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07-13-2015, 06:40 PM
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#41
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,146
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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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07-13-2015, 06:43 PM
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#42
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Guru
City: Seattle
Join Date: Apr 2012
Posts: 1,142
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PS-I think many waves are caused by fat bottomed girls!
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07-13-2015, 06:46 PM
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#43
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,146
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Quote:
Originally Posted by THD
PS-I think many waves are caused by fat bottomed girls!
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You need a bigger hot tub....
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07-13-2015, 08:23 PM
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#44
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Guru
City: Vero Beach, FL.
Vessel Name: FIREFLY
Vessel Model: Pilgrim 40
Join Date: Nov 2012
Posts: 918
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Quote:
Originally Posted by markpierce
I vote wind.
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hmmmm hasty?
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07-13-2015, 08:26 PM
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#45
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Guru
City: Melbourne, FL
Join Date: Dec 2014
Posts: 1,731
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been there, done that, sorry no go. He does not define the penalty for x extra gallons of fuel.
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07-13-2015, 09:03 PM
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#46
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Guru
City: Vero Beach, FL.
Vessel Name: FIREFLY
Vessel Model: Pilgrim 40
Join Date: Nov 2012
Posts: 918
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I bet a large concrete ball makes a good anchor to use in a tsunami.
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07-13-2015, 09:29 PM
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#47
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Guru
City: Melbourne, FL
Join Date: Dec 2014
Posts: 1,731
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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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07-13-2015, 09:35 PM
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#48
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Guru
City: Hotel, CA
Join Date: Sep 2011
Posts: 8,323
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The answer is 42
__________________
Craig
It's easier to fool people than to convince them that they've been fooled - Mark Twain
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07-13-2015, 11:41 PM
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#49
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Guru
City: Carefree, Arizona
Vessel Name: sunchaser V
Vessel Model: DeFever 48 (sold)
Join Date: Apr 2008
Posts: 10,185
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Quote:
Originally Posted by stubones99
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.
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07-13-2015, 11:45 PM
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#50
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Guru
City: Beaufort, NC USA
Vessel Name: Sylphide
Vessel Model: Kingston Aluminum Yacht 44' Custom
Join Date: Aug 2014
Posts: 2,228
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Question Hull speed.
Quote:
Originally Posted by cpseudonym
the answer is 42
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don't panic!
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07-14-2015, 01:15 AM
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#51
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Veteran Member
City: Laidley
Join Date: Mar 2015
Posts: 91
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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.
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07-14-2015, 05:03 AM
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#52
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Guru
Join Date: Oct 2007
Posts: 22,553
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"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.
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07-14-2015, 06:45 AM
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#53
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Technical Guru
City: Wilmington, NC
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 6,194
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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".
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07-14-2015, 07:05 AM
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#54
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Guru
City: Vero Beach, FL.
Vessel Name: FIREFLY
Vessel Model: Pilgrim 40
Join Date: Nov 2012
Posts: 918
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Quote:
Originally Posted by stubones99
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.
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07-14-2015, 07:30 AM
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#55
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,146
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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.
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07-14-2015, 11:33 AM
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#56
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Guru
City: League City, TX
Vessel Name: Pelago
Vessel Model: Wellcraft 3300 Coastal
Join Date: Aug 2011
Posts: 1,069
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Quote:
Originally Posted by psneeld
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.
__________________
Larry
M/V Pelago
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07-14-2015, 11:44 AM
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#57
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,146
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Quote:
Originally Posted by LarryM
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.
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07-14-2015, 12:30 PM
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#58
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Guru
City: League City, TX
Vessel Name: Pelago
Vessel Model: Wellcraft 3300 Coastal
Join Date: Aug 2011
Posts: 1,069
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Quote:
Originally Posted by psneeld
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 . .
__________________
Larry
M/V Pelago
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07-14-2015, 12:41 PM
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#59
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Guru
City: Flattop Islands
Vessel Name: Blackfish
Vessel Model: custom
Join Date: Dec 2012
Posts: 724
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"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.
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07-14-2015, 12:54 PM
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#60
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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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.
__________________
Eric
North Western Washington State USA
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