Deep vs shallow keel

[Laminar Flow]

I can well imagine that watching other boats sail past you to windward would be quite maddening as a racing sailor, it is rather counterproductive to the purpose of the entire activity.

It does not invalidate the real advantages of shallow draft. I remember reading the a (cruising) report on a large Swan; the vessel had a draft of 3.5m. While, as expected, a sparkling performer, the rest of the report was about all the places the boat couldn't enter or even get close to. That kinda defeats the whole purpose and pleasure of cruising to me.

While my own boat will never be a performer to windward, below 50 degr. she has nothing to be ashamed about, but she will float in 1.35m and dry out comfortably on her legs. We have enjoyed places where you simply couldn't go, not with a 6'3" draft; up Breton rivers and many of the Sunds in southern Denmark, areas in Holland and the Waddenzee. For that I'm quite happy to make some (fairly) modest concessions as to her weatherliness. She could still cross an ocean if I had a stroke or covid restrictions got to me.

A lot of the perceptions around boats are just that. For that reason I like the research done by by places like the Delft institute, it quantifies things and, all things being equal (which of course they are not necessarily or always, but mainly so), the difference is at worst 5%. In life we happily make many decisions for a lot less.

 

[geem]

.A lot of the perceptions around boats are just that. For that reason I like the research done by by places like the Delft institute, it quantifies things and, all things being equal (which of course they are not necessarily or always, but mainly so), the difference is at worst 5%. In life we happily make many decisions for a lot less.
[/QUOTE]
I have to say that my experience with sailing to windward in company with shallow draft boats of similar size is that the difference in performance is considerably more than 5%. I appreciate that a theoretical study says 5% but in the real world my experience is somewhat different. There may be other factors that play in to this such as hull shape not just draft. Also in mast reefing does you no favours going to windward. Windage is also an issue. We have 2.2m draft, folding prop, tall solent rigged mast, slab reefing. Compared to a similar sized yacht with shallow draft, fixed prop, in mast reefing and a large reefed genoa going to windward in a 2m sea and 20kts over the deck, we will make typically 10-15deg better to windward at the same or better speed. The point is, that draft is only one aspect of windward performance. If you have all the negative aspects listed above I very much expect that kind of boat to be motoring to windward in those conditions. Even in lighter winds and flat seas we often see quite a difference in windward performance

 

[anoccasionalyachtsman]

I have to say that my experience with sailing to windward in company with shallow draft boats of similar size is that the difference in performance is considerably more than 5%. I appreciate that a theoretical study says 5% but in the real world my experience is somewhat different. There may be other factors that play in to this such as hull shape not just draft. Also in mast reefing does you no favours going to windward. Windage is also an issue. We have 2.2m draft, folding prop, tall solent rigged mast, slab reefing. Compared to a similar sized yacht with shallow draft, fixed prop, in mast reefing and a large reefed genoa going to windward in a 2m sea and 20kts over the deck, we will make typically 10-15deg better to windward at the same or better speed. The point is, that draft is only one aspect of windward performance. If you have all the negative aspects listed above I very much expect that kind of boat to be motoring to windward in those conditions. Even in lighter winds and flat seas we often see quite a difference in windward performance

The biggest differences are the ability and motivation of the people in charge.

 

[Laminar Flow]

.A lot of the perceptions around boats are just that. For that reason I like the research done by by places like the Delft institute, it quantifies things and, all things being equal (which of course they are not necessarily or always, but mainly so), the difference is at worst 5%. In life we happily make many decisions for a lot less.

I have to say that my experience with sailing to windward in company with shallow draft boats of similar size is that the difference in performance is considerably more than 5%. I appreciate that a theoretical study says 5% but in the real world my experience is somewhat different. There may be other factors that play in to this such as hull shape not just draft. Also in mast reefing does you no favours going to windward. Windage is also an issue. We have 2.2m draft, folding prop, tall solent rigged mast, slab reefing. Compared to a similar sized yacht with shallow draft, fixed prop, in mast reefing and a large reefed genoa going to windward in a 2m sea and 20kts over the deck, we will make typically 10-15deg better to windward at the same or better speed. The point is, that draft is only one aspect of windward performance. If you have all the negative aspects listed above I very much expect that kind of boat to be motoring to windward in those conditions. Even in lighter winds and flat seas we often see quite a difference in windward performance
[/QUOTE]
All things being equal... The original premise of the thread was about the variance between sister ships with different keel options. The Delft study concerns the performance of different keels only. For all I know they stuck them on the bottom of a board before dipping them in the test tank.
No other aspects were considered.

As you point out, there are many factors that influence windward performance. Windward ability is, in simple terms, the sum total of resistance, parasitic drag and lift generated by the forward motion through the water.
As lift increases with the square of the speed, the designer's obvious goal would be to increase speed while keeping drag and all factors that detract from it to a minimum.

Ergo, we need to eliminate inefficiencies in the rig, reduce parasitic drag in it and the superstructure. Optimize hull shape and appendages, remove, modify anything (through hulls, propellers, shafts and struts etc) that raises resistance; keep the bottom smooth and clean.

Some of these measures are doable, some are not practicable and some we are willing to compromise on for obvious benefits.

Therein lies the skill of a designer, all of whom are not created equal, to come up with the most optimized shape and the best compromise that meets most of an owner's needs.

 

[anoccasionalyachtsman]

All things being equal... The original premise of the thread was about the variance between sister ships with different keel options. The Delft study concerns the performance of different keels only. For all I know they stuck them on the bottom of a board before dipping them in the test tank.
No other aspects were considered.

As you point out, there are many factors that influence windward performance. Windward ability is, in simple terms, the sum total of resistance, parasitic drag and lift generated by the forward motion through the water.
As lift increases with the square of the speed, the designer's obvious goal would be to increase speed while keeping drag and all factors that detract from it to a minimum.

Ergo, we need to eliminate inefficiencies in the rig, reduce parasitic drag in it and the superstructure. Optimize hull shape and appendages, remove, modify anything (through hulls, propellers, shafts and struts etc) that raises resistance; keep the bottom smooth and clean.

Some of these measures are doable, some are not practicable and some we are willing to compromise on for obvious benefits.

Therein lies the skill of a designer, all of whom are not created equal, to come up with the most optimized shape and the best compromise that meets most of an owner's needs.

I remember the Delft study (and have a feeling it was run by Scheel and van Oosanen?) and saw pictures showing and adapted an adapted dynamometer that allowed the hulls to be be run with heel and/or yaw. The memory's pretty vague on it, but I don't think they ran waves. I've just asked a friend in the business to have a look for a copy of the report, and will put up a copy if we find it.

 

[Laminar Flow]

I remember the Delft study (and have a feeling it was run by Scheel and van Oosanen?) and saw pictures showing and adapted an adapted dynamometer that allowed the hulls to be be run with heel and/or yaw. The memory's pretty vague on it, but I don't think they ran waves. I've just asked a friend in the business to have a look for a copy of the report, and will put up a copy if we find it.

Thank you, I would appreciate that. My source was the excellent book: Principles of Yacht Design by Larsson and Eliasson, first published in 1994. Delft also did the tank testing for Australia's keel at the time and I remember that Van Oosanen was instrumental in that.

The Delft study on keels indicates that there might be substantial benefits in terms of shallow draft, had the differences between the deep keels with 2.29m and the shallow ones with 1.38m not been quite as extreme. In the higher wind speeds (hence, boat speed) the results for the wing keel and also the Scheel keel to some extent were closer to that of the deep fins.

A study done in Southampton on behalf of Laurent Giles for a large sailing vessel indicated that even quite subtle changes to an original shallow draft plan form can significantly improve keel performance. The main features were a very slight increase in depth and giving the keel some drag (i.e. deeper at the back). Ironically, this is a feature found in many traditional craft as the entire length of the keel becomes the leading edge, generating lift along it.

 

[johnalison]

I found this article about Sadlers, which I thought interesting. From the data give towards the end, it appears that shallow fin and lifting keels only suffer slightly in terms of tacking angle for the 32. This doesn't quite correspond to my memory of sailing in company with them, but it may be that the shallow fin versions I knew were not well sailed and had poor gear. The proportional difference for these boats' draft is greater than for my model so generalisations are unlikely to be accurate.

https://www.lucasyachting.co.uk/wp-content/uploads/2017/10/keel-choice.pdf

 

[anoccasionalyachtsman]

We'll have to wait to see it, but I also vaguely remember thinking that the testing was purely on the hydrodynamics of the keel and a 'standard' hull, so yes, every inch deeper is going to be better for the aspect ration and the CofG. I can't really think of any reason that it wouldn't be more or less linear though, so interpolation of the results should give a reasonable guess at 'deeper shallow'!

btw. It was the Australia II testing that got Delft thinking, and it wasn't really until Scheel hatched his that they did the test we're talking about a few years on. Van O was definitely involved in both sets, but made himself look a bit daft later when he claimed more than his share of the glory.

 

[Laminar Flow]

I found this article about Sadlers, which I thought interesting. From the data give towards the end, it appears that shallow fin and lifting keels only suffer slightly in terms of tacking angle for the 32. This doesn't quite correspond to my memory of sailing in company with them, but it may be that the shallow fin versions I knew were not well sailed and had poor gear. The proportional difference for these boats' draft is greater than for my model so generalisations are unlikely to be accurate.

https://www.lucasyachting.co.uk/wp-content/uploads/2017/10/keel-choice.pdf

Thank you for that article, johnalison.

The comparison between deep and shallow fin is within 1% of the Delft figures which makes sense either way.

What I was missing were the relative speeds and ballast ratios; I doubt the centreboarder would have shown itself to be quite as stiff without some extra weight.

As you have earlier intimated, full size tests, even when boats are sailed head to head, have quite a number of variables to contend with and that are not easily controlled, such as variations in gear or the skills of the skipper.

Interesting points for me were the better directional stability of the longer fin as well as those of the wing keel. The dampening effect of the wing keel was also cited in the German comparison of the two Contests and I suspect this action causes considerable turbulence in a seaway at the cost of some speed.

 

[anoccasionalyachtsman]

Thank you for that article, johnalison.

The comparison between deep and shallow fin is within 1% of the Delft figures which makes sense either way.

What I was missing were the relative speeds and ballast ratios; I doubt the centreboarder would have shown itself to be quite as stiff without some extra weight.

As you have earlier intimated, full size tests, even when boats are sailed head to head, have quite a number of variables to contend with and that are not easily controlled, such as variations in gear or the skills of the skipper.

Interesting points for me were the better directional stability of the longer fin as well as those of the wing keel. The dampening effect of the wing keel was also cited in the German comparison of the two Contests and I suspect this action causes considerable turbulence in a seaway at the cost of some speed.

We had another thread on winged keels recently, and I mentioned the difference offwind there (I sailed one particular type fairly frequently with both wing and deep fin) . It really is quite astonishing how effective it is, and I wouldn't mind betting that the cost in drag is made up by the reduction of roll and pitch on the aero's of the rig. As I said there, it's a pity I never got to sail one against the other.

 

[Graham376]

The Delft study on keels indicates that there might be substantial benefits in terms of shallow draft, had the differences between the deep keels with 2.29m and the shallow ones with 1.38m not been quite as extreme. In the higher wind speeds (hence, boat speed) the results for the wing keel and also the Scheel keel to some extent were closer to that of the deep fins.

A few years ago, we sailed in company with a long keeled Warrior maybe a couple of feet shorter and very different design to us. Quite a lively day but not sure of highest wind speed as our anemometer packed up at something over 40 kts. We couldn't catch him until wind speed dropped further north.

Did the Delft study do any comparisons on roll resistance? I seem to remember a paper many years ago which indicated deeper keel boats were more likely to be rolled in large seas than long or shallow keels. IIRC it was due to the power of wave below surface.

 

[anoccasionalyachtsman]

A few years ago, we sailed in company with a long keeled Warrior maybe a couple of feet shorter and very different design to us. Quite a lively day but not sure of highest wind speed as our anemometer packed up at something over 40 kts. We couldn't catch him until wind speed dropped further north.

Did the Delft study do any comparisons on roll resistance? I seem to remember a paper many years ago which indicated deeper keel boats were more likely to be rolled in large seas than long or shallow keels. IIRC it was due to the power of wave below surface.

Delft had the same limit as we did - tank width. To get a breaking wave in the tank meant a minimum height, and a correctly scaled model would have been too small for meaningful experiments really. Memory says it was NMI (as was) at Teddington, post '79 Fastnet - they had all sorts of pools to play ships in.

 

[Laminar Flow]

A few years ago, we sailed in company with a long keeled Warrior maybe a couple of feet shorter and very different design to us. Quite a lively day but not sure of highest wind speed as our anemometer packed up at something over 40 kts. We couldn't catch him until wind speed dropped further north.

Did the Delft study do any comparisons on roll resistance? I seem to remember a paper many years ago which indicated deeper keel boats were more likely to be rolled in large seas than long or shallow keels. IIRC it was due to the power of wave below surface.

Resistance for any given hull can be, simplified and without considering increase in resistance due to wave action, be divided up into frictional resistance due to wetted surface and form or wave making resistance.
Wetted area obviously is a concern for a long keel boat; however, this type of resistance increases fairly modestly with rising speed and in a shallow curve.
Wave making (form) resistance is predominantly a function of displacement, assuming that we are talking about hydrodynamically optimized hull shapes, when comparing and this type of resistance rises exponentially at a relative speed of about 1.

Given enough propulsive power and comparable sail area displacement ratios, there is absolutely no reason a well designed long keel yacht needs to be slower in displacement terms, unless of course the lighter model can, as some designs are capable today, reach higher than displacement speeds by planing. In relative speeds up to 1, a nicely shaped long keeler with the same SA/Displ. ratio will certainly give a light flyer a run for it's money.
In rough going, a superior directional stability might also give an edge in individual cases, allowing a long keel to carry more sail and longer. All in displacement terms, of course.

To your second question. As far as I know, Delft didn't do any roll/capsize tests in regards to their keel study.
C.A. Marchaj in his book "Seaworthiness, the Forgotten Factor", addressees your point in some detail including the evaluation of tank tests.
The short of the long: the two main contributing factors to capsize in waves are light displacement and wide beam, whereas generous displacement, moderate beam, larger lateral plane and the greater inertia of a substantial rig provide better capsize resistance (all of which are not exactly speed enhancing factors and hence not popular in contemporary design).

The dangerous part of the wave is the spill zone, where the water moves at a relatively greater speed than subsurface. In a lighter, beamier boat the risk of the deck edge becoming entrapped in the slower or in relative terms stationary subsurface water is apparently a greater risk as a "tripping" hazard. At such extreme angles the "grip" of the keel would be quite irrelevant.

A friend of ours tried to weather a severe storm in the North Sea by raising the keel on his Southerly. As a consequence the boat would regularly roll to such extreme angles that the lee side winches in the Southerly's very high centre cockpit were well under water, which, as he explained, made reefing his head sail rather hazardous.