First Sail on Spinnaker

[catsketcher]

Yes fatter hulls have LESS wetted surface for a given displacement/volume.

The best hull for a very,very slow boat is a half sphere. This gets the maximum volume for minimum wetted surface. At slow speeds wetted surface is a major contributor to total drag.

This is why 2 handed monos like 505s and 470s could beat Tornados in very light conditions. The Cats would glue to the water. (They also were unable to tack well at all in light conditions).

At faster speeds wave making resistance becomes much important and it is this reason we have thin hulls that are not affected by hull speed limitations.

cheers

Phil

[UpOnStands]

Originally Posted by Training Wheels

Don't fatter hulls have less wetted surface for a given displacement?

yes. but fatter hulls have greater transverse cross section and of course the Cd of the fat hull is worse.
This problem should become worse as speeds rise but my guesstimate is that as the fat hull is much shallower than the thin one, the high drag you would expect to get at high speed is being weakened by the free surface effect.

[tp12]

The slim hulled cat won't be at the same displacement though, surely. For the same length it would be lighter; which negates that argument. A longer, slim hulled cat might have the same displacement but then it would have more waterline length and likely be significantly faster in light airs.

No?

[UpOnStands]

urban legend? Does anyone have proof that fat hulls are superior to thin hulls at high speeds - as applied to catamarans? I've seen several statements to that effect but proof? As you note, it seems impossible to create a rigorous comparison.
if we take a fat hull (L/B = 8.5) and scale it to equal the WLOA of a thin (L/B= 12.8) hull, the fat hull has extremely small draft to yield the equal displacement. So small that any real-world comparison becomes impossible?
for 10m WLOA with 3000 kg loading per hull
thin has draft of 53cm
fat hull draft is 10cm

[44'cruisingcat]

Gave it another run today. (Didn't take video.)


Set up a bridle between the bows, so I can move the tack from the cockpit.


Tried sheeting the double reefed main in to the centreline, and was able to sail right down to 180' with the tack off a bow. In fact I could go a bit past 180, with the sail flying nice and stable. In 2-3 metre swells too.


Later on discovered our 'chicken out" windspeed - when it starts getting over around 20 knots I'm starting to think about socking it. We socked it in about 22-23, no difficulty. Just after hitting 18.7 knots on a surf! That was enough!

[catsketcher]

Gday Guys

There seems to be a crossed thread here.

Cats can be faster than equivalent monos at because their hulls are thinner. It allows most of them to operate at higher speeds than displacement craft usually can - heavy monos are limited by their hull speed. It seems that if you get to 8-1 or even better 10-1 fineness ratios you can break through the hull speed barrier. You do not have to be a catamaran to do this.

The Olympic K1 kayak is 5.2 metres long and the world record for the 500m is 96 seconds- That is 10 knots - way above the 5 knot hull speed a normal 5 metre mono has. This can occur because the K1 is impossibly narrow - try just sitting in one and you will fall in - without doubt. It has an approx 30cm waterline beam giving a very high fineness ratio. They go much faster than the Tk1 kayaks which have a minimum waterline beam of 50cm which because of their shorter length gives them a fineness ratio of about 9-1. TK1s really hit the wall when you paddle them - you can feel the boat wants to climb the bow wave.

If you want to go fast you want fine hulls. There is a little caveat to this in that you also want the hulls to be full at the ends to go fast but this does increase drag a little at slow speeds.

For a GIVEN VOLUME/DISPLACEMENT a hull with a higher fineness ratio will have a higher wetted surface area. So if I design a 4000kg cat with rounded 6-1 hulls and you design a 4000kg cat with 12-1 hulls your cat will have more wetted surface area than mine. You can check this out using an ellipsoid area calculator. The nice thing about the ellipsoid is that a sliced in half ellipsoid is pretty much the same shape as the underwater section of our cats. Start with every dimension at 1 - that is a sphere. You get a certain value.

The nice thing about ellipsoids is that if you increase one dimension by X and reduce a second by X (keeping the third constant) the volume stays the same. Play around with it and you can see that the surface area - wetted surface if it is underwater - goes up even if the volume stays the same.

https://www.google.com.au/webhp?sour...ind%20S&skip=s

In light winds wetted surface is a primary component of the drag of a boat. This is because when going slowly wavemaking and form drag are not much of an issue. Look for waves when a 12 metre cat does 3 knots - if the sterns are floating free then there will hardly be a ripple - so almost no energy is needed from the boat's engine or sails to be converted into wave energy. You still have to push the hull through the sticky water and this is a major part of drag downwind. (There is induced drag that is more important when developing lift from a foil upwind but we are sailing downwind).

So after this long monologue we have the fact that shorter and fatter hulls carry a certain displacement more efficiently when moving slowly. As soon as they make waves they are turning motive power into wasted energy. They will also hit the wall earlier and more forcefully. Your nice fine hull cat will hardly notice if you get it near its theoretical hull speed. (Look up Froude ratios).

On top of this a Lagoon 440 with its waterline of about 42ft has a hull speed of about 8.6 knots. Above what was shown on the Lagoon's speedo.

I first learnt about fat hulls being fast in light winds when I was sailing from Moreton Bay to Mooloolaba in my 31ft racing trimaran. A 42 ft fat Harris cruising tri left at the same time. I thought we would leave him for dead but in the light and square conditions he was very hard to leave behind. When the wind came up the next day we left him over the horizon but he had double beds, a private head, dinette and huge storage. He could get up to 8 knots easily and was happy with that. It was what I asked my designer for when I got Kankama's plans. She goes a lot faster than that but going fast can ask a lot of a boat if you go too extreme.

cheers

Phil

[catsketcher]

Gday 44

Pulling the main on tight on a square gives me the shivers - it is so wrong on any other boat. It must be making the boat much harder to steer for the autopilot.

Only a cat could do this. On a mono pulling the main in tight on a square is asking for a broach. In nasty conditions you may end up broaching a cat which is very hard to do. Why not just pull the main down totally? It does nothing but round you up when sheeted in hard.

When I taught sailing I used to teach that we steer with the main and follow with the helm. It is the only way to keep a dinghy or light yacht on its feet in heavy air. ease the main out to steer downwind, tighten to go upwind. Pulling the main in tight on a square is like fingernails on the chalkboard.

[44'cruisingcat]

I had the same concerns. But our friends have been doing this for years.


The main in this case is really only there to blanket the spinnaker when socking it. They had a couple of instances when they didn't have the main up where they had real difficulty getting the sock down on their symmetrical kite. They were ready to get rid of the spinnaker altogether when they tried this. Now they use their spinnaker constantly.


To play it safe, I put a couple of reefs in. It does work, the boat steered fine, although the main flopping from one side to the other was annoying. As a bonus, when the wind gets up and you decide to drop the spinnaker, you already have the main up, ready to go. Keeps the boat moving, reducing apparent wind too.


And that technique allowed our assy to run way down deep, dead square, very nicely. Intuitively it seemed like a bad idea. But it's hard to argue with how it works.

[catsketcher]

Knock me down with a feather. First a thread where a Lagoon goes fast in light wind and then I learn that doing the exact opposite of a concrete law of trimming is helpful.

[sea-bee]

Originally Posted by 44'cruisingcat

... able to sail right down to 180' with the tack off a bow. In fact I could go a bit past 180, with the sail flying nice and stable...

On our trip back from Qld last month, we tried different arrangements each day looking to be able to carry our little ASSY DDW together with the main. We found that with the tack poled out really wide, and the main's boom pulled out and down with a preventer, that we could sail right up to and a bit past 180. It was significantly faster with the main than the spinnaker alone. We already had the pole for holding out the genoa when running wing and wing, and even with the beam of our cat it made the difference in being able to stick the luff out wide.

What's the advantage in keeping the main sheeted on the centreline? Is it just in heavier winds when you don't need the extra power?

Chris

[sailjumanji]

Originally Posted by yeloya

Screacher is not an upwind sail. See wikipedia below:
"
A screacher is a sail that combines the features of a spinnaker and a reacher.
Its similarity with a spinnaker is that it is not attached to a stay along its luff, and typically has a slightly larger curvature than a genoa"
Yeloya

Wow, I raced Corsair trimarans for eleven years in Nationals and even Key West Race Week events. All of our boats had screachers on bowsprit, and the better-cut sails were very effectively raced in both light air beating conditions, as well as reaching. On downwind we flew asymmetric spins. These sail combinations, including square top main, and jib, made these trimarans very effective/fast on all points of sail.

Wikipedia is wrong, regarding screacher not used upwind. And it's not a terminology issue - the Class Rules and sail makers for these boats have always referred to these sails as screacher. I don't think Code 0 came onto the scene until later.

[tomfl]

Originally Posted by sailjumanji

Wow, I raced Corsair trimarans for eleven years in Nationals and even Key West Race Week events. All of our boats had screachers on bowsprit, and the better-cut sails were very effectively raced in both light air beating conditions, as well as reaching. On downwind we flew asymmetric spins. These sail combinations, including square top main, and jib, made these trimarans very effective/fast on all points of sail.

Wikipedia is wrong, regarding screacher not used upwind. And it's not a terminology issue - the Class Rules and sail makers for these boats have always referred to these sails as screacher. I don't think Code 0 came onto the scene until later.

That has been my impression as well. Part of the confusion may be that while the true wind direction would look like a reach, or even a broad reach, the apparent wind direction turns into more of a close reach because the boat speed moves the wind direction forward. And faster boats, like an fboat, really shift the apparent wind forward. Especially in what I will call light wind (less than say 12 knots) a six or eight (or even more) knot boat speed will result in a huge shift in the apparent wind direction. I figure my Seawind is a 2/3 wind speed boat and the fboats often move at the wind speed or greater.

When I bought my Seawind the PO had ordered a new screecher from Mack and returned it to be recut so it was flatter to better go up wind. I suspect the cut of any sail makes a big difference in how well it goes to windward.

In any case I ran across this vid of monos (I know this is a multi forum) that is worth a click to see how mains and head sails can be flown. Really nice views from what seems to be a camera on a UAS.

link

[mikereed100]

Originally Posted by 44'cruisingcat

To play it safe, I put a couple of reefs in. It does work, the boat steered fine, although the main flopping from one side to the other was annoying. As a bonus, when the wind gets up and you decide to drop the spinnaker, you already have the main up, ready to go. Keeps the boat moving, reducing apparent wind too.

We do something similar. If we expect the wind to come up we carry a reefed main with our symmetrical spinnaker so that when we sock the spinnaker we need only unroll the jib to keep on our way. If the wind is light and we expect it to stay that way, we leave the main down. I really don't find the main much help in socking the spinnaker, even in 20 knots. I have gotten into trouble before when the main caused the spin to suck back into the rigging. Now, I just have my wife release the windward sheet and guy then lay on my back on the tramp and pull.

[44'cruisingcat]

Originally Posted by catsketcher

Knock me down with a feather. First a thread where a Lagoon goes fast in light wind and then I learn that doing the exact opposite of a concrete law of trimming is helpful.

This guy has been a national champion in OTB cat's, as have his kids in cat's he built them. Maybe being willing to try stuff is how we improve?

[44'cruisingcat]

Originally Posted by sea-bee

What's the advantage in keeping the main sheeted on the centreline? Is it just in heavier winds when you don't need the extra power?

Chris

Having the main sheeted on the centreline basically keeps it from upsetting the spin, sailing DDW. As said before, it's only really hoisted to help in getting the spin down later.

Poling the tack out wide probably gets your spin clear of the main's wash. Being a small sail probably helps too. I don't have a pole though, and don't really plan to.