What speed does your boat motor forward at lowest rpm?

Yup - thats how you do it!

A good narrowboat steerer can be surprisingly delicate or surprisingly brutal.

First Mate could steer a 65 foot modern job into a narrow - 7 foot - lock witout touching the sides almost every time. She could steer through the Blisworth Tunnel - over 3kms long - with a cheese and pickle sandwich in one hand, a mug of coffee in the other and the tiller in the small of her back, without touching the walls.

She remarked that it was easy as the tunnel was 15 feet wide!
 
Daydream believer said:
On tickover in the Dutch canals my boat goes embarrasingly fast. If we are in convoy approaching a bridge or lock there is often a boat that slows down long before we get there. Typically this often happens in smaller canals like the one leading to Goes, where one always seems to crawl along.
Normally, I would be quite happy to follow in convoy, but it usually means that I have to keep dropping in & out of gear. Even though my engine ticks over at 800RPM. My Brunton Autoprop just pitches up & gives max drive.
So I usually end up overtaking the convoy, but trying to explain to the boats, that I am overtaking, that they are going too slow just makes it worse. So I end up looking like a mad Englishman. :unsure:
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Hence the reason why some Dutch boats have "trolling" gearboxes to keep the prop speed low. I sympathise with the problem as I hit 3.5 to 4 knts on tick over and it doesn't feel good for the engine. Tow a drogue?!
 
Supertramp said:
Hence the reason why some Dutch boats have "trolling" gearboxes to keep the prop speed low. I sympathise with the problem as I hit 3.5 to 4 knts on tick over and it doesn't feel good for the engine. Tow a drogue?!
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Yacht engines and props are set up maintain good forward speed against strong headwinds , breaking waves and chop. As I have commented else where a small yacht will do quite a surprising speed just on its dinghy outboard motor in flat water and still air
 
I had a Dolphin engine in my Mystere some time in the last century. While an excellent engine in many ways, it didn’t take kindly to idling for long periods. On occasions such as when doing the Amsterdam night passage in convoy, it was my habit to stop the engine instead, and I enjoyed the consternation that this caused in onlookers.
 
Laminar Flow said:
There are a number of things to pick apart here:
A fixed, non variable pitching prop is designed to work effectively in a surprisingly narrow range of RPM and speed, usually around 90% of max speed. 70% efficiency is about as good as it gets for a displacement prop.

A clean displacement hull has around 25-40% slip, but in a high speed planing hull this will trend towards 0%.

Total resistance is not a linear curve, but exponential. Consequently, the RPM or power needed to reach a certain speed is not a linear progression, i.e. : 1000 RPM per knot achieved. This explains why very little power is needed to move a boat at slow speeds, even though the prop will not be all that efficient or designed for such a low RPM/speed.

The ideal prop would be a large diametre, slow turning, single bladed one. For various practical and obvious reasons this is not or not always possible. A sailing boat's (fixed) prop will naturally be a compromise to reduce resistance under sail.
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Agree with this description. Good write up. Although I can tell you that I and countless others have been aiming for 0 prop slip all my life. An impossible target, haha. Managed 3% so far, after hiding the cost and ownership of far too many fancy props !!
 
Just side thrust from a rudder on full lock and a prop. Forward motion prevented by the edge of the canal, so the stern pivots around the bow. When you get far enough round, back off a bit then same again to get your bow out of the hole, which is just a wide bit of canal.

Here's how someone who knows what he's doing does it. It's 8 mins long, so a bit of judicious fast forward might be in order

 
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