What boatspeed should I expect, as a percentage of True wind?

[arsenelupiga]

Originally Posted by fxykty

Hull speed is where the wavelength of the bow wave equals the waterline length. For heavier boats the stern DNA tends to squat and you have to apply a fair bit of power to climb over the bow wave. Refer to The Secrets of a Catamaran’s Performance: The Length on Water Line - Lagoon inside for a decent description.

For a lighter boat there is much less of a bow wave so very little extra power is needed to exceed hull speed - such that we can say that it barely applies. Our nominal hull speed is about 9.5 knots and exceeding that is painless and extra power is not required.

I don’t understand your spinnaker comment - what directions do the two clew lines go? At least the one on the leeward side will have a significant forward component - that’s physics. If it wasn’t so your spinnaker would be flying horizontally from the halyard, which of course it doesn’t.

ok thanks for first hand report from O 55. No doubt designed to easily achieve above hull speeds.

I started looking into semi-planning and how to achieve on our boat. I will cancel exercise if forces too large, or dont feel comfortable and for now just planning how to do. Manual says 15 kn boat speed should be limit but at > 130 true so see how it goes.

Sym spi run DDW without main - both clew lines basically point straight up and same length, with slight forward component and spi attachment points almost direct forward. That is the only angle i plan to use Sym for when i get new asym.

[44'cruisingcat]

Originally Posted by arsenelupiga

hum, you say you do not have hull speed ? According to VPLP fast cruising cats are displacement boats like condomarans and same rules of the game apply. Sorry, only foil cats escape this...

Regarding SPI's I fly symmetric only DDW and in this case majority of forward force is thru spi halyard. Rest of angles use assym where forces are split better.

During tank testing years ago, Edmund Bruce found that hulls with LWL:BWL greater than about 8:1 do not create a large enough bow wave to cause a "hull speed" restriction.

This is why boats such as rowing sculls, kayaks, and multihull sail and power boats can greatly exceed the "hull speed " for their length without planing.

Here we're exceeding hull speed easily without needing much sail or strong breeze, and not planing:

https://youtu.be/o4PoObnQrpc

As you can see there's minimal bow wave formation.

Here's an Oram power cat at 30 kts, more than double "hull speed" clearly not planing, again minimal bow wave creation:

https://youtu.be/yrCh8NKMXyo

[chrisr]

Originally Posted by 44'cruisingcat

During tank testing years ago, Edmund Bruce found that hulls with LWL:BWL greater than about 8:1 do not create a large enough bow wave to cause a "hull speed" restriction.

This is why boats such as rowing sculls, kayaks, and multihull sail and power boats can greatly exceed the "hull speed " for their length without planing.

that is incredibly interesting and new to me...but sounds perfectly reasonable

however i suspect most lagoons and similar will have a hull LWL : BWL of less than 8:1 anyway.

cheers,

[jmh2002]

Originally Posted by 44'cruisingcat

During tank testing years ago, Edmund Bruce found that hulls with LWL:BWL greater than about 8:1 do not create a large enough bow wave to cause a "hull speed" restriction.

Yes, even James Wharram covers this, and most Wharram designs should be around 12:1 if I recall correctly (and if they aren't overloaded...).

https://www.wharram.com/articles/how...rd-and-windage

"Most people are aware that a monohull yacht (apart from recent skimming racing designs) has a limiting speed of between vWLL x 1.3 - 1.5, due to the built up of wave drag resistance.

What is not generally realised is that Multihulls also suffer from wave drag resistance. In the 1960s I first observed that catamaran designs with different cross sections (flat bottoms, V-eed and semi circular hulls), all pulled visible drag waves at speed if they had an individual waterline length/beam ratio of 8:1, i.e. a limiting speed factor. (For interest, my 63ft. SPIRIT OF GAIA (see article by Mark Smaalders, Cruising Helsman - August 2002) has a WL length/beam ratio of 17:1.)

Ancient Pacific catamarans, as in Fig.1c, had waterline length/beam ratios ranging from 12:1 to 20:1. Modern racing catamarans also have length/beam ratios of 12:1 to 20:1!! Wim's 12 metre catamaran hulls, for accommodation and load carrying purposes, have a waterline length/beam ratio of 9.5:1, only slightly better than the wave dragging 8:1 hulls I observed in the 60s! My studies have shown that the average modern accommodation cruising catamaran hulls has a WLL/Beam ratio of between 8:1 and 10:1 (most designers never give this vital figure)."

[arsenelupiga]

Originally Posted by chrisr

that is incredibly interesting and new to me...but sounds perfectly reasonable

however i suspect most lagoons and similar will have a hull LWL : BWL of less than 8:1 anyway.

cheers,

yes, that is very interesting. Nice sailing 44c. I measured from manual and for L 400 you get 1:7.1 full loaded and if loaded like us at around 2T then ~ 1:7.8.

If 8:1 tipping point then 1:7.1 and 1:7.8 could mean reasonable difference as it is fairly close to that point.

How do you explain this promotional L 42 clip 16kn boat speed in 15 apparent ? Assume v ery similar hulls to ours. Probably empty and ratio would be say 1:8.1

https://youtu.be/CwwIrqyWdLg

[arsenelupiga]

here you go before i delete, this is how these ratios can be calculated - by counting. One square 7.4cm in this case. Use microsoft paint and superimpose net.

this one has net

[44'cruisingcat]

What's to explain? It's not like 8:1 is a line in the sand. Obviously as you get significantly lower than 8:1, bow wave generation increases, but it's not like a switch. Boats at 7:1 can still exceed "hull speed", it's just that you might notice it takes more power to do it. (Like flying a huge spinnaker in a near gale.)

[Donkey]

How do you explain this promotional L 42 clip 16kn boat speed in 15 apparent ? Assume v ery similar hulls to ours. Probably empty and ratio would be say 1:8.1

Simple. Look at the breeze strength and the sail area. Not many average cruisers are going to sail their boats like that. 44C would be hitting 20-25knots in those same conditions.

.....and the spinnaker does not pull from the head only, nor is the mainsail driven through the battens.

[arsenelupiga]

Originally Posted by Donkey

How do you explain this promotional L 42 clip 16kn boat speed in 15 apparent ? Assume v ery similar hulls to ours. Probably empty and ratio would be say 1:8.1

Simple. Look at the breeze strength and the sail area. Not many average cruisers are going to sail their boats like that. 44C would be hitting 20-25knots in those same conditions.

.....and the spinnaker does not pull from the head only, nor is the mainsail driven through the battens.

I look at forces by 'separating' sail from the boat and observing connections. Spi pulls via all three attachment points but mainly sheet and halyard. Main pushes via battens and pulls from end of boom. This way get better idea where stress points are.

I dont believe 44C can sail faster than wind speed in 23 kn true no matter how much sails. Too unstable for starters.

I am planning this type of sailing in suitable conditions and approaching it carefully for obvious reasons. If I can make it so that stresses are reasonable and stability okay will sure use it. Using main and spi creates lower stress loads and spreads loads than using SPI only.

[44'cruisingcat]

Originally Posted by arsenelupiga

yes, that is very interesting. Nice sailing 44c. I measured from manual and for L 400 you get 1:7.1 full loaded and if loaded like us at around 2T then ~ 1:7.8.

If 8:1 tipping point then 1:7.1 and 1:7.8 could mean reasonable difference as it is fairly close to that point.

How do you explain this promotional L 42 clip 16kn boat speed in 15 apparent ? Assume v ery similar hulls to ours. Probably empty and ratio would be say 1:8.1

https://youtu.be/CwwIrqyWdLg

How about 7-8 kts boat speed in 4-6 kts apparent?

https://youtu.be/c5Ry0s2vhPU

[Apollo19.5]

Originally Posted by 44'cruisingcat

How about 7-8 kts boat speed in 4-6 kts apparent?

https://youtu.be/c5Ry0s2vhPU

That is nice speed is 12-13knt true at that depth. That is where there performance boat have a real benefit in my mind. Not top end but able to make good speed in lighter airs. Nice

[arsenelupiga]

Originally Posted by 44'cruisingcat

How about 7-8 kts boat speed in 4-6 kts apparent?

https://youtu.be/c5Ry0s2vhPU

[44'cruisingcat]

Originally Posted by Apollo19.5

That is nice speed is 12-13knt true at that depth. That is where there performance boat have a real benefit in my mind. Not top end but able to make good speed in lighter airs. Nice

Exactly! I'm really not interested in 20 kts.

[GRIT]

44C: is that a sym or asym?

Sock or roller furling?

We're planning our next light air sail, and are looking for comparisons.

Great video, thank you.

Cheers.
Paul.

[44'cruisingcat]

Originally Posted by GRIT

44C: is that a sym or asym?

Sock or roller furling?

We're planning our next light air sail, and are looking for comparisons.

Great video, thank you.

Cheers.
Paul.

It's an assymetric. In a sock.

If we had a screecher I'd choose a symmetrical spinnaker.
(I'd love a screecher but can't really justify the cost.)

The assy gives us a bit more reaching ability.

The sock works well. But we (almost) always have the main up when we're using the spinnaker, even DDW, primarily because the main can be used to blanket the spinnaker to make socking it easy.

DDW, we'd have the main double reefed, traveller centred, sheeted tight. This allows the breeze to pass by the main and keeps the spinnaker stable.

To sock, I go forward, Tania dumps the mainsheet, the spinnaker collapses, sock comes down virtually under its own weight. Works even in a bit over 20kts.

The main also helps by keeping the boat moving, reducing apparent wind.