Help me troubleshoot my outboard motor position challenges

[patrickza]

I bought an old Catalac with dead diesels and converted it to electric drive with two ePropulsion 6kW outboards. I also added 3.2kW of solar and for the most part the boat is running absolutely brilliantly, even though I didn't have time to get the sails functioning, the efficiency of these motors combined with the large solar array means I have an unlimited motoring range dependent on speed, as long as the sun is shining (

). We also have enough power to make water, heat water, cook electrically, run the Starlink and pretty much live like we're in a house electricity wise.

Where the problem comes in is related to the mounting of my motors. The prop center lines are only 1 meter apart, and that means I don't have the leverage to use differential thrust, something I've previously relied on when docking catamarans, lifting the anchor single handedly and picking up mooring balls single handed. As the throttles are wireless, I like being up front at the anchor when lifting anchor, but steering with the throttles means this isn't always possible. The motors also tend to cavitate in lumpy seas.

The way I see it, I have a few options:

1) Cut my losses, sell these motors and fit pods. That should solve both issues, with the downside being the inability to lift the motors out of the water in case I need to clear seagrass or fishing line. Estimated cost to implement $3500 after selling the outboards.
2) Make the motors steerable. This likely solves docking issues, but unless I make the steering wireless (I probably could with an actuator) it won't solve the anchoring/mooring ball challenges or the cavitation in lumpy seas. Estimated cost to implement $1000
3) Fit anti-cavitation plates to the motors. A large part of the differential thrust challenge comes from how easily the motors cavitate in reverse. I can't get anywhere near full or even half throttle in reverse before I start sucking air, so just maybe having the plates fitted would mean that I can use the massive torque these motors provide to steer the boat. Estimated cost to implement $500
4) Try and find a new location for the outboards. I could move them around 30cm further apart, but I'm not sure that'll be enough. Any thoughts or any other locations you can think of?

Money isn't the main driving factor here, but what I don't want to do is waste any, so I'm hoping to choose the right solution straight up. My gut tells me to go to the pod motors, with my only hesitation being that they might be less efficient as they push into the rudders rather than into clean water like the outboards do, as well as the fact that a constantly submerged motor might mean things can go wrong more easily.

Belowis a picture from before I launched to give you an idea of how the motors are placed:


Any ideas?

 

[Thistle]

Longer shafts? Angle the motors to move the props outboard?

 

[dunedin]

Struggling to see what the issue is for anchoring or picking up mooring buoys single handed. Most of us do that fine without twin engines or remote controls.
For docking you don’t need a lot of thrust power. If you can control the engines independently, would it work by rotating one motor by 90 degrees prior to docking? Then one motor provides fore and aft thrust, the other provides sideways thrust, to port or starboard.

 

[patrickza]

Most boats have the prop wash flowing over the rudder, so in effect some steering even without movement. In my case if I'm not moving I have no ability to steer. My only hope right now is differential thrust, and in the current setup it's simply not enough to overcome any wind. That's what I need to rectify.

 

[AntarcticPilot]

Why not move the mounting points to the back of the hulls? That's where the drive would be for an inboard setup.

 

[Stemar]

Why not move the mounting points to the back of the hulls? That's where the drive would be for an inboard setup.

That's what a friend's Catalac has. It works, but you'll want the engines linked to the steering if at all possible.

An alternative, possibly rather brave, solution might be to make a cut a hole in the hulls just forward of the rudders and build an outboard well.

 

[rogerthebodger]

That's what a friend's Catalac has. It works, but you'll want the engines linked to the steering if at all possible.

The P could connect the top of the rudder so an arm fitter to the outboards so the outboards would turn when the ruccer turned this could be done under the hull first

 

[B27]

Many single-screw yachts, somewith twin rudders, manage fine.
I suspect a large part of the issue here is that this boat has excessive windage and no keel so at low speed, its windage will pivot it about the rudders?
If you pivot the outboards, it will still mostly blow downwind, just sideways instead of being turned by the wind.

You could fit rudders to the outboards.

A mate has a big flybridge motorcruiser, twin inboards. He's fitted bow and stern thrusters and can park it inch-perfect in a crosswind.

There are quite a lot of twin screw mobo's with the screws not much further apart, and tiny little rudders. One big difference is the turning moment they can produce is pretty high, because they have very substantial engines.

You either adapt the boat or your handling techniques.
Sort the anchor so you can operate it from the cockpit?
Pick up buoys from alongside the boat?
Get a crew?

Adapting the boat, you could put a 3rd motor on the front crossbeam to drop down as a bow thruster?

Steering the outboards and remote controlling that is easy, a tillerpilot or two and an off-the-shelf remote.

 

[Tranona]

There really is not a simple solution as the basic issue is that you do not have enough power to do anything other than go slowly in flat water and no wind. Simple physics made worse by all the extra weight and windage you have added. The original typically had 2*8kW engines (seem to remember yours had 2*23kW!) with deeply immersed propellers in front of the rudders - perfect for helping steer the boat at low speed.

Clearly you cannot fully replicate Diesel engine performance with electric power, but replacing with pods will likely solve the cavitation by placing the (more efficient) propellers low down in front of the rudders. If you can have independent control of each engine speed and direction you would have far more chance of using power to steer the boat. Don't think making the existing motors steerable will help much as they are simply in the wrong place to have any real effect. Moving them to the end of the hulls might be better, but will make coming out of the water worse. Equally not sure anti cavitation plates will do much to help get thrust in reverse as the basic problem there is the propeller does not have enough thrust to move the boat so just spins where it is.

If you really can do the swap to pods for a net $3500 then to me that is a no brainer.

 

[Snowgoose-1]

Pods (360 degree) , seems an ideal solution. Tiltable to offset drag whilst sailing would be a bonus. In fairness, most multihulls live on hammerheads or linear pontoons . Great that everything else is working for you other than parking. I have had linear pontoons in the past with an unhandy monohull and still , on occasions, had to raft up and then warp the boat in. Either due to wind/tide or variable gap to get in. My current boat has an outboard in a well. I have 45 degree twist left or right which makes a massive difference .

 

[patrickza]

Thanks all, so far I think the best answer as a few of you have confirmed is to bite the bullet and go for pods. I'm going to stick with the same brand and same power, as I think they've got plenty power for moving the boat. They claim to be 9.9hp equivalent, but I suspect that's more to do with legislation and marketing than reality.

They also claim their 3kw motor is a 6hp equivalent, and their 1kw is a 3hp equivalent. Knowing how much more efficient electrics are than internal combustion, and going by feel, I would guess the motors are at least 12hp equivalent. The amount of torque off the line is quite astounding. I've been in other cats with significantly larger motors which don't leap off the line with nearly as much authority as these electrics, they're just in the wrong place.

 

[Sticky Fingers]

Assume the pods will be fitted in / through the hulls? Are they steerable?

 

[B27]

Thanks all, so far I think the best answer as a few of you have confirmed is to bite the bullet and go for pods. I'm going to stick with the same brand and same power, as I think they've got plenty power for moving the boat. They claim to be 9.9hp equivalent, but I suspect that's more to do with legislation and marketing than reality.

They also claim their 3kw motor is a 6hp equivalent, and their 1kw is a 3hp equivalent. Knowing how much more efficient electrics are than internal combustion, and going by feel, I would guess the motors are at least 12hp equivalent. The amount of torque off the line is quite astounding. I've been in other cats with significantly larger motors which don't leap off the line with nearly as much authority as these electrics, they're just in the wrong place.

I think much of that might be the way an electric motor develops full torque in a fraction of a second, while a diesel engine takes time to build up revs.
You should think in absolute terms, how many Newtons of thrust, and forget about equivalence to HP of diesel.

The turning moment from a pair of opposing props is the thrust times the distance between them. Torque = FxD

The problem is, you have a lot of windage which can exert a big turning moment on the hull. If it exceeds what you can counter with two outboards, it would likely be a pig with two diesels.

 

[patrickza]

Assume the pods will be fitted in / through the hulls? Are they steerable?

Correct, they will need 3 small holes and one larger hole in the hull. I'm not a fan of holes in my boat, I don't even have one for the watermaker, but in this case I suspect the outcome will be worth it. They're not steerable, but will be much wider apart, and be in front of the rudders, so hopefully I'll have tank drive steering this time:

 

[Sticky Fingers]

Sounds good. Wonder if you can get LH and RH props, so you can exploit a bit of prop walk steering too?

 

[patrickza]

Sounds good. Wonder if you can get LH and RH props, so you can exploit a bit of prop walk steering too?

It would have been nice, and should be easy to do with some software on the controller and a mirror image prop, but unfortunately they don't have that option available. At least with both props being the same I can get away with just needing one spare.

 

[Refueler]

I assume the old diesels shafts exited hulls in front of rudders ?

Why not put shafts back - power the old shafts with E-motors ?? You then have them ahead of rudders - widely spaced etc.

 

[Alan S]

Before spending a lot of money on pod drives which will compromise sailing performance and lose the ability to easily access props in the event of obstruction, I would try a variation of Dunedin's idea in #3.
When you need to manoeuvre fix port outboard to starboard about 20 or 30 degrees and vice versa. This should give you much better differential steering.
Anti-cavitation plates might help too.
I have a (single) motor same as yours and agree that when moving very slowly the prop easily sucks in air in reverse.

 

[srm]

If you want a more efficient engine installation/power use I am inclined to agree with @Refueler, inboard E-motors, shafts, props, rudders. Should you want to try the boats fairly moderate sailing performance it will be helped if you fit folding or feathering props.
The cheapest improvement will be @AntarcticPilot suggestion, though I would extend the hulls' flattish planes above the props with a large anti-cavitation plate mounted on the hull. A slot in this plate with rubber flaps to close it would allow the props to be lifted when sailing or for cleaning. They could be linked and a simple tiller pilot used for steering.

 

[Mister E]

Our Catalac 8 has a single central outboard motor and it is linked via ropes to the rudders.
This system gives really good slow speed steering but not quite as good as twin inboard props.
If you can link them to the steering that would be the cheapest.