Safety of Shore/Props

[Mistroma]

Side loading on cradle

Aerodynamics is certainly not my field but I wondered if I could get a rough handle on the loads involved. Ignoring the effect of sudden changes in load, the worst case scenario should be with the hull at 90 degrees to the wind.

I have a spreadsheet I used ages ago to get some rough figures for likely forces on a mooring. I took out factors relating to shear, wave action etc. and just left the basic calculations.

Boat: Jeanneau 42DS
Est'd Cross Sectional Area 33m2

Drag coefficients:
Flat sections of hull, rudder & keel guestimated at 0.9
Curved section at stern & below waterline approx. 0.7
Mast, boom approx. 0.6
Rigging approx. 1.0

This is the fuzzy area as drag coefficients aren't really fixed, vary with 3 dimensions not just CSA and I'm only making a guess to start with. However, I doubt any would lie outside 0.6 - 1.1 so figures should be in correct ballpark. Hopefully, someone with more expertise might be able to correct some of my assumptions.

Wind(kn) Drag (kg)
10 42
20 168
30 377
40 670
50 1047
60 1508
70 2053
80 2681
90 3393
100 4189

The calculation is very simplistic and just assumes that the wind is pushing the boat sideways. Obviously there will be leverage from a high mast & rigging and these accounted for around 12% of the final figure. However, that hasn't been factored in to the figures.

As the wind at the boatyard probably peaked near 80kn that would mean approx. 2.7 tonnes of force pushing the boat sideways. CSA on 3 support pads on leeward side might be around 300cm2 and that gives a point loading of 9 kg/cm on the hull.

Of course it need not be evenly spread and even so still works out at almost 1 tonne per pad.

Unless I've made an error somewhere it does look as if side loads can be measured in tonnes.

Now that I've arrived at a suitably scary big figure there's good news for me to consider before I go down to check the boat.

1) The wind should only have been side on towards the end of the bad weather
2) The boat is surrounded by others and so not exposed to the full force of the wind (probably more like 10%-20%).

So no hull damage with any luck but I imagine the windex and anemometer might not have survived. Meant to take them down before craning out but forgot (make note for next year).

 

[Seajet]

The modern trend towards keeping rigs up ashore has a lot to answer for; the vibration set up by a mast in a very strong wind will dislodge most props ( and maintenance checks on the rigging tend to get skipped too ).

Stepping and unstepping a mast is a bit of a pain and is added expense, but it's nothing like the hassle our forebears went to in maintaining wood, mild steel & canvas rigs; the term 'seamanship' springs to mind.

I remember the October 1987 blow, with whole rows of boats knocked over like dominos; a steel cradle will always be better, but is relatively expensive and one has to stow it somewhere when not in use.

I was lucky in that my boat was on a sheltered mooring, but the leaving afloat for the winter idea has its' own snags, the mooring has to be super-secure against more severe gales for a start, and in some theories a GRP hull needs time ashore to dry out as an anti-osmosis measure, or at least to be gelshielded.

There is also the point that boats left on moorings in winter can attract thieves.

I know someone whose boat is damaged thanks to a yard worker driving a wedge in too enthusiastically, so the steel cradle for large single keel boats wins every time for me...

Fortunately I have strong trestles for my lift keeler, and am able to lower the rig with the assistance of one chum.

When I had a fin keeler with a keel stepped mast I went through all the shore storage hassles mentioned, plus on re-stepping the mast a crane driver decided to ignore the hole in the deck the designer had provided, and create another one !

 

[Elessar]

I reckon in extreme conditions I'd add to a cradle, a couple of cross-braced wooden shores/props with wedges.

I see lots of boats out of the water.

The only time I've seem an accident, which was right opposite my workshop, was when owners had a fiddle with the shores.

One of my staff works part time in a boatyard, so is trained to drive the crane, shore the boats etc. He knows that if he touched them when working for me, he is uninsured. He knows he would be doing his last day of work for me if he did so.

Now I'm talking professional yards not clubs here.

The boat yards prop lots of boats, have good systems for training in place and are insured if the worst happens.

The modern 4 leg metal cradles are cross braced and I've never seen one fail.

The wooden shores rely on friction. The nails in the cross braces help stop them wiggling loose but no more. After a blow and periodically anyway, the wedges need to be hammered in to make sure they're tight.

In summary, worry about something else, and if you must worry about this speak to the yard and DO NOT fiddle yourself.

 

[Elessar]

When owners know best

Owner changed the shores without the yard knowing. This was 100% not the yard's fault.
This yard does not charge for moving shores when painting etc.

 

[Poignard]

I assume therefore that your mast is down.

Too right. I would never lay up my boat with the mast up unless I had absolutely no choice but to do so.

 

[DaveS]

Mast up while ashore?

Too right. I would never lay up my boat with the mast up unless I had absolutely no choice but to do so.

+1

At Craobh masts have to come down - no option. The shoring can creak quite alarmingly in a blow (I occasionally sleep aboard) but they claim not to have lost one yet...

I thought this was universal practice in Scotland until seeing the sad thread on Clyde Wanderer's current misfortune. There were pictures earlier this year of a line of shored boats with their masts up in a marina in the S of England that had been felled like dominoes. IIRC the same thing happened last year, and in several previous years. Taking the mast off is a hassle, and costs, but if peace of mind has a value...

There are other benefits. Accessing the mast while horizontal makes a lot of jobs much easier.

 

[NormanS]

The yard that I am in, on the West Coast of Scotland, insists (quite rightly) that all boats must be in cradles. The yard, however is a former gravel quarry, and so is 20ft lower than the surrounding land, and also has trees all round it. It is therefore very sheltered, and there is no requirement to take masts down, and in practice few do.

 

[Clyde_Wanderer]

I've just seen the posting of the 'accident' where Clyde_Wanderers boat has been hit by another whilst ashore.
http://www.ybw.com/forums/showthread.php?t=296397

My own personal opinion is that the modern steel stand/props are not as reliable to stay gripped to the boats bottom as the traditional timber shores with a wedge.

I have seen many boats ashore temporarily sat with just 4 modern props holding them up, which makes me very anxious.
I have often found a modern prop will not hold well against the hull and has been found very loose.

The one prop that I notice commonly drops away is that which sits under the bow.

Although my yard regularly taps home the wedges as the boats 'settle', I still check tightness of things myself.

I would advise any boat to have at least two wooden shores with wedges either side and to be cross braced, even though steel stands may also be used.

Boats that tend to be bows heavy really do need that shore/prop under the bows at all times.

Not getting exactly you mean here, are you saying that timber shoring is better than cradles?
My boat was on a cradle which would never have moved due to the wide footprint, unfortunate that the boat on timber shores next to her fell on her.
I would agree that some of the narrow box shape cradles are not good as their footprint is narrower than the boats beam which could mean that the whole cradle goes over with the boat.
When my boat was hit she was heavily pushed over to starboerd and down, yet the cradle never moved and she stayed on it.
As for a shore under the bow, I sat and watched my boat lifting about 1 inch of the two forward cradle pads during gusts, this was readily rectified by placing a shore under the counter.
I know of another boat on a cradle which had to be straped down to stop the bow lifting during gusts.
C_W

 

[prv]

Too right. I would never lay up my boat with the mast up unless I had absolutely no choice but to do so.

I take mine down - it needs varnishing, and I wouldn't like to miss the opportunity to overhaul all the running rigging which is relatively complex. But nearly all the other boats in the yard leave theirs up, occasionally with sails still bent on. Must admit I sometimes worry about one of them falling over on top of me.

Pete

 

[Seajet]

Leaving masts up is dubious at best; but leaving sails on is just downright unseamanlike, and inconsiderate to neighbouring boats; their owners might actually care, so they and their pride & joy should not be the victims of lazy ******s !

 

[stav]

Leaving masts up is dubious at best; but leaving sails on is just downright unseamanlike, and inconsiderate to neighbouring boats; their owners might actually care, so they and their pride & joy should not be the victims of lazy ******s !

+1, but another aspect is the over chocking of boats and the damage to its hull but at least that is damaged limited to your own boat. That would be my concern with a modern cradle, potential for only 4 points of contact, I would want a minimum of six, eight would be better. ALso with acro style adjustment the ability to put a high point pressure on must be greater than with wedges. It was interesting to see a modern boat (cored)? with at a least a 19mm deformation to the hull around the support when walking around hamble point over the summer. This must have been the death knell for many older wooden boats as well.

 

[pvb]

I would never allow my boat to be ashore in anything other than a proper steel cradle. Having said that, the boat stays in the water all the time (except for a week every other summer for polishing and antifouling).

 

[Coaster]

Seems to me that the support needed for a boat out of the water depends primarily on the degree of exposure. A boat in an exposed yard in west Scotland will be subject to hugely greater wind loads than one in a sheltered Welsh or English location.

I'd always choose a good cradle in preference to individual wooden shores and wedges. As others have said, when boats have strong winds blowing onto and around them they move slightly. It doesn't take much movement to dislodge wooden wedges.

 

[davidej]

Aerodynamics is certainly not my field but I wondered if I could get a rough handle on the loads involved. Ignoring the effect of sudden changes in load, the worst case scenario should be with the hull at 90 degrees to the wind.

I have a spreadsheet I used ages ago to get some rough figures for likely forces on a mooring. I took out factors relating to shear, wave action etc. and just left the basic calculations.

Boat: Jeanneau 42DS
Est'd Cross Sectional Area 33m2

Drag coefficients:
Flat sections of hull, rudder & keel guestimated at 0.9
Curved section at stern & below waterline approx. 0.7
Mast, boom approx. 0.6
Rigging approx. 1.0

This is the fuzzy area as drag coefficients aren't really fixed, vary with 3 dimensions not just CSA and I'm only making a guess to start with. However, I doubt any would lie outside 0.6 - 1.1 so figures should be in correct ballpark. Hopefully, someone with more expertise might be able to correct some of my assumptions.

Wind(kn) Drag (kg)
10 42
20 168
30 377
40 670
50 1047
60 1508
70 2053
80 2681
90 3393
100 4189

The calculation is very simplistic and just assumes that the wind is pushing the boat sideways. Obviously there will be leverage from a high mast & rigging and these accounted for around 12% of the final figure. However, that hasn't been factored in to the figures.

As the wind at the boatyard probably peaked near 80kn that would mean approx. 2.7 tonnes of force pushing the boat sideways. CSA on 3 support pads on leeward side might be around 300cm2 and that gives a point loading of 9 kg/cm on the hull.

Of course it need not be evenly spread and even so still works out at almost 1 tonne per pad.

Unless I've made an error somewhere it does look as if side loads can be measured in tonnes.

Now that I've arrived at a suitably scary big figure there's good news for me to consider before I go down to check the boat.

1) The wind should only have been side on towards the end of the bad weather
2) The boat is surrounded by others and so not exposed to the full force of the wind (probably more like 10%-20%).

So no hull damage with any luck but I imagine the windex and anemometer might not have survived. Meant to take them down before craning out but forgot (make note for next year).

I don't feel qualified to comment on your calculations but the fundamental assumption is that the full wind speed is impinging on the full sideways area.

Apart from the fact that wind speed decreases at lower levels (down to zero at ground level), there is also that yachts are normally in neat rows each shielding the next one.

I would hazard a guess that most of the toppling force comes from the mast and actual forces are only a fraction of your calculation.

Stiil no excuse for not supporting the boat properly.

 

[Bandit]

Mast windage is significant, look at the heel produced in a gale with with yacht on a pontoon in a marina.

Bulb and wing keels are a problem as there is no flat load bearing area of the keel to support the boat.

With a fin keel yacht I would not use props and wedges, in high winds, boats ashore vibrate and wind is not constant it gusts and changes direction and prop and wedge not under pressure will fall down.

If using a cradle with four arms, support should be given at the front and the back of the yacht to stop it falling forwards or backwards. Its easy with a yacht with a fine entry to wedge apart the legs and once a 10 ton yacht starts moving it takes some stopping.

In exposed areas masts should be removed.

On a rigid T shaped cradle such as Roodberg I prefer a 5 ton lashing strap over the front leg under the hull over the other leg and then tight to the cradle frame again.

 

[Seajet]

As mentioned above, point loading at shoring points is crucial - as in to be avoided !

A friends' boat has had the hull damaged by a berk driving a wedge on a timber prop in too hard; on the other hand I was quite impressed by the large size of the pads on my chum's trailer from Hayling Trailers, with the acro style supports, modifications ( like removing the axles ! ) and care he was able to raise the hull enough to inspect the lift keel - which stayed with its' weight on the ground.

No connection to Hayling Trailers.

 

[Clyde_Wanderer]

I was down having a look at the situation today and as the mast has been removed of the boat that came down on mine I looked at its rigging.
After noticing aluminium swarf among the strands of my backstay and then checking the other boats rigging I was shocked to discover that it was his roller reefing foil that had slid down my back stay starting at a point aprox 10-12 ft above my aft deck and continuing down along the back stay for aprox 4-5ft before snapping my mast.
The deep gouge in his foil is where my backstay cut into it.
In light of this, I reckon the wind got under his hull,lifted the front of his boat high in the air, (high enough for his foil to ride nearly half the height of my mast) her bow then spun 90 degree to her starboard side and over my boat when her rr foil came down on my backstay at the same time her keel slid out from under her to port and after my mast broke her bow came down on my ps toerail amidships bending her pulpit rail out to port snapping the foil to drum joint and damaging my toerail, she then slid away and down until she came to rest on her sb hull near the waterline area and her mast stopped short above my cockpit.
Because of the downward/starboard force she excerted on my backstay the lower half of my mast permenantly bent to sb side slightly before it snapped just above the spreaders.

 

[chubby]

I would never allow my boat to be ashore in anything other than a proper steel cradle. Having said that, the boat stays in the water all the time (except for a week every other summer for polishing and antifouling).

If you are a longstanding customer of a particular yard then you have to assume they know what works for their location and trust them, the recent and likely coming winds are indeed strong but not that exceptional in the catergory of the 1987 storm: my yard does use wooden shores cross braced mainly but the boats are also huddled together, I presume shelter each other and Dabchick is amongst the huddle and bows into the prevailing wind, it doesnt stop me worrying and going down to check but I don`t presume to tell the yard what to do and trust their experience.

 

[Seajet]

Clyde Wanderer,

when struggling in the aftermath of the October 1987 Hurricane on the South Coast, I found things had happened which completely defied the normal laws of physics, the forces were incredible !

I hope your problems are sorted quickly and satisfactorily.

 

[Clyde_Wanderer]

Seems to me that the support needed for a boat out of the water depends primarily on the degree of exposure. A boat in an exposed yard in west Scotland will be subject to hugely greater wind loads than one in a sheltered Welsh or English location.

I'd always choose a good cradle in preference to individual wooden shores and wedges. As others have said, when boats have strong winds blowing onto and around them they move slightly. It doesn't take much movement to dislodge wooden wedges.

Totally agree and thats why I built this cradle for my boat.
Six pads and a wide foot print and multi adjustable with a strong spine to hold the whole lot together, and cheaper than buying one.
I have removed my mast every year, mainly to do work on it but had decided to leave it on this year, the luck of the draw!
C_W