The perils of edgy design offshore

Here the ORR races stability index:
http://www.offshorerace.org/images/pdf's/Part Two.pdf
If we take CI+SI as being 15 (maximum figure), then LPS of this boat would be around 85 deg. Let's asume 90....

 

I think you are making a poor assumption.

 

Well, asuming the 100 figure for the SI, the limits for LPS in this case are 100 and 85, as the figure for CI + SI may vary between perhaps 0 and 15 at its most if I'm not wrong.
So 90 seems plausible. But take the figure you prefer: a rather poor LPS for an offshore boat that size anyway.

 

You seem to be ignoring the possibility the CI is at the other limit of -5.

A quick run of the formula with ballpark numbers for the variables might make you rethink this.

 

I'm not ignoring it.

+10 +5 = 15
+10 -5 = 5
+5 (just not to take 10) - 5 = 0

There are more posssibilities, of course, but a variation between 0 and 15 seem to be highly probable.

Thus 90 seems to be a plausible value.

 

Your math seems strange.

The CI range is -5 to +5
The SI Range is 0 to +10

Therefore the range of potential sums is not 0 to 15, it is -5 to 15.


If you would bother to plug ballpark numbers for this design into the formulas you might come to a better conclusion than what you have done.


Regardless, IF there was a requirement for boats to comply with a Stability Index for Cat 0, 1, or 2 there seems to be a problem.

 

Yes, but -5 seems very unrealistic, that's why I took the 0 to 15 range.
And if we take CI+SI as being in the middle of the -5 +15 range (5), we still get LPS as being a low 95.
A poor number anyway.
Do you want to further debate this?

 

Thanks Paul

 

Which would make it an abysmal boat to be in in the conditions they experienced.

I had expected the LPS to be higher (>110) and even then I'd argue that it's still a very poor design for an offshore boat given the extended side decks.

 

This surprises me.

When I look at this design and think about it in a knockdown I think about the total bouyancy in two components. There is the volume in the immersed hull and the volume in the immersed wing. Together they give a total volume.

As the boat begins to heel, the volume in the wing is helping resist heeling. The center of volume of the wing is outboard the center of total volume point.

As the boat approaches 90 degrees the volume of the hull and the volume of the wing begin to line up. So the wing is helping less.

Given the lack of dihedral in the wing it wouldn't be much more than 90 degrees before the center of the volume of the wing is on the wrong side of the total volume, so it then adds to the overturning force.

As the boat heels more the overturning force becomes even greater, at the same time the keel is contributing less and less to the stability. A "perfect storm" for a turtle.

The Stability Index seems to indicate the LPS is about 95 degrees. I could well believe a real life LPS of between 95 and 100 degrees for this design. At your 110+ degrees the immersed wing should be trying very hard to reach the surface in the wrong direction.

 

One of the photos I've seen of the wings from the top shows them to be relatively thin in section, with stainless hinges along the fold line. I was surprised by the hinges, as it basically means the wings are made to fold for storage and travel. There is most likely a lock down provision, but to be strong enough for crew weight, for stanchion mounting & lifelines the wings can't be exactly lightweight and minimal. Since there is no monocoque frame continuation of the internal bulkheads, the wings have got to be very strong with little flotation contribution. Since they have to be a self contained unit, there is a lot more material than an integrated wing would require.

Just a guess, but I doubt the wings are making as significant a contribution to buoyancy as it appears. Yet more conjecture and guessing.

--
CutOnce

 

I expected that from the adopted regulations for offshore racers that no-one would presume to go offshore in a such a craft of lower LPS !

Given the low form stability I expected that the ballast would be chosen to achieve this.

Combining ULDB with an enhanced overturning mechanism a low form stability and an abysmal LPS adds up to one very dangerous design offshore.


The real issues with a design like this should be how much higher the LPS should be above the accepted standard, to cope with the added drag from the projections in a knockdown and the enhanced overturning moment from any sideways translation.

If the LPS is as low as indicated then there is no debate about whether the boat was unsuitable or not. Although all this was apparent enough right from the start.

 

It's a trailerable design with a retractable keel.

 

The Perils of Edgy Weather in Any Boat

I did the rough sketch below to get some idea of the stability, if any, of the K35 when knocked over past 90 degrees. I ran this by two naval architects, one said the reasoning was good but that the result probably wasn't precise enough; the other said that the sketch leaves something to be desired because the only way to get an accurate LPS/AVS is by using a computer program suited for it.
That being said the sketch shows the boat at an angle of heel of 130 degrees at a displacement of 3250 pounds with 1500 lbs. of ballast which is the most accurate info I've found. It is done to a scale of 3/8"=1'.
As shown the boat appears to have significant positive stability, however, the crew is presumed to be in the water.
Scenario 1: if the crew(est. @ 8 X 170= 1360lb) was standing or sitting at the juncture of the wing and the side of the boat at this angle there would be
no positive stability unless the mast was sealed. Just for the hell of it I looked at the effect of 1 cubic ft. of masthead buoyancy and it is fairly dramatic-at this angle it is about 2000 ft.lbs.
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Scenario 2: if the crew were standing at the turn of the bilge the boat would appear to still have positive stability in the sketch as shown.

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Tentative conclusions:
1) I'd sure like to see the designers LPS/AVS because I think this boat has more RM from 90 degrees than it is credited with,
2) The crew can have a lot of effect on this boat when sailing, as we know, or when capsizing and what they do or don't do could be aided by the original designers LPS calculation. This could result in anything from saved lives, to no capsize and ties in with the idea of a practiced crew plan for a capsize in a boat like this.
3) It doesn't appear to me that the crew could right this boat from a turtled position even if they stood as far outboard as possible on the wing on one side. However, if one wing was partially flooded there is a good chance of righting. I believe that a righting system could be devised for this boat.
4) There is enough of the boat likely to be out of the water to amplify the effect of waves tremendously as Mr. Johns has said. I'd say ,based on what people who have sailed the boat have said, that it is possible the immersed wing(acting like a keel) was an asset and not a liability in this case since it may have prevented the boat from getting away from the crew that were in the water. The design of the boat- allowing it to float when turtled- certainly appears to have contributed to saving the majority of the crew-giving them a ready made raft.
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Thanks to my friends for their help in understanding this and in understanding and calculating the ORR Stability Index which I am learning to do now.


Notes on the sketch: the estimated CB of the boat is shown at 130 degrees.
The righting moment is calculated by the distance of a vertical line thru the CG of the ballast bulb from the estimated heeled CB. The heeling moment at this angle is calculated by the distance of a vertical line thru the CG of the boat(not incl the keel) from the estimated heeled CB of the boat.

click on image--

 

Nice work Doug,

As told:
- To approximate equal submerged volumes when righted/heeled is not an easy task with such shapes.
- Vertical position CoG of hull plus masts, rigging, etc, may not be accurate.
- Only a detailed and careful calculation will produce accurate stab curves (and thus LPS)

One question:
I think I see openings in the sides of the cockpit. Are those watertight?