Question Hull speed.

I like the ballast effect of full tanks too.

Interestingly, while doing a crossing of the gulf a week or so ago, I observed on my AIS the usual tug and barge combos doing 8 or 9 knots. While looking out the window, I saw a pusher tug and barge combo that looked like it might not be a conflict, until I checked the AIS and discovered it was doing 13 knots and was certainly going to be conflicting! The tug and barge combo in the pusher configuration provides an effectively huge waterline and therefore the hull speed was way higher than a conventional tug-and-tow config. Beware!

SCOTTEDAVIS said:

Thanks ski, I was dreaming of a 500knot long skinny ship, was gonna call her the t-salami. Now my dreams are dashed.

You and your math stuff.




The formula for wave speed for a deep water wave is:



To figure out whether it's a deep or shallow water wave, you need to find its wavelength.

a) The formula for wavelength vs. period is



T, the period, is in seconds.



Every good surfer should know this crap.

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Having worked my way around this stuff a bit there are several missing links, the most important being propagation and directional changes. In other words winds cause waves and a passing storm has winds of varying strength and directional changes.

As computer models become more sophisticated, short term updates to wave height and amplitude have become more accurate. Great if you are the predictor but with more finger crossing if you are a pleasure boater outrunning a storm or if you are the commercial fisherman trying to make the the fisheries window a payday.

Lots of data and forecasting is available for us to make decisions, most but not all even 3 day accurate. Therein lies the conundrum.

Edit #28,
Re the falling ball example I think the speed of the ball determines the wave's amplitude or height while the size of the ball determines the wave's speed ... outward.

are most waves created by wind?....or concrete balls?

I vote wind.

Wind or mechanically generated I think they have the same chacteristics. A bigger/longer hull or a greater dia falling ball = a wave w greater distance between crests. And greater velocity true or no?

Actually wind is mechanically generated.

Scott your question above is stupid.

Research and report back....


http://www.enchantedlearning.com/subjects/tsunami/


The Speed of a Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entire ocean. A regular wave (generated by the wind) travels at up to about 90 km/hr.


http://earthweb.ess.washington.edu/tsunami/general/physics/characteristics.html


Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

And Tsunamis have what to do w hull speed?

SCOTTEDAVIS said:

The formula for wave speed for a deep water wave is:

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That's the speed of an isolated deep water surface wave (the group velocity of a water wave packet). The speed of a deep water surface wave series (the phase velocity of a plane wave, more like what the bow of your boat generates) is twice that.

Then converting meters to feet, and meters per second to knots, you would get

V = 1.34 sqrt(L),

the familiar formula for "hull speed", the speed at which the wavelength of your boat's bow wave equals its waterline length.

MBs....


Nothing...just like a lot of what is posted.


Just refer to post #36 for similarities of mechanically or wind generated waves...that may be one of the reasons I am linking info.


I didn't start the comparison...just providing links to support what I already know.

For all the NA wannabees...


What shape is a hobie cat hull and in what way does it conform or not to the 1.34 constant?


What speeds are typical for a hobie cat in moderate winds...and what would be the hp equivalent for say 15knts of wind to push one?

PS-I think many waves are caused by fat bottomed girls!

THD said:

PS-I think many waves are caused by fat bottomed girls!

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You need a bigger hot tub....

markpierce said:

I vote wind.

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hmmmm hasty?

been there, done that, sorry no go. He does not define the penalty for x extra gallons of fuel.

I bet a large concrete ball makes a good anchor to use in a tsunami.

Back when I bought my first catamaran, I wanted to add a mooring in a nice quiet cove. I built a plywood box in the back of my pickup truck (4'x 4' x 4' and filled it with lots of concrete and a rebar loop.

I backed the truck deep at the launch ramp and found I could easily drag that heavy block over to where I wanted it. Even though the huge concrete block was heavy above water, and below it was next to nothing. The bobbing sailboat would slowly drag that block ashore.

I wound up using a couple of screw anchors to secure the buoy and block.

The answer is 42

stubones99 said:

I built a plywood box in the back of my pickup truck (4'x 4' x 4' and filled it with lots of concrete and a rebar loop.

Even though the huge concrete block was heavy above water, and below it was next to nothing. .

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Sounds like one stout pickup truck to haul over 2 yds of concrete. And in the water the weight is reduced to just a few mere tons.

cpseudonym said:

the answer is 42

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don't panic!

Thank You all.

The thread has not drifted too far lol.

Eric mention the effect of wave length and its` effect on hull speed, because we know that water has a cohesive nature I am guessing buit do not know for a fact that length of the wave passing the hull has a larger or lesser drag on the hull depending on its` [ the waves ] length in comparison to the length of the hull / boat.

I watched a piece on the Teeve about tuning the hull of the Queen Mary [ new ] and to get the performance / speed the designer wanted they lengthened the bulbous bow underwater, this had the effect of shifting the length of the wave further aft as it passed along the side of the ship. This bought an immediate rise in boat speed and less fuel consumption. The amount they extended the bulb was quite a lot and looks strange compared to what you normally see on the bows of ships.

So I guess it backs up the W L L part of the equasion.

Thank You for the schooling.

Regards.

David.

"What speeds are typical for a hobie cat in moderate winds...and what would be the hp equivalent for say 15knts of wind to push one?"

With the L/B of a Hobie cat the skin friction is mostly what holds it back.

Another Rule of Thumb is a ordinary prop is able to put 20 lbs of thrust into the water

.At 17K of wind speed most sails will make one pound of heeling moment per sq ft of sail.

Going downwind most of this would be thrust.

How fast will a Hobie go, with 300 hp , FAST I'm sure.

Had a fellow power his Hobie with a very tiny high speed motor.

A Mighty Mite or similar.

His purpose was, it was really easy to sail 10 miles away and when the wind dies , 10 miles is a really long paddle.

He was satisfied with his get home engine , which looked like about 8K at his cruise speed.

I had to tow a dismasted Hobie Cat after a storm. Towed with a 20hp skiff. Planed out easily while towing. Amazing how little drag they have going fast but still not "planing".

stubones99 said:

Back when I bought my first catamaran, I wanted to add a mooring in a nice quiet cove. I built a plywood box in the back of my pickup truck (4'x 4' x 4' and filled it with lots of concrete and a rebar loop.

I backed the truck deep at the launch ramp and found I could easily drag that heavy block over to where I wanted it. Even though the huge concrete block was heavy above water, and below it was next to nothing. The bobbing sailboat would slowly drag that block ashore.

I wound up using a couple of screw anchors to secure the buoy and block.

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WOW! your a strong guy, 8000 lbs of ccrt and you dragged it out of your truck down a boat ramp and it still would not hold your boat, seems just wrong.

You should get your money back as you got some dang defective concrete.

My point being...based on the shape of Hobie hulls...certainly not planning or semi-displacement....


There are a variety of factors that go into what "hull speed" is.


I believe there is some kind of 5:1 rule where length exceeds beam by 5:1 and toss the 1.34 rule.


Basic rules of thumb are just that...unfortunately some think you can use it like gospel for every answer without looking at a lot more.


I am NOT a NA....and cant begin to give lengthy, accurate answers....


But I do know enough to spot a lot of oversimplified assumptions.

psneeld said:

For all the NA wannabees...


What shape is a hobie cat hull and in what way does it conform or not to the 1.34 constant?


What speeds are typical for a hobie cat in moderate winds...and what would be the hp equivalent for say 15knts of wind to push one?

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I raced a Hobie 18SX back in the 80's and can only comment on what I observed on my own boat, and those of my immediate competitors. We were competing at the National level, so I can only assume the boats were being sailed optimally or close to it.

The Hobie 18 has symmetrical hulls that are very narrow (see image below) and deep, and are fitted with daggerboards. The dimensions are 18' LOA, 8' Beam, 400# Disp (empty), 240 SF Sail Area.

We always assumed that sails, when properly set and trimmed could produce between .02 - .07 HP/SF. In the case of the H-18 that comes to between 4.8 and 16.8 HP. Or, fully loaded with 650# crew, between 10 and 33 HP/Ton if my math is right.

In moderate air on a broad reach, the H-18 could easily make 15 knots and 17 in higher winds. The fastest we ever sailed was 18.5 knots (by radar gun and knotstick - no GPS back then) but I heard some had seen 20 in higher winds and smooth seas behind a breakwater.

Assuming 18 knots, the Hobie has a S/L ratio of 4.24. Mind you, at 18 knots one hull is almost buried and the other is flying completely out of the water.

Obviously, the old 1.34 S/L Ratio flies right out the window here. The Beam/Length ratio seems to make an enormous difference.

Hmmm . . . I remember I once hit 60, but I don't think the maximum speed on a trailer counts . . . . .

Hope some of this info helps.

LarryM said:

I raced a Hobie 18SX back in the 80's and can only comment on what I observed on my own boat, and those of my immediate competitors. We were competing at the National level, so I can only assume the boats were being sailed optimally or close to it.

The Hobie 18 has symmetrical hulls that are very narrow (see image below) and deep, and are fitted with daggerboards. The dimensions are 18' LOA, 8' Beam, 400# Disp (empty), 240 SF Sail Area.

We always assumed that sails, when properly set and trimmed could produce between .02 - .07 HP/SF. In the case of the H-18 that comes to between 4.8 and 16.8 HP. Or, fully loaded with 650# crew, between 10 and 33 HP/Ton if my math is right.

In moderate air on a broad reach, the H-18 could easily make 15 knots and 17 in higher winds. The fastest we ever sailed was 18.5 knots (by radar gun and knotstick - no GPS back then) but I heard some had seen 20 in higher winds and smooth seas behind a breakwater.

Assuming 18 knots, the Hobie has a S/L ratio of 4.24. Mind you, at 18 knots one hull is almost buried and the other is flying completely out of the water.

Obviously, the old 1.34 S/L Ratio flies right out the window here. The Beam/Length ratio seems to make an enormous difference.

Hope some if this info helps.

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Thanks....I was really trying for the regular asymmetrical, significant rocker Hobie hulls..but I think your efforts should make some thinks about just using the simple "rule of thumb" numbers for hull design, speeds and power.

psneeld said:

Thanks....I was really trying for the regular asymmetrical, significant rocker Hobie hulls..but I think your efforts should make some thinks about just using the simple "rule of thumb" numbers for hull design, speeds and power.

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IIRC, I heard the rule of thumb for the Hobie 14 and 16 were actually very similar. Well sailed, the 14's could hit 14+ knots and the 16's could make 16+ knots. The S/L ratio was the square root of the hull length. I may be way off, but that's what I seem to remember the other skippers saying. Those were the original "asymmetrical, significant rocker Hobie hulls".

A little checking and it seems they are all sailing faster now. That's progress!

Just for the fun of it, here is a video of what its like on a Hobie 18 sailing at speed. Short ad but worth the wait.


Sorry for the thread divert . .

"Hull Speed" for any particular boat is infinitely variable, as Psneeld and Larry have guessed it is dependent on how "big" a hole in the water you are trying to make. Catamarans are skinny (high beam/length ratio), but they are also (more importantly) light (low displacement/length ratio) for their length.

The average production fiberglass trawler yacht will have a displacement/length ratio between 260 and 400, and the typical hull speed formula applies. A beach cat might have a displacement/length ratio of 40, hull speed formula no longer works. The hole in the water is just a bunch smaller, as are the waves generated by it's passing. Thus it's potential speed is much higher, but hull form must be suitable.

Very narrow hulls don't March to the same psysics as a hull w more typical length to beam ratios (LB ratios). And of course a 10' X 37' hull will have different chacteristics than a 14' X 34' hull. At low speeds mostly.

My 18' (20' cut off) canoe w a 6hp OB goes about 14 knots. Definitely wind in the face canoeing. I can do it solo but as one would imagine the bow is about 2' off the water. With a person up front it's a nice ride. I can do figure eights (canoegies) as a witness said if I do them very carefully.