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S&S Swan General - Bow thrust on a 48
07 September 2021 - 07:47
#1
Join Date: 05 August 2010
Posts: 142

Bow thrust on a 48

Dear all,

after another narrow escape in a narrow marina we have (almost finally) decided to install a bow thrust on our Swan 48. My questions to those of you who sail a 48 (or 47) with a bow thrust:
- Where is the thruster located in the boat, measured forward from the mast?
- How deep is the thruster set, measured from the waterline?
- By your experience, are you happy with that position or would you recommend a change?
- What kind of thruster does that refer to, regarding diameter, power, type (i.e., retractable or tunnel)?

I believe we could gain a lot out of the experience we cannot yet have, so I am counting on those of you with similar boats ...

Wishing you fair winds and wide marinas,
Martin (Vellamo, 48/039)

08 September 2021 - 10:56
#2
Join Date: 02 January 2008
Posts: 1547

Dear Martin

It is assumed you are looking at tunnel thrusters. Some 100 kg of thrust suggested
Here some general geometric advice for a proper installation.
The thruster should be as far forward as possible, but sailing yachts have shallow fore bodies, and this restricts the position.
Tunnel diameter  8...10 inches, local hull draught 2...2½ ft, tunnel top submerged one diameter in order not to suck air, and one diameter of hull depth below the tunnel, as well as a tunnel length of two diameters to avoid undesired cross-flow
The tunnel ends should be rounded where they meet the hull with a radius of 10 % tunnel diameter, Making elliptical recesses aft of the tunnel so the ends are parallell with the centerline reduces the added resistance
Kind regards
Lars

12 September 2021 - 05:57
#3
Join Date: 05 August 2010
Posts: 142

Dear Lars,

thank you very much for the detailed information! I haven't talked to the yard doing the installation yet, at least not about details - as of now, I am still open about a retractable thruster provided we can afford it. Do the two types - retractable vs tunnel - compare in strength?
And while the retractable is definitely further below the waterline, there would of course be no hull left under the propeller. In case of a tunnel, you suggested one tunnel diameter of hull left below - would that be for stability reasons, or is it related to the tunnel flow characteristics?
In any case, we will probably have to balance vertical and horizontal position, i.e. a deeper position is traded off against a smaller leverage (position moves aft), so I am very grateful for the details you sent!

Best regards,
Martin

12 September 2021 - 08:20
#4
Join Date: 02 January 2008
Posts: 1547

Dear Martin

You ask about the strength, assume you mean the thrust?
A tunnel thruster often does not achieve the expected thrust, because all the geometric requirements can not be met, and each deviation reduces the output to some extent.
Retractable thrusters are more predictable in this respect.
 
Also the retractable thruster has a piece of the hull below the tunnel - the panel covering the hull opening when closed.
 
A too short tunnel with too little hull depth below it will suffer from undesired cross flow, which reduces thrust.
Best regards
Lars
 

20 September 2021 - 20:10
#5
Join Date: 11 November 2010
Posts: 17

Hi Martin

After a few seasons in the Med getting the bow blown off while trying to dodge anchor lines while mooring up I've nearly decided to fit a bow thruster to Snow Wolf, Swan 48. I've noticed the wind always picks up in the Med just when I'm trying to berth stern to!

We will be going for a vertical retracting thruster just forward of the anchor locker bulkhead. I've been investigating Lewmar or Maxpower.

 

A Swan 48 that was previously called Jocobite had a bow thruster finished when she retired from racing. Not sure what Jacobite is now called but the owners may be members of ClassicSwan.

20 September 2021 - 20:17
#6
Join Date: 29 January 2007
Posts: 913

Dear Martin and Snow Wolf,

she is hull 024 Sleeper

Fair winds!

matteo (47/069 Vanessa)

21 September 2021 - 16:09
#7
Join Date: 05 August 2010
Posts: 142

If Sleeper X really has a bowthrust, that should put to silence every argument that "these things make you slow". I've seen her racing, and that boat is certainly anything but slow (even conceding that Jacobite was, if I recall correctly, a bit lighter on the scales than the other 48s).

I know that Montana has a bowthrust as well - I'd be interested in its position and the crew's experience with it. I'm considering a retractable thruster as well, and the area of the chainlocker bulkhead seems to be a natural position. I just need to see if we can then still store 75 meters of anchor chain as well ...

Best wishes,
Martin

21 September 2021 - 17:25
#8
Join Date: 02 January 2008
Posts: 1547

Dear Martin

Assuming that your anchor chain is 12 mm, 75 m would occupy a volume of  approx 130 liter.
Kind regards
Lars

16 January 2022 - 18:11
#9
Join Date: 05 April 2010
Posts: 9

Dear Martin!

It's some time ago but I know that Dantes removed their tunnel thrust from their 48 S&S because it took too much space in the stern. 

May be they still have it ir willing to sell it!?

Ulli

THEMIS Swan 48 #16

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