Scaling power to weight ratios from human power to models

[Skip Johnson]

The paddles are outside my baliwick, the current state of art on the MR340 is pretty well advanced.

I had somehow missed the Pedyuloh, an interesting thing for sure but I'm personally committed to my AD scull for personal use.

The current project is a variation of Harry Bryon's set up on Thistle rotated 90 degrees for a horizonal foil,

 

[fallguy]

Skip-did you read the Scaling? thread? It has everything you need from Ad Hoc’s prior reply back in 2021.

Really very interesting and I think you can really get the model close if you setup a spreadsheet and use that old thread.

I was somewhat correct on the Fn.

Not sure why we lost Ad Hoc.

 

[tropostudio]

The Scaling thread is good. The formula (or geosim, as Ad Hoc terms it) is P model = P full x (1/scale factor)^3.5. It got me wondering about it's derivation. Google AI (it works for some stuff):

Displacement is based on scale cubed and velocity is based on square root of LWL. The results from the formula give results that don't seem applicable to this scenario, where viscous drag is such a large part of total drag.

Skip's full-scale parameters : 260# displacement; 30' (360") LWL; 100w power (disregarding efficincy losses from inout to output).
For a 1/10 scale model (36" LWL): 100w x (0.1)^3.5 = 0.0316w
For a 1/5 scale model (72" LWL): 100w x (1/5)^3.5 = 0.358w

Those numbers seem ridiculously low. Hence, look at the scale electric boat modelers for 'rules of thumb.' FWIW, they seem concerned with models for displacment or semi-displacement boats that 'push water' to look much like the full-size boats.

@Skip Johnson - can I ask why you are looking at a fin drive over a propellor? As Mark Drela has often noted, a prop can be 90%+ efficient if you can afford a large disc area and turn it slowly.

 

[Skip Johnson]

Theory postulates the highest efficiency comes from moving the largest volume the least amount. Aquatic mammals all have horizontal fin drives, fishes aren't tied to the surface and evolved with vertical fins.
My experience with an AD scull reinforces the idea.
Evolution on a shorter time scale on the MR340 has replaced propellor drives with paddles.
We'll see.

 

[portacruise]

The 24 hr distance record for HPB has never been won by a horizontal fin drive, and none have ever competed, AFAIK. But that may change if Skip is successful, can't wait to see what happens! I think the current record is held by a paddler, but that is not quite the same as (leg?) horizontal drive, which might have additional losses compared to the best paddles. The total power of muscle groups that are used with propeller and all other drives are not likely to be of identical power, even for the same person, which makes comparing total overall efficiency difficult.

 

[Skip Johnson]

You're right, comparisons are iffy. The last three records went to Carter Johnson, surfski; Greg K, pedal driven propellor; and the current record holder from Poland in a kayak.
The common thread is they were all superb athletes and driven to compete.

I'm driven to dabble in the fringes of small boat design. The horizontal fin drive has a lot of theory to recommend it; compare the swept area of a prop with a rectangle the height off the prop diameter and 3-4 times as wide. That being said I've discovered the first little oopsie in my idea, time and effort will tell how far this goes.

 

[fallguy]

If you study the fish; they also vary.

The wahoo, a catchable species here in the gulf can swim about 50mph at 50 pounds. Incredible.

The key to its speed isn’t necessarily just the fin, but the body shape of the fish is torpedo-like.

Of course, one thinks foil awfully fast if we want to reduce drag, but I can’t fathom(hehe) fin drive and foil.

Just making conversation…

 

[portacruise]

Wonder what kind of boat Greg K used? If it was a pontoon, there might be room for improvement with a mono and prop combo?

 

[Skip Johnson]

Wonder what kind of boat Greg K used? If it was a pontoon, there might be room for improvement with a mono and prop combo?

Skinny mono with outriggers.

 

[portacruise]

Yes, I forgot Rick W. likes that design... Thanks!

 

[Skip Johnson]

Yes, I forgot Rick W. likes that design... Thanks!

He should, it's his ;-)