Four Candles 2012

[Jeremy]

Seems a pity to give up on the air props, Alan, as they have so many potential advantages.

Do you actually need the shrouds?  I flew aircraft and microlights for years and none ever had shrouds around the props as they just added drag.  The safety risk is very small with your outriggers, as I'm guessing that the props are inside the total beam.  I'd be inclined to save the weight and do without prop shrouds and just go with a pair of open props.  If you can rig up some way of providing differential thrust you could probably do away with the rudders, to, so reducing drag a bit more, or, perhaps, you could use a water rudder for even more authority.

[Alan]

I don't feel the need for shrouds to protect me or other competitors, but I feel they are needed to protect the spectators. I suppose I could leave them off and have a "safe operating procedure" (you see, it's just like being at work) such as staking and taping a no go area around the boat when on land, and testing. And removing the batteries at other times of course. I have deliberately used the props in pusher mode so that even if a chuck loosens they would still be safe, and I am confident in the supporting structure. You are right about the width Jeremy, within the stabilisers.

What do people think regarding bin the shrouds and have an SOP?

I still have the water rudder option, it will connect very easily to a bellcrank which operates the air rudders.

[Jeremy]

If you can turn the drills off quickly, then I'd have thought that a SOP would be OK.  

Alternatively, as the danger is only really from contact from the sides, how about a couple of vertical poles on the outriggers as last-ditch guards?  They don't need to go right around the props, as the only likely area of contact from another boat would be from either side.

A water rudder may well have a fair bit more authority than the air rudders, plus would probably have less drag.  Rudders immediately downstream of a prop have a fair bit of drag, as they are working in a higher velocity flow.

Jeremy

[Alan]

Progress has been slow and difficult and I nearly resorted to posting my woes just to get some nice humour and help from HBBers. At least I have made another prop shroud and finally mounted the stabilisers to the hull. After a long talk with an HBBR member about safety I have decided to retain the shrouds and try to negotiate their removal with the scrutineers by promising safe operating procedures etc. Better than turning up without and being banned. For reasons of fore and aft balance I have mounted the stabilisers very far forward which means that they do not protect other competitors on the water.
Thanks for all advice and comments so far.

Several HBBers seem to like trikes so I hereby present my trike project from 1981, made for the Aspro Clear Speed Trials which were run as sprints on Brighton promenade and as a road race at Goodwood. My friend Steve White who was a talented amateur rider rode the trike and was the fastest unfaired machine in the sprints but suffered an unrepairable puncture just before the road race.

Front wheel drive and successful rear wheel steering.

[BrianP]

Here's a distraction



Brian

[Dennis A]

Hi Alan

I did not understand your comment on the outrigger position as I thought that these were to prevent roll overs. Mine are set quite far back and only give any problems when hit by a wave coming to the stern at around 45 degrees.

Your Trike
I raced at the Aspro event and it gave me a interest in pedal power for many years I still play with boats and get a lot of pleasure when having a pedal outing on the Thames.

[Alan]

Dennis; sorry, I didn't make myself very clear in that previous post. Last year, Four Candles was very stern heavy due to all the gear fitted aft and me sitting further back than I was supposed to due to removing the original seat. The boat was a bit sluggish to steer. As all the "machinery" is fitted aft again, I have fitted the stabilisers as far forward as possible, just above the waterline so that their weight forward will balance the weight aft and the boat will hopefully trim correctly fore and aft. It should steer better now.

Ah, now I get what you mean! I wrote that the stabilisers would not now protect other competitors. What I meant was that they would not now act as a shield between my air propellers and a competitor in the event of a collision on the water as they are further forward than the propellers.

What sort of bike did you use in the Aspro Clear? I have lots of photos, I might have a picture of it. I remember trying to keep up with my friend on "normal" bikes as he practised at Goodwood and I discovered just what a huge difference there was between an athlete and an averagely fit person (me).

BrianP, what's the story of the trike? Is it for a race?

[BrianP]

Hi Alan, the trike is a Velayo, a commercially available trike. Fixed front wheels and rear wheel steering. I just really like the design. Having found a Whindcheetah just too low to feel safe on the road, I like how high the design is.

 http://www.fortschritt-fahrzeugbau.de/



Brian

[Alan]

The Velayo is a nice design but I thought it a little bit expensive although it must be virtually hand built. Cheap compared to the Boris bikes though at £19,000 including vending device, and after Barclay's sponsorship; I feel that a lot of my tax money went straight into a consultants pocket.

Back to the subject. Four candles is now ready to test, rudders and throttles all connected and working.

Here are the pesky prop shrouds, enthusiasm returning now that they are fitted.

Not sure where to test, probably Medway, Cuckmere haven or Thames in that order.

[Jeremy]

Looking good, Alan, you're further ahead than me - I'm still glueing bits of foam to make the internal bits of my hull!

Those shrouds look fine to me, and I doubt very much if that have any negative impact on performance, they may actually reduce the tip losses a bit.  Now you've gone to all the trouble to make them I'd be inclined to keep them on, just as a belt-and-braces approach to safety.

[Timmo]

Really looking forward to seeing her in action at Beale.

Tim.

On 13 May 2012, at 12:31, Jeremy [via UK HBBR Forum] wrote:

> Looking good, Alan, you're further ahead than me - I'm still glueing bits of foam to make the internal bits of my hull!

[Alan]

Thanks Tim. I'm echoing your thoughts exactly Jeremy, regarding safety.

[Dennis A]

Hi Alan
Four Candles looks great, I will not mind being beaten by that. Somewhere to test is a problem, I normally use the Thames at Windsor but it has been flowing too fast. I tried the local canal but my turning circle is far too big and the tow paths too small. I shall check out the Thames again to see if the water has gone down.
Trikes
At Aspro I used my Scorcher trike, finished 2nd. from last but only raced at the sprints as I ran a half marathon the next day.
I was interested to see that both yours and the Velago were rear steering trikes, as this type is normally considered to be unstable.

[Alan]

Took Four Candles to its first test on water today, at the lake of a small dinghy sailing club in the S.E. The first thing I noticed was that someone had failed to pack the batteries so I had a return  trip up the dual carriageway while wearing a slightly too small wetsuit, to collect the missing batteries.
The conclusion after testing is that the boat is a bit slow, trimmed down by the stern, and steering is not very effective with air rudders. They are very far forward and this combines with the trim to give the turning circle of a destroyer. So I will ditch the pretty rudders and go back to a water rudder, and sit as far forward as possible for best trim. I could also buy propellers with more pitch to get more power but they are expensive and I have spent enough already.
So, I don't think that I will win this year, but it should at least be entertaining.

[Jeremy]

Well, to look on the bright side, at least you've had your boat on the water.  I'm still trying to finish the hull, have yet to start even painting, and we've only got three weeks to go..........................

I suspected that you might have a rudder problem, hence the suggestion earlier that differential thrust or a water rudder might be more effective.  Speed is hard to judge when you're not running against another boat, plus you may find that conditions on the day (weed?) mess up the competition, so I reckon you're still in with a chance (probably better than mine at my present rate of weather-restricted progress).

Jeremy

[Dennis A]

Hi Alan
If you think you require more pitch, you could carve two new props from pieces of straight grained 4 X 2. This is quite often done for home built wind turbines and I think would not take to long.

[Alan]

Jeremy, you were right about the rudders, although I don't think differential thrust would work because the props are too close together.
Dennis, I decide that I haven't the time or energy to carve props, but then I thought that I could make them for LH rotation which would allow the drills to run in the forward direction and give me another 100 rpm, and I could make them very slightly larger diameter with a wide tip chord which might increase their efficiency within the shrouds, so this might be an option.
I did experiment today with using a steamer to curve pvc downpipe which worked well, which gives me the option of making lighter stabilisers.

[Jeremy]

What you really need is more rpm, I believe.  If the props are the ones I think they are (24 x 16?), then Javaprop suggests that the efficiency at 1500 rpm and 2.5 m/S air speed will only be around 25%.

[Alan]

The props are Xoar 32" X 10" sport props in beech. Here are the secret data:
no load - 18.9v, 3.27A, 61w, 1590 rpm
with prop - 17.1v, 12.2A, 208w, 1380 rpm

I can get 12" pitch props but they are £60 each and I'm not willing to spend more money at the moment.
If I could turn the drills round and run them in the forward direction (which has safety issues) I would get another 100 rpm.

I've just had the nutty idea of turning the whole power assembly back to front and mounting it at the front, between the cross beams. I would have to have a strong mesh screen to guard against a loose prop but it would give a little more power due to the "correct" motor rotation, and probably solve the trim/steering problem. Am I still sane?

[Jeremy]

I've just done a quick Javaprop run for that absorbed power and rpm (147 W and 1380 rpm), with an assumed airspeed of 2.5 m/S (just under 5 kts) and it suggests that a 32" prop with a pitch of about 14" would be optimum, but even that would only work at about 37% efficiency (I assumed Clark Y foil section, as that's what most wooden props seem to use).  The thrust at this power and efficiency would be about 22 N per prop, 44 N total, which sounds reasonable, enough to get a canoe to 5 kts for sure.

I couldn't force Javaprop to give a result for a 10" pitch at that rpm, the lowest pitch I could get was around 12" and that gave figures not far off those above.

If the rpm is increased to 1500, then the pitch comes down a bit, but still only to about 12", and this means a lower airspeed of just 1.4 m/S, although thrust does increase to about 35 N per prop.

If your design speed is around 5 kts, then the 32 x 10 needs to turn at around 2150 rpm to absorb 147 W, but will only be around 33% efficient, with a thrust of about 19 N at that speed.

It seems that you need much larger prop diameters to give reasonable efficiency at these low boat speeds.