Help me troubleshoot my outboard motor position challenges

[OneTwoSail]

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 (https://youtu.be/YlJPHILNVoI). We also have enough power to make water, heat water, cook electrically, run the Starlink, washing machine 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.

Attached is a picture from before I launched to give you an idea of how the motors are placed.

[Chotu]

Fit the pods.

You want the props where they were designed to go for the best performance/handling.

I have petrol/gas outboards. It was vital to put the props where they were designed to be if it was a diesel powered boat. So… I built wells to locate them there.

Wells are way too big of a modification in your case. Too much effort and would eclipse the cost of the pods.

Put the pods where the props are supposed to be and you’ll be quite happy with the boat’s restored handling characteristics.

Also, you don’t have prop wash over your rudders with the current installation. You need this for good handling. The props should be forward of the rudders, directing a stream of water over them.

Note: a bow thruster is also an option

[smj]

Go with a single steerable outboard. Unless you mount the twin propulsion units through the hulls you won’t gain much maneuverability.

[team karst]

Originally Posted by Chotu

Fit the pods.

Also, you don’t have prop wash over your rudders with the current installation. You need this for good handling. The props should be forward of the rudders, directing a stream of water over them.

A lot of cat owners will disagree with that one. Rudder(s) not needed for excellent low speed yaw authority.

[Chotu]

Originally Posted by team karst

A lot of cat owners will disagree with that one. Rudder(s) not needed for excellent low speed yaw authority.

it sure makes it better to have both things going on at the same time though.

they handle like a dream when you can use flow over the rudders as well as the typical tank type of rotation from the props alone.

[fgd3]

Looks to me like your propellers are plenty deep. Since your cavitation problem appears in reverse I think you should first try replacing the propellers. Yours appear to be optimized for forward thrust and therefore may not work well in reverse.

[catsketcher]

My two cents.
My 38ft cat has had a centre mounted outboard and twin outboards in nacelles, about 4 metres apart. I have found no need for any rudder flow. The cat manouvres very well just under differential throttle. It could be even better I guess but I am very happy not having wells.
One major problem you may be having is the prop. We had a Honda outboard with a normal looking prop and it was fine in forward but terrible in reverse. I would look at installing props that look like a normal shape found on diesel yacht, or a Yamaha high thrust. Going to Yamaha outboards and props made a huge difference to reverse thrust.
I tend to do innovative , or alternative setups. I see no reason why you shouldn't move the outboards further apart and then angle or cant them, so that the props are further apart when down. I would check if there is an issue with oil levels but being electric this should not be a problem. But the props should be massive for your heavy and higher resistance type cat.
Cheers Phil

[Pete7]

Originally Posted by Chotu

Fit the pods.

Nah, you would loose the ability to dry or and remove one engine easily if it needed a service or repair.

Originally Posted by Chotu

Note: a bow thruster is also an option

Yes possible, or angle the electric outboards so the props are further apart. Electric outboards don't need to be vertical like a petrol outboard, though Husacat did try twin petrol OBs being angled years ago with some success.

Pete

[purvisgs]

Originally Posted by catsketcher

would look at installing props that look like a normal shape found on diesel yacht, or a Yamaha high thrust. Going to Yamaha outboards and props made a huge difference to reverse thrust.

I tend to agree that the props look like minnkota inspired type, not suitable for the task on a heavy/high windage boat. If better props (large diameter as possible) can't be fit or adapted to your current setup-

Another option might be to find a pair of old yamaha HT/9.9's (extra long 25"), remove the gas powerhead, and swap electric motors under the cowls. Some good "outboard conversion" examples out there youtube etc if you dig around.

Overall though my feeling is you probably undersized them for real world conditions, shouldn't have listened to the "marketing"//// So you could also just try bigger. 6hp tohatsus would/might be a little under-performing in the same configuration, ~10hp (not "10hp equivalent") might be normal/adequate. ~15 might be better/

[mako]

I’d keep the e-propulsion.

So you don’t have leverage (too close together) and there is no prop wash over the rudders.

How about lowering them about 300mm, spreading apart 300mm as you stated, and then angling outwards. Even 20 degrees out will gain you a couple hundred mm of more leverage. New props of course as stated.

[OneTwoSail]

So I've been giving it a lot of thought. At the moment I'm going to give it another season before I make a major change. For now I think I'll add the anti-cavitation plates. Another boat has the same motor, and the plate allowed to increase the power by over 50% before he started cavitating.


If that still doesn't work then I'll look into linking them to the steering bar. It seems to me like that would be an easy thing to do, the challenge is the steering bar moves a fair amount, and I don't know if the motors would have the turning arc to make it happen. I'm sure there's a clever mechanical solution to that, but I don't know what that is just yet.

[OneTwoSail]

Thought I'd add the solution I came up with here. In the end I followed the suggestion of someone to have the motors point "toe-in" 25 degrees each. This gives me a sideways thrust component when docking and is working brilliantly! Some pictures below.