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You're going to want a wide front end with a lot of bouyancy. All of your thrust will be comming from above the centerline of the hull, where as normally it is below the centerline of the hull which helps lift the bow. All airboats that you will see have a very wide footprint in front because there will be a lot of downforce thrown to the front of the hull until it get up on plane.
 
Thanks for the info guys.

I've been researching a fair bit lately, and I've found that Ken Warby's record holding boat was some 8.5m long, yet only 2.5m wide - a length/width ratio of around 3.5.

Sounds a bit too 'skinny' to me. That's probably because it had to lug that rather large Westinghouse J24 turbojet engine around. His second boat was marginally longer still. I'm not sure how that would translate to an RC twin-turbine boat. After examining a lot of pictures of Spirit Of Australia and Aussie Spirit, I've noticed that it has a really low and narrow bum - probably to keep the thrust as close to water-line as possible, and to minimise rear-end drag.

I pulled the tape-measure out and tried to imagine a 1/4 scale of such a boat - it would come out about 2.10m long by 0.63m wide. I don't have a lot of hydroplane experience (or boats in general), but that looks and feels pretty unstable to me. Then again, I'm not trying to facilitate a 2.8m half a ton jet engine behind me. My Turbines only need 300mm length and about 350mm width to work in.

I've been drafting a prototype in TurboCAD, and something around 1.5m long by 0.8m wide is starting to look good. And with a transom about 600mm wide by 125mm high (0.6 x 0.125m). I, too, am aiming to keep the line of thrust just above waterline and to minimise drag. Apparently, the greatest source of drag on Ken's boat/s was the rudder. So he hacked off 6" off the bottom of it, and promptly went out and set the World Record.

Kudos Ken...

BJ:)
 
How much does one of your jet engines weigh, including fuel tank, mount, etc.? What is the thrust of one engine? I'm thinking that your project might be more manable with just one engine, at least for your first attempt at designing and building a jet hydro. Hydros tend to need a bit of tinkering and adjusting (even redesigning) to get them to run right. You might want to design in an adjustable / removable planning surface for the rear of the boat, similar to what is on the J.A.E. riggers? A rule of thumb for center of gravity would be 90% of the distance from the transom to the sponson transom (for a start). Remember to build flotation into the hull, and build the motormount light, yet strong, in case of a blow-over or stuffing.
 
The Turbines only weigh roughly 3 pounds each (1.5kg) and push out around 30lb (15kg) thrust. Fuel is the main weight issue - they drink about 16oz (call it 450ml) per minute. So, 5 mins at full tilt is going to require around a gallon of fuel - close enough to 10lbs (5kg).

I can see how Ken has designed his rear end - changing the dimmensions on the centre tappered box looks like it changes the planning characteristics.

I'm not sure what you mean by 'stuffing' ??

BJ:)
 
Back in the early 80's Jack Waltrip made a Don Pinkert style MIXER a 4 point hydro

wide front sponsons with rear sponsons he used a very noisey pulse jet to power it.

well it ran fast and was stable turning was a bit hairy however after a couple of runs he put it away.

I know NAMBA insurance don't covered thrusted not sure if IMPBA does.
 
Probably not the right place to ask, seeing as no-ones really had a lot of Jet Thrust boat experience...

But - am I better off opting for a slightly longer hull, than a shorter one?

I'm tossing up between a 1m long main hull v's 1.2m. With side sponsons around 600-650mm and a centre, slightly wider sponson of around 750mm.

1.2m for the main hull just looks too long - even though it's only 200m longer, it seems to add quite a bit to the bulk of the hull.

Nothing's been glued yet, so I still have the option of cutting 1.2 Ali tubes to replace the 1.0m ones in the pic.

Any ideas?

BJ:)



 

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Still has no idea of what 'stuffing' means...

BJ:)
When your boat quickly buries it self in the water , in one form or another . Hope to see you get er done "and not stuff it".
Oh... like the front end dips down and under? As opposed to lifting the nose and back-flipping.

That kinda thing?

BJ:)
Yes, exactly. Stuffing a hydro is not as common as "blowing it off the water". But when they do stuff, it usually happens after hitting another boats wake (or aquatic life?), then the boat can take a hop, skip, then nose dive and disappear under the water. They usually float back up after a few seconds.

Hey, that's a nice photo of the rear sponson on the jet boat! Interesting.

As far a hull size goes, you could maybe split the difference, and make it 1.1 meters long. That would make it similar in length to our 1/8 Scale Unlimited Hydros, which are about 44 inches x 24 inches (or 1.1 x 0.6 meters) and weigh 12-18 lbs ready to run. They run a 0.67 cubic inch two stroke engine on ~50% nitromethane, put out about 5-6 hp, and run around 60 mph on an oval course.

If your hull is built too small, it may sit so low in the water (at launching) that it may not get on plane. If the hull is too large, it may hold the top speed down a bit, but at least you could have fun zooming it around the pond.

Also, you may want to keep the amount of aluminum in the hull to a minimum, to keep the weight down, and maybe do less damage in the unlikely event that you hit anything. This sounds like a fun project.
 
you can try to use what the military uses on their planes thrust vectoring from side to side to help turning ? just a thought..
 
Thanks for the comments guys...

The Ali tubes will remain at 4, as in the pic. If I wanted to really save weight, I could always use CF rod, but that's a tad on the expensive side.

And I've been considering some thrust-vectoring - to compensate for the vertical direction of 'push' (for ride trimming purposes) as well as to help cornering (as well as yaw trim), not that I anticipate turning all that much.

BJ:)
 
Ahhh....."stuffing"....

Got it now - like the lil red boat at the start, and the yellow-bummed thingie near the end ..???



BJ:)
 
Been watching this thread with interest, anyway visited the National Maritime Museum here in Sydney today with the kids, Ken Warby's record breaking boat lives here and has now been mounted up in the air so took some detailed photo's of it. Some interesting things going on here don't know if it's design scales down or not..... If you need any other angles let me know.

http://www.pbase.com/gtmurph/national_martime_museum

Cheers,
 
Thanks for the heads-up Murph :D

Any pics of 'Spirit Of Australia' you have would be much appreciated.

And if there are any underside pics around of Ken's second boat (Australian Spirit), that would be a bonus.

From the feedback I've been getting on other threads, there's a lot of suggestions to build a long and skinny BlueBird K7 type shell. Others have suggested taking a standard 3-point prop design and putting a Turbine on it.

Neither of those approaches particularly appeal to me - so I'm building a design based on a modified version of Australian Spirit (boat #2). Like you, I'm not sure how well it will scale down, but most model aircraft seem to scale ok, so I can't really see why a boat shouldn't do the same. I guess time will tell.

I've made a bit of a compromise in my design - Ken's boats run a length:width ratio of around 3:1. I'm aiming for a boat around 2m long - so in theory it should only be around 600mm wide. But I want to run my Twin Turbines on it, so I've gone for a width at transom of 750mm, with a max of 900mm where the front sponsons start. Hopefully, that's not too wide and it doesn't turn into a 'wing', but that's the road I'm travelling down at the moment - mainly due to the distinct lack of info on RC Jet Thrust boats. And by that I mean real Jet Thrust boats, not prop boats modified to accept a Turbine engine.

The reason I've picked Kens designs as a basis is that they have been proven as successful - for over 30 years now no-one has come up with anything better. Sure, lots of hoo-ha and talk, but other than himself, no-one else beaten his record. So that says something to me.

While I'm not out to set or break any RC records, I've been captivated by the subject and I'm totally hooked on RC Turbines. I've received a lot of negative input around the traps, and what really bugs me is that not one of those people have built and tested a Jet Thrust boat. You will have seen first-hand that his design varies markedly with the other designs of the past and present. Designs that, in the past, have proven fatally flawed, and 'current' designs that have been all-talk-no-action for almost 10 years now.

Unfortunately, we never got to see Ken's second boat run in anger - only a few shake-down runs at Taree etc. He built that boat with 600kmh in mind. I've read that he was also working on a third boat, with a mooted ceiling of around 800kmh. Ken is 71 now, and I don't think we'll see him running any of his designs himself any more, should someone finally break his 317mph/511kmh record set way on back in 1978. His two main rivals - Arfons and Taylor - sadly succumbed to the temptation of joining the most elite club on the Planet of which Ken Warby is the only member: To have officially recorded a World Water Speed Record of faster than 300mph on water, and lived to tell the tale.

Will my boat work? Who knows. What if it doesn't? Who cares.

But at least I'll be in a position to say to myself "This bit worked...this bit needs improving..."

And I'm getting quite a buzz from the whole process.

Here's a coupla pics of where I'm at at the moment...

Cheers,

BJ:)





 

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Whoa - Murph!

I just clicked on your link (thinking it was just a link to the Maritime Museum homepage - which doesn't have all the much info on Spirit of Aus, considering it was and still is the World Record holder). What a surprise to find it's shots that you have taken of her. Sensational photo's.

Thank you so much for that.

There's some really nice detail in those - particularly the 'wedge' and the piss-ant tiny amount of rudder that he ended up running.

Again, much apprecitated. Thanks mate :D

BJ:)
 
Hey Brian,

No problems with the pics hope they helped, I remember the wedge was somewhat key to the design of the boat in a doco I once watched about the run.

I've had a great interest in these turbine engines for some time and I think one of the first things I asked about when I got into boats had anybody run one. My goal one day (when I'm rich and famous :lol: )is to build a scale boat with a genuine turbine but running through a gearbox to a prop eg: like a real one.

Have you thought about going down the EDF (Electric Ducted Fan) first to dial in the hull design, thrust line, etc, could be a lot less expensive should the boat take a spill a full note. These are pretty powerful now and could be a cheap way to get you going in the right direction.
 
Yah Murph - I've thought about getting one or two 90 or 100mm ducted fans to test things out a bit before whacking a pair of Turbines on it.

If I could find 1...maybe 2 at a decent price (including ESC & LiPo's etc), I'd go for it. I did a quick look around on flea-bay etc. and man, I'd be up for a couple of hundred a pop. Now, if I could go "pre-loved" for a modest sum, that would be a better option.

BJ:)

PS *ahem* Cough! what do you mean 'a real one'?? Warby's boat was sure as eggs real-as-they-get... Props. PAH! LoL
 
Say...Murph...

Is it ok if I post your pics link on some other sites?

They're great photo's that I'd like to share with some other enthusiasts...

Regards,

BJ:)
 

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