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Old 19-06-2010, 14:39   #61
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Originally Posted by tsmwebb View Post
As to the loss of control, it could be a trim issue or a design issue or a badly manufactured rudder. The last is common and an asymmetrical, poorly formed or twisted foil can be have very unforgiving stall characteristics.

Tom.
If I remember correctly, the Lipari has the rudder forward of the saildrives. It may be placement of the rudder more than size. BTW, this is common in both FP and Lagoon cats under 40'.
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Old 19-06-2010, 15:33   #62
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if the into-wind yaw (heading change) is great enough gyroscopic procession could easily add to the rolling force to exceed the static stability limits. As the yaw rate decreased the procession decreases to point of stability and you are hanging there. Just a theory if you are looking for reasons why this could happen short of the theoretical limits. Going from 90 deg off the wind to 60 could increase apparent wind a lot - so probably good your sails were not (I assume) trimmed for optimal into-wind power or you may have gone right over. If that was me I would now have a wind alarm at 20 True and reef at or before then. (I'm guessing - I don't know your boat.)
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Old 19-06-2010, 15:45   #63
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Originally Posted by Event_Horizon View Post
10kts exceeds the hull speed for a FP 43'?
I don't know about the specifics of the Lipari, but guy I know that had an FP Venezia (might have spelled that wrong) claimed he regularly sailed it at 18 knots in the Carribean and once achieved 22 knots. Depending on the hull design and weight, cats are not limited to displacement hull speeds.

I don't think you can describe the OPs experience as flying a hull, but more of a trip caused by a loss of control. The sudden turn to windward at speed caused the leeward hull to dig in and the sudden increase in drag nearly stopped the leeward hull but the boats momentum was still driving it forward (now nearly sideways). As the CG was above the CP on the leeward hull the momentum tended to lift the windward hull. This in combination with the wind pressure on the sails lifted the hull out of the water. From a design point of view I would be more concerned about the loss of control than the inherent stability of the boat. I seriously doubt that the Lipari as enough sail area to actually fly a hull, which in a racing cat is a stable position. It does however require enough rudder to maintain control of the boat.
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Old 19-06-2010, 16:05   #64
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I don't know. I've never found a formula to figure it out. I do know that weight plays a huge factor. Load it up for an extended cruise and fill the water tanks (1280lbs) and you'll lose ~1kt in speed.
The length of the wave created by your hulls approaches their sailing length at V=1.34*sqrt(LWL). That's true of all displacement hulls and is what I call hull speed, but despite its name it is not in any way shape or form a predictor of boat speed. There is a long and horrible history of arguing this point on the internets and a little googling should uncover a wide variety of opinion.

What I think you may be looking for is a velocity prediction algorithm. The KSP number is about as simple a rule as can be created. It is a rule of thumb and should be treated as such but assuming you don't capsize it says a multihull should go V=0.5*WS*sqrt(SA*LWL/D). You can combine it with the capsize formula WS=9.48*sqrt(0.5B*D/SA*CE) to get an estimate of top speed.

Where:
V= boat speed in knots
WS = apparent wind speed knots
LWL = load water line feet
D = displacement pounds
B= beam between centers of buoyancy feet
SA = sail area ft^2
CE = vertical distance between center of effort of hull and sails in feet

Apologies to all for the hijack.

Tom.
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Old 19-06-2010, 18:35   #65
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tsmwebb,
I'm confussed by your capsize formula, how can apparent wind speed be calculated without a variable for true wind speed and what is the origin of the constant 9.48? Also I understand the center of bouancy of a hull and the center of lateral resistance but not the CE of the hull. Dave
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Old 19-06-2010, 19:17   #66
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...and back to the cruisers' perspective...

With great respect to the engineers and their formulas -- We do appreciate, read and attempt to understand them, but a lawyer's background warns that a little knowledge can be very dangerous! -- we'd like to go back to what perhaps was the 'core' of Yeloya's thread here...what does Yeloya's experience mean to the cruisers out there?

For us, some lessons are clear, but others not so clear...

'Reef early' is clear. Even there, however, Yeloya's experience is unsettling as we all have enjoyed the 'buzz' of our vessels scooting along on a fine day in flat waters as the breeze gusts up to reefing levels. Still, if ever there was an experience to 'sober up' the thrill seekers in us, Yeloya's was just that...and we are very grateful for him sharing it here.

Releasing the sheets is also clear...and unloading the jammers when dark water shows gusts looming is also obvious, but very important.

What is not so clear, however, is what to do with the helm. There are two seemingly-contradictory directions here...head up or fall away. Which is it? The monohull background here probable ingrains the instinct to head up, but we can certainly see the logic in the various posts suggesting the best response would be to fall away. We're probably persuaded to the fall away camp intellectually...and thus hoping the intellect rules over the instinct if the need arises! But we will also be very grateful for any further comments on this really black-or-white choice in a critical area...
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Old 19-06-2010, 20:08   #67
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Quote:
Originally Posted by tsmwebb View Post
The length of the wave created by your hulls approaches their sailing length at V=1.34*sqrt(LWL). That's true of all displacement hulls and is what I call hull speed, but despite its name it is not in any way shape or form a predictor of boat speed. There is a long and horrible history of arguing this point on the internets and a little googling should uncover a wide variety of opinion.
V=1.34*sqrt(LWL)

This equation has always been considered a very rough estimate for monohulls, and outright incorrect for multihulls. There have been some attempts to tweak the K-value (1.34) to have a version more suitable for the thinner hulls on a catamaran. We are talking a typical K-value of 1.5+.
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Old 19-06-2010, 20:15   #68
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tsmwebb,
I'm confussed by your capsize formula, how can apparent wind speed be calculated without a variable for true wind speed and what is the origin of the constant 9.48? Also I understand the center of bouancy of a hull and the center of lateral resistance but not the CE of the hull. Dave
Credit John Shuttleworth for the formulas. I apologize if I am misrepresenting them. As I recall (but I'm too tired to look up right now) ISO uses a functionally identical capsize formula but with metric units. Basically what it is saying is that a moment is set-up between the sail force (assumed to be a flat plate) and the hull/keel force that causes a rotation. The coupling of hull force and the sail force is what causes the rotation, if there were no hull force then you would get a translation. Hence the distance between the ce of the hull and ce of the sail plan as the lever. When that moment is equal to the righting moment it is assumed you capsize. 9.48 is a magic number Sorry, but to some extent that's true. Mostly it is the constants moved from inside the formula and converted to consistent units. However, there is also a bit of a safety fudge thrown in there which reduces the constant. To simplify things the set up is envisioned as a stationary boat broadside to the wind. So, true wind equals apparent wind. There are many problems with the formula -- perhaps the biggest being the one you may be alluding to which is that fast boats and slow boats create very different amounts of apparent wind from true wind so the formula doesn't tell you at what true wind to reef. It is, however, the industry standard and it can be used to get a reasonable idea of how much apparent wind it will take to lift a hull which is useful info.

Tom.
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Old 20-06-2010, 02:53   #69
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I view the rudders as the most important component and the absolute last place to compromise. I'm no designer but when you put the helm over the boat must respond. If you know those limits in advance you are fore warned but in this case I'm not sure it would have helped. DaveOnCudjoe Post 51.

As was pointed out earlier in this thread, driving the bows down with a lot of jib can also contribute to weather helm by moving the center of lateral resistance forward. Dave of Calgary Post 55.

There are issues here that need to be considered.
1. If the helm is not neutral then the sailor has got something wrong. It's a first warning, sort it out.
2. The Aft mounted mast of the Prout's and BB's give a lift component to the genoa. It helps to counteract the bow diving syndrome.
3. Lifting a hull is quite serious, more so on a cruiser, that it corrected itself by over powering the rudders may have been a deliberate design feature.
4. Many a mono too, flying a full sail set, may well have been close to a knock down under these circumstances, certainly a scary heel. They too, would normally round up pretty sharpish in spite of having adequate helm authority. It's a safety feature.
5. Rounding up in a Cat DOES de-power the sails if they were originally trimmed correctly.
6. It is VERY important to put the second reef in the main when running downwind as ANY course change can the apparent wind, and change the mains effect from a blanket in the wind to an aerodynamic wing with a marked increase in forces. Not a mistake I'll make again, for a while!
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Old 20-06-2010, 03:22   #70
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Originally Posted by Event_Horizon View Post
V=1.34*sqrt(LWL)

This equation has always been considered a very rough estimate for monohulls, and outright incorrect for multihulls. There have been some attempts to tweak the K-value (1.34) to have a version more suitable for the thinner hulls on a catamaran. We are talking a typical K-value of 1.5+.
The speed of a wave in deep water is approximately V= sqrt(g*L/2pi). So a boat will be traveling at the same speed as the wave it creates when V = sqrt(g*LWL/2pi). That's hull speed. Move g and 2pi out of the root and convert the units and you get 1.34 (unless you're a metric person in which case you get 1.25). Anyway, that's where 1.34 comes from. Works the same for narrow hulls, fat hulls, deep hulls and multihulls. It just doesn't say anything about how fast the boat can go. As a speed predictor it is useless for monohulls and multihulls alike. It doesn't include enough information to say anything about performance. It does motivate an understanding that like hulls have a drag component that varies with the sqrt(LWL). Specific instances of that relationship are described with speed to length ratios or Froude numbers. That's useful.

What's a "K" value? Where does 1.5 come from and what does it mean? How is it used?

Tom.
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Old 20-06-2010, 03:40   #71
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I am an engineer and have a basic understanding of physic and all these "empirical" formulae.
To the buyer of a cat or charterer who wants to enjoy sailing, they are totally irrelevant. You pay tons of money for buying a boat from a reputable manufacturer and you have the right to assume that some naval architects, engineers have made their jobs using whatever formulae and test necessary to make yr "cruising" boat safe enough at the range of 20-25 knots of wind..I know that there is no such a 100% safe boat. In the geography we are sailing, the wind is unpredictable and can change suddenly because of countless small bays, valleys, straits, sharp mountains. The conditions I face with are rare but can happen any time. But, a cruising cat should never fall in such situation with this kind of wind and sea state in even under full sail whatever the trim is. To those who are continiously advocating reefing; the day we sailed the wind varied between 0 and 18 knots max, generally around 12-14 knots. There have been times when I pushed the limits but always with a professional crew with me, not while I had my wife and guests on board enjoying their coffee in a lovely afternoon..
Bottom line;

a-I've got my lesson (don't make assumptions and projections basing on yr previous experience with "apparently" similar boats, get ready to sheet the main out in a second if you see the gust is coming althought it seems a monohull practice..)
b-I have warned you..
c-I will further investigate to find out the exact reasons, talk to FP and if necessary make some amendments on the boat and rigging to make it even safer for charter purpose. I risked my life and I don't want to put at risk any charter customer.

I want to conclude by saying that, I still believe that FP's are great and extremely safe boats. But even big companies can make errors. (remember Mercedes A-160 which was rolling over at virtually any curve. They were recalled and supplied with ESC system later as a standard equipment. Toyota recall, etc.. ?)
If I get any news on that, I will update you.

Thx again for all yr contributions. Happy (and safe..) sailing

Yeloya


I am not blaming FP, they made are still making great and very safe boats.
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Old 20-06-2010, 04:09   #72
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What is not so clear, however, is what to do with the helm. There are two seemingly-contradictory directions here...head up or fall away. Which is it?
Both?

It depends on the boat a bit. But, my rule of thumb is to head up when the apparent wind is forward of the beam and down when it is aft of the beam. With the apparent wind on the beam if I feel like I'm in immediate danger (big squall or something) I ease the sheets. Steering out of danger can be ineffective on a beam reach because on a fast boat (relative to the true wind speed) heading either way may result in an increased heel and even on a slower boat it can take a while for the turn to provide any relief.

Tom.
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Old 20-06-2010, 04:37   #73
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I am an engineer and have a basic understanding of physic and all these "empirical" formulae.
To the buyer of a cat or charterer who wants to enjoy sailing, they are totally irrelevant. You pay tons of money for buying a boat from a reputable manufacturer and you have the right to assume that some naval architects, engineers have made their jobs using whatever formulae and test necessary to make yr "cruising" boat safe enough at the range of 20-25 knots of wind..
...
To those who are continiously advocating reefing; the day we sailed the wind varied between 0 and 18 knots max, generally around 12-14 knots.

Yeloya
Yeloya, it is very kind of you to have posed your experience. Thank you.

It is probably just me, but I am still a little confused about the conditions on the day. In your fist post you said
Quote:
25-27 knts just at 90 degree (apparent) on my starboard, boat speed on water 10,5-10,7 knots, almost flat water (2 max 3 ft swells from the wind direction) no reef
That would suggest the true wind was between 27 and 29 knots. That's a lot of wind to be sailing with full sail and no hand on the sheets. But from your last post quoted above it is not clear to me if it was blowing "18 knots max" or 20-25 knots... I completely agree that at 18 knots no boat marketed as a "cruising" cat should fly a hull.

Tom.
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Old 20-06-2010, 04:45   #74
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For some real design formulaes take a llok at a 'catalac' site as follows:
Try Catalac, an affordable cruising catamaran
There's a range of stuff there, should be of interest to you....... there's loads of stuff in there. Data on prop drag too if you were considering going to a folding or an auto pitching prop.
I'm happy you found my web site helpful.
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Old 20-06-2010, 06:46   #75
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my rule of thumb is to head up when the apparent wind is forward of the beam and down when it is aft of the beam. With the apparent wind on the beam if I feel like I'm in immediate danger (big squall or something) I ease the sheets.
Thanks Tom. Very neatly put.

We agree 100% with all of that...and perhaps that perspective may go toward explaining what seemed to be comments heading in both directions earlier in this thread.
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