Water line vs uxter plate

[Telstar]

Hi All,

 

Just wondering how far the water line should be above the uxter plate on a narrowboat.

I recently had to remove some of my engine room ballast after running aground.

I noticed that the narrowboat was considerably slower at my normal cruising engine rpm.

On re-instating the ballast, boat speed is up again.

Hence the question of optimum water line/uxter plate level.

Hoping there's some interesting technical data on the matter.

 

Cheers, Andy.

 

[Tony Brooks]

People who know often say about one inch but I don't think it's that critical within an inch or two. The important thing is to have the uxter plate deep enough to prevent air being drawn down into the prop disk (ventilation). Yours may well have been ventilating with the ballast  removed.

 

FWIW, when I ballasted my uxter plate a bit lower I found I get far less of a cockscomb spewing up behind the boat and it seemed to swim a bit better. That may have been to do with the shaft angle now pointing down a little though.

 

Others may well have different experiences and a different view.

[BEngo]

The ideal position for the uxter plate is just sufficiently immersed that it does not go 'slap slap slap' all the time when moored stern to wind.

 Any deeper and you just get more drag, particularly as the stern digs in when moving.

Not enough immersion and you will get ventilation- when the prop draws in air. (Not cavitation; that is caused by pressure differences across the bats allowing steam bubbles to form and then collapse).

N

[Keeping Up]

When our boat was built, the plate was only just below the water line at the extreme stern. I always reckoned it would be better if it were slightly lower, and this was proved to be the case when the extra weight of overplating lowered it by between 1 & 2 inches. The boat goes slightly faster at the same engine revs, steers more easily and positively, and most of all stops in a much much shorter distance because going into hard reverse doesn't lift it clear of the water to suck air in.

[Tony Brooks]

I suspect the answer may well be influenced by the shape of the sternpost and the length of the rear swim. A blunt sternpost and a short swim would make it more difficult for the prop to suck water into itself from alongside the boat so may tend to draw more water and thus air down from the surface.

[Murflynn]

2 hours ago, Tony Brooks said:

 

 

FWIW, when I ballasted my uxter plate a bit lower I found I get far less of a cockscomb spewing up behind the boat and it seemed to swim a bit better.

 

 

you're driving too fast.    

 

narrowboats are not hydroplanes. 

 

 

as a non-narrowboat driver I am always impressed by the occasional boat that sweeps past with next-to no disturbance of the water, while others seem to make a huge wave whilst making only average progress   ......   any ideas which are the hulls that swim well, and which are the worst offenders?

Edited April 9, 2021 by Murflynn

[Telstar]

2 hours ago, Murflynn said:

as a non-narrowboat driver I am always impressed by the occasional boat that sweeps past with next-to no disturbance of the water, while others seem to make a huge wave whilst making only average progress   ......   any ideas which are the hulls that swim well, and which are the worst offenders?

 

Hi Murflynn,

As a seasoned boater (but only recently narrowboats) I can answer this one.

The longer the boat, the smaller the bow wave at any given speed.

 

Some good reading here.

https://en.wikipedia.org/wiki/Hull_speed

 

Andy.

[Telstar]

Thanks for the comments on waterline vs uxter plate everyone.

 

With my ballast removed, the uxter plate is still an inch or so below the surface.

When under power (3-4mph) there is a clear thrusting tail of water from the stern.

No audible ventilation.

 

With the ballast in place, I guess the uxter plate may be 2-3 inches below the surface.

Under power, the thrusting tail is very short and more of a boil.

But as previously stated appears faster on the gps per shaft rpm.

 

I'll leave in place but good to understand.

 

Andy.

[Murflynn]

16 hours ago, Telstar said:

 

Hi Murflynn,

As a seasoned boater (but only recently narrowboats) I can answer this one.

The longer the boat, the smaller the bow wave at any given speed.

 

Some good reading here.

https://en.wikipedia.org/wiki/Hull_speed

 

Andy.

 

Fully aware of that, thanks, but some hulls are better than others of the same length.

[Jen-in-Wellies]

Next time you see a boat that seems to swim really well, without a lot of wash, ask the owner to lift it out, or turn it upside down so you can take a look at the hull shape. ?

[Bee]

Narrowboats are pretty much the only boat I can think of that have a big flat plate over the prop. I can't think of any benefit to this apart from, as Bengo says, to prevent air being drawn into  the prop and even then the modern device of a weed hatch negates this. My boat has a counter more like a sailing boat (or Titanic) the boat goes perfectly well in forward but its right on the borderline of prop immersion, if I stick it in reverse the stern lifts and doesn't hold back as well as it should. I think that its not the immersion of the uxter plate that is the issue but just getting the prop down that last inch or so to prevent it sucking air.

[Murflynn]

22 minutes ago, Bee said:

Narrowboats are pretty much the only boat I can think of that have a big flat plate over the prop. I can't think of any benefit to this apart from, as Bengo says, to prevent air being drawn into  the prop and even then the modern device of a weed hatch negates this. My boat has a counter more like a sailing boat (or Titanic) the boat goes perfectly well in forward but its right on the borderline of prop immersion, if I stick it in reverse the stern lifts and doesn't hold back as well as it should. I think that its not the immersion of the uxter plate that is the issue but just getting the prop down that last inch or so to prevent it sucking air.

 

I really don't understand why you think there is a distinction.  From experience with various types of work barges, including deck-mounted outboard propulsion units, I can assure you that a propeller will create a negative pressure on the 'upstream' side which will tend to draw in air, which tendency is limited/controlled either by being deeply immersed or by being separated from the air by either a cowl or by a structure like an uxter plate.  With an uxterplate the air is 'socially isolated' from the prop by about 60cm of water, which is not easily overcome.   A prop with no isolation (like a conventional outboard) needs to be 'well deep' to achieve the same result. 

 

In the case of a narrowboat lowering the uxter plate by an inch probably has the same effect as lowering an unguarded prop by at least 5 times as much.

[TheBiscuits]

42 minutes ago, Bee said:

a big flat plate over the prop. I can't think of any benefit to this apart from, as Bengo says, to prevent air being drawn into  the prop and even then the modern device of a weed hatch negates this.

 

That's why most of them have a plate across the bottom as well as the top ...

[ditchcrawler]

56 minutes ago, TheBiscuits said:

 

That's why most of them have a plate across the bottom as well as the top ...

Even without it where is the air going to come from to feed the prop. any locked in there will soon be drawn away. Lifting my weed hatch often breaks a vacuum 

[Jen-in-Wellies]

1 hour ago, Bee said:

Narrowboats are pretty much the only boat I can think of that have a big flat plate over the prop. I can't think of any benefit to this apart from, as Bengo says, to prevent air being drawn into  the prop and even then the modern device of a weed hatch negates this. My boat has a counter more like a sailing boat (or Titanic) the boat goes perfectly well in forward but its right on the borderline of prop immersion, if I stick it in reverse the stern lifts and doesn't hold back as well as it should. I think that its not the immersion of the uxter plate that is the issue but just getting the prop down that last inch or so to prevent it sucking air.

The purpose of the uxter plate and the motor narrow boat curved stern was to protect the propeller and rudder when towing an unpowered butty. Now most people don't tow a butty, but the design remains for the look of the thing. It also gives extra space on the back deck and provides some protection to the rudder and prop when in reverse.