Knots generally necessary to plane a C420 downwind?

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BearDown185

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With the caveat that it will necessarily depend on crew weight and wave conditions, is there a rough rule of thumb for when a C420 will start to plane when running or on a broad reach?

Something like a 220lb crew needs X kt wind or XX mph boat speed; a 275lb crew needs Y kts/Z mph boat speed?

People's subjective experience at given crew weights would also be very useful.

 
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BearDown185

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See this.  Dinghies with a relatively flat aft will begin to rise out of the water one they hit a certain boat speed.   This results in a decrease in drag from the hull/water interaction, which results in a further increase in speed, which results in even more rise, which results in even less drag, which results in even more speed, until the boat is eventually riding across the top of the water.  

Once you hit this point, the boat's maximum speed is no longer limited by its natural hull speed as it is now skimming across the surface of the water and will take off like a rocket.

The actual speed at which this happens differs from one boat design to another and is further influenced by the overall weight of the boat (which is where crew weight comes into play).

 
fastyacht

fastyacht

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A week or so ago I was steaming upriver back from a fishing trip. Winds were gusting to 20. When a fleet of college crews in their 4-twinkies broad reaching home sailed into a gust front each crew got up and scooted along smartly, one after the other as they got into the gust.

 
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fastyacht

fastyacht

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7 knots boatspeed is not planing (transonic though LOL). Also 7 knots windspeed defilutelymy not planing

who knows what the original poster was meaning. Seemed to me windspeed but then he quited shaomopedia silliness so who knows.

"Planing" is an overused term. But then again people think it is like pigs. It isn't really. But whatever.

 
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maxstaylock

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If the info coming out of this thread seem a bit squiffy, it's because dinghy racers don't generally measure boat speed or wind speed.  No idea how fast I go upwind or down, only care if it's 1 knot faster or slower than you.

If you're wondering if you are planing, then you are not, it's not something you need a speedo to judge.  

Does a C420 have a kite?  International 420's with trapeze, spinnaker, and a couple of caffeined up gym bunnies could probably plane with about 9 knots true, a couple of fat inept losers like me would struggle to not plane, even without trapeze and spinnaker, on a broad reach in about 18 knots?

 
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Emilio Castelli

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Green Valley, Russian River
fastyacht said:
7 knots boatspeed is not planing (transonic though LOL). Also 7 knots windspeed defilutelymy not planing

who knows what the original poster was meaning. Seemed to me windspeed but then he quited shaomopedia silliness so who knows.

"Planing" is an overused term. But then again people think it is like pigs. It isn't really. But whatever.
Click to expand...
Hull speed for a 4.20 is approximately 5knts. If you're going faster than that, you're planing.

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BearDown185

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Thank you for the helpful feedback.  It sounds like there is a dearth of solid numbers.  

As for tracking boat speed, these days various personal electronic devices (cameras, smartphones, watches) have GPS support and can track your speed and course during practices.  

I was looking for a rule of thumb to give to Club 420 sailors in various weight ranges as to when they should be feeling out a broad reach for planing conditions.  As in "the forecast predicts the wind should reach a constant 12 kts around 11am, so don't forget to start feeling out the conditions after the 3rd race."

 
Steam Flyer

Steam Flyer

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BearDown185 said:
Thank you for the helpful feedback.  It sounds like there is a dearth of solid numbers.  

As for tracking boat speed, these days various personal electronic devices (cameras, smartphones, watches) have GPS support and can track your speed and course during practices.  

I was looking for a rule of thumb to give to Club 420 sailors in various weight ranges as to when they should be feeling out a broad reach for planing conditions.  As in "the forecast predicts the wind should reach a constant 12 kts around 11am, so don't forget to start feeling out the conditions after the 3rd race."
Click to expand...
Make planing part of your practice. When it's possible, or seems like it might be possible, give them a chance to do some fast reaching and pump it up. They will race the way they practice.

As for speed O meters, useless. It does no good whatever to know that you're going 5.3 knots instead of 5.1 knots, unless you also know why. Big expensive boats use expensive instruments to show speed relative to true wind, velocity and angle.

FB- Doug

 
fastyacht

fastyacht

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Why would you need to tell your sailors when to seek out a broad reach?  I mean, you just feel the conditions and play out there anyway. That's part of the mystery of it all and fun.

As to "planing" you are not understanding that it doesn't work the way that wikipedia article says it does. There isn't some magical number. Sailboats such as the club 420 almost never actually plane (as defined by naval architects). They do "plane" somewhat by "sailor's" terminology (see Emilio's definition above).

As you get more power, the boat just goes progressively faster. As the stern wake leaves the stern, the boat trims aft, the bow rises etc. The specific behavior depends on the boat. A GP14 will sky the bow quite early. When you want to really go you sit way aft to get the rocker out of the way--but only if you have enough power to actually get up. Otherwise you keep the bow down. On a laser you will feel this strange bow lift kick in if you are light weight but it wont happen if you are heavy, etc. A V15 if driven hard will not lift off if you keep the crew at the shrouds but will find another "gear" if you move aft at the appropriate time (and no, you can't measure this you have to learn by feedback. You don't need instrumentation and it wouldn't help anyway). A 505 just goes faster and faster, it doesn't go through marked changes in trim like the other two examples I gave you. As a 505 really gets going you move aft like all boats as much to balance the sail forces as anything else. On a thistle if you don't go all the way aft with the chute in 25+ you will do down the mineshaft. The club 420 has a somewhat strange bow shape that makes for optical illusion of the bow drooping. It also has a lot of Vee up there so the bow doesn't lift like a laser.

But none of this really matters. You go sail the boat and learn how to feel the changes. You can see the wake changing--that's the most useful sailor's judge of how fast you are going. When the stern wake starts to leave the stern....

 
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crashtack

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BearDown185 said:
I was looking for a rule of thumb to give to Club 420 sailors in various weight ranges as to when they should be feeling out a broad reach for planing conditions.  As in "the forecast predicts the wind should reach a constant 12 kts around 11am, so don't forget to start feeling out the conditions after the 3rd race."
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Not sure this is ever relevant in a c420. There's a very slim range of conditions where heading up and doing wild-thing seems to be marginally vmg optimal on a i420 versus just sending it as low as possible, but I've never seen that on a c420. The only situation I can think of where this would be useful is when heading up to a broad reach allows you to maintain planing while surfing, but that's a more general rule of thumb that's applicable in any surfing conditions.

Especially with a kite, I think you'd be better off teaching your kids how to go downwind more effectively in breeze

 
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