Deep vs shallow keel

[johnalison]

In practical terms most of us work to 2m depths when close in. Knocking 20-40 cms off this with a shallower keel to enable ditch crawling with greater confidence is actually immaterial. Same argument with bilges, which I sailed for years before going fin.

But having a good fin not compromised on depth is a force for stability and pointing that cannot be denied, and one that trumps the occasional mini benefit of creeping over some shallow bar with greater confidence.

PWG

You can only speak for yourself. Many of us sail from areas where depths are limited and a few inches can make quite a difference. 2m is a good average expectation, but my 1.5m gives me a considerable advantage locally over the standard 1.8. Extensive crusing in the Southern Baltic has also left me very pleased with my choice. I would describe my boat as 'semi-shallow', and there are certainly some models, mentioning no names, where the shallow version is quite extreme, and you can see them on the water struggling to keep up with bilge-keelers. Stability should not be a problem if properly designed, and many fins have weighted bulbs to compensate. My 34 is perfectly happy sailing with full main and jib to windward in 20knots true wind, if that is any guide.

 

[Kelpie]

Something I don't believe was mentioned upthread was the benefit of shallow draft when drying a boat out deliberately. If you are accustomed to working on your boat between tides, half a metre less draft makes a very significant difference. It opens up more drying spots, you can work further from spring tides, and crucially you get a longer working window.

 

[convey]

As an aside, for the sake of an honorable mention, I was looking at some old 50s sailing magazines and came across a photo of a Fairey Atlanta being lifted while its keels were down. Looked the same as this drawing. Floats in 18" of water. Very curious boat. Clearly modeled on some species of whale ...!

Don't make'em like that any more.

 

[Laminar Flow]

In practical terms most of us work to 2m depths when close in. Knocking 20-40 cms off this with a shallower keel to enable ditch crawling with greater confidence is actually immaterial. Same argument with bilges, which I sailed for years before going fin.

But having a good fin not compromised on depth is a force for stability and pointing that cannot be denied, and one that trumps the occasional mini benefit of creeping over some shallow bar with greater confidence.

PWG

Each to their own.
I like shallow draft and it opens up cruising areas, anchorages, rivers and canals not accessible to deeper vessels. With a 5', better 4', you can still go through the French canals; with 6' or more you can't. With 2m draft there are many Danish fjords and harbours that are off limits, as are large parts of the Waddenzee and and a number of the Breton harbours and most of the rivers, just to name a few. Many consider 5' the max for the Bahamas.

I would be cautious about designs offered in various, different keel configurations and drafts, in particular if the shallower keel was not specifically modified (bulb, wings) to achieve the same righting moment. Some designers/yards simply ad more ballast to make up for reduced righting moment and overburden the hull. This can adversely affect performance and load carrying ability. Keel/centreboarder variants are notorious for this.

A boat designed from the outset to have shallow draft is by far the best choice and there is no reason that they be poor performers or unweatherly. The Dutch in particular and also the Americans have a long and successful history of designing shallow draft ocean going boats. The French as well have many formidable designs on offer.

I used to own a 50' cruising cutter that had a lifting keel. With it I could vary the draft from 1.2m to 2.85m. The keel weighed 5t and was hydraulically operated. I took this boat through the French & Dutch canals, the Waddenzee, through the Baltic, North Sea and across oceans. It could also dry out. Deep draft is fine for crossing oceans, but it is really the edges that I find the most interesting. As far as I am concerned; every centimetre less in draft opens up my cruising range by a hundred miles or more.

 

[Laminar Flow]

In practical terms most of us work to 2m depths when close in. Knocking 20-40 cms off this with a shallower keel to enable ditch crawling with greater confidence is actually immaterial. Same argument with bilges, which I sailed for years before going fin.

But having a good fin not compromised on depth is a force for stability and pointing that cannot be denied, and one that trumps the occasional mini benefit of creeping over some shallow bar with greater confidence.

PWG

Looking forward to seeing you in Lannion, Brittany, then (for example). No measure of "confidence" will get you in there; (shoal draft) "ability" might.

For what its worth, I'd rather enjoy the "mini benefit" of ability for crossing a bar or the sill to a tidal port than having my boat pounded to bits on said bar no matter how "confident" I felt about it.

Shoal draft is about ability, not about merely about "confidence".

 

[coopec]

You can only speak for yourself. Many of us sail from areas where depths are limited and a few inches can make quite a difference. 2m is a good average expectation, but my 1.5m gives me a considerable advantage locally over the standard 1.8. Extensive crusing in the Southern Baltic has also left me very pleased with my choice. I would describe my boat as 'semi-shallow', and there are certainly some models, mentioning no names, where the shallow version is quite extreme, and you can see them on the water struggling to keep up with bilge-keelers. Stability should not be a problem if properly designed, and many fins have weighted bulbs to compensate. My 34 is perfectly happy sailing with full main and jib to windward in 20knots true wind, if that is any guide.

I've only sailed socially but I recall being an a 50ft racing yacht. The wind was blowing a bit and I know the skipper wanted to drop anchor but he dilly-dallyed around. I asked a fellow crew mate why he didn't go into the sheltered cove where all the other boats were and he said it was too shallow as the yacht we were on drew 7'9". I figured then that excessive draft could be dangerous on a cruising yacht.

I believe 5ft is about the limit for canals in Europe.

 

[geem]

. My 34 is perfectly happy sailing with full main and jib to windward in 20knots true wind, if that is any guide.

Is that in flat water or in a 2m sea? The performance could be fine in a flat sea but impossible in a 2m sea. My own boat would be in full main and genoa at 14kts T in a 2m sea. Once we see over 20kts apparent wind we switch down to working jib then progressively reef the main as the wind increases. We would never carry full sail to windward in 20kts T.

 

[johnalison]

Is that in flat water or in a 2m sea? The performance could be fine in a flat sea but impossible in a 2m sea. My own boat would be in full main and genoa at 14kts T in a 2m sea. Once we see over 20kts apparent wind we switch down to working jib then progressively reef the main as the wind increases. We would never carry full sail to windward in 20kts T.

There is an old saying that you should reef to the sea and not the wind and there is a lot of truth in this. I was essentially referring to sheltered water, but in general I prefer to keep the boat fully driven. With my 110% jib and full main the boat becomes fully driven at a ridiculously precise 19kn true wind, my experience over twenty years with her. I have carried this sail to windward in up to 28kn, if only briefly, but in open water would reef earlier.

One designer, I think Chuck Payne, had a maxim that a cruising boat should satisfy the rule of 20s. That is, it should be able to carry full working sail in twenty knots of wind and heel no more than twenty degrees. This is easier in a larger boat, of course, but my current boat, which nearly fulfils this, is far more relaxing to sail than my previous boats that didn't.

 

[Graham376]

A lot of sailing boats are offered in two versions; deep keel and shallow keel. What are the disadvantages and advantages of each type besides of a shallow keel boat been able to sail in shallow waters?
Thanks

Our boat is offered with shallower Scheel keel (4'6") which we have and a deeper conventional keel (5'6"). Sailing in company with deeper keeled one from Northern Portugal to Camaret, there was no noticeable difference in sailing performance or leeway but we were cruising, not racing. In beam seas, we seemed to roll less than the deeper keel.

 

[johnalison]

Our boat is offered with shallower Scheel keel (4'6") which we have and a deeper conventional keel (5'6"). Sailing in company with deeper keeled one from Northern Portugal to Camaret, there was no noticeable difference in sailing performance or leeway but we were cruising, not racing. In beam seas, we seemed to roll less than the deeper keel.

That was not my experience. I have sailed with and against sister ships several times, and the only time I felt disadvantaged was when sailing with a rally from Cherbourg through the Alderney race. It was a broad reach and fairly light wind, and a boat that we had pulled away from in flat water the previous leg overtook us. It was obvious that this boat was keeping more wind in its sails as it was rolling less in the difficult conditions. Earlier, I had thought that a foreign-owned sister was rolling less when it was the next boat along on an alongside berth.

 

[RJJ]

It's one of those "horses for courses" things. We draw 2.3m. so far I have more greatly valued the extra pointing ability. If you point 2 degrees lower in certain conditions, then over a 15 mile beat that's enough to delay your supper significantly. For passage-making and weekending that makes a difference to our lives.

As I think is stated above, it may or may not vary the CoG but almost certainly affects leeway. A longer foil gives you more lift for the same drag. If it doesn't affect the CoG then the keel is presumably heavier, which again affects performance, upwind and down.

It surely depends where you are and what you need to do. For central Channel, there aren't many places where 30cm would make much difference. If you want to go up some of the Normandy rivers you would just choose a smaller boat. For the east coast, or the Bahamas, there are probably enough occasions when draft calls are marginal.

 

[Laminar Flow]

Our boat is offered with shallower Scheel keel (4'6") which we have and a deeper conventional keel (5'6"). Sailing in company with deeper keeled one from Northern Portugal to Camaret, there was no noticeable difference in sailing performance or leeway but we were cruising, not racing. In beam seas, we seemed to roll less than the deeper keel.

The Scheel keel is a 'type' of ballast bomb/endplate and as such it allows for a lower CG than a standard fin and less draft. A longer (fin) keel is also more stall resistant than a deep narrow one and it is quite possible that such a keel will be more able to resist rolling. Passive roll attenuation fins also tend to be long and shallow.

There was quite some enthusiasm for wing keels after it's debut on Australia in 1983(?) and all sorts of boats were fitted with one. I remember reading a comparison test by a German sailing mag done on two Contest sister ships.; one of them with a wing, the other with a standard fin. The fin was faster on all accounts. It was found that the wing likely generated additional turbulence and drag in a seaway, slowing the boat down. It did have a much shallower draft.

Delft university did an interesting comparison evaluation for different keel configurations including: deep fin, shallow long fin, Scheel keel and wing keels. (and others)

The results are in above order and in hours of time on an Olympic course.
First number at 15kt wind, second at 25kt
Deep fin, 3.96; 3.52
Long fin, 4.14; 3.72
Scheel keel, 4.10; 3.64
Wing keel, 4.01; 3.53

Edit, shallow added

 

[Laminar Flow]

Just to further the point regarding the Delft institute keel research: the comparison was between deep keels with 2.29m draft and shallow keels with 1.38m.

The shallow long fin with 40% less depth was 5% slower over a course of which 60% is to windward.

In the higher wind speeds the Scheel and wing keels were able to somewhat narrow the gap to the deep keels.

In cruising terms a 5% advantage in windward performance for a deep keel over the benefit ofa 40% reduction in draft is immaterial. IMHO

 

[johnalison]

Just to further the point regarding the Delft institute keel research: the comparison was between deep keels with 2.29m draft and shallow keels with 1.38m.

The shallow long fin with 40% less depth was 5% slower over a course of which 60% is to windward.

In the higher wind speeds the Scheel and wing keels were able to somewhat narrow the gap to the deep keels.

In cruising terms a 5% advantage in windward performance for a deep keel over the benefit ofa 40% reduction in draft is immaterial. IMHO

That's a much greater difference in draft than that of my boat, 1.5/1.8, which is also about the same as that of my friends who have had shallow-draft versions of Bavaria and Hanse. For this kind of comparison, the speed differential would be much less than the 5% quoted. In very light winds I did well in some racing, which we thought might be due to the lower wetted area.

 

[James_Calvert]

There must have been another thread like this quite recently because I've said some of this before.

When we bought our Sadler 32 we were East Coast based so specified the shallow fin (4'6") rather than the deep (5'6"). It made a useful difference accessing sills, transiting spitways, and taking short cuts. Also it provided a longer and so more stable keel base for drying out alongside.

I had always wondered whether we missed out to any degree on windward ability, but although all things being equal it may be the case, the boat proved itself more than capable in a Round the Island race a few years ago. We were beating into a F5, gusting 6 all the way to the Needles, then a quiet run, but no chute up behind the island and then another beat past Bembridge to the finish.

My Laser racing son mainly helmed and brought some keen dinghy sailors who were great at getting us quickly through the tacks and resuming their spots on the rail. This gave us a terrific edge over similar cruisers, and in fact we came second among the Sadler 32s, and did very well against most other cruising type boats of up to 36 foot on uncorrected time.

We might have done even better had we not split the mainsail under the second reef at the start line, so couldn't shake it out for the run.

My point is it's not so much what you've got, but how you use it that made the difference for us that day. The shallower keel just wasn't material.

 

[geem]

There is an old saying that you should reef to the sea and not the wind and there is a lot of truth in this. I was essentially referring to sheltered water, but in general I prefer to keep the boat fully driven. With my 110% jib and full main the boat becomes fully driven at a ridiculously precise 19kn true wind, my experience over twenty years with her. I have carried this sail to windward in up to 28kn, if only briefly, but in open water would reef earlier.

One designer, I think Chuck Payne, had a maxim that a cruising boat should satisfy the rule of 20s. That is, it should be able to carry full working sail in twenty knots of wind and heel no more than twenty degrees. This is easier in a larger boat, of course, but my current boat, which nearly fulfils this, is far more relaxing to sail than my previous boats that didn't.

May be Chuck Payne designed his boats with big engines and large fuel tanks! For me the ability to sail in light winds is far more important than being able to carry my full sail plan in stronger winds. You can always reef you sail plan when you need to.

 

[Laminar Flow]

What the Delft studies show is that at lower speeds the Sheel keels and wing keels obviously have more form resistance than a deep trapezoidal fin. As speed builds, the individual resistance of appendages, including props etc, becomes a less significant contributor to overall resistance.

Frictional resistance due to wetted surface rises quite gently and very much parallel to form resistance up to a relative speed of 0.7. At a relative speed of 1.0 the curve for form (wave)resistance diverges sharply and exponentially. For the short rise from a relative speed of 1 to hull speed (1.35) will require four times the propulsive force.

Overall, the increased resistance for a larger wetted area due to a longer, shallower fin is immaterial. The difference in performance is caused by less lift at slower speeds and hence greater lee way and a possible reduction in righting moment in strong winds. To compensate it pays to increase light air speed and performance with a larger sail area, as Geem pointed out, and this is the single best improvement to an average cruising performance possible.

To be clear, the well documented average (relative) speed for modern (a loose term) sailing yachts is 0.9 or 1.63 if you're metric (That is for a 28.2' DWL 4.8kts, for example). A quick glance at a resistance curve clearly shows why and, a decent sail area/displacement ratio provided, all or nearly all boats, including long keels, long shallow fins can achieve this average speed under normal cruising conditions.

Racing is a different matter as it is for ascetics, who thrive on freeze dried foods, don't carry intoxicating beverages in heavy glass containers and cut the handles off their tooth brushes to save weight. Form or wave making resistance is closely linked to displacement and difficult to reduce if you want to live respectably on an average size boat.

 

[LONG_KEELER]

Just a thought.

You would think that long keeled boats would roll less because of the damping. Not so for me. I expect that most have a narrow beam which doesn't help.

 

[johnalison]

May be Chuck Payne designed his boats with big engines and large fuel tanks! For me the ability to sail in light winds is far more important than being able to carry my full sail plan in stronger winds. You can always reef you sail plan when you need to.

Hmm. I have no apprehension about reefing, in fact, I abandoned the old principle of reefing early when modern slab reefing came in. However, moving from a boat that needed reefing at the top of F4 to a boat that would handle much more was an enormous step in cruising enjoyment. In spite of this, my boat generally outperforms many AWBs with less ballast, blousier lines and less sail, in light conditions. It is, however, 20 yrs old, and I am not pretending that some new flyers would not be faster in these conditions. Our impression, sailing around northern waters, ie Channel to Baltic, is that winds have increased in recent years, and that much of our cruising was done in reefing or near-reefing conditions during this period.

 

[Laser310]

as a racing sailor, i find the leeway made by shallow draft cruising boats to be quite maddening - even when i'm not racing.

if you are not a racing sailor, it probably won't bother you

i own a 30ft boat that draws 6ft 3in. its performance upwind is quite satisfying - a pleasure, actually...

with a huge masthead asym chute, it is fast downwind too.