Monday, 27 September 2010

A danger to myself.

Alarm, danger Will Robinson!

Anyone else remember "Robbie the Robot" in "Lost in Space" which graced our TV back in the pre Startrek days of the late 60's. Lost in Space was a modern take on the "Swiss Family Robinson" story. A modern day space family marooned on a far away planet, trying  desperately to return to Earth.

Am I a danger to myself? I ask the question because, I have been thinking about propellers. Sad, I know, but someone has it to do. Now, what has become apparent is that selecting a propeller for a narrow boat is like needing a wizard qualification, just like Harry Potter. This is because everyone say's selecting the right propeller is something like mastering a "Black Art

Now I don't lay claim to being a boffin on propellers or anything of that ilk. I work with many boff's or "propellerheads", well enough of them to realise, I don't want to be one.

However, by reading messages posted on various canal forums and other such stuff on the net being a bofin would help. It's not altogether clear just what actual data I need to decide on the correct size and pitch of the prop. Not only that, there is very little information on how to use the information, once I have got it, to make a calculation anyway.

Here are a number of items gleaned from the net, that I am given to understand, I will need to specify the correct propeller for my boat.

Picture from NarrowBoat Swallow blog.

1. Boat Weight, now that must be a significant factor to consider.

2. Boat Length, must be less of an issue than the boats actual weight - but the longer the boat the more friction is going to be involved.

3. Boat Beam, is going to be a significant issue as the cross section displacement profile must also be a very significant factor if only for the profile of the bow wave.

4. Boat Draft, how low in the water can the propellor be fitted. The deeper the better, however you need to take into account that most narrow craft are shallow draft so that it can travel along canals that are infrequently dredged.

5. Engine BHP, what is the minimum and maximum power available in bhp.

6. Engine RPM Range. Minimum and maximum usable RPM range. Fuel efficiency will come into mind when selecting the - maximum RPM for sustained usage.

7. Engine maximum torque. What is the maximum amount of torque available.

8. Engine maximum torque RPM. Where does the torque maximise in the rpm range.

9. Gearbox Ratio. Rotation speed of the prop shaft will be a significant figure.

10. Prop Diameter. What size of prop will physically fit in place.

11. Prop Blades. Three is the norm, however 2 or 4 is not unknown.

12. Prop Pitch. The angular distance in the blade pitch.

13. Prop Slip. Whilst a single rotation should move the boat "the pitch distance" the reality is that the distance moved will be reduced. The difference is the prop slip.

14. Shaft size.

15. Left-hand or right-hand thread. Dependant on which way the prop shaft turns for forward movement.

Alternatively there is a much simpler way and that is to use the Castle Marine propeller pitch calculator.


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