Propulsion suggestions

We have a 30’ x 12’ x 3.5’ steel barge all .25” plate which weighs approximately 17,000 lbs. It is used for dredging around our dock area; when working it has a 20,000 lb excavator on deck along with a 5 yard container for spoils. The total weight is approximately 45,000 lbs loaded. It is currently powered by a single 150 hp outboard engine which even at half throttle supplies plenty of power for our use. We would like to replace the engine with either hydraulic drives or submersible electric drives because the outboard cooling system has trouble operating in the silt filled shallow water.

Hydraulic Marine Systems, Inc., Harbormaster, and Thrustmaster all make really nice commercial systems but they are all out of my price range.

Our primary use would be for movement in and around the dock and boat house. Most travel would be limited to about two hundred feet from the shore and is on a lake so current is not a concern. This is all personal use and limited to about 80 hours a year.

We are exploring one of two systems:

1: Using twin electric submersible motors (similar to trolling motors but much larger) powered by battery packs.

2: Using single or twin electric motor(s) mounted inside the barge with a hydraulic pump mounted directly to it (them) which would drive the two hydraulic motors mounted in an outboard configuration. This would also be powered by battery packs.

The weight of the batteries is not a major concern; we would probably use AGM batteries because they would be below deck. The batteries would be kept on maintenance charge while not in use from shore power while tied to the dock. Considerable thought was given to using a small diesel to power the hydraulic pump but with the limited use we are leaning to battery power.

Obviously with a barge speed is not a concern even empty it would only travel about 5 MPH, we are only looking for pushing power. We think a pair of 18 to 20 inch diameter propellers with a shallow pitch; slow turning would give us the thrust required.

Considering we only use the current outboard at half throttle or less (it starts to cavitate above ½ throttle, most likely because of the small diameter high pitch prop) and still have more than enough performance; it would seem we could design the system for a total of around 30 to 40 HP and still have good results.

Thank you for reading, any thoughts or suggestions would be welcomed.

Mark

 

Mark
The power and prop size you have in mind with the electrics seems ideal.

Getting into proven marine units will be expensive even for the smaller units you are thinking about.

There are some reasonably economic EV motors and controllers such as these:
http://www.thunderstruck-ev.com/AC_drive_performance.htm
It starts to get complicated when you think of the gearing required and the shaft sealing. One factor in your favour is the slow speed so the housing below water does not have to be tremendously streamline. I think some sort of pod on arms that allow them to be easily lifted out of the water would be worth some thought.

It depends on the engineering skills available and if you want to do a bit of design and development.

There are some decent size systems commercially available but you need to google around and be seated when you check the prices. There are certainly some referenced in threads on this forum.

Rick W

 

You could replace the engine on the outboard with an electric motor. That can be a low voltage DC48V or an AC motor running off the generator

 

Rick and Gonzo,

Thank you for your replies.

Do you have a recommended RPM at the propeller? If it helps I attached a few photos of the barge as it sits.

For planning purposes am I better to start at the prop and work back to the engine that would be able to supply the torque needed to power the prop or the other way around?

What were your thoughts about using hydraulics powered by an electric motor.

Rick,

Is there a reason you recommended an AC motor over a DC. I will be running directly from battery power.

Thank you again,
Mark

 

I do not like the brushes in DC motors. They are a source of maintenance that is avoided in AC. The electronics are so good these days that even the cheap stuff gives reliable efficient operation. No more complicated than a DC controller.

Most of the electric motors used in radio control and EV these days are AC.

You would normally start with an estimated drag on the hull at the required speed. You could do a reasonable back calculation from your existing prop if you know the engine rpm and the gear ratio. So record the engine rpm at the desired barge speed. Get the prop details for pitch and diameter and the bottom gear ratio in the leg. These are usually in the outboard spec.

The design then goes into prop selection based on the required thrust at the target speed.

Rick

 

Outboard engines run at high rpm which makes for a cheaper replacement. From your description, you need something like a 50HP motor. It should run at the same rpm as half throttle which is probably around 2800 or so.

 

One big question would be endurance.

How many hours will this unit need to operate at one time.

Sounds like the power requirements are small and any 4 stroke outboard with a big prop would do fine.

Some mfg create geared OB models that swing big props , mostly for displacement sail boats, 1st choice.

FF

 

Fred,

Thank you for the reply, as far as enduarance that should not be an issue. Almost all the time would be at or near the dock itself. Once a year I would take it to the marina to clean the bottom, about 4 miles (an hour run at the barge's break neck speed) we would have our regular boat incase there was an issue with the batteries going dead. The only problem I have had with the outboard was the cooling system, while digging there is so much silt disturbed that the system clogs.

Someone suggested converting the outboard to a remote water pickup. That is a possibility but I think we are still leaning towards the twin electric drives, most likely 48 volt motors with around a constant 12 to 15 HP each and peak in the mid 20's. With the correct reduction I should be able to swing 18" to 20" inch props. We are also still considering both direct electric and electric over hydraulic. Most of my current research has been in which motor to select. I'm sure I will have a few more questions as the choices get narrowed down.

Thanks again,
Mark

 

Cheap AC motors, cheap (Chinese) Diesel AC Genny (aircooled 1.200$) direct driven. et voila..............
These gennies live only for about 700 to 1000 hrs. But that maybe centuries in your application.

my 2c (€ Cent of course)
Richard

 

Rigs like what you have are commonly nudged around by a keel-cooled small boat. For as often as this thing moves, I wouldn't give it any repower but use the outboard when possible and get a push at other times. Maybe harbourmaster, http://www.harbormastermarine.com/ , makes a drive to suit your purpose...

 

Since the power required will require a motor to operate anyway , the weight and expense of generators and pricy electric motors and dieing batteries is hardly needed.

I would use a hyd package , air cooled probably used , and cheap.

For a drive a hyd motor on whatever scrap big outboard , with blown motor you can find would do.

While hyd motors are waterproof , there really not built for sea water immersion,

An outboard leg IS.

Run it in the muck, all you fix is a prop, since efficiency is not a criteria lots of used spares can be changed out quickly.

FF

 

That Yamaha is probably a great engine, designed for your app with suitable torque and prop. Your local service man or a competent gadgeteer should be able to make a closed loop cooling system for it. Even a 55gal drum of fresh water and a separate electric circulating motor might work.

 

Can you add a big fast oil connector to drive an hydraulic motor just like another hydraulic accessory you connect ?
Some Oyster boat are done this way with a vertical lifting
Can be made out of used part to be more affordable

 

why not use an air cooled mud motor
http://www.mudbuddy.com/HyperSport.htm

 

Oops, never thougt I would be on the same train as Kistinie, guess the end is near.........but he's right: since you always have an excavator on board: They practically always have an oil power take off available. Use this, with quick connection fittings to hydraulichoses and a hydraulic motor at the bottom end of a vertical tube, or even mounted as a longtail engine.

Alternatively, mount a used Volvo or Merc I/O drive to the barge and connect your hydraulic motor to the inside shaft stub.