Square top Mainsails

I found a full on gaff rig gave my boat bed helm when reefed, the CoE moving aft as the reefs were put in. On a fat head, the area of the CoE will still be moving forward, giving better helm balance.

All rigs being a compromise.

 

Yes on a reefed fat head the C of E moves forward when reefed but "better" is not necessarily true.
It reduces weather helm, but that may not be what you need. I sail on narrow rivers, pointing ability is everything, we sail with more weather helm than most, as it helps pointing ability .

It seems strange that reefing your gaff rig would alter the helm that much as the jib stays the same size, becoming proportionally larger, most of your reef being aft of the C of E and the peak of the gaff being much reduced in height catching less wind.

The above being said, on a normal Bermudan, particularly on my new main with its longer foot, reefing will move the C of E forward as well. I've deliberately chosen to have two rows of reefing points.
It's going to be a whole new leading curve in handling my boat, with a new main, a new rudder, and I'm considering increasing the jib slightly...

 

With 3 reefs in, the gaff span is just above the booms gooseneck, so the triangle of sail that is left, looks more like a traditional storm trysail, but rather than having the luff on the mast, its the equivalent of having it on the boom. And so what would normally be a short foot, becomes the leech, and is way aft on a long boom, rather than up close to the mast. Hence the helm balance got worse as the sail was reefed. That particular boat would have benefitted from a bowsprit OR changing to a fathead main.

Certainly not implying its a master stroke to performance on every boat, but certainly a rig i would think about at the design stage.

 

Squaretop mains https://www.boatdesign.net/threads/squaretop-mains.46803/

 

Here’s the Saphire sq vs pin article. There’s a translation available I think:

Kopfsache: Squaretop vs. Pinhead-Grosssegel — Saphire Segelschule https://www.saphire.swiss/blog-/kopfsache2021

 

They are not comparing apples to apples: The square top main is 7 sqm bigger, on equally long masts. Additionally, the pinhead is a (rather poor quality) cruising panel sail, while the square top is a fully battened membrane (and would cost about the double of the pinhead). Not surprisingly, in light airs, < 12 kn, the SQT is faster, while above 15 kn the pinhead would win.

If you add the width of the sqare top to the length of the pinhead mast, the areas will be fairly similar, the pinhead mast will be 1 m taller and the pinhead will always win.

The aerodynamic explanation of the blog text is mostly nonsense, there is no laminar flow over sails, the induced drag is not because of the pressure equalization between windward & leeward, but rather loading variation along the span and cannot be reduced to nothing, unless the mast is made infinitely high. Inverting the top of the sail certainly doesn't decrease induced drag, but increase it (a lot), for a similar lift. Here's an example of realistic turbulence behind sails:

 

Nice work Mikko ...

Squaretop is great, but so much voodoo being attached.

 

How/does drag effect a square sail going downwind? I thought it was mostly lift?

The sim-flow, if accurate, does look draggy.

 

Thanks Mikko

I was thinking that Saphire was explaining the difference between 2 stock rigs (racing vs cruising) for the same hull, which was something, at least. But getting boat manufacturing companies to offer boats with different mast heights is not widespread, which is why we went custom 20+ years ago. Backwinding the top of inverted full batten roachy pinheads (phatheads?) is something that those of us with 30 degree swept spreader rigs have been dealing with for years when trying to decide when to reef a stiff mast concept. Or looking at the top main plastered on the spreaders in the dreaded S shape, during a gust, and wondering if the battens are going to break, although carbon battens can put up with that kind of abuse.

Could this explain advice to reef a square head in different wind speeds/situations/ sailing angles than a pin head? For example UK, who’s making my upcoming small Squarehead main have mentioned earlier reefing than our present balky pinhead, upwind & down, but I’ve heard nothing about handling differences between our previous Lidgard roachy pinhead ( think ~ Hobie 16 main planform) and current small head Squareheads. I’ve been assuming that reefing earlier (Squarehead) has to do with controlling flow/ drag at the top of the sail, as well as at the leach, 3/4 blade, 9/10 flying jib / Yankee / Code zero / whatever it’s called. I’m not asking for free advice, but there must be some flow theory based design differences afoot here, similar to the changeover in windsurfing sails between sails controlled by leach tension vs square head twist a few decades ago, although mast stiffness was a large part of that, and mast stiffness for monohull keelboats is still ~ at early Div 2 aluminum spar windsurfing levels, kind of like when Finn mast went from Bruders to the early stiff Needlespars? The future appears in some places earlier that others….

 

I have tried to compare apples to apples. I used my program UliSail (New software to calculate and optimize sail forces https://www.boatdesign.net/threads/new-software-to-calculate-and-optimize-sail-forces.67844/) and calculated the driving force for the original rig of a Dehler 33. Then I shortened the mast by 2 meters and added the cut-off sail area at the top of the main. The sail area is in both cases 35.4 m^2
The result was a driving force that was 6% lower than in the case of the original rig. The results are listed in the attached file. Interesting is also the vertical distribution of the circulation.

 

Remmlinger,
Your inputs for the squaretop look a bit extreme, e.g. there is no way that the main twist difference is optimally almost 50%.

Why dont you rerun with a more real life scenario, say:
1. Reynolds number sensitive squaretop width 0.9 m approx
2. Mast height reduction 1.0 m approx
3. Squaretop twist around 12 degrees

I am working on a similar project here now. The squaretop, trimmed optimally should have slightly less induced drag and lower CE for a given heel angle.

Cheers

 

Oops

... lower CE for given forward force

 

Oops #2
... and the traveller should be higher as well, so try above CL of 1 to 2 degrees...

 

And after thinking some more..

1. At this windspeed and approaching full heel, partial L/D optimisation rides along with RM utilisation.
2. Max heel should probably not be above 24 degrees
3. 6.1 knots is too slow for the waterline length, 6.4 and 2 degrees wider twa would likely be better. (Target speed ~ 1.2*rootLWL)
4. Leeway and induced drag would then reduce, leeway to < 4 degrees
5. Change in jib twist between configurations should only be +/- 1 degree
6. The heel angle should not be the same for both configurations.

See what you can do! I will make some time to wade through the detail you have provided.

AJB

 

If I make only half the change (mast shortened by 1 meter), the driving force will only be reduced by 2%, compared to 6%, when doing the full change.
You can download my program and play with it yourself.
Regards
Uli