What are the drawbacks of bow thrusters?

[Group9]

The cost is the main drawback to me. There are other things that are more useful to spend our limited budget on. Now, if someone gave me one? I'd say thanks and would use the hell out of it!

[double u]

...if it were on the-secondhand-boat-that-we-absolutely-must-have I'd take it off, close the holes & sell the damn thing...

[Hobie_ind]

Originally Posted by Auspicious

My experience has been with the microswitches at top and bottom. Leakage is when the switches shut off retraction before the thruster is up against the seals. Talk to Bill Rouse. End of the world? No. Frustrating and time-consuming? Yes. The speed limit is in the boat manual. By the way, Amel produces the best boat manuals I have ever seen.

If you had leakage it also means you also didn't have the locking pin adjusted properly. The limit switches simply alarm and shut off the retracting mechanism. There's also a manual pin that ensures the entire assembly is locked up AND tight against the neoprene gasket that's on top of the retracting unit. After re-installation and new seals/gaskets, a quick check by squeezing the cable assembly would indicate whether the micro-switches and locking pin assembly required minor readjustment. A 5-10 minute job in my experience with reasonably good access to everything, so I wouldn't call it 'frustrating and time-consuming'.

[Auspicious]

Originally Posted by Hobie_ind

If you had leakage it also means you also didn't have the locking pin adjusted properly.

Yep. Remember delivery skippers get what they get, much like new boat owners.

The receiver for the locking pin had slipped down. Not worth turning around for but pretty much an irritant.

[IslandHopper]

Originally Posted by Auspicious

Simply not true. The drag is far from negligible. Any decent hydrodynamics course will show that, and the accounting department of a shipping company can show you the fuel costs.
.

Please re read my post! no shipping company i have worked for, including the one i currently work for, have justified the expense and added complexity of fitting doors over thruster tunnels to offset any performance loss, the cost of the drag is negligible compared to the expense of trying to eliminate it, hence why the overwhelming vast majority of commercial applications don't utilise doors and rely on best installation practices to minimise drag and fuel consumption.

Plus every ship i have been master off in the last 23 years has had multiple bow and stern thrusters and not one has had doors fitted, my present employer has about 100 ships fitted with thrusters and none of them have doors, in fact i have never worked on a ship fitted with doors, i know there out there but they are few and far between...

On vessels the op is talking about, any performance loss with a correctly fitted and fared thruster would be negligible and not an issue IMHO...

[a64pilot]

Originally Posted by Group9

The cost is the main drawback to me. There are other things that are more useful to spend our limited budget on. Now, if someone gave me one? I'd say thanks and would use the hell out of it!

That I think is really the gist of it more often than not.
I’d like one, wouldn’t want to lose the storage, but if I were often Marina hoping, I’d want one even more.
However after buying the stuff I wanted, and new upholstery for the Wife and an enclosed cockpit for her to shower in etc., it fell to the nice to have list.
However for people that rarely visit a Marina and most often anchor out, I think they are less useful. However we all visit the pump out or fuel dock on occasion and there were many times when I’ve watched a thruster equipped boat and listed for one.

[svmariane]

Bow thrusters? Please sir, thank you sir, I'd love that. Sir.

So okay.... We'd just bought our new (to us) 48 foot Celestial center cockpit.

All the razz-a-ma-tazz over, we slipped mooring lines from our side-tie and headed out for DAY ONE! And got caught by a cross breeze that drug us alongside the sailboat innocently docked ahead of us. Costly experience.

Whatever I thought I knew, whatever experience I'd had, only helped reduce the level of damaged.

Well motivated, I learned to drive my boat.
THIS boat with HER configuration.

Still...... Wish she'd had bow thrusters that day.

[Auspicious]

Originally Posted by IslandHopper

Please re read my post! no shipping company i have worked for, including the one i currently work for, have justified the expense and added complexity of fitting doors over thruster tunnels to offset any performance loss, the cost of the drag is negligible compared to the expense of trying to eliminate it, hence why the overwhelming vast majority of commercial applications don't utilise doors and rely on best installation practices to minimise drag and fuel consumption.

In school it was pretty clear that there is significant drag from an open tunnel. The analysis was supported by testing in the flow channel. In practice the customers we worked for had the same conclusions. Military ships, where speed is a priority but fuel economy still counts, fit doors. Commercial ships, where fuel economy is a priority but speed still counts, fit doors also.

I can't explain why your lines didn't use doors. I can speculate because not everyone does but that wouldn't be useful.

I won't sail around dragging a bucket behind a recreational boat. That is essentially what a thruster tunnel constitutes.

The science is clear. Think what you like.

[DumnMad]

This from a shipping article:

Appendage Resistance
For cargo vessels in calm water conditions, appendage resistance is about 2 to 3 percent. Roughly half of the appendage resistance is
attributable to the rudder and half to bilge keels. Rudder resistance can increase substantially in severe wind/weather conditions or for directionally unstable ships.
Added resistance from a bow thruster tunnel can be signi cant (in the range of 1 to 2 percent of calm water resistance).

[IslandHopper]

Originally Posted by Auspicious

The science is clear. Think what you like.

It ain't a case of what i think ok, it's a case of whats out there, and no matter what you think the overwhelming majority of ships plying the oceans with tunnel thrusters do not have doors fitted........fact!

[IslandHopper]

Originally Posted by DumnMad

Added resistance from a bow thruster tunnel can be significant (in the range of 1 to 2 percent of calm water resistance).

And yet every year an ever increasing number of ships are fitted with tunnel thrusters, new builds and retro fitting. It used to be that tunnel thrusters where mainly in the domain of Offshore Supply and Support Vessels (where i work) or tugs but more and more ship types are seeing the value in there use, shuttle tankers, cruise ships, gas buggies, container ships etc are all routinely fitted with them regardless of performance figures, why is this so when ship owners are notoriously tight fisted with money?..... because they can see the benefits far outweigh any drawbacks thats why, they crunched the numbers and fitting thrusters comes up trumps

[double u]

could the reason be that cargoships operate very much slower than their theoretical hullspeed- so main resistance is surface drag - & that their hullshape - parallel vertical sides! - are optimized for something different than maximum hydrodynamic efficiency?

[Jim Cate]

Originally Posted by double u

could the reason be that cargoships operate very much slower than their theoretical hullspeed- so main resistance is surface drag - & that their hullshape - parallel vertical sides! - are optimized for something different than maximum hydrodynamic efficiency?

Excellent point! I'd appreciate one of our resident NAs commenting on the difference in the effect of such parasitic drag when operating at very different speed/length factors.

Jim

[a64pilot]

I would assume big cargo ships have thrusters because either
Increased fuel burn is less than what a tug would cost and with thrusters, they may not have to pay a tug?
Or, they go places where no tug is available, and they need thrusters.
Doors may add to cost and complexity and maintenance, and the costs may be more than the decrease in fuel burn would be, cause I can assure you from what little I Know about aerodynamics, there is increased drag cause laminar flow is interrupted.
How much? I don’t know, may well be so little with a properly faired tunnel that it’s irrelevant at our speeds, or it may be like a fixed prop.
I suspect neither, I suspect it’s in between.

However I guess it’s interesting, but sort of irrelevant what a big ship does, cause not everything scales? For example I assume you don’t see bulbous bows on boats our size, cause it doesn’t work for little boats?

Do many big ships inject air into the boundary layer of their hull? Cause that should reduce drag, but the cost of the system and operating it may cost more than the fuel saved as a for instance.
Sometimes based on costs, fuel is cheaper than fuel saving measures. I bet their cruise speed is calculated on how much more do they make if crossing times are shorter vs fuel costs of going faster. They go with what gives them the most profit of course.
1960’s I think an experimental jet was built that vacuumed the boundary layer. It worked, decreased drag significantly, but was expensive and I guess a real bear to maintain, so it was never fielded.

[Pelagic]

I find this a silly question.
A mechanical aid that can de-stress any complicated cross wind maneuver in tight places has any meaningful disadvantages?

It is like arguing that using GPS will make you a less safe navigator.....