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Fit issues on paddleboats ...(12 posts)

Fit issues on paddleboats ...Humma Hah
Jun 29, 2002 10:26 AM
... Yesterday was my wife's birthday, and she decided to take a day off and go to Hershey Park. My B'day is about a week off, and I joined her to make it a double celebration. This meant losing a day of riding.

The closest thing I could find to exercise was the paddleboats. We rented one for half an hour. Fun, but uncomfortable. The seat to BB distance is fixed, at a distance probably optimal for a 10-year-old. The crank length seems to be something like 100 mm. I had a devil of a time coupling any real power to the pedals.

Not that it would have done much good. Those things seem to have an optimal hull speed of about 2 mph. Pedal gently, and you go 2 mph. Pedal briskly, and you may increase that to 2.1. Pedal at max output, and you go 2.1 mph with a foamy white wake.

It seems to me that the pedal boat industry needs some cycling talent. I'd propose the hull design be switched to kayaks, with adjustable seat position and other ergonomics adapted from 'bents, and maybe an internal centrugal pump for jet-boat propulsion, instead of the inefficient paddles.
re: Fit issues on paddleboats ...DougSloan
Jun 29, 2002 4:56 PM
My brother had one of the those paddleboats at his houseboat. I used to take it out early morning for a workout. I discovered the same thing. Pedal too fast and the paddle cavitates (I think is the term). It just spins and doesn't do much. So, I'd just find the sweet spot and go further to make up for it.

I think there are some techy bicycle crank-type water craft out there, but I can't remember what they are. One is inflateable and fits in a backpack; you attach your regular bike to it.

Doug
re: Fit issues on paddleboats ...Skip
Jun 29, 2002 8:21 PM
HH,

Give this a look; may inspire some thoughts.

http://www.hydrobikes.com/Hydrobike_Video/Products/Explorer_I/explorer_i.html
positionDougSloan
Jun 30, 2002 7:11 AM
It seems like a recumbant position would be much better on a boat. Lower center of gravity would help reduce roll and make it more stable.
positionSkip
Jun 30, 2002 11:47 AM
I agree. Interesting to see a top speed of 10 MPH though. Now, maybe with an aero-winged keel, and lighter COMponents, hummmmmm.......... - Ha!
Looks like an exercycle on floats ...Humma Hah
Jul 1, 2002 2:44 PM
... topheavy, high wind drag, and it looks about as much fun to ride as that exercycle I donated to charity last year, after dusting off the cobwebs.
theoretical hull speedmr_spin
Jul 1, 2002 6:29 AM
One problem you had is that with a boat of that size, you are never going to go all that fast, so you might as well not even try. You need to know about the concept of theoretical hull speed.

I copied this description from a web site:

"Hull speed is controlled by the length of wave a boat hull makes as it passes through the water and the longer the hull, the longer the transverse wave the hull can generate. The basic math that discribes this is deceptively simple; hull speed is the squareroot of the waterline, multiplied by 1.34."

I know this applies to sailboats, but I'm not sure about paddle boats. Let's just assume it does. So, the water line of your paddle boat (the length of the hull where it touches the water) was maybe 6 feet?

SqRt(6) X 1.34 = 3.2 knots

That's roughly 4 mph. Wheeee! Quite the thrill ride.
power?DougSloan
Jul 1, 2002 6:50 AM
But what if I put a 200 hp outboard motor on it? Seems like it would go a little faster than 4 mph. I think the equation that applies to sailboats might not work for propelled boats.

Doug
well...mr_spin
Jul 1, 2002 7:03 AM
A paddle boat with a 200 hp outboard seems kind of silly, don't you think?

The equation might apply only to sailboats, I'm not sure. For the purposes of estimating paddle boat speed, I'll bet it comes fairly close.

Considering that paddle boats tend to be floating boxes, with very wide, barely shaped hulls, it would be quite a feat to sustain more than 4 mph for a reasonable length of time. As your speed incrases, the drag on the paddles would increase and eventually the two forces would max out. Plus, the faster you go, the more the boat rises out of the water, making the paddles even less efficient. You can't beat physics.
Applies to boats that keep their hulls in the water ...Humma Hah
Jul 1, 2002 11:30 AM
... a powerboat that gets up in a plane, or a hydrofoil, are immune from the equation, but it applies to sailboats, canoes, most large ships, supertankers.

Yes, you can push a hull past hull speed, but the energy cost is high. Hull speed is a point at which there's minimal drag, and you get peak efficiency.

Recent racing yachts have gone to "winged keels". My old physicist boss got a peek at the New Zealander's keel, and recognized it was significantly different from most. Its not for drag reduction, he thinks it is for lift, an attempt to go partly hydrofoil and beat the hull speed problem.
They're closer to 5 ft ...Humma Hah
Jul 1, 2002 11:25 AM
... so hull speed is slower.

I worked for some years for a physicist who loves sailing. Bought himself an Olsen 40 racing yacht, a cute little 40-footer that once won a TransPac racing against the big "sleds" the multimillionaires own. The sleds are faster due to higher hull speed, but there's a handicapping system. After that race, Roy Disney had the handicapping rules changed so a scrappy little 40-footer would never beat his sled again!

I figure a nice long, sleek hull like a kayak or canoe would be considerably faster.
Something about displacement hull vs. planing hull...cory
Jul 1, 2002 7:57 AM
...but I don't remember everything I used to know. My dad's paddleboat (typical, I assume; it looks like every other one I ever saw) plugs along about about 1.2 knots no matter if it's pedaled by me or my 6-year-old nephew. The hull design (displacement type) means it has to push the water aside to move through it. A planing-type hull rides on top, has less contact and goes like hell. But I don't think you could generate enough power to get it up "on the step" the way racing hydros do, no matter how it was shaped.
That paddle has to be an inefficient propulsion system, but I wonder what would work better. I don't remember everything I learned during a brief period of interest in boats, but prop design would be critical with such low output. Got no idea at all about a jet drive, but I'm pretty sure that unless you could get that hull up on plane, its max speed is severely limited.