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What, more salt to add??!!

 

Ad Hoc,

Perhaps I didn't phrase the question well.
What L/B or L/D ration would you consider good, from the standpoint of not having much difference between square and round sections, which was the topic of discussion. You offered the opinion, based on testing that there was nothing overall better about one or the other (over a range of speeds).

Obviously the graph was from some text, which does not illustrate any differences between square and round.

So.

At what L/B ratio do you think square and round underwater cross sections are substantially equivalent at a higher speed? And I am fairly sure you are talking about in flat water, not waves.

I

 

There is no "definitive" cross over point. However, L/D ratios from 8 and L/B ratios from 15 we have not discerned any real differences.

Again, one assumes there is no sudden abrupt change in the facet angles.

 

Thanks, it is the nature of such things that such a "cross over" is not abrupt, but I appreciate your input.

Marc

 

C'mon, they can't be THAT amusing. Fun, yes, but SERIOUS fun. This is an orphan MicroCat I rescued last year in Rhode Island. There are similar unamusing yet apparently enjoyable boats in Australia called Aqwakats. And this one's kinda cute: http://www.youtube.com/watch?v=5HjKHrcNY04&feature=related

 

Thought you may find this of interest.

These guys are no spring chickens and 26 minutes is pretty good sustainability.

Hope that helps.

http://www.youtube.com/watch?v=9iRnSHIrCa0&feature=youtube_gdata_player

 

The best airfoils have a lift/drag factor of 54, using laminar airflow. As much as I know, the best laminar foils as the NASA NLF0215 (NLF series: natural laminar flow) are not the best chose for hydrofoils. I tried to google the L/D for hydrofoils and they seem to be in the area of 20.

This means, that for a boat weight (including driver) of 100kg, you need 5kg of thrust. I think this is equivalent to Ricks V15 if you add additional drag for struts and prop.

As the L/D is dependent on AOA and given for the best value (most times 3°-4°), you will have to vary the foil area to match this angle for the target speed. Then you will have a drag of 5kg no matter for what speed.

So, where are the propellor specialists who know what speed can be reached providing 6kg (1kg for losses and struts) of thrust?

 

One very good way to iteratively design a propeller for a niche requirement like this is to use Javaprop (see here: http://www.mh-aerotools.de/airfoils/javaprop.htm). This software was originally designed for model aircraft propellers, but if the fluid parameters on the "options" page are changed for water, rather than air (density 1000 kg/m³ for fresh water, kinematic viscosity 0.0000013 m²/s, speed of sound 1447 m/S) then it gives good results for human powered boat props. Rick validated the output from Javaprop with a range of prop designs and found the real-world props to perform closely to the Javaprop predictions. My experience has been the same.

The process is iterative and a bit time consuming, as you need to work through the various trade-offs between diameter, rpm, etc to get the most efficient design. Choice of aerofoil section also makes a fair difference, My props use the E193 section from around 25% out (it's too thin a section to be used near the root) and this seems to work reasonably well (around 85% under optimum conditions).

 

I already did that before my last post, but it seemed impossible to reach more than about 3m/s at 150W.

 

58.8 N of thrust (your "6kg" requirement translated from mass to force) at 3 m/S is 264.8 W, from Newton, so this would be the power required from a 100% efficient propeller. If the prop was 80% efficient, then to produce this much thrust at this velocity would require an input power of about 331 W.

Alternatively, if you had an 80% efficient propeller that was absorbing 150 W at 3 m/S, then it would be producing 40 N of thrust.

 

I believe the power target here would be more like 100 watts at the prop.

We already know it can be done for short duration with high output athletes.

What about your average joe for an hour... I think 100 watts is reasonable.

Opinions?

 

I think you're about right, Tom. I've never accurately calibrated my output (and I'm not that fit anyway) but a comparison with the power from my electric bike quickly shows that 250 W is way more power than I can deliver!

 

Back tracking a bit to the idea of a foil to reduce displacement rather than lifting the hull from the water have you seen this website http://teamtarka.co.uk/category/design/
If I understand correctly, in order to use the same electric boat hull for both sprint and endurance events they have a planing hull to get the sprint speed and added a foil to the rudder in the endurance race to stop the stern from squatting.

 

Working with the 6kg drag I generated a set of graphs of power in watts vs speed in MPH, KPH and m/s. For power I showed the input power (assuming 80% drive train efficiency) and the effective output power propelling the boat. These graphs represent a family of hydrofoils each of which is tuned for a specific speed where its lift balances the 100kg boat weight. Before the hydrofoil lifts the hull out of the water more power is required to overcome the combined drag of the hull and hydrofoil.

Dave

 

Jeremy,

Have you mixed up your units again??

Power = trust x velocity = (6x9.81) x 3 = 176.6W.

But, the velocity of 3m/s this is the velocity of the water into the prop too, not the velocity of the vessel. (The thrust is at the prop only too). As the shape and run of the hull aft for the flow of water into the prop comes into play, as the as well as the efficiency of the prop itself. Since the effective horsepower required is different from the thrust horsepower developed by the prop. All this can all be broken down into ever smaller ratios of interaction between the prop and the hull.

This can be rather cumbersome and not all the values are easy to calculate. To simplify the whole thing naval architects generally use the term PC, the propulsive coefficient. Which is simply:-

PC = EHP/SHP = effective horse power / shaft horse power.

So typical props in typical installations, for example, the PC ranges from 0.4 to 0.6. The ratio of PC takes up the whole hull efficiency, prop efficiency, appendage coeff’s, transmission and relative rotative efficiencies all into one simple ratio. It provides a quick engine selection guide.