats.

The hidden keel

Keel we call the central backbone of the hull - even if there are no outer fin to see.
In historic time very few sail ships have had a typical outer keel, because all ships were build on the shore - or in the larger cases, on a slipway. Nevertheless, the shape of a hull gave the needed sail-stability. Long and slim for speed - or more wide for better transport capacity - but all crafts were sailing well in the pointed sail-direction.
Streamlined the hulls were for more speed, and with only one clear axis - and not square nor circular, as these forms could have more than one sail axis.

The spell between Hull and Sail

The old-fashioned static way to use CE and CLR:
Estimation of heeling - or calculation of ballast boulders

Only few centuries ago we in shipbuilding went from extrapolation of experience and model building to calculation of consequences.
The calculations of CE and CLR started in the waning era of sail-ships, mostly to calculate worst case of the heeling of tall ships and large yachts - setting the forces on those centers in balance with the weight of the vessel concentrated in the Gravitation center together with the buoyancy forces through the Metacenter.
That was the time when the ballast still was placed in the bottom of the ship as boulders.

Cites from Thomas Walton's book: 'Know your own ship', 1899 Web link
In that time the CE = Center of Wind's Effort was calculated with "only such sail as could be carried safely in fresh breeze" and "those sails were supposed to be braced right fore and aft" (worst case).
Finding the CLR = Center of Lateral Resistance was a bit more tricky, but it was "approximately and sufficient correct for all practical purposes as the center of the immersed longitudinal section passing through the middle line of the ship" = the Center of Broadside Plane.

Under this conditions were calculated the heeling -
Or rather, they calculated the boulders needed in ballast to keep within certain heeling:
A fresh breeze attacking in the CE will turn the ship over the calculated CLR and the ship will heel down until the geometry of the heeled hull has moved the buoyancy center sufficient out from the central plane and together with the weight of vessel concentrated in the gravitation center have reestablished a balance.
When in balance, we have the searched heeling.

So it was then; but since naval architects and yacht designers still use the CE and CLR for general design purposes, because they are convenient and easy to calculate.

That is so, if a ship is heeled because of wind, then a certain percent of winds force on sails alone will press downwards and augment the weight of ship - and with that increase the seize of submerged hull:   10° heeled - give +17% of winds force, 20° heeled give +32% of winds force - and 30° heeled give +44% of winds force to be added to the weight of the ship.

Heeling and ballast isn't of much interest for balsa-rafts, but the core of the above mentioned theories around CE and CLR developed hundred years ago seems to be a useful tool to explain the Guara-steer system and the static turn in a more technical way - but too as a supplementary theory as is special useful for wide-beam vessels, as rafts and catamarans.

The DYNAMIC influence of streeming water

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When sheeting out, the vessel begin to sail, and she will accelerate until the hydraulic resistance against this forward movment has grown to the same size as the forward force from sail. But too she wil gain leeway until the sidewarts force from same sail will match the growing lateral hydraulic resistance.

Technically the contribution from the dynamic forces, as created by the streaming water when sailing, is explained by the few sketches
- and that is the same whatever you have a pointed, a rounded or a square-off bow.

First figure - The static CLR is more or less in the geometrical centre of the underwater hull

Second figure - Sailing ahead:
The vessel will plunge her bow into the sea and due to the press of water in the bowwave the bow is seized and hindered in side-sliding - whereas the water is slipping the aft-end without hindrance of nothing there.
The result of this hold of water around the stem play together with the old static CLR and create a new = a dynamic CLR (Center of Lateral Resistance) more ahead. And with more sail-speed = even more ahead.
The new dynamic CLR = Center of Lateral Resistance is here indicated by a blue ring and is the combination of old CLR + the water pressed around the bow.
The arrow indicate here a common hydraulic resistance with its attack point - and nothing around movement.

Third figure - Drifting leeward:
The same hydraulic rules reign if the vessel drift sidewards; but due to the fact, that the most vessels hold a high lateral resistance, the leeward speed are much smaller, the pressure in the "bow-wave" therefore low and the CLR move only a bit against lee.

Fourth figure - Resulting situation:
And when the wind blow abeam, the result of those two movements - as are connected to the headway and the leeway respectively - place the dynamic CLR more ahead and a bit to lee.
The truth is, that we seldom calculate with any sidewarts "bowwave", because it is totally dominated by the changes in the shape of a heeled underwater-hull. - but it exist!

Generally we only say: "When sailing ahead, the CLR moves ahead" - without to say much more.

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Sequence of events:   When hoisting the sail, we establish a CE = Center of Winds Effort, and this will play together with the actual CLR - first the static CLR and then the dynamic - and as the vessel gain speed the CLR move ahead.

The classic rudder and CLR

 
Note:   Any board /Guara set down + and any rudder /steer oar angled out - will brake the speed of headway.
      - and both board and rudder work together with the wind by moving ahead or abaft CLR = Center of Lateral Resistance.
Note the difference: A sail powered vessel often sail with rudder angeled, to keep a course - on motor powered vessels, the rudder nearly always is neutral.

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Resumen of the 4 different steer systems.

The sailed course is a result of those two centres: CE and CLR. They define the course, and a correction is due to manipulation of one or both of those two centres.

On a rudder-steered sail vessel, the nature of the dynamic component cause, that the helmsman nearly constantly need his hand on the tiller
On the Guaras controlled raft, the static component will do that the raft will stay stable as a weathercock as long as the wind blow stable from same direction

Kon-Tiki raft sailing:   counteractions against weather helm

With more sails you can counteract (or trim) by move more ahead (or abaft) the CE = the common Center of Effort. That you can do by adjusting the sails (when more sails), setting a foresail, a jib or similar.
With only one square sail skipper can't do much with his CE - but he has the option to tilt (what is easier than move) the mast forward with parrel, yard and perhaps with tack and sheet too.
The alternative to move ahead the wind-center CE is to move backwards the water-centre CLR, what is the most normal to do; and that counteraction is the same, whatever you have of steer-system.

The relaxed explication: Balance the leeway of AFT-end against that of FORE-end.
The more technical /scientific explication: Move backwards CLR by one or more of these three ways:

And after that the wind will do the rest as ever: blow CE leeward of the actual CLR - to point your boat /raft.

Advantage of a cutwater = a POINTED bow

- and from the kites in blowing wind it isn't difficult to see, what will happen with raft-bows of same V-shape in streaming water -
- even a minor change (here shown 10 degrees) in pointing raise immediately a course-correcting waterpress -
- and too we could ponder what influence the prow-angel has on the reaction-force against a minor yaw -
- and furthermore we could think over what a bulbous nose will apply to that -

Raft Prow Review

Attention on Square-off rafts

 

Understand more spatial the old CE and CLR

Note the simple fact, that the dynamic contribution of both water and wind will oscillate due to gust and waves, and that make exact calculations of CE and CLR impossible; and therefor we can't make any beforehand calculated tuning.
But there is no need to do that, because with these theories, then skipper know in what direction he has to move CE and CLR to obtain the wanted reaction from his vessel, and that he will do by adjustments on sail and Guaras.

With other words:

"The Wind-center will blow to leeward of the Center of the Hydraulic Resistance" - (and that simply is so, because there is no other forces as influence on the situation)
What we do, is to "steer". We react on the 'now-situation', with what we know about the nature of CE and CLR, and we move either CE or CLR by changing one or both of the dynamic components - alternatively one or both of the static components.
Too the classic rudder work with the CLR.

And if you by an incident or foolishness have build your craft, and the lateral resistance as should be high seems insufficient for your purpose, then you may try to increase this by mounting a skeg, a permanent fin, a retractable keel, a centerboard, a daggerboard, a leeboard or you could mount a row of boards as Guaras along the sides of your craft - or perhaps you could mount an outrigger.

Final Conclusion:

The Evidence:

A raft is as a flat-bottomed sail-craft, just as every one of the sail-ships with lee-boards depicted earlier Ref: flat bottom
The vessels shown are all flat-bottomed sail crafts without keel, but nevertheless they without greater problems are able to beat against wind and keep a stable course - but of course, a lee-board of some type will make her beat even higher to the wind - or as in the Humber Keel-case give a turn-point for a slab-sided and stable craft.

Technical comment:

Rudder-steering and Guara-steering both do the same: they move the CLR = Center of hydraulic Resistance.
Rudder-steering work dynamically pushing on the streaming water - as push back and deflect on the rudder.
Guara-steering change the static CLR-position of underwater hull

And if you don't like to work with CE and CLR, then you could say:
Sailing ahead with wind abeam, the bow is hindred in leeway by the press in the bow wave - and that is the leeway of aft-end you contrawork and bring in balance with that of for-end, by using either your rudder, steer-oar, Guara or 'dip a paddle' - or whatever you have.

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the Sail