Mast aft rig

[Moss]

Anybody out there with experience of mast-aft rigs, such as the single-masted ketch design of Running Tid Yachts? (See http://www.runningtideyachts.com/sail/)

I'm keen to hear how they sail compared to other rig types.:rolleyes:

[Brent B]

My only mast-aft experience was to design and build one for a Junkyard Boats competition. It used a 35 sq ft jib from a Jet-14 as its only sail (required). Hull was 8 ft long, 10 deg constant deadrise, modest rocker. We built it in one day, won all 3 races with it the next day, each with a different skipper.

Performance was quite surprising. It jumped up on a plane a few times in puffs while reaching. This with a 180 lb adult on board and only 35 sq ft of sail.

I keep thinking about designing a properly built 14 ft boat with this sort of rig. What size boat do you have in mind?

Brent
Benson Sails

[DanishBagger]

Not being a naval architect, I'm not sure I'm right, but that backwards-pointing spreader, and with the tilt of the mast, doesn't that put extreme bending forces on the mast just where the two are connected?

Also, the bloke states, that there is less windage, but how can that be, there's still three sails, still a mast, there's even a boom (although windsurfer-like)?

One thing, though, I really like the looks of it, strange as that may seem.

[SeaSpark]

The bending moment can partially be reduced by the back stay, but this will put great compression load to the mast.

Anyway, the profile in the upper part of the will have to be very heavy compared to a conventional rig, the spreaders will also have to be heavy becouse of the luff tension of the mizzen.

All this weight at this height can not be good for performance.

[Moss]

Brent B - It's a 20ft ply-on-frame dory. Still finalising the keel configuration, but leaning towards asymmetrical twin keels so I quite like the low aspect ratio of a mast-aft rig. How efficiently were you able to point?

Bagger - I'm not interested in the three-sail plan, just the efficiency of a genoa on its own, with the mast set well aft to resolve any CE problems. Kind of like a catboat without the mast getting in the way or the air flow.

[SeaSpark]

You could also look into the lattice and A-mast options, they have some of the advantages you are looking for.

(lattice-mast picture is from MikeJohns gallery)

[Brent B]

"Brent B - It's a 20ft ply-on-frame dory. Still finalising the keel configuration, but leaning towards asymmetrical twin keels so I quite like the low aspect ratio of a mast-aft rig. How efficiently were you able to point?"

Our tiny little mast-aft boat would tack through 90 deg. Overall the performance was quite amazing. Four different skippers sailed it.

One disadvantage is that the sail, if boomless, is not self-tacking like a conventional mainsail.

Your effective J would be about 15 ft on a 20 ft boat. Consider a single sail of 200 sq ft. It could have a foot of 15', and a mast height of 27 ft.

The keel location is critical to helm balance. On our boat I made a long daggerboard slot so the board could moved fore or aft to balance the helm. Interestingly, my original placement was correct, and we never adjusted the board position.

I assume you would use roller furling. Remember that this does not work well for roller reefing because the reefed sail shape is way too full. Installing a banana shaped foam layer along the luff helps a lot to flatten the partially rolled sail, making it more effective upwind.

Brent
Benson Sails

[Moss]

Thanks Brent, that's given me some food for thought.

SeaSpark - That A-mast looks like it's stepped a little forward of amidships. What shape sail/sails does it take, and how are they rigged?

[SeaSpark]

I made the picture very quickly and did not think to much about mast positon.

A have seen one with a normal sloop rig, the mainsail on a vertical wire from the top of the mast to deck level. Other rig types are possible ofcourse.

Could not find much information on the internet on this rig type, problem is that "a mast" is a very common word.

[windward]

I've been interested in this design for a while. Here are some sites for anyone wishing to look into it further.

http://boatdesign.net/articles/mast-aft-sailing-rig/
http://www.runningtideyachts.com/monohull/
http://www.barefootsworld.net/windwalker/aftmastsailingcalc.html

I believe that sometime in the next few years someone is going to show up at a big-name race with a boat with a mast-aft rig, probably with an aluminum hull and carbon-fiber mast. At that point we’ll start seeing them everywhere, especially if they perform well. We've seen more radical developments than this in the last few decades.

Of course, the real advantages of the design aren’t so apparent against spare-no-expense racing yachts, with their rotating, aerodynamic, hydraulically-tuned masts and olympic athlete crews. For more modest boats, however, the benefits are more readily apparent.

The mast-aft rig designs I’ve found online are monohulls or cats with the mast placed just in front of the cockpit, at about 2/3 of the LWL, canted forward 10 deg. and hinged to fold down for clearing bridges. They use anywhere from 2 to 5 sails. Typically a masthead genoa is used with a standard jib, sometimes with a fore staysail and/or mizzen.

Anyone taken aback by the idea of a canted mast should take a look at some pictures of clipper ships like the Flying Cloud - now there's some canted masts!

Advantages of the mast-aft rig
(cribbed from one of the sites above)
1) A rig that – is more aerodynamically efficient, enabling more speed with the same sail area and using a shorter mast.
2) – delivers an aerodynamically clean, parallel leading edge for all the sails.
3) – allows the whole sail plan to be roller-furled away.
4) – allows the furling of the sails without turning to weather.
5) – divides up the total sail area into smaller manageable sizes.
6) – maintains its balance center (CE) with different sail combinations.
7) – produces less heeling moments due to a lower center of effort.
8) – can be operated single-handed without leaving the cockpit. All the sails self-tack. The genoa and jib need to be tacked over past the forestays, which can be done quickly with roller furling from the cockpit.
9) – has the leech of all the sails operating clear of the mast, spreaders and shrouds, allowing sheeting for optimal performance of each sail, and eliminating spreader wear.
10) – simplifies rigging by eliminating mast track, slides, and mainsail boom with all its rigging and hardware, and the lubber's bane - an accidental gybe.

[SeaSpark]

The A-mast is better know as bipod mast Procyon project had one:

http://boatdesign.net/forums/showthread.php?t=10771

sorry to break into this thread but had to correct my mistake.

[SailDesign]

Windward,
You are going to have to defend those assertions.
Nos 1, 2, 3, - naah, all of it!
Even No.10 is suspect, especially the gybing part. I would imagine the accidental gybe with a socking great genoa out there would be just as bad, being wrong-sided and all.
Steve "not a believer"

[SeaSpark]

5,6,7,
Schooner rig has same advantage.

8, The genoa and jib need to be tacked over past the forestays, which can be done quickly with roller furling from the cockpit. Did you ever try this yourself? If so you must have noticed it takes quite some effort.



My conclusion: If you are looking for a rig with all the advantages you mentioned consider a schooner with free-standing rotating wing masts.

[AleX`G]

I am going to be entering a school model boat race next year. We are gathering ideas and have decided to give the aft mast rig a go. We will be building other rig types like standard bermuda and perhaps a lateen rig with simmilar hulls.
Might show some of the advantages.
Then again alot of things are different scaled sown so much.
It certainly looks different though. Can this rig sail closer to the wind than a standard bermuda with a jib/genoa.

Any advice is greatly appreciated on any subject relating to our project.

[DanishBagger]

I really don't have any doubts that the aft rig would work on a model. However, when making things life-size, "weird" stuff happens ...

_Not_ being a designer, nor a sailmaker, but I doubt that this will point any higher.

[AleX`G]

Would there be any point in using an aft mast setup on a model. Id try it just because its different but would there be any advantages

[messabout]

Phil Bolger did a mast aft design some time ago. Not sure what he called the boat...Canard maybe. His boat is typical Bolger, it is boxy but he has managed to make it look sexy nonetheless. I believe one or more of the boats have been built. A little digging might turn up an owner who has real world experience with this idea.

[brian eiland]

I was looking back thru the forum and ran across this subject thread that I either had not noticed before, or maybe I just thought I would let it go along on its course and then enter some defenses to the discussions...so here goes
...Also, the bloke states, that there is less windage, but how can that be, there's still three sails, still a mast, there's even a boom (although windsurfer-like)?

One thing, though, I really like the looks of it, strange as that may seem.
Thanks for the compliment on the looks.

I think if you look back at my website and my postings on this subject, you will NOT find that I have claimed "less windage"





...the spreaders will also have to be heavy because of the luff tension of the mizzen.
Don't know that I understand this comment? Are you speaking of the conventional spreaders or my aft facing 'jumper strut'. Certainly it will have to have some beef under the luff tension of the mizzen





I'm not interested in the three-sail plan, just the efficiency of a genoa on its own, with the mast set well aft to resolve any CE problems. Kind of like a catboat without the mast getting in the way or the air flow.
Check this mast aft design out in my gallery of photos:
http://www.boatdesign.net/gallery/showphoto.php/photo/7043/cat/500/ppuser/399





Advantages of the mast-aft rig
(cribbed from one of the sites above)

8) – can be operated single-handed without leaving the cockpit. All the sails self-tack. The genoa and jib need to be tacked over past the forestays, which can be done quickly with roller furling from the cockpit.
I never said "all sails self-tack"

What I did say, "One single person could sail this fairly large rig from anchor up to anchor down. All the sails roller-furl, and the mainstaysail and the mizzen both can self-tack. Only the genoa needs to be tacked over, and this could be delayed until the boat has come about. Even motorsailer folks should appreciate this rig."





..Even No.10 is suspect, especially the gybing part. I would imagine the accidental gybe with a socking great genoa out there would be just as bad, being wrong-sided and all.
I might first dispute the "socking great genoa". In reality it is not that big, only equivalent to maybe a 110%. And many a boat has run downwind wing-and-wing with a big genoa and no problems.






5,6,7,
Schooner rig has same advantage.

8,
Quote:
The genoa and jib need to be tacked over past the forestays, which can be done quickly with roller furling from the cockpit.

Did you ever try this yourself? If so you must have noticed it takes quite some effort.

My conclusion: If you are looking for a rig with all the advantages you mentioned consider a schooner with free-standing rotating wing masts.
But a schooner rig has TWO mast.

As I've said previously I did not claim both headsails needed to brought over past headstays...in fact only the genoa: "the mainstaysail and the mizzen both can self-tack. Only the genoa needs to be tacked over, and this could be delayed until the boat has come about."

A freestanding schooner is a good prospect. Look up Eric Sponberg's





Phil Bolger did a mast aft design some time ago
I will be adding some of Bolger's info and dwgs to the other discussion on this subject at AftMast http://www.boatdesign.net/forums/showthread.php?t=623.

And you might visit this discussion of Wishbone Masts, A Mast, Bi-Pod Mast
http://www.boatdesign.net/forums/showthread.php?t=1999