Re engine 16M trawler yacht

[MapisM]

The ballast was to stop her leaning over in a cross wind. The fin stabs ought to have done the job, but given their modest size and 7 knots of boat speed in a big sea they weren't enough. The ballast did substantially solve the problem

Wow, does SD support the RNLI in his spare time?
In none of the few occasions when the sea was big enough to make me reduce the (already low!) cruising speed of 8/9 knots, down to 5/6, I never experienced leaning due to cross wind.
Of course, the lower the speed, the less effective the stabs are, but even at 5 knots, in my boat they are still more than good enough.
Just a thought: since for each hydraulic system size/power, stab builders normally offer a choice of 2 or 3 fin size, aren't by chance available some bigger fins?
That's a relatively inexpensive upgrade, compared to upgrading the whole system.
I'm mentioning it because, on paper, the ballast could have made also the boat "harder" to stabilize, when the waves do gets her rolling.
In other words, she might be less prone to leaning on one hand, but also more prone to rolling, for any given fin size...
...though that's a bit of armchair naval engineering, of course!

 

[RogerRat]

Interesting stuff roger. It's well known that increasing the angular inertia, aka putting weight high (or low - in a keel bulb) increases roll period ie makes for slower rolling.

Silver Dee's problem, pre ballast, was partly small engines and high windage. For normal cruising it didn't matter, but Silver Dee went to sea in some horrific wether. The ballast was to stop her leaning over in a cross wind. The fin stabs ought to have done the job, but given their modest size and 7 knots of boat speed in a big sea they weren't enough. The ballast did substantially solve the problem

Hi JFM,

Yep, I agree, but from my first read of the problem I thought there were two issues, one of stability at anchor for diving or even a bit of fishing. Wing stabilisers can't help here of course and @ 7 knots, there isn't enough water flow. Your point about capsize potential is noted, the experts would reduced roll moment to the minimum but obviously not risk moving the C of G too high to make it unstable. The second, was to increase economy with most efficient engine at a modest speed, whether the engines were big and heavy or lighter and lower duty cycle, a Late Starter special. :ambivalence:

Personally, two things I would NOT touch are replacing engines and / or Props, well not without consulting a witch doctoré.

Strangely enough, when I re-did my yachtmaster course after 25 years, (without Morse code) there was a nautical engineer discussing stability of yachts. Did you know that a dis-masted yacht is more likely to capsize than one with a mast?

Now that really threw me until it was explained. I doubt if most of the readers on Scuttle-butt would know that as weekend sailors.

Roger.


PS. Sorry about getting back on thread. Silver Dee :encouragement:

 

[MapisM]

Did you know that a dis-masted yacht is more likely to capsize than one with a mast?

I can see that, but it's also more self-righting, surely?

 

[vas]

Strangely enough, when I re-did my yachtmaster course after 25 years, (without Morse code) there was a nautical engineer discussing stability of yachts. Did you know that a dis-masted yacht is more likely to capsize than one with a mast?

Now that really threw me until it was explained. I doubt if most of the readers on Scuttle-butt would know that as weekend sailors.

Roger.


PS. Sorry about getting back on thread. Silver Dee :encouragement:

RR,

demasted yacht with engine running and under control, vs demasted yacht bobing around in a F10?
I'm not a naval engineer but I cannot see how a dunno 200kg rigging up on top with a CG 4-5m above WL would affect the behavior of a craft with a ton of ballast with a CG a meter below WL.
If the boat is NOT under power/control they may just have a point else I don't get it. Anyone care to explain?

cheers

V.

 

[RogerRat]

RR,

demasted yacht with engine running and under control, vs demasted yacht bobing around in a F10?
I'm not a naval engineer but I cannot see how a dunno 200kg rigging up on top with a CG 4-5m above WL would affect the behavior of a craft with a ton of ballast with a CG a meter below WL.
If the boat is NOT under power/control they may just have a point else I don't get it. Anyone care to explain?

cheers

V.

Blows your mind don' it?

Moment of enertia, heeling moments etc. blah blah. No I don't think I can but it's true and proven mathematically and in practice. Confirmed by life boats too.

The link above will help explain stability and roll but I don't think that's the whole story with a dis-masted yacht.

Here it is again:
ftp://ftp.fao.org/docrep/fao/011/i0625e/i0625e02d.pdf

RR

 

[RogerRat]

I can see that, but it's also more self-righting, surely?

You would like to think so, but I think that would depend on how much water had been shipped down below in the period just prior to being flipped. :ambivalence: Spooky eh?
RR

 

[vas]

Blows your mind don' it?

Moment of enertia, heeling moments etc. blah blah. No I don't think I can but it's true and proven mathematically and in practice. Confirmed by life boats too.

The link above will help explain stability and roll but I don't think that's the whole story with a dis-masted yacht.

Here it is again:
ftp://ftp.fao.org/docrep/fao/011/i0625e/i0625e02d.pdf

RR

<grumpy mode=on>
sorry RR. it didn't blow my mind at all!
I stand by what I said, the example you post has NOTHING AT ALL to do with a demasted yacht. If you draw the righting forces on a similar section in a yacht with the ballast at the keel way below wl it's going to be a completely different story (all imho)
Mind I didn't spot where the pdf deals with inertia. Further (and no offence) I don't believe that they expect fishermen or leisure moboers/raggies to understand that mess they've printed.

<grumpy mode=off>
five hours sleep and off for a 12h supervising,meetings,teaching trek, wish me luck

V.

PS I'm looking fwd to the opinions of the other forum armchair engineers!

 

[westernman]

Why I think this is relevant to 'Silverdee' is that the increase ballast was strapped in to the boat to give stability at anchor.

For stability at anchor, you will be wanting some flopper stoppers.

 

[scubaman]

I hven't bought or ordered a replacement Whopper. I had a string of lovely cars every 2 years or so for 10 years up until 2005 when I got the e60 M5 then I kept that 8+ years becuase I could see anything better for what I wanted. The F10 M5 is the obvious choice (or M6) but I haven't ordered one (yet)

Apologies for the OP for taking the car-drift a one post further but as said earlier, I can perfectly see from you are coming from J. I've had my more modest e61 530d for eight years now and still haven't found the irresistible urge (or the desire to pay the ridicilous car taxes here) to find a replacement.

As a side note, I had the pleasure of testing the diesel M5 along with the Merc E 63 AMG on a track few months ago. Both were of course very nice cars but also overshadowed by the Audi R8 which was just fantastic fun on track (but would be completely wasted in road use, imho). Btw, I'm not even an Audi fan.

Have you considered a Merc CLS?

 

[RogerRat]

<grumpy mode=on>
sorry RR. it didn't blow my mind at all!
I stand by what I said, the example you post has NOTHING AT ALL to do with a demasted yacht. If you draw the righting forces on a similar section in a yacht with the ballast at the keel way below wl it's going to be a completely different story (all imho)
Mind I didn't spot where the pdf deals with inertia. Further (and no offence) I don't believe that they expect fishermen or leisure moboers/raggies to understand that mess they've printed.

<grumpy mode=off>
five hours sleep and off for a 12h supervising,meetings,teaching trek, wish me luck

V.

PS I'm looking fwd to the opinions of the other forum armchair engineers!

Good Luck Grumpy!

I note that you say it is in your humble opinion. I can only say that these posts are not my opinion, just as stated to me and explained 10 years ago, so I can't remember the physics of explanation. Apologies.

RR

 

[MapisM]

demasted yacht with engine running and under control, vs demasted yacht bobing around in a F10?
I'm not a naval engineer but I cannot see how a dunno 200kg rigging up on top with a CG 4-5m above WL would affect the behavior of a craft with a ton of ballast with a CG a meter below WL.
If the boat is NOT under power/control they may just have a point else I don't get it. Anyone care to explain?

Vas, I don't think it makes a lot of difference whether the yacht is under control or drifting - though of course it's more likely that a drifting vessel is grabbed by a beam sea, but that's true regardless of the mast.
The reason why I said "I can see that" happening is because it's a well known fact that the rolling movements of any dismasted yacht are much more hectic/sudden/snappy (feel free to choose the best wording), due to the missing inertia of the mast, that contributes a lot in making rolling movements softer/slower/more gentle (same as before re.wording! ).
Now, a logical consequence of this is that if it takes the force of an X high breaking wave to capsize a yacht with its mast in place, thus contributing to the roll resistance with its inertia, it could well be that an Y high breaking wave (with X>Y, obviously) is enough to capsize the same yacht when dismasted. It simply starts to roll, and keeps rolling, much faster.
I'm sure there must be a way to explain that with vectors and drawings, but I'm not going to try, life's too short...

Anyway, I maintain that a dismasted yacht - AOTBE - has better self-righting properties, for the very same reasons as above, i.e. if and when a wave makes it go belly up, it's bound to snap back much faster than a yacht with a mast.
@RogerRat, re. "that would depend on how much water had been shipped down below in the period just prior to being flipped": yeah, right, but that's a different matter altogether.

 

[Pinnacle]

Vas, I don't think it makes a lot of difference whether the yacht is under control or drifting - though of course it's more likely that a drifting vessel is grabbed by a beam sea, but that's true regardless of the mast.
The reason why I said "I can see that" happening is because it's a well known fact that the rolling movements of any dismasted yacht are much more hectic/sudden/snappy (feel free to choose the best wording), due to the missing inertia of the mast, that contributes a lot in making rolling movements softer/slower/more gentle (same as before re.wording! ).
Now, a logical consequence of this is that if it takes the force of an X high breaking wave to capsize a yacht with its mast in place, thus contributing to the roll resistance with its inertia, it could well be that an Y high breaking wave (with X>Y, obviously) is enough to capsize the same yacht when dismasted. It simply starts to roll, and keeps rolling, much faster.
I'm sure there must be a way to explain that with vectors and drawings, but I'm not going to try, life's too short...

Anyway, I maintain that a dismasted yacht - AOTBE - has better self-righting properties, for the very same reasons as above, i.e. if and when a wave makes it go belly up, it's bound to snap back much faster than a yacht with a mast.
@RogerRat, re. "that would depend on how much water had been shipped down below in the period just prior to being flipped": yeah, right, but that's a different matter altogether.

I can't prove it, but intuitively that "feels" right.

MapisM - you seem to be spending a lot of time on here of late - don't you have something more important like varnishing to do?

 

[MapisM]

Nah, just doing stuff with the PC, and keeping an eye also here, among other things....
The boat is now sheltered, being prepared for recaulking.
And you know I hate brushes and sandpaper with a passion, anyway!

 

[jfm]

Mapis I generally agree. Problem here is that RR's "Did you know that a dis-masted yacht is more likely to capsize than one with a mast?" as stated to him by a loadmaster (which frankly doesn't mean it has come from the highest authority) is a bit of a twee simpleton claim. The only relevant thing you can say of a dismasted yacht is that it has lower angular inertia (and lower mass) than the same yacht with a mast. Whether that makes it more or less likely to capsize is a whole different question. It could in some conditions, but it is not true to say that universally it does.

Having the mast would help soften the roll (in physics terms, it would increase the roll period and reduce the angular acceleration) which is a good thing, but at the same time it increases materially the windage and the COG height, which are bad things in the context of capsize resistance.

If being dismasted were a bad thing so far as the boat's dynamics were concerned, there would be no point in having weight reducing carbon masts and dyneema standing rigging, which seek to emulate somewhat being dismasted (by reducing rig mass and weight). But these are exactly what the best sailboats in the world DO have. So the loadmaster's claim is as I say a bit mickey mouse.

 

[TimBennet]

So the loadmaster's claim is as I say a bit mickey mouse.

I'm afraid it's your assertion that is mickey mouse.

It's impossible to capsize (strict definition) a yacht by wind pressure, because when the heel angle equals 90 degrees there is zero wind pressure on the rig. A boats ability to resist this 'heeling' even to 90 degrees, can certainly be improved by reducing weight aloft and is the primary concern of any high performance yacht.

Capsizing takes the boat beyond 90 degrees. Here the energy required to do this must come from a more complex, dynamic regime including that from breaking waves. The ability to resist this regime does include a significant contribution from the weight of the rig with more weight being advantageous.

So 'stiffness' and the resistance to heeling is helped by a carbon rig, but a lightweight rig, of which the ultimate 'lightweight variety' is no rig at all, will lessen a boats resistance to capsize. The Wolfson published all these findings in the late eighties in response to questions about why yachts that have lost their rigs were found to be rolled repeatedly, despite having lower centres of gravity and much less windage. These findings were independent of any free water effect from shipped water.

 

[jfm]

The ability to resist this regime does include a significant contribution from the weight of the rig with more weight being advantageous.

Just to make the picture complete, it's the mass of the rig multiplied by the square of the distance from the roll centre that does the job here

a lightweight rig, of which the ultimate 'lightweight variety' is no rig at all, will lessen a boats resistance to capsize.

No, not in a "like night follows day" way. What dismasting does is reduce the boat's angular inertia. Now in waves, AOTBE, that will make the boat more capsize-prone so far as the angular inertia effect is concerned. However, AOT are not E, due to losing the weight of the rig but retaining the keel, which makes the boat less capsize prone so far as the balance effect is concerned.

In other words, when you remove the rig you have one factor increasing tendency to capsize and another reducing it. Depending on the balance of those factors, you might find a dismasted boat is less capsize prone. But it is not a universal truth; it is circumstance specific.

 

[westernman]

I'm afraid it's your assertion that is mickey mouse.

It's impossible to capsize (strict definition) a yacht by wind pressure, because when the heel angle equals 90 degrees there is zero wind pressure on the rig.

There is the wind pressure on the hull as well. In 60 knot winds this will be quite considerable.

 

[TimBennet]

Depending on the balance of those factors, you might find a dismasted boat is less capsize prone. But it is not a universal truth; it is circumstance specific.

I suggest you read the work from the Wolfson Unit. Resistance to being rolled is a function of the sum of the inertia of all contributing parts of the boat. The mast is a significant contributor because as you say, the formula includes a square function to the radius of gyration. If you remove any contributing factor, resistance to capsize will diminish, but the bigger the contribution the bigger the loss will be felt.

Any improvement in dynamic capsize resistance by a lowering of the C of G by the rig loss is insignificant in comparison.

 

[jfm]

There is the wind pressure on the hull as well. In 60 knot winds this will be quite considerable.

Yup. In fact there are loads of factors, like as the capsize starts (I mean the hull goes just beyond 90 deg rotation) the rig acts as a flopper stopper and its drag resists the roll. So the rig resists capsize, but not only because of the angular inertia effect. This drag operates on a modified linear relationship with rig height, whereas the inertia effect is a pure square law.

Then when you are well past 90 deg of roll and approaching the point (say 160deg or whatever) where the hull is about to want to become stable at 180deg inverted, the rig acts as a "trip up" or sea anchor device making it easier for the wind on the keel and the next wave to perfectly capsize the boat, ie the rig has (at this point in the proceedings) made the boat more capsize prone not less.

The answer to being more or less capsize prone is way more complex than removing a rig or not.

 

[RogerRat]

Mapis I generally agree. Problem here is that RR's "Did you know that a dis-masted yacht is more likely to capsize than one with a mast?" as stated to him by a loadmaster (which frankly doesn't mean it has come from the highest authority) is a bit of a twee simpleton claim. It could in some conditions, but it is not true to say that universally it does. So the loadmaster's claim is as I say a bit mickey mouse.

Ooops, Sorry to jump back in, but this was misunderstood!

This was not the claim of the Big Lift 'Loadmaster.' The discussion with him was about loading to reduce GM and reducing roll rate by raising the centre of gravity and how this was a general misconception. He was not a stereo typical docker either, very knowledgeable, interested and pleased to discuss when interest shown.

The dis-masted yacht scenario was based on actual research and was passed onto me and others by a marine engineer graduate from University of Southampton on a yachtmaster course. The info quoted by TimBennet from 'The Wolfson Unit, Resistance to being rolled, etc.' sounds to me spot on with how I remembered that discussion but it was 10 years back now.

I probably shouldn't have mentioned it as most people with a basic understanding of physics find it hard to get their heads round, me too. btw.

It's a little bit like, "How many forces does a rotating propeller exert?" - Minefield. :ambivalence:

RR