Have been thinking about mast angles on land yachts. I understand that in some designs the mast can be raked deliberately to leeward - to lower the CE and to introduce a downward component to stop skidding, but has it ever been done with some kind of spring in the rigging so that in a gust the mast moves over to leeward? Like relieving the pressure by heeling the mast..
welcome to the forum gofaster. any clues as to where you sail,what kinda landyacht?
we have tried heeling rigs before. it resulted in a soggy yacht, lack of speed and acceleration. instead we build chasssis that twist and store then return the stored energy. on bigger stayed rigs , this done by the rear axle
Aha.
My experience too has been that I got less acceleration - call it reluctance to go - and rather stodgy performance, but it did handle the gusts. The mast just tilts releasing the pressure.
I am a mechanical engineer. Windsurfer. Interested in why things are and always looking to make things go better.
I have been building land yachts for a while - but they are radio controlled ones!. I use them as test platforms to try out my ideas. and have an absolute ball doing it. Nothing beats a screaming reach on 2 wheels in a strong northerly.... ( well actually maybe a windsurfer does..)
The wind here in Wellington is often very strong and gusty - 4 days in the last 2 weeks I think it was over 40knots, nearly 50.
I've been looking at ways to cope with this. Easiest is just sail with a smaller sail. But I have begun looking at the side rake.
Many years ago there were springy harness cords used on windsurfers, to help with speed sailing in gusty wind. Don't know if it's still done?
I'm very interested in what you said about letting the chassis absorb the pressure. How do you do that?Twisting chassis or just sprung rear axle?
Fantastic picture!
I have been experimenting successfully with flexitip masts to relieve pressure during strong gusts but maybe I need to revisit the real bendy mast...
Question - in this yacht design are you allowed any mast? ie is this mast used because its found to be the best or is it because you have to?
Select to expand quotegofaster said..
Fantastic picture!
I have been experimenting successfully with flexitip masts to relieve pressure during strong gusts but maybe I need to revisit the real bendy mast...
I wish the photo were mine! I don't even know who took it; it's been in the DN class archives for many years...
The DN rig is unique (to my knowledge) in how it uses mast-bend.
I just wanted to show one (extreme) example to get people thinking about bendy parts.
The Finn rig is probably a better analog to the typical unstayed landsailer rig.
Another place to look for inspiration about chassis flexibility is the go-kart racing community. In many cases they use frame twist/flex like a suspension.
Most sailboats and landsailers allow the tip to bend to leeward which increases twist in the sail and decreases the angle-of-attack at the top of the sail. This reduces the lift at the top of the rig (i.e. moves the center of effort lower). There are even some rigs that allow the top of the sail to generate "negative" lift (i.e. pushing the mast to windward) to move the CE even lower.
It's the opposite effect in the DN, the mast-tip is actually "hooked" to weather which keeps the top of sail in-line with the mast (i.e. reduces/eliminates twist). Usually when you look at a DN directly from the front or back along the boom the sail "disappears" because there is no twist and you are looking directly along the sail from the leach to mast (the boats in the photo look like they have much more twist than normal these days).
The bend in the mast and plank both contribute to effectively lengthening the side-stay and angling the mast to leeward. The curve of the mast/sail reduces the amount of spanwise flow (i.e. "spilling air") so the forward drive of the sail does not reduce much. However, the side-force vector now points down reducing the tendency to hike and putting more force downward on the leeward runner to keep it from sliding sideways.
Select to expand quotegofaster said..
Question - in this yacht design are you allowed any mast? ie is this mast used because its found to be the best or is it because you have to?
The mast bend is not controlled by the class rules, only the length, weight, cross-sectional dimensions, and material are proscribed in the rules.
Super-bendy composite masts are the result of observations that wood and aluminum masts were "fastest just before it broke".
There was a period in time before the rules were changed to allow composite masts when breaking a mast during a race was a commonplace occurrence as people tried to balance the speed potential of bend against the strength limitations of wood and aluminum.
The rig is rreally the result of the antiquated sail dimensions. There is way too much sail-area for the size of the boat and no effective way to reduce it within the rules, so so sail and rig-development over the past 30 years or so has centered on controlling the power from the sail and reducing the drag.
Cheers,
Geoff S.
Thank you for your detailed analysis. I'm particularly interested in the lift and downward force generated by the angled mast. I'm wading through my Marchaj book on sailing theory to try to understand what is going on.
Another good book is "High Performance Sailing" by Frank Bethwaite.
I haven't read his more recent book, "Higher Performance Sailing", but I'll bet it is also outstanding.
Cheers,
Geoff S.
