changing the prop

[emlclcy]

my boat at 1500rpm in still water makes a brisk walking speed. when I bought the boat I was told it had a small prop, so cruising upstream is very slow, I'd like a bit in reserve especially on the K&A this time of year!

soooo I want to swap the prop for one more suited.

I need to remove the existing one to measure the taper so I can spec the new one.

looking for tips to remove the prop whilst still in the water.

what sort of puller do you use?

how to stop the prop turning whilst undoing the large retaining nut?

do it all through the weed hatch?

cheers

carl

[Tony Dunkley]

my boat at 1500rpm in still water makes a brisk walking speed. when I bought the boat I was told it had a small prop, so cruising upstream is very slow, I'd like a bit in reserve especially on the K&A this time of year!

soooo I want to swap the prop for one more suited.

I need to remove the existing one to measure the taper so I can spec the new one.

looking for tips to remove the prop whilst still in the water.

what sort of puller do you use?

how to stop the prop turning whilst undoing the large retaining nut?

do it all through the weed hatch?

cheers

carl

 

All of today's so-called narrowboats have relatively small propellers because they draw so little water, and whatever speed any boat makes in still water, it can make exactly the same speed through a river current or tide.

Before you go to the trouble and expense of changing the existing prop on you boat, it would be best to establish whether or not it is already correctly matched to the gearbox reduction ratio and engine.

At the moment it sounds as though you intend to match the propeller to your aspirations on boat speed, which may not improve anything at all if the engine can't reach it's maximum governed rpm turning a bigger, or more heavily pitched propeller.

[jake_crew]

If you need to remove it - there may be threaded holes near the boss of the prop to enable you fit draw screws.

 

These go through a bar over the end of the thread on the shaft and you can then pull the prop off.

Edited March 3, 2016 by jake_crew

[bizzard]

There are proper propeller extractors available.

[emlclcy]

this is a picture of the prop during the boat survey, looks like it doesn't have those holes.

good call tho

I'm going to measure the prop radius this weekend and see if there is a way to read any numbers stamped on it

oops sorry didn't attach the picture!

[emlclcy]

anyone done this before?

 

Aldermaston lock is large, it has a very large cill which after the lock is empty sits proud of the water by 6" or so and is about 4ft into the chamber.

so with several people to hand, gradually empty the lock with the back of the boat over the cill just enough to expose the prop. Aldermaston lock empties itself over time so someone would be at hand to open the top gate paddle slightly to maintain a constant level whilst the prop is removed.

to risqué?

[mrsmelly]

anyone done this before?

 

Aldermaston lock is large, it has a very large cill which after the lock is empty sits proud of the water by 6" or so and is about 4ft into the chamber.

so with several people to hand, gradually empty the lock with the back of the boat over the cill just enough to expose the prop. Aldermaston lock empties itself over time so someone would be at hand to open the top gate paddle slightly to maintain a constant level whilst the prop is removed.

to risqué?

 

I suppose you could ask CART to put a stoppage in place for you to fascilitate this plan of action?

 

Tim

[pophops]

What you do with your own boat is your own business. Just be sure that these 'several people' are willing to risk serous injury or death on your behalf in order that you can avoid the cost of a dry dock.

[Tony Dunkley]

anyone done this before?

 

Aldermaston lock is large, it has a very large cill which after the lock is empty sits proud of the water by 6" or so and is about 4ft into the chamber.

so with several people to hand, gradually empty the lock with the back of the boat over the cill just enough to expose the prop. Aldermaston lock empties itself over time so someone would be at hand to open the top gate paddle slightly to maintain a constant level whilst the prop is removed.

to risqué?

 

Yes, times without number.

But we were doing it with 70' [+] boats which were watertight to deck level, and more height under the counter to work in.

If you do try it yourself, then make sure you've got the boat well roped back to the top gates as the water level drops, . . off both dollies and round either the handrails or with the shallower hull that you'll have, round the mitre posts.

 

Before embarking on this, though, I do suggest you give some serious thought to what I said in Post #2, and to what Old Goat has just put in the next Post down.

Edited March 3, 2016 by Tony Dunkley

[OldGoat]

emlclcy, on 03 Mar 2016 - 10:53 AM, said:

this is a picture of the prop during the boat survey, looks like it doesn't have those holes.

good call tho

I'm going to measure the prop radius this weekend and see if there is a way to read any numbers stamped on it

oops sorry didn't attach the picture!

The prop: in your picture looks about the correct size for the boat (OK - the pitch may be wrong, but I have my doubts). The important point to note is the tip clearance and that to me looks about the least it should be.

 

Methinks you need to consider other matters first - engine size / gearbox ratio / how much marine growth you have on the hull.

 

There have been threads on here indicating that getting a propeller off requires major effort requiring hydraulic pullers.

[Tony Dunkley]

The prop: in your picture looks about the correct size for the boat (OK - the pitch may be wrong, but I have my doubts). The important point to note is the tip clearance and that to me looks about the least it should be.

 

Methinks you need to consider other matters first - engine size / gearbox ratio / how much marine growth you have on the hull.

 

There have been threads on here indicating that getting a propeller off requires major effort requiring hydraulic pullers.

 

Sometimes they can be very reluctant to come off, sometimes not, but changing a propeller without first establishing that it isn't correctly matched to the engine, gearbox, and the boat, doesn't make much sense however you look at it.

[Neil Smith]

I think I read on here about a slipway that will pull the stern up just enough to get the prop off, cant remember where but others might.

 

Neil

[Paul C]

I think I read on here about a slipway that will pull the stern up just enough to get the prop off, cant remember where but others might.

 

Neil

 

If you drop the nut, does it roll off at a comically slow pace down the slipway and into the water?

 

I've heard (and seen) a staircase lock be used as a temporary dry dock for changing a prop. But unless you knew it was going to come off quickly, you'd probably be better in a dry dock or the boat slipped, since it may take some time. For example you might want to remove the old prop, measure it, calculate a better size, obtain it, then refit it. You might be able to measure it (or find its measurements) through the weedhatch, saving a lot of time.

[Tom and Bex]

You might be able to measure it (or find its measurements) through the weedhatch, saving a lot of time.

Any tips for measuring pitch through the weed hatch?

 

Tom

[Paul C]

Look for the marking on the prop, which says what pitch it is?

 

Take an accurate picture from above, then use a protractor on the screen or printout?

[Tony Dunkley]

Look for the marking on the prop, which says what pitch it is?

 

Take an accurate picture from above, then use a protractor on the screen or printout?

 

That won't work. If you intend to calculate pitch using the angle across the face of a blade in relation to a line at right angles to the tailshaft axis, then you have to know the distance along the blade [radius] at which the angle was measured. To maintain the pitch at a constant value along the whole length of a propeller blade, the angle [measured across the face] decreases from root to tip. The exception to this are those peculiar Axiom propellers on which the angle remains constant from root to tip, thereby increasing the pitch from root to tip.

Edited March 3, 2016 by Tony Dunkley

[Paul C]

So how's it done then?

[Tony Dunkley]

So how's it done then?

 

By measuring the radius at which the angle is measured, . . . usually [when measuring a constant pitch propeller, on which the pitch is the same from root to tip] at about two thirds the blade length from the root, and it wouldn't be possible to determine that point on a photograph.

The same position along the length of a blade is used when determining the pitch with the propeller laying on a flat surface and measuring the height difference between points on the leading and trailing edges of the blade, and the horizontal distance between those points, on a circle of the same radius.

Edited March 3, 2016 by Tony Dunkley

[Pete & Helen]

this is a picture of the prop during the boat survey, looks like it doesn't have those holes.

good call tho

I'm going to measure the prop radius this weekend and see if there is a way to read any numbers stamped on it

oops sorry didn't attach the picture!

 

You could try using something like this to read the numbers on the prop. Probably worth a couple of quid to save you messing about

 

http://www.ebay.co.uk/itm/3-5M-Android-Phone-Endoscope-Borescope-Inspection-Probe-Camera-Support-OTG-ET-/141912994131?var=&hash=item210aac7153:m:mCPyrTiY1QeEZBDB7d6lQcg

[emlclcy]

You could try using something like this to read the numbers on the prop. Probably worth a couple of quid to save you messing about

 

http://www.ebay.co.uk/itm/3-5M-Android-Phone-Endoscope-Borescope-Inspection-Probe-Camera-Support-OTG-ET-/141912994131?var=&hash=item210aac7153:m:mCPyrTiY1QeEZBDB7d6lQcg

blimey that's cheap!

asking around at work, someone has lent me a gopro in a waterproof case so i'll have a go with that

thanks tho

[Iain_S]

Following on from post #2, how close does the engine get to its rated max. r.p.m. in open and deep water?

 

If it gets there easily, then you may be under propped; if not, probably not.

[Bee]

As Old Goat has said it looks about right, 17 or 18 inches in dia. and looks fairly coarse pitch, I would doubt if it is far off being correct. If you do decide to change it then I have a couple of pullers you can borrow.

[emlclcy]

As Old Goat has said it looks about right, 17 or 18 inches in dia. and looks fairly coarse pitch, I would doubt if it is far off being correct. If you do decide to change it then I have a couple of pullers you can borrow.

thanks Bee, i'll get in there with the go pro and see if I can find the numbers on the prop.

If you have your boat flat out on still water does it go faster than someone walking on the tow path?

[Alan de Enfield]

thanks Bee, i'll get in there with the go pro and see if I can find the numbers on the prop.

If you have your boat flat out on still water does it go faster than someone walking on the tow path?

 

What do you mean by 'flat out'

 

1) Full movement on the throttle lever

2) Full (max) engine revolutions for that model of engine.

3) A huge bow wave that the boat is trying to 'climb over'

 

Someone walking along the towpath will probably be about 3 miles per hour (brisk walk), a fast walk is 4mph

A NBs max speed is pretty much dictated by its shape and probably would be unable to achieve much more than 7 or 8 mph with any engine prop combination.

 

Our last NB would not do more than 5mph (GPS measured) but I put that down to a 3:1 gearbox. Ex hire boat 'probably' geared down to stop speeders.

Edited March 3, 2016 by Alan de Enfield

[RLWP]

All of today's so-called narrowboats have relatively small propellers because they draw so little water, and whatever speed any boat makes in still water, it can make exactly the same speed through a river current or tide.

Before you go to the trouble and expense of changing the existing prop on you boat, it would be best to establish whether or not it is already correctly matched to the gearbox reduction ratio and engine.

At the moment it sounds as though you intend to match the propeller to your aspirations on boat speed, which may not improve anything at all if the engine can't reach it's maximum governed rpm turning a bigger, or more heavily pitched propeller.

I'll leave the technicalities on matching prop/engine/aspirations to Tony. Do answer his questions though as at the moment I can't see any evidence that you'll make a scrap of improvement.

 

Briefly on the practicalities last time I fitted our prop I tightened the nut up by standing on a six foot long bar attached to the spanner. Previous to that I had tried several times to tighten that nut through the weedhatch with no success

What do you mean by 'flat out'

 

1) Full movement on the throttle lever

2) Full (max) engine revolutions for that model of engine.

3) A huge bow wave that the boat is trying to 'climb over'

 

Someone walking along the towpath will probably be about 3 miles per hour (brisk walk), a fast walk is 4mph

A NBs max speed is pretty much dictated by its shape and probably would be unable to achieve much more than 7 or 8 mph with any engine prop combination.

 

Our last NB would not do more than 5mph (GPS measured) but I put that down to a 3:1 gearbox. Ex hire boat 'probably' geared down to stop speeders.

Probably not. Ours will do a bit over four and a half, but not at full throttle and only on certain canals. On others, she would probably go faster at lower revs

 

Richard