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View Full Version : What's more traditional: Square mast or round mast?



gmeadows
02-08-2016, 11:05 AM
I am building a 20 foot knockabout with a mast shown to be a maximum 4 inch diameter on the plans and 20 foot in height.

What would be more traditional, a round mast or a square mast? It's a 1915 BB Crownenshield Dark Harbor 12. I plan to make the remaining spars round.

Part of my reason for asking, is because I have a 2 x 7 ( actual dimensions) x 20' long piece of Sitka spruce. Believe it or not, the section properties of a 4 inch diameter master versus a 3 and 1/2 inch diameter mast are almost double. A 3 1/2 inch square mast would provide me with section properties that would be adequate for the strength required.

David G
02-08-2016, 11:18 AM
Are you looking for most 'traditional'... or most 'period/designer-appropriate'??

Most traditional of all is back to the roots. A grown stick. Debarked, Shaved a bit to fit the mast step & partners. Beehole at the top for a halyard dumb-sheeve.

Ian McColgin
02-08-2016, 11:23 AM
"Traditional" is probably the wrong word. Affordable, buildable, and good use of available wood are better thoughts. But before any of that, is the design for hollow or solid.

Rectangular sections, such as LastBoat will get, are often called for to combine well engineered lightness with affordability and ease of construction. It's also a very efficient use of the wood. I'd thought about oval bird's mouth for LastBoat but the masts would require more raw lumber, more epoxy, and a great deal more labor for a structure that's no better.

On the other hand, it sounds like you have the material to lay up a solid round stick with little waste. Before you do the glue up, router an inch or so U on each opposing face to give a wire channel and/or to get rid of a little wood that contributes nothing to the ultimate structure. If you aim the grains (you don't say how the grain runs) rightly, even if you have to resaw and make the blank of four pieces rather than two you'll end up putting less of the wood into sawdust than most any other method. You can also build the tapers on the fore side and port and starboard sides in before you glue. Then eightsiding and rounding go quickly.

G'luck

Jay Greer
02-08-2016, 12:12 PM
Round is certainly much older than square in the traditional mast cross section department. The square mast was introduced by L.Francis Herreshoff who advocated, correctly, that a square mast is nothing more than an elongated box and, anyone who can build a box can make a mast as well as it is just as efficient as a round one. Certainly, as Ian, mentions, building a square mast, is a much more effient use of materials. One is much better off building one on a spar bench which, is quite simple to construct.
Jay

Peerie Maa
02-08-2016, 12:13 PM
^Then again square is about as aerodynamic as a brick, and as a sail is supposed to be aerodynamic . . .

Go with Ian's built up solid round stick.

Jay Greer
02-08-2016, 12:15 PM
Herreshoff made extensive tests of mast shapes and found that square and round don't make much difference.
Jay

Thorne
02-08-2016, 12:16 PM
Traditional = round

andrewdarius
02-08-2016, 12:41 PM
There are the gaff jaws to consider too.

Peerie Maa
02-08-2016, 12:44 PM
Herreshoff made extensive tests of mast shapes and found that square and round don't make much difference.
Jay

With the technology and sail cloth available at the time, that could be true. ;) :D

Ian McColgin
02-08-2016, 12:51 PM
Round, oval or rectangular all present turbulence to the sail's luff. Round has the same turbulence no matter what direction the wind. Rectangular might present less on a beat. Some ovals might present more on a beat but some ovals are designed to present less. If you really care, make a rotating foil mast and g'luck on the engineering.

Peerie Maa
02-08-2016, 12:55 PM
Round, oval or rectangular all present turbulence to the sail's luff. Round has the same turbulence no matter what direction the wind. Rectangular might present less on a beat. Some ovals might present more on a beat but some ovals are designed to present less. If you really care, make a rotating foil mast and g'luck on the engineering.

Don't forget, the only time that a rectangle or oval presents minimum frontal area to the wind is when the wind is dead on the nose. In which case you are going backwards.
The very best shape is the "strawberry" section patented a few years ago.

Jay Greer
02-08-2016, 01:07 PM
Unless one is seeking America's Cup efficiency where fractions of seconds will win or lose a race, a rectangular cross section is more efficient on the wallet! Square is easier to rig in that tangs can be fashioned and fastened much more easily and more cost effectively. It is also much easier to paint or varnish if material is to be appied by brush. I, personally, prefer the look of a round mast which, is certainly a must for gaff rigged boats. One might argue that if square masts are better for boats then God would have grown square trees. But, each form has its virtues! In the end, I would not choose to put a round set of masts in my H28 as skipper's ashes would boil up out of the sea.
Jay

Peerie Maa
02-08-2016, 01:13 PM
^ If birdsmouth had been invented when H28 was being drafted, she would have had round or oval. :D

JoshuaIII
02-08-2016, 02:54 PM
I've build several masts, small to 45+ft.
It's a fun job to do, and a piece that you actually work with everytime you sail :)

Square is a bit quicker, but it's not really long doing it round too. Once the table saw is setup, you do all your cuts and glue.
Steps are important.

I would say round, just because it look more "right" ;)

P.I. Stazzer-Newt
02-08-2016, 04:29 PM
Sleeve luff - then nobody will know.


More seriously, there is something to be said for square to the gooseneck - provides nice flat space for mounting winches, cleats or whatever, also makes tabernacle design easier.

Ben Fuller
02-08-2016, 05:08 PM
The OP noted that he is doing a known 1915 design by Crowninshield. Period correct would be round spar, spliced rigging, no tangs.

Bob Cleek
02-08-2016, 05:31 PM
Unless one is seeking America's Cup efficiency where fractions of seconds will win or lose a race, a rectangular cross section is more efficient on the wallet! Square is easier to rig in that tangs can be fashioned and fastened much more easily and more cost effectively. It is also much easier to paint or varnish if material is to be appied by brush. I, personally, prefer the look of a round mast which, is certainly a must for gaff rigged boats. One might argue that if square masts are better for boats then God would have grown square trees. But, each form has its virtues! In the end, I would not choose to put a round set of masts in my H28 as skipper's ashes would boil up out of the sea.
Jay

Quite true, but I'd say only applicable really for jib-headed sails set on a track. Gaff isn't going to work with much of anything besides a round mast (unless, of course, you are going to set the gaff boom on a gooseneck affair that calls for a lot of weight aloft!) If the sail is laced to the spar, mast or boom, round makes for far easier setting the sail and less chafe on the luff lacing line. No question, though, if the spar is going to carry a sail track and a lot of "Marconi" rigging, I'm in the box-section camp.

Ben Fuller
02-08-2016, 05:37 PM
Quite true, but I'd say only applicable really for jib-headed sails set on a track. Gaff isn't going to work with much of anything besides a round mast (unless, of course, you are going to set the gaff boom on a gooseneck affair that calls for a lot of weight aloft!) If the sail is laced to the spar, mast or boom, round makes for far easier setting the sail and less chafe on the luff lacing line. No question, though, if the spar is going to carry a sail track and a lot of "Marconi" rigging, I'm in the box-section camp.

Dark Harbor 12 of 1915 was a gaffer similar to the NGH boats of that era, all gaffers with round sticks, spliced rigging. BB Crowninshield would have accurately speced the spar and rigging. You don't start seeing tracks and box sections until after WW1. As someone noted above, LFH claims to have invented the hollow rectangular spar.

Bob Cleek
02-08-2016, 06:00 PM
Dark Harbor 12 of 1915 was a gaffer similar to the NGH boats of that era, all gaffers with round sticks, spliced rigging. BB Crowninshield would have accurately speced the spar and rigging. You don't start seeing tracks and box sections until after WW1. As someone noted above, LFH claims to have invented the hollow rectangular spar.

I can't say if LFH claimed to invent box spar construction, but I believe his father, NGH did lay valid claim to have invented metal sail track. I'd say the box spar only made sense after the invention of sail track and so I'd give the point to NGH. :D

Jay Greer
02-08-2016, 07:02 PM
By documentation and his own admission, LFH did invent the box section mast. Here is an example of an eliptical mast I designed for a competitive wooden boat. The mast is what is known as a German Box mast in that the mast is a hollow ellipse made up of four sides that were pre-hollowed and shellacked internally before glue up. The wall thickness on the sides is 9/16" and the fore and after staves are 5/8" on the forward side and 3/4" on the after side to allow for track fastening screws. The entire mast is 44' long from butt to masthead and weighs just a bit over 154lbs minus rigging. The halyards are internal and the main halyard has a locking device at the mast head to eliminate the strain of the main halyard on the column of the mast. What is seen here are the bending molds that were used to form the Everdure tangs.
Jay
https://im1.shutterfly.com/media/47a6dc29b3127cceeb05afe8574000000030O00QYsmrNy5bsQ e3nwg/cC/f%3D0/ls%3D00107990352120160208234750773.JPG/ps%3D50/r%3D0/rx%3D550/ry%3D400/

Measures Once Cuts Twice
02-11-2016, 12:30 PM
Partly unrelated but pictures of DH build are requested by all (well just by me but no one ever posts that they don't want pictures so it's implied (except if someone starts a "selfies in speedos" thread then all bets are off))

Pics!
Pics!
Pics!

......../|\
. ...../ | \.........
. ..\=====.....
...... ==}......

Pics!

Canoeyawl
02-11-2016, 12:39 PM
Drag coefficient

https://upload.wikimedia.org/wikipedia/commons/3/37/14ilf1l.svg


Drag coefficients in fluids with Reynolds number approximately 10(4)

Wiki

Peerie Maa
02-11-2016, 01:07 PM
Drag coefficient

https://upload.wikimedia.org/wikipedia/commons/3/37/14ilf1l.svg


Drag coefficients in fluids with Reynolds number approximately 10(4)

Wiki

OK, now find some with the angle of incidence for the best a yacht can make to windward:D

Its going to be between 0.8 and 1.15 for a square section.
The streamlined body will rise to about 0.3 or more.
http://upload.wikimedia.org/wikipedia/commons/2/22/Lift_drag_graph.JPG
And will create turbulent air over the lift side of the wing until it reattaches.

Canoeyawl
02-11-2016, 10:33 PM
[QUOTE=Peerie Maa;4797911]OK, now find some with the angle of incidence for the best a yacht can make to windward:D

Its going to be between 0.8 and 1.15 for a square section.
The streamlined body will rise to about 0.3 or more.
http://upload.wikimedia.org/wikipedia/commons/2/22/Lift_drag_graph.JPG
And will create turbulent air over the lift side of the wing until it reattaches.[/QUO

Some between the diamond and square in the above picture, eh?

(What would a similar graph say about centerboards without a foiled or streamlined shape? I suspect a very limited angle of attack)

Peerie Maa
02-12-2016, 04:25 AM
OK, now find some with the angle of incidence for the best a yacht can make to windward:D

Its going to be between 0.8 and 1.15 for a square section.
The streamlined body will rise to about 0.3 or more.
http://upload.wikimedia.org/wikipedia/commons/2/22/Lift_drag_graph.JPG
And will create turbulent air over the lift side of the wing until it reattaches.

Some between the diamond and square in the above picture, eh?

(What would a similar graph say about centerboards without a foiled or streamlined shape? I suspect a very limited angle of attack)

The data is out there :D
http://www.aerospaceweb.org/design/waverider/figures/fig08.jpg

In stead of the table of spheres and cubes, we should be looking at the right hand column of bars.
http://www.aerospaceweb.org/question/aerodynamics/drag/drag-shapes.jpg

Jay Greer
03-10-2016, 12:32 PM
It was my friend Carter Pyle who began experimenting with wing masts that incorporated a full batten double luff sail with a mast that pivoted to align with the wind. It was quite impressive and the cat he designed was very fast. This was in the mid to late 1950's. Still unless a boat can approach foil speeds, the streamlining afforded by the shape of the mast is of little influence on efficiency. This was proven in Germany by wind tunnel testing by the aerodynamic professor Manfield Curry. This testing resulted in L. Francish Herreshoff inventing the box section mast that was used on the R boat "Yankee" that LFH designed. The box mast also incorporated internal halyards which reduced windage as well as did the elimination of the head stay where a jumper was incorporated to support the mast head and apply leverage to the jib stay. LFH also did away with internal blocking proving it to be unnecessary also, creating a cleaner curve along the length of the spar when loaded.
Jay

Peerie Maa
03-10-2016, 01:08 PM
^ Rectangular or elliptical spars can also offer the advantage of designing the transverse strength to suit the transverse staying and bending loads, and the longitudinal strength to suit the longitudinal loads. Be careful though as the best shape to resist the compressive loads is cylindrical, so you can only move away from that form with lots of intermediate stays and struts to reduce the panel length for stability. I think that now we have reliable birdsmouth construction the justification for rectangular sections is superseded.

http://static.rcgroups.net/forums/attachments/5/2/8/4/3/0/a7026237-222-image-f3210665.jpg

Stays and struts going every which way. ;) I like the club on the foot of the jib though.

Jay Greer
03-10-2016, 01:49 PM
Yes, that is a copy of the drawing made by LFH which is a revision of the original concept in order to give more support and control to the shape of the mast head and to prevent it from bowing forward as well. since the jumpers on the original do not reach as high as does the secondary set of jumpers above the lower one. Although these drawings are published in Roger Taylors new book, I did have the good fortune of seeing them when I was a guest in the Herreshoff home many years ago.

I agree very much with the concept of the birds mouth method of mast building. It was superseded by Nathaniel Herreshoff's method of spar construction that was best known for its use of building S boat masts. The construction required a very complex spar bench with a clamping system that held the staves together under pressure until the hot hide glue had set. The use of hide glue was only because of the fact that no other adhesives that would support the load were available at that time. It was necessary to be very judicious with keeping the spars protected from moisture as hide glue is water soluble. There can also be no arguement that the attachment of tangs to a box mast is simpler than that of a round one. When we stop to see how simple the construction of a birds mouth spar is, it is a wonder that NGH did not think of it!
Jay

gilberj
03-10-2016, 02:30 PM
Nearly everything has already been said. I'd just like to mention that the box spars advocated by LFH had the corners well rounded. The box shown on those drag diagrams had sharp corners which vastly increases the drag. LFH's box spars were a big improvement on what was going on before.
Very often the masts were solid sticks. On more developed boats the stick would be ripped down the middle and the two halves would be flipped and glued back together so any natural warping would counteract itself. It was still a solid heavy spar. On some the halves would be hollowed out before they were glued back together, a lot of work, which was a little tricky to get it exact.
The birds mouth masts are a real step ahead and its a little surprising as Jay says, that someone did not come up with them before.
With regards to the OP I'd suggest the LFH box pattern is the easiest and has the least waste, but I'd suggest a birds mouth mast as the way I'd go for that boat. I'd definitely go for a hollow mast as the much lighter weight is easier to handle and allows the boat to be stiffer and better able to carry sail, with no real losses.

Andrew2
03-10-2016, 02:45 PM
Interesting to see the charts with the Coefficients of drag. My father designed a 12 ft Dinghy in 1948 that had a teardrop mast. The goose neck only hinged in the vertical plane, so the boom rotated the mast to face the wind. OK it should have had a bit of over rotate to provide a good section, but the boat was aimed at first time builders and was quite popular on the east coast of the UK. Around 2K built. I cut my teeth on #1 at age 3 and was allowed out alone at 6 yrs. Mentioned in the press at the time as the fastest boat for under 100. Named Fleetwind. Another feature was the mast step. Rather than have adjustable shrouds, they were fixed length and there was a brass screw jack at the heel of the mast, so you just heaved it up onto the tufnol socket and turned the included bar until the rig felt tight. Diamond bracing was standard too.
Masts were Sitka spruce, hollowed out in two halves, then glued together with Aerolite First built in-house, then farmed out to Collars, of oar fame.

He would have welcomed birdsmouth construction, as better use of material. Primary thought was performance, but accessable to the (then) home builder.