Scow stability explained

This has to be one of the best descriptions of the dynamics at play in scow sailing and why many people find the boat to behave so, well, kindly at high speeds and fresh winds.   The Melges 17 scow referenced here has many of the high tech innovations found in modern skiffs, but all scows enjoy the hull characteristics. 

This post was originally made on Sailing Anarchy.

Hello LS,

I’ve seen your attempt to discuss the M17 on the Sportboat Forum (talk about a hostile crowd!), and as Dinghy Anarchy is probably the closest fit you’re going to find for the M17 on Sailing Anarchy, I thought I’d write something to support your efforts (and as you can see from my posting history, I hardly ever write anything here). So if you’ll forgive my making a few comments, here goes:

1) A Scow is a Scow

First off, I understand why you’ve been describing the M17 as a sort of a cross between a sportboat and a dinghy and a catamaran (in terms of the sailing experience) — but as you’ve discovered on Sportboat and are getting hints of here (e.g., Pete M’s posts), doing so can irk those who love a particular type of boat and feel you’re somehow insulting their genre by making the comparison. So (if you’ll excuse my offering unsolicited advice) I think you should embrace the M17’s “scowness” and shift instead to explaining to folks why scows are so fun (e.g., as in Fugu’s observation that M16 husband & wife crews seem happy on heavy air days).

Now LS, as you regularly sail a scow, you can do a far better job at this than I (I’m a trapeze dinghy sailor, and my scow acquaintance extends only to MCs on an afternoon’s visit to Minneapolis); but from my own meager scow exposure and from what I’ve heard from talking with scow owners, here’s how I’d describe the scow experience to fellow dinghy sailors:

A. (Presumably) an M17 won’t have the light, “danceable” feeling of Fugu’s Swift, Pete M’s International 14, or (many peoples’) 505s; nor will it have those boats’ ability to punch through big waves, nor their hair-trigger balance, nor (in the case of the more extreme dinghies and skiffs) their outright speed, nor the 505’s all-weather ability. But what it will have is accessible speed in light-to-moderate wind: it has a (2006) US Portsmouth rating of 79.0 (compared with the 505’s 79.9), so it’s no slouch around the race-course — but more importantly it can be fairly easily sailed by couples, father/child teams, and folks who are no longer teenagers even at heart. 🙂

B. The accessibility lies partly in how a scow feels: unlike high-performance dinghies, which have to be balanced flat (which is part of the appeal!), upwind the M17 will heel over and take a definite set; that is, while it’s of course possible to under or over-heel the thing, the scow has a natural heel angle into which it likes to settle, and as such it’s forgiving to less-than-zenlike crews. In addition, it tracks upwind in a way that’s uncommon for many dinghies: when heeled, the underwater hullform is essentially that of a narrow 17-foot long canoe, and whilst the bow will certainly rise and fall with the waves (perhaps excessively so for those used to skiffs and modern dinghies), the boat won’t be easily thrown off course, and the steering isn’t affected by small changes in heel angle. This combination of speed, steady-tracking, and forgiveness-in-heel makes the M17 feel almost akin to a narrow (and unusually fast) keelboat when close-hauled, and the feeling of power is surprising in something so small and unballasted.

C. Off the wind is when a scow can feel most like a dinghy — but even there it’s a little “odd”. If there’s enough wind to plane, you’ll stop heeling and you’ll sail the thing flat. When you do that, the hull’s odd rocker (fairly flat fore and aft, with a pronounced bend in the mid-section) will bring much of the (flat) bow sections off the water, and the hull will sit on the flat aft sections, with the mid-rocker acting as a sort of shovel-nose. This is a fantastic planing shape in the old-fashioned sense of the word: contrary to the “stepless” transition of ‘9er skiffs, there’s an enormous gulf between planing and not-planing; and once planing there’s an enormous amount of noise, spray, and bouncing over the waves. This does not only feel extremely fast (because of all the commotion), it is extremely fast — not skiff-fast, but faster than many trapeze dinghies if the water’s fairly flat (and heaven help you if it’s not). For a dinghy sailor, the surprising thing here is how controllable everything remains even at top speed: not only does a scow’s (completely flat) hull shape naturally tend to plane upright, but when it does roll or heel, the heel angle doesn’t try to steer the boat and overpower the rudder. Hence broaches and death rolls are not a dominant concern, and as the boat is fairly heavy and the downwind sailplan fairly immense, overall there’s a feeling of tremendous power.

In short, while an American lake scow can be said to (sort of) feel a little like a big dinghy, and (sort of) a little like a small sportboat — in reality it’s unlike either, and it’s its own thing. And on a sheltered bay or lake, it’s great fun — have I got it right, LS? 🙂

2) The Melges 17 is Exciting Because it Brings Dinghy Technology to the American Lake Scow

I think part of the misunderstandings that have arisen in this thread have to do with the scow-focused marketing material that Melges has published, and which has been somewhat repeated here. To wit: as you (and Melges) have pointed out, the M17 is unique among American scows for having shaped (non-flat-plate) high-aspect ratio leeboards, a lightweight carbon rig, a one pull-hoist asymm/pole arrangement, and other pieces of technology drawn from the high-performance dinghy/skiff/sportboat worlds. When these elements are integrated into the basic scow platform (which has some formidable starting advantages), the result can be pretty amazing(!) — but if one cites these innovations without the “first time in a scow” proviso, it can sound like you’re making much broader claims, which then incites skepticism and hostility. (That being said, this has mostly been a kindly thread; folks like TeamFugu, Bistros, GybeSet (and 505 sailors in general) are — as can be seen in their many other posts — decidedly a class act!)

Now, the above notwithstanding, the M17 is a fascinating boat, as it’s the first real attempt anyone’s made to create a truly modern American lake scow in a manageable 2-person size. As mentioned above, scows have some major things going in their favor to begin with (for lake sailing in typical American summer conditions), and from all the reports, R/P have done an excellent job at bringing the genre up to date. It’ll probably be easiest to explain why by directly addressing peoples’ comments — and so if the rest of you will permit me….

3) Hiking a Scow is Not Like Hiking a High-Performance Dinghy

Bistro’s has pointed out that “Australians have been sailing deadly fast two man hiking boats for twenty to thirty years with similar performance and rigs.”:

Fast 2-man hiking boats have indeed been around for decades (in the UK and elsewhere as well as in Australia, and the Aussie-born Tasar has adherents in N.America)) — but as many people here have already found, they’re not the happiest boats to sail when fully powered up. There’s a reason so many posts in Dinghy Anarchy extoll the virtues of trapezes: when you race a high-performance hiking dinghy in heavy air, your speed through the water is highly dependent upon how hard you can hike. Upwind legs are thus a pain (literally, in the back, thighs, and abdominal muscles), and in a very short while you start thinking “Trapeze. Yeah. Good idea.”.

Scows are different. If you look at LS’s pictures on the first page of this thread, you’ll see that whilst people are (sort of) hiked out, they’re not killing themselves — instead, they’re mostly employing a lazyman’s / middle-aged sort of sit-hike, with their rear ends over the water and their upper bodies tilted at a reasonably comfortable angle. Now, the young’uns in the Glen Lake 2009 photo are hiking moderately hard — but even they are bent-legged and holding a position they (being young & fit!) can maintain quite easily. But more to the point, you don’t need hiking heroics to work a scow to weather in strong wind; hiking does help (obviously), but the performance gain per additional inch of body leverage is much lower with a scow than with a high-performance hiking dinghy, and if you want to relax a bit so your mind can focus on something other than how much pain your body is in, then you can do so without a huge performance hit. That’s why so many couples, father/offspring, and older sailors can be competitive in the M17 class — the hiking is much less misery.

4) Dinghy Intuition Does Not Fully Transfer to Scow Hulls and Righting Moments

I think highly of most everyone who’s posted here (and I’ve enjoyed posts from many of you for several years now) — but on this particular thread, GybeSet is displaying the best intuition. 🙂

As LS’s photographs show, the M17 (like most scows) has zero deadrise, little flare, and hard (though rounded) bilges. There is no skiff or high-performance dinghy with this set of characteristics, and indeed some dinghies possess the exact opposite. Couple these characteristics with the scow’s rectangular planform, and you have a hull shape that doesn’t behave as dinghy intuition might suggest:

A. The Center of Buoyancy Shifts Greatly With Heel
With most dinghies, the center of buoyancy lies more or less near the center of the waterplane area. That is, if you heel a dinghy (or skiff) hull so that the chine (or bilge) is immersed and the centerline is about to come out of the water, then the center of buoyancy lies (very roughly!) about halfway between the centerline (keel line) and chine. That presumably is the basis of Pete M’s assertion that the M17’s CB shifts only a foot with heel (Post #33). But with the scow hull shape, the CB shifts laterally much farther than that. To get a rough idea, look at the photo on Post #28, and tilt your head so that the leeboard is “vertical” in your field of view; that perspective shows you the immersed hull shape when the scow is sailing, and the center of buoyancy lies slightly inboard of the leeboard. Eyeballing the photo suggests the offset from centerline lies somewhere in the 20-24″ range….

B. Flowlines Don’t Distort With Heel
All high-performance dinghy shapes show marked waterline and flowline distortions when heeled — that is, if you heel very much at all, the immersed hull takes on a warped shape, and the passing water has to take strange paths to get around it. As a result, two (bad) things happen: the boat is slow; and the hull tries to steer itself in a direction the helmsman doesn’t want to go. In short, Heel Is Bad, and high-performance dinghy (and skiff) sailors have learned to sail their boats flat.

With a scow (and also with things like Open 60s), the hull is designed so that heeling yields an immersed body that’s straight and true — and which is narrower and has markedly less wetted surface than that of an unheeled hull (unless we start talking about things like International Canoes and narrow Moths). Hence scows go faster when they’re heeled (unless they’re planing flat), and they maintain control when heeled.

C. The Hull Adds Considerable Righting Moment
Because dinghy hulls have to be sailed flat, the weight of the boat has no beneficial effect at all: boat weight does nothing to promote stability, and all it does is inhibit boatspeed.

With scows, however, the (considerable) weight of the broad, flat hull adds considerable righting moment of its own (this is one of GybeSet’s points, which was subsequently ignored). To do a quick back-of-the-envelope estimate: the published weight of the M17 is 335 lbs. If we use the lower of the above estimates for lateral CB shift (20″), that yields a righting moment of ~550 lb-ft (note: the cosine effect at 15-deg heel is less than 5%). To put that in perspective:

* If we were to turn the M17 into a round-bilged keelboat, with a 3-foot deep keel bulb supplying stability, then to achieve 550 lb-ft of righting moment at 15-degree heel, we’d need 700 lbs of lead in the bulb.

* Or, if we made the M17 a sail-it-flat conventional dinghy and put one of Pete M’s 185-lb crewmen on a trapeze (i.e., assume 5’6″ beam, 6′ crewman with CG halfway up his body, laying perfectly horizontal), then that crewman would provide ~1060 lb-ft. That is, the M17’s weight-induced righting moment is worth half a trapezing crewman (on a conventional dinghy).

D. The Total Righting Moment is Not Bad

Finally let’s put (via back of the envelope) the M17’s total righting moment in perspective. If we put 320 lbs of tired sit-hiking crewmembers on the deck of the M17 (LS, is 320 a typical crew weight?), with their CG about a foot beyond the deck edge, then the total righting moment is:

Crew Righting Moment: 320lb * ((5’6″/2) + 12″ + 20″) = ~1700 lb-ft
Hull Weight Righting Moment (from above) = 550 lb-ft
Total Righting Moment = ~ 2250 lb-ft.

Compare that to a 5’6″-beam conventional sail-it-flat dinghy with single trapeze and equal-sized (6′) crew:

Hull Weight Righting Moment = 0
Sitting Crew Righting Moment = 160 lb * ((5’6″/2) + 12″) = 600 lb-ft
Trapezing Crew Righting Moment = 160 lb * ((5’6″/2 + 36″) = 920 lb-ft
Total Righting Moment = `1520 lb-ft

Pretty neat, eh? 🙂 You can play around with the above numbers (e.g., if you make the M17 crew completely lazy, and have them sit-“hike” with their CG right on the gunwale, you get a total righting moment of ~1950 lb-ft; if you make the conventional dinghy twin-trap, you get 1840 lb-ft, etc.), but the overall picture remains the same: a scow like the M17 has more righting moment (considerably more righting moment) than a single-trapeze sail-it-flat dinghy of comparable beam, and roughly comparable righting moment of a double trapeze dinghy (again of comparable beam). Of course, balancing that you have the scow’s additional weight (and trapeze dinghies can have hull flares, wings, and racks to significantly increase their beam), but in the end, scows have surprising amounts of righting moment for their size, and that’s why they can carry so much sail.

5) Sundry Points

A. LS, asymmetric high-aspect ratio leeboards are used by many “Open” ocean racing classes (e.g., Open 60, VOR 70), so the M17 is not unique among monohulls here — but your point is well taken. When talking with dinghy sailors, you can mention that these leeboards are like jibing centerboards without any of the problems of such; plus, as you’ve pointed out, by being asymmetric, they can run at smaller AOA than symmetric boards, with concommitantly less drag.

B. On rereading the above, I see I’ve not made clear why the M17 can be less painful to hike on than things like a 59er: it has to do with how far away from the center of buoyancy is the crew CG, and how much the hull weight already contributes to righting moment. With a conventional dinghy, hull weight helps not at all (not if you’re sailing it flat like you’re supposed to), and a hiking crew is sitting so close to the CB that every bit of lean makes a significant addition to the overall righting moment. With a scow, the hull weight is already doing something, and you’re starting already pretty far away from the CB; hence the % change in righting moment is less greatly affected by leaning. (To give a practical example: on a Laser, you can feel every bit of hiking angle on the boat trim and speed — but if you’re on a trapeze and make similar changes in upper body position, the effect on the boat trim is far less….) Hiking still helps of course — but the incentive to kill yourself is far less than it is with a conventional hiking dinghy.

C. If I’ve made a misstatement anywhere above, I apologize! Ditto if I’ve flubbed a calculation, and double ditto if I’ve managed to offend anyone. And apologies in advance if anyone writes anything in expectation of a reply: it’ll probably be a good part of a year before I post again here, so best wishes to everyone, and may you all please have a great winter & spring (and summer & fall for the Aussies and Kiwis)!


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