Print Friendly
[ img - print2.png ]
LayOuts
Casa
de
Dina
[ img - tumi2.gif ]
Hotel
in
Lima
< The inca's balsa log raft
- Seamanship and rafts
- A desktop exercise
- The Pharaoh's reed boat
- Catamarans with trimboards

Addendum II to
Inca's sail powered Balsa Log Raft

Sailing catamarans with Guaras


Some catamarans are born with Guaras - even if they call them "trimboards"


[ img - Pahi-52-sailing.jpg ]
oceangoing catamaran of renown mark
A rudder steered vessel is normaly considered as "well trimmed", when she with wind directly abeam can sail with rudder neutral
- but of course skipper can trim her for any wind

There is no doubt, that in the European sail culture we principaly used rudder steering to our boats. And a rudder in correct dimension is an effective and quick working steer system, as we have no reason to leave - but that is not the only way to controll the steering of a sail vessel. There exist too Guara-steering.
Some catamarans (Wharram Pahi for example) are born with 4 trim keels, as even not created for that purpose make a compleete Guara-steering.

The natural static centre of hydraulic resistance of the hull will move out in direction of the trim board as is set down - and when sailing on, this excentric CLR will move ahead due to the grip from bow-wave + a little to leeward due to leeway created by the wind abeam.
And this dynamic CLR = 'centre of hydraulic resistance' will work together together with CE = the 'centre of winds effort' and decide the direction of your cat.

When you have balanced (trimmed) your leeway of aft versus that of your bow, you can tie your tiller and go to make something other - as a cup of coffe. Your catamaran will keep her course in relation to the wind - just as a windvane - a weather cock itself.

Trimmed, this Guara-steering will not impede use of rudder. The rudder-steering will play together with the trimboards and permit correcting to both sides of the trimmed course.


The situation on the minor cats is a little different. Those Wharrams are not born with trim keels, but have two steer-oars as seems much alike those of the Pharaohnic vessels. Because these steer-oars are not fixed by any withy through the side of hull, they can be dipped or lifted and in this easy way give a trim, as still permit a steering by twisting the oar - just as the pharaohnic boats did on the river Nile.

- small explanatory hints -

Some remarks:

1):    Guara (trimboard) steering will only work with wind on sails - and not with neither paddles, oars nor motors -
2):    Use if possible the two centerboards in windward hull, and not those in lee hull, and with them you can balance the leeway of aft-end versus the fore-end.

If you are experienced sailor, then note that rudder-steering and Guara-steering both do the same: they move the CLR = Center of hydraulic Resistance.
Rudder-steering work with the dynamic streeming of water
Guara-steering change the static CLR-position of underwater hull

And if you don't like to work with CE and CLR, as you feel rather indefinable, then you could simply 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 whatever you have.

If your catamaran NOT is born with build-in Centerboard of type trim boards, as the above illustrated Wharram Pahi, it could be an easier solution to mount these as leeboards outside on luff hull - for and aft - what all together means on that side of the hulls, as turn against the other hull. With that your hulls at least will not need submit to any serious "surgical" operation.

[ img - cat-all-stars2.png ]
Wind is blowing and the sea resist movements - each one working in own center CE and CLR. And that is the position of these two centres as give the pointing of the craft.

Any steering of the pointing skipper can do by move one or both of the two centers - and that could be done by a rudder, adjustment between more sails, moving Guaras, angeling or dipping a steeroar etc

On every sail-powered ship it is so, that due to gusts and big waves the dynamic component defining the centres of both water and wind will oscillate and that make exact calculations of Center of Wind and Water Resistance impossible with any degree of accuracy. Just as with rudder-steering we neither need to know that. We only need to know their nature to be able to counteract the actual situation by adjustments of sail and Guaras - and that is what skipper has to do.

We have heard of skippers steering with wind-rudder, as usually is a wind-vane attached by ropes to the tiller. And thus moving the classic rudder of the boat by this ropes, the boat will keep its course in relation to the wind. A Guara-steered craft act as a big wind-vane in herself.

a practicable example

Steering catamarans with Guaras

This web-page is a manual dedicated to explain the function of the Incas' old steer system for their sail-powered rafts. Guara is the board called, (or vara as sound igual in Spanish) and Guara is the same as a daggerboard.
Under the way round in the raft sailors world Thor Heyerdahl has been a central figure. Not only for his deeds, but too because he by his Kon-tiki raid put the focus on Guara steering, as the sailing world with roots in the western cultures has known centuries, but never understood.

That steer phenomenon with Guaras is explained in details at earlier page.

Tuching a theme as Thor Heyerdahl brought us around in the existing world of sailers - and by Heyerdahl's reed rafts, too to the ancient Egypt, and all the way discovering many things, as not are common known.

For example:
1): that there are catamarans sailing around with a complete Guara steer system - without knowing it - or -
2): other catamarans sailing around with a steer-system similar to the genial steer-oar of ancient Egyptians - too without to know the further virtues. That is what I will tell about in this chapter - and exemplify.

- - - In chapter 5 was shown that a Kon-tiki raft with a Guara in each corner is able to point statically all the compass around. The same is of course possible too sailing a catamaran with a Guara /daggerboard /centerboard in each corner. That means that a catamaran equipped as a Wharram Pahi63 can navigate alone with her trim-keels as for the purpose are placed in each corner.

The explication for creation of CLRs:
4 trimboards[ img - cat1.png]
can be combined[ img - cat5.png]
A vessel with trim-boards in each corner can create and move her CLR all around the compass
 
 
 
 
[ img - zephyr-red.gif ]
blowing of wind
Wind abeam
[ img - cat2.png]
Broad reach
[ img - cat3.png]
Running downwind
[ img - cat4.png]
 
 
 
[ img - neptun.gif ]
resistance of water
The result is that when we place our CLR = 'Centre of Hydraulic Resistance' anywhere all around the mast.
then with a simple rigging and sail we can point our raft where we want -
Rule No.1 for Guaras (here 4 trimboards):
By use of your bow and stern Guaras /trimboards, you can define CLR = your hold in the water
= the pivot point for your vessel

Rule No.2: The basic idea in Guara steering is:
Whatever you have of sail, the CE = Centre of winds Effort will blow downwind of the Centre of Hydraulic Resistance (as is the hold in water) and thus define the pointing of the craft
- and if sails are adjusted for the now pointed course, you are sailing -

Pointing just as a weather-cock on his pivot!

One thing is a static turn, but sailing on, the condition changes, because now the bow is pressed into the bowwave and that press will hinder a sidesliding - whereas nothing will hinder the aft-end from drifting sidewarts.
Weather helm is a phenomenon as occur with all sail boats when they start sailing, whatever they use of steering. Here the skipper has to act: angle his rudder or adjust his Guaras /trimboards.
Because of the press in bowwave skipper not need so much trim in front, and he can therefore lift upwards his fore trimboards.
Abaft he has either to counteract with his rudder or he can plunge downwards aft trimboard - bringing the sidesliding of aft end in balance with the sidesliding of bow - keeping his pointing.
With a rudder steering he probably need to have a hand on tiller. Only if his craft is balanced (trimmed), he can let go, and the craft will keep her mean course only disturbed by gust and big waves.

NOTE: Sailing on, the CE will still blow leeward of CLR, and still point the vessel, as it have to - only difference is, that by 'sailing on' the press on bow has moved CLR ahead!

[ img - pahi-63.jpg ]
the interesting is that this Wharram Pahi catamaran is born with four centerboards,
placed as trimboards in the ends of each hull

That is the shape (profile) of the underwater-hull as define the CLR = 'Centre of Lateral Resistance'
- and with the trimboards this catamaran can move her CLR against for and aft

[ img - pahi63-profile+CLRs.jpg ]
One thing is that it is possible to steer this catamaran by her trim-boards only. Another ting is to do it. In this case the greatest hinder is , that access to the trimboards is rather difficult and far away from the helm.
If the trimboards should be used for steering, what could be rather interesting, the lifting-lowering handles should be mounted near by the place for the watch on duty.
tracking the origen of the Heyerdahl reed rafts.
Once more Thor Heyerdahl by his reed raft sailings inspired us to pay this visit in the land of the Pharaohs.

The first astonishing observation from the old land of Pharaoh was their handheld steer-oars on their reed rafts. That steer principle looks much, as what Dirch from the house of Wharram demonstrated on Carnon river in Cornwall: Dipping a steer-oar on lee side of aft end to brake the side-sliding of aft-end; and the further study verified, that it was what they did in old Egypt in their first period of river-sailing.

dipping steer-oars painted in Tomb of Merab - 4500 years ago [ img - tombOfMerab-24Ghizeh.jpg ]
dipping steer-oar demonstrated at Carnon river
[ img - dirch.jpg ]
note that this boat as is steered with a dipping oar is the same as shown as number three down in the stip - there with a classic twisting steer-oar
The Egyprians boats grew in size, and the sailing capasity developed. With their bigger boats they had to use up to 4 helmsmen working with each his dipping steer-oar to give the needed size of hydraulic resistance to balance of side-sliding. Then they changed to one oar only - but now so big, that no man could wield it in hands. Therefore they relieved the human force by fastening the oar to a support on the side of the boat.

Outside the dipping of a huge steer-oar the new fastening technology gave option for a new use: twisting the steer-oar, what is very useful when rowing.

The relief from tomb of queen Hatshepsut can be rather difficult to read clear after the many thousands of years - but some scolars and specialist have done the work to make some rather detailed drawings
[ img - TombOfHatshepsut5.jpg ]
[ img - punt21-ship.jpg ]
[ img - punt11-ship.jpg ]
Oceancoing Egypt vessels
-
two sections of the tomb-relief showing details around the stee-oars and their fastenings
Note the detail on the drawings as are showing that the longshafted steer-oar on all boats are fasted to the gunwale by a "parrel" as both permit a twist of the oar but too a displacement along its axis (dipping)

[ img - lapita-08.jpg ]
[ img - lapita-10.jpg ]
[ img - tahitiwayfarer-sailing-18.jpg ]
[ img - Wharram-amatasi-oar.jpg ]
[ img - amatasi-06.jpg ]
[ img - amatasi-07.jpg ]
an oceangoing catamarn and two minor ditto for daysailing show their steer-oar arrangement as looks rather similar to those from the old Egypt
- but the newer boats can't the same as the old ones. Even if they easily can be changed, they are born to 'twist only' - they are not build to dip for trim of the vessel


The Pharaoh steer-oar is rather genial. A Nile-boat you can point by tilting her steer-oar - back and forth
This oar combine the stable Guara /daggerboard /trimboard-steering with a classic rudder. Dipping the oar the Egyptians first trimed the vessel for awry load and cargo, adjusting to neutral leeway of aft - and then the oar was tied. Under sailing it could work as a normal rudder by twisting the blade.

Lift up your steeroar will move ahead your CLR as give 'weather helm'   -   Deeper down and more aft will move your CLR aft, as give 'lee helm'
[ img - farao+CLR-A.png ]
Tilted up = Minor impact on the CLR of hull
[ img - farao+CLR-B.png ]
Tilted down give more impact = moving more the CLR
[ img - farao+CLR-C.png ]
Pushed down and aft = Most impact, but need a long strong shaft - as the Nile boats had
- and with sails hoisted, whatever type of sail you have, you will learn what what the position of CLR means for your boat -
An example: If all your crew is joining aft in your boat, then the aft end will be loaded down, and you will get lee-helm. That you simply can lessen by lifting the steer-oar upwards and the boat will balance again - or you of course can do as we all times have done: press your tiller sidewarts to compensate. Skippers choice.

Note that the Pharaonics only needed to use one oar - that placed on lee side of a hull. That observation probably means that the new generation of catamarans with two steer-oars don't need to join their tillers, as sometimes seen. That is okay for downwind sailing, but by normal wind abeam they only need that steer-oar in action, as is mounted on a lee side af a hull.

Some considerations around the asymmetric hull shape of the pacific proa

The speedy Flying Proa


[ img - venice-gondola+canal.jpg ]
The Venice condola is a well known asymmetric hull shape - but is never seen with sail

The flying proa and her asymmetric hull

Cite Wikipedia:   During his 1740–1744 circumnavigation, Lord Anson also saw the proa. His fleet captured one in 1742, and Lt. Peircy Brett of HMS Centurion made a detailed sketch of the proa - Rev. Richard Walter, chaplain of HMS Centurion, estimated the speed of the proa at twenty miles per hour (32 km/h)

[ img - anson_proa.jpg ] [ img - Prao-Mariannes-3v.jpg ]
the interesting point here is the asymmetrical hull shape, the shunting instead of tacking is a consequence of that

'The flying proa' of the Marianer Islands keep allways her outrigger to wind side - sailing-on and heeling she lifted her outrigger out of water and gained speed by reducing her forward hydraulic resistance. Furthermore they by balasting this outrigger with crew, they could balance their heeling.
To keep the outrigger always windward has as consequence, that she has to shunt and not tack, and that they did by changing tack and sheet from bow to stern - and in same operation tilting the mast the other way.
With other type of rigging as the symmetrical square sail or faroe sail, they probably could have shunted in an other way keeping the mast untuched - but they used crab claw sails (sometimes by Europeans mistaken for latin sail) and had to do as they did.
They used a steer-oar for steering - probably a dipping steer-oar placed on lee side of hull - and of course too changing to the other end when shunting.

The most interesting is NOT their rigging and way to tack, even most spectacular.
The most interesting is their asymmetric shape of main hull, as make it necessary to shunt.
An asymmetric under water hull we know very well from all our mono-hulls, as all are asymmetric when heeled.
Here at the Marianas Islands they had a asymmetric foil formed hull-shape as they kept with same side to the wind independently which pointing. Such a hull shape is supposed to act as a wing and give a dynamic lift across the streeming of water. In this way they seems to had created a ship-shape as counteract the traditional leeway.
More speed = more lift. But just around this "anti leeway lift"-phenomenon we have no account.


Have a look at some NASA foils:
[ img - airfoil.png ]
a collection of standard NASA floils
[ img - conventional-airfoils.gif ]

Going on with this wry hull shape:
If a asymmetric hull shape is good for a Flying Proa, then next step is to have a look on a catamaran
[ img - cat-A.png ] [ img - cat-B.png ] [ img - cat-C.png ]
First figure:   The classic catamaran hold two rather symmetric hulls tied together with some beams
Second figure:   If we make those hulls asymmetric with camber against the opposite hull, then nothing will happen, because the lift of each hull will neutralize the other
Third figure:   A catamaran don't heel, we say, but that is not quite right, because if we sail with wind abeam, then the wind force on sail will press down and submerge the lee hull - and lift the luff. And with that, the underwater hulls change size - both length and depth, and will yield different lifts, and we will have a resulting dynamic force as work against leeway.
I have still not met any proved knowledge about streeming water around an asymmetric foil-shaped double hull.
Therefore I don't know, if that is possible to create a hydro dynamic lift - like an aeroplane wing - as can match and counteract the static lateral force from wind.
That means: permit the catamaran to beat higher to the wind by reducing her leeway.
That theoretical challenge is hereby handed over to next generation - to be verified or rejected.
[ img - zephyr-red.gif ]
Asymmetric
Catamaran
heeled 4°
[ img - cat-heel.png ]

 

If she sail with less leeway is not verified
- but it seems so

[ img - mail-kly-runasimi.gif ]

Lima - Octubre 2018 - Seventh edition of this note around catamarans