Advantages of hard chined, human powered, stabilized monohull, over round bottom?

Rick W (and others) use a hard chined, boxed section for human powered pedal boats. I have read most of the 138 page thread on this subject but perhaps missed the reasoning on why this is preferred over a round bottom, someone even said that it was an advantage. Roll stability is not a concern since it has amas for stabilization. Other than ease of construction, why is the rectangular cross section preferred over a round section that would have less wetted area?

 

Ease of construction comes to mind. Oops, you want additional reasons ! I think the construction issue would be the main one.

 

Rick found it lower drag in Michlet or Java or something.
Reduces beam for the same displacement so better l/b ratio.
Out-weighs the penalty of the increased wetted surface area.

And roll stability was a consideration because the better balanced while making way, the less work the training wheels... oops, I mean pontoons had to do which means less drag, more speed , less work, blah, blah, blah, blah.

You may want to reread the early parts or look for that specific topic as a Rick W thread prior to.

 

Of course, there are not just two options, "round" bottom or hard chine, you could have something intermediate.

 

I found a whole lot of the old content gone.

Is that pedal power thread still available ?

 

Pedal Powered Boats thread is still available.
It doesn't appear to be missing any content, but how can you tell?
The dialogue seems to flow okay.

W9GFO, why do you ask?

I can tell you I used a square cross-section hull with two kayak style paddlers to win a boat race.
Rick also holds the human powered endurance world record with his design.
Mine was a proa with a small hydrofoil flying the pontoon at 4-knots.

 

I'm thinking of building a single person pedal boat...

Getting a better l/b ratio makes sense, not sure about the roll stability thing though. Are you saying that a narrower hard chined hull has more stability than a wider round bottom? If so, how would you quantify the difference?

 

It's basic hull theory.
Roll a round hull and you get no change in buoyancy.
Roll a shallow rectangular hull and the centre of buoyancy shits creating a corrective righting moment.

 

I think that round hulls do create righting moment when rolled. If they didn't, they would stay heeled when you rolled them. I think only a cylindrical hull that is rotated on its longitudinal axis would result in no change in buoyancy.

 

There are a whole raft of drag sources in a pedal boat. As you pedal, the boat surges and heaves because your center of mass is moving as you pedal. Chines can dampen that out, particularly if you end up near a natural frequency of the bobbing boat. And this can definitely come in to play on a pedal boat. Let's say you have 1.1m^2 of Aw, and are at 100kg all up. Natural heave frequency (sans added mass) is about .6 seconds. This would be in resonance with a pedal cadence of 50/min. Using a box section reduces the Aw and increases the added mass term, pushing the natural frequency to a lower value, and reducing the amplitude of heave for cadences above 50. Pitch also has a natural frequency that you want to avoid. I looked at using some trick amas to tune the natural frequencies of a pedal boat so that it could be set up for different durations and crank rate. The boat was half built when I moved and I ended up tossing it in a dumpster.

The second reason, as already mentioned, is build accuracy. Lengthwise fairness should be mirror perfect at this scale. And you want dimensional tolerances to the thousandths of an inch to hit the displacement curve accurately. This is way easier to build and verify with flat panels than with round shapes. You really don't want to add 10 pounds of bog trying to fair the thing after you built it.

 

No.
A long slender narrow hull has less resistance than a wider hull form.
But it boils down to 2 apsetcs
1) WSA - wetted surface area - friction
2) Length-displacement ratio - the lighter the hull for its length. This reduces residuary resistance.

A narrow hull has low stability because its metacritic height (KM) is proportional to the waterplan inertia, which is dictated by the beam. Less beam lower metacritic height
KM = KB + BM where BM = I/V. The I being the waterplane inertia.

 

Okay but does the narrower hard chined hull have a practical advantage when it comes to roll stability compared to the wider round bottomed hull of the same length and displacement? My gut says that the hard chined version's advantage (if the advantage really exists) is so slight that in practice it would make no difference.

 

But.……" Roll stability is not a concern since it has amas for stabilization". So why bother with discussing that ?

 

That makes sense but how would the damping of the wider round hull compare to the narrow hard chined hull?

 

If there is better stability with narrow hard chines then you can spend more time with both amas clear of the water, which means less drag. I am trying to figure out if this is a real reason for choosing narrow hard chines over a wider round bottom, or if it just theory.