Bilge Keels?

oldmanbitter

oldmanbitter

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I have a 36ft dutch steel boat. A standard inland waterways cruiser from the 70's. I have taken her out to sea many times and the roll in any kind of swell is a bit scary, as like all similar boats she is top heavy and fairly flat bottomed. My question is, will fitting bilge keels make a big difference? any advice would be appreciated. Thanks
 
it sounds as if the design has low "form stability". i.e. the shape of the hull makes her roll easily. There could also be probs with the relationship between metacentric height and the CoG.

Adding bilge keels - preferably long ones along the fore and aft axis - will dampen the roll and also lower the CofG a bit.

Probably worth running some drawings past a yacht designer (can you contact the builders ? Stevens ?) for a handful of gold coins or beer, to be sure.
 
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it sounds as if the design has low "form stability". i.e. the shape of the hull makes her roll easily. There could also be probs with the relationship between metacentric height and the CoG.

Adding bilge keels - preferably long ones along the fore and aft axis - will dampen the roll and also lower the CofG a bit.

Probably worth running some drawings past a yacht designer (can you contact the builders ? Stevens ?) for a handful of gold coins or beer, to be sure.

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I have no idea who the builders are, and a yacht designer is probably financially out of my reach! It wouldn't cost me a fortune to fit them though and being able to sit on the hard without support would be great for emergencies.
 
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I have no idea who the builders are, and a yacht designer is probably financially out of my reach! It wouldn't cost me a fortune to fit them though and being able to sit on the hard without support would be great for emergencies.
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you would have strengthen the bottom of the boat in the area of your proposed new keels or else its going to deform in that area. Also if you are just going to use steel plate you would have strengthen the root and foot of the new keels.

good luck
 
I would get my local boatyard to do the work. They work with lots of steel fishing boats, so I trust that they would do a good job. I guess I really need to know if it will make the boat much more stable. I took her round Mizen Head last year and broke most of my cups and plates!
 
Have you considered active stabilisers or towed paravanes? The structural work involved in putting on ballasted bilge keels sounds quite extensive.
 
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Have you considered active stabilisers or towed paravanes? The structural work involved in putting on ballasted bilge keels sounds quite extensive.

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I had considered stabilisers, but not paravanes. I will look into that - thank you
 
Stability is affected by many items. A yacht that is normally very stable under engine will "whip" horribly when the mast is lowered. The movement becomes VERY rapid due to the boat weight being concentrated around the CoG. It's a bit like the spinning ice dancer/ ballerina who pulls her arms & legs in and so spins much faster. I think that your problem is due to a similar issue, all the weight is concentrated close to the CoG.

My point is that short, heavy keels may not help all that much. And I suspect you won't want long keels with a lead bulb either as it will impact on your draught. How about a mast & riding sail? That will give you a lot of additional stability & maybe even some slight drive. If it is mounted properly it will be easy & quick to lower when bridges are about. And it may be an attractive addition to your boat.
 
I'm not sure that adding bilge keels will dampen the roll sufficiently to make a big difference. Blue water motor boats tend to use paravane stabilizers like THIS but they do need booms to deploy them so they're not exactly convenient. If you search thru back copies of MBM you'll find articles by the Jauncey family who have sailed their steel motor boat all over the world and they seem to be paravane fans
The other possibility you have is a steadying sail which at least has the advantage that it won't add as much drag as keels or paravanes
 
"The movement becomes VERY rapid due to the boat weight being concentrated around the CoG. It's a bit like the spinning ice dancer/ ballerina who pulls her arms & legs in and so spins much faster"

It's normally accepted good design to have the mass concentrated as close as possible to the CoG, not at the extremities. The ballerina is a different physical principle altogether - called conservation of angular momentum - and doesn't at all prove or illustrate that something with its mass close to its CoG will (all other things being equal) spin or roll faster

But the steadying sail is a great idea imho and would be a good addition to the boat
 
Angular momentum is the same property relevant to both situations. The reason that good design concentrates mass around the CofE is that less energy is wasted in creating angular momentum.

However there are unique circumstances where the opposite is desirable. For example very light weight dinghies like the 49er really suffer in light winds because excessive pitching movement destroys the aerodynamic airflow on the sails. The solution is to move crew weight to the ends of the boat, therefore increasing the effort required to build angular momentum, which is what Frank Bethwaite recommends in his book High Performance Sailing. The logic being that the loss of energy through not pitching easily is offset by the increased power from the sails.

What works for sails is being stationary and the same thing would apply to comfort - being more stationary increases comfort. Weight on the bottom and in the form of a mast and sail will definitely help, however there are lots of other factors involved including shape of hull, so I would speak to a designer to see if it makes enough difference to warrant the cost/effort.
 
We have been here before and I hesitate to leap in again but......... this rolling is caused by too much stability, the COG is low down and the Cb (centre of buoyancy) moves out to the centre of the immersed area (in the transverse model) thus giving a large righting lever causing the violent rolling you note.

The only remedy is to find the original stability data and calculate the weight required to raise the CoG and reduce the righting lever. This weight will normally be applied at deck level.

This will increase the rolling period, making the boat more comfortable and with less whipping about, but the amount of roll will remain about the same.

This exercise should only be carried out after consultation with a marine architect. In fact, if the data is not available then he can generate the data by conducting a series of heeling tests (essentially suspending a weight at varying distances from the centreline and measuring the heel with differing weights).

Note I am NOT advocating that you should start applying weights to the deck without proper knowledge.

Tom
 
Have a look at http://www.free-marine.com/i8transtab.htm and note that the lower the CoG the larger the righting lever GZ when the boat is heeled by external forces (waves).

Note that the CoB always acts vertically upward from the centre of the immersed area and this moves from side to side as the boat rolls.

Raising the Cog reduces the size of GZ and therefore the righting moment, reducing the rate of recovery and increasing the roll period while retaining stability.

Tom
 
Apart from your very first sentence, I agree that 100%. I can't however work out if you're agreeing or disagreeing with what i wrote! :-)
 
Having had a very scientific education, the written word was never my strong point. Was suggesting that it is actually the same principle behind the ballerina and boats. The conservation of angular momentum is just the by product of no force being applied when Newton's laws are in effect.

The ballerina is a great example to illustrate Newton's laws as it is a relatively frictionless situation, whereas boat behaviour is heavily influenced by friction and energy lost to waves.
 
Maurice Griffiths designed a lot of sailing boats with bilge keels. Many were either hard chine or very full rounded bottom with low deadrise and shallow draft. This hull form is prone to rolling so his bilge keels were long low aspect ratio and set parallel to the centre line and at right angles to the hull, so splayed outwards relative the water line. They were no deeper than the central keel. The idea is that they would act as dampers to reduce roll. They also allow drying out upright.

Some evidence that they work. However, I removed them from my boat because don't need the drying out bit and they used to slap when sailing on the wind. Don't notice any real difference in rolling, but I did increase the draft by 30cm by lowering the ballast keel at the same time, so difficult to make a direct comparison.

Many displacement MOBO hulls have bilge keels so not new territory. They are easy to make and fit provided you re-inforce the bilge area. If they are bolted on you can always remove them if they don't work.
 
The need for roll stabilisation came with the advent of power. Bilge keels or plates were amongst the first practical measures to reduce roll on steam/motor vessels. They do this by adding damping to the vessels rolling motion thus changing the resononant rolling frequency of the vessel. From a quick look the paravanes are basically doing the same. To ensure that you create enough damping will involve a little calculation but with a little research that should be a DIY job.

Active fin stabilisers will always beet simple damping systems but at a cost in terms of both cash and power.

Changing the metacentric height also has an impact as the strenght of the rightim moment has a significant impact on the roll period and thus the resonant frequency in roll. I am not sure that DIY calcs on this would be a good thing unless of course you did really know what you were playing with.

I would expect that structural changes that were not properly documented may also have an insurance impact, particulaly when you are playing with stability figures
 
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Steadying sail seems to be the least expensive and most user friendly option. I already have a stainless steel mast. Would I need a boom, or is the sail supposed to be fixed?

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The bickerers seem to have forgotten you /forums/images/graemlins/laugh.gif

You do not NEED a boom as any sail (including a steadying sail) may be loose footed. BUT you do need to be able to sheet it in at the appropriate angle (depending on wind direction relative to your heading). Make sure you can set the sail at a wide range of angles to the centreline of the boat.

You could do this with a metal bar or track across the beam of the boat to which you could fasten the sheet. A cheaper but less convenient method might be 2 seperate sheets from the sail fastened to a fixed point either side of the boat. adjust the two sheets to get the appropraite angle.

I must admit I tend to use my sails for drive rather than stability (that's just a minor bonus to me) so someone else may be able to advise you better.

Stuff the technical arguments, it's more important to try helping each other. /forums/images/graemlins/smirk.gif
 
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