designing a fast rowboat

[Guest625101138]

Originally Posted by helen07

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 !

The boat in that clip is not one of my boats.

There is quite a lot of discussion on various boat configurations on this thread:
Pedal Powered Boats
If you want to explore these more radical ideas then it is probably best to take them to that thread.

This clip gives you an idea of how little my outriggers actually contact the water. Water drag is next to negligible. Give it a few minutes to download:
http://www.boatdesign.net/forums/att...igger_load.wmv

Rick W

[nordvindcrew]

I was going to suggest that the thread had gotten off base a bit, but changed my mind. True, I am concerned with convention oar-on -gunnel rowing, but getting all this great feedback on other forms of water power is great. That forward rowing thing really looks to be moving. Having Leo and Rick commenting puts a light on boat form and efficiency that I appreciate greatly. I asked Dave Gerr to look over the thread and give us some imput if he would care to on the gereral design parameters special to conventional rowing boats. Right now i'm fantasizing about one of Rick Willoghbys (?) hard chine hulls with a box on top to get out to a 4' beam to mount the oarlocks on. It would be sort of a box keel boat taken out to a ludicrous degree. The box keel would be the only part in the water with the seating portion on top not in any contact at all except in rough water.

[Guest625101138]

Originally Posted by nordvindcrew

.... Right now i'm fantasizing about one of Rick Willoghbys (?) hard chine hulls with a box on top to get out to a 4' beam to mount the oarlocks on. It would be sort of a box keel boat taken out to a ludicrous degree. The box keel would be the only part in the water with the seating portion on top not in any contact at all except in rough water.

The extreme box keel arrangement is getting a lot of attention for powered craft. I refer to it as a stabilised monohull or faux-tri. There are a few threads discussing it. The most recent is shown here:
Trimaran motorboat / stabilized monohull
Post #26 has many photos of various versions.

One problem you encounter in a seaway is flop roll when the main hull is suspended between wave crests. To overcome this you need to have the sterns of the outboard hulls located near the stern of the main hull.

For a rowing boat I would think about mounting the oarlocks off the bow of the outboard hulls. The beam supporting these hulls would be elevated to avoid dragging through wave crests.

I designed the hard chine flat-bottomed hull for ease of building, expecting it to perform a little worse than my optimised rounded section hull. However in practice it is proving to be a better hull overall. I think you would find it an outstanding rowing boat. Would need some fairing to deflect water off the hull as it drives through waves.

With stabilisers a slender hull is more practical for ocean rowing than a scull.

Rick W

[nordvindcrew]

my idea is to have the final result look more like a rowing shell except that the beam would be wider, maybe 24" and the Washboards ( I think that's the right name) would be more substancial and almost form the shape of a small dinghy on top of the main hull. Am I totally nuts?

[Guest625101138]

Originally Posted by nordvindcrew

my idea is to have the final result look more like a rowing shell except that the beam would be wider, maybe 24" and the Washboards ( I think that's the right name) would be more substancial and almost form the shape of a small dinghy on top of the main hull. Am I totally nuts?

That would work quite well. The attached is a clip of one of my earlier boats with a wave piercing bow and a canoe style "washboard". It carved through waves quite well and the cockpit kept dry. The limitation was the ama beams clipping waves. With oars it is a matter of how high you set rowlocks to avoid clipping waves.

Rick W

[Easy Rider]

Hey guys,

So there has been an incredible amount of chat on how to design the fastest rowboat ... for a few messages could we switch to the easiest boat to row. Of course the are highly related but what if I want to pack a load or, like a fellow here in Alaska that wants to fish (as in troll) w a row boat. A really big and heavy dory can be rowed amazingly well w it's low wetted surface, it's relatively low wave making form and at slower speeds it's short WLL is not an issue. I row a 10' Whitehall type and I think I could row it w two fingers and frequently think it's such a waste I should get a considerably longer boat but now I think maybe just plain bigger would be better. Is'nt out country founded on that principle?

Easy Rider

[nordvindcrew]

I've rowed quite a few boats, and they all are pretty easy at, maybe, 3 knots or less. That is given no current and no wind. All the different types are good if you choose for the use intended. I like the dory type and think the Gloucester Light Dory is a good choice. They handle moderately rough water well, can easily carry three people, are capable of respectable speed, yet, row easily at 3 knots with out a whole lot of wind resistance

[Easy Rider]

Thanks for your thoughts Nord. I don't fully agree w the windage, it's got a low WLL and relatively high PC. Modern canoes mostly have a planform (view from above) that is said to be and is quite diamond shaped. That would (in my opinion) minimize the PC. I theorize that if a boat had a diamond shape in three dimensions it would be optimized for minimum resistance. If the boat had a half diamond cross section (vee bottom) and a side view half diamond shape of the keel drag would be minimized. The mid section would obviously need to be curved in fair lines for about 20 to 30% of the length from the fwd straight lines to the aft. Like most modern sail boats it would need to be wider amidships to compensate for the loss of volume at the ends and it would perform badly if not at it's optimum displacement. What do you think .. or anyone.

Easy Rider

[Guest625101138]

Originally Posted by Easy Rider

T......... I theorize that if a boat had a diamond shape in three dimensions it would be optimized for minimum resistance. If the boat had a half diamond cross section (vee bottom) and a side view half diamond shape of the keel drag would be minimized. The mid section would obviously need to be curved in fair lines for about 20 to 30% of the length from the fwd straight lines to the aft. Like most modern sail boats it would need to be wider amidships to compensate for the loss of volume at the ends and it would perform badly if not at it's optimum displacement. What do you think .. or anyone.

Easy Rider

You do not need to theorise there are very good analysis tools that can arrive at lowest drag hull form under various operating parameters and hull constraints. The boats I build these days are product of this approach.

For a single person a rowing scull is very close to the lowest drag form you can get for a strong rower without compromise for length, beam and stability. Surf skis are close to the lowest drag for a hull form that needs some inherent stability. Racing kayaks (K1) are maximum length constrained and minimum beam constrained but are about the lowest drag hull given these constraints.

The paddle presents a considerable proportion of the windage in a kayak. I expect it is similar for oars in a rowing boat. Going to windward in a kayak the upper blade is advancing through the air at about twice boat speed plus the windspeed over water. The power consumed in moving it through the air is a function of its airspeed cubed and its area so, although only a small area, it still costs a lot of energy due to the higher velocity.

Rick W

[ancient kayaker]

Originally Posted by Easy Rider

... I theorize that if a boat had a diamond shape in three dimensions it would be optimized for minimum resistance ...
Easy Rider

My canoes are fairly quick despite being somewhat beamy. I let the sheer planks define the plan shape. The sheer planks are simply bent around a middle form and secured at the stems; then I add the rest of the boat. The middle form can be centered for a symmetrical shape or offset typically 10-15% aft to move the max beam location to where the hips are, for a snug-fitting boat. The shape that results is a section of a hyperbola and has the diamond look that you noted. Of course, the sheers being mostly above the surface do not generate much drag but they go a long way to defining the waterlines.

Originally Posted by Rick Willoughby

... The paddle presents a considerable proportion of the windage in a kayak. I expect it is similar for oars in a rowing boat. Going to windward in a kayak the upper blade is advancing through the air at about twice boat speed plus the windspeed over water. The power consumed in moving it through the air is a function of its airspeed cubed and its area so, although only a small area, it still costs a lot of energy due to the higher velocity.
Rick W

Wow! I hadn't thought about the square law drag; I learned using a paddle with blades that could not be feathered and have never bothered with feathering. Perhaps I should make the effort. In a kayak, if the blade is about 1/4 of exposed body area and is moving at twice the speed it will have similar drag to the body. That's a lot.

[Easy Rider]

It's wonderful to get so much imput from so many extreemly knowledgeable guys. Rick .. I suspected as much but I'm curious how "diamond shaped" the lowest drag form is in all three dimentions. does the "most efficient form" have hollow ends? About the kayak blades most modern kayak paddles are adjustable so you can change from non feathered to feathered in a few seconds. I paddle w non-feathered blades untill the wind gets strong and then switch. Definitely makes a difference. May be a bad habit as when one needs to brace .... . Happy T day Easy rider

[Guest625101138]

Originally Posted by ancient kayaker

....


Wow! I hadn't thought about the square law drag; I learned using a paddle with blades that could not be feathered and have never bothered with feathering. Perhaps I should make the effort. In a kayak, if the blade is about 1/4 of exposed body area and is moving at twice the speed it will have similar drag to the body. That's a lot.

Terry
This is something I have a lot of time to ponder when I am competing in the Murray Marathon. The River meanders a lot but in December when the race is run there is predominantly a head wind.

I have considerably more area exposed to wind than a kayak because I sit on top of the hull rather then in. If there is a paddler who just has my measure in a sheltered or downwind reach I can make ground on them into the wind. I wondered why for quite a while. There may also be some loss of rhythm due to the wind and waves but waves are mostly small.

Rick W

[Guest625101138]

Originally Posted by Easy Rider

It's wonderful to get so much imput from so many extreemly knowledgeable guys. Rick .. I suspected as much but I'm curious how "diamond shaped" the lowest drag form is in all three dimentions. does the "most efficient form" have hollow ends? About the kayak blades most modern kayak paddles are adjustable so you can change from non feathered to feathered in a few seconds. I paddle w non-feathered blades untill the wind gets strong and then switch. Definitely makes a difference. May be a bad habit as when one needs to brace .... . Happy T day Easy rider

The diamond shape in both plan and longitudinally is not far off the lowest drag hull when there is no constraint. Meaning the only parameters controlling the shape are power and displacement. There is usually some slight concavity in the ends. This outcome is not hard and fast. At higher power relative to displacement the ends get fuller.

As noted before racing canoes are usually length constrained and the ends get fuller on the really narrow ones than the case that has no length constraint.

If you had a displacement and power level in mind I can show you the shape of the hull that gives lowest resistance.

The hulls I design for my use are usually optimised for 130W applied at overall efficiency around 80%.

Rick W

[ancient kayaker]

Originally Posted by Rick Willoughby

... There is usually some slight concavity in the ends ... Rick W

Interesting. I went back to the FreeShip files for some of my canoes looking for concavity, although I have never specifically designed for it. I found a relationship between the amount of twist in the bilge planks of my 5-plank designs and fine entry/exit.

The one with the most twist built to date has virtually straight entry lines, perhaps a hint of concavity. Looking at the hull it looks distinctly concave but that is an illusion due to the viewing angle. I tend to use bilge plank twist as a tool to maximize waterline length and reduce rocker for fast boats, other designs are specifically intended to be easy to build and are constant angle, no twist at all, fuller stems and incidentally much easier to manouver.

The canoe I plan to build next is a Wee Lassie by J. Henry Rushton and it has about as much twist as it is possible to imagine, at least 80 deg from midships to stem. The entry is extremely fine and concave. It will be interesting to see how she paddles. I should pay more attention to waterlines.

[Guest625101138]

Originally Posted by ancient kayaker

...

The canoe I plan to build next is a Wee Lassie by J. Henry Rushton and it has about as much twist as it is possible to imagine, at least 80 deg from midships to stem. The entry is extremely fine and concave. It will be interesting to see how she paddles. I should pay more attention to waterlines.

Terry
To put some perspective on this my V11 design actually has concavity in both the waterplanes and buttocks. It is designed for power of 150W at about 82% efficiency - something like 113W on the hull allowing for windage and other minor losses. The design speed is 12kph (say 6.5kts). This hull is 24ft long and 9" beam. So while it has concavity it is low power and quite long slender hull.

If Wee Lassie is typical canoe/kayak length of about 14 to 16ft then it is probably close to a lowest drag hull for 3 to 4kts at very low power and a stability requirement. So it will be a long course where you are likely to experience a benefit. It will not be a boat for sprinting. That would have fuller ends.

Rick W