Riser length.

Mudisox

Mudisox

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It has been suggested that 1.15 X max depth is sufficient for a heavy riser chain for a grid of 50 + boats, in 2.5m tidal waters, fairly exposed with summer winds of 50+kts.
This "solution" to a problem of their own making has come from the ODA, in Portland.
We all know that it is far, far too short and the snatch in even 1m waves will pull out the fittings of any unfortunate boat attached but does anyone have specific, scientific evidence to refute their claims that this will do the job?
 
The riser only goes to the mooring buoy doesn't it? If so, then I suspect a decent strop length will solve the problem, won't it?

It doesn't really matter that the mooring buoy doesn't float on top of the biggest waves, as long as your strop allows for the pitching of the boat. I would use 2 strops of the largest size your cleat will hold. Perhaps even a chain strop. If rope, use a smallish soft eye that has to be lightly forced over the cleat as figure 8 knots will make it hard to accomodate 2 strops. I would also recommend that you use some anti-chafeing hose or similar.

Lots of mooring in the Straits (OK so they're not as exposed, but do have very strong & large tides & wind over tide can be short & steep) have "tight risers" with long chain strops connected to the top of the riser, just below the buoy. In effect this gives you the length you need to prevent snatch, yet the buoy will lift the swivel on springs to stop it getting buried.

Sorry if this is not an opinion you want to hear. /forums/images/graemlins/laugh.gif
 
I feel sure that 1.15 of max depth is far too short. That means that a wave of 37.5 cms will see your chain tight trying to lift the weight or mainb chain of the mooring. That sized wave can come from a passing mobo or a bit of wind. As you say it will rip out the fittings and destrroy the boat.
IMHO the riser should have a good secion of nylon or polypropelene rope to provide spring and should be longer than 1.15 perhaps another metre longer.

I would suggest that if the hardware is provided by the mooring owner then you attach additional rope from the boat (doube up for safety) to the chain then adjust the length to see what you cna get away with in terms of how long you can have without bumping other boats. Or get the management upset.

You might consider running the rope over a roller at the bow (from the chain which is very close to the bow) then back to a fixture as far back as possible (even via pulleys to the stern) so that you get the max amount of rope or rubber shock absorbers in the circuit while not necessarily making the riser longer.

But again double up the system in case of failure from chafe
good luck olewill
 
Agree with Searush, it just means the bouys may at times submerge but you need to treat the bouy as if it where a fixed item and allow a long enough strop to absorb the boats motion. In effect you will have an inverted cantinary effect, the chain goes up to the bouy then along to the boat instead of along the seabed and up to the boat. It will put more stress on the grounf fixing though, what is on the bottem?
 
It sounds as if they are trying to keep the footprint down (boats moored close together).

Lots of factors here: type of boat (keel depth, windage, etc.), range of tide, empirical data related to height of tide under surge conditions, wave frequency and boat length, significant wave height at the location, natural and wind-induced current...

All these ( plus costs !) will contribute to the decision to go for a small riser.

I'd be concerned that a short riser will encourage boats at low tide to settle too near to the sinker (if there is one).



There's lots of stuff on deepwater moorings (e.g. http://otrc.tamu.edu/pages/DW%20mooring.htm) but it's related to mooring of oil platforms and tankers.


Why not ask them for the technical basis (including risk analysis) for their decision, on the grounds that empirical knowledge suggests that (say) 1.5 times max tide height has a significant effect on damping the surge of a moored vessel ?
 
I should explain more.
We had perfectly good mooring arrangements based on a trots of Battleship level ground mooring chain well spaced out. The chain risers were 2-2.5 x depth and supported by a buoy. Rope mooring penndants have a history of chaffing through at Portland and this must be restricted in length to get all the boats in. The only problems we ever had were in nil wind conditions with boats touching each other if wind direction as different for when they last arrived at the mooring.
The area left, after pinching our crown estates seabed for the new Olympic facility is 25% of the original area, and the replacement chains all round about 30-40% smaller in diameter! Cost cutting to the extreme.
They are also late- lifted last oct and the final date WAS 1st April. Still no consensus on design and the parts aren't even here!! So no boats on the water yet but don't fear folks - you the tax payer are compensating us whilst they fiddle.
 
A forumite who left not so long ago gave us a copy of harbour masters mooring requirements, as example when we wanted to lay ours.
It consisted of large sinker shackled to 3/4" galvanized chain, length = to highest spring tide depth expected. Next was the galvanized swivel folowed by 3/8 or 1/2" chain as riser which was also length = to highest spring tide depth expected. Topped by mooring buoy and then pick-up buoy on strop. Strop length = to freeboard and deck length to fastening point.
He did advise us to use more than one strop and that is what we did, making sure we fastened to different points on the boat.
Our boat survived many storms before moving her years later to marina base. As far as we know that mooring is still in use by subsequent owners unchanged from our set-up. Other boats that owners thought they knew better did break loose in storms, strong weather.
Needless to say I used to sip Martini while Hubby used to paddle about in the mud servicing the gear !
 
Couple of points:
Dragging the buoy under puts a lot of load on it, generally above manufacturers rating.
Long pendant from buoy to boat gives a bigger swinging area than putting a bit more length in the riser.
I think you have real problems, as I guess there is not a consistent current, so boats will swing in all directions!
Is fore and aft mooring an option? Probably too exposed?
Also short risers will likely move the ground chains around in a strong wind across the ground chains.
As has been pointed out, the loads from waves can be large.
The jolt as a chain pendant goes tight is not kind to boat, rig or mooring.
A summer of drysailing perhaps.
Make sure your insurance company is happy with what you end up with, I reckon there will be some bruised transoms!
 
Hi muddy sox disregard my comments if you have a buoy which remains in the water with a tether rope or ropes from it to the boat.
I was assuming that the riser would be pulled on to the deck of the boat along with the buoy. Obviously that would be disastrous with a short riser. Not so bad if the buoy takes the load and the boat is horizontally tethered to the buoy.
good luck olewill
 
<<< Lots of mooring in the Straits (OK so they're not as exposed, but do have very strong & large tides & wind over tide can be short & steep) >>

Were you there when the fleet review was held off Moelfre? Force 10 north-easterly straight down the Strait against a spring flood? I've never seen boats pitch so much.

I agree with you though. Where moorings are tightly placed the riser is often barely more than the depth of water at high tide. We see sunk mooring buoys in many places, suggesting that this is a common practice. So long as the pick-up line is long enough it shouldn't be a problem.
 
[ QUOTE ]
<<< Lots of mooring in the Straits (OK so they're not as exposed, but do have very strong & large tides & wind over tide can be short & steep) >>

Were you there when the fleet review was held off Moelfre? Force 10 north-easterly straight down the Strait against a spring flood? I've never seen boats pitch so much.

I agree with you though. Where moorings are tightly placed the riser is often barely more than the depth of water at high tide. We see sunk mooring buoys in many places, suggesting that this is a common practice. So long as the pick-up line is long enough it shouldn't be a problem.

[/ QUOTE ]

/forums/images/graemlins/laugh.gif I was off Moelfre; anchored there until the Navy ran for cover. Then I decided to make discretion the better part of valour & dodged round to inside Gallows Point. It did blow a bit didn't it!

Conway was a place to see 3' diameter mooring bouys dragged under the by the current - absolute pig to avoid, all you see is a swirl in a constant location.

I once had to cut my previous boat off a mooring. It must have got tangled & I picked it up at half tide. I watched it go tight & reckoned that the floatation of the boat would pull it up out of the mud & release the tangle . . . .


But it didn't!


When the stern was 2' out of the water I bottled out. There was no way I could untie or release my temporary strop - so for the one & only time I cut it. It didn't half go with a bang!

SWMBO personally checked all moorings I picked for several years afterwards! /forums/images/graemlins/laugh.gif
 
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