Bilge keel draught when heeled

[Barton Seagoon]

Hi everyone. When discussing the relative merits of fin keel vs bilge keels, people often say that a bilge keel will get deeper (more draught) as the boat heels over, while a fin keel will get shallower. Thus the lateral resistance will change accordingly. Has anyone ever measured this on a real boat to see if this is true, and how they compare? I understand the theory, but what is the situation in practice? (Of course there are lots of other pros and cons of both designs, which we don't need to repeat in this thread)
I originally posted this question in the Westerly Owners forum, and Concerto has sent a helpful reply, but I am keen to hear further thoughts (and maybe real experience) from this august bunch of know-it-alls

 

[Sandy]

I think some people need to get some bits of paper and a pencil out and think about what they are saying.

 

[johnalison]

Some early ones were built with upright keels, (or should that be downright?), in which case the draft would certainly get shallower and LR reduce, but with splayed out keels it must be more complex, with initial deepening and improved LR perhaps.

 

[srm]

I don't think there is a simple answer. It depends on the cross section geometry of the hull and keels and movement of the centre of buoyancy as the boat heels.

If your concern is lateral resistance of the same hull with fin or bilge keels the upright draft of bilge keels would generally be less than the draft of a fin keel so you are going to need to make detailed drawings for comparison at different angles of heel and will also need to account for the different lateral area and hydrodynamic lift of the two different keel configurations. The intuitive response is that the lee keel will become nearer vertical if splayed and thus more efficient, but this may be offset by a smaller lateral area than the fin keel version and drag/turbulence of the weather keel. I have read comments in the past that for some specific boats the bilge keel version was as good to windward as the fin keel, but do not have references.

As a generalisation it was regarded that bilge keels could be a problem if the boat grounded. Forcing a single keel boat to heel would always reduce the draft whereas a bilge keel configuration would initially increase the draft of one keel. A couple of centimetres one way or the other could make the difference between floating off and waiting for the next high water.

 

[srm]

but I am keen to hear further thoughts (and maybe real experience) from this august bunch of know-it-alls

You can, no doubt, hear plenty of "know-it-alls" propping up yacht club bars.

People with real world experience know first hand that there is always something new to learn/discover/catch us out. But then I have only owned and sailed cruising boats for around 50 years and am happy that, despite a varied sailing/maritime career, there is always something new ahead.

 

[B27]

If you run aground in a bilge keeler, it does tend to turn around the keel which hits the bank, which might generally be the leeward one.
If you look at a bilge keeler end-on, you can visualise that the leeward keel will dip as it heels, but exactly how much will depend on the hull shape. Hard to quantify but I reckon mine might go from drawing say 1 metre to 1.3, maybe 1.4?
At the same time, the depth transducer is also closer to the surface due to the heel.
That doesn't worry me as the sea bed varies by more than that, around here it's rocky and unless I'm aiming to dry out, I want a metre of clearance.
I don't go charging into shallows, powered up with gunwhale under water, in fact I don't think I've been shallower than 3m under sail this season? We have an engine for getting into port, so we'll be upright.

Some bilge keelers sail well enough and might be hard to differentiate from a shoal draft fin.
A deep fin will be more powerful and a 'racier' boat.
In waves I notice the effect on the windward keel when heeled.
On the whole I'd say having a folding prop and a clean bottom is a bigger deal than keel type, but a bilge keeler is always a compromise upwind, a little more so in a choppy sea.

I would say a bilge keel or shallow keel boat is also disadvantaged, compared to a deep keel, down wind in waves and breeze. I think I would be able to sail closer to a dead run comfortably with a deeper keel and more stability. That matters when the breeze blows along the coast.
But I am used to other people's, bigger boats.

East Coast people who want to sail over mud banks with the keel cutting a groove may see things a bit differently from us in the South West where most of the water is deep and much of the bottom is hard.

 

[johnalison]

East Coast people who want to sail over mud banks with the keel cutting a groove may see things a bit differently from us in the South West where most of the water is deep and much of the bottom is hard.

There is nothing we like better on the East Coast than grinding through the mud with one or both keels. On the other hand there are probably more places along the South Coast where bilge keels can take advantage of their ability to dry out and remain upright and still be able to walk away from the boat.

 

[mjcoon]

Some early ones were built with upright keels, (or should that be downright?), in which case the draft would certainly get shallower and LR reduce, but with splayed out keels it must be more complex, with initial deepening and improved LR perhaps.

I don't think the angle of dangle of the keels is so central. Surely the point is that the keels are both to the side of the centre line. So as the boat heels they will rotate around the centre of buoyancy, which initially at least must take one keel deeper and the other shallower.

 

[srm]

I don't think the angle of dangle of the keels is so central. Surely the point is that the keels are both to the side of the centre line. So as the boat heels they will rotate around the centre of buoyancy, which initially at least must take one keel deeper and the other shallower.

Most likely. However, as the boat heels the immersed volume changes shape and the centre of buoyancy will move to leeward which generates a righting moment. Unless of course the hull is cylindrical in section.
Edit: So sailing at any angle of heel moves the the centre of Buoyancy to leeward of the centre line. The immersed volume remains the same but draft may change depending on the hull shape.

 

[B27]

Most likely. However, as the boat heels the immersed volume changes shape and the centre of buoyancy will move to leeward which generates a righting moment. Unless of course the hull is cylindrical in section.
Edit: So sailing at any angle of heel moves the the centre of Buoyancy to leeward of the centre line. The immersed volume remains the same but draft may change depending on the hull shape.

An extreme case, a wide flat-bottomed boat could lift both keels when heeled.
A very fat round bilge with lots of rocker could lift its ends out of the water and put the leeward keel down further.

It doesn't really matter in general, because the seabed isn't perfectly flat, your keels are not running along the same line as your echo sounder, so you have to allow for variations.
The sea has waves on it. The tide deviates from the predictions....

Get thee outside the 10 metre contour!

 

[johnalison]

I don't think the angle of dangle of the keels is so central. Surely the point is that the keels are both to the side of the centre line. So as the boat heels they will rotate around the centre of buoyancy, which initially at least must take one keel deeper and the other shallower.

That must be true, though to a lesser extent. The great disadvantage of thinking about a problem is that you get nearer the truth, and where’s the fun in that?

 

[Pete7]

Get thee outside the 10 metre contour!

Good grief no, its full of ridiculously fast mobos' terrified of straying outside the channel, much safer inside 2m

So time for a picture in this instance a Moody 31, draft 1.2m. We did once sail her so the rail was nearly in the water. Peering over the windward side I could see the keel just below the surface slicing through the air in a trough and a thumb if it hit the next crest. We only did it once because we all had to hand on and memsahib wasn't impressed, but we made it around Brambles West Knoll without having to tack.

That would put the surface somewhere in the region of the red line with the leeward keel pretty vertical.

 

[mjcoon]

Most likely. However, as the boat heels the immersed volume changes shape and the centre of buoyancy will move to leeward which generates a righting moment. Unless of course the hull is cylindrical in section.
Edit: So sailing at any angle of heel moves the the centre of Buoyancy to leeward of the centre line. The immersed volume remains the same but draft may change depending on the hull shape.

The movement of the centre of buoyancy to leeward surely cannot be enough to take it past the leeward keel which is thus still going to be rotated so as to take it deeper, at least, as I said, initially.

 

[mjcoon]

That must be true, though to a lesser extent. The great disadvantage of thinking about a problem is that you get nearer the truth, and where’s the fun in that?

Hmm, I like a flight of fancy as much as the next nerd, but I still think far too many people are having fun deviating from the truth! ;-)

 

[Pete7]

Hmm, I like a flight of fancy as much as the next nerd, but I still think far too many people are having fun deviating from the big ship channels seeking somewhere different.

I agree, us parking on the sand hills at the Royal Victoria YC. Great club, real shame what the new ferry is doing to the sea bed. Perhaps they need some eel grass.

 

[Daydream believer]

Good grief no, its full of ridiculously fast mobos' terrified of straying outside the channel, much safer inside 2m

So time for a picture in this instance a Moody 31, draft 1.2m. We did once sail her so the rail was nearly in the water. Peering over the windward side I could see the keel just below the surface slicing through the air in a trough and a thumb if it hit the next crest. We only did it once because we all had to hand on and memsahib wasn't impressed, but we made it around Brambles West Knoll without having to tack.

That would put the surface somewhere in the region of the red line with the leeward keel pretty vertical.


View attachment 166233

May I suggest that in that picture the line is drawn incorrectly. From just looking at it , it seems to me that there is a greater volume of hull below the line on the heeled detail as there would be if the line were drawn for the boat in the stable upright state.
Should the area be the same? Or is one taking into account the displacement, caused by the wave form, as the vessel moves forward? Thus, reducing volume & causing it to settle down - initially.

 

[Pete7]

Should the area be the same? Or is one taking into account the displacement, caused by the wave form, as the vessel moves forward? Thus, reducing volume & causing it to settle down - initially.

Yep, that's the problem is sailing in waves. Stationery is easy, there is a tide mark, but at sea in the Solent chop the waterline is all over the place, so the line is a bit of a guess.

I have added a yellow line which if the yacht rolls would put the leeward rail in the water and at the same time lift the keel so comes out in a trough, or there abouts.

 

[B27]

The line is a plane.
The volume below the plane is the displacement,
How the plane moves when you heel is easy to guess, harder to know exactly.

If you take a hull shape like a Laser dinghy, when you heel it, the ends come out of the water, so the buoyancy must concentrate more near the middle of the boat. Take an old IOR hull and the waterline length increases when heeled. It's either an exercise needing some proper analysis or we can make do with guessing. Guessing is near enough for me.

 

[srm]

Should the area be the same? Or is one taking into account the displacement, caused by the wave form, as the vessel moves forward? Thus, reducing volume & causing it to settle down - initially.

It is the immersed volume that needs to be the same, not the cross section area. What an end on view will look like depends on the lines of the hull and how the hull volume is distributed fore and aft.
As noted above wave form complicates things further.

 

[Pete7]

So, if the yellow line is approximately right, then the leeward keel is deeper when heeled shown with the blue lines.