Stern thruster on Blue Angel

[BartW]

After using Blue Angel for 14 season without Stern thruster, I finally decided to add this feature,
Advice from the specialist here is welcome,

Drawing of the cross section of the stern:
Stern thruster (on scale) is BLUE

Problem: there is a support plate (RED), for the rudder cylinder
We need to make a cutout in that plate, to make space for the thruster motor
And we need to bring the thruster more backwards (Orange flange), not to interfere with that rudder cylinder (GREEN), and in order to not interfere the water Thrust with the trimflap cylinders (GREY)



The mounting of the T-tube with thruster assembly, should be done from the outside,
we have schecked all dims, and that should be doable

rear vieuw:



this is a photo looking to the stern bilge,
green dot is rudder cylinder
red dot is the support plate for this rudder cylinder
the black tube in the front is the P & SB rudder connection rod.


Advice welcome; if it is a goodf idea / practically doable to fit the stern thruster here, on that Orange flange (100mm thickness)

 

[Bouba]

The thruster doesn’t have to be on the centre line of the boat....if there is more room port or starboard you can put it there

 

[BartW]

The thruster doesn’t have to be on the centre line of the boat....if there is more room port or starboard you can put it there

left and right from the center position is a GRP box with the rudder shaft, and rudder tiller arm on top, so no space there
on the pic above you can see the P -side box and tiller arm.

more upwards no go bcs water thrust is pushing against hilo platform lift arms

 

[jfm]

My comments would be:

1. Remember the "support plate" as you call it (and I often call it the "rudder shelf") does two things: it supports the hydraulic ram as you say, but it also supports the tops of the rudder tubes. Both important jobs.
2. I would make the orange piece 150mm not 100, so that you chop away less of the rudder shelf. There is no harm in having 150mm
3. The design of the 150mm orange piece is important. You need (I think) to attach the stern thruster to the orange piece, THEN attach the orange piece to the boat. That's ok, if you design it that way. I would make the orange piece as a welded box from thick stainless steel plate, maybe 6mm for the box and 10mm for the transom flange. painted well. With a square flange that bolts to the transom with maybe 20 bolts (6 each side).
4. On the centreline!

Good luck!

 

[benjenbav]

Would it be possible to mount the assembly ~250mm higher to avoid the (red) support plate? Perhaps that would result in too much thrust interference from the hi-lo supports?

 

[benjenbav]

Would it be possible to mount the assembly ~250mm higher to avoid the (red) support plate? Perhaps that would result in too much thrust interference from the hi-lo supports?

Writing this before seeing post #3...

 

[BartW]

My comments would be:

1. Remember the "support plate" as you call it (and I often call it the "rudder shelf") does two things: it supports the hydraulic ram as you say, but it also supports the tops of the rudder tubes. Both important jobs.
2. I would make the orange piece 150mm not 100, so that you chop away less of the rudder shelf. There is no harm in having 150mm
3. The design of the 150mm orange piece is important. You need (I think) to attach the stern thruster to the orange piece, THEN attach the orange piece to the boat. That's ok, if you design it that way. I would make the orange piece as a welded box from thick stainless steel plate, maybe 6mm for the box and 10mm for the transom flange. painted well. With a square flange that bolts to the transom with maybe 20 bolts (6 each side).
4. On the centreline!

Good luck!

1. agree but that "rudder shelf " does already have 2 big cutouts in that zone, zoom in on the picture, just below the rudder ram you see the cutout for one of two stringers
I have been thinking to place a SS plate on top of the remaining wooden plate, but that might not be necessary.
2. yes 100mm was just the minimum, 150 is better, but have tobe carefull that the electric motor is not too deep in a tunnel re ventilation / cooling,
but that is no issue when we make a stainless steel box = next point: = bigger air volume around the motor. and SS housing is water cooled
3. fit a stainless steel box feels like a good idea. !

thanks !

 

[Bouba]

A different (more expensive) idea...is waterjet thrusters.....all pumps etc can be mounted far inboard away from all machinery

 

[jfm]

Would it be possible to mount the assembly ~250mm higher to avoid the (red) support plate? Perhaps that would result in too much thrust interference from the hi-lo supports?

I don't buy the whole "thrust interference" point. The effect is tiny. It's a red herring in my book. The thrust is developed right at the thruster blades themselves and what happens to the pumped water 1 metre away isn't that interesting.

 

[jfm]

A different (more expensive) idea...is waterjet thrusters.....all pumps etc can be mounted far inboard away from all machinery

Urgh!

 

[MapisM]

more upwards no go bcs water thrust is pushing against hilo platform lift arms

Not only that, but the deeper the better for any thruster, in principle.

OTOH, having said that, in your boots I would still consider the higher placement, for several reasons.
First, I agree with jfm that the thrust reduction due to the outlet flow interference shouldn't be high - if any.
In fact, what the thruster really needs is "solid" water to grab all around its inlet side, and I think there's enough space for that.
Second, that would eliminate the need of messing with the rudders shelf - a job that I would really hate doing, almost to the point of giving up the thruster idea for that reason alone!
Lastly, that would also place the motor in a drier and more accessible place, as opposed to the location in your drawing which puts it very close to the bilge moisture and harder to inspect/access.

Just another thought:
did you possibly check if there's any stern thruster driven by an external vertical shaft, connected to an horizontal one that goes through the stern at a higher position (akin to an outdrive)?
That would allow you to place the tunnel as low as as you want without needing the motor alignment.
Not sure if you see what I mean, but I half recall to have seen such contraption somewhere.
Though I can't for the life of me remember on which boat - let alone the thruster brand!

 

[jfm]

Yes I like that idea. Mount it fully 300mm higher, then you have a simple job, no cutting of the shelf and no 100/150mm orange piece.
And just don't worry about the H+B units/water flow. Makes a much simpler/faster job.
Will be a bit noisier with more white water as you use it, but that doesn't matter too much

 

[MapisM]

You bet it doesn't.
No thruster would ever be able to compete with the engines sound from BA straight stern exhausts!

 

[Bouba]

You could put cowlings on either side of the thruster if you feel it’s mounted too high

 

[Bran]

Have you considered mounting all of the stern thruster outiside the hull, with the motor vertical, all housed in an external box mounted onto the stern? Does have some compromises but I have seen it done on some smaller boats.

 

[BartW]

Not only that, but the deeper the better for any thruster, in principle.

OTOH, having said that, in your boots I would still consider the higher placement, for several reasons.
First, I agree with jfm that the thrust reduction due to the outlet flow interference shouldn't be high - if any.
In fact, what the thruster really needs is "solid" water to grab all around its inlet side, and I think there's enough space for that.
Second, that would eliminate the need of messing with the rudders shelf - a job that I would really hate doing, almost to the point of giving up the thruster idea for that reason alone!
Lastly, that would also place the motor in a drier and more accessible place, as opposed to the location in your drawing which puts it very close to the bilge moisture and harder to inspect/access.

Just another thought:
did you possibly check if there's any stern thruster driven by an external vertical shaft, connected to an horizontal one that goes through the stern at a higher position (akin to an outdrive)?
That would allow you to place the tunnel as low as as you want without needing the motor alignment.
Not sure if you see what I mean, but I half recall to have seen such contraption somewhere.
Though I can't for the life of me remember on which boat - let alone the thruster brand!

OK, good idea's and interesting developement here !

above that Ram, under that white cover is the hilo hydraulic pump,
iirc there is some room right below the shelf where this pump sits on, and the pump could go a tiny bit more up,

I just found this old picture with that pump in its position,



the oilcontainer of the pump sits on a grey shelf which is about 20cm above the floor level (grey wooden beams)
we just need to re arrange the mounting of that pump,
and move that White panel with those hydraulic valves.
Apart from the hoses, there is nothing else connected to this white panel.

down below you can see the rudder ram, green paint at that time,
this has been rebuilt and painted black since than.



yes, the wet environment down there was another big concern, that I intended to mention about,
but much less of a problem, when the thruster motor is that high, and above that rudder shelf


now I need someone there to take a few more dimensions

 

[BartW]

You could put cowlings on either side of the thruster if you feel it’s mounted too high

yes, but cowlings also create some resistance on the water flow from the thruster, = a bit less performance,
would be interesting to test with and without them.

 

[BartW]

Have you considered mounting all of the stern thruster outiside the hull, with the motor vertical, all housed in an external box mounted onto the stern? Does have some compromises but I have seen it done on some smaller boats.

not really, too complicated imo.

 

[BartW]

here is a drawing with the stern thruster in the "higher" position.

7



I quite like this, but in this position, most probably we need to remove the hydraulic pump for the platform which is just above, see pic in #16

so in practice it would be better if we could lower the thruster 100mm, until the electric motor is just above the (red) Ram shelf,

would it be a good idea to give the rudder connection rod this: \_______/ shape , in order to make space for and move that ram (green) 15cm to the right ?

 

[MapisM]

I'm not sure to follow your train of thought, B.
By making the connecting rod U shaped (leaving aside the fact that I don't think it's an easy modification), you would only gain a few centimeters of "free" space above it, but that wouldn't affect the steering ram, which as I understand is even more "in the way" than the connecting rod.
OTOH, if you wish to move the ram 15cm to port in order to free the centerline space above the rudders shelf for the thruster motor, I can see your logic, but that's a major modification because it would change the geometry of the whole system, and I think you should find another ram with the appropriate length/excursion - assuming that the remaining space between the ram and the port tiller is enough, because pretty sure you can't move the rudder stock!

I never looked carefully at the platform pump, but I would guess that moving it elsewhere should be the way to go.
After all, that doesn't have any mechanical placement constraint (in sharp contrast with the rudders!), so it should just be a matter of moving hoses and electrical connections, unless I'm missing something.
Not saying it's trivial, but IMHO much less risky than messing with rudders...

PS: on second thought, I think the only (relatively) not so difficult method for moving the ram away from the centerline would be to place it outboard of the port rudder, in a position exactly symmetrical to the current one.
That would allow to keep the same ram and all the geometries, with two BUTs:
1) the rudders shelf doesn't seem to extend much further outboard of the rudder, so you should find another solid base where to attach the ram, and
2) maybe also space and accessibility could be a problem?
Just a thought, anyway!