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Old 13-02-2019, 04:13   #1
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Bow thruster with a long tunnel

Our crew of 2 is pretty proficient with maneuvering Entropy, a 43 ft Beneteau Sense Monohull (11 t) in close quarters. However, there have been tight situations where only luck prevented damage, and in some situations it would be extremely reassuring to have an oh-crap button which might turn the tide.

My take on a bow thruster is that I'd consider it as an insurance rather than a "standard maneuvering device". I'd still plan maneuvers to be done without it and also not use it unless actually needed.

I have read all the existing CF threads about Bow Thrusters and have a pretty good impression on the pros and cons of adding a Bow Thruster to a boat.

For our situation I'd appreciate thoughts and opinions if it actually makes sense technically and from a usability standpoint.

Situation:
We have the boat at a Beneteau authorized workshop, and they have obtained the original Beneteau plans how to add an aftermarket bow thruster to the boat. I am confident that the workshop does a good job, and the quotation (~4900 EUR for thruster, cabling, work, ~2300 EUR for lamination works and ~1750 for woodwork on the berth) looks not exactly cheap but OK to me.

The 43 ft Monohull has a large water tank (360 l) below the front berth. It is not possible to install a thruster in the bow in front of the tank, rather it would have to be done behind the water tank, directly below the "head" end of the bed.

This would mean that the large wooden drawer below the berth would have to be removed, in its place the tunnel and the thruster would have to be installed.

Now the problem I see is that this is a non-optimal place for the tunnel for various reasons
- it is about 3 m from the bow, reducing torque during operation
- the tunnel would have to be quite long, about 1.6 - 1.8 m. The thruster manufacturer recommends a maximum length of 6 times tunnel width, that would be 1.1 m.
- with the thruster mounted in the middle of the tunnel, how would one do the maintenance on the propeller and anodes? I doubt that an arm could reach the propeller from the side. This sounds like a huge maintenance problem to me.

I am interested in opinions of fellow sailors if this sounds more like a "it looks like a problem but in practice it isn't" or "don't do it, you'll waste space and money for a solution which will generate more problems than it solves".

If anybody has experience with a similar layout, i. e. a very long bow thruster tunnel, I'd appreciate some feedback how this works out.
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Old 17-03-2019, 15:14   #2
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Re: Bow thruster with a long tunnel

Apologies to bring this up again. The original article was admittedly probably a case of TLDR.

I am visiting the boat tomorrow and will have to make a go/no-go decision about the bow thruster installation this week.

With such a long tunnel for a bow thruster I fear that maintenance will be difficult or not possible at all if one cannot reach the propeller with an extended arm from the side (e. g. for changing the zinc anode).

I am inclined not to install the thruster primarily for this reason, but also I have doubts about the efficiency of this setup.

Any opinion are appreciated, in particular of persons with first hand experience ("my boat has a 2.5 m tunnel and all is just fine" would help).
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Old 17-03-2019, 15:18   #3
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Re: Bow thruster with a long tunnel

Personally I wouldn’t go near an installation like that — how on earth are you going to maintain it? Bowthrusters need looking after.

Is there no possibility of fitting a drop-down thruster in the same position? At least you’ll be able to get to one of those. And the thrust will be significantly greater without the very long tunnel.

When you say there have been “tight situations”, are those situations ones that you have been able to learn from and avoid happening in the future? If so, why the urgency for a non-optimal bowthruster installation?

I don’t know about your particular boat, but it seems likely it would behave similarly to mine. I tend to do most complex manoeuvres stern-first, which is far more controllable single-handed (and I can hook any line on from the stern rather than on a traditional two-person setup with an older design boat where you would work from amidships). I find that, if the bow is not behaving itself, a small whack of forward power with the rudder set around half-way one side or the other will make the bow leap to the side without significantly otherwise affecting the motion of the boat into the berth. It’s remarkable how much difference this makes to close-quarters manouvering. Since learning how to do this delicately on my particular setup I find that I have my bowthruster after all, and without cutting any holes at all.
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Old 17-03-2019, 15:22   #4
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Re: Bow thruster with a long tunnel

Get a drop down. No worries about fouling and very serviceable when dropped.
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Old 18-03-2019, 00:16   #5
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Re: Bow thruster with a long tunnel

Quote:
Originally Posted by Tillsbury View Post
Personally I wouldn’t go near an installation like that — how on earth are you going to maintain it? Bowthrusters need looking after.
Thanks for your opinion. Those were exactly my worries. Will likely not do it after all.

Quote:
Is there no possibility of fitting a drop-down thruster in the same position? At least you’ll be able to get to one of those. And the thrust will be significantly greater without the very long tunnel.
Unfortunately not. It's a "drawer" below the head end of the cabin's bed, and below it there is a bulk of oddly shaped polyester hull material below it to support the water tank and the bedding. No clean access to the hull "downwards".

Quote:
When you say there have been “tight situations”, are those situations ones that you have been able to learn from and avoid happening in the future? If so, why the urgency for a non-optimal bowthruster installation?
Yes, absolutely, we have learned a lot and avoid or properly address those
situations now.

Quote:
I don’t know about your particular boat, but it seems likely it would behave similarly to mine. I tend to do most complex manoeuvres stern-first, which is far more controllable single-handed
...
Since learning how to do this delicately on my particular setup I find that I have my bowthruster after all, and without cutting any holes at all.
Yes, absolutely the same experience. Stern-first is our go-to approach for most docking, in tight quarters or against the wind. It works amazingly well.

It was the extra insurance I was after - and preparing the boat for less experienced helmsman. (There's family currently acquiring licenses eager to use the boat as well... )
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Old 18-03-2019, 01:53   #6
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Re: Bow thruster with a long tunnel

Not ideal but could they install the unit towards one end of the tunnel to make access easier?

I suspect it won't make a big difference if it's 1.5' from one side and 4.5' from the other when in operation. Does the unit need access from both sides for regular maintenance?

Then again 10k can pay for a pretty big oops (especially if insurance is covering the bulk of it)
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Old 18-03-2019, 02:40   #7
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Re: Bow thruster with a long tunnel

There was a another variation on the theme announced a couple of years ago. Effectively a large pump int he middle of the boat with four outlets, one at each corner. The pump moved water from one side to the other through 4" fire hose like pipes as required by a joy stick. Something like this might be easier to fit and service.

https://www.hollandmarineparts.nl/je...s/bow-thruster
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Old 18-03-2019, 02:53   #8
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Re: Bow thruster with a long tunnel

One thought towards installing the prop of to one side. Be aware that you might suck more air in if you are to close to the side.
One of the reasons it is designed by the yard to go in this location is probably that the thruster needs a certain depth below DWL. Move it forward means likely also to move it closer to the surface which can cause the same issue.
Possibly check with the yard or other owners how they deal with the maintenance.

Good luck,

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Old 18-03-2019, 02:59   #9
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Re: Bow thruster with a long tunnel

Quote:
Originally Posted by Pete7 View Post
There was a another variation on the theme announced a couple of years ago. Effectively a large pump int he middle of the boat with four outlets, one at each corner. The pump moved water from one side to the other through 4" fire hose like pipes as required by a joy stick. Something like this might be easier to fit and service.



https://www.hollandmarineparts.nl/je...s/bow-thruster


Yes, we pondered this briefly. Problem is that the drive is much less efficient than a traditional bow thruster. It also requires some elaborate plumbing in the boat. Does not work for our setup.
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Old 18-03-2019, 03:02   #10
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Re: Bow thruster with a long tunnel

Quote:
Originally Posted by valhalla360 View Post
Not ideal but could they install the unit towards one end of the tunnel to make access easier?

I suspect it won't make a big difference if it's 1.5' from one side and 4.5' from the other when in operation. Does the unit need access from both sides for regular maintenance?

Then again 10k can pay for a pretty big oops (especially if insurance is covering the bulk of it)


Asymmetric installation is not so good, it‘s a dual prop design.

And you are totally right regarding the 10k oops buffer...

So far all points towards „won‘t do it“. Thanks for all your input, folks!
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Old 18-03-2019, 03:26   #11
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Re: Bow thruster with a long tunnel

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Originally Posted by mbartosch View Post
I am visiting the boat tomorrow and will have to make a go/no-go decision about the bow thruster installation this week.
bart, don't do it. You are under pressure to make a decision of huge cost impact with a mediocre installation. Walk away from it.

You guys have been maneuvering your boat adequately such that this is not mission-critical, it is only a luxury. Don't be pressured.

Put the money into a strong engine on an inflatable and use that as a tugboat if you need to. Or teach your wife to toss a line to a piling and use your capstan winch to manage the bow in heavy winds. There are alternatives.
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Old 18-03-2019, 04:31   #12
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Re: Bow thruster with a long tunnel

Bow thrusters should be located as far forward, and as deep as possible.
The optimal tunnel length is 2 to 4 x tunnel diameter, and you should avoid tunnels longer than 6 to 7 times the tunnel diameter, as the performance reduction is then clearly noticeable.
An attempt should be made to locate the propeller in the midship plane. In short tunnels the propeller is located eccentrically on the port side, in order to improve the thruster performance to starboard.
Generally the top of the tunnel should be an absolute minimum of 1/2 x the tunnel diameter below the water surface. It's often recommended that it is at least 3/4 x tunnel diameter below the surface and even more is better.
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Old 18-03-2019, 18:21   #13
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Re: Bow thruster with a long tunnel

Quote:
Originally Posted by mbartosch View Post
It was the extra insurance I was after - and preparing the boat for less experienced helmsman. (There's family currently acquiring licenses eager to use the boat as well... )
Well that’s easily addressed. Inexperienced helmsmen get to do every mooring that’s uncomplicated with plenty of room and little wind. They will then gain experience just like you.
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Old 18-03-2019, 19:53   #14
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Re: Bow thruster with a long tunnel

I don't know your boat but would it not be possible to replace the water tank with a smaller one that would allow you to install the thruster in a better position. If you need more water capacity, an additional tank could be added in the space that you thinking about using for the thruster at the moment.

Sidepower also makes a streamlined external thruster that might work for you. Expensive but it might save you a lot of the cost of modifying your current layout - no tunnel, no modification the the woodwork on the berth.
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Old 18-03-2019, 21:28   #15
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Re: Bow thruster with a long tunnel

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Sidepower also makes a streamlined external thruster that might work for you.
Absolutely that needs to be installed in front of the crash bulkhead! If you're cruising then eventually that big thing sticking down in the bow will be torn off when hitting a log, whale or very determined jellyfish... with a big hole ripped into the hull you'd better make sure to have a sturdy watertight bulkhead behind it.
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