Flopper Stoppers

[TimAbram]

I'm a final year Yacht Manufacturing and Surveying Student and saw this month's article on flopper stoppers. This is the subject of my degree dissertation and was wondering if any one had any practical experience with them. and what size you use compared to the size of your boat ???

If you have do you realy think they work ???

My research so far points to the fact that they don't (in numbers terms) exactly do an auful lot.... But I'll keep you all posted as I go.

 

[santeana]

Tim, I've no personal experience (yet). In case you haven't seen the reference from Lin and Larry Pardey (The Cost Conscious Cruiser pp 156-158) they have very positive views. Significantly, I think, they operate it on the end of a 6m spinnaker pole and "...With this long lever, we find that an 18 inch by 18 inch box, plus 30 pounds of weight, is all we need to cut the snappiness out of the rolling motion and steady the boat...." Maybe, with such an endorsement it's worth suspending judgement. Why not contact them? They do operate on some BBs and correspond via E-mails. I can't recall it but some small research should find them out - maybe someone else can help. Good luck with the thesis!
Ron.

 

[macd]

I've regularly used a drogue, about 3 feet diameter, hanging from the kicked-out boom and using a length of chain for ballast. The ballast is crucial othewise the kit can't sink quickly enough to react to the next wave. The longer the wavelength the more effective it is -- so it works very well in swell, much less so in a shorter waves. It seems most effective of all in preventing rythmic rolling. I suspect the forces involved are some kind of simple harmonic effect, in that it takes only a relatively small effect to stop them initiating, although the rolling forces can become huge if undamped.

 

[jimg]

These chaps would not be without them! They are a standard fit (almost) to these boats.

http://www.nordhavn.com/rally/voyage/welcome.htm

 

[maxi]

Yes, they do help to break ther rolling rythm - they also serve another purpose as they will double as an emergency steering gear, fed through blocks at the midship cleat position and back to a winch.

Not good enough to steer through the Solent but a definite get -you- home where you have sea room.

 

[peasea]

I made one five years ago from a 6mm tufnol triangle of 24" sides with a 2 lb iron weight at one corner. Works very well.

 

[scarlett]

Mine is an 18 inch trangle on a 32 foot lightweight. It gives the impression of damping the roll but without an identical 'control' and sophisicated measurements it could be that just doing something about rythmic rolling makes one feel better.

 

[kandoma]

I used the plastic rocker stoper plastic cones from Davis, 3 on each side , extended by using my two genua poles, extensivly in all weathercondition. I would never go cruising without such an arrangement. The load on the mast and rigging is big, but it keeps the boat from going rail - rail in a chop.
For my next trip, I will carry two stainless steel square rocker stopper, ca. 0.5 m2 each.

Peter

 

[TimAbram]

Update:

The research so far:-

I have thaken a sieries of still water roll runs in a tank with a 1:5 scale model with and without keel, with and without flopper stoppers with different mass distributions to try and obtain a damping factor for a given plate size.

I was expecting some scaling problems but they appear to be negligible.

The primary effects are small on the basic rolling function and seem to be goverend more by the roll gyradius (mass distribution) and basic stability characteristics than any effect of the plate itself.

As many of you have suggested the likely highest effect from the basic maths appears to be in altering the harmonic motion of the boat when waves are at the boat's natural resonant roll frequency.

The flopperstoppers themselves impart considerably less damping force than say the keel or rudder ........

lots more number crunching to do along with lots more testing with the model in waves and a full scale trial .....


Keep you posted

 

[Phoenix of Hamble]

Good stuff Tim,

Thanks for coming back with an update.... most interesting

 

[tugboat]

quote[The flopperstoppers themselves impart considerably less damping force than say the keel or rudder ........]unquote

Not arguing or anything, but I'm surprised that you say the flopper-stoppers impart less damping force than the rudder. Surely the damping is going to be proportional to [the area of the plate X(times) the lever arm].
The lever arm for the rudder will be the vertical distance from the centre of the rudder to the axis of roll (somewhere near the waterline). The lever arm for the flopper-stoppers is going to be the distance between the end of the spin pole and the vessel's centreline.
Or are you just hanging them off the deck edge? Oim feelin' a bit confused, innit? /forums/images/graemlins/crazy.gif /forums/images/graemlins/confused.gif

 

[TimAbram]

True the lever lenght will have a not inconsiderable effect on the force imparted by the plate damper infact it is proportional to the distance from the centreline cubed.

However the hinged plate damper I am testing does not act over the whole of the swing as it has to rotate to be perpendicular to the flow after diving to "bite" the water before comming into full effect

Scondly the plate is pretty small in area in comparrison with the sailboat rudder as it would be unweildy to deploy. The aspect ratio of a rudder fixed perpendicular to the rotational axis gives a high damping coefficient in the region of 1.6 whereas the square damper under test has a drag coefficient of around 1.16

Finally I am only testing a single sided device and so only has an effect over 50% of the roll cycle .......

More soon

 

[Piers]

Flopper Stoppers are used extensively by boaters in the States. Therefore, go to the magazine Passage Maker, log on, and ask your question there.