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How is the bowthruster's power reduced by the boats speed?


Michael Donning

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Hi all!

I plan to add a bow thruster in my little computer game (some people asked for it). I know how it generally works and I also know that they have virtually no effect above at "higher" speeds. (One reason why they use tug boats here on river Elbe.)

But there wasn't one on our hireboat last holidays (didn't miss it) and therefore I have no idea how the effect is specifically on narrowboats.

What is your personal experience? How does the effect of a bowthruster decrease at i.e. tickover, around "cruise speed" (3.5 mph) or even on higher speeds on rivers on a narrowboat (or widebeam)?

I'd be happy to read from you. Thank you!

Michael

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24 minutes ago, Michael Donning said:

Hi all!

I plan to add a bow thruster in my little computer game (some people asked for it). I know how it generally works and I also know that they have virtually no effect above at "higher" speeds. (One reason why they use tug boats here on river Elbe.)

But there wasn't one on our hireboat last holidays (didn't miss it) and therefore I have no idea how the effect is specifically on narrowboats.

What is your personal experience? How does the effect of a bowthruster decrease at i.e. tickover, around "cruise speed" (3.5 mph) or even on higher speeds on rivers on a narrowboat (or widebeam)?

I'd be happy to read from you. Thank you!

Michael

I have always worked on a rule of thumb that  transverse  bow thrust effectiveness drops off quite quickly  as the boat gathers speed through the water and is usually pretty ineffective once around 2 - 2.5 mph is reached. There are other types of thrusterss which are more effective at higher speeds but these types are not usually fitted to shallow draft canal boats.

 

Howard

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33 minutes ago, Michael Donning said:

Hi all!

I plan to add a bow thruster in my little computer game (some people asked for it). I know how it generally works and I also know that they have virtually no effect above at "higher" speeds. (One reason why they use tug boats here on river Elbe.)

But there wasn't one on our hireboat last holidays (didn't miss it) and therefore I have no idea how the effect is specifically on narrowboats.

What is your personal experience? How does the effect of a bowthruster decrease at i.e. tickover, around "cruise speed" (3.5 mph) or even on higher speeds on rivers on a narrowboat (or widebeam)?

I'd be happy to read from you. Thank you!

Michael

Directional stability increases with speed through the water requiring a greater turning moment to be generated by a bowthruster to achieve the same rate of turn than at lower speeds. 

The apparent reduction in bowthruster effectiveness at higher speeds is illusory. In reality it is that the fixed amount of side thrust (turning moment) is overcome by increased directional stability.

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4 minutes ago, howardang said:

I have always worked on a rule of thumb that  transverse  bow thrust effectiveness drops off quite quickly  as the boat gathers speed through the water and is usually pretty ineffective once around 2 - 2.5 mph is reached (...)

Thank you. That helps much.

Just now, RLWP said:

What on earth would you use a bowthruster for at cruising speed?

I won't ask people for reasons why the use the bowthruster at a specific speed. I just want to be sure to simulate the correct effect (= nil ) when somebody tries it out.

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4 minutes ago, Michael Donning said:

I won't ask people for reasons why the use the bowthruster at a specific speed. I just want to be sure to simulate the correct effect (= nil ) when somebody tries it out.

Can you simulate reaching inside the boat to find two badly located buttons out of sight from the tiller, often one above the other so you have to guess which is left and which is right, then not being sure if they are working because you don't notice any difference?

:D

Richard

Edited by RLWP
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10 minutes ago, RLWP said:

What on earth would you use a bowthruster for at cruising speed? 

Richard

 

Steering the boat. 

One occasionally sees (or more particularly, hears!) a NB progressing slowly along the canal whooping away repeatedly as the BT is used instead of the rudder. 

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3 minutes ago, RLWP said:

Can you simulate reaching inside the boat to find two badly located buttons out of sight from the tiller, often one above the other so you have to guess which is left and which is right, then not being sure if they are working because you don't notice any difference?

Sure, I could simply display a black screen for some seconds. Would not be much different than simulating the effect of a bottle of wine on the helmsman (consumption, not the other).

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15 minutes ago, Mike the Boilerman said:

 

Steering the boat. 

One occasionally sees (or more particularly, hears!) a NB progressing slowly along the canal whooping away repeatedly as the BT is used instead of the rudder. 

I often see people reversing at speed and using the bow thruster for steering. :)

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Mine only has any effect when the boat is moving at very low speeds, but for single handling a big boat it can be very useful indeed. It's also great for reversing. After 13 years it's never failed me (touch wood) - unlike the main engine which I stupidly came to rely on but which failed on a tidal river! :lol: Any equipment can fail so that's not really an argument for not having it. 

11 minutes ago, Chewbacka said:

Once the bow thruster has sucked in a carrier bag or two it doesn't work at all.  To make your simulation more accurate, don't forget to often have the canal too shallow which plays havoc with the finer points of steering.

Yes, to my annoyance on several occasions I've found the same applies to the main prop. I've had all sorts wrapped around it, coal bags, bike tyres, etc. Yet oddly I've never once had anything caught around the BT prop. Probably because it's not spinning much of the time. You're much more likely to foul your main prop than a BT if you use it correctly and sparingly.

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Put a grill over the bow thruster tube to reduce the crud thats sucked in . Reduces the thruster very slightly but your blades last longer  . Also have control buttons on the control (morse ) handle , no need to go inside then . Bunny 

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On 29/12/2017 at 10:06, Michael Donning said:

Hi all!

I plan to add a bow thruster in my little computer game (some people asked for it). I know how it generally works and I also know that they have virtually no effect above at "higher" speeds. (One reason why they use tug boats here on river Elbe.)

 

Could it be a Bernoulli effect thing that causes this? The speed of the water rushing past the ends of the bow thruster tube as the boat moves decreases the pressure in the tube. The bow thruster has to use so much effort to overcome the pressure drop that the force of water coming out the end drops and eventually ceases? Dunno. My brain isn't working too well this New Years Day morning for some reason :unsure:.

Jen

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Just now, Jen-in-Wellies said:

Could it be a Bernoulli effect thing that causes this? The speed of the water rushing past the ends of the bow thruster tube as the boat moves decreases the pressure in the tube. The bow thruster has to use so much effort to overcome the pressure drop that the force of water coming out the end drops and eventually ceases? Dunno. My brain isn't working too well this New Years Day morning for some reason :unsure:.

Jen

I think it’s simply momentum, which others have described as ‘directional stability’. 

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1 minute ago, WotEver said:

I think it’s simply momentum, which others have described as ‘directional stability’. 

Yes that makes more sense. The momentum makes the turning effect of the thruster seem a lot less as you travel a long distance while it happens.

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I think if you are just still the thruster pushes with only the momentum to fight, when going forward the shape of the bows and hull will make the boat go straight and you then have to overcome the resistance of the flowing water over the hull.  Not sure if this makes sense.......

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3 minutes ago, Chewbacka said:

I think if you are just still the thruster pushes with only the momentum to fight, when going forward the shape of the bows and hull will make the boat go straight and you then have to overcome the resistance of the flowing water over the hull.  Not sure if this makes sense.......

Yeah, that makes sense. So not simply inertia but the shape of the hull working along with that momentum to keep it moving straight. 

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1 minute ago, Chewbacka said:

I think if you are just still the thruster pushes with only the momentum to fight, when going forward the shape of the bows and hull will make the boat go straight and you then have to overcome the resistance of the flowing water over the hull.  Not sure if this makes sense.......

 

It does, but it’s hard to explain the process by which this happens. 

Similarly in a crosswind, my boat gets blown sideways when dead stationary, but even the slightest forward motion stops the boat being blown sideways. 

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3 hours ago, Chewbacka said:

I think if you are just still the thruster pushes with only the momentum to fight, when going forward the shape of the bows and hull will make the boat go straight and you then have to overcome the resistance of the flowing water over the hull.  Not sure if this makes sense.......

Bowthrusters seem to work well when reversing. 

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On 12/29/2017 at 10:06, Michael Donning said:

Hi all!

I plan to add a bow thruster in my little computer game (some people asked for it). I know how it generally works and I also know that they have virtually no effect above at "higher" speeds. (One reason why they use tug boats here on river Elbe.)

But there wasn't one on our hireboat last holidays (didn't miss it) and therefore I have no idea how the effect is specifically on narrowboats.

What is your personal experience? How does the effect of a bowthruster decrease at i.e. tickover, around "cruise speed" (3.5 mph) or even on higher speeds on rivers on a narrowboat (or widebeam)?

I'd be happy to read from you. Thank you!

Michael

Are you using a physics model in the game?

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46 minutes ago, Richard10002 said:

Bowthrusters seem to work well when reversing. 

See the comments above about how  thrusters becoming less effective when speed through the water increases - especially after it reaches around 2-2.5 mph.  This applies equally to moving ahead and astern, but as a generality astern speed is usually slower and therefore doesn't reach this "fall off" speed. Try going faster in reverse and you should find a significant drop off in BT effectiveness.

Howard

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5 hours ago, Chewbacka said:

I think if you are just still the thruster pushes with only the momentum to fight, when going forward the shape of the bows and hull will make the boat go straight and you then have to overcome the resistance of the flowing water over the hull.  Not sure if this makes sense.......

 

2 hours ago, Richard10002 said:

Bowthrusters seem to work well when reversing. 

These observations seem to fit well together, as going astern the water is moving across the flat sides of the hull, rather than across the shape of the bows.

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