Where is the pivot point ?

[Petrolia]

on a twin shaft motorboat if you have one engine forward, one aft and the rudders straight ?

My common sense seems to indicate the boat should pivot around the props which are quite close to the back of the boat. Ie the back of the boat would stay in about the same place and the rest of the boat would swing around.

In practice though the pivot point seems to be much further forward and the back of the boat swings out.

It's not a problem and the boat handles really well it's just that I don't understand why it doesn't pivot at the props.

 

[peterb26]

If you think about it theres a huge amount of resistance to be beaten if the boat were to pivot about the props.

Imagine a big hand holding the boat and turning it round. It would take far more energy to move it round pivoting via the props. The least resistance to the turn is a point someway forward of this - and thats where it will pivot.

It will be different on every boat - depending on keel design, size of props - width of props etc.

 

[Jonmendez]

if you have one ahead and one astern with the rudders straight then it will pivot roughly in the middle but as it turns it will have a tendency to creep slightly forward as one end is pointy and one blunt!

 

[peterb26]

And that one from Jon1m is from a thoroughly professional professional Petrolia. /forums/images/graemlins/smile.gif

 

[mlthomas]

The pivot point on the boat will change whether you are going forward or astern using the engines for manouvering.
Forward the pivot point will be around the 1/3 boat from the front and going astern 1/3 from the back (approx.)

 

[peterb26]

Mark - Petrolia meant when the engines are opposed and the boat is rotating.

 

[mlthomas]

Oooops /forums/images/graemlins/blush.gif
You can tell I'm new around here

 

[peterb26]

No worries Mark - thanks for the contribution. /forums/images/graemlins/smile.gif

 

[fisherman]

On one of the older lifeboats, possibly a Barnett, it was thought that to spin the boat to stbd it was best to put the stbd engine ahead as it acted on the rudder better, with the port engine astern.....never tried it.

 

[Bandit]

When I had a semidisplacement Halmatic 34 it was half way.

My old Fairline Sedan 36 a planing hull with no keel was two thirds of the way back.

My Aquastar 48 is semidisplacement and it is about half way.

Interestingly when i bought teh Fairline Sedan it had been retro fitted by a twat of a frenchman with a keel that went past the props to alow drying ( normally a keel finishes well short of the props) and it would hardly turn at all , you need the water to flow from the ahead prop to the astern prop past the back of the keel to turn properly.

 

[Petrolia]

I understand what you mean.

Also thinking more and partly off the top of my head, when moving forward on the port engine (rudder straight) the boat turns slowly to starboard. When moving aft on the starboard engine the aft of the boat turns to port.

So there is obviosly a pivot point in both directions although not necessarily the same point. So both of the above tend to turn the stern to port so maybe that's why with port ahead & starboard aft it turns about same said pivot point rather than the props.

Also there is the prop walk which will have the same affect. Port prop going forward walks to port. Starboard prop astern also walks to port.

I think I've got it !!

 

[Petrolia]

Yes. And I also read somewhere that the props are more efficient going forward so another reason the boat will creep forward.

 

[dombuckley]

If you are turning at rest with the rudders amidships, the principal turning action actually comes from the propwalk of both props turning in the same direction. This therefore moves the stern sideways, and the boat turrns around her normal pivot point (depending on underwater profile, usually about 1/3 of the way aft of the stem), rather than twisting the whole boat round the props.

As regards putting the inboard shaft ahead, outboard shaft astern, this would work on an older slipway-launched lifeboat. But this is because of its fairly unique arrangement with a single rudder between twin props running in tunnels, where propwalk has little or no effect. Whatever you do, don't try it in your AWB in a marina: could be quite expensive!

 

[Petrolia]

Hmmmmm. Will have to think about that one. Other way round ?

In fact if I want to turn sharply say to starboard, without moving forward and at the same time swing the aft around to port I turn the wheel hard starboard, port ahead, starboard astern.

In fact it shows that method in the Nimbus manual. Works OK.

 

[Petrolia]

When did you find out about the Frenchmans keel.

After you bought it ?
/forums/images/graemlins/laugh.gif

 

[Petrolia]

OK. 100% happy abut the prop walk now. /forums/images/graemlins/smile.gif

Also see how Fishermans old lifeboat worked with twin props but one rudder.
Don't worry I won't try it with twin rudders /forums/images/graemlins/blush.gif

 

[Bandit]

No I knew about the keel already but i discounted the pice to suit.

 

[Bandit]

When you are manovering on engines it is generally in a restricted area so use reference points pontoons, a point on the quay a moored boat and add more power one side or the other to suit.

If I am going ahead and want to turn slowly to starb i just knock the port engine into neutral and vice versa. This gives you a gentle curve in the way you want to go and it halves your manoevering speed and if you hit anything you do it a lot more gently.

Too many people rely fully on bow and stern thrusters, they should really learn to handle the boat without and then learn with them as well at a later date. That way you would not hear the bowthrusters running from the moment the boat came into the pierheads as some do.

 

[nonitoo]

On a twin screw with the screws outward turning then whichever way you elect to turn, port or starboard, you will have favourable transverse thrust.

However if you have inward turning screws then all transverse thrust is cancelled out when you try to turn and therefore the turn is more difficult to achieve.

The pivot point is very much a moveable feast, moving forward when making weigh ahead and moving aft when coming astern.

Therefore your bow thruster will be much more effective when coming astern as opposed to moving forward.

Tom

 

[Petrolia]

You know I could do with a keel on Petrolia. We went fishing on her for the first time a while ago and she rolls like you wouldn't believe. On the drift or at anchor. My mate was sick all over the place.

It's the same if we have to wait outside a harbour for the IPTS. We were outside Calais once and it was a nightmare !

I need some kind of drop down keel just for when stationary. Maybe Nimbus would fit one for me between the engines ?