stabiliser angles?

[bowthrustermick]

I have a 53 steel dutch tsdy which is a real roller. To slow the rolling I am thinking of welding on an additional steel stabiliser between the lifting points but how wide and at what angle. the boat is a deep vee. The existing stabiliser is miniscule

 

[sarabande]

any bilge plate needs to be in an area which is not "sheltered" by a keel, or which will impede water intake, as the boat rolls. You need to think of surface area not how far from the hull it sticks out (to reduce underwater snagging.

The angle is usually normal (at 90 degrees) to the hull, but if you are going to use them to help dry out on hard ground, you need to check from the section drawings.

FWIW on a 16 tonne, 35 ft round hull, the plates measure 2.1m by 0.3.

 

[nonitoo]

Violent rolling is usually a symptom of too much stability (the righting lever is large) and can be reduced by adding weight on deck to raise the Centre of Gravity and so reduce the righting lever between that and the Centre of Buoyancy when the boat is heeled by an external force.

Careful calculation is necessary and it helps if you have some stability data for the vessel type to hand.

Tom

 

[sarabande]

Woah ! Stop !


Would you like to check that statement first before someone does something disastrous ?

Rolling is a function of hull shape, with the relation between CoG and CoB / Metacentric height being a contributory factor.

 

[nonitoo]

Exactly what I said, careful calculation is necessary but it does work.

I am well aware of the connection between CG, CB, GZ & GM in the transverse hull model and it applies to a small vessel as it does to a large one.

After compiling a GZ curve for my father's boat 40 years ago the rolling period was increased by 50 % by adding 300kg (IFIRC) to the deck. The angle of roll was not reduced only the period was changed.

I was certainly not suggesting that anyone randomly strap weights to the boat without calculating first. I would imagine that there must be stability data available from the builders for a boat of the size mentioned.

Tom

 

[MapisM]

On such boat, bildge plates can do very little to reduce rolling.
Active stabs are the only effective way to go, though obviously much more expensive.
But I'd rather keep it as it is, if you don't want to go that route.

 

[Deleted User YDKXO]

I agree with Nonitoo. Yes, at first sight, his statement does'nt make sense but actually it's correct. Excessive rolling can result from too much righting moment. Nordhavn (who should know a thing or 2 about stabilizing displacement motor cruisers) say

Stability and comfort in rough seas are a major concern for those venturing offshore, and P.A.E. has given a great amount of attention to determining the right combination of ultimate stability and roll period with the Nordhavns. Too much initial stability, and the boat will feel like it's "snapping" from one side to the other once it encounters rough conditions. Too little, and the boat will roll easily even in small seas. Finding the right roll period, or the frequency with which it rolls from side to side, is as much an art as it is a science, and it is important that a comfortable rate of roll is achieved through proper hull shape and location of the ship's center of gravity

For example, Nordhavn recommend on some of their models that the ship's tender be carried on the flybridge rather than on the stern, which would seem to go against common sense, but they have found that this leads to an easier motion at sea

 

[Grajan]

John Bain of "Silvers" fame had a anti roll plate design which seemed to work on the two boats that we fitted them to
Speak with Glasgow University's archive section at the, Ballast Trust, Johnstone by Glasgow for dimensions etc.

 

[sarabande]

Happy to agree with what you say. What I wanted to avoid was some twerp with a rolly boat just adding another storey to the wheelhouse or something.

Apologies if I misinterpreted your comments.

 

[PCUK]

Steel boats roll because they have a higher centre of gravity due to the weight of the material they are built from.

Now, how about a bit of lateral thinking!
Have you considered the stabilisers used on American fishing boats. They consist of torpedo like fins that are suspended from retractable outriggers each side of the boat. They are designed to dive easily with the down roll but resist rising on the up roll, thereby dampening the roll. Similar things of a different shape,often termed 'flopper stoppers' are also used when stationery. They all need very strong mountings!

 

[bowthrustermick]

What is the opinion regarding steadying sails to slow down the rolling movement ?

 

[Sundays_Child]

PCUK, you're referring to paravanes.

I've heard that they are pretty effective, but not especially safe to be pulling in in a swell when returning to port. Best for longer journeys.