Myth: Hull Speed

[Talbot]

Originally Posted by s/v Jedi

cats don't plane because they do not have the flat underwater surface area for that. The formula you use is only valid for the type of hull used for creating that formula.

That depends on the cat. The old catalac hull form has a lot of drawbacks, but it does allow one thing - if you study the hull it actually has a lot in common with the old MTB hulls.

Now Jedi will say that they dont plane - who am I to argue with this.

However, if the boat accelerates to more than three times hull speed, rises a bit out of the water, overtakes the wave train, and feels like it is riding on rails for about half an hour, I dont know what else to call it

If you call a boat that is planing a duck, my boat was definitely quacking

[s/v Jedi]

Hi Talbot: that certainly sounds like planing ;-) So there are exceptions like always.

But the term hull speed should only be used for a number that was determined to be that hull speed for that particular hull. So not that 1.34 times sq.rt. of waterline for a cat.

cheers,
Nick.

[Stillraining]

If I close my eyes and just feel and listen I always exceed the 1.34 rule...

[44'cruisingcat]

Originally Posted by Joli

Well, here is what a S/L ratio of 1.8 to 2.2 looks like.

Here is what a S/L ratio of 3 looks like. (ie 600 mile day, 25 knot average)

Can modern mono's sail beyond a S/L ratio of 1? Yes, easily but the real question is do we want to? If you look at rally results or cruise logs you will find very few cruising boats sailing faster then a S/L ratio of .85.

Want a faster passage? Buy waterline.

Seems so much more civilised on a multi:

YouTube - Banque Populaire V Smashes Transatlantic Record

[daddle]

The graph above doesn't "go vertical". Proving the original point and nicely echoing actual on-the-water behavior. Go sailing in a boat with reasonable performance. Observe.

[roger.waite]

Originally Posted by Liam Wald

Are you saying that, in perfect conditions, you can make 360 nautical miles in 24 hours?

Achievable with perfect conditions, perfect crew, and need for speed.
We have had great daylight runs averaging 12+ kts in a 30 footer. In these conditions, speed is driven by how far you surf each wave. Which is just a function of how close you are to the edge of control!

As an ultralight, we surf at 15+ with wind on the stern quarter, and get 8.5 knots (ratio~1.6, allowing for increased LWL) powered up and heeled on a beam reach (when flat stern sections are no longer working for us).

But I seldom cruise short-handed like that. The ride is more comfortable at 7.5 kts, the wife is kinder when she gets a decent rest, and there is a far bigger margin for the silly stuff that happens at 4 am in the morning.

[roger.waite]

Originally Posted by s/v Jedi

But the term hull speed should only be used for a number that was determined to be that hull speed for that particular hull. So not that 1.34 times sq.rt. of waterline for a cat.

Agree.

My theoretical hull speed, light ship, is 2 times sq.rt LWL. In practice, this comes down quite a bit when loaded with water, fuel, food, etc.

[barnakiel]

Re: planning cats / tris.

I believe that when looking at maxi tris at speed the center hull is planning, while the amas are still using their elongation.

What I like a lot about various designs, is how easy (or difficult) it is to achieve a serious portion of the hull speed. I bet the ease of achieving just this is a great 'to have' of any good cruising boat. Sort of like I do not need a boat with great potential hull speed but rather one with great ease of achieving the first 1 of the 1.34.

b.

[Tropic Cat]

Originally Posted by daddle

The graph above doesn't "go vertical". Proving the original point and nicely echoing actual on-the-water behavior. Go sailing in a boat with reasonable performance. Observe.

The graph does go vertical indeed.

The nice thing about CF is anyone can spout anything they like. Readers believing it is something entirely different. If someone were to tell me they have the perfect 12 knot 30 foot monohull, I'll say that's nice and ignore any post they ever make.

Prove a 360 mile day and I'll believe you. Until then ...

Bull crap

[s/v Jedi]

Hi Roger,

When you surf a wave and try to keep with it but just don't make it, your speed over ground is just below the speed of the wave traveling over ground. This would put you in the 15-20 knot range. The power of the wave is awesome and can surf almost any boat. Even the oldest designs have to tow warps and stuff during storms to prevent it because their hulls loose stability at these speeds.
But remember that surfing isn't planing... planing is when you do not need the wave for that speed to sustain the speed... even though you might need the wave to bring you into plane because a lot more power is needed for that than for sustaining it.... if you can follow me ;-) Think of planing with the dinghy: getting it to plane is the hard part; once planing it's easy to keep planing even when you take the engine throttle back.

Yes, ultra light displacement puts you outside the old formula. I suspect your boat also has little wetted surface for it's length. We are somewhere between light and ultra light displacement (depending on who's definition is used) and we can only get to 1.5 on hull speed, but we can plane and the boat was designed to handle 26 knots. I think the fastest sustained speed a Sundeer 64 clocked was 23 (we only saw 14.5 sustained but carry our house with us).

Barnie: the maxi tri's that I see going fast only have their leeward ama in the water ;-) I'm not even sure because they are gone before I can get a good look... I have seen one doing 11 knots in not even 8 knots wind.

cheers,
Nick.

[roger.waite]

Nick - agreed. Surfing at speed in a short-ish boat is much more of a roller coaster that you see in the videos of larger boats. It takes a truly exceptional boat to plane without a little help from the waves.

As for perfection, its as hard to find the perfect boat as it is to find the perfect woman. Yet men can always be relied upon to perform (badly).

[Ex-Calif]

Between displacing and planning is "getting on plane."

Add enough power and any boat with even a marginally flattish bottom starts to "get on plane"

On the first 40.7 I was sailing on last year we routinely saw 11's and even the odd 13 on the gauge. Here's a shot of us fartin' along pretty good under the Asail. Not the bow wave we are pushing along...

And no, we are not bow low because of our fat bowman (me - LOL) - I lost 15 kilos after this and moved to main trimmer. The replacement bowman was like 5'7" 130#

[Joli]

Well this one probably can but it'll be hard to keep the wifey happy on a cruise.

The Cone of Silence: Go The Cone!

Originally Posted by Tropic Cat

The graph does go vertical indeed.

The nice thing about CF is anyone can spout anything they like. Readers believing it is something entirely different. If someone were to tell me they have the perfect 12 knot 30 foot monohull, I'll say that's nice and ignore any post they ever make.

Prove a 360 mile day and I'll believe you. Until then ...

Bull crap

[Joli]

True enough, the 130 foot Bank Pop is one smooth ride. Not so much the 60 foot Bank Pop. LOL

YouTube - banque pop chavire

Originally Posted by 44'cruisingcat

Seems so much more civilised on a multi:

[sandy daugherty]

Consider the lowly paddle. It really doesn't care what words we use to describe what it does; it may actually predate language and has a grandfather clause. But it can help resolve this apparent (but in fact non existent) difference of opinion with a little hands-on experience.

If we hold a paddle the wrong way, it slices through the water easily, at great speed, with little contribution to the forward motion of the boat we are sitting in. It clearly exceeds its theoretical hull speed. Rotate its handle 90 degrees, and it resists moving through the water so much that the boat we are sitting in starts to move. That happens because a bunch of water piles up in front of the paddle, and we leave a hole in the water behind that fills in as the water around the hole overcomes it's inertia to respond to the laws of gravity. Interestingly enough, the boat we are sitting in moves as if its being pulled by a force equal to the amount of water that is piling up in front of the paddle. If we pull the paddles with much more force, we don't really go faster, we just make a longer hole in the water behind the paddle.

Now if, by mistake, we let the paddle stretch out to the side, it suddenly darts to the surface and skips across the top of the water. Not much force gets applied to that imaginary rope pulling the boat we are in. That is because the paddle rose out of the water to skip over the top. The paddle is moving faster than the water can get out of the way, (not pushing a bunch of water in front of it,) so it climbs over the water like a snow ski over heavy powder. Inertia has more effect than displacement.

A narrow boat, with a fine entry, is a paddle going sideways. It doesn't push much water aside. A relatively short, fat boat pushes a lot of water aside, creating a bow wave. The dip behind the bow wave is the hole behind the paddle, and the stern wave is the over-reaction of the water responding to gravity. If a catamaran slices through the water without pushing very much water aside, there isn't much of a bow wave, so there isn't much of stern wave, and so on. But since the laws of gravity aren't confused by an extra hull or two, the catamaran is still floating based on how much water it displaces*. But by going much faster, its dancing on top of a longer hole in the water. So is it planing like a snow ski? Kinda, but its not riding on the top of water that can't get out of the way fast enough. Is it displacing as much water as it weighs? Kinda. Is that what is happening when a B42 First gets the wind in her teeth? No. This is more like that semi-displacement hull with a big engine climbing over its bow wave, which reduces the bow wave, which makes it easier to get over its bow wave ad nauseum. That's planing , even if it isn't doing thirty knots.

Is anybody breaking the law of hull speed? No. It has been rescinded by Wikipedia, an authority by consensus.

*Which is more important, being skinny and cool -looking, or just not carrying around its own weight in lead dangling way under the boat in structure with surface, induced and parasite drag?