designing a fast rowboat

[Guest625101138]

Originally Posted by helen07

Hello again , maybe 2 weeks ago i suggested twin hulls with a centre cradle and today i happened to chance upon this video which features a row boat remarkebly similar to what i had i mind .

http://video.google.com/videoplay?do...&hl=en&view=2#

The video is of a Rocat:
http://www.rocat.com/
There was considerable development on the design and construction and a few boats were produced and sold. I understand there was not enough interest to have a viable business.

The boat is about as good as you can do with a catamaran but the performance against a rowing scull is relatively poor. About 11% slower or 35% more power to do the same speed. This is a fundamental of catamaran versus monohull as Leo L noted above in post #546. My figures are based on sustainable power for a fit person and total displacement of 114kg. The difference varies a little as design power and weight shifts. If there is a length constraint then the catamaran has an advantage below about 16ft for a single person rowed or paddled boat.

The Rocat would be competitive against a beamier ocean rowing monohull but a few owners were expecting to be competitive against OC1s and surf skis. Paddles cope better with rough water than oars. A long monohull with pedal powered propeller is the most efficient human powered boat in any conditions.

This clip shows the conditions where a beamy cruising pedal boat will outperform a rowing scull:
http://www.openwatercycling.com/vidphotos/PICT0722.MOV
A long slender pedalled monohull is even better than this as it does not pitch as much.

Rick W

[ancient kayaker]

The catamaran configuration provides extra stability allowing a sailboat to carry more sail for more power. With a human powered boat the amount of power is limited.

An experienced rower (or paddler) can balance a narrow shell using the oars so the additional stability of a cat is of limited use to him. When the boat is powered by the legs which provide more power, extra stability is needed to handle heavier conditions hence a cat works best. Would a trimaran with small amas that can be above the surface much of the time be an even better choice?

[Guest625101138]

Originally Posted by ancient kayaker

... When the boat is powered by the legs which provide more power, extra stability is needed to handle heavier conditions hence a cat works best. Would a trimaran with small amas that can be above the surface much of the time be an even better choice?

Terry
To clarify.
Rowing with a rigger is more powerful as both legs and arms are in use. This is a feature of the ROCAT. I expect it gets close to the biomechanical efficiency of pedalling or paddling; both measured around 26%. A sliding seat sculler has been measured at 22%.

Using both legs and arms in a coordinated way enables the highest power in anaerobic sprinting but pedalling is not far behing and requires less coordination. For longer duration, pedalling and paddling have similar biomechanocal efficiency but a good prop will give 85% efficiency compared with about 70% for a paddle.

A pedal prop driven stabilised monohull is the most efficient for long distance and a pedal powered hydrofoil is the most efficient for sprinting:
http://www.youtube.com/watch?v=0yk7t...3B10A6&index=2
http://www.youtube.com/watch?v=l2UOH65QOI4

Apart from the better efficiency from a propeller over a paddle or oar, there is also the advantage of steadier application of power with pedalling meaning that the fluctuation in speed is much less. A good recumbent cyclist will "spin" with about 30% variation in torque (and less variation in thrust) at slightly higher frequency than a kayaker and almost three time the frequency of a rower. The force in both paddling and rowing ranges from zero to a maximum and there is significant dead time at zero thrust particularly with rowing.

Given the power is a cube function of speed, there is a significant penalty associated with speed fluctuation.

Rick W

[helen07]

So put your pedals and prop between the hulls and maybe (from an unqualified point of view) you have minimal drag , persistant power and if you add a bicycle gear system perhaps the potential to reach far greater speeds ?

Though if i remember correctly this is a rowing competition we are talking about ?

This guys sitting way too high but you get the idea .

http://images.google.com/imgres?imgu...a%3DN%26um%3D1

[Guest625101138]

Originally Posted by helen07

So put your pedals and prop between the hulls and maybe (from an unqualified point of view) you have minimal drag , persistant power and if you add a bicycle gear system perhaps the potential to reach far greater speeds ?

Though if i remember correctly this is a rowing competition we are talking about ?

This guys sitting way too high but you get the idea .

Yes it is a rowing thread. Rowed, paddled or pedalled the lowest drag catamaran will be around 11% slower than a monohull of the same displacement for equivalent method of propulsion.

I speak from personal experience per attached video clip with pedalled cat v my numerous monuhulls; the comments of a ROCAT rower competing against monohulls and knowledge of the physics involved as quantified by a few extraordinary mathematicians.

It is interesting that the developer of ROCAT did not realise the disadvantage would be so significant. In fact 11% is not all that significant if you are a casual rower but in competition it is a big gap particularly when you realise it takes 35% more power to achieve the same speed.

Rick W

[helen07]

A canoe like hull with a rowing machine , similar to those found in a gym , set up to power a prop , and 2 stablizers on out riggers ?

[Guest625101138]

Originally Posted by helen07

A canoe like hull with a rowing machine , similar to those found in a gym , set up to power a prop , and 2 stablizers on out riggers ?

A colleague was heading that way with his forwardface drive system before he sadly passed away:
http://forwardface.com/includes/movi...teering_01.AVI
His son may continue the endeavor. They were working on the stabilised monohull to lower drag at the time he passed away.

The drive system is very efficient but still suffers the speed variation of rowing in the configuration shown in the clip. I used a pedal variation of the drive system using swing arms in my V12 boat. It has very high biomechanical efficiency but runs into problems at sprinting type power level.

Rick W

[helen07]

I'm way out of my depth here but what if the return motion was set to pull water and the power motion set to push ? or perhaps the return motion could be used as power also at a much lower gear ratio just to keep up momentum ?

Just suggestions while trying to be helpful and probably worthless but they might possibly plant a seed in the mind of somebody who really can help ?

I can't access the link you posted , it just says page can't be displayed ?

[helen07]

Heres an idea that might be denied by our preconceptions about rowing , as we have no oars we don't actually need a full stroke , so what if we used 2 half strokes left and right to provide continueous power to the prop - imagine a rowing/running exercise machine ?

[ancient kayaker]

Alternatively, if the power stroke is used to pull the seat forward and the legs to push it backward there are twice as many power storkes to harness for driving the prop.

[sabahcat]

Originally Posted by Rick Willoughby

The video is of a Rocat:
http://www.rocat.com/

Found this in my long quest for a 12 ft catamaran dinghy



http://www.duckworksmagazine.com/08/...kiff/index.htm

[Guest625101138]

Originally Posted by helen07

...
I can't access the link you posted , it just says page can't be displayed ?

I have attached the video file to this post. It is 4Mb so will take a while to download depending on your connection speed.

Rick

[Guest625101138]

Originally Posted by sabahcat

Found this in my long quest for a 12 ft catamaran dinghy



http://www.duckworksmagazine.com/08/...kiff/index.htm

That would perform well against a 12ft monohull rowing boat but would not get close to a 20ft monohull.

If you have a length constraint for storage or transport then this would be a good choice.

Rick W

[seawalkerboat]

feature:
*this boat can divided in two parts,it adds up to half the weight to manipulate,half the storage space,but twice the stability.And it can be put into the business car.
*FRP boat,3 years warranty for the boat hull
*PVC tube,can be attached & removed easily.But it can add safety and stability
*more parts for the boat made of stainless steel,like oarslock,cleats on the bow and so on

[helen07]

Originally Posted by Rick Willoughby

I have attached the video file to this post. It is 4Mb so will take a while to download depending on your connection speed.

Rick

Thanks i got your vid Rick and looking at your hull it seem pretty quick and it seems a shame to add stabilizers so i got to thinking , how about adding flares at rail hieght so they don't come into contact with the water (thus slowing you) unless the boat rolls significantly . I'm thinking a hammerhead shark like shape . If they were designed to use your momentum to throw the boat back up might that help ?

The first person to suggest men could fly was locked up in a mental institution and i'm starting to know how he must have felt !