Thread: GIS sail tweaking

Sounds like downhill skiing. All the grip has to be on the uphill edge of the ski. Let the downhill edge dig in and it'll take 15 minutes to find and collect your skis, poles, gloves, hat, etc. from where they all land.

Originally Posted by paulie

Sounds like downhill skiing. All the grip has to be on the uphill edge of the ski. Let the downhill edge dig in and it'll take 15 minutes to find and collect your skis, poles, gloves, hat, etc. from where they all land.

The problem in an open boat though, is that if you lean it up wave, and the wave breaks, it'll all come on board. That's better though than tripping on the down wave side and rolling the boat.

(Though rolling down wave, doing a quick 360 degree barrel roll and coming up still in the wave is a valid sea or surf kayaking trick, I had to do one of those with three weeks of gear in the boat when I got caught in a big surf tube on the outside of Calvert Island on the BC coast.

Ian

Great analogy to surfing a longboard! That makes just too much sense.

Now if we can perfect the bottom turn, drive back up the face and turn back down off the lip ...

What is happening when you're on a beam reach and the swell lifts you from the side?
(1) the mast head tilts away from the wind as the swell starts the lift
(2) then the masthead falls towards the wind as the swell passes underneath

What I anticipate for a change in apparent wind when this happens seems to be the opposite of what actually does happen. I expect that falling over the far side of the wave and having the masthead tilt towards the wind would be like pumping, and would produce a temporary lift, but in reality it seems the other way around, you get headed and have to fall off a bit.

That was a great adventure, Christophe. Thanks for sharing it.

To your question--do you think what may be happening is your stern is being pushed to leeward by the wave you have crossed? I'm thinking your rudder is probably all or most of the way down so that's a nice big surface catching the water on the front of the wave.

IOW, perhaps it's not your bow digging in but the stern being pushed to leeward.

Howdy,

The forces when the bow of a boat is deep in a wave and the boat is moving fast are substantial. The bow can overcome the steering capability of the rudder. This can be termed "bow steering" (of the unintentional kind).

On John Goodman's Goat we had 5 days of 20 to 25 knots from behind in the Texas 200 and the boat didn't show a moment of worry in terms of steering. We found also that the goat makes very good time even when deeply reefed.

But that was with the wind quite close to behind so we were always able to trim for no heel.

Also we had two people and about 100+ pounds of food, camping gear etc. This gives much more stability of course, though most of our weight was inboard.

On many boats loading can cause more problems because it forces the bow deeper.

Also in design terms the Goat has been optimised to reduce this tendency. As we all know you can still steer with a balanced helm when the boat is well heeled. Something that is not so easy with many modern boats. Also in more moderate, but still quite strong winds and wave/chop the boat behaves with good manners and does nothing startling.

But every boat has its limits! The wind can get very strong and the waves can get big. Particularly when sailing single handed.

Again with the rudder ... I suspect that when stuffing the bow into the back of the wave in front that the rudder will be deeply immersed if the boat is level ... it is still in the face of the wave behind. I think heel is the bigger culprit for lifting the rudder out.

SAILING
The counter in terms of sailing is to try and make sure the bow meets the wave symmetrically ... by that I mean the boat is level so the boat doesn't get more force pushing the bow one way rather than the other.

Heel also means the rudder tends to come out of the water somewhat.

This can be easier said than done in the conditions Christophe was out in. Reefing will help ... and also reduce the speed that waves will be overtaken at.

If going close to downwind and you are competent to drop the sail in such strong conditions you could just drop the sail and blow downwind ... in extreme conditions the boat will still do 4 or 5 mph and with the centreboard down will go maybe 10 degrees each side of straight downwind. I would be very reluctant to go offshore by myself in a strong breeze ... but Christophe was caught out here as can happen... and triple that and add three for the wind being offshore (which it wasn't).

Another trick with bare poles sailing is if you stand up with feet well spread you will be reasonably stable and you will act as a small sail too.

Reaching you can often just let the sail flap partially or fully and keep your speed down.

Another strategy would be to make sure the wind direction is not changing fast, reach with sail flapping or luffing as needed to a point where you can sail near straight downwind. And drop the sail to sail in under bare poles. I've sometimes used just the top of the mainsail raised just very slightly (usually leaning against the mast) with the rest of the sail tied down in the boat.

A little bit of extra speed increases the centreboard's efficiency so you can point a little higher, but it is hard to get effective sail shapes to reach under this sort of jury rig.

DESIGN
The elements to the design are that in Australia we found that the normal tendency to try to do something at the bow to reduce this tendency to "bow steer"

But what to do when the wind gets stronger?

Many modern boats with wide sterns become difficult to handle, or at least have substantial helm loads when heeled even in flat water. They can be fast, but are cows upwind in stronger winds when you have to have a strong bloke on the mainsheet easing sail to prevent the boat from rounding up. This is purely because of heel and the wide stern.

The normal path to downwind speed was normally "long flat run with wide planing sections aft". I've discussed elsewhere that this was a mistake ... though you still see plenty of boats with wide sterns being produced thinking that is the way to high performance. It is 1970s thinking.

In about the mid 1980s several Australian classes started to realise the previous thinking about trying to fix nosediving and bow steering didn't require changes to the front of the boat.

You can see lots of discussion about increasing bow freeboard or increasing bow "reserve buoyancy". And it is kind of natural to think this way.

But I was woken up by the moths in the very early 80s by my friend Alan Downes who was one of the builders of lightweight scow moths. 11ft plywood boats that sail very fast and had hullweights around the mid 30lbs. They have now evolved into the hydrofoil moths.

Because they are fast and were scows it was very easy to stick the nose into the back of the wave in front after surfing the one behind. They tried lots of thing to reduce it until maybe Ian Ward started pinching the sterns in really tight. So the boat might have a bow 3+ft wide, but the stern might only be a shade over 18 inches or something like that. The stern is still wide a couple of feet in front of the transom but suddenly the lower chine sweeps in with quite a radical curve to narrow the transom. That way .. when you move back there is a relatively large change in trim.

Have a look at the Goat's bottom panel and you will see the same thing. The tendency is that designers would normally like to run the chines at the stern at close to parallel to the water flow. But that leads to wide sterns and crappy handling.

From the Moths this went into the various skiff classes where the bottom chine or only chine was pinched in laterally.

Around the same time multihulls started to realise that they could reduce nose diving tendencies by moving the point of maximum rocker aft. Just like the OzRacer/PDRacer bow sticks up in the air when going faster because of the excessive stern rocker ... the multihulls became much less prone to sticking their noses under.

This is the stern of a neat little catamaran built in South Australia that was one of the first to exploit it. It was well know for being good mannered downwind where you could drive it much harder plus it would tack much faster than previous cat generation. Though look carefully at the tornado and that is similar from the early '60s.

Alpha Omega


A-class


I remember the A-classes having this type of rocker from about 1975. But the Boyer designed and built boat here refines it.

Another example was when I was working for Duck Flat in the very early 90s. The second Norwalk Island sharpie 23 had been launched and raced in the biggest yacht race in Australia (in terms of entrants) the Marlay Point race. An overnight race for trailer sailers. All the hot production boats set up special boats for the race, sailmaking companies are there. The wind came in at up to 50 knots in some places. The narrow sterned Norwalk Island Sharpie 23 planed and planed and planed with perfect control, but the wide sterned boats were uncontrollable and most failed to finish. There were 6 sinkings. The NIS broke the course record, won on scratch and handicap.



(re other discussions you can see that the mainsail is sheeted well in beyond the 10 degree mark - slow slow)

This is also the secret of the perceived seaworthiness of double ended boats. Many places on the net they take the old chestnut that the stern "parts the seas coming up from behind". It is actually because the shape has more balanced volumes and the boat doesn't stick its nose in when heeled or when a wave is under the stern.

So the goat has this feature too.

Another thing that comes from this observation is the use of computer software for design. In addition for all my boats I do the same trick as the double ender. I do a simulation in the computer to see what happens to the pitch of the boat as the boat heels. If the boat stays level as it heels - I am happy. This often takes some fiddling with bow lines and stern lines to achieve because the wider stern always wants to overcome the narrower bow. But with care it can be done.

This also ties into the idea that you use the fore and mid body of the boat to plane on ... if you use a wide stern you need the bow up in the air at a crazy angle to get the right angle of incidence. So modern boats have a narrow but flatter section in the mid body and bow ... which is something the flat bottom of the Goat approximates. As we all know it doesn't stick its nose up or start kicking spray when it planes ... it just goes faster without much fuss.

CHANGES
I wonder if it makes sense to experiment with a rudder blade about 200mm deeper for such bad conditions. There still will the same tendencies in the same position ... the rudder should be deeply immersed if you are flat but there is a sudden drop in steering ability
1/ the boat is slowed, rudder forces for the same angle go with the square of the velocity. So if the speed halves the potential rudder force is only 25% of a second ago.
2/ the rig loads up. same thing as 1/! The wind is 20 knots and the boatspeed 12 giving an apparent wind of 8 knots. Stuff the bow and speed is now 7 knots so apparent wind is 13 knots. Sail force when going fast is 8 squared = (actually proportional to) 64. Sail force when the bow is stuffed is proportional to 169 ... double.
3/ Heel is the most likely reason for the rudder area being reduced
4/ extra person doubles the possible righting moment. Ballast is nowhere near as effective as the boat has to go over to such a big angle before it starts working ... BUT IT IS BETTER THAN NOTHING! (particularly if it can't move about). Maybe lots of extra righting moment potentially means more speed when stuffed so the chance of getting out of it better.

So you can see why reefing works as top speed won't be that diminished (as we found with a heavily laden boat in the Texas 200. With one, two or three reefs the speed was about 10 consistently with jumps up to 12 and more.

Sailing downwind in strong winds and big waves is one of the most tricky things you can do. There will always be the point where any boat will be compromised by the conditions and downwind it will happen at lower windspeeds than if you are heading upwind.

The old saying is
Toward the wind the boat can take more than the crew can
Downwind the crew can take more than the boat can

But the old sailors habit is
When you head off - go into the wind
That way you get a speedy return.

(Or I would add - you get SOME return because worse case you can sit on the bottom of the boat until it gets to shore) and always let someone know what your plan is.

MIK

I wrote the most ... but doesn't mean the other parts of the discussion should be discounted!!!

MIK

Waves do have a cirulation. One chap said my argument about hullspeed, displacement yachts and surfing was compromised by it.

I am sure it is a factor, but I think it is outsized by other factors.

The example I used is that the 12 knot speed of a 12 metre yacht surfing can't be down to the speed of the water circulation because if you are a surfer or a fisherman you know the boat just doesn't move backwards and forwards with much velocity as the waves pass under.

But certainly a factor if the boat is close to compromised already because of design characteristics or sailing style.

MIK

Originally Posted by Boatmik

I wrote the most ... but doesn't mean the other parts of the discussion should be discounted!!!

MIK

I have SO gotten my money's worth! I don't know crap about physics or fluid dynamics, but I follow what you're saying (mostly) and am enjoying every bit of it. Thanks professor Storer!


Sent from my iPhone using Tapatalk

Originally Posted by Boatmik

...Ballast is nowhere near as effective as the boat has to go over to such a big angle before it starts working ... BUT IT IS BETTER THAN NOTHING!....

If you do decide to carry ballast on a flat-bottomed boat, would it be better to distribute it along the chines rather than down the centerline?

Not that I'm advocating sandbags for the Goat. I wouldn't know how to affix them properly so they don't slide to leeward. Just wondering if that helps the physics more than having the mass along the axis of rotation.

75# sandbags do not slide to leeward. Neither do 50# ones. Gravity holds them in place.

Good point from Paulie,

It doesn't need to be in the middle because if balanced the effective centre of gravity of the bags is in the middle.

So that opens out the options a shade.

MIK

One way to do the ballast might be to make a tube and wind it around the CB case and tie the ends together.

A feature of the GIS that amazes me is that the faster it goes, the more impossible it seems to bury the bow. MIK and I tested this on Grahamstown Lake when we both sat quite far forward while reaching in around 12-14 kts, and the foot stayed above the water.

Can't compete with Christophe's epic tales (they are the best!), but I did have a fun adventure today sailing on Lake Hodges, a spot close to my house that is surrounded by high hills and terminates at a dam towards the western end. That is also the most constricted part of the lake, and the direction of the prevailing wind, so even though I had to make it through spots with almost no wind at all, and areas where the wind direction shifted 180, when I made it up into the slot I had sustained 15 knots to work into with the full sail. By feathering, pinching like crazy, and not caring that my leech was fluttering away, (and not being in the ocean today, so I could hike from the rail!!), I got to the dam, and the payoff - a nice fast downwind run back. I took a vid of the run back, but there was no way I could film upwind, no available hands!

Testing new stuff: I noticed that it was impossible for me to test Wood's tight traveller, because I put a ratchet block on the end of my boom, and I installed it backward, so you can guess what it did when I tried to tension the leech. With the higher wind and the full sail though, I found better performance with the sail sheeted out beyond the point where I could tension the leech anyway! I just let it flutter away and drove the leading edge.

Going upwind with lots of sail, Feather performs as she always has - very predictable, and stable. Even in gusty canyon wind, you can adjust without losing control - zero weather helm issues, and I mean zero. It's amazing when you think of how much weather helm you would have in other boats in similar conditions. With the sail on the leeward side of the mast, if I got a gust that I could not handle with the sail full, I backwinded the leading edge a bit, and no problemo.

My new "slab" boom, though much stiffer still flexes, but I'm happy with it anyway! You can see the fex in the vid, as I had not bothered to release the outhaul, or ease the downhaul yet. I was fast and stable. (Didn't even raise the board, though I should have a bit!) Full speed downwind I messed with shifting my weight to the opposite side from the sail to balance the helm, and it worked, but I swear it was faster to keep the boat flat and suffer a little weather helm.

Here are three pics as well, that are solutions to my own dumb mistakes:
(1) the perfect use for the ugly Christmas sock-like present you never wear - prevents scratches.
(2) the preventer, in case you forget to engage your trailer winch ratchet before hitting the highway
(3) how to use your wind indicator, in case you forget to install it before raising the mast (doh!).




GIS Feather downwind run on Lake Hodges - YouTube

edit: I switched my vid audio to the Moonlight Sonata, because I can't stand listening to myself, and the sonata has the right tempo to match my camera jerks.

I would recommend that EVERYONE uses a trailer "preventer" as in the above post. If that one piece of rope is there there is zero chance you will lose the boat. You could potentially lose half the trailer and not notice until you got home.

MIK