Keel depth...

[andymcp]

Excuse my ignorance, but as far as I can tell the main real world difference between a standard and deep keel (assuming overall ballast etc remains the same between the two) would be a higher pointing ability on the deeper version.

Is that about right, or are there loads of other benefits of deeper keels that I am missing? Any wisdom gratefully received.

Cheers,

Andy

 

[Tranona]

You've got it! The shallow keels usually have similar stability curves because they have more ballast or as in the case of my boat a large bulb at the bottom of the keel.

Generally the deeper keel allows you to carry sail in higher wind strengths because the ballast is lower down. The main advantage is usually deeper keels are more efficient foils so improve windward ability in particular.

In normal "pottering" mode in gentle conditions, and particularly off the wind it might be difficult to tell the difference, but as soon as the wind gets up and you want to press the boat upwind you realise the benefits of a deep keel.

Choice depends on the way you want to use the boat. We chose shallow draft because we wanted the ability to go though the French canals and the boat was used mostly in benign conditions in the Ionian. However it does get frustrating on the odd occassion trying to make progress to windward in heavier weather.

 

[andymcp]

Excellent, thanks. Alas we seem to be frequently heading to windward, and it's almost always windy. For some the difference seems reasonable and probably makes quite a difference - circa 30% more depth so you would expect very apparent benefits - but on others the difference seems so minor as to make me question how much of a recognisable improvement it would be, e.g. less that 10% increase in depth but a princely sum to receive it.

 

[johnalison]

I have a "shallow" fin of 1.5m c.f. the standard 1.8m. I have sailed in company with standard boats and raced against them, in moderate conditions. On the whole, there is not a lot of difference between us, though I have formed a few impressions.

Pointing ability in normal conditions seems to be identical, though I would expect a little more leeway to become apparent in a seaway with the shallower keel.

If anything, the shallow fin may be slightly faster off the wind due to a smaller wetted surface.

The deeper keel results in less roll, both under way and anchored. On one run round Cap de la Hague with a light quartering wind we were unable to keep up with a deep-keeled boat, though he also had a larger foresail. My impression was that his mainsail was holding the wind better than mine.

The ability to carry sail is the same in both boats.

Not all designs will show the same variations, and in some, such as Sadler 32, the shallow fin is definitely inferior.

 

[Twister_Ken]

At a tangent - lead is better than iron. Denser, thus smaller (but not shallower) keel for same righting moment = less wetted surface area. And it doesn't rust as soon as you breath on it.

 

[flaming]

A few years ago a new model of a very sucessful cruiser racer was delivered. The Boat it was replacing had won everything, so expectations were high.

It bombed. It seemed, despite rating higher than the old model, to be slower in all conditions and both upwind and down.
Pro sailors were hired, rigging gurus were brought in, new sails were ordered, but nothing made any difference. Finally the owner paid to have the boat lifted with the intention of fitting a new folding prop, as the prop not folding properly was the only other thing that could be thought of.

When it was in the slings someone commented that the keel looked quite small.

A quick check confirmed the mistake - the yard had accidentally fitted the shallow draft keel to the boat! A new keel was quickly dispatched to the dealer, with the owner understandably upset!
Next race, the thing was quick... It finally started beating the old models on the water.

So yes, there can be a huge difference that would be picked up on the race course, but in most cases a family crew won't notice.

 

[Resolution]

One other factor to check is the weight of the keel. Sometimes the yacht manufacturer chooses to keep the righting force of the shallow keel the same or near to that of the deep keel. Mechanically this can be done by using a diferent shape, eg with a lead bulb, but more likely just to be a thicker splodge of cast iron. This is likely to be heavier than the deep keel. Check the overall weights.

 

[Seajet]

The point about deep keels is not just a more efficient aspect ratio, it's that they go deep enough to get through the 'surface drift' layer of water near the surface, which is generally reckoned to be about 1 metre deep as a rule of thumb.

As I'm sure you're aware, especially during and after a strong wind the whole surface layer of the sea is set in motion to leeward, which needs to be taken into account in dead reckoning.

So a deep keel allows grip on the more static water below the surface drift, and much better progress beating into strong stuff.