Catamaran tacking angles , really that bad???

[JustMurph]

Originally Posted by arsenelupiga

your VMG is 5.37 as per above. we do 6 kn at 30 app in 15 kn. Our VMG is 4.5 kn. This is 19% better. Not bad but not that great difference, given comfort penalty you had to take.

There’s little to no comfort penalty in this boat in my experience so far. It’s sits pretty damn flat and doesn’t hobby horse. I’d go so far as to say on a lot of points of sail the faster she goes the smoother the ride.

[Factor]

Originally Posted by arsenelupiga

your VMG is 5.37 as per above. we do 6 kn at 30 app in 15 kn. Our VMG is 4.5 kn. This is 19% better. Not bad but not that great difference, given comfort penalty you had to take.

Have you been on a Mumby? -Its one of the more comfortable boats I have sailed.

[django37]

Originally Posted by arsenelupiga

impressive ! using jib and main ?

Yes, but note that TWS is in m/s

[arsenelupiga]

Originally Posted by Factor

Have you been on a Mumby? -Its one of the more comfortable boats I have sailed.

not been on one but saw it in anchorage. Looks actually pretty good. Made from aluminium - different purpose boat than ours i guess.

[arsenelupiga]

Originally Posted by django37

Yes, but note that TWS is in m/s

yup, understand the numbers. How do you achieve such great numbers ?

I have studied my track on second by second basis and even at 6 kn upwind when one can keep hulls glued to sea, there is very little slipping. Maybe 3 deg or even less. Once bows are flying here and there, that is when slippage becomes large.

[Factor]

Originally Posted by arsenelupiga

not been on one but saw it in anchorage. Looks actually pretty good. Made from aluminium - different purpose boat than ours i guess.

Mumby's are designed to be long distance live aboard cruising boats. The designer has circumnavigated at least once on his own design and has lived aboard for many many years. In fact I don't think I have come across a mumby that hasn't been a live aboard boat.

[JustMurph]

Originally Posted by arsenelupiga

you could be onto something here. because larger boats gets harder to build stiff enough and get quicker structural damage if trying too hard in chop. in flat water any cat goes well upwind.

Utter nonsense.

Originally Posted by arsenelupiga

we are discussing rough seas here, not flat water. You will not do 8 kn in 20 kn of wind here unless on suicide mission.

We can't sail to windward in a messy sea state quite as fast as fxykty, but not too far off an average of 8 knots. Our tacking angles open up a bit more, but we certainly aren't thinking about slowing the boat down.

[mark_morwood]

Originally Posted by arsenelupiga

we are discussing rough seas here, not flat water. You will not do 8 kn in 20 kn of wind here unless on suicide mission.

That was not my experience on our Catana 48. As one example we (family of 4) spent quite a few days of our passage from Bermuda to the Azores in 2013 sailing upwind into a 20 to 30 kt NNE wind at around 8 knots without any real trouble.

Here's a video if you're interested of what that looks like in a Catana 48. Under the clock on the bulkhead you can see an instrument repeater showing 8 knots boat speed in 28 knots of wind in the first video. For comfort and speed we were cracked off quite a bit from close hauled to maximise VMG.

https://youtu.be/TWEUyKMk0z4
https://youtu.be/rY4-lNKxF4E

[fxykty]

Originally Posted by mark_morwood

That was not my experience on our Catana 48. As one example we (family of 4) spent quite a few days of our passage from Bermuda to the Azores in 2013 sailing upwind into a 20 to 30 kt NNE wind at around 8 knots without any real trouble.



Here's a video if you're interested of what that looks like in a Catana 48. Under the clock on the bulkhead you can see an instrument repeater showing 8 knots boat speed in 28 knots of wind in the first video. For comfort and speed we were cracked off quite a bit from close hauled to maximise VMG.



https://youtu.be/TWEUyKMk0z4

https://youtu.be/rY4-lNKxF4E

I don’t understand how you can bear away from close hauled and increase VMG. How does that work?

I can’t see the wind angle in the video, but assume it’s 28 knots AWS and between 40* - 60* AWA given the position of the boom. Using the L-36 VMG to Wind Calculator, I get the following numbers:
[LIST][*] AWS 28 knots[*] BS 8 knots[*] AWA 60*/ 50* / 40*[*] VMG 1.92 / 3.38 / 4.79 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is faster despite the slower speed.
[*] AWS 28 knots[*] BS 6.5 knots[*] AWA 50* / 40* / 30*[*] VMG 3.07 / 4.15 / 5.1 knots

Of course, leeway may increase at the slower speed, smaller angle.

Now, all the same figures but TWS, as I don’t know which you had shown. We reef to AWS so that’s what our instruments show as primary wind.
[*] TWS 28 knots[*] BS 8 knots[*] AWA 60*/ 50* / 40*[*] VMG 2.16 / 3.68 / 5.08 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is again faster despite the slower speed.
[*] TWS 28 knots[*] BS 6.5 knots[*] AWA 50* / 40* / 30*[*] VMG 3.23 / 4.30 / 5.21 knots

[Tupaia]

Originally Posted by fxykty

I don’t understand how you can bear away from close hauled and increase VMG. How does that work?

it can be an advantage, but not in all cases, the same as gybing downwind doesn't always benefit VMG.


It can be beneficial in rough seas to enable you to keep the power up and limit leeway rather than getting slowed with each wave when leeway will be proportionately higher. In all cases you need to be cognizant of the actual VMG. There will be a sweet spot for every wind speed and sea state, something that is not always evident from polars.


There are other factors like aerodynamic drag that increases by the square of the area the faster you go. A reason why many condormans with huge above water profile, high bows that also sport angular sharp edged bridgedeck cabins do so badly to windward.


I remember that when the bows on Play Station were raised to limit wave action on the forward beam there were discussions about the effect this would have on the windward performance in flat water. Everything is a compromise.


There is an interesting article here.



Ocean Sail Articles: Velocity Made Good Trading off course against speed

[arsenelupiga]

Originally Posted by fxykty

I don’t understand how you can bear away from close hauled and increase VMG. How does that work?

I can’t see the wind angle in the video, but assume it’s 28 knots AWS and between 40* - 60* AWA given the position of the boom. Using the L-36 VMG to Wind Calculator, I get the following numbers:
[LIST][*] AWS 28 knots[*] BS 8 knots[*] AWA 60*/ 50* / 40*[*] VMG 1.92 / 3.38 / 4.79 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is faster despite the slower speed.
[*] AWS 28 knots[*] BS 6.5 knots[*] AWA 50* / 40* / 30*[*] VMG 3.07 / 4.15 / 5.1 knots

Of course, leeway may increase at the slower speed, smaller angle.

Now, all the same figures but TWS, as I don’t know which you had shown. We reef to AWS so that’s what our instruments show as primary wind.
[*] TWS 28 knots[*] BS 8 knots[*] AWA 60*/ 50* / 40*[*] VMG 2.16 / 3.68 / 5.08 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is again faster despite the slower speed.
[*] TWS 28 knots[*] BS 6.5 knots[*] AWA 50* / 40* / 30*[*] VMG 3.23 / 4.30 / 5.21 knots

that is very true. My current thinking for upwind work is to maximise combination of best VMG and least stress on structure. That point comes when one bears away so that hulls are not launched off the wave regularly. Longer hulls will do better here. Sometimes waves are naughty and one has to go 50 + AWA. Sometimes 30 awa will do even in large seas. One can steer tighter angle manually (say 40 awa) and avoid 'launching' waves to improve VMG a bit. I think it works but is a bit of work i am not prepared to do unless emergency.

Any other tricks to improve VMG in nasty seas?

[wingssail]

Originally Posted by fxykty

I don’t understand how you can bear away from close hauled and increase VMG. How does that work?

I can’t see the wind angle in the video, but assume it’s 28 knots AWS and between 40* - 60* AWA given the position of the boom. Using the L-36 VMG to Wind Calculator, I get the following numbers:

• AWS 28 knots
• BS 8 knots
• AWA 60*/ 50* / 40*
• VMG 1.92 / 3.38 / 4.79 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is faster despite the slower speed.

• AWS 28 knots
• BS 6.5 knots
• AWA 50* / 40* / 30*
• VMG 3.07 / 4.15 / 5.1 knots

Of course, leeway may increase at the slower speed, smaller angle.

Now, all the same figures but TWS, as I don’t know which you had shown. We reef to AWS so that’s what our instruments show as primary wind.

• TWS 28 knots
• BS 8 knots
• AWA 60*/ 50* / 40*
• VMG 2.16 / 3.68 / 5.08 knots

Now, let’s go 10* closer to the wind and 1.5 knot slower. Notice in all cases the VMG is again faster despite the slower speed.

• TWS 28 knots
• BS 6.5 knots
• AWA 50* / 40* / 30*
• VMG 3.23 / 4.30 / 5.21 knots

A couple of comments about the figures you're presenting:

In each of your lists you are showing a constant boat speed (i.e. 8 knots) while varying the wind angle (i.e.[*] AWA 50* / 40* / 30*). This produced quite high VMG numbers at the closer wind angles. In reality all boats slow down as they head up. Best VMG is achieved at the wind angle which is shown on the boat's polars, such as TWA 45+, and heading up will always produces lower VMG. Heavy seas often slow a boat and require further bearing away.

Secondly, looking at the videos I don't see 28 knots of true wind. It looks lower than that, let's accept AWS 28 and considering the BSP figure you're giving, 8 knots, I'd guess the TWA would be around 48 degrees and VMG slightly over 5.5 which is still impressive and a realistic scenario, in my view.

[fxykty]

Originally Posted by wingssail

A couple of comments about the figures you're presenting:



In each of your lists you are showing a constant boat speed (i.e. 8 knots) while varying the wind angle (i.e. [*] AWA 50* / 40* / 30*). This produced quite high VMG numbers at the closer wind angles. In reality all boats slow down as they head up. Best VMG is achieved at the wind angle which is shown on the boat's polars, such as TWA 45+, and heading up will always produces lower VMG. Heavy seas often slow a boat and require further bearing away.



Secondly, looking at the videos I don't see 28 knots of true wind. It looks lower than that, let's accept AWS 28 and considering the BSP figure you're giving, 8 knots, I'd guess the TWA would be around 48 degrees and VMG slightly over 5.5 which is still impressive and a realistic scenario, in my view.

We need the actual TWA or AWA figure. The mainsheet is quite eased, so extrapolating the boom angle to our boat (only slightly bigger) the AWA must be around around 50*, which at 8 knots boat speed is a TWA of 65* and TWS 23 knots.

Notice that I did drop the boat speed for the narrower angle comparison to 6.5 knots, as the poster wrote that they cracked off to speed up and improve VMG. That’s what I’m wondering about.

It’s really hard to judge wind speed and wave heights from video.

[fxykty]

Originally Posted by arsenelupiga

that is very true. My current thinking for upwind work is to maximise combination of best VMG and least stress on structure. That point comes when one bears away so that hulls are not launched off the wave regularly. Longer hulls will do better here. Sometimes waves are naughty and one has to go 50 + AWA. Sometimes 30 awa will do even in large seas. One can steer tighter angle manually (say 40 awa) and avoid 'launching' waves to improve VMG a bit. I think it works but is a bit of work i am not prepared to do unless emergency.



Any other tricks to improve VMG in nasty seas?

Slowing down is a more effective tactic to stop launching off waves than is bearing away, if you still want to go somewhere upwind. To bear away far enough so that the increased speed of the wider angle doesn’t make launching off waves more of a problem you would be on a beam reach.

Assuming that you are properly reefed for the conditions, fastest VMG is provided by slowing down and heading higher - as the smaller wind angle increases VMG faster than the slower boat speed decreases it. Keep track of your leeway and keep just enough boat speed to prevent leeway dramatically increasing. In bigger seas that will be a wider angle than in flatter seas.

Use both daggerboards (if you have them). Be mindful of the sea state and whether tripping is a risk.

Try dropping your boom to 10-20 degrees below the centreline, just far enough that it just starts getting backwinded by the headsail. Keep the sheet tight and drop the main on the traveller. Flatten the mainsail through more outhaul and more halyard, if possible. This will produce less side force and decrease leeway.

Run one engine and motor sail - very helpful to keep the boat speed from slowing down too much at the tighter angle.

I’m not sure that steering around waves is a thing for catamarans, at least for upwind where you’re approaching waves at an angle. You would turn up to set up your windward bow to spear through an approaching wave, then you would bear away along the top of the wave to avoid launching off it. By that time your leeward bow encounters that same wave at a wide angle that will launch it of that wave.

The reality is that for cats you generally have to sail wider angles to have more speed to overcome higher leeway (compared to monohulls), due to the increased aerodynamic drag at higher wind speed and increased wetted surface drag at slower wind speeds. Bear away far enough and the increased boat speed and reduction in sideways force due to wider wind angle will practically remove leeway. But for upwind, you need the tighter angle.

[mark_morwood]

Originally Posted by fxykty

I don’t understand how you can bear away from close hauled and increase VMG. How does that work?

Sorry that was an oversimplification. I was not trying to start another VMG v AWA v TWA v boat speed discussion and I do not have the data to support one from that passage. I was just saying it is quite possible to go upwind on a cruising catamaran at 8 knots in 20+knots for many days without it being suicidal.

To be more accurate I should really have said to maximise the combination of VMG, comfort, strain on the boat, strain on the crew, etc, taking into account sea state, the autopilot's steer to wind capabilities in rough conditions, and the forecast which was for a shift to the east in a couple of days time which meant we'd be able to flop over onto a starboard tack and fetch the Azores as planned.