Random (cold weather induced) question about sailboat rigs

[rgranger]

Hey All

So I had a random thought (not an uncommon condition) when looking at the lanteen rig vs the cat rig.

My question was this.... why don't you ever see the mast at the back of the boat? So I did a google image search and found a few... but not many and only on multi-hulls.

Is it just athetics or is there a practical reason why a cat rig is acceptable but a reverse-cat is not?

 

[shemandr]

Interesting question. I'll have to give it some thought. Which is a good thing since I don't need more synaptic deterioration.

 

[PaulK]

Lee helm?

 

[shemandr]

Lee helm?

So less of a problem on multihulls? Also prolly a strictly an off wind rig?

 

[rgranger]

So those are interesting questions. I guess I'd want to know if lee helm or off-wind-only are issues with the lanteen rig. To my eye (I've never sailed a lanteen) the lanteen in the picture above has a sail that could be supported by an aft mast and you would have the added bonus of not having a "bad" tack due to the mast cutting the sail area when on the leeward side.

 

[shemandr]

Yeah but ... the double head sail rig leading the boat off the wind is such an easy peesy way to go. And even wing on wing is almost as good. If you manage jibes.
The aft mast rig kinda seems like rear wheel drive on black ice. I don't know.

 

[heritage]

It does eliminate the need for a boom. This would be nice, especially on smaller boats where the boom is at the crews head level. The large foresail could be designed with a self-tacking strut that would act as a boom to improve sail shape going upwind and make tacking easier.

 

[PaulK]

Looking at only the rig ignores how the hull design and the center of its lateral resistance impacts the balance of the boat working in tandem with the CE of the sail.

 

[jssailem]

I believe it is a desire to achieve optimum performance to windward. The aft Mast location when tried was not as efficient as the mast forward position.

If you get down i the weeds of the aerodynamic performance of sails you'll discover that a single sail is not as good as a jib and a Main to move a boat through the water.
Here is Arvel Gentry's look at the issue.

http://gentrysailing.com/pdf-theory/Aerodynamics-Of-Sail-Interaction.pdf

The explanation of how he arrived his conclusions takes a bit of time to comprehend. The quick answer is on page 10 in the discussion of what happens to the sails alone and together.

For the cat rig it is a matter of being able to shape the sail and reduce wind turbulence (Drag) on the leach of the sail.

Either of the mast positions on a Cat rig off the wind (greater than 90º's) is about using the wind as a pusher. Then it is about sail area to capture the breeze.

 

[rgranger]

Yea the slot effect thing makes sense
So if we ignore Bermuda rigs and just compare a lantern rig to an aft mast… what then?

 

[DArcy]

In theory, the interaction between the jib and main do increase efficiency but also using higher aspect ratio sails does. A 20' wide by 20' high sail is not as efficient as 10' wide x 40' high sail and will provide more lift even though they are the same sail area. This is why gliders have long slender wings. If you put one large sail on a boat with a given mast height you will have a lower aspect ratio than two sails that split the sail area.

 

[jssailem]

Clues to considering your question lie in a study of history and geography.

The rig you discuss is the “Lateen” . I.C. Campbell outlines the origins

https://web.archive.org/web/20160804061252if_/http://www.uhpress.hawaii.edu:80/journals/jwh/jwh061p001.pdf

Development of sailing boats, I believe, evolves based on the conditions sailors experience. We are a lot that creates better for the conditions we attempt to master. Once a tree branch filled with leaves, to a mothers shawl, to square rigged raft floating downwind. Wanting to get home we had to construct a way of sailing into the wind. The Lateen is a simple sail design and I can see how the evolution occurred aboard the boats in the Med and Indian Ocean, the Long boats of the Vikings, or the sailing cats of the Polynesian sailors.

 

[thinwater]

Here is the story of building an aft-mast double headsail rig. An interesting read, even if you would never build anything similar. A lot of problem solving.

A $6000 Cruising Catamaran

 

[Will Gilmore]

The aft-mast rig, also called a delta rig or a stays'l rig had been toyed with on and off for decades and longer. The stays'l schooner has been around much longer.

Creole, built 1927

Creole was built for the American, Alexander Smith Cochran. Launched as ‘Vira’ she was the largest yacht C&N had ever constructed. Her Bermudan staysail rig was used so that the yacht could be sailed with limited crew who would not have heavy gaffs to hoist.

My guess is that aft-mast rigs are chosen for modern catamarans to reduce complexity for the sailor. Make them a simple as possible for a boat designed for comfort and automation over efficiency.

A few delta rig pics:

A wind tunnel test:

Wind Tunnel and CFD Investigation of Unconventional Rigs

-Will

 

[DArcy]

Those aft mast designs look like they would be difficult to get good forestay tension due to the steep angle of the backstay.

 

[Will Gilmore]

Those aft mast designs look like they would be difficult to get good forestay tension due to the steep angle of the backstay.

Masts are generally supported like a B&R rig. There are definite issues to deal with when there's no sheet on a boom working with the back stays.

I remember, in the seventies, I was told that the headsail was more efficient than the main due to no mast to cause turbulence. I don't think that has proved to be true in more recent studies.

-Will

 

[DArcy]

Masts are generally supported like a B&R rig. There are definite issues to deal with when there's no sheet on a boom working with the back stays.

I remember, in the seventies, I was told that the headsail was more efficient than the main due to no mast to cause turbulence. I don't think that has proved to be true in more recent studies.

-Will

B&R rigs usually have most of their power in the main and are not known for having good forestay tension. For a cruising boat that doesn't sail upwind that really isn't a concern and the simple rig could outway the reduction in upwind performance. But then a Freedom stayless rig is also very simple.

 

[jssailem]

the headsail was more efficient than the main due to no mast to cause turbulence

In a sloop rig, when you sail Main only you find that there are limits to how much you can go to weather. With a Mast Head rig, which you likely were on, the jib can be (and is often) larger than the sail area of the main. When you unfurl the jib (or raise the jib), the boat points higher, and speeds up. This often leads a sailor to consider the jib as more efficient. What we learn from Gentry is the combination of jib and main are what improves the boat performance. The main acts to force more wind to move over the lee side of the jib. The jib helps to move the stall point on the main further to the leach. This combined aerodynamic activitiy is observed on the boat as going faster and the sailing closer to the wind.

The angle of attack for the jib is tighter and the main is trimmed a little looser. This combination appears to be the goal.

Sailing off the wind (below 90º) the size of the sail becomes a factor. The sloop starts to utilize the feature, more like the square rigs of long ago, the size of the sail to capture the breeze.

 

[rgranger]

@Will Gilmore
That was a great post and a lot of interesting reading. Thanks!

I found this photo interesting

It reminds me of a crabclaw sail (something we hashed out in a post about this time of year two years ago). I wonder if you couldn't borrow some design properties and applications of the crab claw but support it with an aft mast instead of in a lanteen fashion. That would allow the crabclaw to avoid having a "bad tack" side. I also think that if you could figure out a way to tip the mast athwart, you could use the tip as a way to spill air on the crabclaw in a manner similar to the T-shaped support ideas we saw floating around a few years back...

What I'm seeing is something like this...

Where the bottom spar (boom) is snubed down by a vang on a traveler that goes the entire width of the beam. Then if the mast is designed with stays that allow you to adjust the athwart pitch (this would be the tricky part) you would move the vang to the leeward side and pitch the mast to windward and you should have a sail that looks like a crabclaw profile to the wind. If it worked, the advantage would be a crabclaw sail you could set up on a roller furler, the leading edge isn't spoiled by a spar, AND you cut the sail flat on a crabclaw for ease of sail production.

All I need now is infinite time and money to try out some of these ideas.

 

[Will Gilmore]

... the T-shaped support ideas...

I really like this sail plan. I can imagine a number of benefits, from ease of wind spillage, uninterrupted airflow on either tack, lift generation that would reduce wetted surface underway, and improved resistance to heeling.

If you used a spar on the leading edge, you could go higher with the aftmast and use less backstay support. You could still have roller furling around the luff spar.

-Will