Hull shape for simple sailing scow

Hello,
I'm looking to build a simple sailing (and rowing, and maybe motor sometimes) scow (about 8' long, 4' beam) and I'm wondering about hull shape.

I've seen a lot of plans around the internet for little scows like this made from plywood with vertical sides and sharp corners between the side and the bottom of the hull. I'm planning on building my boat with fiberglass over a foam shell, so I have a little more design freedom than the plywood builders have.

My question is mainly, how will a smoother bottom shape (rounded chines instead of the sharp chines on a simple plywood scow) affect the behavior of the boat (stability, speed, sailing performance, etc).

Most pictures of "professional" small-to-medium size scows I've seen appear to have a fairly rounded hull. While I want good sailing performance (emphasis on ease of sailing more than speed), I also want to be able to fish from the boat, use it as a swimming platform, row it, motor it, etc, so stability is important.

 

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Welcome to the forum! I think at an LOA of 8' you can't really take advantage of a scow shape-something like an Optimist pram will probably be as close as you can get. A sailing scow is designed to sail at an angle of heel because that dramatically reduces wetted surface.
If you want maximum stability then a hard chined shape like the Opti is best on such a small boat. The rounded shapes on some scows are there so that when the boat is heeled to its best sailing angle wetted surface goes to a minimum-the idea is to achieve a long narrow shape to the immersed portion of the hull when heeled.
Goog luck!
Butterfly
Current Specifications
Crew 1-2
Type Monohull
Design Scow
LOA 3.632 m (11 ft 11.0 in)
Beam 1.372 m (4 ft 6.0 in)
Draft .660 m (2 ft 2.0 in)
Hull weight 61 kg (130 lb)
Mast height 5.486 m (18 ft 0 in)
Mainsail area 6.967 m2 (74.99 sq ft)

Just for the heck of it the following is an RC model with a partial tunnel hull and very rounded shape so when heeled it was the lowest possible wetted surface. The third picture is the smallest real scow I know of the 12' Butterfly:

click on image:

 

I'll probably end up going with some happy medium - part of the reason I wanted to round the chine is just for looks, and I suspect the fiberglass would be less apt to get damaged without the sharp corners. Since I don't need the maximum speed I'll probably keep it a little on the boxy side for stability.

Thanks for the info!

 

8' is too short for what you plan. 11' works nicely. Here is one I designed, built, and have been trying to destroy or wear out for 16 years. It's more of a Garvey than a scow and is derived from the 9'6" Claud Worth Auray rowing dinghy lines.

 

You may want to take a look at the OZ Racer from Michael Storer or the original PD Racer. These are square, right angled and beautiful to the right eyes (and a sharp stick in the eye to others).

Hard chines and somewhat brutal lines do contribute to very high form stability and exceptional weight carrying ability - at the expense of traditional "pretty". I'm a fan of Michael Storer's Goat Island Skiff - it is a lot bigger, but you end up with a far more versatile boat that actually can sail, row and motor well. It isn't a stable swim platform though.

There are lots of choices to look at ... look at Duckworks and all the other sites out there as well as the galleries posted by designers here. Try to keep your first build simple and cheap - you will learn a lot and also get fast gratification in the form of a boat. Your first will not be your last build.

This web site has got a lot of great resources in the people that frequent it. The building part is pure fun for me - even the days of sanding and fairing.

I'm beginning to admire the simple genius of a boat like the Goat Island Skiff which can do more than one thing adequately. I've got four boats and now I'm thinking of building another. I guess I'm going to have to sell off some to make room.

--
CutOnce

 

small light sailing scows with large flat surfaces sail faster because the hard chines actually reduce the drag when at planning speeds. The drag is lower because more of the hull is out of the water when up on plane. Smooth curved and rounded corner hulls have reduced drag when in displacement mode, so it depends on your design intent.

If it will be mostly used with oars or a small motor at slower speeds, or heavily loaded, a rounded displacement hull would have less drag. If it will be faster and lightly loaded, a flat bottomed planning designed hard chine hull would be better.

Stability is a complex issue as well, and more to do with max beam, and center of gravity location, as well as hull shape.

If you want to build something your self, find a successful design about the size you want and build it according to the plans. Do not worry about finding the "perfect" design, just build it and enjoy it.

You might go down to a public boat yard and see if you can rent or borrow various different small row boats/sail boats to get a feel for how the different hulls behave. Similar designs usually behave in a similar way.

 

Before you design even a simple scow, really deeply examine how you will actually use it. Rowboat fishing with kids? High speed sailing to make you excited? Motoring into a bad chop with 3 adults? None of these are really the same boat. The job designs the boat, always remember.
I'm sure you have been inundated with designs by now, but stay focused on your design goals.
What does this boat actually do for me?

 

Hospadar, if you have to ask the questions you've asked thus far, then you are nowhere near skilled or experienced enough to consider designing your own boat. This isn't an intentional dig at you personally, but simply a blunt observation.

Plans for these types of craft are cheap and many are free. You can attempt self design and end up with a boat that can't possably balance and sails horribly or you can build from a set of $50 or less plans, with a very probable successful end product.

The short of it is, you can build a boat that is a God awful thing to sail, just as easy as one that is exciting and well balanced, right out of the box. Trust me, launch day can be embarrassing enough, but it's much worse if the boat can't remain upright or flops over on every tack or can't tack or doesn't steer or doesn't respond the way you want, etc., etc., etc. Download the free plans for a Tern or a Sea Flea or what ever and have fun. After some hydrodynamic and engineering study, maybe you can take a shot at self designing your next project.

 

The smallest scow I know of is the Northbridge Junior, designed by Frank Bethwaite (father of 49er designer Julian). It's a sweet and ultra-simple little thing, very much like an old scow Moth. I think it's 2.4m long and claimed to be 15kg in the hull.

It's very much in the Optimist class (the Opti is replacing them although they are slower and in some ways inferior) and therefore probably way too small for what you want.

http://davesworkbench.blogspot.com/2008/07/northbridge-junior-revamp.html

Another size up and well proven is the old ('40s) Mk II Moth, still sailed in New Zealand as a one design class - and getting more boats to its national titles than the foiling Moths do!

http://www.moth.asn.au/download/mkII_moth_plans.pdf

These are much simpler than later Moths, of course, but with their 80sq ft (approx) rig, these are roughly comparable with Laser Radials for performance. Again, quite a small boat but they could row and motor OK on fairly flat water.

With respect to some of those old plans in boat-design books, you may well find that these boats, designed by top class designers in classes where a vast amount of information was shared, are much better performers.

 

http://www.youtube.com/watch?v=wWkd2e8apSI

Scow moth plans:
Download the last know blueprint plans of the Mk II (PDF - 2.9Mb)

Download the Cole Super Moth plans (PDF - 54Kb)

Moths are 12 ft instead of the 8 ft you're looking for, but you might be able to adapt the shapes. It might even be possible to put in a bulkhead at 8 ft so the whole bow would be removable, like a nesting dinghy. Then you might be able to store and transport it in 8 ft, and even use it as a dinghy, but still be able to restore the buoyancy and waterline length for sailing at 12 ft.

 

Howdy,

If you are looking at a straight sided scow - the two side panels straight and parallel you have to sail the boat pretty level or the corners of the bow and stern transom dig in.

It could be counteracted by adding more rocker, but that could result in hobby horsing.

That's the down side.

The upside is the huge stability allowing quite large sails to be carried safely.

The OzRacer/Oz PDRacer has a mainsail of 82 square ft and handles it very happily.

Best wishes with your project!

 

You can radius the chines, which will subtract a little displacement and stability and add a little performance, depending on how large you make the radii. I would think 1 to 3 inches might be good if you are planning on a turtle or pdracer type hull. The fiberglass will go around the radii much easier than it will take sharp corners.

Your boat may need some kind of frame work that is more elaborate than a plywood model, due to the lesser stiffness of fiberglass. But this framework can also be made of fiberglass and incorporate seats and decks.

 

With chined raceboats, generally we would round off the chines up to the widest point of the boat - normally behind halfway, then make them and the transom edges sharp for water release when planing.

But it is struggling over a tiny bit of a percentage.

Rounding is a good theory, but I don't see any evidence that there is any particular disbenefit from sharpish chines (meaning rounded just enough for glass tape) along the whole length. I can sometimes see a little vortex of the chines of the OzRacer and other PDR variants, but never off the also chined Goat Island skiff or BETH sailing canoe, which should theoretically have more of a vortex because of the chine intersecting the direction of travel.

And all these boats perform very well by any reasonable standard. (body hiked dinghies of about the same length)

And we know there are serious vortexes around the sail, and the foils, which are probably a scale or two of magnitude higher because of the lift being generated.

I don't think rounding the chine gives enough benefit to make the extra complication worthwhile.

At least until somebody gives me some significant numbers

Best wishes
Michael Storer

 

Hi Mik, good to see you here again. I agree in that rounding over a chine makes little difference in planning craft. In fact, I'd be willing to argue that this has the potential of robbing speed, not improving it, on boats capable of good speed/length ratios. This said a heavy radius (large diameter) is a different matter, which will on some hull forms make it appear much like a round bilge, though I still don't see an advantage, other then looks. Parasitic drag will increase on a heavily "softened" chine and you lose the ability to shear this flow off the hull.

I will still argue that the original poster is in way over his head, if needing answers to these types of design element questions. He hasn't been back to this thread since I suggested this the first time, so I've probably pissed of another one . . .

 

Inland Lake Scow

This thread was about a simple scow at ,really, too short a length. I thought I might give the design rationale that was given to me many years ago by a great scow sailor. This design theory is NOT applicable, at least very well, to a scow design below about 16'.
It's interesting to note that every performance scow-designed as an inland lake scow has a rounded chine and I think it is the M20 that is a tunnel hull.
These shapes -at least from 16' on up form narrow low wetted surface shapes when heeled which is how a "real" scow is sailed most of the time. The shape allows the boat to sail at substantial multiples of hull speed without planing though planing can occur in stronger winds at boat speeds begining around 15 + knots. From personal experience the 28' E scow was equivalent or just a bit faster in speed to a 20' Shark catamaran in 1968.

See the rough sketch below of a tunnel hull inland lake scow. The design rationale is to get the immersed section of the hull to be long and narrow at the waterline-10/1 L/B or better at about a 15 degree angle of heel:

(click on image)