Gludy
Active member
I do, but it does not bother me at all 
I am not here to go with the crowd, I learn by asking questions.
I am not here to go with the crowd, I learn by asking questions.
Boat Stability
- Thread starter Gludy
- Start date
victoraspey
Member
Practical Boatowner June 2004: Tests show that a well designed catamaran is more resistant to inversion by breaking waves. Whats more the comparison with sail powered craft is between ballasted monohulls and unballasted catamarans - whereas ballast does not enter into the equation with semi displacement or planing motorboats.
victoraspey
Member
Practical Boatowner June 2004: Tests show that a well designed catamaran is more resistant to inversion by breaking waves. Whats more the comparison with sail powered craft is between ballasted monohulls and unballasted catamarans - whereas ballast does not enter into the equation with semi displacement or planing motorboats.
Wiggo
New member
I'm not wound up. I'd rather be discussing something where we might learn about taking a boat out to sea, but whatever...
Wiggo
New member
Re: Stability, Seaworthiness and Seakeeping
Which one do you have in mind, out of interest?
I'm not agitated, BTW. Personally I'm a big fan of this Care in the Community stuff, and if these little chats help you to work through your personal issues, then great...
Which one do you have in mind, out of interest?
I'm not agitated, BTW. Personally I'm a big fan of this Care in the Community stuff, and if these little chats help you to work through your personal issues, then great...
Gludy
Active member
Re: Stability, Seaworthiness and Seakeeping
Wiggo
I do not have any in mind.
I just gave a clear one line answer to your question so that we could be atalking about the stability of the same thing.
Wiggo
I do not have any in mind.
I just gave a clear one line answer to your question so that we could be atalking about the stability of the same thing.
Wiggo
New member
Re: Stability, Seaworthiness and Seakeeping
OK, it's just that I can't actually think of a displacement mobo with a 'deep keel' - it sounds a lot lke a dismasted yacht to me. Otherwise, we're talking purely hypothetical hull shapes. In which case, my hypothetical planing boat is rock steady in any seas, underway or not, by virtue of a huge gyroscope. Admittedly turning it is a chore, but I can make tea and bacon butties in a hurricane.
OK, it's just that I can't actually think of a displacement mobo with a 'deep keel' - it sounds a lot lke a dismasted yacht to me. Otherwise, we're talking purely hypothetical hull shapes. In which case, my hypothetical planing boat is rock steady in any seas, underway or not, by virtue of a huge gyroscope. Admittedly turning it is a chore, but I can make tea and bacon butties in a hurricane.
gcwhite
New member
Gludy, Considering the Stability bit only. Why is it that two boats with the same seaworthy category 'B' say the Sealine S42/5 & the Fairline Phantom 43 have completely different
stability characteristics when at anchor. The Sealine being all over the place in any sea thats moving and the Fairline sits there with hardly a twitch?
stability characteristics when at anchor. The Sealine being all over the place in any sea thats moving and the Fairline sits there with hardly a twitch?
Gludy
Active member
I have already stated that the seaworthy Class A, B etc is not by any means a precise guide to seaworthiness. However stability is just one part of that classification.
Stability is a basic measure of the amount of energy required by a wave that would turn the boat. The measure of this is the STIX number.
Class B stix is from 23 to 31 so even on this scale one boat can be the same Class B but almost 50% more stable than the other!
However even that is not the complete picture because I am sure that the motion at anchor may be related to many other aspects of boat design than just the stability.
Stability is a basic measure of the amount of energy required by a wave that would turn the boat. The measure of this is the STIX number.
Class B stix is from 23 to 31 so even on this scale one boat can be the same Class B but almost 50% more stable than the other!
However even that is not the complete picture because I am sure that the motion at anchor may be related to many other aspects of boat design than just the stability.
gcwhite
New member
I am sorry to drift away from the definition re stix but when one sees a boat which is frisky on its mooring compared to another which is stable it is tempting to draw a layman's conclusion that the latter is more seaworthy than the former. Would this be an unfair conclusion?
Its_Only_Money
New member
Re: Stability, Seaworthiness and Seakeeping
"I am talking baout a displacment boat with a deep keel. "
Aren't they yachts /forums/images/graemlins/confused.gif
Or do we mean just-enough-deep-keel-to-avoid-being-called-something-else?
"I am talking baout a displacment boat with a deep keel. "
Aren't they yachts /forums/images/graemlins/confused.gif
Or do we mean just-enough-deep-keel-to-avoid-being-called-something-else?
Gludy
Active member
Re: Stability, Seaworthiness and Seakeeping
Well take a look at the keel on the SD hull of the Trader - its fairly deep, lower than the props, which is why it can dry out.
Well take a look at the keel on the SD hull of the Trader - its fairly deep, lower than the props, which is why it can dry out.
trev
New member
No No !
Stability is the ability of a vessel to recover from being displaced by a wave, or other external force - it is not necessarily a measure of seaworthiness (or seakindliness). To put it simply it is generally measured by the size of a vessel's Geometric Metacentre (GM) which in turn governs the size of the 'righting lever' applied when the boat is displaced from vertical.
A boat with a large GM (most motor cruisers and yachts) will tend to be 'stiff'- ie return quickly to upright if displaced, a boat with a small GM would be described as 'tender' and would roll sluggishly, however each would be stable so long as a positive GM remained active.
By overloading the boat above the centre of gravity you raise the height of 'G' and thereby reduce the size of the 'righting lever' - it can even become negative resulting the vessel capsizing.
That's very simply put, but as I said in ealier post - So long as GM is satisfactory I aint bothered.
Stability is the ability of a vessel to recover from being displaced by a wave, or other external force - it is not necessarily a measure of seaworthiness (or seakindliness). To put it simply it is generally measured by the size of a vessel's Geometric Metacentre (GM) which in turn governs the size of the 'righting lever' applied when the boat is displaced from vertical.
A boat with a large GM (most motor cruisers and yachts) will tend to be 'stiff'- ie return quickly to upright if displaced, a boat with a small GM would be described as 'tender' and would roll sluggishly, however each would be stable so long as a positive GM remained active.
By overloading the boat above the centre of gravity you raise the height of 'G' and thereby reduce the size of the 'righting lever' - it can even become negative resulting the vessel capsizing.
That's very simply put, but as I said in ealier post - So long as GM is satisfactory I aint bothered.
Gludy
Active member
I believe you are right in one way and wrong in another.
Size really does matter and it does come into the stability calculations ... at the end of the day a bigger boat, everything else being equal and good design, will withstand bigger seas....... on that art we agree.
But there is much more to the STIX number than that:-
STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.
So stability at sea is more than a function of size/GM.
Size really does matter and it does come into the stability calculations ... at the end of the day a bigger boat, everything else being equal and good design, will withstand bigger seas....... on that art we agree.
But there is much more to the STIX number than that:-
STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.
So stability at sea is more than a function of size/GM.
Wiggo
New member
[ QUOTE ]
But there is much more to the STIX number than that:-
STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.
[/ QUOTE ]
I've seen you quote this a couple of times, Paul. What does it actually mean? Wind Moment? Downflooding? Reserve Buoyancy? Knockdown recovery surely relates to blowboats, doesn't it?
But there is much more to the STIX number than that:-
STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.
[/ QUOTE ]
I've seen you quote this a couple of times, Paul. What does it actually mean? Wind Moment? Downflooding? Reserve Buoyancy? Knockdown recovery surely relates to blowboats, doesn't it?
kimhollamby
Active member
Paul,
You have to be careful here. Working on the basis that you are trying to learn and might therefore be looking for further references you need to start to look at the essential differences between dynamic and static stability. You also need to consider why a deep keel works well (righting movement, not water resistance per se).
STIX might have something to say about a planing hull at rest or slow speed but once the hull starts generating lifting forces it takes on a life of its own. Ditto a genuine semi-planing hull, although in that case the balance is probably very different because there is often little in the way of lifting surface from midships forward but a much greater lifting surface (often completely flat) aft.
For some of these reasons things like STIX and RCD categories can be as misleading as they are useful.
As I think might already have been said here you cannot even judge seaworthiness by how 'tippy' a boat is at rest. In reality a hull that is extremely initially resistant to movement (a 'stiff' hull) might in fact cause quite undesirable characteristics in terms of crew comfort at sea with the motion being more snatched (typically vomit inducing).
Weight distribution on motorboats and what it does is a fascinating subject. For example (and with apologies for the generalism) a single engine boat with its engine lower down generally leads to a great righting moment (and hence arguably greater seaworthiness) but it might appear less stable at rest because it also tends more prone to list when your 20 stone auntie takes up the invitation for a marina visit and steps on the side deck. Twin engines (weight higher up, spread out to sides might initially have greater resistance to roll but ultimately have less righting movement, keel or no.
Unlike some who appear to be losing patience here I find these topics facinating - but you do have to be extremely careful about adopting bar wisdom as truths.
I think the most sensible thing I have seen here so far has been the assertion that the line between planing, semi displacement and displacement is, at best, blurred and the really important things are specific hull design, fit out and weight distribution. Plus of course (and I don't think this has been mentioned) helmsman skill. To be frank you can take a tea tray across the Channel if you get the human factor right.
You have to be careful here. Working on the basis that you are trying to learn and might therefore be looking for further references you need to start to look at the essential differences between dynamic and static stability. You also need to consider why a deep keel works well (righting movement, not water resistance per se).
STIX might have something to say about a planing hull at rest or slow speed but once the hull starts generating lifting forces it takes on a life of its own. Ditto a genuine semi-planing hull, although in that case the balance is probably very different because there is often little in the way of lifting surface from midships forward but a much greater lifting surface (often completely flat) aft.
For some of these reasons things like STIX and RCD categories can be as misleading as they are useful.
As I think might already have been said here you cannot even judge seaworthiness by how 'tippy' a boat is at rest. In reality a hull that is extremely initially resistant to movement (a 'stiff' hull) might in fact cause quite undesirable characteristics in terms of crew comfort at sea with the motion being more snatched (typically vomit inducing).
Weight distribution on motorboats and what it does is a fascinating subject. For example (and with apologies for the generalism) a single engine boat with its engine lower down generally leads to a great righting moment (and hence arguably greater seaworthiness) but it might appear less stable at rest because it also tends more prone to list when your 20 stone auntie takes up the invitation for a marina visit and steps on the side deck. Twin engines (weight higher up, spread out to sides might initially have greater resistance to roll but ultimately have less righting movement, keel or no.
Unlike some who appear to be losing patience here I find these topics facinating - but you do have to be extremely careful about adopting bar wisdom as truths.
I think the most sensible thing I have seen here so far has been the assertion that the line between planing, semi displacement and displacement is, at best, blurred and the really important things are specific hull design, fit out and weight distribution. Plus of course (and I don't think this has been mentioned) helmsman skill. To be frank you can take a tea tray across the Channel if you get the human factor right.
Alastairdent
New member
Re: Stability, Seaworthiness and Seakeeping
There is a displacement mobo with a fin keel moored behind me. It's an ex-pilot boat, and as usual with these is very skinny. The keel is original, must have been designed in to increase stability.
There is a displacement mobo with a fin keel moored behind me. It's an ex-pilot boat, and as usual with these is very skinny. The keel is original, must have been designed in to increase stability.
duncan
Active member
Kim,
thansk for putting so eloquently what I think many have been trying to explain - short of copying a load of diagrams from Dag Pike (or other) authors - which I have assumed would be a copyright issue.
On the bright side I have never before found myself agreeing completely with every word posted by Wiggo ( /forums/images/graemlins/smile.gif)
thansk for putting so eloquently what I think many have been trying to explain - short of copying a load of diagrams from Dag Pike (or other) authors - which I have assumed would be a copyright issue.
On the bright side I have never before found myself agreeing completely with every word posted by Wiggo ( /forums/images/graemlins/smile.gif)
Gludy
Active member
Kim
I am on a learning curve here and have actually just ordered some books on hull design!
I am a chartered engineer and although not in this field, and so do understand that any particular classification can be misuses and/or relied on too much. Indeed I have actually pointed out that the classification A, B etc system is a very rough system, an indication rather than an absolute. The STIX is a bit better but again is just a simple index covering many aspects of stability.
The question about the 20 stone aunt - that had already been covered in the threads when the direct question was asked about a boat like a Sealine may tip more when someone gets on ... I answered that this is not the same as stability - which is exactly your point.
"Unlike some who appear to be losing patience here I find these topics fascinating - but you do have to be extremely careful about adopting bar wisdom as truths."
Its very difficult to get a serious discussion on any subject going here. Some cannot separate the message from the messenger. Others simply are not interested and seem to resent anyone that is. So at times the debate was bar style point scoring with some humour but because of that I made sure that after that I clearly stated, for example, that I myself thought the RCD rating system just a crude measure.
"Plus of course (and I don't think this has been mentioned) helmsman skill. To be frank you can take a tea tray across the Channel if you get the human factor right."
Helmsman skill was mentioned and agreed by all that it matters a lot but the discussion had to assume that the same good helming standard was being applied so that the hull was the variable ... otherwise you get nowhere!
"To be frank you can take a tea tray across the Channel if you get the human factor right. "
Not in a f10 whilst making bacon butties you can't
I fully appreciate that the subjects that I bring up during the last four years on the forum are not everyone's cup of tea. Frankly what really messes up the discussion is the fact that as soon as I raise a subject, always technical, the personal attacks start. The forum is not just about technical discussions I agree but there should be room for it.
You well know that many of the subjects I have raised have resulted in massive postings and i believe we have all learned from them.... I certainly have. It will be a pity if that element of the forum is hounded out so as to conform to the clique that insist only on light bar room chat and nowt else..
I am on a learning curve here and have actually just ordered some books on hull design!
I am a chartered engineer and although not in this field, and so do understand that any particular classification can be misuses and/or relied on too much. Indeed I have actually pointed out that the classification A, B etc system is a very rough system, an indication rather than an absolute. The STIX is a bit better but again is just a simple index covering many aspects of stability.
The question about the 20 stone aunt - that had already been covered in the threads when the direct question was asked about a boat like a Sealine may tip more when someone gets on ... I answered that this is not the same as stability - which is exactly your point.
"Unlike some who appear to be losing patience here I find these topics fascinating - but you do have to be extremely careful about adopting bar wisdom as truths."
Its very difficult to get a serious discussion on any subject going here. Some cannot separate the message from the messenger. Others simply are not interested and seem to resent anyone that is. So at times the debate was bar style point scoring with some humour but because of that I made sure that after that I clearly stated, for example, that I myself thought the RCD rating system just a crude measure.
"Plus of course (and I don't think this has been mentioned) helmsman skill. To be frank you can take a tea tray across the Channel if you get the human factor right."
Helmsman skill was mentioned and agreed by all that it matters a lot but the discussion had to assume that the same good helming standard was being applied so that the hull was the variable ... otherwise you get nowhere!
"To be frank you can take a tea tray across the Channel if you get the human factor right. "
Not in a f10 whilst making bacon butties you can't
I fully appreciate that the subjects that I bring up during the last four years on the forum are not everyone's cup of tea. Frankly what really messes up the discussion is the fact that as soon as I raise a subject, always technical, the personal attacks start. The forum is not just about technical discussions I agree but there should be room for it.
You well know that many of the subjects I have raised have resulted in massive postings and i believe we have all learned from them.... I certainly have. It will be a pity if that element of the forum is hounded out so as to conform to the clique that insist only on light bar room chat and nowt else..
