Pi Hi by John Pizzey
Hi John, very intriguing rig concept. The movable mast must mean that the jib “forestays” have to change length. How do you make furling gear effective when you can’t use a fixed length rod to pull evenly on the jib? It seems to me that keeping proper tension in wire luff jibs would be difficult, possibly making it difficult to furl the jibs when really needed. I guess the other jib forestay becomes a backstay to keep tension high to allow reliable furling. How do you make it work?
I think hanked jibs with downhauls might be more reliable, and less demanding of high luff tension. But not so elegant either.
John—this is very interesting and exciting stuff! Needless to say, I have some questions.
Why you use furlers for the fractional jibs? Why not use a smaller version of the same type of Isoceles jib, but smaller and hoisted fractionally?
How long is this design? It looks to be in the 8-9m range—is that right? Care to share any other nubers? I’m interested in sail area for the various rigs, BWL for different loading conditions (the steps), center to center BOA, etc.
I think you are saying that both rudders stay down all the time—with the back one steering and the front one adjusted to the amount of lift you want. Is that right? Is the front rudder backwards (using a two way crossection) or does it turn around (using a naca section)? I’d love to see some more explanation of that.
Are the forestays continuous, running through blocks?
ok…that’s enough for now.
Its great having you on this forum!
best,
Chris
Hi, in response to CP and Chris, the Jib forestays are fixed from the masthead down to the furlers which are winched directly down to the forebeam, double tackle if you want, big winch or whatever. When so winched down at the front the front strop between the furler and the leeside does nothing except lie around and wait for action. Its length does limit the athwartships positioning of the furler but the strop can be place anywhere for a suitable staying angle so that’s not a problem. The length of the strop (the back and active one) determines how far forward the mast pivots.
You could use hanked jibs but it would be messy and as Chris suggests you could simply use a smaller version of the drifter but at some time you have to settle on a design to meet the chosen paramaters and thats what is illustrated, in concept.
Until I cut Flight into halves and work out how much to add in, probably involving a few trials with screwed in panels, standing back and looking, nothing is fixed. The sail on Flight is around 300 ft² and this one will be bigger, I think I have drawn it at about 400 ft².
Both rudders stay down and freely rotate 360° as illustrated by the circle on my cockpit drawing above and that’s why I was so pleased with the control thereof, a breakthrough in my deliberations over the years as I have always had trouble with needle like tiller extensions, in my view they are a weak link, and by reference once more to the successful Russell design, one needing solving. Provided the linkages from the rudder slope up to the top of the bellcranks the linkages may stay attached to allow the rudders to be raised for beaching, accompanied by some movement of the tillers, hopefully.
Happy sailing
John Pizzey
Hi John,
I have answered to your comments about the A-frame mast here: http://proafile.com/forums/viewthread/116/
A heeling rig, as I remember, was first built in the trimaran Sigma made by the German designer Xaver Worthmann about 40 years ago. It works well, but at that time it was to much ahead.
Othmar
Hi John,
#13
In response to Rudolf, sailing bow down to some extent appears to be just about inevitable in a proa because of the fore and aft symmetry required unless you can use live crew weight to counteract it.
I don’t really understand why fore and aft symmetry would lead to bow down attitude. Exept when straight keeled. That I have experienced.
Some rocker in the keel could help keep the bow up by dynamic lift I would like to think. Also having the mast centered would leave more hull length in front of the mast to counter the thrust of the sail and help dynamic lift with a long lever arm.
The rocker would make it easier for the bow to lift as the stern will go down without any fuss.
This effect can be seen very well on Farrier tris sailing fast with their bows pointing at the sky.
I understand that in your configuration you want the boat to bear off to be able to counter that with you front rudder. Bow down attitude would cause the boat to luff as the bow has more lateral area then the stern.
regards,
Rudolf
I don’t really understand why fore and aft symmetry would lead to bow down attitude. Exept when straight keeled. That I have experienced.
At http://www.john-shuttleworth.com/Articles.html, in the article “Multihulls on Performance”, John Shuttleworth describes how he designs trimaran outrigger hulls so that the centre of buoyancy moves forward as the boat heels. That keeps the rudder in the water (in the days before foils and triple rudders) and makes pitchpoling less likely. In his article on the Tektron and Dogstar 50 catamarans, he writes:
In addition to increasing the accommodation space the flare creates a number of additional benefits to the sailing properties of the hull. The aft sections rise vertically off the water, while the forward sections are flared out so that, as the hull sinks into the water (as the boat heels), the centre of buoyancy moves forward. This has the effect of keeping the bows up when fast reaching, and also dampens pitching.
This shift of the static centre of buoyancy is not available with longitudinally symmetric hulls.
Some rocker in the keel could help keep the bow up by dynamic lift I would like to think.
I remember Shuttleworth mentioning this in an article on the Tektron 50 design, though I can’t find it on his web site. I guess it is an article I read in print, not online. He also uses flare. Newick used spray rails on Cheers. Fritz recently told us that his boats go bow up at speed. That can only be dynamic lift, because of the longitudinal symmetry, and I don’t remember seeing either flare or a spray rail on his hulls, but I do think the hull bottoms curve up at the end, and they are very rounded, not V-shaped.
Also having the mast centered would leave more hull length in front of the mast to counter the thrust of the sail and help dynamic lift with a long lever arm.
I know that keeping the bow up is one reason to put the mast far back on A class catamarans, and also on Idec II, Sodebo, Groupama II and Banque Populaire V, all designed to go downwind. However, I am not convinced that the reason is a longer lever arm between mast and bow. These are boats that routinely heel quite a bit while going fast. On page 2 of the “Multihulls on Performance” article (http://www.john-shuttleworth.com/Articles/mhonperf/multihull_perf_p5.jpg), Shuttleworth shows the downward component of a heeled rig. That is one way the longitudinal placement of the rig can make a difference.
But look only at the forward, not the lateral component of sail force, or assume the boat doesn’t heel enough that the lateral component develops enough downforce to worry about. If you are also happy to assume that the forward component remains parallel to the hull, then longitudinal placement makes not a blind bit of difference to how much the rig pushes the bow down. If the forward component remains parallel to the water, you want to place the rig forward, because aft placement and a rising stern would increase the lever arm. If the forward component is inclined, you want lifting sails forward and sails that push down aft. You probably want a pyramid rig. Like this, with perhaps greater spacing at the lower ends to increase the vertical forces:
Robert Biegler - 09 March 2013 04:08 AM
But look only at the forward, not the lateral component of sail force, or assume the boat doesn’t heel enough that the lateral component develops enough downforce to worry about. If you are also happy to assume that the forward component remains parallel to the hull, then longitudinal placement makes not a blind bit of difference to how much the rig pushes the bow down. If the forward component remains parallel to the water, you want to place the rig forward, because aft placement and a rising stern would increase the lever arm.
I agree with that.
But wouldn’t the effect of dynamic lift benefit from placing the rig aft?
Rudolf
“This shift of the static centre of buoyancy is not available with longitudinally symmetric hulls.”
If a hull was longitudinally symmetrical, as well as with flared bows, wouldn’t this effect be even more pronounced? Because as the one bow sinks and becomes fuller, the other rises and becomes much skinnier, hence big shift in buoyancy.
Hi Rudolph, the Farrier tri used to be an exception in multihulls. I am not up to date any more on recent design trends, but in his Trailertri design and against convention at the time Ian opted for a broad stern semi-planing hull in a light boat. I have some knowledge of them as I sailed in the first prototype and prepared and prosecuted Ian’s patents which served him well. I doubt that you could create this arrangement in a proa, but my life has been spent getting patents for people who have made advancements so I never say it can’t be done and hopefully someone will come up with the answer, if they have not already done so.
I am no expert on hydrodynamics so I can’t fully answer your questions. Non-symmetrical boats have much more buoyancy aft than forward, yet they still manage to sail bow up. Whereas, if most of the buoyancy is centred as in a proa hull with a lot of rocker, any force applied by the rig high up will tend to rock the boat forward about this central buoyancy and perhaps this can or can’t be countered by selective shaping of the bottom of the hull.
When you introduce speed and dynamic effects into the equation, who knows, but I do know that there are less variables for a proa than a cat or tri. As I said earlier relying on amateur advice is risky business but discussions are interesting and help us see other points of view. I particularly noted in his article referred to, John Shuttleworth said “Other types. Proas (Atlantic, ama to leeward, and Pacific, ama to windward) and trimaran foilers. In general these are development types almost exclusively for racing, as far as modern multihulls are concerned, and they have special problems that require particular knowledge, experience and seamanship for safety at sea. Due to lack of space these types will not be dealt with in any detail in this article”
Perhaps he should be asked to contribute to the discussion.
The simple, but not the only amswer is to keep the hulls long and the rig low, minimum rocker may also help and can be used in proas but less so in any other sailing craft as tacking would be a problem. If you want a tall rig for speed, the mast back with a reacher or spinnaker set as I have proposed may give the lift we are seeking. Being an amateur I can’t predict that so I will have to build it and report the outcome, if only I could wave a magic wand.
Cheers
John Pizzey
Thanks John,
You’re right, it will have to be built to see if it works. This seems to be unkown territory.
It is nice to see different approaches form different people,
towards the forces involved and towards dealing with/using these forces.
regards,
Rudolf
Hi Rudolph, the other most important factor which I failed to mention is that in a “conventional” craft which has lots of buoyancy aft, it must also have lots of load aft, that is a comparatively light bow. In proas we are stuck with symmetry in buoyancy and load unless we want to perform differently on different tacks.
Cheers
John
“This shift of the static centre of buoyancy is not available with longitudinally symmetric hulls.”
If a hull was longitudinally symmetrical, as well as with flared bows, wouldn’t this effect be even more pronounced? Because as the one bow sinks and becomes fuller, the other rises and becomes much skinnier, hence big shift in buoyancy.
True, but that’s a change in trim. Shuttleworth was trying to design boats that either don’t change trim or else trim bow up as the lee hull sinks. He wants a shift in centre of buoyancy with immersion, not trim. The background is that Shuttleworth was working for Kelsall when they designed racing trimarans in the late 70s. These were powerful enough to get problems with the rudder lifting out. Shuttleworth’s solution was to design amas that shifted the centre of buoyancy markedly forward when immersed, giving his 1980 designs a characteristic bow-up trim when pushed hard, which kept the central rudder in the water and also made pitchpole less likely.
Then he found a way to create the same effect in catamarans.
He explains his thinking in the articles I linked to.
His article on seaworthiness is also very much worth reading.
Regards
Robert Biegler
But wouldn’t the effect of dynamic lift benefit from placing the rig aft?
I don’t know. I had assumed that the effect were additive, instead of interacting. But you could be right, because the dynamic lift from rocker in the bow should depend on the angle of attack, so if the rig pushes the bow down too much, you might even get the water flow sucking the bow down.
Thinking more about the explanation for why the rig being placed aft helps keep the bow up, I am no longer satisfied with it. If the rig’s centre of effort is moved aft, the lateral resistance has to follow suit. If boards and foils are parallel to masts, then the rig pushing the stern down should be countered by board and rudder pushing it up. If the mast is canted to windward, placing it aft should make things worse. That would leave only the weight of the rig as an explanation.
Regards
Robert Biegler
Hi Robert and Rudolf, maybe as I suggested in my sail forces article the rig aft gives a greater inclination to the forestay and more lift from the headsail. I seem to recall reading an account of Russell’s trip across the Pacific indicating they often sailed under foresail only but I haven’t checked to see if that is correct. Russell appears to be everyone’s barometer.
Regards
John.
Robert Biegler
Thinking more about the explanation for why the rig being placed aft helps keep the bow up, I am no longer satisfied with it. If the rig’s centre of effort is moved aft, the lateral resistance has to follow suit. If boards and foils are parallel to masts, then the rig pushing the stern down should be countered by board and rudder pushing it up. If the mast is canted to windward, placing it aft should make things worse. That would leave only the weight of the rig as an explanation.
Is the downforce of the rig you are talking about caused by heeling?
I do not take any heeling into account. That may be a mistake though. To skim the ww hull over the water
you don’t need a big heeling angle. Also downthrust is opposed by lift of the boards. Otherwise the boat would be pushed straight down in big winds.
(Of course this is just my simple way of trying to understand what’s going on.)
To keep the bow up I would keep weight away from there and shape the botom to provide dynamic lift.( rocker)
That should oppose the forward thrust of the sail and keep the bow up.
In my view this lifting of the bow is related to speed, so if the bow digs into a wave and speed remains lift is still
there. To help slicing through the wave deck area has to be smaller than the bottom area and preferably not flat.
Of course a long bow (lw hull) would be a big help.
When sailing Blind Date I had the feeling that at 14 kts it was clearly held back from further accelerating.
At that stage it would have a bow down attitude. Linking that to the straight keel from the lw hull led me
to this line of thinking.
But any other solution to this problem would be welcomed of course.
regards,
Rudolf