The Hull: LOW forward hydraulic resistance combined with a HIGH lateral ditto
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.
Venice gondola showing her asymmetrical hull
Not every shape of hull can be sailed by the wind
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.
If a free floating body hold a LOW forward resistance together with a HIGH lateral ditto - then we have the basis to power her by sail
the arrival of CLR and CE
The spell between Hull and Sail
Boat - CLR
Raft - CLR
The static Center for Lateral water-Resistance (the underwater hull) is defined for any vessel
The old-fashioned 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
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.
wind and water up against weight and buoyancy
normaly heeled ship
Vasa with bolders in bottom
And such simple calculation could probably have saved the famous oldtimer Vasa as in 1628 capsized just outside the shipyard.
HEELING: the torque made by wind against lateral water-resistance - in balance with the torque of bouyancy against gravity - but the only parameters as skipper can influence are sails and the weight of boulders in bottom (sailing in ballast)
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.
A consequence of those heeling calculations was the recognition, that with the ballast and cargo placed most down, the ship could carry more sail to drive her forward - and as soon the shipbuilding technology permitted the constructions, we saw deep aspect keels and fin keels with deep-down ballast replacing the boulders in hold.
From a static CLR to a dynamic CLR
The DYNAMIC influence of flowing water
The CLR - Center of Lateral Resistance was in the beginning defined alone by the static situaton - without sailing.
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.
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.
the STATIC situationas CLR was defined more than 100 years ago
sailing aheadthe pressure in bow-wave increase the LATERAL resistance in bow
the drift leewardof course the leeway too have a similar increase of forward resistance
resulting true coursethe result is that the CLR move ahead and a bit to leeward
First figure - The static CLR is more or less in the geometrical centre of the underwater hull
Sailing with a wind abeam, the wind as will try to blow ALL the craft sideways - but with the grip from bow-wave the wind will mostly be able move the aft-end.
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.
Strip of START-occurences
fig.1the STATIC CLR is defined as the Center of Lateral Resistance - no movement ahead -
fig.2when hoisting sail and sheeting out, the wind will blow its center CE to lee of the actual CLR
fig.3that means that the vessel before gaining speed will be prone for lee helm
fig.4more speed ahead bring more press in bowwave, and CLR move more ahead
fig.5full speed ahead place CLR most ahead - now prone for weather helm
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 now modified common rule
The actual CE will always blow leeward of the actual CLR and thus define the pointing of the vessel!
what are we doing when we steer
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 tack and sheet.
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.
Dipping Oar steering
Classic Rudder steering
Three steer systems doing the same
Start your sailing sailing ahead - next step will be adjustment of Guaras: more Guaras aft against aft-end sliding
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:
1): Set down some paddel-oars down on lee side to hinder sidedrift - or
2): Plunge down some Guaras /Daggerboards into their AFT-slots to do the same.
3): Turn your rudder to push sidewards on the streaming water.
And after that the wind will do the rest as ever: blow CE leeward of the actual CLR - to point your boat /raft.
Note: Guaras in bow are only needed for a static turn
When sailing ahead, you can lift upwards your FORE-guaras (but not take away) - because the squeeze from bow-wave will take over
- all steering will have to be done in AFT-end by regulating of leeway - as explained above - or as demonstrated.
The classic rudder and CLR
A rudder is a wonderfull steer system, as angeled out push on the streaming water and move the aft end sidewarts until you have your new course.
A motordriven boat then have to set the rudder neutral to keep this new course.
Whereas a sailpowered vessel have to keep the rudder angeled to sail on the new course.
What happen here is, that the static CLR is influenced of both the squeezing press in the bow wave, as hinder sidesliding of bow, but too of the sidewarts press on the angled rudder, and these two forces move CLR either against for or against aft. If you are angeling the rudder away from the wind the CLR creep closer to the mast - if you are angeleing against the wind the CLR move more ahead.
CLR is the Centre for Lateral water-Resistance - and "with sail braced right fore and aft" she is "heaved-to"
Sailing ahead is added a lateral resistance from the pressure in the bow-wave - and the center move ahead
Is the rudder angeled against the wind, then the water-resistance aft is reduced and the center move further ahead
Angling the rudder away from the wind, then is added a waterresistance and the center move now closer to aft
And keeping angled the position of rudder, then we have moved CLR, and the wind together with the sea will do the rest. The rule is as always: the wind will blow its center to lee of the centre of water resistance.
This permanent angeling of rudder /steeroar was why Thor Heyerdahl had to struggle so much to keep his course - and a steer-oar is much harder to angle out than a rudder because of the long oar-shaft.
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.
1): The rudder controlled crafts we are adjusting course by turn out the rudder, as push at the streaming water = the dynamic component of the CLR
2): A steer-oar as we either can dip deeper or twist (or both) - is a combination of a rudder and a aft-Guara
3): The Guara controlled rafts we change the course by changeing the shape of the under-water hull = the static part of CLR
4): The rudderless sailors are moving their Center of Wind by balancing the wind press on their sails, but too - when possible - moving the CLR by moving a ballast (crew) between for and aft or tilting a centerboard.
In all 4 cases the rule is:
The CE = the Winds Center will blow leeward of CLR = the Water-center
- and that is that, as define the pointing of your craft -
- and if your sail(s) are adjusted for the now pointed course, you will sail -
On a rudder-steered vessel, the nature of the dynamic component cause, that the helmsman 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
Comment around CLR of motor boats
We don't talk much about CLR in relation to motor boats, but of course the underwater hull of a motor powered vessel have a center for hydraulic resistance - and of course a motor have to thrust directly against this CLR to sail straight ahead. And of course both small and big ship are vulnerable for vind abeam, specially when slowing down missing the support from bow wave.
If you want to change or correct this course you have two ways: 1): Move direction of thrust as you do with an outboard motor - or - 2): move your CLR with rudder, steer oar or what you have of steer system.
After that a motor-powered boat have to set the rudder (the thrust if outboard-motor) neutral to keep this new course.
Shape your raft-bow as you want
Advantage of a cutwater = a POINTED bow
sharp stem cleaving the water in two streams - going to both starboard and port
bow wave of pushed square-off barge
A pointed bow give a high lateral resistance of the bow
- a rounded less
every boy (or girl) who has flown a kite know that a V-shape stay more stable in the sky- the V-shape is self-stabilizing because, when tilted the enlarged wing /side will take more wind and correct the imbalance - too valid for a box-kite -
- 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 -
Rafts with different bow-angel - all drawn 10 degrees inclined
A transom bow will not go back to central sailing, the raft will find a balance when the two water-streams are equal.
Whatever you have of angel on your bow, it will split up the instreaming water in two and thus create a reaction against a yaw
Whereas a bow-wave as is unilateral deflected by a transom bow will promote a crab-sailing along the diagonal
Hull shape - a study of raft-prows
Raft Prow Review
Kontiki 1947 Thor Heyerdahl mounted a "snow plough" in the stem
replica 2011 of Kontiki (Tangaroa2 converted for Film)
Las Balsas 1973 in Ecuador preparing for Australia
Illa Tiki 1995 - draft
Manteña Huancavilca 1998
Tangaroa2 raft 2008 with her classic shaped prow with figurehead
Kontiki2 2015 was born snub-nosed - raft Rahiti Tane -
An-Tiki 2011 - Could be a "4-bodied raft" - or a "double catamaran" ?
How to form the underwater body of a craft, we have discussed in hundreds of years and seen many solutions. That has given basis for much new-thinking and many innovations. The advantage of a balsa raft is its large load-carrying capacity - and the disadvantage is its slow headway. The Spaniards all described the balsa rafts having one big and long central trunk as a stem (just as a modern bulbous bow ?) And with minor and shorter trunks lashed alongside this. Always an odd number of trunks. As a hand !
The pointed stem is not a need for the bow of such a flat-bottomed hull, because the steering is done by Guaras, as statically change the underwater hull if needed. A raft doesn't heel, and loss because of capsizing is never heard.
The photo of Kontiki is from the book: 'American Indians in the Pacific' by Heyerdahl. On his raft Thor Heyerdahl mounted a "snow plough" in the stem to pass better through the water - or perhaps to reduce the splash. The Kontiki-raid showed us an awful seamanship - but was a wonderful adventure to hear about - and this gave impulse to many later adventurous raids.
The three 'Las Balsas' were build in Guayaquil in the classic Ecuadorian shape. Later on in 1973 they were sent over the Pacific Ocean, where they after a hard passing landed with the most of the rafts in North-East-Australia.
The 4 brave old men on the An-tiki-raft crossed the Atlantic in the wake of Columbus. They reduced the forward water-resistance by employing only 4 plastic tubes as floaters for buoyancy, so we feel doubt about it was a four 'bodied raft' - or a 'double catamaran'
structure of An-tiki
Square-off bow or pointed bow - the rules are the same
Attention on Square-off rafts
A square-off bow play a special play as not exist by the pointed bow
Running downwind, a square-off raft within few degrees can flip-over and change to sail on the other bow
raft.C): a square-off raft will probably be able to keep a straight ahead course sailing downwind - but if any wry wind it get unstable.
raft.B and raft.D): Unstable positions.
With minor unbalance, the lee corner take over the governance and divide the incomming water two unequal streams - the raft then will tumble over, due to the press from the one-sided deviation of the water and end as indicated at raft.A or raft.E - and each time the bow-wave change side, the raft will tumble over to the other side.
raft.A and raft.E): When the two streams reach same size, a flat-bottom raft will find her balance and sail stable on along a diagonal - just as the boy's kites.
The counteraction for this slalom-course is the same for every vessel as is running for the wind: Set the CE = Wind Center ahead - and keep the CLR = the Hydraulic Center well back For a Guara-raft that means: with the AFT-Guaras plunged down - and the rest up.
Just for every running vessel, the rule is:
a CE will blow leeward of a CLR
With wind abeam, a square-off raft need more Guaras down aft than a raft with a pointed bow - just to counteract the press from the one-sided bow wave.
With wind abeam
heaved-to sailing diagonal sailing straight alternative ahead
fig.1): the wind press is directly up against the HIGH lateral resistance = that means nearly a STOP of the vessel even with sail hoisted. And without streaming water, there is no displacement of the Hydraulic Center CLR
fig.2): sailing-on, the Hydraulic Center is moved ahead; and pressed by the unilateral deviation of bow-wave the raft will turn to diagonal sailing
You can sail on this diagonal course, but you have to adjust your sail and rigging for diagonal - and not centerline. But a diagonal sailing will NOT yield the LOWEST hydraulig resistance.
fig.3): A Guara-raft can point in every direction you would like!
Therefore the wry pointing can be compensated to centerline-sailing by move backward the CLR by plunging more Guaras down aft - until CLR match CE. Don't forget that Guaras plunged down brake the headway
fig.4): An alternative could be to move CE forward to match the position of CLR - by mounting a foresail or a jib in the stay. That too give indeed more forward force from sails.
This combination of the four presented is expected to give the raft her fastest headway.
As always - the rule is:
a CE will blow leeward of a CLR - and if your sail is adjusted for that pointing, you will sail -
A more pragmatic explication:
Sailing ahead the stem is pressed into the sea and of this reason hindered in side-sliding - whereas the water is slipping the aft-end without hindrance of nothing there.
And that is why Dirch only is handling his 'dipping paddle' from the aft-end of his vessel:
That has as consequence, that sailing with a wind abeam - the wind as will try to move ALL the craft sideways, but will mostly be able move the aft-end.
Result: With water press on stem, the CLR = Center of Water Resistance has moved forward, giving the ship weather helm, and we have to correct with our Guaras.
But either that is a greater problem. As explained earlier: On a raft with Guaras, we can controll sidesliding individually of for and aft, and we can move around our Hydraulic Centre as we will by plunging in or lifting up Guaras - and in that way correct any pointing.
In all cases the sail has to be adjusted for the real way through the sea + how the wind hit the raft - and if the sail is adjusted carefully for this - we will sail. If not, we will have a problem!
TRUE course and APPARENT wind are the parameters for adjustments of sails TRUE course is the direction of your wake
APPERENT wind is the direction of your flag or vind-vane
And this moving ahead of CLR is perhaps the reason, why newer speedy sails-ships seems to carry rather much sails in front of the hull + eventually equiped with foresail, bowsprit and jibs - and too have rather much skeg and keel aft - as the famous 'Bluenose' from Newfoundland - called the "Queen of the North Atlantic".
The core of sail boat steering
Understand more spatial the old CE and CLR
Just as the windsurf-board sketched, we today understand CE and CLR more spatial - as the conditions all around a sailing vessel.
CE we use as acronym for 'Center of winds Effort' - or simply the Wind-center - of the vessel, whatever static or dynamic wind-forces on hull, hut, rig and all sails in the position as the sails just now are adjusted.
CLR is a metaphor for the 'Center of Lateral Resistance' and is now the attack point for jointed static and dynamic forces on the underwater hull - the point of pivot. Of course we can split-up and distinguish the water resistance of Lateral resistance as control the leeway - and Forward resistance restricting the headway.
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.
And too the classic rudder work with the CLR
when things go wrong
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.
But don't destroy your Guara-steering
If you want to sail, it is as in any other case of life: What count is the result you get - and not what you could, should nor would have done.
The existence of an outer keel is no condition for exercise of sail powered sailing
If a Guara-raft can't beat to wind it is NOT a Guara-problem - the problem is either the sail or missing seamanship (knowledge).
A raft is as a flat-bottomed sail-craft, just as every one of the sail-ships with lee-boards depicted earlier
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.
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.