Stern Thruster Performance

R

R Baker

Well-known member
Joined
May 15, 2015
Messages
196
Fluid Motion Model
C-24 C
I posted awhile back of the performance of the stern thruster on my 29. I t works well when moving the stern to starboard but poorly to port. I believe the rudder partially blocks the flow from the thruster. I imagine this must be common to most tugs as the thruster is located on the port side of the stern. As well the cowling reduces the effectiveness of the flow. It is however necessary due to the shallow installation. Can anything be done to improve its performance to port ? I wonder if a larger rudder would be an option in lieu of a stern thruster.
 
The rudder size is engineered for the boat. I know of another tugger who experimented with using a larger rudder. He went with the original. The larger rudder was way too sensitive and over steering became the issue.
 
Turning the wheel hard over when operating the thruster should help. That will minimize the effective surface area that the thruster flow impinges on. I investigated options for changing rudder size. Unfortunately we operate these boats across a wide range of speeds and no one rudder size is ideal for all. A larger rudder is ideal for docking but not so for running. As the saying goes, pick your poison.
 
The previous owner of my boat installed a rudder extension that doubles the size of the rudder. I have never tried maneuvering the boat without the extension so I don't have a comparison to what improvement it might contribute. I don't have any problem with steering over sensitivity and the auto pilot works well. The thruster shroud was removed on the starboard side - also done by a previous owner. He claimed both of these modifications made a major improvement. There is a picture of the rudder extension in my album (page 7). My docking skills are not what one would call great..... so I need all the help I can get....:roll:
 
our 31 seems same both directions
 
I think the first experiment will be to remove the inboard deflector. As to a larger rudder, I think there is some merit in examining that option. Some rudder area forward of the pivot point will reduce the load on the steering system. However, I do agree this must be an engineered proposition. Ah, boats, boats.... So many boats so little time !

Bill46 ....what is different in the 31 installation ?
 
Make sure your trim tabs are all the way up as well.. I have noticed they will effect proformance as well..the deflectors are to allow water pick up with avoiding cavitation being the thrusters are so close to the surface.. I have thought of trying that myself but figured in the end that if they where not necessary they would not be there..
 
A very good thread with regards to rear thruster performance and purpose of cowl mounted to both the port and starboard side of the tube. After reading and looking closer at our tugs thruster it is pretty clear the water force/movement to starboard is disrupted by the rudder to a degree. I thought it may be worthwhile to follow up with Imtra on the pros and cons to removing the side cowls. Their reply was very interesting especially the comment on potential to snap a shear pin if wheel is hard over to port. Below is a photo which i send them plus their response.


Response from Imtra:
"The cowls are to prevent the stern thruster from sucking air from above if the thruster install is mounted too close to the waterline. If it is, it causes ventilation and creates a lot of bubbles but not much thrust.
The cowls restrict the water intake slightly so we say about 10% reduction in performance. (but keep in mind even though there is maybe a 10% reduction, you actually might improve because of sucking air or other obstructions that it is diverted away from.
After looking at your pictures, I am not sure how much you can rotate the cowl without possibly sucking air from above. You will need to check on your installation to see if that is the case and then go from there.
The Rudder does present a big obstruction and if the wheel is hard over to port, rudder closer to the thruster, you can “create” back pressure that may cause the shearpin to break."
 
On the 29, both thrusters seem to be about 10" or so below the water level. This is from examination of pics I took when I bought my Ranger. These dimensions need to be confirmed In the field. The specs for the 60 Sidethruster specify a minimum of, I think, 7.5 "; measured from the thruster centre to the water level. If I am correct in this, why then are the deflectors needed. I couldn't find any info on the Sidethruster site advising when the deflectors are needed. A bit of a black art, it would seem. Give me a larger rudder, and with the relatively flat stern sections, the boat should back and fill easily.
 
Regarding turning the wheel hard over to improve thruster preformance to port, to my mind less obstruction to water movement would put less stress on the shear pin not more.
Please explain; Thanks
 
Just passing along how Imtra responded to my question regarding the cowls. I am not an engineer so do not know all the vectors and angles that set up back pressure which may potentially lead to a shear pin breaking so not able to explain any further than what the experts responded with.
 
How about making a tubular extension that routes over the rudder and redirects the flow of water to the stbd side of rudder?

There are also thrusters available that use jets. A centra pump sucks in water and then redirects water out of small openings in the hull. No shear pins, no complicated access acrobatics
 
On the Cutwater site, there are pics of the stern, out of water, of the 28 and 30. Both boats have the stern thruster installed at an angle so the thrust to starboard is directed somewhat over the rudder. Presumably this is an improvement. I wonder if it would work on the 29 and does it in fact result in any significant improvement. Maybe Andrew knows ?
 
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