Bow Thruster!

Dear Friends,

I am contemplating the thought of fitting a bow thruster to Farouche. I would like to launch the question in our forum and to collect points of view and advises on the issue as per the following:

1-      First, I feel a little embarrassed to think of a bow thruster on such beautiful and original yacht as the 47! Would anyone else consider a bow thruster at all? Is the thought unholy?

2-       If possible at all, would a 130kg Thruster, or 8.7 HP, be sufficient? It is indeed according to the manufacturer Sleipner.

3-      The casting of the tunnel should be done with some sort of Epoxy, such as the West System Epoxy, using biaxial glass and high density filler. Is this correct?

Thank you for your points of view and advises.

Philippe (47/050 Farouche)

Dear Phillipe!

Well, I have always consideret a thing like that a thing for Chicken Sailors!!

Well, in that case, I am a happy chicken sailor!!! :-)

I LOVE MY BOW Machine!!!! So go and do it!! I will make your life easy!! And habour monouvers a wonderful easy game!!

Just one thing! You are heavy!! go for the biggest one you can fit!! Go for 12 or even 16 HP if you can!! (I have 12) You will see, that your tunnel might be placed too high, due to the construction of the bow of your 47.

You will love your new toy!

Many regards

Carsten Overland  Cygnus Peña 47 #20

Dear Carsten,

Thank you very much for your kind words and encouragement.  The norwegian manufacturer, Sleipner, refuse to talk in terms of HP but rather in terms of Torke. The bow thruster they suggest to me is the SE130/250T/12V with a torke of 130 kg and with twin 5bl Q props. They say it is more than sufficient if not overdimensioned!

However, I will investigate further and I thank you for smoothing down a bit my embarassment!

Cheers

Philippe (47/050 Farouche)

Dear Philippe

Forgive me for being critic, but I feel compelled to point out he problem with tunnel thrusters on sailing yachts with shallow forebodies. In order to function properly there needs to be enough water above, and enough hull depth below the tunnel. Serious loss of thrust will result if the requirements are not met. The shape of the tunnel ends is also important.

I would appreciate to hear what side thrust Cygnus Pena achieves, and what the manufacturer brochure says

Kind regards

Lars

Dear Lars,

Sorry for not answering earlier. I thank you for your accuracy and for pointing at important details. My examination of the issue shows the following. Please consider and do not hesitate to correct any mistake:

The tunnel is 250mm in diameter. The manufacturer says that half of that diameter above the tunnel would be enough. In my case it will be more than that: 350mm. The thrust of the recommended propeller is 130 kg - 6,5 KW output or 8,7 HP. The front edges of the tunnel will rounded and augmented by a deflector which will push the water flow out from the hull. Standing directly in front of the tunnel at the angle of the boat's centerline, the back face of the tunnel will not be visible.

All this might seem fine in theory! As to the practice, well, we'll see.

Cheers

Philippe (47/050 Farouche)

Bonjour Philippe

Some additional suggestions.

The tunnel ends should be rounded off all around the circumference as shown at the left end.

There is an alternative to building up thickness in front of the tunnel, i.e. to make a recess aft of it, so deep forward that the front edge is not visible from straight forward, and tapering off aft, as shown by the red area.

 Meilleurs salutations

Lars

PS

Right end, sorry !

Bien le bonjour mon cher Lars.

Thank you for your accurate suggestions and added clarifications. In my case here are more specific data for your kind consideration:

1-I will have more than 300mm of water (to the water line) above the tunnel, that is more than its diameter, which 250mm. This is ideal according to the manufacturer.

2-I will have a little more that 1/2 the tunnel diameter under the lower edge of the tunnel to te rounding of the hull. This seems OK (?) since the minimum required is 1/3 of the tunnel diameter under it.

3-The tunnel length will be about 3 times the tunnel diameter, thus staying within the ideal of a length 2 to 4 times the diameter.

4- I understand that the best solution to reduce the drag and to increse (or keep) the efficiency of the thrust power is to make a recess in the hull at the back  of the tunnel. True. However, this is what is being done on newly build boats, facilitating the casting of the recess. I imagine - I am not sure - that this operation might be quite difficult on an existing hull. Thus the recess, in order to be efficient, must corespond to about 10% of the diameter of the tunnel: ca 25 to 30mm! What about the hull thickness on this area as well as many other considerations that come to mind? Therefore, I will probably - unless you prescribe otherwise - go for a deflector in front of the tunnel.

Wecome with your comments and meilleures salutations amicales.

Philippe (47/050 Farouche)

Mon cher Philippe