OK by allowing twist... which is like feathering the top of a sail, and in effect stopping it working.
It is good to watch a video of sailboards sailing with a lot of twist in the sail... I once saw a sailboard speed week championship video, the tops of the sails were flapping like a flag... the battens and tension on them stopped a fair bit of it but they were flapping. Next time your on the water watch other sails for the top flapping about just a little. (you won't see it on your board as your looking almost straight up)

I normally don't take into account twist to much because its one thing that the sailor can control with sheet pressure and angle.
And yes if you allow lots of twist of sail top feathering it will lower the CE, but it will also cause air drag.



I have done a picture of what would occur if the tops of these sail were twisted or feathering, the twisting line would be about where the black line is.... and if we then do the same line drawings on the effective sail area we end up with a totaly different CE point.

This picture is of a yacht has wind coming from behind the yacht at considerable strength... the main sheet has been let out and the mast has flexed forward indicating a tail wind, that sail is acting like a spinnaker rather than an aerodynamic wing. The yacht would be traveling at a speed less than the wind. Why? ... If he was at the same as wind speed the mast would be straight, if he was faster than the wind the mast would be flexing back.

Now not knowing the beach, if that guy wanted to go faster he would need to steer slightly to the left, and sheet in a bit to start to get the use of the apparent wind he would be starting to generate, the would then jibe the yacht and zigzag down the beach in l-a-r-g-e sweeping jibes. As he would end up faster than the wind those jibes become gentle tacks and HUGE speed [at that point I hope he's carrying a GPS] because he would be hooting along.

There was earlier mention of Sail Feathering and Air displacement over the top rear of the sail. I have mounted Long Woolen Telltales at the top of the new sail and it would seem that the tendency is for a fair bit of wind movement upwards and off the sail. This is where I was going to try a Truncation/Stall Packs to redirect the Air straight back over the sail. I am sure it would cure the p[roblem but whether it is worth the effort is the question?? How much efficiency it would gain?
Ron]

Gizmo said...

This picture is of a yacht has wind coming from behind the yacht at considerable strength... the main sheet has been let out and the mast has flexed forward indicating a tail wind, that sail is acting like a spinnaker rather than an aerodynamic wing. The yacht would be traveling at a speed less than the wind. Why? ... If he was at the same as wind speed the mast would be straight, if he was faster than the wind the mast would be flexing back.

Now not knowing the beach, if that guy wanted to go faster he would need to steer slightly to the left, and sheet in a bit to start to get the use of the apparent wind he would be starting to generate, the would then jibe the yacht and zigzag down the beach in l-a-r-g-e sweeping jibes. As he would end up faster than the wind those jibes become gentle tacks and HUGE speed [at that point I hope he's carrying a GPS] because he would be hooting along.

Gybes in a blokart at speed can get very interesting. Depending on surface they readily spin out or just slap themselves down if you are not careful.

I usually sheet right in coming into the gybe and ease it out as I round onto the other leg to save getting overpowered.

As you say Gizmo "l-a-r-g-e sweeping gybes" are the order of the day as sharp ones will usually get you into trouble.

Sharp tacks also get you into trouble and usually with a blokart you will not change direction of travel but just do it backwards. Not good if you are tacking to avoid running into a fixed object. Ouch!!!

Unfortunately theories are fine for solid objects ... soft sails are VERY variable things the CE and the position can be changed up / down and forward / back.
Sailboards use this principle to the maximum for steering etc.

Knowing about CE, CB, CG, CLR / CR etc it's more about knowing what your yacht is doing and why its doing it......

Ya missed one... CE One third up One quarter back
In verticle line or thereabouts with the other two
Go sailing

As close as i can make out it would have to be a point directly on the top of the three

on the left hand third(RedSquare)

Hiko said...

CE One third up One quarter back
In verticle line or thereabouts with the other two
Go sailing

What have you found the results to be by raking your mast forward and back?

This is what I got:





Note that this is assuming full twist under load -- if the sail is cut so that the head still provides lift then this point will move slightly up and backwards.

when I laid it right back the back wheels pushed sideways(on a loose surface). If I stood it right up there was no speed. Somewhere around 10 to 12% seems to work best

Not quite sure what to make of this but i9t all sounds very familiar. Could it be that we are getting the mast Balance and tension to the point that it is acting like a Mast and Sail on a Square rigger only sideways Most certainly feels that way, particularly through a turn as one is bringing the machine through stronger winds and the Apparent Wind increases the whole unit slides sideways
Ron
PS; Is that what you are feeling AUS?

nebbian said...

Hiko said...

Gizmo said...

What have you found the results to be by raking your mast forward and back?

I have found in strong winds on the beach that the yacht balances better
upwind with the mast further back than its balanced position downwind
thats how it seems to me Someone may be able to make sense of this

I find the same thing on a windsurfer. Going upwind the sail foot is often brushing my toes, but going downwind it's normally raked a lot more upright. Not sure of the physics though!

I guess I should clarify this a bit
Downwind to a landyacht travelling at three times or thereabouts the windspeed the apparent wind is still from in front but probably less so

Clarifying once again [sheeesh] the yacht I was referring to above was a class5
also with tall skinny wheels
The mini with 4,00x8 wheels doesnt seem to be so sensitive to the CE position
or is that due to the unsurpassed skill of the designer? [landy]