Using dinghy to stop rolling at anchor

[mcdonald_ajr]

Had an idea and would welcome opinion. At a rolly anchorage or mooring, use the gybe preventer and mainsheet to lock the boom out at right angles and secure the dinghy to the end of the boom. Instant catamaran! Well almost.

I tried this last Summer and it was fairly easy to do. The sea was flat at the time, so it wasn't a fair test of whether it would actually reduce the rolling motion.

I'd be interested if anyone else has tried this and does it actually work?

Anthony

 

[prv]

I can imagine it working to some extent - as you try to roll away from the dinghy, it will be lifted and its weight will act against the roll. A lightweight inflatable might work better if part-filled with water (but watch you don't rip the lifting rings off).

What you don't have is anything to stop the roll towards the dinghy, but perhaps damping half the cycle is enough. I think most flopper-stoppers are lowered on one side only.

Pete

 

[johnalison]

I've tried it with a bucket in a small boat with a little success. I once saw an article in YM which described coping with the Braye roll by tying an additional shear line to the buoy fro about midships to turn the boat head to swell, but never had a chance to try it. Not easy when doubled up I imagine.

 

[RupertW]

I've tried it with a bucket in a small boat with a little success. I once saw an article in YM which described coping with the Braye roll by tying an additional shear line to the buoy fro about midships to turn the boat head to swell, but never had a chance to try it. Not easy when doubled up I imagine.

I've used that technique (bahamanian moor) a few times at anchor and found it works really well if the angle of the swell is not too great, and if the swell direction is pretty constant. I have never had the courage to keep it in place in strong winds as it obviosuly creates mre windagage so have then just reverted to anchoring from the bow.

The only true cure is find another place to anchor/moor or buy a cat.

 

[john_morris_uk]

I've laid a kedge out from the stern to alter the angle of the boat to the swell which worked quite well for a time.

I caw a design for a 'flopper stopper' that involved a weighted bread tray with two weighted plywood flaps secured inside to act as big 'one way valves'. The designer said it worked very well, although where you put the bread tray when on passage is another matter!

 

[prv]

I caw a design for a 'flopper stopper' that involved a weighted bread tray with two weighted plywood flaps secured inside to act as big 'one way valves'. The designer said it worked very well, although where you put the bread tray when on passage is another matter!

The "standard" design is a triangular piece of plywood with lines from all three corners and a weight on one corner. On the downroll, the weighted corner pulls it down vertically through the water; on the uproll it's lifted by the bridle in a horizontal position and resists movement. Essentially just the valve flap without the bread tray, and because it's a flat board it should be easy to stow.

Pete

 

[dom]

Had an idea and would welcome opinion. At a rolly anchorage or mooring, use the gybe preventer and mainsheet to lock the boom out at right angles and secure the dinghy to the end of the boom. Instant catamaran! Well almost.

I tried this last Summer and it was fairly easy to do. The sea was flat at the time, so it wasn't a fair test of whether it would actually reduce the rolling motion.

I'd be interested if anyone else has tried this and does it actually work?

Anthony

Here's my tuppence worth. The physics of paravanes (as used on trawlers) is straightforward: paravanes are shaped to resist the water strongly if one tries to quickly lift them vertically through the sea, thereby imparting a big resistance to rolling as it say rises 1m upwards through the water. The energy created is mostly dissipated into the sea.

Now imagine you attach a dinghy to the end of the boom. As the boat rolls the boom will suddenly hit a "shock point" whereupon a sudden load will hit the tether between boom and dinghy. This will damp 1/2 of the roll cycle as long as the dinghy is not lifted out of the water, although the shock loads created would be very damaging to the dinghy. But in any waves/swell the dinghy will lift into the air (given the big mass differential); the dinghy will thereby convert its roll resisting energy into potential energy (via its height above the water); unlike the paravane which dissipates energy into the sea. As the boat rolls back the other way this stored potential energy will act to increase the roll in that direction!

Depending on the frequency of the waves you might enjoy a small net reduction in rolling; more likely you will suffer periods of light rolling followed by periods of really heavy rolling. You will almost certainly damage your dinghy (especially if you ballast it with water) so you will be stuck on your boat for the evening! So I guess no I wouldn't do it in any weather that I'd be irritated by the roll

 

[Blueboatman]

I have used a stout sculling oar tied at toerail and cabin top so inclined into the water at 45deg. It certainly worked and the flexing of the oar gave testimony to the work it was doing.

I did at first wonder if I had created a monster that would auto-paddle the boat round in circles all day long....

In the Caribbean where dinghies and outboards are mighty tempting to those of a nocturnal light fingers pursuasion, it is common to hoist the dinghy just clear of the water when asleep. This also discourages fouling. I do not remember boats getting into a cyclic roll when doing this in a rolly anchorage. You would occasionally hear the slap-splash as the dinghy kissed the water on the down roll...

 

[dom]

I do not remember boats getting into a cyclic roll when doing this in a rolly anchorage. You would occasionally hear the slap-splash as the dinghy kissed the water on the down roll...

Your oar technique will be better than a paravane as it resists and dissipates both up and down forces! And I'm sure you're right about not getting into a cyclic roll when say boat is say >10T and the dinghy <50kg -- it prob wouldn't do much either way.

 

[Uricanejack]

.
I sneaked a peak at the motor boat site yesterday and discover they now have stabilisers which work at anchor.

On a ketch we often used the mizzen at anchor to keep us head to wind. I think a hard sheeted sail would have helped dampen any roll, though it was never our purpose.

 

[Rigger Mortice]

The "standard" design is a triangular piece of plywood with lines from all three corners and a weight on one corner. On the downroll, the weighted corner pulls it down vertically through the water; on the uproll it's lifted by the bridle in a horizontal position and resists movement. Essentially just the valve flap without the bread tray, and because it's a flat board it should be easy to stow.

Pete

i tried that. No matter how I adjusted the lines and the weight I couldn't get it to do anything.

 

[prv]

i tried that. No matter how I adjusted the lines and the weight I couldn't get it to do anything.

I guess that's why there are cunning alternatives to the "standard"

Pete

 

[TQA]

Best is a stern anchor to keep you lined up with the swell.

If the wind and current are consistent a spring line from the stern to the anchor rode works, adjust till swell is on bow or stern.

 

[Iliade]

Using a rigid GRP dinghy, fit an old seagull on the transom, tie a bridle to each quarter and the bow, then secure to the boom in the traditional flopper stopping manner. Remove bung and allow dinghy to settle 4' below surface.

All problems solved :0)

 

[KellysEye]

>On a ketch we often used the mizzen at anchor to keep us head to wind.

We used a riding sail on our mizzen mast, it cut the swing from up to 90 degrees to 35 degrees and thus significantly cut the snatch loads. Our solution to a rolly anchorage was to go somewhere else where it doesn't roll.

 

[mcdonald_ajr]

Thanks for all the suggestions. I have decided to make a flopper stopper using a triangle of plywood, with some lead at one corner, as per this link
http://offshore-sailor.com/boat-gear/27-flopper-stopper-you-can-make-yourself.html

It will store flat, and even if it doesn't work, it will keep me happy playing. Last time I spent a night in Braye Harbour, I was awake all night with my head banging back and forth against the hull, trying to think of a solution to the rolling I might sleep better knowing I've done all I can, even if it keeps rolling.

I agree a spring line to align the boat into the waves is very much worth trying.

Back to the dinghy idea. I had some tension in the painter so the nose of the dinghy was out of the water. There was no snatch load as a result (at least with the size of the swell we had). It usefully corrected the slight list to Starboard that all Westerly Storms suffer from (as the boom was held out to the Port side). I think it helped a bit, but I take the point that in a large swell, probably not a good idea! Hence the plan to build a flopper stopper.

Anthony

 

[flaming]

Just a thought, but if you had something solid like strong boathook, or an emergency tiller etc that you could use to link the end of the boom to the dinghy, you would then have something that resisted both upward and downward rolls.

Probably work best with a dinghy with something solid to attach to though.