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Proa talk


rob denney

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Reference to Freeboard is that a Cat3 Inspector must firstly assess the overall seaworthiness of a vessell before completing the Cat3 inspection.

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So if you got an inspector who thouight like Rod Boy you'd never be allowed to race :lol:

 

Greg - good post, got a meeting this morning, I'll have another look this arvo.

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Squid: In a storm longer/wider is always going to be better, do I have to explain that?

No what I would like you to explain is whether there is any difference, due to the short hull on one side, between the way a harryproa behaves in rough weather and a catamaran of the same length. If you look at the harrigami photo below you can see there is very little displacement in the ends of the long hull. I can’t see how the extra length (meaning the 1.25 metres from the bows back) of such a hull contributes much towards seakindliness when its tied to a short hull on the other side of the boat.

 

 

Is fora the plural of forum? Maybe fori? guess it depends on whether it is greek or latin, better than forums.

It is a plural of forum. The other is forums, I like fora better too.

 

I think Greg made the right choice of boat with the tri.

It was the interior layout that clinched it. Comfortable, if close, accomodation for 2, with room for 4 or 5 people to sit around in the cabin and have a drink and a yarn if desired, without having to be wedged into bunk shelves or down tunnels.

Also I do not consider myself prejudiced against proas. I made an informed decision some time back that they weren't a type of multihull that appealed to me in the LOA size range I'm interested in owning.

 

 

Rob: Storm comparison is a bit tricky without knowing anything about your boat,...

My boat is (or more correctly will be) 7.6 x 6m. But a comparison between it and an HP is not what I was getting at. I posted this somewhat in jest to squid “Also while you have one longer hull in a Harryproa you also have one short one. Is one side of the boat therefore less seaworthy than the other? ”, but I guess it underpins what I'm getting at. I’m asking whether, in the context of seaworthiness, the long hull/short hull configuration of the HP has some disadvantages in comparison with a similar sized cat with two equal length hulls. See above for more details.

 

It's an interesting question, is the resistance to pitchpoling the same from one long hull and one short one as it is for two long ones, or an average??? I don't know. I do know from sailing mine the motion upwind is much closer to a "non-leaning monohull". As the two bows are offset they both hit the oncoming wave at the same time. Did that make sense?

 

 

And these are now officially fora.

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I was thinking about the paint/shiny vboat thing while in the basement sanding my proa.

 

I think maybe if someone decides to build a proa they have given up on resale value already.

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Looking forward to seeing it built and sailing.

 

Can't see why you guys are dumping on it so readily.

 

I agree that people should let results speak for themselves.

 

But it's pretty hard to pass technical judgment (even as skilled engineers) without knowing much about how they behave and the loads they see.

 

Eg: Seems pretty clear to me the beams don't see anything like the loads in a tri or cat. In a tri the pontoons take rig loads and can effectively have to suspend the centre hull between wave crests, including during impacts. In a cat the beams are being bent by the rig loads and the uneven wave effects on the two long hulls. In the HP there are no rig loads across the beams, and the twisting effect is reduced by the shortness of the windward hull. Pretty hard to judge what the real loads will be. Seems to me there's only one guy in the room who has that data.

 

Shunt v tack is the same. If you haven't done both, how can you know? Multi's aren't reknowned for keeping their forward momentum through the tack and they still have to bear away to accelerate afterwards - the shunting proa has to lose all its speed, but it gets to put the brakes on with the mainsail, and is immediately in the power zone afterwards. I sense that there is huge variation available in the shunting manouvre and the difference with skill and experience could be much greater than when tacking a typical multi or lead swinger.

 

My question would be can you tack away when someone calls starboard on you?

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Shunt v tack is the same.

 

I would respectfully disagree with that comment :D

 

What about Gybe vs Shunt? :lol: imagine doing 20 knots downwind, stopping then going backwards.... Im fairly sure its slower than Gybing, but I dont know iv never tried it... :clap:

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Looking forward to seeing it built and sailing.

 

Can't see why you guys are dumping on it so readily.

 

I agree that people should let results speak for themselves.

 

But it's pretty hard to pass technical judgment (even as skilled engineers) without knowing much about how they behave and the loads they see.

 

Eg: Seems pretty clear to me the beams don't see anything like the loads in a tri or cat. In a tri the pontoons take rig loads and can effectively have to suspend the centre hull between wave crests, including during impacts. In a cat the beams are being bent by the rig loads and the uneven wave effects on the two long hulls. In the HP there are no rig loads across the beams, and the twisting effect is reduced by the shortness of the windward hull. Pretty hard to judge what the real loads will be. Seems to me there's only one guy in the room who has that data.

 

 

I've sailed on 'W', been involved with Bi-Maran design, as well as other cats and tri's so I think respectful comment is what these sites are all about.

 

I am not at all convinced however that taking the stays off the mast and making it freestanding reduces the loads on the beams or beam mounts. Comparing a cat to a proa lets say. The side stays have an angle to the mast of around 3 or 4:1, (much better that a keel boat's 10:1 for instance). Now a free standing mast has to take that bending load within the hull structures, and transmit to the other hull (for stability) through the depth of the beam. Think of the entire rig load going through the beam mount next to it as twist...

Given that the weight of the platform is the same, (just for the discussion), the righting moment of a cat or proa will be the same. The loads on the hulls from twisting and racking will be the same. So other than making it lighter, in my opinion the beam landings and beam structure will be similar.

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Greg, it broke, guess the beams weren't strong enough :lol: trial and error there.

 

If someone calls you swing the rig around and stop, then head off in theother direction. More interesting if you want to tack and there are boats to leeward.

 

TimC, that's beyond me, we'll have to wait till Rob gets back.

 

I would think that shunting is slower than gybing, I'm intrigued that Rob says he can match an "ordinary " Multihull in a tack, can't wait to see it.

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Tim C touched on this aspect of Proa design before but I thought I'd have a crack at explaining it.

 

The righting moment of a cat (excluding crew for a second) equals half the beam x the displacement, then plus the crew weight x the beam.

 

When the windward hull flies, the mast, boom etc is being lifted hence the righting moment figure.

 

What about the Proa.

When the gust strikes that the hull with the mast in it is not really adding to the righting moment (in fact when it leans over far enough it is reducing it).

So the Righting moment of the Proa is basically the weight of the windward hull with the crew in it x the beam.

 

In basic terms this means that on a cat the rig weight is a benefit in terms of righting moment where as in the Proa it is not.

 

So if a cat and proa are the same weight and beam, and had the same sail area, the two would have the same Power to weight ratio but the cat will have a higher righting moment (will therefore carry her sail better).

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All very good points.

 

After posting I remembered that the catamaran beams are probably under compression rather than bending from the rig loads, cause the vertical loads are taken by the dolphin striker.

 

Theoretical modelling the loads involved in a yacht is impossibly complex. Most yacht designs (monos, multis, canting keels etc) seem to have evolved through the try it/break it/ make it stronger rule. I guess the same will happen with the HP.

 

Tim,

 

I'd be surprised if the hull loads from a free standing rig are even half that of a stayed rig. Most of the static rig loads are to tension the forestay, pretension various parts of the rigging and induce bending. On top of that, while sailing you have extra wind and wave induced dynamic forces which are not the same as for a flexing free standing mast. Basically you trade large rig forces for a straight forestay, a light weight rig and the ability to control the mast shape.

 

On a free standing rig all the loading is dynamic, but the maximum load you have to handle on the rest of the boat is governed by the bending strength of the mast where it leaves the deck.

 

Think of a laser versus a phase 2. I haven't seen many problems with the socketed laser mast, but I have seen gunwhales ripped apart by the side stays of the phase 2 (without the mast coming down).

 

Except for the impact of waves (probably a big exception) this must also apply for the beams. In terms of basic righting moment the beams need be no stronger, in combination, than the bending strength of the mast where it leaves the hull.

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Thats a very good explanation Bogan!

 

for anyone unsure what a shunt involves here it is

 

around 45 seconds into the clip.

 

As far as I know (I have no experience with these speed machines) you do the same shunting trick both upwind and downwind.

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All very good points.

 

After posting I remembered that the catamaran beams are probably under compression rather than bending from the rig loads, cause the vertical loads are taken by the dolphin striker.

 

Theoretical modelling the loads involved in a yacht is impossibly complex. Most yacht designs (monos, multis, canting keels etc) seem to have evolved through the try it/break it/ make it stronger rule. I guess the same will happen with the HP.

 

Tim,

 

I'd be surprised if the hull loads from a free standing rig are even half that of a stayed rig. Most of the static rig loads are to tension the forestay, pretension various parts of the rigging and induce bending. On top of that, while sailing you have extra wind and wave induced dynamic forces which are not the same as for a flexing free standing mast. Basically you trade large rig forces for a straight forestay, a light weight rig and the ability to control the mast shape.

 

On a free standing rig all the loading is dynamic, but the maximum load you have to handle on the rest of the boat is governed by the bending strength of the mast where it leaves the deck.

 

Think of a laser versus a phase 2. I haven't seen many problems with the socketed laser mast, but I have seen gunwhales ripped apart by the side stays of the phase 2 (without the mast coming down).

 

Except for the impact of waves (probably a big exception) this must also apply for the beams. In terms of basic righting moment the beams need be no stronger, in combination, than the bending strength of the mast where it leaves the hull.

 

My comments are applicable to cats or tris with free standing rigs as much as proas.

 

Modern multihulls carry relatively little stay tension, compared to a keel boat. So the static load is relatively low on a stayed rig. The forestay tension really comes from the mainsheet tension, and the dynamic load on the main leach being transferred to the forestay.

 

The healing force from the rig is transferred to the windward hull via a low angle stay. It is a nice big triangle. On a 9m boat the hounds might be at 9m up, and the half beam 2.5m. The Centre of Effort would be say 6m off the water

 

On an unstayed rig of a similar size, the CoE is still the same height off the water, but the rig load has to be transferred to other hull (to windward in a cat or proa, to leeward in a tri) via the dimension of the beam. Sure some of that load will be sheer, but with the CoE so far above the hull, much of the load will be twisting the hull the mast is in, and so the beam landing and beam structure have to take that load, to take advantage of the righting moment. The beam height would say be 200-700mm high.

 

So I can't see how the beam structure of an unstayed rig boat can be lighter than a stayed one. If it is the same weight then much has been achieved. The storm wracking loads would be the same on both structures.

 

I am not saying one is much better than the other (stayed vs unstayed) I would suggest the best boat to put an unstayed rig in is a monohull, as they do not have the advantage of the beam of a multi.

 

Even with current carbon mast technology, unstayed rigs seem to end up heavier than stayed ones, as the disadvantages of extra complexity and windage still appear to make a lighter total rig and platform package. Also, all those stays give lots of opportunities for different sail options. Having a rig that performs in 15-20 knots is one thing, but it is the extreme ends of the wind scale that challenge a performance yacht.

 

At this stage we should applaud people like Rob for pioneering new thinking in rig design, and challenging us all.

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Modern multihulls carry relatively little stay tension, compared to a keel boat. So the static load is relatively low on a stayed rig. The forestay tension really comes from the mainsheet tension, and the dynamic load on the main leach being transferred to the forestay.

 

Tim, my understanding is that static means unchanging and dynamic means changing. I'm stretched to imagine any static loads of a boat that is under sail. Perhaps in test conditions in perfectly flat water with unchanging wind strength and no change to the point of sail - conditions that don't really exist outside a wind tunnel. But my real issue is with your over-simplification of the forces concerned. Yes, mainsheet tension increases forestay tension, but it also places the mast under increased compression. At least two other factors change loading: wind strength and wave action. Wave action will also for example make the rig weight a dynamic loading, which I believe can be mistaken for a static one.

 

Note that some of these forces exist on unstayed rigs, and that with lasers there is much evidence of damage over time to the mast socket, the most common is seen with a slight bulge seen under the hull - but cracking at the top is not uncommon.

 

Anyhow, back to these points made about proas. The points have been made about the advantages of the beam and having a leeward hull to take advantage of gusts. Slingshot (http://www.dcss.org/speedsl/crossbow.html) was an interesting design, as it was sailed both in trimaran and proa configuration. The proa configuration was faster.

 

Also, Crossbow II, with a twin rig, seemed to have little concern about taking full advantage of its width to hold up the masts, and rightly so - the forces require use of surprisingly little beam. Lower centre of effort was seen as more important.

 

And the points that wolf made:

In basic terms this means that on a cat the rig weight is a benefit in terms of righting moment where as in the proa it is not.

 

So if a cat and proa are the same weight and beam, and had the same sail area, the two would have the same Power to weight ratio but the cat will have a higher righting moment (will therefore carry her sail better).

 

I entirely agree with your logic above Wolf, so long as the boat concerned is overpowered where the additional righting moment will directly translate into speed (though the righting moment is far lower than say the windward hull or crew). In fact the proa for the reason you state will be overpowered in slightly less wind than the cat. However in conditions where the boats are not overpowered, the righting moment of the rig is still relevant - but as a negative for the catamaran and a positive for the proa. This is because the proa's windward hull will be flown earlier than the cat one, not only because the hull is lighter, but because of the position of the rig. In these conditions, the proa design has found to be significantly faster.

 

One last point on proas. The rig can be inclined to windward so that when flying a hull it is directly upright - or even inclined to windward. This is faster than one leaning to leeward, as downward forces that result will translate into greater displacement. Rigs inclined to windward is one of the reasons that windsurfers are so fast...

 

Overall, if I had to choose which was faster - a cat or a proa - I'd say top speed would generally go hands down to the proa in under 20 knots of wind, and over 20 knots differences would diminish. With a short course around the buoys - the cat would win (imagine a tacking dual!).

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My reference to static loads was the stay tension on a rig while sitting on the mooring/marina.

The load on any rig, given the same sail area, will be the same. Just the forces and directions will be different, compression and tension on a stayed rig, bending, sheer, and twist on a unstayed one. The transmission on the later can only be through the mast bast, and mainsheet, so from there to the hulls and beams. The stayed rig on a multihull spreads the loads to various areas, chainplates, mast base, sheets etc.

Yes this may be an simplification, but a complex subject of forces that I'm sure even engineers wouldn't agree on....

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An analogy for beam loads on stayed and unstayed rigs.

Which hurts your legs more, hiking on a Laser, Finn, OK, or trapezing on a 470, 3.7, 49er, (or any small yacht)? Your legs are transferring sail healing force into power to carry sail.

It could be noted which rigs have the more sail carrying ability and options, and that neither type is significantly lighter than the other, for sail are or speed.

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That is a really great post NZL3166.

 

Here's the way I see that play out though.

we are talking a "low stability" boat with a very high bruce number.

So I'm not thinking they are going to be anywhere near 20 knots before they are overpowered.

 

Frantic Drift has a bruce number of 2 and is comfortably powered up in 12 knots.

 

Rig weight:

There are two aspects to this, the Proa because its loads generally are lower will get away with a lighter mast section than the cat would so while the rig weight is light it is more a function of that than it is of it not having stays.

 

That discussion pops up all the time:

If you decide you are happy with no Headstay, Code 0, Genoas or Gennakers, then quite possibly the stayless mast is the lightest option.

Can't really say I see a lot of difference myself, as an overall drag package, for sure the stays and diamonds are a dud idea.

But we are using our sail combinations to cover all sailing angles from light airs upwind to heavy airs downwind.

By leaner standards we have very few sails, but in the past 20 years the big holes that existed in Multihull sailing (mostly up and downwind in under 10 knots) are being filled, we have tighter forestays (better pointing upwind), and both bigger sail plans but especially bigger gennakers and big top mains, give us better performance offwind.

In stronger winds we are using staysail (storm jibs) with deep reefed mains which bring the CoE much lower.

 

our Bruce Number is also modified in very light air by using a Tight luffed Code 0 upwind in up to 6 knots of wind (in my own boats case our upwind light air Bruce # is 2.05, whereas in normal trim its only 1.86).

 

I have priced up a number of Stayless rigs, especially the aero-type rigs that Carbo-spars used to build. Everyone knows that Carbo spars went under because they lost a lot of money on those rigs, and the reality is that they cost a lot more to build than most people imagine (and are willing to pay).

There is usually a great deal of Carbon required at the bottom (as well as additional structure in the boat itself). And due to the increased labour required the tube ends up costing a lot more. In Robs case this is offset by doing the labour himself, and the lack of stays and sails.

 

Finally I love that photo of Phillips Innovator, and as always Bad Kitty's wit has enabled it. It is quite similar in a way to Davids beam disaster, it really shouldn't have broken, but it still did !

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I have an image of Rob Denny on a ridge somewhere cocking his AK47 with 8x 'scope sight and methodically picking off one by one a gang of ranting mouth frothers who are throwing clumps of horse defecation at him. But he is aiming not to kill but to shoot their big Texas-like hats off, and slowly they start to see reason -while the few hard core diehards require a second hole through their big hats for them to finally retain their senses.

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