ST21 'Skinnydip' build, and boating adventures, Noosa, Australia

[Fuzz]

While I very happy you have the tranny problems in back of you I do wonder how much performance you are giving up with the slower turning prop. I know the normal answer would be to use a larger prop with more pitch but I do not believe those options are available to you on this build
On a side note I regret for you Aussies that one of your own did not win the AO. It did work out for us pretty well so I will not complain.

[glossieblack]

Thanks guys.

Skinnydip was stored on her build cradle in the shed while we went cruising in Great Sandy. Fortunately the bushfires ignored her.

She's still there, awaiting the availability of a trailer to relaunch her. That should be any day now.

[cape man]

Glossie,
We just had two weeks with my father in law, a 89 year old, retired helicopter engineer. His last job was working on the V22 Osprey and figuring out how to make the wiring harness below the wings that rotates 90 degrees do so without the 500 or so wires in it not tangle (guess he figured it out).

He and my wife spent most of the time visiting aeronautic and train museums around the state... He came home, and for his last two days here was drawing the engineering diagrams of how a 12 cylinder rotary engine can operate with a single cam...

Send me an email with everything you can about the transmission, motor, prop, etc. and I'll give him something else to ponder. If nothing else, you will be exercising a brilliant mind that's about to turn 90.

[Jeff]

Very cool Cape Man!!!!! Jeff

[OrangeQuest]

That is a great idea!!

[glossieblack]

Thanks cape man.

What I’d really like to know is if it is possible, by getting a bespoke gear or gears cut, to rebuild Skinnydip's current Twin Disc MG 340 from a 1.5:1 to a 1:1 box.

Do you think your father-in-law could be interested in figuring this out? Any explanation of why the setup with the ZF Hurth box failed would be a bonus.

The following gives some the background.

The designer of ST21 specified a 20hp diesel, a 1:1 transmission, and a 10” diameter by 10” pitch propeller. The tunnel’s configuration limits the propeller diameter to 10”, so the 1:1 transmission is specified to spin the small prop as fast as possible. About the only 1:1 transmissions available these days are large and heavy hydraulic ones designed for engines up to 300hp. The only 1:1 mechanical box I could find still be in production was a Turkish brand Koysan, but it was as heavy, and after months of trying to contact Koysan to no avail,I gave up.

So I built Skinnydip with a marinised 20hp Kubota diesel and a ZF 45C 1:1 hydraulic box. On three occasions the 20hp diesel Kubota with the Hurth ZF 45C 1: i transmission was trialled without success. Each time progressively stronger damper plates failed in less than one hour. The first and third tests were on the water, the second on the hard.

Between the first and second trials, the engine and gearbox were sent to a specialist engineering works where every aspect of alignment and fit between engine and transmission was checked and found to be satisfactory. After destruction of the third damper plate the only hypotheses left standing was that a large hydraulic box like the ZF 45C, rated to 300hp, is incompatible with a 20hp diesel engine because the small flywheel of the small diesel is insufficient to stop the engine pulse hammering the damper plate to destruction. It was going to take expensive experimental testing with bespoke heavier flywheels to disprove or validate this, so I decided to move on.

I've now installed a Twin Disc MG 340 1.5:1 mechanical transmission to replace the ZF 45C, which has been sold on. The 20 hp diesel with a TD MG 340 1.5:1 is the same combination commercially marketed by Nanni and Beta. It has not failed after approximately 10 hours running time. The 20hp Kubota can be continuously used up to 3,000 rpm, so with a 1.5:1 box, a 10" diameter prop can be turned up to 2000 rpm. The designer advised that with the 1.5:1 box and a 10” diameter prop, a 12” pitch should be used.

While I’m curious as to why the hydraulic ZF Hurth 1:1 box didn’t work out, I’m now focussed on getting the best results from the Twin Disc 1.5:1. Tweaking the 10” prop’s pitch will hopefully provide some improvement in top end speed.

What I’d really like to know is if it is possible, by getting a bespoke gear or gears cut, to rebuild the Twin Disc MG 340 box to 1:1, and if so, what’s involved.

[OrangeQuest]

By any chance did the company checking the clearances and or alignments of the transmission/adapter/engine write any of their readings down? Other than just saying everything checked out ok.

[cape man]

Jim Smith, retired Boeing aeronautics engineer (long before the 737 max... ), has been given the challenge! Again, no promises that he will come back with a solution, but a Bateau boat half way around the world will give him mental exercise for sure.

...is possible, by getting a bespoke gear or gears cut, to rebuild the Twin Disc MG 340 box to 1:1, and if so, what’s involved.

My only problem is that I will now have to listen to the problem every time I speak with him...

"Jim, can I get you some more coffee?"

"Well, sure, but that reminds me of the transmission and damper plate problems you brought to me 5 years ago..."

"Jim, isn't that a nice sunrise?"

"It certainly is. The red color reminds me of what the connections on that damper plate in Australia must have looked like just before they failed."

The really good news is that marine transmissions may now be included in the conversations about helicopters, planes, and trains.

[glossieblack]

Thanks cape man and thanks Jim.

[glossieblack]

It looks as though the trailer I've been borrowing to launch and retrieve Skinnydip is unavailable this coming week, so today I started tweaking the e-drive subsystem of the diesel-electric hybrid drive system, in light of the river trials before we went sailing late last year.

There is insufficient cool air being delivered to both the e-motor and the e-controller. In hot weather conditions, after running at around 3-4 knots for a time, the controller senses heat overload and shuts the system down.

I spent today pondering a number of different fixes, all involving increased forced cool air flows, and all doable . It's an experimental boat.

I'll sleep on the fixes tonight.