Sail trim quiz question

[Jackdaw]

Its better to have a very long wing with a short chord than it is to have a very short wing with a long chord. Think sailplane and u2 spyplanes

EXACTLY this. For a long time, rating rules encouraged boats to have squat sail plans with overlapping genoas. In the absence of these rules, a modern designer will ALWAYS create a tall rig with high aspect sails with square ends (where ever possible). Usually 9/10s fractional. Look at everything from the AC boats to Volvo, TP52s, modern racing monohulls. Its fastest.

 

[shemandr]

...Its better to have a very long wing with a short chord than it is to have a very short wing with a long chord...

That is, it's better to have a high aspect ratio foil then a low aspect ratio foil. This is dogmatic today. But why then does a 70's C&C go upwind like a banshee with a 150?

 

[Will Gilmore]

Yeah! What Shemandr said.
- Will (Dragonfly)

 

[Jackdaw]

That is, it's better to have a high aspect ratio foil then a low aspect ratio foil. This is dogmatic today. But why then does a 70's C&C go upwind like a banshee with a 150?

Fun ride. But with the same sail area flying, a modern First or J will tear it up.

A fract non-overplapping First 35 rates 68 in PHRF. A C&C 35 MkIII rates what; 115??

 

[BobbyFunn]

@BobbyFunn I would love to see the video on your test but don't see a link?

See if this works.

Blue tape on ground marks wind direction.

 

[DayDreamer41]

Works like a charm thankyou

 

[Will Gilmore]

Very very cool
The tinsel shows wind direction. I assume the driving force is the component of the normal force with respect to the surface of the sail that points towards the bow. It's been awhile since I had to calculate force vectors but it doesn't look like there is a large component pulling forward on the main.
- Will (Dragonfly)

 

[BobbyFunn]

I assume the driving force is the component of the normal force with respect to the surface of the sail that points towards the bow.

You saying it looks like the air is deflected back and that drives boat forward? The wind is pretty intense for the scale. Could have poured sand and it would probbly look like that.

I dont really know the whys, just showing a cool observation.

The sails are drafting mylar. They are pretty stiff. If they move its significant.

The main is at as low an angle of attack as i can get it here. I see the leech of main get pulled to lee when the lee telltales get thrashing.

The headsail leech acts like the business end of a leaf blower, with a cupped hand over it to nozzel the flow.

Question for experienced...see the airfow coming off the headsail? Has it ever made sense to play the headsail rather than the main to keep heeling under control. If power comes from jib then so does rounding up. If the headsail is let out the airflow should be vectored out as well, countering the forward drive and wasting airflow to lee. That would also reduce the airflow on main weakening it.

 

[Jackdaw]

See if this works.

Blue tape on ground marks wind direction.

WAY too much wind!

 

[Jackdaw]

Also remember that when doing seat-of-the-pants empirical testing, if your results do not match what experts predict, it is probably your testing methodology at fault. In particular, scale models to not act exactly like full-scale boats, so keep that in mind!

 

[Will Gilmore]

WAY too much wind!

scale wise, the tinsel is heavy. Need to see it flutter.
I'd like to see it with tinsel attached to the leading edge of the mast.
- Will (Dragonfly)

 

[Jackdaw]

Also the tinsel is way too long. On the leech, short telltails will show if the flow has stayed attached to both sides of the sail by streaming aft if yes but sucked to the detached side if not.... but if it is too long, it is dragged aft in the general flow (attached or not) and shows nothing.

 

[Owen Thistle]

+1 to Jack's comment about scale models. How the fluid interacts with the objects has a lot to do with the viscosity (Reynold's number) compared to the dimensions of the objects. This has a lot to do with where the flow separates from the sail etc. I found this article that says you need to compress the air by the scale factor (i.e. if your model is 1/10 then please run the test at 10x atmospheric pressure). http://www.daviddarling.info/encyclopedia/R/Reynolds_number.html Can you crank up the pressure and try again?

 

[BobbyFunn]

if your results do not match what experts predict, it is probably your testing methodology at fault

Jackdaw you are exposed as a flat earther.

Ill see what i can do with shorter tinsel more tinsel in other locations, and lighter wind.

The problem with short tinsel is it lays flat if it isnt hanging.

 

[BobbyFunn]

Can you crank up the pressure and try again?

Thats brutal. The decompression chamber is too far in the back of the garage to make a path to for that., besides the crew would never agree to being lock up in there to make sail adjustments.

Whoa, wait..thats a functional sailboat. That does a lot of what my 17 footer does.

Please let there be a lurker crew from a volvo ocean racer who has also sailed a mini trans and noticed a diiference.

 

[Owen Thistle]

Thats brutal. The decompression chamber is too far in the back of the garage to make a path to for that., besides the crew would never agree to being lock up in there to make sail adjustments.

C'mon, in the name of science!

 

[Will Gilmore]

BobbyFunn, notice how no one else has model air flow over sails but, none of us has any problem telling you how you could do it better.
I say, forget the pressure chamber and make the model bigger. That tinsel would flow quite well over a full size sail. It doesn't need to be full size like a 40' mast, the same conditions that work for a 7' pram work for an 80' AC boat and, if that is true than the same conditions that work for a pram should work for your model. So, really you don't need to change a thing. That wind tunnel test was the absolute of AWESOME!
You are tee king

- Will ("astounded", Dragonfly)
Now it's time to really get into the analysis.

 

[BobbyFunn]

C'mon, in the name of science!

All the ear popping, the fog as the pressure is let out, muffled voices on the other side. Miss that life.

Of course at 350 feet we have to use mixed gas as well. That wont bode well for the model.

 

[Jackdaw]

Jackdaw you are exposed as a flat earther.

Oh please. I've got and BS in both computer science and software engineering. I know the drill.

Normally I'd say that in a GOOD empirical test, that the test rules over an unproven theory. Scientific Method 101.

But you're doing a SOTP test, any disparities against a well regarded theory should be very suspect.

Not wanting to be discouraging at all, I love this sort of thing!

 

[Rick486]

I knew this thread was headed for an ugly end. If anyone wants to match credentials, I'm happy to do so. However, I backed away from this thread as I knew it would degenerate to guys questioning each other's credentials, ideas, and experience. Please mods, shut this thing down.