1. Boats
2. Sequana
3. Bill of materials

Bill of materials

Although the Ramborator kit comes with lots of parts and fittings, you still need your own RC and propulsion.

Here are the parts, tools and glues/primers/paints I used on my model.

Propulsion : Motor

As brushed motors and ESC tend to disappear, while brushless motors (and LiPos) are all the rage these days, I decided to go the brushless route.

A tame little tug is probably not the typical application for a brushless motor. But brushless motors are extremely efficient. You get tremendous output while current draw is below what a brushed motor would burn. And brushless ESC now come at limited cost.

I settled for a 800Kv brushless outrunner from HobbyKing: Turnigy Aerodrive SK3 - 3542-800KV.

So what is this all about ?

The motor is an outrunner : the outer shell is connected to the shaft and will rotate with it. Clearance is needed around the motor. Outrunners offer greater torque and most low-Kv motors will be outrunners.

Product reference number for brushless motors usually reads as follows:

35 (mm)

Diameter of a classic brushed 540/550 motor. This motor is actually 37mm but still uses "legacy" 25mm mount points which means you'll probably able to installed on older motor mounts (keep in mind that outer shell will turn !)

42 (mm)

Length of motor body (sometimes just the "bell" for outrunners)

800 (kv)

Velocity constant in r/min/V. How fast motor will spin per input Volt

Velocity constant or rating (Kv) is 800r/min/V. This means that when the motor is supplied 1 volt, it will rotate at 800r/min unloaded. I'll power it from a 7.2V 6-cell NIMH battery. Max RPM with no load would hence be 7.2V * 800r/min/V = 5760r/min.

This motor is rated 625W (on higher voltage though) which is total overkill. But low-Kv are mostly bigger motors. Anyway the load and current draw will be limited compared to what the motor can output.

Form-factor and bolt holes are the same as a 540/600 brushed motor. Installation requires no modification compared to the original/intended motor.

Update: now that I've moved to a 2-cell LiFePo4 battery, the motor here is a little too fast. Something in the 600-700kv range would be a bit more suitable.

We nowadays have a lot more options than a mere two years ago when it comes to low-kv motors. Drones are moving to high voltage setups which offer higher efficiency. Higher voltage means that they now need slower/low-KV motors.

Propulsion : Speed Controller

Brushless motors require dedicated controllers which turn DC input into high frequency AC "signals".

As microcontrollers and high power transistors got exponentially cheaper, a technology that was only available to deep "industrial" pockets is now available at astonishingly low cost.

I settled for a Turnigy Trackstar 1/10th 60A Car ESC V2 again from HobbyKing

Rated at 60A it is agains overkill, but a less than $30 there's little point in getting a lower rating. It comes with a small fan although it is unlikely use of the springer will ever get it warm let alone hot.

Obviously this is a car ESC. Most ESCs are designed for planes and don't feature reverse. While this might be acceptable on a fast electric boat; the lack of reverse makes these ESCs useless on "work" boats.

You either have to find a boat ESC, but most of them are designed for high current draw applications (speed boats) and again sometimes don't have reverse either.

Another route is car ESCs. Most will have reverse. Most of them have a "brake" feature which is useless on a boat. A few won't let you disable said feature. On these ESCs you might have to actually pull the throttle stick gently in reverse until the ESC stops de motor (which on a boat takes 0.1sec ...), set the thottle to neutral for two seconds and then reverse again to actually get the motor to reverse.

That kind of behavior obviously makes little sense on a boat and is very annoying. But again, most car ESCs offer the basic "forward/reverse" mode in which the ESC reverses the motor as soon as it has been halted which emulates the behavior of a "classic" brushed setup. This is called "rock crawler" mode on some ESCs as off-road trial trucks use this as opposed to track cars.

As most - if not all - brushless ESC its feature can be programed either trough the remote or with the programing card. Programing through the TX is more tedious but gives you access to some settings not available to the programing card. The programind card is easier and gives you access to settings not available through the TX (ahhh !!) but needs to be purchased separately. Here are the settings:

• Brake/Reverse type
  • Reverse lockout : annoying mode which requires 2 seconds of neutral before reversing ...
  • Forward/brake only : no reverse
  • Forward/brake/reverse : "direct" forward and reverse. ESC brings motor to 0r/min before going reverse or forward. Virtually allow you to go from Full Ahead to Full Astern.
• Brake amount : amount of brake when you pull the stick from forward into reverse. A boat prop has little inertia (compared to a car) which makes that setting quite useless in a boat. Keep in mind that the harder the brake is set the more it draws power.
  • 25%
  • 50%
  • 75%
  • 100%
• Reverse amount: props are less efficient on reverse, but on a tug you might want +100%/-100%. Up to you !
  • 25%
  • 50%
  • 75%
  • 100%
• Punch control : power draw when increasing speed.
  • High
  • Medium
  • Low
  • Lowest
  • Disable/unlimited: as much as the battery can deliver
• Drag brake : with no brake, the propeller will slow down to zero in probably less than half a second. A higher drake brake setting will stop it almost instantly but will draw a little current.
  • Disable/None : no brake, prop will slow down on its own
  • 10%
  • 20%
  • 30%
  • 40%
• Throttle dead band: how much you can wiggle the throttle stick around neutral before the ESC actually starts the motor
  • Large
  • Normal
  • Small
  • Very Small
  • Smallest
• Voltage Cutoff : whether and when to slow-down the boat to let you know battery is almost done.
  • None : as I run a NIMH pack, which is more tolerand to discharge than LiPo, I don't need the cutoff feature.
  • Auto-detect LiPo : LiPo batteries don't tolerate deep discharge. Using the cut-off feature is then required.
  • 5V
  • 6V
  • 9V
  • 12V
• Motor timing : defines when the "current pulse" is sent by ESC, relative to the position of the rotor. Outrunners are supposed to run best with higher angle/timing. I haven't been able to notice any difference between 'Normal' and 'High' (haven't measured actual amp draw though) I decided to stick with 'Normal'.
  • Lowest
  • Normal
  • High

Propulsion : Propeller

No propeller is provided with the kit as the type of propulsion is left at the builder's discretion.

I decided to go with a Raboesch Type #147 40mm 4-blade right-hand brass propeller.

It fits nicely on the M4 shaft and does not interfere with the rudder. Pitch is 41mm.

Speed is given by the following formula: RPM*60*P*S where:

• RPM : motor/rotational speed in r/min. 5760r/min (see above)
• 60 : there are 60 minutes in one hour !
• P : propeller pitch (theoretical travel of the prop in a solid) : 41mm = 0.041m
• S : slip rate (amount of "loss" compared to theoretical speed) : 40% is a reasonnable value for a tug boat, trawler, ... Can vary greatly though. Fast electrics have much lower slip rates.

Keep in mind that actual motor speed will be lower than unloaded speed. We'll guesstimate a 10% loss and consider maximum motor speed is 5180r/min.

Maximum speed would hence be: 5180*60*0.041*0.60 = 7645m/h or 7.6km/h (4.1 knots).

Slip rate is also a wild guess. I'll record actuall speed with a GPS to check how off theory is. :)

While this figure might look way too high, keep is mind that while our boats "scale", water and wind don't. And I'll use my tug as a rescue/recovery boat and the extra bit of power will be useful.

And you can easily maintain "scale speed" by exercising some restraint on the throttle stick.

Update: I got a new Raboesch prop for "extreme towing". Had the opportunity to play with a huge (in size and awesomeness) tow : a semi-submersible heavy lift ship (think Mighty Servant). At 4.00 meter in length and ballasted down with 200+ liters of water it was a nice big tow !

I got a #174 45mm 4-blade prop from Raboesch. Slightly less pitch (40 vs 41mm) but a massive increase of blade aread. The extra thrust proved very useful when towing the Spirit of Archimedes. Amp draw slightly increased but even running my springer a constant full throttle I still got one hour and a half / two hours of run time out of the 6V/7.2Ah battery.

Propulsion : Battery

I already run my good old Carina on a 6-cell 7.2V NIMH battery. I don't want to store and maintain lots of different batteries so I decided to use the same battery on my spring tug.

A 3300mAh 7.2V battery weighs around 350g. I could also use a 5000mah+ LiPo 2-cell 7.4V battery which weighs slightly less (330g) while giving more run time. The ESC is LiPo-ready, it would just require programing.

Another option is to go with a sealed 6V lead-acide battery. I have yet to check what size of battery I would be able to get through the main hatch. Maybe up to 10aH which would give two or three hours of run time while serving as ballast.

Update: considering the huge amount of ballast required to get the tug down (see 2014-02-11 and 2014-02-16 build log entries), I decided to get a 6V 7.2Ah lead-acid sealed battery.

That's around one Volt down from the NIMH 6-cell battery. Hence top speed will be down. Even at 6V there's more than enough power (see first trials).

Top speed should be as follows as per the formula given above:

• RPM: 6V*800Kv*90% = 4320r/min (10% sag under load)
• Top speed: RPM*60*P*S = 4320*60*0.041m*60% = 6376m/s or 6.4km/h (3.5kn)

As seen during the trials, springer tends to dive when full ahead. Maintaining top speed seems a little unreasonable with current ballast configuration. The extra punch will probably help when pushing/towing though.

Update #2: I just got a Zippy Flightmax 2-cell 4200mAh LiFePo4 battery.

Why the change?

• Higher actual capacity under heavy towing
  • I've been to a couple day-long events, with towing. Actual run time on the SLA battery was down to less than 2 hours and battery had to be charged overnight ...
  • Now that I have the barge, power draw is getting higher
• Ability to trim the boat better
  • Because of it's weight the SLA battery was the only ballast I had near the bow with virtually no adjustment possible
  • With the LiFePo battery (which is around 300g), extra lead was needed and could be located to get the balance I wanted
• Ability to fast charge (< 2 hours) which is convenient for events

Radio : TX

I use a Spektrum DX7s 7-channel DSM2/DSMX transmitter which may sound like overkill. But a single TX lets me control multiple receivers/models. And even on a simple boat as a springer tug, I use features not found on a simple two-channel radio. I programmed a mix so that I can control the throttle both with the throttle stick and the proportional knob (potentiometer) on top of the radio. I also use dual rate/expo on the rudder with two settings: one for "long range" cruising with limited rudder range and one for maneuvering with full rudder movement.

Radio : RX

I got a cheap Orange RX R620 DSM2 6-channel receiver from HobbyKing. This is a spektrum compatible/clone receiver. At $13, I think there's no point getting a 2-ch only receiver. I'm still amazed how cheap these are.

Antenna is taped to the coaming of the main hatch (wheelhouse). That's maybe 5cm above warterline and range is very good. I can easily pull off 100m. We'll see whether range becomes an issue if I ever decide to try FPV.

Radio : Rudder Servo

Again I went the cheap route with a standard size HobbyKing waterproof servo.

Glues

Three types of adhesives were used.

• Waterproof wood glue (Titebond III Ultimate; any brand would do) for main assembly
• 5-min epoxy (cheapo HobbyKing) and 30-min epoxy (Z-Poxy) for critical assemblies and to add reinforcement filets
• Cyanoacrylate (Loctite Superglue) for initial assembly and small parts

Sealer / Primer

I decided to use Yachtcare G4 marine sealer/primer (FR EN). G4 is polyurethane single-component resine developed for marine applications as a sealer and primer.

Once the thinners/solvents have evaporated the resin will harden by reacting with ambient moisture.

Multiple thin coats are applied. When used as a wood sealant the first couple coats can be thinned with a PU-thinner to really soak up the wood inside out.

A new coat can be applied once the previous coat is touch dry (usually 2-3 hours).

G4 is a good primer for polyester resins or epoxy resins. Polyester resin must be applied before G4 has fully cured. Epoxy resin on the other hand can only be applied after G4 has thoroughly cured which takes a few days depending on temperature and moisture in the air.

Paint

I decided to use the same paint as on the Odin. Less waste as I can reuse paint across boats. Plus my fleet will look consistent !

Plastic primer

Dupli-Color plastic primer, Clear

Universal primer

Dupli-Color Platinum Universal PRimer, Grey

Hull

Dupli-Color Platinum RAL3020, Traffic Red, satin matt

Freeboard, bulwark

Dupli-Color Platinum RAL9005, Deep Black, statin matt

Deck

Dupli-Color Platinum RAL7001, Silver Grey, satin matt

Wheelhouse

Dupli-Color Platinum RAL9010, Pure White, satin matt

Satin/matt finish was chosen over glossy which I think would look a little off on a work boat.

Five to six thin coats of paint were applied, ~4 minutes apart (at 15°C) after parts were degreased with silicone remover.

Dupli-Color offers for an extra 3€ a set of spray valves with various spray patterns. The wide pattern proves very usefull and is virtually required when painting large areas ("large" being anything larger than 10cm x 10cm) or else you'll end up with most of your paint as dust on the floor rather than on your model ...

No varnish was added over the paint. Paint supposedly doesn't require it but only time will tell.

1. Boats
2. Sequana
3. Bill of materials