> Mike wrote
>>Hm. Are you sure the prop-wash isn't picking up gravel or whatever
>>from the bed of the canal?
>
> I dont think so as this has been happening on all shallow canals. I
> think
> Sean has his finger on the problem, the flat stern as opposed to V
> stern, I
> think a phone call to Peachment to try and get info from other
> purchasers of
> their hydraulic system may give further clues.
>
> Terry
>
Having suffered from both problems (as my boat wasn't built to be
powered and I don't want to alter the original stern lines), picking
stuff up off the bottom creates *irregular* noise without much
vibration, if it's just gravel etc being driven against underwater
surfaces, or irregular shocks to the boat if she's in contact with a
hard bottom (soft mud just leads to mysterious slowing and difficult
steering). Interaction between propeller and sternpost leads to regular
vibration which can be detected if the propeller is run with the boat
tied up in forward gear but not when in reverse gear (at 150 rpm I can
distinguish between propeller-related vibration at 3 times per
revolution and engine-related at 2 times).
As ever, the Victorians were onto this problem - in 1846 Lloyd and
Brunel tested the DWARF with her original fine stern lines and she did
9+ knots at 32 rpm; they then added three layers of planking to make the
stern bluff and speed dropped to 3.25 knots at 24 rpm (engine speeds
were a bit different in those days). Removing one layer took speed up to
5.75 knots at 26.5 rpm; removing the other two restored the original
speed.
I don't think these effects are nearly so obvious with modern high-speed
engines with relatively small propellers - I've noticed that even
putting the rudder over hard will reduce rpm due to the added
resistance.
Sean
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