Pulsejet Kayak

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Bruce Simpson
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Pulsejet Kayak

Post by Bruce Simpson » Sun Apr 25, 2010 7:17 am

Pulsejet powered kayak

From a TV show I did here in NZ.

PyroJoe
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Re: Pulsejet Kayak

Post by PyroJoe » Tue Apr 27, 2010 7:51 pm

Good show,
Was that a Lockwood of typical dimensions or was it slightly modified?
Thanks, Joe

Bruce Simpson
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Re: Pulsejet Kayak

Post by Bruce Simpson » Tue Apr 27, 2010 9:45 pm

It was slightly modified ;-)

metiz
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Re: Pulsejet Kayak

Post by metiz » Tue Apr 27, 2010 10:22 pm

Bruce, not to be critical but have you ever contemplated using a design other the that age old lockwood? you've used it in about 4 shows as far as I can count. How about a big thermojet? nice and powerfull and no U bends. I asume fuel consumption in these applications in irrelevant.
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Bruce Simpson
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Re: Pulsejet Kayak

Post by Bruce Simpson » Wed Apr 28, 2010 11:18 pm

I have all the jigs and tooling here to knock up a lockwood in next to no-time so that's generally what I tend to build when I'm working on a fixed-fee such as the contracts with TV shows.

It might surprise you to learn that I did originally build a ThunderChine using the updated plans from this website for this project but it was a huge disappointment (a bitch to start and very unreliable) despite being very precisely made (I can see why Andreas had problems and eventually gave up). Subsequent analysis indicated that there are a number of serious "issues" with the TC design so it now sits in the corner of the workshop and gathers dust -- maybe I ought to flatten it :-). That left me on the back-foot and very much short on time so I knocked up a lockwood in about 4-hours at the very last minute.

So long as you mount them properly and ensure the exhaust cone is strong enough along the first 1/3 of its length (so they don't implode) the LH work perfectly well and are about the simplest "useful" valveless (from a construction perspective) to make. We had to wind-back the power on the engine used in that show because it had a little "too much" thrust -- the kayak reaching over 70kph on the ground-run, at which point it would have been positively dangerous with a 12-year-old kid at the controls.

metiz
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Re: Pulsejet Kayak

Post by metiz » Wed Apr 28, 2010 11:28 pm

Bruce Simpson wrote:the kayak reaching over 70kph on the ground-run, at which point it would have been positively dangerous with a 12-year-old kid at the controls.
Pshh this kid sounds like a pussy :D
But I see where you're comming from. What are the issues you were talking about, besides the obvious? I know the Thunderchine is an EXTREMELY tuned engine that will (probably, no experience) function poorly or not at all with any construction flaws but I can't think of any other problems. (not defending the design, just making observation and assumptions)
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Bruce Simpson
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Re: Pulsejet Kayak

Post by Bruce Simpson » Thu Apr 29, 2010 12:45 am

metiz wrote:Pshh this kid sounds like a pussy :D
But I see where you're comming from. What are the issues you were talking about, besides the obvious? I know the Thunderchine is an EXTREMELY tuned engine that will (probably, no experience) function poorly or not at all with any construction flaws but I can't think of any other problems. (not defending the design, just making observation and assumptions)
Yes (IMHO) one major problem is the design's 'Q' -- it is indeed to sharply optimized for power and has very little latitude for inaccuracies introduced by construction issues but in my modelling, the flare of the tailpipe is also partly to blame. Running through the models I've created here over the years, I get significant break-up of the exhaust flow and some nasty parasitic eratic drag caused by vorticies, partly due to the degree of tailpipe divergence and partly down to the asymmetry inherent in the tailpipe design. This causes the tailpipe impedance to fluctuate significantly and that (I believe) was partly to blame for the difficulty in obtaining reliable operation.

Of course my models could be wrong (that has to be admitted) but it looks as if the designer was too focused on making a really short engine with good power and did not apply enough weighting to creating a practical engine that was sufficiently tolerant of the many factors that can result in a hi-Q engine failing to sustain.

For the sacrifice of just 10% in raw power and a little extra length, the design could have been made far more forgiving.

It is clearly an engine that has been very nicely and well designed by someone who (unfortunately) has had not so much experience with turning theory into practice. It's good work but is a design that needs some "cut-n-try" optimization and a little less theoretical excellence.

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