Upwind boat speed equals true wind speed

[AngusMcDoon]

Upwind at 30 degrees to apparent wind, boat speed equals true wind speed.

Stick that in your 'multihulls don't go upwind' bias and spill your mug of tea.

 

[MisterBaxter]

A quick doodle suggests that the course would have been 60 degrees off the true wind direction, so not pointing that high... But VMG of half the true wind speed, which feels pretty quick to me! However, my trigonometry is extremely rusty so could be entirely wrong.

 

[AngusMcDoon]

As boat speed increases as a proportion of true windspeed it's diminishing returns unfortunately. This is an inescapable consequence of the vector maths. When boat speed equals true windspeed the tacking angle is 4 times the apparent wind angle - so 4 times 30° is 120°. The only way to improve this is to reduce angle to apparent wind but not many boats can get below 30°. This is not a feature of multihulls but applies to any wind powered craft.

This diminishing returns is why ice yachts, which sail at multiples of true wind speed, are almost always sailing hard on the wind, even not far off downwind. If the boat speed was an infinite multiple of the true wind speed the tacking angle would be 180° and you'd never get upwind at all.

 

[johnalison]

As boat speed increases as a proportion of true windspeed it's diminishing returns unfortunately. This is an inescapable consequence of the vector maths. When boat speed equals true windspeed the tacking angle is 4 times the apparent wind angle - so 4 times 30° is 120°. The only way to improve this is to reduce angle to apparent wind but not many boats can get below 30°. This is not a feature of multihulls but applies to any wind powered craft.

This diminishing returns is why ice yachts, which sail at multiples of true wind speed, are almost always sailing hard on the wind, even not far off downwind. If the boat speed was an infinite multiple of the true wind speed the tacking angle would be 180° and you'd never get upwind at all.

YouTube's algorithm recently decided that I wanted to look at ice yachts. I have to admit that they are impressive, and, as you say, sail close-hauled most of the time. What impressed me was that in the USA there is a culture of preserving and racing vintage ice-yachts from the early part of the last century. From what I can see, they are beautifully-crafted and still go pretty well.

 

[B27]

YouTube's algorithm recently decided that I wanted to look at ice yachts. I have to admit that they are impressive, and, as you say, sail close-hauled most of the time. What impressed me was that in the USA there is a culture of preserving and racing vintage ice-yachts from the early part of the last century. From what I can see, they are beautifully-crafted and still go pretty well.

I believe they cracked the 100mph record in the late 1800s.

I've raced dinghies against foiling windsurfers for a laugh, they don't point high but still do two laps to our one.

 

[fredrussell]

An impressive feat, but those Tacktick displays are ‘orrible.

 

[Chiara’s slave]

An impressive feat, but those Tacktick displays are ‘orrible.

Dragonfly have s9me sort of pact with the devil, ie Raymarine, who offer their oldest shite to the factory it seems. Mine has 40 series displays on the hatch housing. We have up to date Garmin stuff on the bulkhead, it’s only speed and depth. As speed is always much too fast enough, and she’ll float on wet mud, neither are relevant.

We find VMGmax at 27 degrees apparent usually, so in those conditions we’d point a tad higher, But our sails are new, which helps, and Chiara is a smaller, lighter boat. Impressive numbers all the same, for a 35 footer with a lovely interior. The display says about 52 deg true. It’s actually quite hard to say what’s going on cos you’ve obviously got some tide from somewhere behind you. Unless you know the tidal vector, the true vs apparent calculation cannot be accurate.

 

[AngusMcDoon]

An impressive feat, but those Tacktick displays are ‘orrible.

The display heads take no power from the boat at all, which for a type of boat that motors less than most, is an advantage.

 

[Chiara’s slave]

The display heads take no power from the boat at all, which for a type of boat that motors less than most, is an advantage.

I guess it is. Our battery is never flat though, with a N2000 network, a fridge, a stereo, heater etc, with just a bit of solar and the world’s most pathetic outboard alternator output. Solar panels these days seem to be the dogs danglies. Maybe not so good in the frozen north?

 

[AngusMcDoon]

We find VMGmax at 27 degrees apparent usually, so in those conditions we’d point a tad higher, But our sails are new, which helps, and Chiara is a smaller, lighter boat. Impressive numbers all the same, for a 35 footer with a lovely interior. The display says about 52 deg true. It’s actually quite hard to say what’s going on cos you’ve obviously got some tide from somewhere behind you. Unless you know the tidal vector, the true vs apparent calculation cannot be accurate.

True wind is relative to the water. Tide doesn't come in to the calculation.

My sails are new.

 

[AngusMcDoon]

I guess it is. Our battery is never flat though, with a N2000 network, a fridge, a stereo, heater etc, with just a bit of solar and the world’s most pathetic outboard alternator output. Solar panels these days seem to be the dogs danglies. Maybe not so good in the frozen north?

I sometimes spend 2 to 3 weeks between shore power, only coming in from the drizzle when all the Pringles have been nibbled & I'm hungry.

 

[Chiara’s slave]

True wind is relative to the water. Tide doesn't come in to the calculation.

My sails are new.

You use stw not sog? And heading not cog?

 

[Chiara’s slave]

I sometimes spend 2 to 3 weeks between shore power.

Shore power? What’s that? We last connected to shore power about 18 months ago. I reckon we must have used that up by now.

 

[AngusMcDoon]

You use stw not sog? And heading not cog?

I use stw because true wind is relative to the water. Heading doesn't come into the calculation. If SOG & COG are used you get ground wind. In a previous life I was a software developer for a major marine electronics manufacturer & wrote code for this stuff. In the high end kit there is also measured wind which is what the MHU measures. It is converted to apparent wind by calculating out the effect of pitch & roll which on a big boat in a big sea is significant.

 

[dunedin]

A quick doodle suggests that the course would have been 60 degrees off the true wind direction, so not pointing that high... But VMG of half the true wind speed, which feels pretty quick to me! However, my trigonometry is extremely rusty so could be entirely wrong.

So does that make the VMG 3.0 knots?

According to our Polars we are supposed to make 3.46 VMG in 6kts TWS, at a true angle of 45deg.
Of course Polars are targets in race mode, not in fully laden cruise mode. But in flat water we certainly can make good progress upwind in 6kts True.

 

[AngusMcDoon]

So does that make the VMG 3.0 knots?

According to our Polars we are supposed to make 3.46 VMG in 6kts TWS, at a true angle of 45deg.
Of course Polars are targets in race mode, not in fully laden cruise mode. But in flat water we certainly can make good progress upwind in 6kts True.

Yes, about that.

From the vector diagram your polar figures would need you to sail at 5 knots through the water at an AWA of 24°. That sounds a bit optimistic.

 

[Neeves]

I guess it is. Our battery is never flat though, with a N2000 network, a fridge, a stereo, heater etc, with just a bit of solar and the world’s most pathetic outboard alternator output. Solar panels these days seem to be the dogs danglies. Maybe not so good in the frozen north?

In the frozen north in the winter the sun might not be very useful but when its not frozen they get more sunlight (depends on how far north they are). The trouble is that they cannot harvest in the land of 24 hour sunshine and keep enough for the winter.

Birds overcame the issue by migrating.

Jonathan

 

[Chiara’s slave]

In the frozen north in the winter the sun might not be very useful but when its not frozen they get more sunlight (depends on how far north they are). The trouble is that they cannot harvest in the land of 24 hour sunshine and keep enough for the winter.

Birds overcame the issue by migrating.

Jonathan

The sun is in rather short supply in Scotland (which is not really the frozen north, but we soft southerners like to big up our Scottish cousins by pointing out how tough they are sometimes) in the summer, let alone the winter. It is possible that our solar works well because the boat is parked in one of the sunniest places in UK, on the south coast. Though those instruments are solar powered, a tiny strip across the top. Obviously they manage. I’m just wondering on the logic of relieving the load on the boat electrics when the instrument draw is obviously very low. As they were doubtless there when he bought the boat (I think I have been on board years ago, I don’t remember that) I daresay it’s also about the cost of replacing them. Something to do when they finally pack up rather than just because you may not like them much.

 

[AngusMcDoon]

Though those instruments are solar powered, a tiny strip across the top. Obviously they manage. I’m just wondering on the logic of relieving the load on the boat electrics when the instrument draw is obviously very low. As they were doubtless there when he bought the boat (I think I have been on board years ago, I don’t remember that) I daresay it’s also about the cost of replacing them. Something to do when they finally pack up rather than just because you may not like them much.

They draw very little & can be solar powered because they are segmented transflective LCD's. They need no backlight in daytime. Consequently they draw only a few mA compared to the i70s displays which draw 160 mA each with full backlight. The graphic multicoloured multifunction displays look great, but there's a cost with power consumption. A plain segmented digit shows values just as well. It's this feature that provides the relief to the boat's power supply rather than their being solar powered.

When I worked for a marine electronics manufacturer I was developing software for a race display for Open 60's, & they use segmented LCD displays for the same reason.

 

[Chiara’s slave]

I’m only a bit surprised you need to find that kind of power saving, I’m aware it can be done.