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Quote...."If you have an idea of the wetted area of the hull, perhaps by doing some measurements around it from the waterline, then I could do a very rough resistance curve using just viscous drag from the wetted surface, corrected roughly for the length and beam of the hull. It wouldn't be too accurate, but might give you a ball park feel for the thrust needed for a given speed".

OK, I can give you some pretty accurate numbers.

Using the fomula which I have found to work quite well for this type of hull; Hull speed = square root of WL length times 1.3333 I get a max hull speed rowing of 5.8 mph (about 2.6 meters/sec.). That feels right intuitively as well. I've built and rowed enough to be good at that. That would be the max. though, really pushing it.

Wetted area for the proposed hull is close to 40 square feet.

The prop I would like to use is the Torqeedo 12 X 10, or I could use another 12" rugged and easily available prop., and I'm open to suggestions.

I just want to know if it is reasonable to expect at least 4 mph when pedaled at a maintainable cadence of 60 cps and using a 1:8 increase by a reasonably fit 65 year old.

Thank you for your input so far.

Gib

 

Some rough data:

2m/S (about 3.88kts or close to 4mph) would need very roughly 22N of thrust (around 5 lbsf), which is an effective power delivered as useful thrust (output power) of around 43W.

I don't have any data on the blade chord, foil section, twist etc for the Torqeedo prop, so can't give an estimate as to what sort of rpm and power input would be needed to give this much thrust or what the prop efficiency might be. I would guess, based on experience, that it wouldn't be very efficient at this low a blade loading, so might be around 40% or so efficient. The input power would be around 108W, which is about right for the sustained power output for an averagely fit person with no athletic training.

I would suggest that there are better props for this sort of power and speed requirement, as the Torqeedo prop is designed to work most efficiently at a higher blade loading.

 

Thank you Jeremy. I ordered an APC 12 x 14 x 2 blade. It looks kind of flimsy to me but maybe I can reinforce it with carbon fiber and epoxy.

 

Gib:

Can you give me the address for the APC prop, as I couldn't find 12 diameter X 14 pitch on their website. Maybe 14 diameter X 12 pitch? They used to make a wide blade version of 14X12 which was even stronger. It should be strong enough if it is for gas power; not the electric version. Also, they are low cost, so you can keep plenty of spares around if they do break. Coach Dave has even used the very larger diameter electric versions in the past....


Porta

 

Oops, found it in pattern props. Thanks.

P.

 

For rough comparison Andreas proposed the requirements for a competition boat:
650Watt @ 5m/s (104Newton drag)
150Watt @ 3m/s (35Newton drag)

Power scales with velocity cubed while drag scales with velocity squared so:
44Watt @ 2m/s (16Newton drag). With 108Watt of input power if you can go to 80% efficiency instead of 40% then your operating point moves to:
88Watt @ 2.5m/s (25Newton drag) The 2.5m/s is 5.6mph. Note that these are rough numbers. I suspect your boat would be heavier and have more drag than Andreas' proposed competition boat and therefore slower. Modeling your hull would give a more accurate speed/power/drag characterization.

Dave

 

Scheny /Andreas,

Thank you.

 

Coach,

After reading through your post about 4 times I get it that by increasing my efficiency by 100% I should be able to increase speed by about 25% (2.5 m/s divided by 2 m/s = 1.25), and from Jeremy's reply I get it that that would best be done by using a more efficient prop.

Would anyone be willing to hazard a guess, just an educated guess arrived at from your experience, as to the ratio of efficiency between the APC 12 X 14 and the Torqeedo 12 X 10?

And Jeremy.....I get 2 m/s as 4.47 mph. That would indeed be sufficient, and that's with the Torqueedo prop. Is there something I'm missing?

I would be a fool not to go with a more efficient prop, if that's the case with the APC 12 X 14, but it would be good to know in case the APC is too flimsy.

 

You are welcome!

@Porta:
Jets have completely other design rules. The in-/outlets should be rather short (drag) and easy to reach for repair. That's why they build them on the rear or under the wings. Small and light engines come to the rear, as they weigh not so much, but thrust is always nearly symmetric, also when an engine fails. Big planes have them under the wing, because everything in and outside the wings (like fuel) is not counting on bending moment of the wing root, helping structural integrity of the wing.

@Gib:
In my opinion, both props are suited for you. The Torqeedo is optimized for 400Watt @2.8m/s and 1200rpm which means relative high loading. For lower loadings (=> your project) the needed power will go down as it mainly depends on the thrust you produce (=> equals drag).

I personally would take the Torqeedo if money (99$) is no argument (stronger, weed shedding), but you will require a gearing of 1:20.

You can also take one of my designs as I proposed, but up to date, it also costs about 100$. I have to make the hub less strong to get it cheaper, but this can only be done after extensive testing.

 

Gib,

I wish I could make that educated guess on your prop question but I am lacking in the torqueedo prop knowledge department.

Like Jeremy, I have no data. I've seen them in the store but that's it.

A complete guess would be 65 - 70% vs 75 - 80%.

Don't underestimate your cadence.

I think you'll find over time it may increase.

Maybe not but I suspect it will.

Although, perhaps not more than 70 - 75.

 

I've found my comfortable cadence when out riding has dropped a fair bit over the years. I tend to spin at around 65 - 70 now, whereas twenty years ago it'd have been around 80 to 90. Mind you, I don't ride as fast as I used to, either, plus all my bikes, except one, now have electric assist!

 

Hope the comments above are of some help.

Good luck.

Porta

 

Well that's sobering Jeremy.

I used to pedal the same, 80 - 90 but that was twenty years ago as well!

I don't ride much any more and certainly never with a computer like I did before.

Thanks for the warning / insight...

 

Gib,

A heads up on using gym bikes to measure your sustained output.

They are notoriously inaccurate on the high side telling you more than what is there.

The average guy can put out 100 -120 watts sustained comfortably.

If one is really fit that might be slightly higher but the endurance will be outstanding.

If really unfit, again, similar numbers but no endurance at all.

Hope that helps.

 

Scheny / Andreas,

I wanted to repost my sketch as I'm unable to send it by PM.

It is to scale. The hull length being 21' (~7m).

There is a small aft horizontal wing that would bare 10 - 20% of the lift.

The main wings (50 - 60% of the lift), fully pivoting near centre (balanced) independantly if necessary

and jointly through a mechanical mixer responding to height control from the surface sensing wand.

The vertical "canard" under the pod (bulb) will provide effective roll control providing it gets correct

and timely inputs which are vital because as the bulb fills with air (~30% of lift) at speed, roll stability

becomes volatile. The more quickly this is done, the smaller the AoA and fin size needed.

I hope this brings some clarity Andreas.