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  #1601  
Old 10-30-2011
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Quote:
Originally Posted by bobperry View Post
Ballast does not have much effect at all at very low heel angles.
Hull form has a lot to do with stability at low heel angles. I call that "initial stability".
What is this "optimum heel angle" stuff? Most designers will tell you its zero degrees. In any boat in order to get max performance you try and sail the boat upright. In very light air there can be advantages to heeling the boat as the sails fall into their natural shape and there can be a reduction in wetted surface. But this is only for very light air.
In order for a boat to have any righting moment is has to heel. You only get Rm when the Cb moves of to leeward and the VCG moves off to weather. So at zero degrees the boat is in an unstable state as Cb and VCG are lined up.
Bob, you are more than welcome to this thread

If you have followed it you have seen that it is about all types of sailing boats and their sailing characteristics and your expert opinion would be very valuable.

Regarding the information that is provided by the boat designers (GZ and RM curves) I have some doubts about its comparative value.

Can you give a look here and give me your opinion, I mean about the subject of the first post (for additional information you can look at the other posts from Polux on that thread):

Misleading stability - Yachting and Boating World Forums

Regards

Paulo
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  #1602  
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Pacific Proa

I hope this design meets the thread criteria for "Interesting Sailboats."

Test Sailing a Pacific Proa
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  #1603  
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Quote:
Originally Posted by bobperry View Post
What is this "optimum heel angle" stuff? Most designers will tell you its zero degrees. In any boat in order to get max performance you try and sail the boat upright.
I don't know how you say this. For skiffs and many dinghies, of course what you say is true, but I wasn't referring to them.

I am positive that you of all people know that a keelboat (unless crew weight is a large percentage of total weight) will in fact heel when there's much breeze, that for best speed one doesn't trim and adjust amount of sail carried to maintain overly-low let alone flat heel but rather best speed will be with some significant heel, that the boat can be designed to offer reduced wetted area when heeled and usually is designed to do this, that when shaping the hull for best speed while actually sailing this surely would have to be with heel taken into account, etc.

If you want I could find a specific boat for which a specific value has been given. But I am guessing that most likely it is now clear what I was meaning so there may be no need for a particular example (I'd really guess not as you have vast experience of course.)

Last edited by estaban; 10-30-2011 at 05:19 PM.
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Quote:
Originally Posted by PCP View Post
Hi Estaban,

If you really insist and if they see that you would not buy the boat without stability data the factory will provide you with a stability curve made by the boat designer and used to certify the boat.
Good to know, thank you!

Quote:
If you use a RM curve you can calculate all those values.
Yes, if having the curve. I was trying to think of ways to have a limited set of numbers to compare with. Agreed that the curve itself will always have more information.


Quote:
About the resistance to heel at 0º I don't understand what you mean. You have to explain that better. At 0º the boat is not heeled .
I wrote it horribly, not according to what I was thinking. I was thinking of the derivative, the rate that stability increases from zero degrees: the slope of the curve. In other words, does stability increase only slowly as the boat begins heeling from zero degrees, or sharply? A single number could give a reasonably good picture here.
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Quote:
Originally Posted by estaban View Post
I don't know how you say this. For skiffs and many dinghies, of course what you say is true, but I wasn't referring to them.

I am positive that you of all people know that a keelboat (unless crew weight is a large percentage of total weight) will in fact heel when there's much breeze, that for best speed one doesn't trim and adjust amount of sail carried to maintain overly-low let alone flat heel but rather best speed will be with some significant heel, that the boat can be designed to offer reduced wetted area when heeled and usually is designed to do this, that when shaping the hull for best speed while actually sailing this surely would have to be with heel taken into account, etc.

If you want I could find a specific boat for which a specific value has been given. But I am guessing that most likely it is now clear what I was meaning so there may be no need for a particular example (I'd really guess not as you have vast experience of course.)
Estaban, What Bob said is right (of course). What you are talking about is another thing: When the boat heels generates a righting moment, initially ballast counts very little for it and RM comes almost all from form stability but when the boat heels more and more, ballast starts to count on the RM.

Max righting moment normally happens around 60º and that means that it is at that point that the boat has more power to carry sails. We know that no boat sails well at 60º of heel and that it will sail faster at lower angles of heel.

Why? Because the drag is so much that will not compensate the more power (righting moment) that the boat can generate with all that heel.

What you are talking about is about the heel angle that represents the better compromise between power (RM) and drag. The heel point were the sailboat will sail faster.

Regards

Paulo
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  #1606  
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Quote:
Originally Posted by estaban View Post
...

I wrote it horribly, not according to what I was thinking. I was thinking of the derivative, the rate that stability increases from zero degrees: the slope of the curve. In other words, does stability increase only slowly as the boat begins heeling from zero degrees, or sharply? A single number could give a reasonably good picture here.
Yes, I understand what you mean

You can see that on a GZ (or RM) curve and it is exactly as you say, a step curve indicates a boat that generates RM with small angles of heel. Multihulls have a very step GZ curve, so step that it don't look like a curve

Beamy boats have more steeper curves comparing with narrow boats but, providing they have an adequate RM to sail, they will have also a higher AVS and a smaller inverted stability.

Regards

Paulo
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Quote:
Originally Posted by PCP View Post
What you are talking about is about the heel angle that represents the better compromise between power (RM) and drag. The heel point were the sailboat will sail faster.
That's right, that's exactly what I was talking about.

If a given boat, say a Finot-designed boat, stays flatter when sailed optimally (best speed for course and conditions) than some other boat, then even if total area under the curve is the same between it and another boat, it has a significant "head start" so to speak and in practice will require more energy to capsize.

Not more energy from zero degrees, but more energy from an angle hopefully similar to what the boat was at before the gust or wave, as the flatter-sailing boat "has more of the curve left."

As you say, it's not irrelevant how flat the boat will tend to stay in the first place. The number for a given condition when the boat is sailed properly is not zero. (Well for some conditions it will be zero. But for example, it's unlikely to be zero at say 20 knots TWS upwind.)

And for the given condition it can differ between boats, sometimes by quite a bit.

Your other considerations you were talking about are on different points than energy required to capsize beyond optimal heel (as opposed to beyond flat, which I think isn't as useful as the difference has already been used up) and I agree with what you're saying.

To summarize really briefly, what I was saying is that while it's impossible to reduce stability to a single number, a small set of chosen numbers could give a reasonable picture. I was trying to give examples of a small set.

The "Q" value posted earlier is I think a nice example of a single number that can give a guideline, but as you said back at that time, not the whole picture.

Last edited by estaban; 10-30-2011 at 05:02 PM.
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Quote:
Originally Posted by JohnRPollard View Post
I hope this design meets the thread criteria for "Interesting Sailboats."

Test Sailing a Pacific Proa
Hi John,

Certainly it meets and nothing had been posted yet about Proas. Thanks for posting it.

By the way do you know that Proa means in Portuguese and Spanish, Bow? Those boats are probably called like that because when the Spanish or Portuguese find the first native ones they found out they had two proas and no popa (stern).

That one looks particularly interesting.

Proas did never manage to get a grip on mass production cruising boats, i don't know why, perhaps because they sail in a different way.

On this site we can find a good explanation about the differences between a Proa and a Cat, advantages and disadvantages:

Pacific Proa Advantages:

The Pacific proa gets advantages over the catamaran by using the same weight of materials (cost!) to create a significantly longer main hull with higher speed potential:

a longer hull is more easily driven for fast ocean passages and more appropriate to the scale of large seas offshore.

transferring up to 100% displacement to the single large leeward hull as the windward hull lifts results in smooth, comfortable speed with minimal wetted surface.

when pressed for maximum speed, the longer leeward hull with similar rig and no additional lateral stability (the ability to resist tipping over sideways) results in greater longitudinal stability than a catamaran. Instead of pitchpoling, the Pacific proa will roll gently onto the leeward pod .

a small, lifting hull to weather for stability is mechanically easier and lighter than connecting two hulls of equal size and weight (catamaran) or using "floats" of 100%+ buoyancy on either side (trimaran).

While modern materials and methods make structure less of an issue, the crossbeams on a Pacific proa are less stressed than a catamaran of similar displacement.

For the very same reasons, however, it is also true that the Pacific proa carries much less weight for it's length than a catamaran.


Catamaran Advantages:

The catamaran has broad flat transoms aft that carry weight well and reduce pitching; the proa is pointed at both ends.

The catamaran has extra privacy afforded by two hulls for accommodations.
Since there is only one large hull, adding length to a proa returns less accommodation volume and weight carrying capacity than the same length added to a catamaran.

The proa's main hull might be narrower than the catamaran, using a length to beam ratio of 17:1 for speed, resulting in a smaller interior space.

For these reasons, a 21 meter (69') proa has barely the same accommodations as a 43' catamaran. When compared to an 18 meter (59') catamaran, the 21 meter proa has significantly less accommodation volume...


Pacific Proa compared to Catamaran


Of course the guy is a bit partial about Proas

There are other advantages in a Cat: A substantially bigger righting moment for the same weight and beam and also a more straight forward way of changing tack, losing less time and distance.

Regards

Paulo

Last edited by PCP; 10-30-2011 at 05:19 PM.
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If we can post about non-cruiseable boats:

The new Marstrom M32 is very interesting.

A 32 foot extreme high performance catamaran that can be sailed by only two.





Torvar Mirsky testing Marström 32 - YouTube

Katamaran M32 in Travemünde | NDR.de - Fernsehen - Sendungen A - Z - Schleswig-Holstein Magazin - media
Taufe des Hightech-Katamarans* - * SAT1 REGIONAL

Marstrom Composite AB
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  #1610  
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Quote:
Originally Posted by estaban View Post
If we can post about non-cruiseable boats:

The new Marstrom M32 is very interesting.

A 32 foot extreme high performance catamaran that can be sailed by only two.

....
This thread is also about racing boats. there are several racing boats posted here. That one is a nice one and the movies are impressed. I hope that they don't pitch pole as easely as the ones from the America's cup

I also was impressed by the fact that a new racing cat is matter for the TV news in Germany. Impressive. That shows the public interest in sailing. I had no idea, I thought that only happen in France

I also noticed that the company that makes the M32 is the same that make all parts for the Seacart 30. That one has just a big defect: Not having a performance cruising version





Regards

Paulo
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