My propeller - thoughts?

[Bee]

Funny things propellers. When I built our boat I bunged on a prop. that was in the shed, an inch too small in dia and an inch too fine in pitch, blades of decent area and all in good condition. A few years later I put the size of prop on that Beta reccomended - no science involved about hull shape or anything, boat went back in the water, started engine, shoved it into gear and the engine note changed to a deeper grunt. Boat now goes much better, stops better and is generally more satisfying to steer. We do spend a bit more time on rivers and commercial waterways and can now  keep up with a peniche which is useful on the Canal du Nord with its funny sized locks where it is a good idea to find a boat you can share with and then hang onto it. Size matters.

[MtB]

1 hour ago, Bee said:

Funny things propellers. When I built our boat I bunged on a prop. that was in the shed, an inch too small in dia and an inch too fine in pitch, blades of decent area and all in good condition. A few years later I put the size of prop on that Beta reccomended - no science involved about hull shape or anything, boat went back in the water, started engine, shoved it into gear and the engine note changed to a deeper grunt. Boat now goes much better, stops better and is generally more satisfying to steer. We do spend a bit more time on rivers and commercial waterways and can now  keep up with a peniche which is useful on the Canal du Nord with its funny sized locks where it is a good idea to find a boat you can share with and then hang onto it. Size matters.

 

My experience too. I've done a fair bit of swapping blades over the years and broadly speaking apparently small changes like you describe, especially when there are two of them i.e. both pitch and diameter, compound up to make big differences in performance. A bigger diameter blade especially makes a boat stop better as well as loading the engine more nicerly I also find an over-propped engine is nicer to steer than one 'correctly' propped for maximum power, as for normal cruising a lower engine speed results. Especially on higher power engines where 35, 40 and 45hp peak power outputs are never going to be actually used, but the bigger engine still has the low speed grunt to turn a big blade. 

[john.k]

Not entirely relevant,but as a kid I used to sit and watch the Gardners by the hour ,and when the boat was over a shallow bank,the governors would give the engines full fuel,due to the suck down effect of the higher water velocity between hull and sandbar......or so I was told.......My industrial deafness started about then too.

[David Horsburgh]

5 hours ago, Alan de Enfield said:

 

But unless you state that, no one is going to know what your level of experience is, folks go to a great deal of effort to try and explain why & what is happening (assuming that by the question you have little experience) when all along you probably have 1000's of hours of Narrowboating on Rivers / Big canals but just not used to the damp, muddy ditches. 

I disagree. Regardless of experience in certain areas I came across a problem that I didn't know the answer to, so asked the members on here and received my answer, the condition of the Oxford on some sections, and my mind relaxed. I thought I had a prop problem but now I know that I don't. I will always ask for help when I have a problem I cannot work out - wouldn't you??