Aftmast rigs???

A few not so obvious questions regarding paper:
http://www.ignazioviola.com/ignazio_..._HPYDC2008.pdf

1) It is not definitely clear, how they account for drag of "bare" mast, when calculating the sails coefficients? (it looks like drag of the mast is excluded, but this to my view, invalidate results, as sails investigated simply CANNOT by flown without a mast behind them, and only coefficients with mast INCLUDED are of real significance );
2) It is not definitely clear how they deal with drag of bare mast in boat speed calculations;
3) dynamic response of such a rig in real unsteady winds is not even mentioned (?!) It is well known that even MAST HEAD SLOOPS with genoas simply somewhat bigger or equal to main, have wrong gust response, increasing belly of their sails, as wind speed increase; for totally main-less rig this could be only worse.
4) three masted boat with only genoas on all masts did NOT perform spectacularly in OSTAR, despite being rigged with "most efficient sails" and being significantly longer as competitors.
5) deep reefing technology for jibs/genoas when belly of the sail is not increased is yet to be invented

Arguments on the mentioned points are welcome.

 

Good grief !

Threatened already

 

I'm afraid it is too late for that.

I genuinely feel sorry for people who make decisions like this based on their best attempt at due dilligence. Sadly there is so much nonsense published and presented as fact that it makes things more difficult for people to make a well reasoned choice.

When it comes time to solve the problems associated with this choice I doubt the cheerleaders for this sailplan will be helping defray the costs that the owner will have to endure.

 

It's going to work. Wait and see.

 

I may just give mt 2 cents worth and leave it at that

the aft mast setup is another option. In effect it creates a sail shape very similar to the common lateen sail used in Arab Dhows. Now the lateen sail has been in use for a good thousnad years, points upwind well, and is efficient.

Is is going to the fastest sail ever invented. Probably not, but that does not mean that it is bad.

It may be useful to look at the sail in it's entirety. Value for money, simplicity or otherwise, low stress materials, easy reefing, less boat structure weight, easy to control single handed, possibly better motion at sea?

It might be interesting to try a bipod aft, something that is cheap to make, no expensive stays, downward forces go onto rear crossbeam, main hull connectives, a naturally very strong part of a trimaran. It would still retain a sharp taught leading edge. The drag inducing uprights would not be in a position to compromise airflow over the sail.

With a bipod, it could be adjusted a little bit foward, a little bit aft, to give ideal sailing balance when sail is rolled up as a reefing mechanism. Thus easy rudder action independent of wind strength. How do many existing trimarans reduce sail? first they reef the main, which is hard work with slab reefing, then they have to take down the jib, and put up something smaller, also hard work too, in total it is a lot of work, and not easy to do single handed, especially if u do not have automatic helm!!!!!

Can u reef a normal trimanan rig sinlge handed without autohelm.... not with ease, especially if the boat is starting to get a little big, say 30ft or so.

Another possibility with a bipod aft, is in a cyclone, pivoting it all the way forward and lashing it horizontally to the deck, no windage. Should be a very easy, simple and safe procedure to do. Try doing that with conventional rig.

So speed is one issue, but there are many factors in the ideal rig. The chinese junk rig is not the fastest, but it has been around for a good 2 thousand years, so perhaps mind blowing speed is not everything?

I personally think it has potential. Will it set speed records, probably not, but is it very easy to use and value for money, probably yes.. I think this is really where the discussion should progress to. (just my opinion.. feel free to disagree)

peter evans

 

Common lateen rigs have a small spar on the leading edge of the sail. Common lateen rigs have correct gust response, the tip flexes and depowers the tip of the sail.

Wire luff rigs cannot do this.

Everything I've read about aft-mast rigs and supposed increases in efficiency are based on steady state conditions. If anyone has shown that they respond properly to real world gusts (as a free-standing rig or fractional sloop can) I've missed it.

 

offcourse only my 2cts as well i feel free to generaly agree on your thoughts above peter
was looking at aft mast, whishbone and bipod bridges, frame drag at various wind and points still in a search and you want that dynamic now RH ough
have been reading up on the crab claw before where on delta wing CFD study's
apart from entree angle and so in peticular the leading edge profile profile was investigated
and a say hard chine profile was found giving better flow separation and vortexlift

 

When I was testing the little tri, the marina owner came over to look my middle africa rig over. His only comment was he saw those up in africa and they can only sail downwind. He was really PO because he had a big mono and couldn't catch us

I never had the sensors all over the boat and the computer to process large and complicated formula, but I was in a couple of gusts if they could be called that. Also the one time Manie went along we experienced a couple of gusts.

Not sure what you expect to happen. If you sail it carries on and when a gust comes it goes faster... when the gust is over you unfortunately slow down again.

 

Yipster, that boat broached the river...

 

My 2 cents

On Fanie's tri with the aftmast :-

What REALLY REALLY impressed me was the unbelievabilly high pointing ability - I have NEVER NEVER EVER been on ANY boat that could sail that close to the wind, at times it almost seemed like we were sailing into the wind.

As a cruising sail it has got huge potential and for those that have spent a lot of time and money on such a set up - IT WORKS !!! you will be happy.

Racing and all the really high tech stuff - No, the racing faternity with all their big budgets would have gone aftmast a long time ago if it could win. The aftmast has got issues, but for the cruiser it is stuff that he could live with

 

Thanks Manie,

Now if I can just have EVERYONE's two cents I can finish the boat !

Re racing - remember the sail I had was seriously undersize, and as you remember the tri wasn't exactly anything near shaped for fast.

Now imagine you take a hobie 18 or 21, put two decent masts and sails on it, yes, one on each hull butt and make the hulls further apart so the sails can 'breathe air', what do you think how would it be then ?

When I visited my friend at Vygeboom I compared to his windrider and with a hobie. They both beat me - but not by far enough ! I'm convinced if I had a sail size that was more in relation with the tri's size it may have been very different.

Keep those two cents coming chaps !

 

One aspect of the lateen rig that doesn't seem to get a mention very often is the lift it provides, which is in a direction that offsets heeling moment to a degree when sailing a reach. This may be part of the reason why it has a good gust response.

 

I disagree the aft-mast makes for better storm stability and survival

Your post assumes that the best way to survive a big storm is to run. While my boat has 3 drogue pouch lines for running, it also has 2 para-anchors (only one deployed at a time). Setting a para-anchor seems to be the accepted last resort to survive the big incoming breaking waves by tripping the waves before they hit the boat. The para-anchor also holds the boat squarely into the storm.
Now think about this for a moment. You have been running on the drogues and the winds and seas have continued to build to the point that it is no longer safe regardless of your mast position. Now it comes time for you to stop the boat turn 180 degrees and face the wind and waves head on.
At this point, where would you like your mast to be? I know I would want it way in the back. The aft-mast would aid my ability to keep the boat into the wind.


So rather then being a liability, the aft-mast should be an asset for survival situations.

Philip Maise

p.s. Another thing. With a Bermuda rig, especially one of the Hood type, you have a big boom that can be caught by the wind. My old boom was nearly 26 feet long and a foot tall. This is all sail area too. Sail area of the wrong type as the wind can catch it and push the boat sideways. With an aft-mast rig and only a small back spar you have a much smaller area aloft for the wind to catch on. Again aft-mast advantage. If you have a conventional Marconi sail hanging on that boom, you have even a bigger problem. Your sail area aloft owing to that boom could be 3 feet high, especially if it was lowered in a hurry. The crab claw sails I have made for my boat are designed to be rolled around their associated booms and lowered right down to the deck level.

 

Interesting point Philip, I haven't thought about that.

 

Multiple Headsails

I don’t think multiple leading edges are an answer to improved performance, particularly when you consider the additional rigging and trimming of these multiple sails. In fact I was negatively impressed by that little mast aft tri’s three headsail arrangement.

You might have a look over at a few of these postings by some knowledgeable aerodynamic guys such as Tom Speer, et al, such as posting #109 and #122 under ‘SAIL AERODYNAMICS’