Owner wins claim against UK builder

[Angele]

So what you're claiming is that builders are wasting their time mouldling 20mm?

But what about a sand bank, mud or some other surface that might 'give'. This is just as likely, or indeed a more likely a scenario, than hitting rocks. It's these cases that I would think that the additional thickness may be beneficial. Don't you agree?

I think it is more about the area over which the force is spread that will determine whether 20 mm is sufficient. I'd liken this to a horizontal sheet of GRP against which you (1) drop a (2.5kg) brick from a height of 10m, or (2) fire a (20 gramme) bullet. The brick will have more momentum (but less energy) than the bullet. I reckon there is a fighting chance of the GRP sheet surviving the brick, but not a chance with the bullet, because the entire energy of the bullet is focussed at a tiny point whereas the brick's energy is absorbed over a wider area.

So, by all means build GRP to a thickness of 20mm if it might receive a whack from a heavy object over a large area, but think of a different material (steel?) if it might experience a large force focussed on a single point.

 

[JumbleDuck]

Yes, but you would then have merely a numerical value for the forces or pattern of forces that would break (i) a 7mm hull and (ii) a 20mm hull. But you would be no further on with answering the question "Did this particular grounding apply the forces computed under (i) or (ii), and THAT is the critical question. Hence, for the respondents I prefer the tactic of saying that the claimants just haven't proved (on BofP) their hypothesis that this particular grounding would not have broken a 20mm hull.

On the other hand, you might (big, big stress on that word) be able to measure the force required to produce the observed damage in the 7mm hull, apply the same force to the 20mm hull and see what happens.

I suspect it would be a lot easier and probably more credible to computer model the grounding scenarios using finite element analysis.

As jfm says, insufficient data. GRP is not easy stuff to model, particularly when cracks are involved, and you would need awful good information about the size, direction and duration of the applied forces, which I imagine were a mixture of direct forces and torques.

I stick with my earlier comments that the force needed to stop a 30 tonne boat quickly is likely to be way more than 40.5kN - you just have to hope that this is spread across large sections of the hull (such as the boat riding up over a rock) and not taken by a single (weak) point on impact, or the boat will surely sink.

It depends on how long it took to stop, what distance it took to stop and what proportion of the force came from the contact. If you just chop power on one of these things, how long would it take to stop? Supposing the back end were hefted up a bit at the same time, digging the bow in?

I think Pearl need to make another one with the right thickness of hull, run it aground in the same place, and show that it sinks. Easy peasy.

 

[Angele]

Ah ok, my apologies for being so numerically mixed up Angele!

No problem.

 

[jfm]

It must be possible to make educated assumptions on that surely?

You can make assumptions of course, but they will in a case like this often be just guesses. If a prop strikes a rock then the shape and depth of the rock (among other things) have a big effect on the forces on the P bracket mounting pad and there was no data about those things. Whatever data claimant assumes/guesses/proposes must (if the case is properly defended!) be correct to a BofP standard. That is sort of a "51% test" so to put some context around it, if two things could equally have happened, then neither of them makes the grade. I doubt there is a single grounding force scenario here that makes the 51% threshold.

 

[MapisM]

Rudder at full angle, boat going straight ahead, I'll admit not for long.

Blimey, do you mean (if I'm now re-reading correctly your post #142) that due to the stress on the rudder stocks, while doing full lock turns at 30+ kts, the rudders actually remained stuck in the center in spite of the wheel turned hard over?
Now that's scary, regardless of for how long it happened! Never heard of that on any boat....

With apologies for the o/t, but THAT is something I wouldn't expect from any boat.
GRP hulls unable to survive a grounding is not exactly breaking news, otoh....

 

[Angele]

It depends on how long it took to stop, what distance it took to stop and what proportion of the force came from the contact.

Indeed it does, as I explained at post #122 when I used a (conservative, I think) estimate of 0.3s, and the calculations I did came up with a massive force - almost 10 times that needed to punch your way through 20mm GRP if applied at a single point. However, what I failed to appreciate at that first of my posts that the majority of this force would have been horizontal, and not the vertical force needed to punch the P-bracket up through the hull.

I think Pearl need to make another one with the right thickness of hull, run it aground in the same place, and show that it sinks. Easy peasy.

.... or perhaps they could have given that other boat to the plantiff and avoided the whole thing coming to court and them losing...

 

[jfm]

you might (big, big stress on that word) be able to measure the force required to produce the observed damage in the 7mm hull, apply the same force to the 20mm hull and see what happens.

I agree that, but of course you need to know what the "observed" damage was in the first place! Had this claim be defended properly it would have been made clear to the judge that there wasn't any "BofP standard" evidence as the nature of the "observed" damage. The damage wasn't "observed" at all, and the only non hearsay observational evidence was admitted to be untruthful. The "6 inch" claim comes not from observation but from deeply flawed net water flow rate calculations. If the thing had been defended properly the judge would have found that claimant didn't show on BofP that the damage was 6 inches to begin with, and therefore all the empirical stuff one could do in a lab based on a 6 inch crack would be meaningless.

Even if all that were not so, you have to deal with the fact that 6 inches of crackery could be produced by force X for duration Y or by force A for duration B, and so on!

 

[Pinnacle]

Blimey, do you mean (if I'm now re-reading correctly your post #142) that due to the stress on the rudder stocks, while doing full lock turns at 30+ kts, the rudders actually remained stuck in the center in spite of the wheel turned hard over?
Now that's scary, regardless of for how long it happened! Never heard of that on any boat....

With apologies for the o/t, but THAT is something I wouldn't expect from any boat.
GRP hulls unable to survive a grounding is not exactly breaking news, otoh....

I don't think so. What I think VP is alluding to was that the rudders suffered from cavitation before they "gripped" the water. Happy to be corrected if I am wrong!

 

[MapisM]

Mmm... I see what you mean P, and you might be right.
Then again, I would be puzzled by VP reference to the stress on the rudder stocks, which has nothing to see with what you are suggesting.
Btw, his post was #141, not 142 (my typo in previous post).

 

[Portofino]

Blimey, do you mean (if I'm now re-reading correctly your post #142) that due to the stress on the rudder stocks, while doing full lock turns at 30+ kts, the rudders actually remained stuck in the center in spite of the wheel turned hard over?
Now that's scary, regardless of for how long it happened! Never heard of that on any boat....

With apologies for the o/t, but THAT is something I wouldn't expect from any boat.
GRP hulls unable to survive a grounding is not exactly breaking news, otoh....

Nope -think aircraft and lift .
Angle of attack increases say 60 degrees and the lift is lost -lamina flow destroyed -wing stalls -plane plummets .
With a boat .
Bomb along @ 32 knt then instead of normally - slowly turning ,ram it hard over as fast as poss ,
The rudders steering rams work ok ,now the blades are 45-60 degree ( or what ever ?) they stall the water flow is disrupted and until it slows with the drag the boat carries on -going forwards ,
Stalls

Imagine the stress / force on the stocks -blades -bolts etc

Of course if attached to 7 mm GRP they will rip off and leave a crack or hole and it will end in court

 

[Portofino]

If Peals legals team where not " crummy " quote JFM ,with the crack / water ingress argument .
then I think the claiment would have simply played "sale of goods act ". Or fit for pupose
Sucessfully arguing -
7mm is really not thick enough for this application -wheeling in a bus load of expert witnesses ( boat builders / designers ) .
Who for the defence jFM would put there name to a report / evidence -that 7 mm is ok ,the industry standard ?

So it was allways going to be a lost cause for Pearl -ending up in court defending 7mm lay up around a P bkt ? iMHO

 

[volvopaul]

Blimey, do you mean (if I'm now re-reading correctly your post #142) that due to the stress on the rudder stocks, while doing full lock turns at 30+ kts, the rudders actually remained stuck in the center in spite of the wheel turned hard over?
Now that's scary, regardless of for how long it happened! Never heard of that on any boat....

With apologies for the o/t, but THAT is something I wouldn't expect from any boat.
GRP hulls unable to survive a grounding is not exactly breaking news, otoh....

No, while doing 30 plus knots wheel turned hard over say to starboard, boat turns, then hard over to port, rudder stalls boat does not change direction immediately , what I'm sayimg is that rudders are hard over yet boat goes straight hence water flow is directly at rudders when there at an angle, giving tremdous stress on the rudder stock. The boat has power steering so no resistance on the wheel. Some boats do stall, the last one I was on prior to this was a windy typhoon 43, that was a scary boat to drive.

 

[rafiki_]

No, while doing 30 plus knots wheel turned hard over say to starboard, boat turns, then hard over to port, rudder stalls boat does not change direction immediately , what I'm sayimg is that rudders are hard over yet boat goes straight hence water flow is directly at rudders when there at an angle, giving tremdous stress on the rudder stock. The boat has power steering so no resistance on the wheel. Some boats do stall, the last one I was on prior to this was a windy typhoon 43, that was a scary boat to drive.

I'm not sure the forces work like this Paul. If the rudders had "stalled" then the water flow is very mixed up, so there should not be much force on the rudder or stock. Forces would return when the water flow had resolved itself, and with the rudder gripping the water. I'm no expert on aqua dynamics, but that would be my reading.

 

[JumbleDuck]

Imagine the stress / force on the stocks -blades -bolts etc

The forces on a stalled aerofoil/hydrofoil are generally a lot lower than you get when the thing is producing lift and drag as intended.

 

[Portofino]

The forces on a stalled aerofoil/hydrofoil are generally a lot lower than you get when the thing is producing lift and drag as intended.

Yeah kind of in an aircraft cos air is not v dense compared to the lift force -plane plummets -gravity takes over .
But in water a blade at 60 degrees to the 30 ton boat direction .Water is dense -put your hand in @ 30 knts wiggle it about like a rudder = feel the force .

 

[JumbleDuck]

Indeed it does, as I explained at post #122 when I used a (conservative, I think) estimate of 0.3s, and the calculations I did came up with a massive force - almost 10 times that needed to punch your way through 20mm GRP if applied at a single point. However, what I failed to appreciate at that first of my posts that the majority of this force would have been horizontal, and not the vertical force needed to punch the P-bracket up through the hull.

The P bracket would have acted as a lever, like a manual tin opener, with potentially much bigger forces at the GRP than applied by the shaft. There would have been vertical forces too - didn't the lawyers argue about the relative size of these?

As a matter of interest, though, did the boat actually come to a complete stop? If - to use your figures - it had a mass of 30,000 kg and was doing 3 m/s when it hit, 40 kN would have been enough to reduce the speed to 1 m/s over a distance of 3 m, which doesn't seem an unreasonable situation.

 

[JumbleDuck]

I agree that, but of course you need to know what the "observed" damage was in the first place! Had this claim be defended properly it would have been made clear to the judge that there wasn't any "BofP standard" evidence as the nature of the "observed" damage. The damage wasn't "observed" at all, and the only non hearsay observational evidence was admitted to be untruthful!

I mean the final observed damage. I'd love to know why they though the crack got bigger afterwards, though. Was the boat dropped or badly propped when salvaged?

 

[rafiki_]

I mean the final observed damage. I'd love to know why they though the crack got bigger afterwards, though. Was the boat dropped or badly propped when salvaged?

There could be other inherent loads on the hull from the engine installation for example. Once a crack has started, propagation will follow, as long as there is sufficient force available.

 

[JumbleDuck]

I'm not sure the forces work like this Paul. If the rudders had "stalled" then the water flow is very mixed up, so there should not be much force on the rudder or stock. Forces would return when the water flow had resolved itself, and with the rudder gripping the water. I'm no expert on aqua dynamics, but that would be my reading.

Yup. That's why stopped props produce less drag than props driving a gearbox or generator around.

 

[JumbleDuck]

Yeah kind of in an aircraft cos air is not v dense compared to the lift force -plane plummets -gravity takes over .
But in water a blade at 60 degrees to the 30 ton boat direction .Water is dense -put your hand in @ 30 knts wiggle it about like a rudder = feel the force .

It's a lot, but not as much as you can get from generating lift. Aerofoils and hydrofoils are very similar in how they behave ... both tend to stall at about 18^o, for example.