Hi Hannu,
I'm in total agreement with you. In fact were I in the commercial business of offering direct drives I would specify a proper thrust bearing. Not only does that facilitate motor removal on the water but it is the best engineering practice, at the expense of some friction/reduction in drive efficiency.
I find it interesting that most small yachts also don't have an axial fail safe, should a half coupling on the prop or motor shaft come loose. I tapped both my shafts 8mm and use a plate washer inside the half coulping so neither can pull through the half couplings.
As regards shock loads, this makes total sense. I'm not aware of Cross & Moore here in UK. Do you have a link as I can't find them? Unless you mean a acompany by that name or similar that made micrometers and stuff? My memory is not what it was....
I agree going by Young's modulus/Hooke's law that shock loads are part elastic too. I found this interesting table which concurs with what you said.
Unwin safety factor α based on tensile strength
| Material | Static load | Repeated load | Impact load | |
|---|---|---|---|---|
| Pulsating | Alternating | |||
| Steel | 3 | 5 | 8 | 12 |
| Cast iron | 4 | 6 | 10 | 15 |
| Copper, soft metals | 5 | 5 | 9 | 15 |
Standard strength: For ductile materials=Yield stress
For brittle materials=Fracture stress
As a young man I was a Royal Navy MEA (P) i.e an Artificer and remember with horror the loads on the ship once we were off the Beaufort scale during the Cod Wars in Iceland. The waves were over 60ft and our 300,000 shp Frigate had the screws out of the water as we surfed over the top and came grinding to a halt in the wave trough below. The whole ship ached and groaned, but being a warship with safety factors above and beyond civil duties, we came out unscathed. Exciting as a young man, but now 40 years later I have no desire to experience that again. And the thought of being out in those conditions in my little 26ft boat scares the wits out of me!
As regards cycle loadings of most leisure craft, again I find myself in agreement. I read a good article on Attainable Adventure Cruising that discussed rig and cycle loadings of a continuous blue water crusier when compared to us leisure sailors.Basically they may experience cycle loads in 1 year that us day/weekend sailors experience in 10 or 15 years.
Were I to re-engineer my system again, I'd look at fitting a separate thrust bearing that transmitted loads to the strong part of the hull/turn of bilge area. One of my issues is physical space. I'd need to reduce my battery size and/or remake various parts to make the room.
Again thanks for your input and valid food for thought.
Out of interest this is my 2nd season with my conversion. I originally had a 2 blade 12 x 10 prop. I calculated that to pull the correct amperage at nominal system voltage at max RPM (reduced from 1250 to 970 RPM loaded now) that I'd need a 12 x 14 3 blade of 50% BAR. I checked the voltage and current at the output of my controller using my programmer this week and found 22.7 Volts and 103 Amps at full throttle, once steady state motoring was achieved - but I also noticed a vibration that did not exist with the old prop.
I put this down to the fact I have a long keel and the 2 blade was shielded at midnight and six o'clock behind the deadwood and when out of that area had balanced forces on it, but the 3 blade is less well balanced to accept the inrush of water from the keel area to it in the aperture as the blade loadings are of unequal areas as they position at ten to, ten past and six o'clock, so putting an unequal load on the shaft. I shall check my shaft alignment, but right now I can see me going back to a two blade because of this and the shaft loadings we have discussed. I'd recalculate diameter/pitch and blade area in a compromise area. I am unable to fit a larger diameter prop, which in fact I would not do anyhow on my sailboat, again because of loadings discussed.
John
http://john.rushworth.com/
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