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Old 26-08-2010, 10:39 PM   #1
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Drivetrain/prop shock absorber??

Here's the crazy thought for tonight then.....

We know that it's the high load spikes caused by re-entry etc that kill outdrives. Most of the time, torque levels are well within spec. Everyone is caught up trying to make drives strong enough to take these spikes and not doing very well.

Maybe the answer is to take away the spikes?

Imagine a car with no suspension driving along a perfectly smooth road, it'd hold together fine. Then imagine driving that same car over endless speedbumps, it'd fall apart pretty fast. That's how I see the issue with outdrives handling big power in rough seas.

I was thinking of a way to make a 'shock absorber' in either preferably the prop hub, or possibly the cush drive. Imagine it like a rubber driveshaft capable of taking the power. Under normal conditions it'd be 50% wound up, directly transferring power to the prop. When the prop leaves the water, it'd release back, then take up the shock of re-entry as it winds back up again to full load. Surely that'd help the drive out a huge amount??

Could be achieved with a torsional spring system with VERY heavy weight springs holding the shaft. Under no load they'd be relaxed, under standard load they'd be 50% stretched, but there would be 50% left to take up any shocks gently. As you leave the water they'd spring back to 0% or there abouts, then upon re-entry rather than a sudden shock, they'd stretch up to 75-80% if needs be, then relax back to 50% again, giving the drive a much easier. I believe a similar idea is used to a smaller extent with compression springs in automotive clutches?

Another way could be a clutch like system which would have to be set quite accurately to maximum normal torque, so it'd slip a bit during extreme load. I'd imagine the first idea would be easier.

Anyone heard of anything along these lines before? Any comments/ideas?

James
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Old 26-08-2010, 11:14 PM   #2
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So what a bit like the rubber damper that's already built into your coupling? Or maybe come away from solid hub props and go for bushed?


James, you really have got to stop analysing this to death, get the damn thing finished, get out there and give your right foot a bit of education.
You know the old man and machine inextricably linked in harmony type of stuff

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Old 27-08-2010, 12:30 AM   #3
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Ha ha, well I was thinking a bit more advanced than the bit of rubber. Similar, but with far more travel. The prop bush can offer a degree or two of shock absorbtion, I'm looking for 180 degrees minimum, if not a few revolutions.

I know, it's all mainly just pie in the sky stuff, but I'm too busy to be working on the boat at the moment, so I'm at home in the evenings thinking about things instead and I'm the kinda person who endlessly tries to find solutions to problems instead of just putting up with them!

James
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Old 27-08-2010, 12:53 AM   #4
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James

For god sake just get on and build the bloody thing because you are distracting me from building mine

Anyway where are the latest photo's of yours, looking forward to seeing how its progressing.
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Old 27-08-2010, 07:48 AM   #5
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Ha ha, well I've definately done this subject to death then! I still believe there's possibility in this one though!

I can post some pics up, but I haven' progressed much lately. Been very busy with work and organising the powerboating, and I've been trying to see more of my family in the evenings as I feel I missed out on a lot of Mia's early months because I was always too busy working on the boat.

There's no way it'll be done this season, so I'm just taking a break for a bit, then taking my time to do it right over the winter. I do need to get cracking though, next year is gonna be here before I know it!! I'm at the stage now that I was expecting to be at this time last year when I bought it... Good structural and empty hull ready for a few cosmetic repairs, repaint and rebuild.

I'll update the thread accordingly tonight...

James
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Old 27-08-2010, 08:35 AM   #6
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SportsBoat and RIB had an article on exactly the sort of prop hub you're talking about in the current issue!
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Old 27-08-2010, 04:17 PM   #7
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SportsBoat and RIB had an article on exactly the sort of prop hub you're talking about in the current issue!
Not a prop hub, but a driveshaft coupling - like a gearbox deal. They are hydralic couplings. I had one in my sports car a long time ago, and when the fluid leaked it was like you had a slipping clutch.

They are commercially avalable. They are also sometimes called Fluid Slip Couplings, and their intention is to allow for machinery to 'jam' or 'judder' without affecting the drive.

You find them commonly in combine harvesters!

Also some older sports cars that had overdrive usually drove the overdrive via a fluid coupling. It is essentially two sets of vanes in a fluid cylinder, and the drive on one os transferred directly to the other. They are designed with a defined amount of 'give' - however, depending on the fluid you use ( changing the viscocity ) you can get more slip than that designed which is what you want. A little slower to get onto the plane, and a little less responcive on the throttle, like a turbo lag or a slipping clutch - but also the protection to the grarbox and drive.

You can buy them here
http://www.twindisc.com/IndustrialProducts/IndFC.html
And about 30 other places.
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Old 27-08-2010, 05:53 PM   #8
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Now THAT is what I'm talking about!! I was gonna try to explain that exact type of fluid coupling, but couldn't think of a way of doing it, or if it even existed for that matter!!

Any idea what they cost? Please please tell me they're not stupidly expensive!!

James
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Old 31-08-2010, 06:12 PM   #9
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Ditching the blower and fitting a turbo might have exactly the effect that you're after but somehow I cant' see you doing that

Just put it all together, then we can go for a play!
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