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Old 18-04-2018, 13:17   #1
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Sitting on keel at low tide

We recently moved our Beneteau 323 to a new marina and have discovered it nestles into the mud at the the bottom of our slip on it's keel such that we have to wait for the tide to move. Will this be a long term structural issue or are we just worrying about it unnecessarily. Could this damage our rudder should we back into the slip then have it become embedded?
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Old 18-04-2018, 13:26   #2
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Re: Sitting on keel at low tide

Couldn't possibly be good. Your keel isn't designed to support your boat and over time there will be damage to the hull and the attachment to the hull, at the very least. You might get away with it for a while in calm weather but over time the damage will accumulate and in a storm or other conditions with surge you could get damage much faster.

Rudder even more susceptible to damage, by a big margin.

Is there a reason not to move to another slip?

Pete
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Old 18-04-2018, 13:51   #3
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Re: Sitting on keel at low tide

Keels definitely can support the whole weight of the boat. All northern boats that come out each winter sit on their keels. And there are areas where marinas routinely dry out or get very shallow during low tide.

The problem could be if there is a lot of movement at low tide. That could be bad. And you definitely don’t want any weight on the rudder. But unless your boat is particularly delicate, there shouldn’t be a problem.
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Old 18-04-2018, 13:53   #4
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Re: Sitting on keel at low tide

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Originally Posted by pete33458 View Post
Couldn't possibly be good. Your keel isn't designed to support your boat and over time there will be damage to the hull and the attachment to the hull, at the very least. You might get away with it for a while in calm weather but over time the damage will accumulate and in a storm or other conditions with surge you could get damage much faster.

Rudder even more susceptible to damage, by a big margin.

Is there a reason not to move to another slip?

Pete
Well this isn't strictly true, right? I mean, the keel is absolutely supposed to bear the weight of the boat when you are out of the water with the jackstands mostly just providing balance. I guess that's not true for every boat, so don't take that as gospel.

Anyways I agree this is a bad slip for this boat, and with the keel somewhat immobilized in the mud some unusual forces could turn up at the hull/keel interface when there's wave action, etc, as you say. Especially if it bounces a bit and ends up putting the rudder in the mud.

I wouldn't loose sleep over it on an older design, but I wouldn't go asking for trouble with Beneteau. Not saying they are bad boats, just seems to be a lot less margin for error with the keels than an older or more overbuilt design.
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Old 18-04-2018, 14:21   #5
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Re: Sitting on keel at low tide

The biggest problem you will have is the antifouling paint won't work if its covered in mud. We used to have this problem until they dredged our marina.

As for the keel and rudder, what will happen over time is the yacht will dig an oval hole for each as the boat settles down twice a day in a slightly different position each time. There may already be a hole there if there was a similar keeled yacht previously on the pontoon.

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Old 19-04-2018, 04:28   #6
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Re: Sitting on keel at low tide

It may not be a problem right now but just wait until a storm or a super low spring tide or both together. Then it will be a big problem. I’d move the boat.
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Old 19-04-2018, 04:48   #7
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Re: Sitting on keel at low tide

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It may not be a problem right now but just wait until a storm or a super low spring tide or both together. Then it will be a big problem. I’d move the boat.
If both occurred together then the boat will be sat in a mud berth. Many a yacht has spent the winter safely moored year on year in a mud berth in the UK.

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Old 19-04-2018, 08:48   #8
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Re: Sitting on keel at low tide

Do you get large waves where you are? If you do, there is a risk of stress as the boat rises and comes back down. If the waves are small then there's little to no risk from slamming into the mud.

I had an issue with a silted up slip and Pete7 is right about the scouring of a hole. I had a lot of silt but the keel and skeg hung rudder kept a nice hole for themselves. I then did some midnight dredging. I double lines my boat and turned on the engine and let it sit in gear for an hour or two a number of times as the tidal current was running. That took care of the problem enough that I was able to get into the deeper channel behind me at all times.
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Old 19-04-2018, 09:10   #9
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Re: Sitting on keel at low tide

There are loads of boats in UK and France on hard and soft bottoms that take the ground twice a day on bilge keels and also fin keels with legs. If the bottom is hard then you cannot pound due to waves or you will do damage. There are boats in our club that have done this for 50 years. Soft mud is the kindest bottom to a boat. Traditionally, many boats were laid up for the winter in a soft mud birth.
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Old 19-04-2018, 09:13   #10
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Re: Sitting on keel at low tide

I always wondered about this.

20 or so years ago in Marsh Harbor, Bahamas we were walking the docks and about a third of the boats were listing at low tide. An older man told us this happened every day, but he had no idea of the effects since most of the boats hadn't moved in months, if not years.

I think without support some of the lighter boats wouldn't be able to handle the stress - all the boats I saw settled at an angle, not "on an even keel" (did I just say that?).
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Old 19-04-2018, 09:17   #11
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Re: Sitting on keel at low tide

The are many Harbor or Marina where boats sit on there keel and ruder, generally in mud, all over the world. Here in Rivière-du-Loup, on the St-Lawrence River in Québec, Canada, the marina dries out at low tide. Not a problem, all types of boats (Bénéteau, Albin Vega, C&C, etc) of all sizes (20 to 45 feet) just make their "beds" in the mud. To my knowledge, bad wheather, heavy storms, don't affecrt the boat at all. Boats are content to wait for the next rising tide so they get free and sail away :-).
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Old 19-04-2018, 11:52   #12
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Re: Sitting on keel at low tide

This depends on the nature of the bottom and the ‘grounding’. But I would be cautious.

I had a Beneteau First 285 in the UK for 10 years. She had a wing keel. I had removable beaching legs made as for the first 18 months of my ownership she was on a drying mooring. My previous boat had beaching legs. But she had a lifting keel and thus would have been designed to beach. My Beneteau grounded on hard mud twice a day from April to October for a year and a half. Some years later I had to get a survey done and that found some stress fractures round the keel bolts. The keel had to come off and some GRP reinforcement done around the keel bolts. Although she was 25 years old at the time, I kind of feel that the groundings probably contributed to the stress fractures.

If you are on soft mud it may not be so much of an issue. But I would be wary of what’s happening to your rudder as well. I was a offered a mooring in Poole, Dorset where I am now with the risk of grounding at low water springs. I refused that and went for an all tide deep water mooring, principally because I didn’t want the risk of pressure on the rudder if she grounded.
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Old 19-04-2018, 13:58   #13
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Re: Sitting on keel at low tide

due to the tide range in central queensland 6mtr+ we dry completely every day / the boat handles it alright / can be a problem with boat wakes or wind over tide chop just as she is sitting down or starting to float / to soften the bounce we fitted railway sleepers with conveyor belting rubber nailed to them / not sure if a modern boat would handle bouncing for long but they will take the weight on the keel / they sit on their keels on the hardstand
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Old 20-04-2018, 01:25   #14
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Re: Sitting on keel at low tide

its definitely not a good idea on this kind of boat, the rudder will get damaged if there is any hard matter at the bottom and the keel will get a pounding if there is any wave action, they are designed to hang down, not support the weight of the boat unlike a lot of other makes. Check for any hairline cracking under the sole around the keel as this will indicate any damage
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Old 20-04-2018, 02:09   #15
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Re: Sitting on keel at low tide

I looked into this issue when designing a dock. I anticipated all sorts of problems where a vessel bottoms out in mud at low tides (rudder damage, blocked through hulls, fouled impellers, engine issues, etc.) but in talking to people who actually do it, I found no such problems reported. I think that if the berth is protected from significant wave action and you're willing to time departures and arrivals accordingly, it's more of a theoretical issue than a practical one, and you'll end up burrowing out a secure mud "nest." I met a guy with an older big Lagoon who keeps it on a canal, and it has bottomed out twice a day for many years with no apparent damages. It's one of those issues that on its face sounds like a profoundly bad idea, but deep water dockage can be hard to find, or expensive, in some locales.
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