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Author Topic: Restoration of Molendinar Pilot Boat  (Read 4304 times)

Laker

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Re: Restoration of Molendinar Pilot Boat
« Reply #25 on: November 16, 2018, 09:28:15 PM »

Today was mild and calm in Portishead so I thought I would take the model for a sail.  The photos show it on Portishead Marine Lake which is the main sailing water for the Woodspring Model Sailing Club.  The light level was quite low and so the photos aren't quite a sharp as I would like, but I took quite a few and picked out the best ones.  The model was motoring more or less continuously for 50 minutes and there was still still some life in the NiCd battery at the end, so the 5 pole 385 motor really doesn't draw much current.  The motor was also barely warm by the time I finished.
As I mentioned above, I have had to set the tension in the rubber o-rings on the drive pulleys quite carefully.  They are 1.0mm width o-rings which means they are quite stretchy and it is possible for the motor pulley to slip on occasions, for example if you hold the prop and then operate the motor.  I don't think it was slipping while the model was underway today, but it is hard to know for certain.  I have also bought some 1.5mm width o-rings which are quite a bit stronger and I might make up another set of aluminium pulleys for these in due course, just to make sure that the slipping doesn't become a problem.
Otherwise the Molendinar Pilot boat ran very well today.  I need to get on with making a proper box for it and then I will be more inclined to travel around to float it with some of the other local clubs.  So there it is, another model in the collection and maybe time to start thinking about the next build?  Happy sailing.
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chas

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Re: Restoration of Molendinar Pilot Boat
« Reply #26 on: November 17, 2018, 09:03:44 AM »

Great job, it's nice to see it sailing again after all that work.
Chas

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Laker

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Re: Restoration of Molendinar Pilot Boat
« Reply #27 on: March 09, 2019, 12:30:23 PM »

I mentioned above that I had some concerns about the thin 1.0mm cross-section O-rings slipping on the pulleys in the original drive system.  Well having sailed the model a few more times it became obvious that the O-rings were slipping and that the motor was strong enough to stretch these very thin O-rings as well.  On one hot day the O-rings slipped so much that the poor thing almost stopped in the water.  I therefore had to make a new set of pulleys to take some thicker, stronger O-rings and I am posting these notes in case it helps anyone else who is thinking of using a pulley drive in a small model.
I have made several changes for the second set of pulleys:
(1)  I have reduced the gear ratio from 1:2.0 to 1:1.7.  The original pulleys were giving the motor a very easy time and the top speed of the model was barely enough, and certainly a bit slower than I remember it being originally.  Therefore I have reduced the gearing by a small amount.
(2)  I have found some 1.5mm cross-section o-rings (x 32mm bore size) which are substantially stronger than the original 1.0mm cross section ones.  It doesn't sound like much difference, but 1.5mm cross section has more than twice the cross-section area of 1.0mm.  They are still quite flexible so hopefully will not introduce a lot more power loss.
(3)  The original pulleys had square section grooves cut into them, so that the O-rings ran on a flat surface at the bottom of the groove.  Experience has shown that this only just produces enough friction and that any slackening of the o-ring will make it slip.  For the new pulleys I have therefore machined in Vee grooves using a screw-cutting tool with a 60-degree included angle at the tip.  The Vee grooves give a much more secure grip on the o-ring without any sign of jamming or grabbing the o-ring.
I have attached a drawing showing the cross-section of the two new pulleys, which I turned from plain aluminium bar stock.
The new pulleys have been fitted in the model and I didn't need to put much tension into the O-rings at all to get a good level of friction on the pulleys.  The drive seems very secure now.  I took the model to the lake and tried it out yesterday morning.  Thankfully the new pulley arrangement seems to solve the problems - the model is noticeably quicker in the water and I can't see any signs of slippage now.  Top speed is now perhaps a bit more than scale speed, so I can throttle back a bit for cruising the model around and have a bit to spare if needed.  After a good run the motor was still barely warm as well.  Hopefully that will now be the final arrangement and I'm looking forward to sailing it around the various local lakes this summer.    I hope that extra bit of info might be useful to someone as well.
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