Really ???

I can't make my mind up about this crowd... is it :idea: or is it

http://www.vismaramarine.it/en/yachts/under-construction.html

Strong on sales hype... but I wonder about some of their claims... The 56' Navetta, for instance, is claimed to displace 20 tonnes, is powered by 2 x 435hp engines and does 28 knots....

But the one that really got me is their Navetta 68. I'm no naval architect... but isn't it a little unusual to have a pair of pod drives located amidships???

 

It is a renown yacht design firm, I have to say... But yes, it is rather unusual choice and somewhat unlogical from the engineering point of view. Possibly driven by an odd SOR.

I can understand (to some extent) the "logistic" rationale behind it - the engine room serves as a separation between the owner/guests area and the crew cabins aft. They also claim that the engine room accessible from the main deck cabin was a request by the owner...

Well ok, but why choosing pods then? The pods in that position (close to the CoG) will be rather ineffective as steering devices because they lack the necessary lever arm - the distance between the CoG and the point where side-force is applied. I would like to see that boat's behaviour in a following sea, where it will need all the steering authority it can get.

If the engine had to be located where it is now, then this is imho a case where a classic drivetrain (shaft, aft props and the rudder behind) would have been a better choice.

 

I haven't viewed the web-site but by your text it sounds like they may be using differential thrust (tank steering) to steer the boat.

-Tom

 

There are no specifications regarding the propulsion plant, just that layout Mat-C has posted.

 

If the pods are of the IPS type, would not steering be fine from the CG location? Set a course with the toggle or set a compass course and the computer should steer the boat just fine with differential thrust. Don't know about the efficiency relative to stern drives though.

 

No way that steering from near-CoG point can be better than steering from the stern, Tom. Just consider the difference in lever arms in two cases. Given the same thrust, if the point of it's application is at the stern the turning moment will necessarily be much higher. Only when two pods are pushing in opposite directions with the same thrust (pure couple) will the turning moment be equal in both cases.
See the attached pic.

 

Actually during turn the boat does not turn around CG but around pole of rotation that is about 30% from forward perpendicular. Yes, on planing boat it might be different due to running trim, but...

Just consider that those boats are not used too often. Interior is a priority, not performance

 

Yes, you're right.

For the sake of general info - please correct me if I'm wrong in this: shouldn't the position of the pivot point P depend on how far is the point of application of the steering force from the CoG, and on the boat's rotational inertia?
If the lever arm is sufficiently long, the rotation starts with very little (almost no) sway motion. In that case, the point P is initially at or near the CoG (the situation in the picture) and then progressively moves fwd to 20-30% aft of forward perpendicular.
On the other hand, if the lever arm is short or the boat has high rotational inertia, then a sway motion is observed before the rotation - implying that the initial point P is at infinity.
Is my reasoning correct?

Looks like that's the case here.

 

Tom, I was replying to the other Tom (Mr. Lathorpe).

 

Oops, that explains everything.

Cheers,

-Tom

 

I might not be looking very hard, but I cant see any pictures of where the screws are in relation to the stern.

http://www.vismaramarine.it/en/yachts/vismara-motor/vismara-motor/my-navetta-68.html


The engine pictures look like pods, and might be under the centre - but you cant tell for sure.

Central engine positioning is great to avoid excessive hobby-horsing at speed.

 

Well... used often or not... I'd be pretty dissappointed if it was my boat and the only way I could get it to turn is to toss one engine in reverse!!
Thanks for your thoughts guys

 

There is no notion about pod drives in description and in video as well.
However, in quite elaborate floorboards in the engine room could be seen.
So the GA plan in the first post could simply show that nice floorboards in engine room , parts of gearboxes that protrude above them, and nothing more.
I would think that pods in the middle could be just false impression from not-too-technical GA plan.

 

This site says that the propulsion is a traditional line shaft: http://www.poweryachtblog.com/2011/05/new-model-vismara-my68-navetta.html

 

Actually the equations of manoeuvrability are solved around the CG, but this does not mean that boat is rotation around CG.