Aftmast rigs???

[CT249]

.....which is perfectly reasonable on a blast reach and from which no real overall conclusion can be drawn.

 

[Gary Baigent]

Since he has a broken starboard foil I'm wondering whether he would use then same sail setup when on port with the intact foil doing its job. Just conjecture but maybe he would go to full main and smaller reacher. But I know nothing.
PS: that boat is getting a hiding?

 

[tgtony52]

There is a subtle but major difference which most people seem to understand. When comparing working sails, these boats still have large mainsails compared to the headsails like they have always had, so are aerodynamically similar. The same rig is being moved back to prevent nosediving by way of longer buoyant bows for a hydrostatic/dynamic difference. The mains are not tiny and being replaced by headsails. The longer bows mean similar "flying" sails are now connected to the deck as bow spits are not needed or can be much smaller. Overall there is less nosediving. The rigs are similar, there placement is different. The standard mainsail is a crucial piece of the rig as always, and has no sign of becoming vestigial equipment soon to be phased out.

Totally agree with you here Brian. It is clear when comparing the sail plan of Wild Oats pre and post surgery (see pic attached) that the main part of the rig has just moved further aft without the main changing shape or size and without it hanging over the stern. It was possible in this case as the previous position of the mainsail clew was well forward of the stern and the new position could still sit forward of the stern.

However, in comparing the rigs of the current Vendee Globe yachts with a winning boat of the event 20 years ago, moving the mast further aft on these newer boats has resulted in the foot of the main getting smaller and the foot of the staysail and/or foresail getting longer. So it would seem that the idea here was not to increase the buoyancy up front but to reduce the size of the main and increase the size of the foresails. (see pic attached)

A subtle but major difference.

 

[CT249]

Nice illustration, thanks.

Perhaps the movement of the mast aft in the 60s is driven by seakeeping factors related to the extremely high speed of these boats (not just their intrinsic speed, but the fact that the course allows them to spend so much time reaching in a breeze) which are not relevant to most craft. Given the obvious shroud base issues, maybe staying width is a factor too? And finally, has the actual centre of area moved aft or has the arrival of the square top moved the mast aft but not the centre of area?

Many of the same factors apply to WOXI, too, although she's more of an all-rounder.

Since the early '80s I've been sailing a class where the mast tip is often aft of the stern. It's good in some conditions, not so good in others. The development class we're mainly racing now, which is arguably the most popular high performance class in the world, doesn't seem to have changed the rig position.

 

[gggGuest]

I'm not sure you can look at the rigs in isolation from the hulls. There's been a general tendency for boats to get more and more wedge shaped with the waterplane further and further aft. Its unsurprising if such craft have masts further aft too.

Amusing to note how this topic has drifted from the original proposition.

 

[CT249]

/\

Yep, the wedge shape was one of the seakeeping factors I was referring to.

 

[brian eiland]

Still Here Defending orginal proposition

Amusing to note how this topic has drifted from the original proposition.

....the original posting that started this subject thread:

Can anyone comment on aftmast rigs. I read some of Brian Eiland's stuff and recently saw a motorsailer with a aftmast bipod flying a huge genoa. I liked it a lot and want to rerig my 48 steel yawl this way if I can. Any help would be appreciated.

Yes, this now very long subject thread has had its share of digressions. But I am stlll here defending my original 'aftmast' rig idea, and trying to provide a depository for as much related information, ideas, etc that might lend support to the idea.

And I want to thank those few brave individuals that have gone off and experimented with variations and alternatives to this idea. I even want to thank the naysayers that keep us looking for improvements and justifications for the concept.

I do fear that the subject thread has gotten so long that few people would bother going back thru all the pros and cons, ...but when I think about breaking it up into a 'new/updated' discussion thread on the same subject, this old subject thread might just get lost in the wasteland of old forum subjects. That would be a shame as I have seen MANY subject threads on many different subjects that offer a wealth of information. get lost in that abyss.

(PS: the original poster made only 4 postings about modifying his steel yawl, and then disappeared from the forum??)

 

[tgtony52]

And finally, has the actual centre of area moved aft or has the arrival of the square top moved the mast aft but not the centre of area?

Without actually doing any geometric calculations, the fact that the keels are further back suggests that the sail centre of effort has also moved back. Putting a square top on a main would move it back whether the mast is moved back or not. Moving the mast back need not necessarily change the centre of effort if the sails are recut to keep the balance.

When I moved the mast back 2 metres on my gaff cutter, I didn't want to change any of the underwater profile, so the centre of lateral resistance was fixed. Therefore I had to increase the area of the foresails and decrease the size of the main by the same amount in order to retain the same balance as before. It worked better than expected.

 

[markstrimaran]

Any thing new, in the full stay sail, or cutter rigged aft mast multi hull.
Only three more months left to step the mast, and recut a head sail.

 

[CT249]

Without actually doing any geometric calculations, the fact that the keels are further back suggests that the sail centre of effort has also moved back. Putting a square top on a main would move it back whether the mast is moved back or not. Moving the mast back need not necessarily change the centre of effort if the sails are recut to keep the balance.

When I moved the mast back 2 metres on my gaff cutter, I didn't want to change any of the underwater profile, so the centre of lateral resistance was fixed. Therefore I had to increase the area of the foresails and decrease the size of the main by the same amount in order to retain the same balance as before. It worked better than expected.

I think that, assuming the same area and mast height, moving to a squaretop reduces the boom length and therefore reduces the distance between the sail's CLE and the mast. Therefore if the CLE remained at the same point, the mast would move aft. However, I may well be wrong on this as I'm just doodling rather than calculating.

As you say, sail shape can affect CLE too, and in my experience that makes eyeballing all this very difficult because a squaretop is normally flatter and more open in the leach.

You're right about the keel position being an indication of CLE, but the long term trend to move keels and rigs aft (probably in reaction to advances in other areas of technology rather than as a speed-increasing factor in itself) blurs the issue.

 

[brian eiland]

I don't have anything new at the moment, and I am presently getting ready to go on a 2 month trip to Thailand for a winter break. Hopefully I will get busy with my outdoor test when I return,...been collecting materials.

 

[tgtony52]

Aft mast remote control sailing boat

Converted my remote control "Nirvana" sailboat into an aft mast rig and gave it a test run today. Still a few bugs to iron out in the rigging, but first impressions were more than encouraging. Acceleration and handling are noticeably better. Top speed seems higher - I have never outrun the wind waves before. And doesn't look half bad either.

The mast is 100mm shorter than the original (on stand in picture) and the sail area is about the same overall. I have positioned the mast where it was most easily possible to do so and this gave a reasonable size for the smaller mainsail. Essentially the main is just a tool to reduce the drag on a bare mast and to provide the balance in the overall sail plan.

 

[brian eiland]

Aftmast Design from Finland

Just recently there has been a submission to the this 'wishbone sailing rig' subject thread,...a reference to another aftmast vessel design,....postings here:
http://www.boatdesign.net/forums/boat-design/wishbone-sailing-rig-1999-22.html#post795388

Rather nice looking vessel design. There is an attempt to get the designer/owner to join the forum and submit his contributions about the design. Should prove interesting.

...more pics and dwgs over on the other subject thread.

 

[sharpii2]

Pretty remarkable. Seems like a lot of stress on that back stay. If it parts or th mast buckles inward, it's all over. But a lot of the likelihood of that depends on how stiff the stability of the hull is, I suppose. The actual angle of the Back Stay seems quite small.

How long has this vessel been around?

 

[brian eiland]

How long has this vessel been around?

I don't know at this time. I was just made aware of if most recently when it was posted on that other subject thread. I have asked one fellow to contact the designer/owner to see if he might be willing to share his experiences.