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PostPosted: Sun Dec 04, 2011 7:22 pm 
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Roadrunner wrote:
frisbee wrote:
Tom Kirkman makes alot more sense than the rest of you.... "hull speed" hocus pocus deals with the sailing sector...at least that's the only circles I hear it being discussed.
Hull speed has nothing to do with method of propulsion (prop, pedal, paddle, sail or fan). Maybe this will help -- it deals specifically with kayaks (for a quick overview skip down to the charts -- speed, drag and HP required):
http://www.keelhauler.org/khcc/seakayak.htm




First, tossing a report in its entirety as Proof is unacceptable in all arenas. Now YOu go read the third and second to last paragraphs of your source...it pretty much makes the "maximum speed/hull length" belief go bye bye as a reason the kayaks don't go faster when a bigger trolling motor is used. he clearly states that the hull speed only belongs to the "rowing" crowd, and that hull length is not a limitation when adequate energy is utilized.
I can't believe I bothered to read that piece.


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PostPosted: Sun Dec 04, 2011 9:31 pm 
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frisbee wrote:
... "hull speed" hocus pocus deals with the sailing sector....
...he clearly states that the hull speed only belongs to the "rowing" crowd....
We may disagree on that, but I appreciate your thoughts on the matter. Thanks. 8)


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PostPosted: Mon Dec 05, 2011 9:21 am 
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I saw an outback just the other day being towed by an aircraft carrier (don't ask what aircraft carrier. I am not allowed to disclose the particulars.).
When the outback's speed approached the maximum speed the hull would allow per some weird formula what Happened?
A) the aircraft carrier's anchor chain that was used as the tow chain Snapped from the strain.
B) the Outback became longer as the speed increased ( a new take on the Theory of relativity. I discussed it with Albert and he was amused.)
C) the outback became the Delorean in Back to the Future, and the rider went back to the days when there were no legal limits on inshore fishermen (Minus the fire streaks on the water)
D) The outback went as fast as the aircraft carrier pulled it.
E) No matter how hard the Aircraft carrier tried it could not overcome the hull limitations of the Outback, and even at full throttle the aircraft carrier could not go faster than 7.rediculous MPH.


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PostPosted: Mon Dec 05, 2011 11:25 am 
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frisbee :

Jeez Frisby can you lighten up a little, we are only discussing kayaks here, not open heart surgery, If things aren't exact nobody dies, and some of this stuff is a grey area.
I think in that article ( I read the whole dang thing also) the guy was pretty much describing human powered kayaks, which with only human power your not going to run into hull speed limitations anyway so why even think about it (that was his point).
However there are alot of us crazy's out there that 6 mph is too boring so we add sails, motors, and hydrofoils to our boats to try and increase our range. For example I have had my Tandem Island (just a kayak) up to 15-17 mph many times with my huge sails, but because the sails are also pushing the boat down as well as forward, it is impossible to get onto a plane. Instead there is a 6 foot rooster tail spraying 3 feet into the air and out the sides at the bow. This is because as they stated in the article the floatation ability of a displacement hull remains constant regardless of speed. So when my sails are driving hard the downward force increases and the bow sinks deeper into the water. Before I added my hydrofoils under the boat the bow would submerge completely under the water ( I called it nautilus mode). It only slowed the boat down slightly but I would get totally drenched with the wall of water in my face. The bow would not come back up until I either released the sails or turned off the wind a little ( I'm a hard ass so I seldom did either). So there is no hard magic hull speed limit, but the amount of horsepower required to drive that hull increases exponentially as you get closer to and exceed the hull speed. Then if you are able to transition into planing mode, the horsepower required goes back down. For example my sails to drive my TI to the speeds I go are generating in excess of 30 hp. Where when I'm going 5-6 mph it only requires 1-2 hp to drive the boat.
Now there is a huge difference between sail and powerboats. Because the power on a sailboat is from above the downward force increase with force. On a powerboat since you are powering from underneath, as you apply more power this allows actually creates less weight on the bow. Thats why on my old racing 3 point hydroplanes I would have to climb all the way onto the bow to get the boat up to speed, otherwise the bow would just go straight up and the boat would sink. This makes it much easier to get the boat up onto plane where it is skimming over the water. Now this is where Tom's explanation comes into play about losing control of the boat. On my racing hydroplanes I had a small keel/centerboard that was used for steering when the boat was skipping over the water at 70 mph. Without that little keel the boat was like trying to steer a saucer on the water (no control whatsoever). Tom is completely correct in what he says. because as you increase the speed on a displacement hull and it rises out of the water and starts skipping on the surface, there is no longer anything forcing the boat in any particular direction (as Tom describes well). John boats have those small ribs that run the length, and V hulls have a v point that keeps control, but a flat bottom kayak has nothing, that why you lose control when it gets on plane, because the designers never thought or imagined anyone in their right mind would ever want to take their boat to those crazy speeds, otherwise the hulls would be designed as what we call planing hulls.
So basically everyones explanation is correct believe it or not depending on your perspective.
Hope this helps clarify
Bob


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PostPosted: Mon Dec 05, 2011 1:44 pm 
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frisbee:
Actually you may be on to something with the aircraft carrier. It's a displacement hull yet can go in excess of 40 mph (probably more). Maybe it has something to do with the nuclear reactor that delivers several hundred thousand horsepower.
That's your solution, just put a nuclear reactor on your outback, then you can go any speed you like. I can't because I have already exceeded my lifetime limit
on radiation screwing around with that kind of stuff.

Alternately they could remove the nuclear reactor and place 1000 mirage drives on the hull bottom, and probably do well. But it would suck to be stationed on that boat.

Humor
Bob


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PostPosted: Mon Dec 05, 2011 6:06 pm 
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I found out this weekend that a Pro Angler will go 6.3 MPH and still handle perfectly. I have no idea how much faster it might go but will keep plugging away at it. I'm afraid, however, that my propulsion power may have topped out and I'm not going to mount an outboard on it.


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PostPosted: Mon Dec 05, 2011 10:15 pm 
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I thought I was being "light" by giving the Multiple answer question..perhaps i should effuse smilie faces through out my post...but then again I figured I did not need to play down to the readership. But for those that need it... :D
There happy?
and when you talked of the sail pushing "Down" didn't you hit the nail on the head about my reference of hull length/speed being a limitation in Sailing.
And when you mentioned paddled power yaks...well we aren't discussing raddle powered yaks, we are discussing electric motor driven yaks (and that was another of my points....the article that we both read was talking about the limitations of paddle-powered (actually we should open this up to Hobies and call it human-powered) and NOT alternate energy driven systems....and who said the electricity wasn't created by a nuclear reactor (even if it is stored in a battery before being used by the trolling motor....so your yak could be nuclear power driven.)?
Let me not forget :lol: Just keepin it light :)


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PostPosted: Mon Dec 05, 2011 10:39 pm 
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Tom Kirkman wrote:
I found out this weekend that a Pro Angler will go 6.3 MPH and still handle perfectly. I have no idea how much faster it might go but will keep plugging away at it. I'm afraid, however, that my propulsion power may have topped out and I'm not going to mount an outboard on it.
That's an excellent speed for the PA, and you'll be pleased to know the handling is also totally solid at 7.1 MPH. At 6.3, I you are stuck in your wake and will need a massive dose of additional power or a downhill run to climb out of it. I would imagine battery life is pretty short at that speed. You must be drawing about 700 to 900 watts?
fusioneng wrote:
frisbee:Actually you may be on to something with the aircraft carrier. It's a displacement hull yet can go in excess of 40 mph (probably more). Maybe it has something to do with the nuclear reactor that delivers several hundred thousand horsepower.
Or maybe one of these marvelous devices!
Image

Here's an excerpt from GlobalSecurity.org
Image
Displacement hulls push water aside as they move. The water that is pushed aside forms a wave that begins near the bow. The length of this wave is proportional to the speed of the hull through the water-the faster the hull moves, the longer the bow wave. At some speed, the length of the bow wave increases to the point that it matches the length of the hull, and the hull operates in the trough of the wave, with a peak near the bow and another peak near the stern. This speed is called the "hull speed" and it is approximately 1.34 times the square root of the waterline length of the hull.

The purpose of bulbous bow is to change the nature of this bow wave to reduce the drag it induces on the hull The bulbous bow creates it's own wave that is farther forward and "out of phase" with the natural bow wave created by the hull, effectively subtracting from the normal bow wave and reducing it's drag-inducing effect.


The "hull speed" concept apparently does not apply to frisbee's boat though since it evidently has no such limitations. 8)


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PostPosted: Tue Dec 06, 2011 8:01 am 
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Interestingly, if I get on the pedals while making 6.3 MPH under other power, I can gain another 1/2 MPH.

I haven't given this sufficient thought just yet, although on the surface it would seem impossible for the Mirage Drive unit, already beyond what it will do with a PA under human power, to add anything additional at that speed. Sort of like sticking your leg out of a car door at 30 MPH and thinking that if you stroke the road with your foot you can push the car a little faster (you can't). There is obviously a difference, but I haven't spent any real time thinking about it, yet.

Another thing I've found interesting is that under power, with the Mirage Drive installed and the fins feathered, versus having them down and in line (1/2 the frontal drag) there is no change in my powered speed. Sunday, I even pulled the drive completely out of the drive well thinking I would almost surely pick up a little additional speed. But, nothing. The top speed remained unchanged.


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PostPosted: Tue Dec 06, 2011 9:47 am 
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Tom, the Mirage Drive (properly tuned) still has lots of juice left at that speed. It can sprint an Adventure up to 8 MPH in still water and pull at least beyond 12 MPH.

With your result of no change in speed with fins up or down, I believe you demonstrated that fin position makes no difference; they are "feathered" in any position as long as they are not active. Technically, the boat should be slightly slower in the fin-up position due to increased interference drag, although of doubtful significance.

Running with the fins out may be trading one type of drag for another, for the open drivewell scoops up water like a bucket at speed (notice the pile of water flooding the front cockpit on the TI to the left of the wheels):
Image

Here's a "hull speed" problem for frisbee to puzzle over:
Image
8)


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PostPosted: Tue Dec 06, 2011 10:37 am 
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I doubt many folks can propel a PA at 8MPH with the Mirage Drive! The limitation, of course, may be the human powering it.

Frontal drag is generally of more concern than overall surface drag, and this is why I was surprised that the Mirage Drive in the feathered position didn't reduce speed versus having the fins in-line.

I did install the drive well plug to see if that brought the speed up, but again, no difference.

These things are interesting and to play with, but at some point it's time to just get back to fishing.


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PostPosted: Tue Dec 06, 2011 11:38 am 
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Tom:
I kind of discovered the same thing when sailing my TI, Whether I have the Mirage drive in or out doesn't seem to make any difference on my speed, so these days I always just leave it in with the fins straight down, like a center board. It probably has something to do with the TI being rather large.
Also typically when I am out especially in lighter winds I tend to always pedal even when sailing (force of habit I guess).

Here is a question for you, On our Oasis, and Revo we always packed the Hobie sail on the boat whenever we went out (even on rivers) and would put it up whenever there was any wind to help extend our range. I just looked on the Hobie site at the PA and it looks like it has a sail mount hole (called the accessory mount) but have never heard anybody mentioning sailing their PA's. I would think with such a versatile and stable boat throwing a sail up when there is an opertunity to take advantage of the wind would help extend everyones range. After all I think that is why everyone is electrifying all their PA's, so they can greatly extend their fishing range.
Just an observation from an outsider, I don't have a PA and have never seen one up close, but it looks to me like the absolute ultimate platform for any serious fisherman, and adding a Hobie sail in my eyes would be a logical extension.

Another observation is the PA has vertical rod holders just like my Oasis, revo, and TI but I have not seen anyone talking about adding a motor mount to the rod holders (like mine) and just attaching a standard transom mount trolling motor with no modifications needed to anything. The motor mount could have hollow tubes so their function as rod holders wouldn't be comprimised. Basically my motor mount cost me about $10 bucks to build with PVC tubing, a pressure treated 2 x4, and a some steel rod, or they can be purchased pre-made from ( http://islandhopperoutboards.com/hobiekayakmounts.html ). I could go to Walmart and buy an off the shelf trolling motor for a hundred bucks and be on my way.
Or do what I did and add a small 4 cycle gas motor instead ( I got mine at http://www.smalloutboardengines.com/20h ... flage.html) (my range is 50 miles with 1 spare gallon of gas @5mph (TI), or 7 mph (Revo)). I have the motor set to run either on propane or gas. A couple small propane bottles can easily get me 30-40 miles, or I think a 20 lb tank would get me 100+ miles (still lighter than 1 battery).
Converting my gas motor to propane cost me about $5-$10 dollars, you just add a cut off valve to the fuel line, and a T connector to one of the vacuum lines and a little rubber hose to a standard propane torch (with the flame tip removed). Basically you start the motor on gas, turn on the propane, then shut off the gas. You then just adjust the propane valve to control the RPM (with the throttle on the tiller in the locked open position). My thought would be traveling with the motor to get you the five or ten miles to your fishing area, then use the Mirage pedal drive system once you are there.
My situation might be a little different than most, 98% of the time I'm in either Tampa or Sarasota bays (both huge), or off of Key West where the closest Island is 4 or 5 miles to the west of KW. A good day for me is 30-40 miles covered. I have about 1100 miles on my TI to date but have only ran 3 gallons of gas through my gas motor since I bought it, so obviously if I can get somewhere without needing the motor I do, and do not go out in my boat without the motor (for safety reasons in the Ocean).
Just a thought from an outside observer. I'm not a fisherman, but love to snorkel and scuba dive with my spear gun year round (I pick and choose what I want to eat LOL, but honestly I have much more fun just watching).

Bob


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PostPosted: Tue Dec 06, 2011 12:08 pm 
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I've sailed my PA several times and enjoyed it tremendously. In fact, I enjoyed it so much I've been considering buying a dedicated sail boat.

My PA has all the range I need - I'm just trying knocking around trying some various things for my own enjoyment. I doubt I'd ever put any sort of internal combustion engine on it.


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PostPosted: Tue Dec 06, 2011 7:43 pm 
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Gentlemen,
I have nothing but the highest regard for the way so many of you, including myself early on, have taken this conversation. It has at the very least been informative, thought provoking and highly entertaining. I would certainly enjoy making an attempt to bring this endeavor to an even higher level of discourse but alas time constraints prevent me from doing so. I must now devote myself to that which brought me here in the first place, that being my love of kayaking. That being said, may I bid all of you a fond adieu as I prepare for my next adventure on the water this weekend. I will now turn my energies in that direction. Who knows, I may even break out above my own bow wave. Peace to all! :)

_________________
Roger
2010 Oasis
Lucie Belle


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PostPosted: Wed Dec 07, 2011 7:39 am 
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This has been a very interesting discussion. As it relates to the OP, the OP has identified there is most assuredly a point of diminishing returns with regards to power. I suspect most kayaks would be very well served with a 30# thrust motor.


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