Bilge keel draught when heeled

[LittleSister]

. . . 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. . . .

The depth of a keel (or pair of keels) is only one factor in creating lateral resistance. Any increase or decrease in lateral resistance as a boat heels is highly unlikely directly proportional to keel maximum depth.

 

[srm]

The depth of a keel (or pair of keels) is only one factor in creating lateral resistance. Any increase or decrease in lateral resistance as a boat heels is highly unlikely directly proportional to keel maximum depth.

Yes indeed, the windward keel just becomes hydrodynamic drag. (See my comments in Post#4).
However, there seems to be an underlying assumption here that a pair of bilge keels are exactly equal to a single keel and improve as the boat heels. Yet looking at drawings of hulls with the two options this is rarely the case as the two keel configurations have different proportions. There are many times that bilge keels are superior to a single keel, as when the boat has to be kept on a drying mooring. Sailing performance is unlikely to be better than with a single keel though due to a less efficient form to the keel that is doing the work and the drag of the second keel that does nothing to improve performance. @B27 in post#6 gives a good analysis based on first hand experience.

 

[Daydream believer]

There are many times that bilge keels are superior to a single keel, as when the boat has to be kept on a drying mooring.

Many times?
Can you please elucidate, because as far as I am aware the only advantage is being able to stand upright when stood on the keels. Plus, a shallower draft, in certain situations.
Apart from that, I am not aware of any other, "advantages"
I can think of a couple of disadvantages. Typically, when I walk around the club hard & talk to members legs poking out from underneath . One has to be careful walking into the club bar, full of men; walking up to one of them & saying " I recognise those legs"
As for running aground, I am aware that a bilge keeler can be down right dangerous, in certain situations.

 

[Snowgoose-1]

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

Good thread.
It sounds logical and should work, but it doesn't for some reason.
I think the boats that compare well with their fin cousins are boats like the fairly recent British Hunters. But they are really twin fins with a lot more draft than is usual for bilge keeler's. Stand to be corrected though.

 

[B27]

....
As for running aground, I am aware that a bilge keeler can be down right dangerous, in certain situations.

Running aground is generally dangerous.
It's basically what they used to call 'shipwreck'.

 

[oldmanofthehills]

Many times?
Can you please elucidate, because as far as I am aware the only advantage is being able to stand upright when stood on the keels. Plus, a shallower draft, in certain situations.
Apart from that, I am not aware of any other, "advantages"
I can think of a couple of disadvantages. Typically, when I walk around the club hard & talk to members legs poking out from underneath . One has to be careful walking into the club bar, full of men; walking up to one of them & saying " I recognise those legs"
As for running aground, I am aware that a bilge keeler can be down right dangerous, in certain situations.

The advantage of bilge keels is often the advantage of shallow draft. Many traditional harbours and pills dry or have shallow water, thus long fin keelers cannot easily visit them. Which is fine for the racing fraternity but restrictive in exploring the old places. Long keelers with legs have the same advantage as bilge keelers and that is what I have nowadays though using the legs is a slight bit of a faff. Long keelers also readily dry out against walls without owners fretting about stability or whether their deep keels can take the load

There is some suggestion that bilge keelers might have better dynamic stability compared to the same keel area of a deep keel due to mass of water trapped between the keels, and certainly our old bilge keeler was fine in quite choppy seaways when other similar sized boats where plunging and leaping unpleasantly. We however didnt point as well

 

[B27]

Good thread.
It sounds logical and should work, but it doesn't for some reason.
I think the boats that compare well with their fin cousins are boats like the fairly recent British Hunters. But they are really twin fins with a lot more draft than is usual for bilge keeler's. Stand to be corrected though.

British Hunters are what I think of as a 'standard' bilge keel, having sailed Duettes a bit when younger.
Deep bilge keels would be RM and Sadler 290.
Is it fair to generalise that 60s era smaller bilge keelers were generally less draught?

The Duette was shallower than the racing Sonata I'm pretty sure, but when it comes to comparing performance, you needed 3 heavy people on the rail to get the best out of the fin keel anyway.
The Duette 'seemed to go OK' because it had a good sized controllable rig on it.
I'm sure to windward, it's VMG would be less, but I wouldn't want to guess how much.

I think a lot of older bilge keel designs don't have great rigs. The people who sail them are not looking for windward performance so they can be in a spiral of not achieving the potential the hull has. But then lots of people with expensive fin keel cruisers tend to end up motoring up wind as well.

 

[johnalison]

The advantage of bilge keels is often the advantage of shallow draft. Many traditional harbours and pills dry or have shallow water, thus long fin keelers cannot easily visit them. Which is fine for the racing fraternity but restrictive in exploring the old places. Long keelers with legs have the same advantage as bilge keelers and that is what I have nowadays though using the legs is a slight bit of a faff. Long keelers also readily dry out against walls without owners fretting about stability or whether their deep keels can take the load

There is some suggestion that bilge keelers might have better dynamic stability compared to the same keel area of a deep keel due to mass of water trapped between the keels, and certainly our old bilge keeler was fine in quite choppy seaways when other similar sized boats where plunging and leaping unpleasantly. We however didnt point as well

The shallow draft doesn’t always work. The sill at the Pond at Walton on Naze has a V-shaped sill, which can catch out a bilge-keeler, or at least its skipper.

My experience of sailing bilge-keelers is fairly limited but my impression was that their directional stability was less than comparable fins.

 

[oldmanofthehills]

The shallow draft doesn’t always work. The sill at the Pond at Walton on Naze has a V-shaped sill, which can catch out a bilge-keeler, or at least its skipper.

My experience of sailing bilge-keelers is fairly limited but my impression was that their directional stability was less than comparable fins.

As others said on this thread, running into things is never desirable whatever your keel. Certainly our old bilge keeler was fairly directionally stable compared to our present long keeler, and both make or made more leeway than a deep keeler. I suspect directional stability is influenced by hull form

 

[Daydream believer]

The advantage of bilge keels is often the advantage of shallow draft. Many traditional harbours and pills dry or have shallow water, thus long fin keelers cannot easily visit them. Which is fine for the racing fraternity but restrictive in exploring the old places. Long keelers with legs have the same advantage as bilge keelers and that is what I have nowadays though using the legs is a slight bit of a faff. Long keelers also readily dry out against walls without owners fretting about stability or whether their deep keels can take the load

There is some suggestion that bilge keelers might have better dynamic stability compared to the same keel area of a deep keel due to mass of water trapped between the keels, and certainly our old bilge keeler was fine in quite choppy seaways when other similar sized boats where plunging and leaping unpleasantly. We however didnt point as well

I already mentioned shallow draft & the ability to stand upright. That is just 2 possible advantages.
I find your third point somewhat dubious. But without scientific evidence to the contrary, I cannot deny your claim. Can you prove it?
In the meantime, SRM said that there are "many times" etc.- I am still waiting to hear what they may be.

 

[Daydream believer]

The shallow draft doesn’t always work. The sill at the Pond at Walton on Naze has a V-shaped sill, which can catch out a bilge-keeler, or at least its skipper.

Can be awkward in some parts of the Dutch canals as well where the sides slope a bit. Not much fun if pushed to one side by a barge coming the other way & having to get right over. One does not normally moor alongside in those canals but sometimes one has to whilst waiting for a bridge. If there are no pontoons the bank is the only option & then one can catch a bilge keel.. I have clipped my rudder a couple of times when turning in crowded situations heading up the canal to Goes & I have a fin..

 

[Snowgoose-1]

British Hunters are what I think of as a 'standard' bilge keel, having sailed Duettes a bit when younger.
Deep bilge keels would be RM and Sadler 290.
Is it fair to generalise that 60s era smaller bilge keelers were generally less draught?

The Duette was shallower than the racing Sonata I'm pretty sure, but when it comes to comparing performance, you needed 3 heavy people on the rail to get the best out of the fin keel anyway.
The Duette 'seemed to go OK' because it had a good sized controllable rig on it.
I'm sure to windward, it's VMG would be less, but I wouldn't want to guess how much.

I think a lot of older bilge keel designs don't have great rigs. The people who sail them are not looking for windward performance so they can be in a spiral of not achieving the potential the hull has. But then lots of people with expensive fin keel cruisers tend to end up motoring up wind as well.

It begs the question should an underwater profile be designed for a particular kind of keel and rig ?
Understandably, yacht manufacturers did seem to offer a one size fit's all I.e. bilge, fin and often lifting without altering the hull much if at all.

 

[johnalison]

It begs the question should an underwater profile be designed for a particular kind of keel and rig ?
Understandably, yacht manufacturers did seem to offer a one size fit's all I.e. bilge, fin and often lifting without altering the hull much if at all.

The Sadlers were possibly the best-known example of multiple options. My test-sail on a 29 was with bilge keels and the boat appeared less lively than the fin that I bought later, but that was just an impression from a single day. YM did a comparison of the four versions of the 32 but as far as I remember they concentrated on performance rather than handling.

 

[B27]

It begs the question should an underwater profile be designed for a particular kind of keel and rig ?
Understandably, yacht manufacturers did seem to offer a one size fit's all I.e. bilge, fin and often lifting without altering the hull much if at all.

Interesting point.
Hull development has largely been driven by racing.
Few people have invested much in optimising bilge keelers for racing. Or for performance.
The average cruising boat owner won't notice a 1% change in speed from A to B, but 1% would win or lose races.

Let's face it, most hull designs are pretty much copies of what's known to work well in racing, compromised to meet requirements. Hunters are evolved from 1/4 tonner shapes and the like.
Somebody wanting to knock out a few dozen budget bilge keel cruisers is not going to embark on an America's Cup style tank testing programme. At least not in 1975.

These days, someone might attack the problem with a computer.

To be clear, I'm pleased with my boat, but the reason I have a bilge keel boat is 100% affordable moorings.

 

[srm]

In the meantime, SRM said that there are "many times" etc.- I am still waiting to hear what they may be.

Do not get so impatient, it seems that we may be in different time zones.

I did not wish to be too dismissive of bilge keel designs. My "many times" refers to the many that have been bought, initially new, then again as it changed ownership. Production builders would not have kept offering the options if the market did not buy them. Presumably, these "many" owners all decided that bilge keels had advantages for them.

The posts above that answered your question seem to sum up the situation so I will not repeat them.

I have sailed on a few bilge keel boats, but would rate sailing performance as adequate. The only boats I have owned with two keels were two catamarans, again with adequate performance. My three monohulls all had longish deep keels with rudders on the trailing edges and sailing performance suited to my preferred style of cruising. As I have a preference to explore away from the well known channels I found being able to lift the keel off the odd unmarked rock by heeling the boat useful.

 

[Daydream believer]

Do not get so impatient, it seems that we may be in different time zones.

Apologies. I tend to forget how far & wide this forum spreads

 

[Snowgoose-1]

Bilge keelers sold in their 1000's on the East Coast .

This was mainly in the seventies and early eighties when sailing was booming and
marina and full tide mooring were scarce. The nature of the area perhaps , and with possibly newness to sailing they offered some comfort if running aground.

I myself bought a used 20' bilge keeler with a 2' 6" draft. The broker did ask me if I wanted to go sailing or just move sideways . Yes it did go sideways on the wind but did everything else extremely well. Bigger boats with increased draft had much improved windward performance.

 

[LittleSister]

Many times?
Can you please elucidate, because as far as I am aware the only advantage is being able to stand upright when stood on the keels. Plus, a shallower draft, in certain situations.
Apart from that, I am not aware of any other, "advantages"
I can think of a couple of disadvantages. Typically, when I walk around the club hard & talk to members legs poking out from underneath . One has to be careful walking into the club bar, full of men; walking up to one of them & saying " I recognise those legs"
As for running aground, I am aware that a bilge keeler can be down right dangerous, in certain situations.

OK. We get that you don't favour bilge keels, but every boat is a bundle of compromises, and those involved with bilge keels suit many owners, as witnessed by their huge sales over the years. Luckily, we all have different preferences and priorities, and a range of boat designs to choose from.

Other potential advantages (all of which come with disadvantages, like every other aspect of boat design) could include -
- able to stand ashore without the cost, inconvenience and risks of cradles
- assuming lesser draft than a single keel, which is typical but not inherent, then less height to climb up or fall from when stood ashore or aground
- the potential to have the two keels at different angles to the centreline of the boat. In principle, if bilge keels are splayed laterally and toed-in forward, when heeled the deeper leeward keel will allow the boat to move straight ahead, or nearer so, than the normal boat track which is a diagonal from the centreline, while the windward keel exerts a downward force countering heeling.

That's just off the top of my head. I'm sure there must be others. (Only one of the 5, I think, yachts I've owned was a bilge-keeler, so I'm not a bilge-keel proselytiser, but I wouldn't rule out having another.)

For amusement only, I'm pondering a non-bilge twin-keel concept, with two parallel high-aspect ratio vertical keels (somewhat like a biplane) close either side of the centreline. This would miss out on several of the advantages of bilge keels (especially the ability to stand upright ashore or aground), but could provide the lift of a deep fin but with reduced draft. The reduced draft would allow thinner keels for less drag, and perhaps also a wider, less costly and heavy, attachment to the hull.

 

[johnalison]

And of course with bilge-keelers you have the joy of crawling along wet ground between the keels to complete the antifouling.

 

[38mess]

And of course with bilge-keelers you have the joy of crawling along wet ground between the keels to complete the antifouling.

It's even more fun with triple keels