Square top mains?

Just looking thru this subject thread, and found this submission fascinating

 

Thank you CT, sorry about the exaggerations. Would you point me at the relevent CFD work on mast airflow disturbance please.

 

Thank you CT, sorry about the exaggerations. Would you point me at the relevent CFD work on mast airflow disturbance please.

 

There's a little something here http://www.wb-sails.fi/news/Ad_aerodynamics/index.htm#mastdrag

 

Thanks Mikko, had a look, useful info.

 

Mikko I can appreciate these nice CFD illustrations, and I can appreciate the fact the mainsail's attachment behind the mast does reduce the drag forces of the mast as your referenced site states:

But all of these newer CFD images can't negate the good old 'in-person touch and feel observation'. When you can walk up to the mainsail and push on the leeward side in that area just behind the mast, and feel almost NO resistance to pushing it to windward, you have to ask yourself "what real force is being generated by this bit of sail area behind the mast??"

Long ago, and with bigger mast sections I deduced that the first foot of mainsail behind the mast was NOT doing any driving.

 

The thing about CFD analysis like Mikko's which gives me great confidence is that its giving us explanations for stuff we already knew but couldn't explain, most notably that aerofoiled, custom sectioned and even over-rotating semi wing masts show little or no advantage over round section masts on the water, even though the techniques we had for analysis before were telling us they should be way superior.

What's important is not how well each individual component of the rig is working, but how well the whole thing works in total. Its like mainsail/jib interaction. They can work together to produce more than the sum of their parts, so if you take the supposedly less efficient mainsail away the efficiency of the supposedly more efficient jib plummets.

 

This is likely the forces that Chris White is pursing with his MastFoil concept

My twin headsail concept was pursing exactly this positive interaction of the leading and trailing sails, but I was trying to line these sails up in a more 'parallel' manner than with the tradition slanted headstay and vertical mast.

 

Hi guys, if the important thing here is that the jib and the main interact as a single unit, what would be the different effects between a self tacking jib (no overlap), a jib that overlaps slightly and a genoa or screecher that overlaps a lot?

 

Hi Clady,
I think this subject thread is entitled square top mains, so this talk of headsails probably best not be here. Perhaps you should visit that subject thread Sail Aerodynamics....it included lots of these discusiions.

 

This micro-cruiser has been flying a Hobie 16 mainsail since 2006. This summer, I cut the luff rope, folded the head down square, and stitched it by hand (broke 3 needles and pierced my thumb with the blunt end numerous times). It wasn't pretty but it proved the point.

The only thing that was less than perfect was that the leech motored between the top and the 2nd batten. I assumed that this was because my McGivered re-config didn't support the top batten adequately.

Right now, the sail is on a Purolator truck heading back from Sobstad's Sails in Barrie, Ontario. I hope to try it this Canadian Thanksgiving long weekend. I'll give you an update next week.

The idea was to prove the square top concept before replacing the bent mast. You can see the difference in mast height due to the square top. 2% reduction in total sail area for an 8% reduction in the length of the mast.

 

SuperPiper, I think that unless you sort the Motering you test will be invalid. Cool idea though. Perhaps a line from the masthead truck and the outer end of the top battern may help.
If I remember rightly, the Hobie 16 main always had a big belly to play with on a bendy mast depending on wind and sea conditions. With a shorter luff, this may influence your results.

 

The sail arrived back yesterday from Sobstad Sails. It now has a proper head panel with a substantial leech tape across the square top. The head panel actually extends below the top batten. All of this should help reduce/eliminate the motoring at the high leech.

The work by Sobstad's is infinitely more professional than the tacked stitch job I did with my wife's sewing kit.

I hope to try the sail tomorrow.

 

Done it. It was good.

I got delayed at work and didn't get to the cottage until Sunday afternoon. After settling in, I unpacked the modified mainsail, slipped in all the battens, and pulled the top one a little extra tight (but not so tight as to distort things). The main was slid onto the boom (Sapphire's main is still not loose-footed), and the boom was tacked to the gooseneck. I hoisted the modified mainsail. It slid up the mast groove without an issue. I had been afraid that any additional layers or stitching would make the luff groove too tight.

The main was pulled down and tied to the boom until Monday afternoon.

In the morning, while draining the cottage waterworks, everything that wasn't bolted to the boat was removed: the cushions, the cruising gear, the battery, the anchor, even the outboard & gas tank. Only the minimum of safety gear was left on the boat. The transom probably popped up an additional 4 inches above the water. She was floating on her knuckle.

We had inherited a fluorescent orange jib from a plywood dinghy that had sailed itself onto the burning pile a couple of decades ago. I bent it on just to compliment the fall colour scheme. It had no hanks so the halyard was pulled as tight as it would go. The tack had to be adjusted to about 2 feet above the deck for her to trim correctly on the Sandpiper.

I pushed off the dock and sailed out of the bay. 5 - 10 knots with gusts to 15. The square top looked good. No motoring on starboard tack and just the odd flicker on port (bent mast, remember). I reached back and forth across the lake for about 3/4 of an hour. It was fantastic. I then doused the dinghy jib and hoisted the Quantum genoa. She hadn't been out of the bag all summer.

What a sweet ride. I didn't have a GPS with me so I have no idea of speed. Sapphire was light, lively and fast. I was waiting for the girl to pop up onto plane. Is that possible?

 

Congratulations, SP! Sounds terrific......