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Old 19-01-2015, 23:19   #31
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Re: Bow thruster on a catamaran, really?

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Originally Posted by Jim Cate View Post
So, having never tried this out at all, what happens when you use a thruster where its outwash hits the other hull? Would that not tend to negate the desired net thrust somewhat?

Jim
no, it works as normal.
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Old 19-01-2015, 23:34   #32
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Re: Bow thruster on a catamaran, really?

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no, it works as normal.
That seems counter intuitive to me... any other opinions?

Jim
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Old 19-01-2015, 23:49   #33
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Re: Bow thruster on a catamaran, really?

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Originally Posted by Jim Cate View Post
That seems counter intuitive to me... any other opinions?

Jim
Why?
If you stand on a dock and push one hull out, both hulls move in the direction pushed.

If you use a bow thruster the water passing through from external to internal, also pushes the opposite hull in the same lateral direction...... its not static.

Its not opinion, it works. I have watched it on Cats many times.

Larger Cats have two thrusters for better and faster control.
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Old 19-01-2015, 23:51   #34
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Re: Bow thruster on a catamaran, really?

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Originally Posted by Jim Cate View Post
So, having never tried this out at all, what happens when you use a thruster where its outwash hits the other hull? Would that not tend to negate the desired net thrust somewhat?

Jim
Possibly, but I doubt if the effect would even be noticeable.
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Old 20-01-2015, 00:08   #35
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Re: Bow thruster on a catamaran, really?

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Possibly, but I doubt if the effect would even be noticeable.
Now that I can believe! But I reckon there must be some effect, since the "downthrust" bow must be pushing against an artificial "current" created by the thruster. As you say 44, likely not a biggie!

Thanks for thinking about it.

Jim
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Old 20-01-2015, 02:21   #36
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Re: Bow thruster on a catamaran, really?

I fitted a bow thruster to our HT 27. The reason was that it has a single outboard which makes manoeuvring difficult. It has other advantages but reducing stress when docking is not one of them.
Try sitting in a lightweight boat in a narrow channel with loads of other boats, waiting for the lock to open, and add a cross wind and you would see the merits of a bow thruster !
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Old 20-01-2015, 05:53   #37
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Re: Bow thruster on a catamaran, really?

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Now that I can believe! But I reckon there must be some effect, since the "downthrust" bow must be pushing against an artificial "current" created by the thruster. As you say 44, likely not a biggie!

Thanks for thinking about it.

Jim
Hi Jim,
You ask a valid question, but it doesn't really make a difference. Effective use of a thruster generally demands bursts of power rather than really standing on the thruster.

For an experiment, watch a friend using his thruster coming in to dock and watch the wash in relationship to how much thrust is required to initiate a rotation. It isn't that much. This is especially true with DC thrusters- just tap the control and you get virtually instantaneous results.

If the guy is really leaning on the thruster he may be trying to advance and counter act a cross current or wind. Which just won't work because of a hydrodynamic phenomenon known as boundary layer. The boundary layer negates the effectiveness of thrusters completely over 3 knots.

If he has 3 knots on, his rudder should be effective for controlling the bow.

Of course 2 thrusters would be better, but 1 should be fine in most situations.
If you've really dropped the ball and the boats gotten away from you, a thruster won't help you, they are for fine motor control.

A note on comparing a twin screw mono to a twin screw cat is lever length. The distance apart of the screws on a cat makes a big difference, and by design a cat will respond much much better to a split than a mono. I've been on twin screw monos with only one rudder as well as on twin screw mono with two rudders, but where the lever length was so short as to render a split ineffective- where you're better off going hard over and putting both engines ahead rather than attempting a split. Where as I've seen cats equipped with no rudder because they respond so well to a split.

I apologise for the lengthy explanation, I know most of you guys are sail oriented cats, but that's some cat handling insight from a power cat perspective.

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Old 20-01-2015, 07:02   #38
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Re: Bow thruster on a catamaran, really?

I believe the loss of efficiency due to interference of the second hull is around 30% which means the thruster- worst case should still have 70% of the thrust it would provide in a mono.

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Old 20-01-2015, 07:09   #39
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Re: Bow thruster on a catamaran, really?

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I fitted a bow thruster to our HT 27. The reason was that it has a single outboard which makes manoeuvring difficult. It has other advantages but reducing stress when docking is not one of them.
Try sitting in a lightweight boat in a narrow channel with loads of other boats, waiting for the lock to open, and add a cross wind and you would see the merits of a bow thruster !
Does your outboard not steer with the rudders?

With our steerable outboard, waiting for bridges is a non-issue. We just point the stern into the wind and leave it at idle in gear and can hold position with ease.

We originally thought of adding a trolling motor on the bow but found with the steerable outboard, we were probably 80-90% of the capability of twin engines once we learned how to use the outboard to our advantage. Way better than single inboard boats we've been on.
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Old 20-01-2015, 09:43   #40
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Re: Bow thruster on a catamaran, really?

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Does your outboard not steer with the rudders?
No it doesn't. The rudders are stern hung and the outboard is in the centre cockpit so tricky to link together.
Agree that it does make a big differenc since we used to hand steer the outboard on an older version HT, but not easy to do.
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Old 20-01-2015, 10:02   #41
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Re: Bow thruster on a catamaran, really?

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So, having never tried this out at all, what happens when you use a thruster where its outwash hits the other hull? Would that not tend to negate the desired net thrust somewhat?

Jim
Yes, some of the wash strikes the other hull partially negating the thrust. Its not much though. The wash from tug hitting the ship partially negates the tugs thrust, but the tugs net effect still moves the ship.
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Old 20-01-2015, 10:03   #42
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Re: Bow thruster on a catamaran, really?

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No it doesn't. The rudders are stern hung and the outboard is in the centre cockpit so tricky to link together.
Agree that it does make a big differenc since we used to hand steer the outboard on an older version HT, but not easy to do.
Ouch, yeah, I could easily see the need then.

I presume the outboard thrust doesn't directly act on the rudders? If so, it's just about the worst situation (without a thruster). Nothing short of forward movement will give you good control.
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Old 20-01-2015, 10:29   #43
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Re: Bow thruster on a catamaran, really?

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That seems counter intuitive to me... any other opinions?

Jim
I believe the thruster is installed at an angle so the wash bypasses the oppsite hull
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Old 20-01-2015, 10:44   #44
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Re: Bow thruster on a catamaran, really?

Right, this is the answer to 'why not?'

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The question isn't does a bow thruster make a boat more manageable, rather is the added weight, power requirements, required maintenance, cost, ect a reasonable trade off for what a bow thruster brings. If you only sail afternoons or weekends and have to move your boat sideways thrusters may make sense. If you are cruising from anchorage and only dock once every couple of years for a haul out and bottom job the boat bucks may be better spent elsewhere.

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Old 20-01-2015, 10:52   #45
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Re: Bow thruster on a catamaran, really?

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That seems counter intuitive to me... any other opinions?

Jim
I just remembered another reason why this works. Bernoullis principle.

When the impeller turns it speeds up the water on the pulling side creating a low pressure zone. The boat seeks the low pressure zone . So the thruster is half pulling and half pushing (I don't actually know the exact ratio).

Same explanation as to why a boat will sail up wind. The wind isn't pushing it up wind, a low pressure zone is created by the airfoil type sail and the boat is pulled into the low pressure zone.

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