Temporary electric bow thruster on a cat?

[Cadence]

Originally Posted by Tornadosailing

Or you could do this:

https://youtu.be/ndWD_I_7hVk

Nice job, but not trying to squeeze in-between two other docked boats with a wind or current. I'm sure the best job I've seen doing that. with twin screws, was done by the wife. The husband handled the lines. He said she is better at it. Finessed it in a 40ish ft. boat with about 5ft. extra room total

[billgewater]

Back in the early days of catamaran evolution, various propulsion systems were tried. Cats with single engines were common. The Easterman (I may have this wrongly named. Easterly?) had a fold up leg centrally mounted. The engine which was actually in the bridge deck adjacent to the aft bulkhead!) was transverse and the gear box junction to the engine allowed the entire gearbox to rotate (on a visible outer ring gear arrangement). A long hydraulic ram pushed the leg down and took the thrust load. It all seemed to work for the owners who traveled everywhere with this arrangement including marinas. I observed them backing into a pen as they claimed better control in reverse. It had a rather large prop, which they said was great when motoring but was completely out of the water when sailing. The engine was remarkably accessible and surrounded by extensive sound proofing!
Twin engines makes some sense - control - redundancy etc, but the cost of a second motor can be frightening along with loss of space. Catamarans are already staggeringly expensive vessels. However throwing heaps of money at a concept does not necessarily mean you get a better design at all - often just more expensive problems and more of them.
The idea of a single engine with a long trailing shaft that can be lifted completely out of the water associated with a bow thruster or two (possibly swing down electric versions), seems an interesting concept. It means a cleaner water flow around the hull and lifting the shaft/prop allows you keep the prop clean. Consider the engine mounted well forward even in front of the mast, so the shaft angle is minimized. It may mean a universal type joint or engine beds that can tilt. Different engineering issues.

Although I did not get a close up, I do remember flying over a very small home made catamaran in the 80's that had this single long trailing shaft arrangement as it motored out of Darwin Harbour. Reminiscent of the fan tail tourist boats around Phang Nga near Phuket. These boats worked shallow water and operated off beaches so the trailing long shafts on a pivot axis, balanced by the motor weight (usually an old car engine) was an exercise in appropriate design for a particular environment. however, the engine and everything else was at the extreme aft of these tourist boats, allowing nearly all the vessel to be used for the seated tourists. These boats were fast!

I guess the issue with sail drives is that the entire package is "off the shelf" thus simplifying design and building considerations. But it does not necessarily mean that sail drives are the actual best solution at all. Sail drive problems are legendary, particularly in the warm tropic waters where overheating of the gearbox or impact damage is common. In the videos above, I am sure you all noticed just how benign the weather was. Hmm
Once when assisting a friend on a catamaran entering a lock gate enclosure we had a problem. The gates were not shutting properly and as the water level outside was higher than in the lock, water streamed in through a central gap about 2 inches wide. This caused considerable turbulence in the lock and the catamaran was violently swung from side to side causing some damage at the bows. The twin engines were useless in this case. But I guess the problem would be the same for any vessel.
Has anyone seen any other interesting alternatives?

[belizesailor]

Geez...this is getting complicated and expensive to solve an almost non-existent problem...on those very rare occassions when a bit of side thrust is needed to park a cat, a RIB makes a fine tug. Also very handy when an engine fails.

[Thalas]

Originally Posted by belizesailor

Geez...this is getting complicated and expensive to solve an almost non-existent problem...on those very rare occassions when a bit of side thrust is needed to park a cat, a RIB makes a fine tug. Also very handy when an engine fails.

Very true. Though I'm a tinkerer at heart. When I see a backup electric outboard sitting there doing nothing, my mind starts coming up with ideas.

[valhalla360]

Originally Posted by billgewater

Back in the early days of catamaran evolution, various propulsion systems were tried. Cats with single engines were common. The Easterman (I may have this wrongly named. Easterly?) had a fold up leg centrally mounted. The engine which was actually in the bridge deck adjacent to the aft bulkhead!) was transverse and the gear box junction to the engine allowed the entire gearbox to rotate (on a visible outer ring gear arrangement). A long hydraulic ram pushed the leg down and took the thrust load. It all seemed to work for the owners who traveled everywhere with this arrangement including marinas. I observed them backing into a pen as they claimed better control in reverse. It had a rather large prop, which they said was great when motoring but was completely out of the water when sailing. The engine was remarkably accessible and surrounded by extensive sound proofing!

Sounds like you are talking about the Stillette outdrive that a lot of smaller catamarans use (still common). It's basically a I/O unit with a really long leg.

That or a an outboard does the same thing. Since you can turn the prop with the rudders, you have directional thrust.

On bigger cats, twin engines start to make a lot of sense as bridge deck clearances get to large for an outdrive or outboard centrally mounted and you need more power anyway...but on smaller 30-35' cats, a single engine with directional thrust gives great control.

I would say our Gemini with single outboard gave us 90% of the benefits of a twin engine design when it came to maneuvering. And yes, it was common for us to back out of a marina around several bends and then put it in forward gear once we were in open water. In addition to great control, it gave you better visibility as the sterns were only 6-8' away from the helm with a clear view.

[Cadence]

Originally Posted by valhalla360

Sounds like you are talking about the Stillette outdrive that a lot of smaller catamarans use (still common). It's basically a I/O unit with a really long leg.

That or a an outboard does the same thing. Since you can turn the prop with the rudders, you have directional thrust.

On bigger cats, twin engines start to make a lot of sense as bridge deck clearances get to large for an outdrive or outboard centrally mounted and you need more power anyway...but on smaller 30-35' cats, a single engine with directional thrust gives great control.

I would say our Gemini with single outboard gave us 90% of the benefits of a twin engine design when it came to maneuvering. And yes, it was common for us to back out of a marina around several bends and then put it in forward gear once we were in open water. In addition to great control, it gave you better visibility as the sterns were only 6-8' away from the helm with a clear view.

Out of curiosity. How large an outboard did you have and what was your opinion about it's performance relative to hull speed?

[Nani Kai]

Originally Posted by Thalas

I saw this recent picture of a bow thruster on a Lagoon. While I can't imagine personally wanting to deal with the maintenance and potential failure for the amount of use it would get, there HAVE been times when docking to winward where it would have been nice to have the bow being pushed into the dock.......

Bow thrusters are useful tools. On a monohull they are built into holes in the bow. On a catamaran they are located at the stern and are otherwise known as twin engines. If you want to thrust the bow to windward on a Cat then apply more forward power to the leeward engine while applying a little reverse power to windward. If maneuvering room is tight and you want to go to windward without drifting fore or aft, then put the helm over to leeward first, hold it there, and engage the engines as noted above. By adjusting more or less power to each engine you can literally walk the boat sideways, even directly into the wind. Add a spring line to the mix and you can put a 100# hound in a 50# doghouse without waking up the ticks.

[Dave_S]

https://youtu.be/g-ilE3ZZp8Q

This might be what your after but with a longer leg. I thought about one. Basically an electric motor on a stick, lower and raise it manually by winch from two pivots on the fore-beam. It could fold back up under the centre beam.

A long lightweight cat is a PITA to park/get off in narrow berths/channels with over 15kn on the beam. 20kn+ forget it, go to the pub.

[valhalla360]

Originally Posted by Cadence

Out of curiosity. How large an outboard did you have and what was your opinion about it's performance relative to hull speed?

Our Gemini came with a 40hp 2stroke outboard. Quite frankly it was overkill as you hit hull speed before you maxed out the motor. Replaced it as it was in rough shape.

Switched to a 25hp 4 stroke. Went from about 8kt top end to around 7.5kt but you don't run that hard or you just burn a lot of fuel. Cruising is normally 6.0-6.5kt, so not really an issue. Some have dropped to 15hp outboards but there have been times fighting a strong headwind or current, the extra power was handy and you don't have to use it all. With a switch to a low pitch work prop, it was actually more responsive around the docks than the 40hp.

The diesel with outdrive is 27hp from the factory, so 25hp is pretty close to the same. The diesel still wins on MPG but not by as much as you would think with the switch to 4 stroke EFI.

Plus if the battery dies, it's dead easy to pull start.

We'll probably get another Gemini in the future and while I wouldn't rule out a diesel version I have a strong preference for outboard in this application.

[Cadence]

Originally Posted by valhalla360

Our Gemini came with a 40hp 2stroke outboard. Quite frankly it was overkill as you hit hull speed before you maxed out the motor. Replaced it as it was in rough shape.

Switched to a 25hp 4 stroke. Went from about 8kt top end to around 7.5kt but you don't run that hard or you just burn a lot of fuel. Cruising is normally 6.0-6.5kt, so not really an issue. Some have dropped to 15hp outboards but there have been times fighting a strong headwind or current, the extra power was handy and you don't have to use it all. With a switch to a low pitch work prop, it was actually more responsive around the docks than the 40hp.

The diesel with outdrive is 27hp from the factory, so 25hp is pretty close to the same. The diesel still wins on MPG but not by as much as you would think with the switch to 4 stroke EFI.

Plus if the battery dies, it's dead easy to pull start.

We'll probably get another Gemini in the future and while I wouldn't rule out a diesel version I have a strong preference for outboard in this application.

Thanks for the reply. You confirmed my suspicions. There was one, close to home, for sale with a 50 four stroke. Seemed like over kill and unneeded wait hanging off the transom.

[smj]

Originally Posted by Cadence

Thanks for the reply. You confirmed my suspicions. There was one, close to home, for sale with a 50 four stroke. Seemed like over kill and unneeded wait hanging off the transom.

From my experience the 50hp will burn about the same amount of gas as the 25 hp when run at the same speed, but it will run at a lower rpm and be quieter. The disadvantage of the 50 vs 25 is the excess weight.

[Cadence]

Originally Posted by smj

From my experience the 50hp will burn about the same amount of gas as the 25 hp when run at the same speed, but it will run at a lower rpm and be quieter. The disadvantage of the 50 vs 25 is the excess weight.

I guess I need to go back to school. Wait.

[valhalla360]

Originally Posted by smj

From my experience the 50hp will burn about the same amount of gas as the 25 hp when run at the same speed, but it will run at a lower rpm and be quieter. The disadvantage of the 50 vs 25 is the excess weight.

Assuming both are 4stroke EFI, it's probably within 10% but so little need for that much excess power the downsides in my mind outweigh the advantages:
- Higher up front cost (and maybe a little extra ongoing maintenance cost)
- Heavier weight. The jump from 25 to 50hp will add around 60lbs (assuming modern 4stroke) The 25hp was about the most I would want to deal raising the motor bucket with unless you add a winch to pull the motor bucket up.

[Cadence]

Originally Posted by valhalla360

Assuming both are 4stroke EFI, it's probably within 10% but so little need for that much excess power the downsides in my mind outweigh the advantages:
- Higher up front cost (and maybe a little extra ongoing maintenance cost)
- Heavier weight. The jump from 25 to 50hp will add around 60lbs (assuming modern 4stroke) The 25hp was about the most I would want to deal raising the motor bucket with unless you add a winch to pull the motor bucket up.

I guess I started this. The one I saw locally had a hydraulic mount. I asked the question to get an owners opinion as to whether of not it was overkill with 50HP. I'd bet a couple of 15HP four strokes with the long leg and "power thrust" would be the nuts.

[Snore]

I agree that if you are skilled a now thruster is not needed.

But curious. Is the bow thruster part of a joystick docking system? I mean if you are going to make something simple complex—— why not completely bugger it up?