Tuesday, 14 February 2012

Bow Thrusters

Well the spray foam contractor didn’t arrive yesterday and as of now, he hasn’t contacted the yard to say he will be here today.  On a more positive note, the welder has arrived.  He and I have discussed the movement of the porthole opening in the galley.  However he has some other tasks to be completed first.  Maybe our porthole and cabin doors will be done later today or tomorrow!

Recently Bob left a comment asking about bow thrusters.  This appears to be another of those cassette –v- pumpout subjects.  Four years ago, when we initially started designing Waiouru we asked ourselves whether we actually needed a bow thruster.  Our hire boats never had them and many privately owned boats don’t.  Obviously the early narrowboats didn’t have one and they managed.   In the end we decided to include the tube for a bow thruster in our shell specifications.  If money was available we might fit one.  It might also be a selling feature when our life afloat ends.

Having decided to fit a tube I then researched size.  IMHO size IS important. Smile  A narrowboat is long, thin and quite heavy.  The purpose of the bow thruster is to push the bow sideways.  I assumed the bow thruster would only be used because the circumstances (wind, current, etc) preclude the manoeuvre being completed using just the engine.   So the bow thruster will need some “grunt”!  I therefore specified a 10hp unit  This appeared to be about the largest available for a narrowboat. 

Bow thrusters can either be electric or hydraulic.  The hydraulic version usually runs from a pump mounted on the engine and it can be operated for prolonged periods.  The electric version is usually connected to lead-acid batteries which are located in close proximity.  The batteries can be quickly drained or overheat if the bow thruster is used for for more than brief periods.

In the end I decided if we had a bow thruster, then it would be infrequently used, (I hope!) so we should fit an electric version.

Obviously we were on the far side of the world when Waiouru was first being built and we had expected the boat builder to ensure our contract specifications would be met.  Waiouru has a bow thruster tube but there was no weed hatch.  The diameter of the tube is 190mm, which isn’t sufficient for a 10hp unit.  My friend (google) informs me the maximum size unit we can fit is 8hp.  This has a blade diameter of 185mm. 

After examining the bow thruster locker I realised there was probably sufficient room for the bow thruster to the left (port side) of the weed hatch.  It would be close to the left end of the tube making it easier to remove any trapped foreign objects.  More importantly, it would free up the rest of the locker for storage.  However the boatyard staff have informed me the unit must go in the middle of the tube.  I remain unconvinced!  All the unit does is push water out of the tube one end and suck it in the other.  I accept there would be greater resistance and some potential loss of power sucking water from the ‘long’ end of the tube.  However I suspect this would be negligible.  So I’m waiting to see the evidence (rather than opinion) that my preferred location for the bow thruster is incorrect.

At the moment the plan is to run the wiring for the bow thruster but delay any decision regarding purchase and fitting until we see what our finances look like towards the end of the build.


Rube said...

Hi Tom,
Thank you so much for your detailed explanation on bow thrusters. It ends up being more complicated than I thought, and also expensive, like 3000+ for Hydraulic. And apparently really not a must have. Looking at it from someone that has never seen a narrowboat up close all I could imagine is a 60' long flat bottom boat in a strong cross wind in a narrow sometimes crowded canal. Probably in those conditions I would rather sit tight and read a good book. Wishing you much good luck with your build, it looks like you have done your homework well. And Jan the blanket is beautiful.

Maffi said...

Hi Tom,
I can't quite get the logic of not putting the BT at one end or the other. Whilst I have never had a problem with crap getting up the thruster tube in the last five years it would certainly be very useful to have it offset now.

I have recently ingested my bow rope. Had the BT been set at one end the removal of the rope would have been considerably easier.

If there is an imbalance in thrust it is not so important. It only ever goes in one direction at a time. Go with your thoughts unless any one can give a 'good' reason why not.

You might like to consider being able to remove the BT while the boat is in the water, something which I cannot do.

Tom and Jan said...

Hi Maffi,
My logic is the principle of the bow thruster is very similar to the jet boat (invented in NZ) where water is sucked in one end of a tube and pushed out the other to propel the craft. I personally see the precise location of the impeller as irrelevant.
I would have preferred to have the bow thruster inside the tube weed hatch, however I was never given that choice. If (as I would like) the impeller is located between the end of the tube and the weed hatch then it should be easier to clear any obstruction by either getting to it via the end of the tube or by reaching down the weed hatch.
I'm still waiting for analytical reason why my location is invalid.

Heth said...

Hi Tom,

Good idea not to go with the hydraulic kind, old, out-dated, slow to react. Basically useless.

However, a word of advice for the electric kind, batteries & the setup. Don't use a battery charger to charge the batteries. You'll have very limited use with them draining quickly like you said. And re-charging may restrict the whole idea of having a bow thruster!

The answer, go for a "splitter" off the engine starter alternator. It will supply an endless source of power to your bow thruster batteries. No need to monitor seconds.

Dave has driven our boat backwards for half a mile using the bow thruster to steer with. We did that a few times last summer, saved us going up 7 locks to wind. We discovered a secret windy hole after 3 locks but it meant backing up to get where we wanted to moor up!! No loss of power. And your bow thruster is ready for use as soon as you start your engine.

It's the best, most efficient setup there is & its thanks to a friend of ours who's a boat engineer that we had ours fitted that way.

Takey Tezey Heth

Tom and Jan said...

Hi Heth,

Good point about the charging system. The only issue I can see would be the size of the cables needed between the stern of the boat and the bow. Too small and there would be a significant voltage drop.