Boat Stability

[Gludy]

Here is an interesting link explaning a bit about the stix number of a boat:-

http://rorcrating.com/stix/stix.htm

Do you know the Stix number of your boat?

 

[trev]

Stix number? Why would I want to know that?
All I need to know is that she has a rather large GM, is a bit on the 'stiff side', and won't roll over unless you do something really stupid.

 

[Gludy]

I want to know it out of pure interest. Its a number that measures the stability of a boat and so give some direct indication of the subject we have been discussing. I am not saying that its the only factor that matters ... far from it. I am saying it is interesting to know.

I cannot see how a planing hull, that depends for its stability on its speed can be as stable when not planing as a displacment hull with a keel. The planing hull is then basically a basin sitting on the water as opposed to a displacment hull sitting in the water and dug into the water - the wave energy required to flip the planing, everything else being equal, must be less that the energy required to flip the displacement.

Sailing yachts achieve their stability through deep keels - is a deep keeled sailing boat more stable than a bilge keeled one? I would think it must be.

I am properly looking at all this for the first time, it may bore some folks to tears and that is fine because they do not have to read the thread do they?

 

[Wiggo]

Stability, Seaworthiness and Seakeeping

Three very important traits in a boat. All different, not necessarily related.

Stable boat: very beamy, doesn't roll about at all, but could slam like a pig, have a low freeboard and get swamped by the wash from a rowing boat. Example: flat bottomed freight barge from the inland waterways of central Europe

Seaworthy boat: built like a brick outhouse, watertight bulkheads, scuppers, low CofG, self righting, but rolls its guts out in anything other than flat calm. Crew die of dehydration brought on by prolonged seasickness. Example: Milltech's ex-lifeboat

Seakindly boat (i.e. good seakeeping): passes the GBB test, but could be wet, could have design flaws that compromise seaworthiness. Example: Trader 575 with plastic escape hatches in transom that could be stoved in by a following sea (sorry, couldn't resist).

RCD Classifications, STIX, AVS and all the other stuff relates only to theoretical stability and some seaworthiness characteristics.

Mathematics of fluid dynamics is only solvable in limited, non-real world scenarios. Any real world hull design is part science, part black magic, part experience. Attempts to classify hulls into Planing, Semi Planing (interestinlgy the Corvette 32, a boat renowned for its seakeeping, is described as semi-planing), Semi Displacement, Full Displacement are relatively pointless. They are fine labels for describing broad classes of boats, but to describe an individual hull shape are generally useless. Most real world hulls exhibit design elements and characteristics of more than one 'type'.

The need to endlessly classify and categorise is probably indicative of something, and is certainly a male trait, interestingly enough. However, if you look at biology, for example, you will find a science that is just realising that it's classifications (kingdom/phylum/species etc) are artificial, and often run counter to the genetic evidence.

Boat designs, like the natural world, are part of a continuous spectrum of forms, and cannot be pigeonholed. Any attempt to do so is ultimately futile, IMHO.

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

Wiggo
I am sorry but, in my view, you are only partly correct.

I agree in general about pidgeon holing. I agree its not just one factor but many. I agree that a hull can show traits of different styl... so I agree on a lot but I disagree that the ,well designed,planing hull boat can ever be as stable as the well designed displacement boat. I ahve already just explained my reasons for this, so need not repeat them in detail. One is a tub sitting on top of the water and the other a tub sitting in the water with a keel dug into the water.

"but rolls its guts out in anything other than flat calm. Crew die of dehydration brought on by prolonged seasickness. Example: Milltech's ex-lifeboat"

Yes abid disadvatage of such SD or D hulls is rolling - we agree and this is why stablisers have come in to cut that down a great deal and this they do with major recent advances in how they do it.

This thread is about boat stability - just one facet of the overall picture. It would be great if we could try and keep it to that. I would like to hear your views on the stability of a planing boat sat on the sea with no power and a displacment deep keeled boat of the same size/weight sat on the sea with no power. Which would be more stable assuming they are bnoth designed well? Once we can establish that we can move forward to different speed sand conditions but I have a feeling you will not allow such a discussion to progress .... prove me wrong.

 

[Wiggo]

Re: Stability, Seaworthiness and Seakeeping

Yes, but trying to have a discussion with you is like trying to nail jelly to the wall, Paul: you started out discussing what I shall refer to as 'seakeeping', the ability to move around and make a bacon butty while underway - that's all to do with the motion of the boat while underway.

Then you squirmed it into a discussion of RCD Seaworthiness, which has everything to do with not sinking, and nothing to do with being comfortable on a passage.

Now, apparently, it's about stability while not underway/making way.

Do you want a boat you like, or do you actually want to drive us all insane? If you do not stick to the topic, I shall be forced to adopt a stern tone, and quote your replies verbatim, refuting them sentence by sentence.

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

Wiggo
There you go again - not dealing with the issue but stating, in effect, that this thread that is headed boat stability should not be about boat stability.

On other threads, I have been forced into discusssing a wide range of subjects - this thread is about one thing - boat stability, if you want to discusss the wider issue then buggar off to another thread that has that wider discussion underway

I am determined as far as I can to stick on this thread to discusiing boat stability with the hope it can get somewhere, I have made it very clear that boat stability is only one factor of many.

 

[Wiggo]

Re: Stability, Seaworthiness and Seakeeping

[ QUOTE ]
This thread is about boat stability - just one facet of the overall picture. It would be great if we could try and keep it to that. I would like to hear your views on the stability of a planing boat sat on the sea with no power and a displacment deep keeled boat of the same size/weight sat on the sea with no power. Which would be more stable assuming they are bnoth designed well?

[/ QUOTE ]

OK. A planing boat sat dead in the water with no power will be rather tippy. A round bilge displacement boat dead in the water will be rather tippy. A deep keel displacement boat (are we talking deep draught, here or deep keel, out of interest? It's just that deep keels seem to be a feature of blow boats, not mobos...) will be more stable when dead in the water.

Happy? Cos if you are, perhaps I can go on...

[ QUOTE ]
Once we can establish that we can move forward to different speed sand conditions but I have a feeling you will not allow such a discussion to progress .... prove me wrong.

[/ QUOTE ]

...as soon as you introduce water flow into the equation, i.e. you make the thing move, the maths goes all Fray Bentos. So now, boats are designed by people who 'think' or 'feel' that a flat section here, a chine there, a bit of keel somewhere else will make a hull with particular characteristics. Usually based on a combination of experience, gut feel and trial and error. Sometimes you get brilliant hulls, other times you get lemons. Then the boat builder comes along and adds loads of gear and buugers up the weight distribution. Or does summat else stupid (remember the QE2 stabiliser debacle a few years ago?)

It's a black art, Paul. You won't solve it by analysing it to death here. Let it rest. Go boating instead.

 

[DAKA]

Re: Stability, Seaworthiness and Seakeeping

Are STIX numbers relevant to planing boats ?
Bath tub owners will not appreciate that while they are at sea for days drifting up and down the coast with the tide making butties; a planing boat will have chosen a weather window, waited for wind with tide and be at the pub for lunch time.

Sorry thought a change in tack required. /forums/images/graemlins/smile.gif

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

Wiggo
I see a chink of light here...

When the boats begin to move (below planing speed - say 10 knots) the planing boat would normally still be less stable than the displacment boat for exactly the same reason - both are in displacment mode but the d hull has a keel dug into the water and is therfore more stable...... agreed?

I am assuming a good design on both boats and noone doing any stupid helming.

 

[Blackfeather]

The STIX values of some popular boats are as follows:

Botnia Targa - STIX to the sea like glue

Bayliner - up STIX and buy something else

Sealine - saleman STIX yer arm up yer back

Fairline - price STIX in yer throat

Birchwood - STIX out like a sore thumb

Any yacht - STIX it up yer bum

Princess - STIX two fingers up to everyone else

Rodman - STIX out as the finest example of European boatbuilding, seakeeping and design (naturally)

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

"Are STIX numbers relevant to planing boats ?"
Yes, is a measure of how much energy is required by a wave to flip the boat.

"Bath tub owners will not appreciate that while they are at sea for days drifting up and down the coast with the tide making butties; a planing boat will have chosen a weather window, waited for wind with tide and be at the pub for lunch time."

That is not really the case with an SD hull like the one I have chosen.

I can go up to 25 knots and so pretty well get to the pub early, I can run from the weather, I can probably maintain a higher speed through the water in bad weather and so beat some planing boats to the pub.

I very much like NordHaven boats but I simply could not tolerate a top speed of 10 knots for my boating. I accept that the seakepping will not be as good as the NordHaven but on balance the choose other factors as more important.

I am not saying that the SD hull is the right hull to go for .... I am only saying that its better for my boating and what I choose to feel is important. However I feel that there should be a better understanding of the alternatives to planing hulls so that when folks choose a boat they make a better informed decision.

 

[gcwhite]

Re: Stability, Seaworthiness and Seakeeping

"OK. A planing boat sat dead in the water with no power will be rather tippy."

Normally I would agree, however I had a sea trial in a Sunseeker Manhattan 50 some months ago and as part of the demonstration the skipper told me to stop the boat in slight (mild chop) conditions. I would have expected a bit of "tippyness" but no she just sat there, definitely in bacon butty land. This was no fluke it was part of the demo which Sunseeker were anxious to show.

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

I would think that that would be the case - we are really talking about the amount of energy a wave needs to turn over the two different hull forms.

I Have sat at anchor for agaes in My Sqauddie making butties, cooking, etc... well the other half has.

I was once caught at anchor in an f7 and boy did we pay the price in moving in every direction at once .... all night, we had to only crawl form a to be to move. Mind you I o not think it was butty making time on any boat of any type.

Butty making in calmish conditions when not underway is not a problem in any boat.

 

[Wiggo]

Re: Stability, Seaworthiness and Seakeeping

[ QUOTE ]
Wiggo
I see a chink of light here...

When the boats begin to move (below planing speed - say 10 knots) the planing boat would normally still be less stable than the displacment boat for exactly the same reason - both are in displacment mode but the d hull has a keel dug into the water and is therfore more stable...... agreed?

I am assuming a good design on both boats and noone doing any stupid helming.

[/ QUOTE ]

As one door closes, another slams in your face...

No, sorry, don't agree. A displacement boat could be a round bilged MFV that rolls like a pig at any speed. Displacement hull form DOES NOT EQUATE to big deep keel. Now answer my earlier question please: are you talking about a deep draught displacement boat i.e. high draught:beam ratio, or something with a deep keel? Cos I don't know of a mobo with a 'deep keel'. Bilge keels, maybe, like you see on Nelsons and others, but not deep fin keels.

 

[miket]

I know you like winding people up but please, if you want a semi/ displacement boat just go and buy one. Don't torture us all as well as yourself!

 

[Gludy]

Re: Stability, Seaworthiness and Seakeeping

Wiggo
you seem to be getting agitated discussing a technical issue ... please calm down a bit.

In answer to your clear question, I am talking baout a displacment boat with a deep keel.

 

[Gludy]

I fail to see how asking simple qustions and having a discussion on boat stability should wind people up. If Wiggo gets wound up then those issues are within him.
If you do not wish to discuss the issues please just do not repspond to the threads.

In his lasy post he asked a straight question and has received a straight answer. I am rarely returned the same favour.

This thread is nothing to do with me buying a boat.

 

[miket]

"I rarely returned the same favour" ......

Perhaps you should read something into this?!!

 

[BarryH]

Re: Stability, Seaworthiness and Seakeeping

In what terms are you defining "stability" Is it doesn't rock a lot for the conditions where small angles of list occur before capsize. Or where a hull rolls and lists to greater angles but the thing has to sit on its beam ends before it capsizes.