With most of the major Mothership Möbius jobs looked after, this week I diverted my attention to our other new boat Mobli. This is our 5m aluminium tender that we designed to match the subset of use cases we have for Möbius.
We have named the Tender “Mobli” as a slight play on Kipling’s character Mowgli.
Basic specs are:
- LOA 5.0m / 16.4ft
- Beam 2.0m / 6.5ft
- Draft 288mm / 13.8 inch
- Weight: 1088 Kg / 2390 Lbs
- Engine: 110HP Yanmar 4JH4-HTE
- Propulsion: Castoldi 224 Direct Drive Jet
Our design intent was not for a typical RIB dinghy to just ferry us ashore in an anchorage and much more so a full on Tender that we designed using the same four SCEM principles we used for Möbius; Safety, Comfort, Efficiency and Maintainability.
This will be our mini eXplorer boat to take us to places we can’t or don’t want to take Möbius to such as up small inlets and rivers, into super shallow bays and enable us to take multi day eXcursions in safety and comfortably dry in most any weather. Should also make for a fun boat when we have grandkids and others that might want to go water skiing or wakeboarding.
Mobli will also be our backup plan if we should ever run into catastrophic problems with Möbius being either unable to move or worst yet, sinking or on fire. As such Mobli will be both our lifeboat should we ever need to abandon ship and also be our emergency “get home” solution by being a little mini tugboat capable of pushing or pulling Möbius at a reasonable speed in reasonable sea conditions.
Hence the 110HP inboard, which also met our single fuel boat design criteria by eliminating gasoline for an outboard engine. I chose the 4JH4-THE model as it is all mechanical injection so no electronics, ECU and such to worry about getting parts for.
After seeing so many jet drives being used the pilot boats being built for Coast Guards, military and police use, we went with a jet drive for added safety, shallow water ability and high mobility in any direction.
Having industrial quality rubber fenders or rub rails wrapped around all sides gave us all the better abilities to be a mini tugboat and be that much safer and cleaner when we make contact with docks or other boats.
Best of all perhaps was that this combination has been put together as a matched set in a joint venture between Yanmar and Castoldi so it came as a very complete kit with literally every part I needed to connect and install in Mobli.
Connecting Yanmar to Castoldi jet drive
Picking up where I had left off last year, I started by working on coupling the Yanmar to the Castoldi via the CentaFlex coupling which you see I have bolted to the flywheel of the Yanmar on the Right. This is a flexible coupling which helps reduce any vibration in the drivetrain and offers some dampening.
This is the CV or Constant Velocity Cardan shaft that connects the output of the Yanmar to the input shaft of the Castoldi jet drive. The bolts are special hardened ones that I had brought back with me from McMaster Carr when we were in the US last year.
These bolts are armor coated socket head and were the Goldilocks choice for bolting the ends of the Cardan shaft to the CentaFlex coupling and the input flange on the Castoldi. 
I had previously and precisely aligned the engine to the jet drive so I could now torque the bolts down and the propulsion system was now fully connected.
Sound Insulation
Next I tackled the installation of the sound insulation for the engine room/bay by covering the inside surfaces with this type of multi layered acoustic insulation.
I had previously purchased two large sheets of this foam and you can see the four layers here in this sample. There is also a peel and stick layer of adhesive under the white covering you see on the far Right so this side goes against the aluminium surfaces inside the ER of Mobli. When I have all the insulation glued in place I will then cover it with thick foil backed cloth for added protection and easy cleaning.
It was a rather finicky job measuring the sizes and angles for the spaces between the fames inside the ER and then cutting the foam pieces to fit just right. Best tool for cutting the foam was my cordless Milwaukee circulation saw and while it was a bit messy it worked eXtremely well at cutting this very fussy and rather fragile foam.
Here is how the foam fits between each frame on the side walls of the ER. I am going to hold off gluing the foam to the ER until I have the whole Tender finished and running to prevent any damage and then I will glue in the foil topped cloth which wraps around all the edges as well and provides a very protective and easily cleaned surface, the same as you have seen inside Möbius.
Hydraulic Steering
The Castoldi/Yanmar combo kit came with a full steering package as well which starts with this manual hydraulic pump that the steering wheel turns. 
First I needed to fabricate the aluminium frames that I have now bolted to the back of this pump which I then bolted to the frame of the cockpit pedestal. Jet drive boats don’t have a rudder and instead you steer by rotating the nozzle of the jet from side to side so two hydraulic hoses will attach here and go back to the Castoldi jet drive to connect the steering wheel to the nozzle.
The angled top of the steering station is hinged to provide easy access to the insides and here I have drilled the hole for shaft of the hydraulic pump.
Bolt those two AL brackets to the vertical panel below the steering station, slide on the steering wheel and presto!
Mobli has his steering station!
Also included in the Castoldi/Yanmar kit was this beautiful and well engineered throttle and bucket control lever.
A bit of work with my jig saw and I had the custom opening cut into the 6mm / 1/4” AL plate that the tender is mostly made from.
Tap four 6mm holes for the SS attachment bolts and the throttle/bucket controller is ready to go. In operation the lever works like a regular throttle lever; forward to increase RPM, all the way back to idle. For a jet drive boat there is also the two way electrical switch which controls the position of the bucket on the jet.
This illustration (click to enlarge) from the eXtremely thorough Castoldi installation manual does a good job of explaining how the bucket move up and down to either allow the jet of water coming out to thrust the boat forward and when the bucket is moved down it redirects the jet of water to thrust the boat in reverse. Neutral or no thrust is achieved by having the bucket half way down so that the water is directed straight down. On the right side top views show how the steering works by moving the nozzle from one side to the other so the thrust of the jet is now out the sides.
Steering, throttle and bucket controls now all installed.
Electrical Wiring
The Castoldi/Yanmar kit came with six different wire harnesses and most of these had high quality marine waterproof connectors on their ends but in other cases where the wire lengths would vary too much from one boat to the next, they were just cut wire ends that need to be connected to fuse and connection panels. 
Fortunately both Yanmar and Castoldi provided me with full wiring schematics and after a few hours I was able to sort out what each wire connected to and get them all labelled for installation.
Four of the wiring harnesses go back into the ER to connect to the Yanmar or the Castoldi wire connectors and this shorter harness is the one that attaches to the bucket controller.
Throttle Morse Push/Pull Cable
The throttle lever is all mechanical and uses a standard Morse Push/Pull cable that most of you would be familiar with and was the next focus of my attention.
These cables need a good radius to bend through so I needed to use my hole saw to cut a 60mm/2.4” hole for the cable to pass through the Al side wall at the bottom of the inside of the steering station.
I cleaned up the edges and then pressed a length of the rubber U channel we’ve used throughout the boat to provide a very good non chafe surface for the cable to rest on.
After a bit of “fishing” to get the 5m long cable down through this cut out and the hole and then back through each frame along the bottom center of the hull I could attach cable to the throttle controller.
One more hole and one more fishing expedition to get the two cable ends through to the cables on the throttle/bucket controller.
Yanmar & Castoldi Control Panels
The Yanmar came with the upgraded control panel that has tachometer, engine oil pressure and water temp gauges as well as an LCD screen for more information such as engine hours and a set of buttons for power, start, stop and glow plugs.
The Castoldi has a bucket position gauge, Neutral bucket switch and backup Fwd/Rev switch so all these needed to be installed next.
A bit more work with my trusty jig saw and some hand files and taps and the dashboard was suitably cut up.
A bit more fishing to pull the wire harnesses up through and start connecting them to their switches and gauges.
There is still some more wiring and connecting to do but everything fits.
This is how the cockpit looks so far.
Time today for one last bit of kit, the VHF radio.
This is where I’ve left off today and will pick up from tomorrow and cover for you in next week’s blog.
Thanks for joining me on the adventure again this week. Hope it was of some interest and if not add your comments in the “Join the Discussion” box below and let me know what else you’d be interested in me covering in an upcoming blog.
See you here again next week.
Wayne
you know things are getting close when attention turns to the tender ! can’t wait to see this little (actually big) guy zip around
Yes, poor big/little Mobli keeps getting pushed down the To Do list by his big sister Möbius but finally getting some time to work on him now. Still a fair bit of work to finish installing the propulsion system, running fuel lines, putting in the wet exhaust system and then getting the whole davit arch system rigged up and working so we can launch him over the side. We have taken a week off from boat work for the most part to enjoy spending time with some dear friends and getting out to an anchorage for a bit but I’ll be back at it and the blog next week so stay tuned for more Andrew.
Wayne, I must admit that I had an “Aha” moment from today’s chapter. When I first saw Mobli, I remember thinking “Why so big?”
But now that you have explained some of it’s practical uses, one in particular stood out for me: Tugboat. I never thought that it could tow or push Mobius.
Wayne, that is just brilliant. Congratulations !
Best regards,
Jake
Hi Jake, yes the design of the tender evolved a lot as we were designing Möbius and the option to use our tender as a potential “tugboat” to push/pull Möbius if such an extreme situation should occur seemed like the best choice for us. We have also had quite a few situations over the years where this would have allowed us to be much more help to other boaters around us.
The diesel inboard also gave us the single fuel boat we had wanted and seemed to add a significant amount of options for our explorations where we can use “little” Mobli for day trips or longer when we want to explore smaller waterways and leave Möbius anchored out.
So we are excited about getting little Mobli launched and seeing how this jet drive tender works in general and try out using it to push/pull Möbius. Stay tuned for all that once we get Mobli in the water in the coming weeks and thanks for joining us on this adventure.
-Wayne
Play d’eau, our Fleming 55, used to have an Avon Jet RIB. After a short time, I swapped it for a ‘normal’ rib with oars (note, with oars), and an electric Torqeedo. Why? When visiting a bay, I drove into some seaweed which was just under the surface and not easily visible due to the sun’s refection, until it was too late. The jet drive sucked it up and jammed it. With no steering, no drive and no oars, I had to call to a passing fisherman (in his old clinker built and Seagull outboard engined boat) to tow me back to the mother ship. The look he gave me was shriveling, to say the least! Nuff said…
Motto of the story? Have a second means of propulsion on board!
I currently have a beast of a tender, half a ton RIB with excessively large outboard. It is big, fast, and I had the same experience, though not with seaweed but a flat starter battery.
I have 100% the same plan as you did.
Hmmm, guess I will have to look into adding a hand crank option to the Yanmar in the Tender to match the one on Mr. Gee. 😉
Good story Piers and yes, we will have other boats onboard to augment our Tender Mobli. Christine has been wanting a Portland Pudgy since she was young so that’s definitely on the list and we already have an inflatable 2 person kayak which we really enjoyed using on our last boat so I think we should be reasonably prepared for those unexpected incidents that crop up.
-Wayne