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Old 11-03-2010, 18:01   #16
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Well I didn't understand those complicated ones....but I understand this formula.
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Old 11-03-2010, 18:21   #17
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Once it comes to doing convoluted computations...it stops being fun.

Theoretical hull speed is just that.....
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Old 11-03-2010, 22:07   #18
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If a First 42 is doing 15 knots or more, I think it is either surfing or planing. I never looked at the under water shape of one but if there is enough flat surface area in the aft section, You might well be able to get it to plane with a spinnaker, especially when helped into plane by surfing off a wave.

We have a very fine entry point, 14 degrees I think. We are also very narrow, just over 15' for a hull length of 64' (better than 1:4 beam-length ratio). Our hull speed is 1.5 times the sq.rt. of the waterline so that isn't too much difference with the 1.4 factor of modern medium displacement hulls. A First 42 might beat 1.4, so let's say 1.45 to be optimistic. I do not know the waterline length but I'll guestimate it at 36'? That would put it's hull speed at 8.7 knots. While I can believe you can get it up to 10 knots by putting a lot of power into it, I can't see it go 15 knots without surfing or planing.

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Old 12-03-2010, 02:21   #19
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From power boat design criteria I remember that it takes three times the power to get over the hump, then just twice the power to maintain it. Modern wide stern sail boats mimic 'speedboats' with interim hulls that sail well and skim readily. Just takes a gust and a bit of downhill water (surfing) to get onto the hump.
Fuel consumption approaching the hump can double for no significant increase in speed, so a driven sailboat is just waiting for the right puff and surf to get going.
Some would say the boat is being overdriven, for a cruiser then perhaps so. I'd like to see an autopilot cope with overhull speeds well enough for 'helm' to go below and make coffee, let alone lunch.
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Old 12-03-2010, 03:47   #20
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Quote:
Originally Posted by Randyonr3
...I tried to tell him that "Hull Speed" is only a factor in the most triditional style of "Full Displacement" boats...
My Island Packet 380 (full keel, 32' LWL) would go right up to 7.5 knots in flat water with the 56 hp Yanmar flat out. At that speed (the theoretical hull speed) the stern would squat until the exhaust was under water. There was plenty of "extra" horsepower pushing her along, but 7.5 knots was "the Wall".

On an offshore passage with about 1/3 of the foresail rolled out in 45 knots on the port quarter and following 24' swells, we hit 11.5 knots a few times. Woo Hoo!

Even a traditional full-keeler can beat the "Rules" in the right conditions.
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Old 12-03-2010, 03:52   #21
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I'd like to see an autopilot cope with overhull speeds well enough for 'helm' to go below and make coffee, let alone lunch.
The modern autohelms with rate gyros should be able to do that easily, afterall Day Mellon surfed down southern wave mountains at well over 30 kts in B&Q while under autopilot.
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Old 12-03-2010, 03:54   #22
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Certainly not a myth :-
\mathrm{Speed\ Length\ Ratio} =\frac {V}{\sqrt \mathrm{LWL} }

where:
v = speed in knots
LWL = length of waterline in feet

The term was converted into non-dimensional terms and was given Froude's name in recognition of the work he did. In France, it is sometimes called Reech–Froude number after Ferdinand Reech.[3]

Square root of LWL x 1.34 = hull speed - applies only to displacement vessels -- not Multillhulls nor non-displacement boats.

Whatever, on most passages most cruising sailboats average 4 knots over 24 hours giving around 100nm/pd.
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Old 12-03-2010, 04:10   #23
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I'm here.

Most cruising boats typically sail at a S/L ratio of .85. It's easy to do, comfortable, you don't push the boat hard, and after all we're cruising.

As an example if a boat has a 55 foot waterline it's hull speed is 10.4 (7.4*1.4=10.4). With that much waterline it's easy to manage 8.8 knots but if you load the power on you can push past hull speed and hit S/L ratios of 1.35 or more. But you are going to work hard, wear things out and break things. So the same boat listed above when loaded up can sail at +14 knots even though it is sailing in displacement mode and you're gonna dig a big hole in the ocean doing this.

As Tropic Cat pointed out cats use light weight and fineness of hulls for speed. Modern mono's go another direction, they are in essence planning hulls using movable ballast (currently water or lead) for stability. The most recent development for a mono's is the horizontal assy lifting foil for stability. As a mono guy it will be interesting to watch this development for the following reasons: 1) The keel no longer has to cant. 2) No water ballast is need (ie no weight) 3) The foil is unobtrusive below the waterline and can be made to self tack. 4) Any mono can be retrofit with this to increase stablity.

Today both an ORMA 60 and a Volvo 70 do 600 mile days which is way beyond hull speed (one in displacement mode one in planning mode). I would guess that the new generation of Volvo 70's will use the horizontal assy foils and may approach 700 mile days. The multi fleet also uses lifting foils but not for stability, they use them to keep the fine ends up and out of the drink. Its not good to stick the pickle fork in a wave face.

Anyway, as a cruiser I'm happy to roll along with a .85 S/L ratio.

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There are other factors which have to be taken into consideration in addition to hull the speed formula (where's Joli?)

The first thing that comes to mind is the fineness ratio of your hull. This one consideration is why catamarans are calculated to have higher theoretical hull speeds than the same LWL mono. A Catamaran might have a ratio of 10 to 1 (length to single hull beam width at the waterline), where a mono will be 3 to 1 or 4 to 1. The old 1.34 times the square root of LWL does not take this into account.

So, a long, narrower hull will have a theoretically faster hull speed than a long fat hull shape.

2nd, any boat can exceed hull speed if it has enough power to climb it's bow wave.

I'm sure there are more factors and I'm certain that our racers will point them out.
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Old 12-03-2010, 04:33   #24
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Quote:
Originally Posted by Hud3 View Post
My Island Packet 380 (full keel, 32' LWL) would go right up to 7.5 knots in flat water with the 56 hp Yanmar flat out. At that speed (the theoretical hull speed) the stern would squat until the exhaust was under water. There was plenty of "extra" horsepower pushing her along, but 7.5 knots was "the Wall".

On an offshore passage with about 1/3 of the foresail rolled out in 45 knots on the port quarter and following 24' swells, we hit 11.5 knots a few times. Woo Hoo!

Even a traditional full-keeler can beat the "Rules" in the right conditions.
We touched 12.8 knots several times surfing downwind during a winter sail along the Swedish south coast. January 2006 I think.... The boat was a 31ft full keel double ender weighing 13000lbs and we had just a little bit of genoa rolled out. Lots of fun but very cold. Had to spend 30 minutes thawing the knots just to get away from the dock.

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Old 12-03-2010, 07:30   #25
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Reading the posts here I think that most of the experiences described are due to surfing down big waves which is always one of the highlights of these passages. The other highlight is the thought that's you're not sailing the reverse course ;-)

But I think that the kite/spinnaker experience on the First was true planing, that is, if the speed didn't drop under 10 knots in between the waves. I once sailed a Dehler 42 sloop on the North Sea, downwind to England with 15-20' seas behind me and I had this feeling that the boat was about to go into sustained planing. I tried my best to achieve that by heading up to a reach while surfing down a big wave, to give the boat more time to accelerate and get more power from the sails. I was allowed to try that for 6 hours but the only result was blisters on my hands from the steering wheel ;-))
I think the Dehler 42 can plane but needs much more wind than what I had (too long ago to remember the specifics).

ciao!
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Old 12-03-2010, 08:14   #26
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Quote:
Originally Posted by s/v Jedi View Post
If a First 42 is doing 15 knots or more, I think it is either surfing or planing. I never looked at the under water shape of one but if there is enough flat surface area in the aft section, You might well be able to get it to plane with a spinnaker, especially when helped into plane by surfing off a wave.

We have a very fine entry point, 14 degrees I think. We are also very narrow, just over 15' for a hull length of 64' (better than 1:4 beam-length ratio). Our hull speed is 1.5 times the sq.rt. of the waterline so that isn't too much difference with the 1.4 factor of modern medium displacement hulls. A First 42 might beat 1.4, so let's say 1.45 to be optimistic. I do not know the waterline length but I'll guestimate it at 36'? That would put it's hull speed at 8.7 knots. While I can believe you can get it up to 10 knots by putting a lot of power into it, I can't see it go 15 knots without surfing or planing.

cheers,
Nick.
Thats the same attitude that I got from the guy who went out with us, as he started running the numbers and said it wasnt possible..
HiLite has just stated that he does 10 in a broad reach.. fast point of sail but the kite is still in the bag..
Our speeds are not abnormal and 10 to 12 is an everyday event in flat water with no current.. With the kite up, 14 to 16..
The point I was making is that the Hull speed Formula was created many years ago and as I believe, was for a oil tanker..or a boat of Full Displacement where the waterline length comes into play..
When new designs of underwater surface came, the formulas went out the window..
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Old 12-03-2010, 08:30   #27
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Surfing

On one occasion in 30 knots of wind aft and 6-8 foot square waves following I surfed my 31 foot waterline at 14 knots for about 20 seconds. That felt pretty much out-of-control. Thought for sure I was going to brodie-out, but didn't.
Other than that, sometimes when I want to feel particularly macho I will set the GPS to read miles-per-hour or better yet kilometers-per-hour.
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Old 12-03-2010, 08:36   #28
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Hey Randy,
Are you saying that, in perfect conditions, you can make 360 nautical miles in 24 hours? (15x24=360)
All due respect but I am finding that difficult to believe.
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Old 12-03-2010, 09:03   #29
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Hey Randy,
Are you saying that, in perfect conditions, you can make 360 nautical miles in 24 hours? (15x24=360)
All due respect but I am finding that difficult to believe.
I havent done it, but I have no doubt that with the right crew, in the right conditions, Yes, 360 is possible.........
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Old 12-03-2010, 09:07   #30
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formulas out the window

Quote:
Originally Posted by Randyonr3 View Post
When new designs of underwater surface came, the formulas went out the window..
My own experience supports this. My boat's hull speed, according to the formula, would be 8.5 knots. However, I'm often able to hit 9.5 on a reach with working sails only. With the gennaker up on a broad reach in flat water I've been able to maintain a steady 10.5.

At first I thought this might be an instrument calibration problem, but it's not. Something about those modern, beamy, light displacement, fin-keeled hulls that SOME cruisers find so abhorrent allows these boats to exceed that old Sq Rt of LWL * 1.34 formula.
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