power cruiser design

Discussion in 'Boat Design' started by dreamer, Apr 14, 2009.

  1. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    (this is somewhat of a continuation of the 'Tumblehome' thread)

    I've done some (more) research into the type of boat I'm thinking of and here are the numbers I've come up with:

    LOA: 41’

    LWL: 40.6’

    BWL: 13.5’

    Displ: 40,000

    Top speed: 15kts

    Those numbers result in a D/L ratio of 279 which would require 160hp to drive at semi displacement speed. At top speed, the Froude number is 1.5 and S/L is 2.35 @ 15kts, which is in the range for semi-displacement. LCB is @ sta. 5.7. I didn’t see too many underwater views of hulls. The ones I did see had a bit of a chine aft which I have as well (may be hard to see).


    Aesthetically, I didn’t want to design another trawler or faux tugboat. I like the plumb bow and tumblehome as well as the slightly reversed transom. For these design cues I looked hard at the True North boats. Their TH is a bit more severe than I’m looking for, but you get the idea. I wanted to try a ‘reverse’ sheer (which makes me think Rybovich for some reason). This allows for some flare at the topsides and gives me a deep bulwark, handy if you have a dog on board I think. Problem is that the ports along the house won’t see anything except bulwark. I was thinking of moving the house up, but that means higher CG and I don’t want that.

    I've attached a simple lines plan and a rough rendering to give you some idea.
     

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  2. Tad
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    Tad Boat Designer

    Rick,

    Your weight seems awfully heavy and the horsepower appears way low for that much weight at that speed. Running very quickly through some semi-D spreadsheets I find 350-375 HP for 15 knots.

    The new GrandBanks 41EU is 2' wider at 15'3" and has a half-load displacement reported as 37,000. She requires 23.3 gal/hr at 14 knots, which is roughly 475 HP. Also note she burns half that much fuel at 11 knots.
     
  3. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    Thanks Tad,

    I will need to review my numbers. I'm new to the tools I'm using for getting this information (Rhinomarine). I think that hp was for two engines (hp each).

    I ran 4 scenarios in RhinoMarine for displacement which ranged from a low of around 30,000 to high at 40,000 depending on the draft I entered (2' - 3').

    It could be that I'm reading the numbers wrong. Before I get a bunch of flak for relying too heavily on 'those damn computers', I have done this by hand using a planimeter (not on this vessel) and would really prefer to do this digitally.

    I think a lot of this too has to do with me being a bit of a noob when it comes to boats.

    Interesting that you compare this to the GB 41 EU as that is the benchmark vessel I've been using.
     
  4. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Agree with Tad. The issue seems to be the assumed displacement. What would it take to try for a displacement of 25K to 30K and see where that leads you. If it is the goal to run at 15kts in a boat of normal B/L ratio, weight as well as hull design will be critical to fuel and power requirements.
     
  5. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    Thanks Tom.

    I went back and had another look at the numbers.

    I first went into Rhino and re-ran my calculations. That gave me a more reasonable displacement of 35,600lbs. @ 2'. I then took my solid model and applied materials to all the solid bodies.

    Hull:
    I can probably lose some weight in the hull. I modeled it at 1" thickness solid e-glass (173lb/cu. ft) = 19,700lb

    Deck and superstructure:
    3/4" e-glass + balsa sandwich (91.5lb/cu. ft) = 13,400lb

    Flybridge:
    1/2" solide e-glass (173lb/cu. ft) = 2,400lb

    total: 35,500lb

    I'm no boat builder, so I can't say for sure whether these material assumptions are right, but it is nice that this appears to jive with the numbers I got from Rhino.

    I could probably get this displacement down quite a bit by using either a thinner schedule for the hull or using a cored hull from the waterline up, but I don't know if I could knock 10k lbs off.

    As for my earlier power requirements, that number (140) was for displacement speeds. I'm not clear on calculations for power. What are some of the calculations for power and fuel consumption? Now, to throw a wrench in the works, my intention is to go all-electric with this boat. That means we may be able to go with lower horsepower as electric motors don't require reduction gears to produce torque.

    In any case, this is a great learning experience. Thanks for the help.
     
  6. Guest62110524

    Guest62110524 Previous Member

    you seem a wee bit confuzzed:)):p are'nt we all, you a re running several different themes

    see the boat my website, huge range low shp, would that suit you better? Not trying sell you anything at all The world has to wake up, green things are here and look at the HUGE roof for solar cells
     
  7. dreamer
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    dreamer Soñadora

    not confuzzed at all. All boats are 're running several different themes'. Unless I'm mistaken, boats have been around for tens of thousands of years. Anyone who says they're 'designing something completely unique' is FOS.

    Your boat is a nice design, but it's not the aesthetic I am after.

    I do not see any info on your boat other than some writing. You have hull lines, Hydro data, etc.?

    Solar is great, except if it's cloudy, night time, etc. I am definitely going green on this vessel. I would say MORE green than yours as I will be solely electric with diesel used only for charging and high house loads. Most charging will be done via solar and a proprietary wind-capture technology.
     
  8. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Rick...

    Your weight guesstimate is way off...minimum solid bottom thickness will be around 3/16", deck and cabin are less plus core. Complete hull structure will be less than 25% of total vessel weight (displacement), house will be about 5% +/-, machinery and fuel about 20-30%, interior 10%, then add systems, fittings, electrical & plumbing.....etc.

    I mentioned the GB 41EU as it's reviewed in the Jan09 issue of Power & Motoryacht, their "green boating" issue, which happens to be lying on my desk. On the cover is the Island Pilot DSe 12m Hybrid, a diesel electric Cat with a huge (4000 watt, 180 sq ft.) solar array covering most of her cabintop. 16 group 31 AGM batteries power the boat for about 6.5 hours at 3.9 knots, which may be offset with diesel and solar charging.
     
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  9. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi Rick,

    First, let me say I like the look of your initial sketch... it is a rather elegant shape, to be sure.

    I like that you're interested in "going green". I'm a former solarcar builder myself and have been involved in some land-based renewable energy research. However, I must caution you to be realistic about it.

    Firstly, you cannot get by with less power because you're switching from diesel to electric. A horsepower is 1.34 kilowatts, regardless of what type of equipment is turning the shaft. If your diesel boat uses 400 hp to reach a certain speed, the electric version will need a 400 hp electric motor to reach that same speed, with an energy source to match. Note that this is about 300 kW, which if supplied by solar alone- even on a sunny day in the tropics- would require at least 1500 square metres (fifteen thousand square feet) of the best silicon PV cells available today, at a cost of about three million bucks.

    A lot of people are really gung-ho on the Island Pilot DSe, and I do like the concept. But she's basically a floating solar farm, and still has to fire up the diesels if the captain wants to go faster than four knots for any length of time. And the DSe is a very low-drag displacement catamaran form, with power requirements vastly lower than a semi-displacement mono of similar size. On electric alone it has only about 10 hp on each of two motors- and that's enough to drain the battery in less than an hour, if you push both throttles to full ahead.

    So I'm not trying to discourage you, Rick.... dreaming is wonderful. But I think you'll need a better idea of how and where the boat will be used, and what capabilities you really need, before you'll be able to progress much beyond the dreaming stage.

    Looking forward to hearing more :)
     
  10. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    Thanks for the valuable info guys.

    Tad, your weight review will be most helpful though 3/16" sounds pretty thin for the bottom thickness. Do you mean 3/16" plus core? If so, what would core thickness be? Does the house weight include the flybridge?

    I have tried to find some difinitive information on what is typcial for the layup schedule, but haven't had a lot of luck.

    Matt, thanks for the info. I have not done any of the numbers for this, my info is mostly speculation based on what I've read is currently in development in other areas besides boats.

    As for power, correct me if I'm wrong. My understanding when it comes to ICE (internal combustion) vs. Electric has to do more with torque than HP. Since a 400 hp ICE doesn't reach optimal torque until a high RPM range, gear reduction is employed. In electric motors, Maximum torque is available right away. So, consider the scenario where you have a 1.5 reduction for the ICE. That would not be necessary for electric and therefore you could get by with 25% less hp (wattage). No?

    Also, consider the batteries. It seems to me that some breakthroughs in battery technology are on the horizon. These would go beyond anything we use now (including AGM) using a smaller form factor for the same amount of storage.

    To be realistic, today I do not think it is feasible to have full-electric unless you have tons of real estate dedicated to batteries and correspondingly huge devices to keep them topped up, including supplementing the charging scenario with off-line ICE generators. In the short term (< 5 years) we will need to be satisfied with hybrids until the battery and charging technology improves.

    In my case, I would have a diesel generator to charge the batteries. The nice thing about this is the ability to distribute weight more evenly.

    I'm glad you like the aesthetic. Here's another image.

    Cheers
     

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  11. marshmat
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    marshmat Senior Member

    You are correct that IC engines often require a reduction gear, where electrics often do not.

    Consider the following:
    Power = (Angular velocity) * (Torque)
    Power = (2 * Pi * RPM) * (Torque)

    For a given power, you can trade off between RPM and Torque. 100 ft.lb at 1000 rpm, or 50 ft.lb at 2000 rpm, give the same power. Slap a 2:1 gear on the second case, and the output shafts of the two powerplants are now indistinguishable- no measurement you can make on the output shaft will reveal which one is which. (Ignoring, for the moment, the 2-3% or so loss that the gearbox might introduce.)

    Now:

    Power = Thrust * Velocity

    Thrust = Drag

    A particular hull will have some characteristic drag (measured in kN, or lbf for those who prefer the old units). Multiply that drag by the speed the hull is going, and you have the power needed to maintain that speed. (Note that a typical propeller will only convert somewhere from 40 to 70 percent of the shaft power into actual, useful thrust.)

    Power is what moves the boat. Torque is what spins the prop. You can transmit 100 hp through a big prop at low rpm with high torque, or a small prop at high rpm with low torque: it's still 100 hp. It doesn't matter what kind of machine made the shaft spin, the power is the same. But each case is appropriate for a different situation: big slow prop for a slower boat, small fast prop for a faster boat.

    You can get exactly the same high-torque-at-low-rpm that you get with an electric drive, by putting a big reduction gear on a diesel. (See "tugboat".)

    Where electric/hybrid has the advantage is when the house loads (lighting, A/C, refrigeration, instruments, etc.) add up to about the same order of magnitude as the propulsion loads. This is true of cruise ships, and possibly of relatively luxurious, low-hull-drag cruisers like the Island Pilot DSe. It is generally not the case for a private, recreational craft that is expected to travel near or above hull speed.

    The worldwide push for electric car development will most likely lead to a wider variety of more efficient, lighter batteries being available at reasonable costs. AGM is a good technology, but the chemistry is still old-school lead/acid. We can do five or ten times that energy density now, we just can't do it cheaply at a large scale yet.
     
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  12. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    Drag.

    what a drag. ;)

    Thanks again for the info Matt.
     
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