What is the HP needed to drive a 20m x 3m 40MT boat at 13kph?

Discussion in 'Boat Design' started by HappyDork, Jan 16, 2025.

  1. HappyDork
    Joined: Jan 2025
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    HappyDork Junior Member

    I am trying to determine how much battery capacity I would need to operate a 20m x 3m 40MT boat @ 13kph for 900km. (65'x10' - 40t @7 Knots) I pulled a couple of equations off on the internets and am seeing around 110kw. (150hp)
    I'm not convinced that the equations are right as when I take the equations to extremes they produce unreasonable results. If anyone can help point me to the right set of equations. Thank you
     
  2. seasquirt
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    seasquirt Senior Member

    Hi Happy, welcome to bd.net, do you have any drawings of the hull shape, the bow, the draught, the stern, the prop size/pitch, things like that ? The experts here can assist you better with as much information as possible, boat lines, pics, materials, construction methods, etc.. More info on what type and size motor, voltage you will be running, and finer details like that. Then bigger chance of getting a more accurate answer. Good luck with your boat.
     
  3. Will Gilmore
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    Will Gilmore Senior Member

    Just as seasquirt is indicating, there is no straightforward, easy answer to that question without knowing specifics about the vessel. How much the boat weighs will also be a big factor. For instance, the formula for Hull Speed is solely based on waterline length, but it only gives the point at which adding more power to push a boat faster begins to have diminishing returns. It does not, in any way, suggest the power needed to get the boat to "hull speed". That is a function of displacement and shape, mostly below the waterline, but not only.

    Prop configuration, RPMs, and location will also make a difference.

    -Will
     
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  4. Rumars
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    Rumars Senior Member

    First you need to differentiate between battery capacity and motor power. Capacity is measured in kWh, you need to multiply the power needed to propell the hull to the desired speed with the time of operation. If the 900km are to be done on one charge we divide 900km by 13km/h = 69.2h. If the distance is to be covered on multiple charges you use the longest distance between charges to determine the baseline time.

    Needed power varies with wind and wave conditions, the extremes beeing a dead flat sea without any wind and powering against wind and waves in a storm. Normally it's the designers job to determine installed power. Displacement hulls can have anywhere between 0.5kW/t to 4kW/t.

    Anyway, we can make some rule of thumb assumptions. A nice slim hull like you propose, driven well below hull speed should not need more then 500W/t in average conditions. For a one charge crossing that means (0.5kWx40t)x69.2h=1384kWh. Plus reserve you are looking at a 1500kWh installed battery as a minimum. If the average operating conditions are better then good, the baseline time is shorter, etc. the needed capacity will decrease.
     
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  5. HappyDork
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    HappyDork Junior Member

    I don't have a boat design as I want to see if there is space for a business plan. I will then hire a designer. If the 500W/t @7Knots holds for boats this size, it is easy to beat trucks by 50% on a $/t/m basis. With a designer that knows what they are doing, it is likely far better than that. Thank you very much.
     
  6. Rumars
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    Rumars Senior Member

    The oldtimers made due with 1hp/t as a general rule of thumb for achieving hull speed in calm conditions. UK narrowboats moved 40t of cargo with 18hp engines, altough not at 7kn. Operating conditions and hull optimization go a long way.

    I'm afraid that beating a truck on money is a completely different thing. $/t/m is different for EV's because you have to amortize the initial battery cost. Cash flow models are completely different and this makes a big difference. For example, if you actually need a 1.5MW battery the installed cost would be at least the same as buying 10 additional semi-trucks. If you have to finance you end up with a very high fixed cost and you must have a good plan how to deal with this.
     
  7. jehardiman
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    jehardiman Senior Member

    Ok, if that is your thrust, what you are specifically looking for is the Gabrielli-von Karman analysis. Here is the original and there are many updated and revised analysis.
    Research report - U. S. Land Locomotion Research Laboratory, Center Line, Michigan NO4 1956. https://babel.hathitrust.org/cgi/pt?id=osu.32435023173206&view=1up&seq=6&skin=2021

    FWIW, if you study transport costs you will find that the real cost is in transhipping, not the actual movement. This is the driving economics behind the dominance of modern over the road truck and seaborne container shipping. The fewer times the cargo is touched, the cheaper it is, regardless of energy costs.
     
  8. philSweet
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    philSweet Senior Member

    900 km isn't doable. 90 km isn't doable either. 9 km is probably pushing it. Look into how the old WWI and WWII diesel electric submarines managed things; and see if you want to go back to doing things that way. Some could manage about 35 miles submerged on batteries. But they were 350 feet long and only doing about 3 knots under water. And they spent 99% of the time on the surface running diesels.

    Do you plan to buy half an acre of land at each port and build your own substations for recharging? You're going to need to do that. Remember you have to recharge the things in a reasonable time. Lets say, with house loads and a reserve margin, that you do run 150 electric hp for 69 hours. You want to recharge in 12 hours. That's 862.5 HP recharging. Call it 635 kW. This is more than three times the size of the largest mains voltage connection that my utility will supply, so you need your own high voltage transformers. They're about $100k a pop FOB. You'll need to put one everywhere you want to recharge. You'll probably want to recharge at off-peak rates.

    What does a 700 kW shore power cord look like?

    Have you decided on a cell size? 500 pound cells, 1000 pound cells, 1600 pound cells? How to you handle them on a small boat?
     
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  9. Will Gilmore
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    Will Gilmore Senior Member

    A business opportunity? Of course, if you built your own recharging stations, it would be very wasteful to not sell the service to other boat owners looking to go electric too. Business would be slow, at first, but once the infrastructure was in place, more electric boats would start popping up.
     
  10. HappyDork
    Joined: Jan 2025
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    HappyDork Junior Member

    I love you guys. This is amazing feedback.
    Transshipping: I can't agree more, this is what is driving the size and shape of the boat. I want to be able to roll-on/roll-off a single dry van trailer not at ports, but at boat launching ramps & docks. Customers would schedule a pickup & delivery, drop off their trailer at the privately owned facility and the trailer would be autonomously loaded onto the boat via a small battery powered "truck". I'm originally an Electrical Engineer who manages a AI/ML R&D team. A small autonomous truck which never breaks 5mph on private property is child's play. Depending on the battery needs, I could have this being a rolling battery pack which drives the power needs of the boat and slow charges at the loading zones. I can also cut down on the boat range requirements by allowing boats to swap out the battery-trucks as needed at the privately owned docks.

    Powertrain - I like electric motors just based on the simplicity of the design and how much cheaper it makes maintenance. There would need to be several repair depos. Being able to pull and replace a small electric motor and get a boat back out making money beats having to repair a built in diesel engine on site. Additionally there is no legislative risk going electric. @philSweet Now for the power source, I'm fairly agnostic. Economic development is the main goal, green is a distant second as a more energy efficient transportation system uses less diesel even if it still uses diesel. The economics and legislative inertia are moving towards solar augmented battery, but an easily swappable diesel electric generator is legitimate, especially if the design would allow the diesel & fuel bays to accept batteries once the economics are clear.

    Cost of capitol - This is where speed matters. What is the most economically efficient speed when the cost of capitol is included. The target of this project economic development primarily in the Mississippi river watershed as well as protected waterways along the gulf and southern Atlantic coast. If you reduce the cost of transportation, you grow the economy... fairly dramatically. The cheaper the better, but cheaper doesn't mean slower. From what I have read power to drive the boat is proportional to the speed squared. The cost of financing the ships, the cost of the depreciation on the ships and the cost of inventory of cargo all factor on the optimal speed. Running these numbers are easy once I get a good estimate of &/m/t @ a given speed. @jehardiman thank you for the publication. I'll dig into that.

    If there is a design which can work, the hardest part is going to be politics. Autonomous boats aren't currently allowed to operate on public waterways. The folks I talked to at the Coast Guard made it very clear that while they loved the idea, they could not approve it without direct congressional action. I have been talking with a municipality which has a couple hundred acres of waterfront land they would be very happy to give to me for a manufacturing site, but I need something more than paper napkin sketches and power point slides. At an average 7 knots, 50% the cost of trucking and an average utilization rate of 75% there is a market need for an additional shipping capacity which could be provided by 2 million of these ships. Now obviously I'm not building 2 million boats, but there would need to be a HQ, design center, operations, several manufacturing sites, and many more repair facilities. All of these sites will provide good jobs for local voters. There is a lot of legislative love to go around. Hopefully it will be enough to get coast guard approval, and grants for the design work and the first manufacturing site.

    Again, thank you all for your input & advice. I sincerely appreciate it.
     
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  11. Will Gilmore
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    Will Gilmore Senior Member

    Love the plan. With the current focus on autonomous cars, trucks and drones, their success should make the argument for autonomous shipping via boat. These things can be successfully demonstrated with non-commercial rated models without CG approval. Just have a pilot present in case.

    -Will
     
  12. seasquirt
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    seasquirt Senior Member

    Hi again Happy, so then, what are the proposed dimensions of this trailer you plan to use, its volume, max gross weight, height, max capacity weight distribution, etc. Saying 'trailer', do you mean a road based unit like off an 18 wheeler combination, as in a soft side flat bed, or freezer, or bulk tipper, or tanker, or 10/20/40 foot shipping container carrier, or do you have a unique design optimised to your transport system ? That will give a basis to which a hull design can be applied. What will they transport, eg. processed metals like pig iron, ore or gravel, grain, or lighter loads. You will need to design for the heaviest and most top heavy loads you will encounter. Ro/ro implies road width plus a bit, at least, to get around for safety access. Will someone need to secure / chain down the trailers to avoid load shifting ? Or will you make special locking securing devices that fit in with your automation plan. Then what will happen when a load sensor sends an alarm ? It will all have to be idiot proof, and vandal proof, if fully automated. That seems like a big ask.
    When you wrote "I like electric motors" I immediately recalled a song called "I like 'lectric motors", it's on youtube, circa 1980's, a bit silly.
     
  13. HappyDork
    Joined: Jan 2025
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    HappyDork Junior Member

    @seasquirt The Dry Van Trailer is the king of last mile hauling in the US. I want to optimize around this to drive transshipment costs to near zero. @jehardiman was very correct for calling that out. Depending on the business vertical it will add 5%-20% to the cost of transportation, sometimes much more.
    Dry Van - Truckers Wiki - Everything You Need To Know https://truckers.wiki/dry-van-trailer/
    53' long x 8.6' wide x 8.6' tall, Max ladened weight 45,000lbs
    upload_2025-1-22_14-35-8.jpg

    The trailer would Ro/Ro into(from) a box just a bit bigger than the trailer. It would look like a box with a boat under it. There would be no lip or other external places for someone to climb on except the roof. (which will have solar if it makes sense). It will be an ugly boat.
    upload_2025-1-22_15-35-25.png Very Ugly Boat example for illustrative purposes only. I'm not an artist or a boat designer.

    For preventing load shifting, I was thinking a series of inflatable bags to secure the load to the inside if the box. Ballast tanks could help with unbalanced loads. The design would be used for calm waters. The boxlike shape of the boat could possibly allow several of these to connect together if there was an eventual desire to do the mainland to Puerto Rico run. I would need to investigate a lot more to see if this made sense.

    As suggested, I looked at canal boats which are very similar in size. They average a 50HP motor for 3.5knot. If this scales with the square of speed, even with a poor design, I should be able to get by with 200HP motor.

    What to do in an emergency... This is a hard problem which gets easier as the number of boats increase. With enough boats, just contract with tug boats along the route. ABC - American Boat Care (AAA but for boats?)

    Again, thanks for the help. Please keep the questions coming.
     
  14. montero
    Joined: Nov 2024
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    montero Senior Member

    Definitely VUB this is a new category of boat. It's not about your floating container. For example rib, sup and VUB.
     

  15. montero
    Joined: Nov 2024
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    montero Senior Member

    Rivers often change level, there is a current, obstacles sometimes appear. How do you imagine the structures on the shore and the loading process itself.
     
    Last edited: Jan 23, 2025
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