Propulsion problems

[ditchcrawler]

But it would be very inefficient. If say a 40mm prop was fitted back to front on a 25mm shaft as a temporary measure and left there. I know pure speculation.

 

Simon

Bet it would wobble a bit

[MtB]

Nor would it produce the white froth described by the OP...

[ditchcrawler]

Nor would it produce the white froth described by the OP...

It probably wouldnt grip the shaft

[Jess--]

quick(ish) method of changing front - rear trim is to empty your water tank (assuming it's in the bow), your bow will float higher but stern should drop a little.

 

my boat is noticeably more efficient with a full diesel tank and an empty water tank (although it is more of a pig to reverse)

[starman]

The OP mentions that the shaft extends "a long way out of the hull" to bring the prop close to the rudder. This would surely cause a lot of vibration in the unsupported shaft.

Edited September 22, 2015 by starman

[Sea Dog]

Won't it be nice when the OP gets this boat out of the water and we can find out what's really going on!

[Dalslandia]

No it will kill the curiosity and fun.

Won't it be nice when the OP gets this boat out of the water and we can find out what's really going on!

There is people for everything, but it seems longsighted, but sure will create the symtoms

Silly question but is it possible to fit a prop the wrong way round on say a large prop bore on a small shaft size?

 

Simon

Edited September 22, 2015 by Dalslandia

[MtB]

Won't it be nice when the OP gets this boat out of the water and we can find out what's really going on!

 

 

It's a shame the OP is so reluctant to post a photo of the swim shape taken from inside the boat. A lot could be gleaned from it in my view. Ruling out a swim shape problem in advance of docking the boat seems a really good idea to me.

 

If hull mods turn out to be required, a plan and a welder need arranging in advance of docking day.

[sjc]

 

 

It's a shame the OP is so reluctant to post a photo of the swim shape taken from inside the boat. A lot could be gleaned from it in my view. Ruling out a swim shape problem in advance of docking the boat seems a really good idea to me.

 

If hull mods turn out to be required, a plan and a welder need arranging in advance of docking day.

 

Not so much reluctant, I am away from the boat whilst it is being painted. Will get a chance to see it in 2 weeks time. Sadly I fear modifying the swim would be too expensive ?

[Theo]

 

Not so much reluctant, I am away from the boat whilst it is being painted. Will get a chance to see it in 2 weeks time. Sadly I fear modifying the swim would be too expensive ?

 

If the boat cannot be cruised, and this would seem to be the case, then altering the swim might be the economical way to get a usable boat, rather than sell the present one for what anyone is likely to pay for what is effectively a narrow unpowered houseboat and buy another one.

 

(Sorry about the hyperlong sentence!)

 

N

[RLWP]

 

If the boat cannot be cruised, and this would seem to be the case, then altering the swim might be the economical way to get a usable boat, rather than sell the present one for what anyone is likely to pay for what is effectively a narrow unpowered houseboat and buy another one.

 

(Sorry about the hyperlong sentence!)

 

N

 

It's a pretty big assumption - the hull is wrong

 

I've seen some pretty awful shaped swims and I haven't come across a problem like this. On the other hand, aeration on a poorly ballasted boat is not unusual at all

 

Richard

[Phil Ambrose]

The OP mentions that the shaft extends "a long way out of the hull" to bring the prop close to the rudder. This would surely cause a lot of vibration in the unsupported shaft.

Very true, long shafts are usually supported by P bracket. I once had a P bracket detach itself and the vibration was horrendous.

Phil

[tidal]

I'd still bet on a combination of aeration and prop/rudder distance.

But the description in the early posts also fits a damaged prop and/or a slightly bent propshaft (at the very back end)

 

Narrowboat swims aren't something I know (much) about so I can't comment

[WarringtonWhite]

The prop is obviously turning OK looking at what has been said.

 

There are things that I would look at if everything else looks OK and both of them rely on something being caught on the bottom of the boat....

 

Look Fwd under the bow and just check that there isnt a tarpaulin or a tree caught under the water line. Trying to push against something like that would cause 'white water'

 

Check that nothing is attached to the skeg. like a tarpaulin etc. This would 'drop out of the way' when stopped but it would only need a very small movement forward to 'lift' the object and servilely obstruct the water flow over the prop. if the prop does not have enough water to screw through, it will draw air.

 

Either of these scenario's will make the tiller shake like a crapping dog.

 

Worth a look

[Murflynn]

maybe the boat has picked up a canal drain plug by the chain, as I believe happened to the Sharpness canal a few years back.

 

I love this speculation, and at least 2 weeks to go before we are any nearer the truth.

[sbillis]

My money is still on the prop fitted the wrong way round.

 

Boatyard has boat with 25mm diameter shaft fitted but only has 40mm diameter prop. Just as a temporary fix boatyard fits prop wrong way round on 25mm shaft and fitted adapted key, large washer and nut. Probably hopes owner will not notice.

 

The result is that the prop just trashes about and cavities in the water (cavitates) and because of the bad fitting causes significant vibration.

 

I now await to be shot down in flames. Can the OP let me know the diameter of the prop shaft?

 

If I am right do I get a prize?

 

Simon

[Alan de Enfield]

 

The result is that the prop just trashes about and cavities in the water (cavitates)

 

It will be easy to see if it is cavitation as when it is out of the water the prop will be pitted - I believe you actually mean 'ventilation' ( sometimes called aeration) rather than cavitation.

 

 

When a vessel suffers propeller cavitation the material’s surface is subjected to a continuous bombardment of impacts from a fluctuating pressure field. The propeller’s material is ductile at normal sea water temperature and, usually, the first sign of a problem is the so-called orange peel effect where the surface suffers ductile deformation leaving it looking like the surface of the familiar fruit.

After that preliminary stage and depending upon the severity of the attack, damage may either cease or continue. Micro-hardness testing of both damaged and undamaged blades shows that, under conditions of cavitation attack, the material in the layers immediately below the surface work hardens and, therefore, becomes brittle.

The tests show that, for undamaged blades, there is a relatively minor alteration in hardness just below the surface probably due to the manufacturing and finishing processes. On the blade surfaces that have suffered cavitation damage, however, a rapid change in hardness can be measured in the two millimetres or so closest to the area of cavitation attack.

The marine surveyor should, therefore, expect that the material will fail under cavitation attack and that the failure will contain a strong element of brittleness. Experiments also show that a major influence on the rate of erosion and damage growth is the local electro-potential of the material.

 

(Short extract from a feature article written by Eur. Ing. Jeffrey Casciani-Wood HonFIIMS)

[matty40s]

My money is still on the prop fitted the wrong way round.

 

Boatyard has boat with 25mm diameter shaft fitted but only has 40mm diameter prop. Just as a temporary fix boatyard fits prop wrong way round on 25mm shaft and fitted adapted key, large washer and nut. Probably hopes owner will not notice.

 

The result is that the prop just trashes about and cavities in the water (cavitates) and because of the bad fitting causes significant vibration.

 

I now await to be shot down in flames. Can the OP let me know the diameter of the prop shaft?

 

If I am right do I get a prize?

 

Simon

Which boatyard do you work for Simon.....☺

[Guest]

Very true, long shafts are usually supported by P bracket. I once had a P bracket detach itself and the vibration was horrendous.

Phil

Isn't that what caused the sinking of that awful yellow thing on the Mersey?

 

Sorry I digress. Having read the thread properly, my money is on counter not sitting in water enough (which means it is most likely to be something else.) I know swim shape makes a big difference to a boat, but it would have to be really bad to cause that much trouble I think. My first narrowboat had a truly awful short, almost rounded swim, but got along OK. It was awful at reversing, but reasonable in forward gear.

Edited September 22, 2015 by Guest

[sbillis]

Thanks Alan, the pitting is I believe the effect of cavitation. Cavitation is caused lack of pressure on the upstream side of the propeller in effect the vacuum caused causes tiny bubble of steam. It is usually caused by high velocity at the propeller tips. It also causes the prop noise.

 

Simon

Which boatyard do you work for Simon.....☺

I have not worked for a boatyard but several have worked on my boat.

 

Simon

[Scholar Gypsy]

Isn't that what caused the sinking of that awful yellow thing on the Mersey?

 

 

 

Yes, more or less - tyre round the prop which ripped off the V bracket and caused a rather large hole in the hull.

 

See page 15 and 19 here

[ditchcrawler]

 

If the boat cannot be cruised, and this would seem to be the case, then altering the swim might be the economical way to get a usable boat, rather than sell the present one for what anyone is likely to pay for what is effectively a narrow unpowered houseboat and buy another one.

 

(Sorry about the hyperlong sentence!)

 

N

Its been like it for 5 years and sold OK

[tidal]

I've seen shaft and bracket damage caused by nets and their associated lines at sea including deflection of the shaft outside of the stern-gland.
Hence my earlier comment. That particular incident gave many of the same effects as described in the OP as did a similar shaft problem after a boat sat down on an isolated stone in a tidal harbour.

In neither case was the deflection particularly visible to the observer

[Phil Ambrose]

Its been like it for 5 years and sold OK

But it only had delivery hours on it ie had done no cruising.

Phil

[Dalslandia]

Any boat (with motor/engine) will do more then a knot in gear at idle, most do 2-2.5-3 knots, I can't think it is ventilating at so low powers, in forward gear. and hardly not is reverse at idle/tickover.

 

Cavitating can happen as said if the propeller is put on backward, the blades is shaped for being efficient in forward gear, a slight cavitating in the short moments it is in reverse gear don't harm the propeller. but if was constant. I seen propellers on RC planes put on backward. so it can happen.