Roll reduction, stability

[Ggohd]

We have a 16m, steel, dutch built ketch, waterline length about 14m and draught of 1.9m. Displacement 22t. Below the waterline she follows a rough curve to the keel which is about 400mm in depth and runs for the entire length of the hull.
In a cross sea she rolls excessively, such that in rough conditions crew rest becomes very difficult. Even with staysails set the rate and magnitude of roll is excessive, although there is no doubt about the inherent stability as she rolls back with alacrity.
It has proved difficult to find advice on line regarding practical solutions. Flopper stoppers might well help on a long passage, but with the amount of rigging required would not be practical for shorter periods.
We have observed that some MFV types are fitted with a flat plane, mini wing, attached to the foot of the keel. What we had in mind was such a plate or wing welded to the entire length of the keel extending to around 400mm either side. This would form an inverted T shape, or if you can visualise the plate laid flat on the ground and the keel lowered onto it. The keel is of 10mm steel in a box containing lead( we think). In our judgement it is quite suitable for heavy welding attachment. But would it help?
We considered smallish longitudinal strakes fitted to the hull at 45deg below the waterline, but they would prove very tricky to attach, needing the internal frames to be stripped back to allow welding.
Any advice or comment from anyone experienced in stability and/or design would be most gratefully received.

 

[alahol2]

Bump and welcome to the forum.

 

[maxi77]

Your rolling can be compared to a spring mass system. To reduce rolling you need to add damping. Bilge plates of some kind are an easy addition and can be arranged to help drying out. The next step up is stabilising tanks,there are various types but effectively the water movement is anti phase to the rolling motion. They also have the benefit for some of not being speed dependant. Finally you have active fin stabilisers which have in general the greatest capacity and most effective stabilisation as long as you are moving ahead.

Mind you theneadfor stabilisers only came about with the advent of steam propulsion, sailing vessels tend no to benefit from stabilisation systems other than sails.y

 

[pmagowan]

Welcome. I am not experienced but have done some research. It is a bit tricky as I think fluid dynamics is not fully understood. There are a couple of things you can do. Weight distribution can help. Depending on the type of roll it can be a matter of moving weight either centrally or, counterintuitively, higher on the beam. The other solution is as you suggest, both increasing resistance of the keel through the water and causing turbulence as it does so. The edge of the keel does this during a roll and this apparently has a dampening effect. If you put on a 'T' then you essentially double this effect.

of course having sail up helps too. I will try and look up some of the books I have been reading and get you their titles as they have detailed stability analyses.

 

[William_H]

A real problem I can imagine. With a beam sea the hull will tend to want to stay level with the water level which is fine until on a wave the water level is sloping so the boat rolls.
The rolling action can be exacerbated or damped by the inertia (weight) of the spars and sails. It might be interesting to remove the mizen mast to see if the roll action changes for the better or worse. I think the natural timing of the pendulum of the rig will be significant. If that matchess the wave frequency then she will really roll. Make that resonance of the rig longer or shorter ie more or less weight aloft might change things.
I might think that a deeper keel with more area down low would damp the motion. The deeper the water the more stable in a wave so the deeper a side area of keel is the better it would dampen the roll. But really I don't know good luck olewill

 

[Heckler]

We have a 16m, steel, dutch built ketch, waterline length about 14m and draught of 1.9m. Displacement 22t. Below the waterline she follows a rough curve to the keel which is about 400mm in depth and runs for the entire length of the hull.
In a cross sea she rolls excessively, such that in rough conditions crew rest becomes very difficult. Even with staysails set the rate and magnitude of roll is excessive, although there is no doubt about the inherent stability as she rolls back with alacrity.
It has proved difficult to find advice on line regarding practical solutions. Flopper stoppers might well help on a long passage, but with the amount of rigging required would not be practical for shorter periods.
We have observed that some MFV types are fitted with a flat plane, mini wing, attached to the foot of the keel. What we had in mind was such a plate or wing welded to the entire length of the keel extending to around 400mm either side. This would form an inverted T shape, or if you can visualise the plate laid flat on the ground and the keel lowered onto it. The keel is of 10mm steel in a box containing lead( we think). In our judgement it is quite suitable for heavy welding attachment. But would it help?
We considered smallish longitudinal strakes fitted to the hull at 45deg below the waterline, but they would prove very tricky to attach, needing the internal frames to be stripped back to allow welding.
Any advice or comment from anyone experienced in stability and/or design would be most gratefully received.

Buy another boat! Seriously!
S

 

[lpdsn]

Bilge keels (along the length of the turn of the hull rather than of the Westerly Centaur variety) is the usual solution with ships. Apparently worked with the early German Dreadnoughts which were infamous for rolling. Does increase resistance though so your speed & fuel efficiency will suffer a bit.

That said, the post that said get a different boat shouldn't be ignored - if it is frequently a problem maybe your boat isn't that suitable for the sort of sailing you do.

 

[Daydream believer]

I think you should read up on the action of waves
I seem to recall an article that suggested that in a large sea the boat stayed at right angles to the wave face as there was an effect on gravity
The article was about stability of cats v monohulls but may be relevant to you
If the article was correct then one has to consider waterflow around the wave & also effect on the hull because it may be that bilge keels are not the way to go only causing yet more drag
Why not just speak to a designer. A few £'s spent with him may well save time & cost in the long run & ( with all due respect to forumites I am not looking to insult) all the opinions in this forum are useless if not founded on some practical/technical knowledge

 

[lw395]

There are two things here, rate of roll and damping.
Adding more keel area can add damping.
Changing the weight distribution can change the roll frequency, a bit like changing the length of a pendulum.

Racing yachts are simply designed to be as stiff as possible, other boats, e.g. fishing vessels are designed to roll in particular ways, which may mean rolling further, but slower.

I recall reading about this in one of the books I have, it's either Larrson and Elliasson (sp?), Gerr, or Marchaj....
I don't think it's a trivial thing that can be answered by a paragraph in a forum.

I have some sympathy with the idea of either doing what your boat is good for, or changing the boat for one that's good for what you want.
But consulting a naval architect might be money well spent, unless you are able to study the theory yourself....

 

[NUTMEG]

My CW28 ketch rolls like a pig. She has long bilge plates but still rolls. Mike of this parish, who is a Colvic Watson expert thinks that the addition of plates may exasperate rolling, although, to me, this seems counter intuitive. Rolling is less when sailing, which is yet another releasing for switching the engine off.

Good luck

 

[simonfraser]

Change to a catamaran.

 

[jimbaerselman]

Hmm. Sounds very like a Brixham trawler I used to sail quite a lot. Except she was rather heavier, and nearly 3m draft. Which slowed down her response. Good for hauling a trawl in through gates on the beam as deck level dropped to the waterline.

These boats have very strong shape stability - a tendency to beaminess, with a flatter bottom shape across the beam. This means the boat tries to conform to the local water surface (every wave . . .). With ketch rig, and only 2m draft on 14m waterline, roll inertia is low, so she responds quickly.

The opposite would be a boat with low shape stability (a rounded cross section under water across the beam) but high pendulum stability. Think deep keel with weight bulb at the end. These also have high roll inertia - high rig and deep weight. They don't respond nearly so much to waves. Add hydrodynamic roll damping provided by the deep keel, and rather lesser aerodynamic damping from the tall rig, and you have boats whose roll response to a seaway is much lower.

There's not much room for adding roll inertia to a 33ton 16m boat without sinking it! That leads you to damping solutions. Sadly, bilge keels don't help if the roll motion is due to waves.

So the advice above - "get another boat" is pretty sound. The other solution is to keep out of waves - stick to Baltic sailing, inland waterways, lakes, estuaries. Sounds a bit like Dutch cruising conditions? Or dose up with seasick pills and live with it. Or add dynamically controlled anti-roll fins . . . a bit of a power drain. Don't forget to draw them in before going alongside!

 

[NUTMEG]

Thank you. That's the most understandable explanation I have come across and explains why bilge plates do not stop my CW from rolling.

 

[KellysEye]

We had a Dutch built 38 foot steel ketch with a long keel and cutaway forefoot, same keel depth and 14 tons fully loaded. Rolling badly in cross seas is normal our solution was don't ever sail in cross seas change direction 20 degrees.

 

[onesea]

One other thought, free surface, as I understand you have a boat that's to stiff.

Keep your tanks less than 90% full let the water slosh around, depending on the circumstances it can help.

Also as others have said look at your wait distribution...

 

[srm]

Even with staysails set the rate and magnitude of roll is excessive, although there is no doubt about the inherent stability as she rolls back with alacrity.

Just some thoughts.

Do you sail or motor? It sounds as if you have a stiff boat that is designed to carry a significant sail area. The comment about staysails seems to imply motoring. Most boats roll when motoring in a beam sea or sailing with too little sail set. Sailing with enough sail to make a reasonable speed should dampen the roll significantly, but will not smooth all of it due to the changing slope of the sea surface and dynamic forces. Once the wave/swell length is more than twice the beam the roll motion will change as the boat follows the water surface, but the roll period will be similar to the wave period when sailing as the boat is held at roughly the same angle to the water surface by the wind pressure on the sails.

Reducing the static stability by moving weights higher to raise the centre of gravity would give a slower roll that may be more comfortable.

Free surface effect from slack tanks may effectively raise the centre of gravity depending on tank shape and baffles, however, there is a danger of rhythmic sloshing of liquid in a wide tank matching the roll and amplifiying it.

Adding wing keels as you suggest will add weight at the bottom and lower the centre of gravity making the boat stiffer and will give a faster roll period unless the hydraulic damping is sufficient to counteract it.

Any structural alteration will change the centre of gravity and is an area that requires professional assistance and calculation. Depending on what you try trial and error could result in a faster roll period that strains the rig and crew or a slow roll all the way to 180 degrees!!!

The simple answer is to consult a naval architect, preferable the boat's designer or possibly the builder, before making any structural changes such as adding keels or moving weights.

 

[single]

I have no idea on the science behind it, but when i re-engined my Colvic long keel, the roll reduced quite significantly! The new engine weighed a lot less than the old one. At that time i also had a huge tidy and reduced the overall weight. If anything there is a lot more weight in the bow now but rolling underway is minimal and not quite as bad at anchor.

 

[jimbaerselman]

We had a Dutch built 38 foot steel ketch with a long keel and cutaway forefoot, same keel depth and 14 tons fully loaded. Rolling badly in cross seas is normal our solution was don't ever sail in cross seas change direction 20 degrees.

Good point. I forgot the frequency issue.

If the natural frequency of the boat in roll (a mix of stablity and roll inertia) matches the timing of the waves arriving, rolling really goes wild. Altering course to change the frequency with which the waves arrive will then cool things down. At the cost of a devious journey.