Four Candles 2012

[Dennis A]

 Alan
I checked out the 32" X10" props on Propsym and can up with very similar answers to Jeremy but I had to assume the cord width and used 100 mm, rpm 1380, power in 163 watts, efficiency 36%, thrust 24 N. If you increase the revs by 100 the thrust only increases by 3 N as the efficiency drops slightly.
I am amazed at the low efficiency as the typical straight water props run at 80%

[Alan]

Those propeller efficiency figures are very interesting, and I wish I knew more about it. I had a look at an aerodynamics book and it seems that you are talking about the Froude efficiency, which seems to be based on the ratio of prop wash velocity to vessel velocity. In other words a hovering helicopter is 0% and a vessel moving at 20mph with a 20mph prop wash is 100%. So this is why bigger props with slower "washes" are more efficient, yes?

I calculated the pitch speed as about 13 mph which probably means a prop wash of about 10 or 11 mph against a guessed speed of about 6 mph.

Last night I turned the props around to run the motor in the forward direction and got 19.04v, 14.0A, 266w
I couldn't get a speed reading because it was too dark, I might test again later to get this. First, the large battery maintained a higher voltage despite the higher current, I think this means that my previous figures were with the small battery. The big battery is really two smaller ones in parallel so it probably has less internal resistance. Secondly, 14A still gives a 12 minute duration, enough to get around the course twice, probably.

[Jeremy]

I'm not sure about Propsym, but Javaprop just calculates efficiency in terms of the power in the propwash (as a simple mass and velocity calculation) versus the power absorbed by the prop at the shaft (which is the sum of the prop drag, tip losses and power in the propwash).  The boat speed gives the inflow velocity, which impacts on efficiency (as when it's zero the efficiency is zero) but more importantly has a significant impact on blade loading.  Blade loading tends to dominate the efficiency characteristic, in as much as a high blade loading almost always means a poor efficiency, as does a blade loading that is too low.

I believe that Javaprop uses blade element theory to determine prop characteristics, by dividing the prop into a number of slices and calculating the L/D at each slice, using the inflow velocity (air speed/boat speed in still air) and the rotational velocity at each point.  It allows for an element of span wise flow,too, I believe, and uses this to determines tip losses from vortex generation.

[Alan]

Had a quick look at javaprop and found that propeller design is quite complicated, but something that I should learn about especially if I do another ccc. I've had enough of development so today I have been doing cosmetic work to make Four Candles look vaguely photogenic from a distance. I did have a minor brainwave yesterday and moved the rudder aft on an extension to get the best possible leverage, and distance from CLR, within the length rule.

And finally, as they say on the news, here is a video of the first water trial:

[Port-Na-Storm]

Alan, That looks seriously impressive to me, much quicker than many of last years's entrants.

Well Done and Good Luck.

[Paul H (admin)]

She is very quick Alan - I'm impressed how well the props work.

Paul

[Jeremy]

Looks good to me, Alan, fast, manoeuvrable, and most importantly finished, tested and working (which is more than can be said for my effort).

[Dennis A]

Alan
I am impress with your acceleration and the position in the water of the out riggers, they hardly tough which keeps their drag low.

[Alan]

Thanks for the comments. The height of the stabilisers was pure guesswork (well, it all was really) but it came out just fine. I could save about 6kg by changing to 68mm pvc downpipe stabilisers but I'm calling it finished now.
With the current stabilisers I think Four Candles would make a rather nice sailing tri-kayak. The stabs would provide all the lateral resistance needed when heeled; there could be another project here.

[BrianP]

Thought you flying boat guys would like this, Carl Cramer's Wooden Boat of the Week. http://boats.woodenboat.com/?p=2820

http://www.youtube.com/watch?feature=player_detailpage&v=x-XXXvFmyHQ

http://www.wfoilsail.com/

Brian

[Alan]

Seen it! It's pretty and interesting but it doesn't seem to be moving any faster than a planing hull of the same size. There is another one, electric powered, out there too; it can be found on Gizmag.

I'm sitting here in a house with broken central heating, no hot water, but it must be much worse for the Thames raiders as they don't have roofs.

[Randonneur]

Some of us have roofs! ( I'm glad I wasn't in a tent on Sunday.)
Pete at Eynsham lock.

On Jun 5, 2012 9:20 PM, "Alan [via UK HBBR Forum]" <[hidden email]> wrote:

Seen it! It's pretty and interesting but it doesn't seem to be moving any faster than a planing hull of the same size. There is another one, electric powered, out there too; it can be found on Gizmag.

I'm sitting here in a house with broken central heating, no hot water, but it must be much worse for the Thames raiders as they don't have roofs.

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[Jeremy]

Alan,

Congratulations on winning a prize.  I'd have liked to have been up against you in a heat, as it would have been interesting to see which of us would have been faster when we were both running on fully charged batteries.

Jeremy

[Alan]

Thanks Jeremy and congratulations for you too; I can't remember what either of us won prizes for, which makes me almost as forgetful as my mum. I really enjoyed the impromptu "race" after the real race, I hope some video turns up of that!
As last year, the variety of solutions to the CCC problem was amazing; the show was so sociable that I didn't even manage to look at all the exhibits and amateur boatbuilding entries properly; and it was also nice to meet and chat with some of the HBBR people.

[Alan]

It's not a race but...

A couple of us have been invited to display our CCC boats at the Thames Traditional Boat Rally which is quite an honour. Assuming I have time to complete the work when I get back from Brazil, I am adding a toothed belt speed increaser to Four Candles. The 1.5:1 step up should give about three times as much power and a corresponding reduction in endurance. The parts and tools required have cost plenty so I really hope it will be worth the effort. It should at least show that my drills are strong enough for next years CCC. And add some excitement to the non-race.

[Jeremy]

Sounds a good move, Alan, as you should get a bit more than three times the thrust at 6 kts, as by my reckoning the prop efficiency will have gone up by around 5% or so from the increased rpm.  

See you at Henley.

[Dennis A]

Alan

I think your increase in revs. will give your boat a real improvement. I made the increased thrust about 2.5 times which increases it to around 50N/prop, but the power requirement increases by 3.5 times. My calcs. showed a decrease in the efficiency. I could be wrong but could not find a mistake. This improvement should give a speed increase of approx. 50%.

[Alan]

Hello all, I was assuming that the thrust would be 1.5 squared, minus a bit because the motor would likely be running well outside its' most efficient range. The boat doesn't make much in the way of waves so I would think there is potential for close to 50% increase in speed. I don't understand how the prop efficiency changes though.  

[Jeremy]

The difference between the estimates I gave and those Dennis gave are probably down to using different assumptions.  I assumed a boat speed of 3.1 m/S (about 6kts).  If Dennis used a different inflow velocity then he'd get different numbers.  The efficiency and thrust is quite sensitive to changes in inflow velocity down at these close to static thrust speeds, making things a little more difficult to guess.

Power absorbed will be pretty closely proportional to the cube of rpm, but that doesn't directly relate to output power (thrust x velocity) because efficiency will change.

The problem is compounded because the some of the models used to determine propeller performance start to get a bit iffy at these very low (for an aircraft prop) inflow velocities, in fact some of them just stop working when the inflow velocity falls to zero and won't give a static thrust figure at all.

The speed increase is unlikely to be 50%, as your hull is already running at a high Froude number, around 1.6 or so, with the existing prop set up.  Anything over about 1.4 is getting into the significant wave making drag region for a hull like yours and so adding 50% more thrust may only give you another 5 or 10% more speed - the thrust/speed relationship won't continue to follow the approximate cube law relationship that it does when resistance is dominated by viscous drag from wetted area.

[Dennis A]

You are right Jeremy, I did not consider the increase of the air velocity. Considering the different air velocities, Propsim shows the same thrust for both different rpm and still shows a efficiency decrease but smaller.  I think this shows a limitation in the model used or my use of it.
I tried the problem in Javaprop but found it more difficult as you have to input a estimated power and this then calculates the blade format each time, which then has to be modified to the design cord.
It seemed that the efficiency still showed a decrease but the thrusts were very low, so again I did not have any faith in the calcs.  
My estimate on speed increase was based on the drag/speed curve for my boat which having a different shape to 4 Candles could get a completely different result.
The real answer is to test it but be careful as you have now the possibility to damage the batteries.