can anyone tell me what this boat is modeled after

well I got today off
and it sounds like I need to go study up on modern keel designs
any suggestions as to good reading
seems like you are pretty up on the subject
thanks
B

oh
Bntii and daiquiri
and I had found the squarekerry site but completely missed the part about the Fara having been found
thats fantastic
I cant wait to see her restored

 

There have been innumerable studies, papers, books, articles, etc. on keel design over the decades. Probably the most concise summary on keel design can be found in "Principles of Yacht Design" by Lars Larsson and Rolf Eliasson, third edition.

For a comprehensive discussion on aero-hydrodynamics, "Aero-Hydrodynamics of Sailing" by C. A. Marchaj (pronounced MAR-ki) is invaluable. This includes the basic development and state-of-the-art science on sailing, including discussions of hulls, appendages and rigs.

Marchaj also wrote "Sail Performance--Theory and Practice" and "Seaworthiness--The Forgotten Factor". These books are also invaluable for the understandign of sailing yacht design. All of these books should be on everyone's bookshelf for those seriously interested in sailing yacht design.

Eric

 

thanks
I think i have Principals of Yacht Design floating around here somewhere
Ive got High speed Sailing round somewhere to
I think
or I used to
its been a while
ok a long while since I read either
and Im not sure I found the time to read Modern Yacht Design
Ill call a friend of mine on this one
Sail Performance--Theory and Practice" and "Seaworthiness--The Forgotten Factor
he's got a used book wholesale house
so chances are he has it
thanks for the tips
B

oh and hey guys I found a lines drawing
and framing details
also a sail plan and static drawing
no wonder they break ribs
thing is build light as a feather

a little bird tells me a few white oak stringers would really help

 

Hey Eric
was reading about heavy seas and boat stability
the more I read
the more I like your boat
I still havnt gotten to keel forms yet
but I had a question
is your keel some kind of variable ogivoil section or something like that
or is it a fixed shape
I notice your mast rotates and was thinkin maybe you got tricky with the keel as well
also notice some wings on the ballast bulb
whats that all about Bart
arent those typically for light wind sailing
designed to lift the hull a little and so reduce friction

 

Boston,

The keel on the SC 40 is a constant section modified GA(W) aerofoil of 10% thickness. The rudder is the same section, by the way, just scaled down. The keel has to be a constant section so that it fits properly in the keel casing (close clearances) inside while it slides up and down.

The bulb is shaped to what I call a Beavertail/Swallowtail bulb (BS bulb, for short--I like the pun). The Beavertail aspect--flat beavertail shape on the aft end--is to form an end plate for the keel blade. Without an endplate, the flow across the keel will tend to slide down the keel and off and under the tip, thereby creating a trailing vortex. The creation of the tip vortex is the very essence or signature of induced drag. The bigger the tip vortex, the more non-uniform is the flow over the keel, the higher is the induced drag. The more you reduce the tip vortex, either by planform shape, foil twist, or by end plate, the lower is the induced drag. Therefore the keel is more efficient. Efficiency is defined as the lift-to-drag ratio, L/D. With lower induced drag, there is less force holding the boat back in speed made good to windward (VMG), therefore the boat is faster in VMG.

Another way to reduce tip vortex is to change the keel planform shape, either to a more elliptical shape, or by twisting the keel section so that the leading edge of the keel actually points a little to one side. Well, with a lifing keel, you cannot use an elliptical planform shape because in a raised position the keel does not fit snugly into the hull. And you can't twist the keel because it has to work on both tacks. To try to twist the keel while underway would be a mechanical nightmare. So for a lifting keel, you are left with a constant section and constant chord length, and you have to fit it with an endplate. Since the boat's stability favors and requires a low center of gravity, it is natural to make the end plate a ballast bulb. This, you can see, helps me have a range of stability (AVS = angle of vanishing stability) of 180 degrees. This is very rare in yacht design. I achieve the same thing on my design Saint Barbara, which you can also see on my website.

As to the swallowtail feature, flat and square-cornered plates shed their own vortices and create induced drag. Theoretically, one can conceive of a keel design whose endplates have endplates, and then those endplates have more endplates, ad infinitum. You'd actually be overcome by form drag and delicate shapes very quickly. But subsequent to the 1983 historic loss of the America's Cup to the Australians, the Americans did a broad array of studies on keel shapes, winglets, and endplates, and they discovered that if you sweep back the tips of the corners of the endplates into a swallowtail shape, you actually reduce the size of the tip vortex, thereby reducing the endplates own induced drag. So I started putting swallowtail shapes on by beavertail buld designs and came up with the BS bulb. The above is NOT BS, by the way, you can look it up, and there is foundation for the science.

I might add that you could conceive of a keel with no bulb, but with a scimitar-shaped tip wherein the tip of the keel (or rudder) is swept aft. This, in fact, is becoming more common. One sees it in airplane propellers a lot these days, and you also see it in the tips of windmill turbine blades. Also, you will notice in my rudder designs, including that of the SC 40, and on Saint Barbara and Bagatelle, that I sweep the leading edge of the tip of the rudder aft. This is contrary to older rudder and keel designs of the 1980s and 90s where, instead of the leading edge at the tip sweeping aft, the trailing edge is swept forward. This is an effort to approximate an elliptical planform shape which tends to give the rudder an elliptical lift distribution along the span, which creates the least amount of tip vortex, and therefore has the lowest drag. This is true in concept. But I am of the opinion that such rudders (and keels) with swept-forward trailing edge tips are actually backwards. Rather, the planform shape should be flipped back to front, thereby giving a more scimitar shape to the planform with a swept leading edge tip. This will be a lower drag shape than the reverse.

Eric

 

The first picture really makes me think 22 1/2 square meter class. (my grandfather had one in Norway)

These boats sail very nice and point extremely high in sheltered water.

In big waves they are deathly. Almost impossible to control, pound to windward, and there is at least one report of submarining (never re-surfaced) downwind.

Good examples of what happens when trying to be as fast as possible within the constraints of a racing rule. Something that hasn't changed, the rules have changed but they produce distorted boats the same.

 

way to answer the question Eric
and ya the part about the vorticies causing drag really hit the spot
I think it might make a nice little alteration in my pond sailor to build it with a steal keel and a bulb
( thinnest of possible forward profile and strong as all hell )
wouldnt the bulb negate the trailing vortex sufficiently to eliminate the need for the platform
I know some airplanes have fuel cells at the wing tips instead of those little vertical stabilizers
or does it have something to do with the density difference of the two mediums that one need additional vortex control

hey T
drove under is what my grandfather used to call that
he taught physics at Norwich university but had worked his way through his own schooling as a shipwright
my family has been shipwrights as far back as anyone can remember
he and my dad beat the craft into me and I ended up doing the same
( working my way through school with it eventually, I just stuck with general contracting )
drove under is rare
usually u sail yourself into a breaking wave or a hard chop and just keep going
but you can also rotate of the top of one and find yourself pointing straight down
takes a really rough sea though
you get that kind of water off the east coast were the Labrador current and the gulf stream meet
I hear it happens in the Bass straight and a few other places, but I never sailed there so what do I know

thats one reason Ive considered a pilot schooner for my run up to ketchikan
or the friendship sloop
either is beyond tried and true in that kind of water
and pilot schooners being specifically made to sail in the roughest of conditions
with a capsize screening factor of about 1.4 on the one Im looking at
and a point of vanishing stability of something like 155deg

and the friendships being fishermen originally



I think Ive decided on this one for my pond sailor
and have found what I think are the lines for it



now to see about lofting it
something Ive not done very much of in say thirty years or so
I dont have room to loft if full size so half will have to do
and Ill fair it on the forms with the stringers since Im keeping em in anyway to stiffen it up some

 

Not sure what you mean here by "platform". There are bulbs and then there are bulbs. Some bulb shapes will work better than others in reducing the tip vortex, and then the bulb itself has its own vortex. To be really accurate one would have to go into the wind tunnel or towing tank to view flow around the keel and bulb. The differences in mediums are taken into account by proper scaling to Reynolds number. This is why keel and bulb shapes can be tested in a wind tunnel where smoke can be seen more readily than dyes water in a towing tank.

Eric

 

well on your site
( nice site by the way )
on the first rendering you show a keel bulb with a flattened area towards the back
rather than a cylindrical section
I called that a platform
I guess I should have called it a wing
didnt have em when I was a kid so Im not so familiar with the terminology for em today

I was looking through keel shapes
which kinda answered the question for itself
as the most modern ones are torpedo shapes
with no wings

 

Apparently not too many Swedish people in this forum.

The yacht in the original question is a M-30.

Belongs to a family of M-15 / M-22 / M-25 / M-30 yachts named after Lake Mälaren, the lake on which the capital of Sweden (Stockholm) is situated.

The number is the sail area in sqare meters.

Both early all-mahogany and later fibreglass versions exist.

The owners club webbsite is at http://mbat.nu (All in Swedish I am afraid)
There is a gallery of various M-boats racing at http://mbat.nu/foto/foto.html

Designed in the 1930's by Lage Eklund.
LOA 11.5m
WOA 2.06m
Draft 1.4m
Displ 2.4t

The original lines drawing:



/jesper

 

way to go
do you have the rest of those lines drawings in there somewhere

most folks have been really helpful about what these boats do well
Ild love some additional input on what they dont do well
obviously they are going to have trouble in rough water

Im likely to try this one



but all I have is these miniature lines for it
Ill try the sweedish site
Ive got a translation program in here somewhere
but any aditional input it greatly appreciated

thanks
B

 

What they do well ?

Let's see....

Look extraordinarily beautiful, go to windward better than downwind, being very small to live 4 people in, and get the crew and interior wet as soon as there are waves.

That's about it

My advice if you're interested would be to contact the M-båt association.
I think there is one wooden new being built. And the forms for GRP building are available.

kristina.lundevall@mbat.nu is the official contact. I'm sure she speaks english and can help you further.

...that's gotta be worth some rep ;-) !

/j

 

Well, I do have the benefit of being of 100% Swedish descent, even though I am third generation American.

This lines plan that you present was also offered to me by my client for the SC 40 as a representative example of the kinds of lines that we should use as a model, so that particular plan did provide me with some inspiration.

Eric

 

thanks guys
I just lost my job this morning
so Im kinda "dead in the water" till I think of something
maybe I should start building boats for a living
its always been what I wanted to do anyway
that or marine biology
( neither likely here in Colorado )

well done working with it Eric
drawing is all of about three inches by eight
would drive me nuts to have to extrapolate the fuzzy bits
its dam near starting from scratch

hey Jesper
good info
does kinda look like it would be easy to broach
and the right wave would help that out just fine
little push and round you go
pretty sure they dreamed up fin and skeg for just that reason
and the lack of free board means your going to be bailing
course I'ld just be goofing around in the thing and not likely to set up a pole and a spinnaker anyway
hell Im kinda lazy about my sailing and sorta get things set and just tack my way about on a course that lets me not even touch the sheets
its a lake
not like Im actually going any were anyway
kinda like my job scene
argggg

dam the hole job thing is killing me
I was really looking forward to building something

there is a star class races round here all the time
pathetically easy to build ( chine form = plywood, cheep cheep cheep )
and its rigging is locally available
also I could get a wrecked one and salvage the bits and pieces
maybe

ugh
I gotta quit thinking about building anything till I get this job thing resolved
what a pain

on the bright side
I think Ive got enough stuff round here to whip up a nice Bloody Marry

 

Just don't end up hanging in the same thread with SH
Sorry Sean couldn't resist
pouring Capt. Morgan, stirring in my mouth, no shaking.. Got promoted last week.. no more norwegian moonshine..