Twin engine CPP

Discussion in 'Powerboats' started by marcoh, Sep 18, 2020.

  1. marcoh
    Joined: Sep 2020
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    marcoh New Member

    I am keen to get some feedback on my thought and pros/cons of controllable pitch propeller (CPP) on a twin engine displacement boat.

    I am in very early concept design stage of a new boat which hopefully will be built one day! I have read all the pros/cons of single engine boats, and twin engine boats. My boat will be in the region of 60ft/60t and I like the idea of two engines for redundancy.

    For discussion sake say I need a 300hp engine, or two at 150hp, and boat need 120hp (40% of full load) to push the boat at 8knots and 1400rpm, and WOT at 1800rpm and 10knots and engine is M1 continuous duty rated . So my query is what will happen if operated the boat on one engine? I have the hp to run at 8 knots, but a prop will be pitched wrong so will I overload the engine with a normal prop? Often people operating a twin engine boat on one engine and reducing fuel consumption by say 15% but what got me thinking was seeing a picture of a similar boat as I am envisaging with black soot all over the top of the boat which could overloading of the engine.

    If both props are CPP then the operating engine/prop can be pitched to the most appropriate pitch with either one or two operating engines, and the non-operating prop fully feathered.

    So if this all makes sense why are so is this not more common (or i have found zero examples), is it just cost? but the costing I have so far indicate only a small additional cost.
     
  2. nemier
    Joined: Jan 2004
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    nemier fascinated with distance voyaging, power or sail.

    Interesting question, I'm keen to see the responses. Lots of commercial boats are set up with this exact configuration (I work on them), but I do admit to not seeing any recreational boats (yachts) set up like this. I agree with your points put forward.
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Marco.
    Other folk more knowledgeable than I will be along soon, but in the meantime here are a few random thoughts.

    I think the two main reasons (against CP propellers) on pleasure boats are complexity (relatively more, compared to a FP propeller) and cost (a lot more!).

    CP propellers are especially useful for fishing trawlers and tugs, but even so, many tugs nowadays have FP propellers.

    In your example above with the 2 x 150 hp engines vs a single 300 hp engine, you would have to see the propeller curves to do a proper analysis.
    But common sense would tell you that there is no point in running one engine hard to achieve 8 knots, if you can run two engines at a speed where they are both very happy, and have the engines sipping fuel (relatively).
    If you shut down one engine, you also get into that (never ending!) discussion about if you should lock the propeller shaft, or allow it to free wheel....... some gearboxes are not happy if the shaft is allowed to spin with the engine not working.

    Some years ago Bruntons brought out an automatically self pitching propeller - more aimed at sailing boats, but it could maybe work for what you have in mind?
    https://www.bruntonspropellers.com/autoprop
    But then why would you want this extra complexity and expense on a motor boat when a pair of FP propellers, used sensibly, will do the job very well?
     
    Last edited: Sep 18, 2020
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I suppose the alternative could be a two-speed gearbox, but you don't see much sign of that.
     
  5. bajansailor
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    bajansailor Marine Surveyor

  6. marcoh
    Joined: Sep 2020
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    marcoh New Member


    Thanks for the comment Bajansailor, right now I am still looking at the options. With the right CPP selection to enable the prop to be fully feathered, the question with locking or free wheeling the shaft becomes a moot point.

    I agree the need to have close look at propeller power curve and engine power/torque curves and that is what started me thinking. My take thus far is that you need two larger engines (maybe not 2 quite at 300hp but larger than two at 150hp) to not overload the single engine in single engine operation, so there is a cost saving to be had there.

    I am in the process of pricing the option so will be able to confirm cost soon, but not sure if a CPP is really that much more complicated. The gearbox is a simple reduction gearbox (nor reverse required) and the prop pitch is simple change by the inner shaft, to me the complexity issue is not significant.

    My goal will be to cruise remote coast around Australia, Indonesia, and the South Pacific Islands, so will be long way from help, and a large main with a small wing engine does not give me confidence should the main fail. And before someone says commercial fishing boats go out all the time with a single main, and NO wing engine, a 1 minute google search will also find multiple examples of broken down fishing boats being towed in!

    Anyway appreciate all the comments, and will keep on doing my homework! :)
     
  7. nemier
    Joined: Jan 2004
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    nemier fascinated with distance voyaging, power or sail.

    Hi Marco,
    Regarding the engine HP selection, please advise the anticipated fully-loaded displacement of the vessel.
     
  8. missinginaction
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    missinginaction Senior Member

    While I understand the sense of security having a couple of engines might give you I would argue that with proper maintenance a single will give you more economical performance overall. Have you ever considered a larger main engine and possibly a "wing" engine? Smaller wing engines can be used as propulsion in emergencies, for low speed operation and to charge batteries if properly set up (high output alternators and smart charging regulators).

    I'm a perfectionist regarding maintenance. In over 40 years of boating I was stranded exactly once (in 1983) on a runabout I owned at the time. The problem was a defective part that had been recently replaced. I was out on a test run after replacing the part and it failed.

    As you do your research I think you will find that for pleasure boats the complexity of controllable pitch is too expensive, too maintenance intensive for the typical owner and impractical for those reasons. Recreational boats are typically lightly used. They're not crossing oceans. Commercial vessels are heavily used. Commercial operators certainly do a cost/benefit analysis to determine the practically of using something as complicated as a controllable pitch propeller.

    When you're actually out there, far from shore you want robust, simple engineering. You're operating something made out of metal in a highly corrosive demanding environment. You can easily run into or over submerged objects. Yes there is sonar but even so there are objects out there below the surface that are almost impossible to detect as you're motoring along. More than once I've had both depth sounders sound their alarms while I was in 600 feet of water just minding my own business. Something was down there very close to my running gear. Maybe it was harmless but who knows? I'd hate to hit something substantial with a fixed prop let alone something as complicated as a controllable pitch wheel.
     
    Last edited: Sep 21, 2020
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  9. nemier
    Joined: Jan 2004
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    nemier fascinated with distance voyaging, power or sail.

    Marco,
    I have a different perspective from Missinginaction, yet we share the same maintenance and operating views.
    I have experience with NOGVA CPP systems, and they are robust, reliable, and cost effective. Some CPP are are actually simple systems, and as you know, offer beneficial features (no reverse gear / ability to feather, etc). I have a recreational vessel that is designed to cross oceans, and is configured with a single main and wing engine and has never failed me in 30,000 miles traversing oceans. HOWEVER, I am always fearful of the main quitting (unfounded fear) and being forced to rely on the underpowered wing for 1200 miles, and in our case with no stabilization. So fearful in fact, that my next boat (if I go that route) will be a twin engined vessel, and the icing on the cake would be a NOGVA CPP system. This is why I am so interested in this thread. I tell you right up front that my fears are irrational, but everytime I'm offshore, dead-of-night, blowing 40-50 kts - howling rigging, and 6m beam seas smashing the hull sides, I always find myself praying that the big boy keeps going. It always does, but I've grown to hate that feeling. What would stop that feeling? Another main engine ; )
     
  10. missinginaction
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    missinginaction Senior Member

    Well Nemier, you're about as far out of my league as it's possible to get. It's interesting though. You own a blue water boat with a single and a wing. You said it yourself. It's never failed you. If I were a betting man I'd wager you stay with your current rig. Take care of that boat. She's a beauty. We're all different, I suppose that's what makes the world such a fascinating place.

    After I wrote the paragraph above I had a thought though. I generally boat alone and while I'm never more than about 20 or 30 miles offshore, I've found that your mind can play games with you as you're alone with your thoughts. I've had to learn how to discipline myself and not let unfounded fears ruin my time aboard.

    I remember once having a problem on my little boat. I won't bore you with all of the details but while all of my gauges told me things were fine, my ears told me that something wasn't right. I stopped. "What do I do now?" I thought. I couldn't understand why what was happening was....well, happening. I could feel myself getting a little panicky. "Just breathe" I thought to myself.

    Then I made a sandwich.

    By the time I finished lunch I had an idea and the problem was quickly solved. I was soon on my way. I always take comfort in that experience and promise myself to remember it. Sometimes you need to just sit with a problem for a bit. These days everything seems to move so fast.

    Nevertheless if twins and controllable props make you sleep better who am I to judge?

    Regards,

    MIA
     
    Last edited: Sep 21, 2020
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  11. marcoh
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    marcoh New Member

    Hi Nemier, I had been doing some high level estimates assuming 60,000kg displacement, but just change my assumption to match a Nordhavn N60 as per my next post so I have a good reference point.
     
  12. marcoh
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    marcoh New Member

    I found MIT course work for Hydrodynamics on line so have doing some homework 2.016 Course Info http://web.mit.edu/2.016/www/courseinfo.html

    I have also been playing with this online propeller calculator from Vicprop and using the NordhavnN60 measurements/displacement (LWL: 57.3ft; displacement: 62.6T) as a baseline, the N60 has a reported WOT speed of 9.8 knots with 325hp@2200rpm engine and 3.958:1 gearbox the Vicprop calculator predicts 9.54 knots, so less than a 3% difference, and therefore seem fairly accurate. Vicprop - Prop calculator for Displacement and semi-displacement hulls https://www.vicprop.com/displacement_size.php

    With two 160hp@2300rpm engines, the prediction is a WOT speed of 9.49knots with a 35.8" diameter / 28.7" pitch prop, and a single 160hp engine should achieve a WOT speed of 7.53 knots with a 35.8" diameter / 24.0" pitch.

    So based on these numbers are 160hp engines enough? and would a 28.7" pitch prop overload the single engine? More homework to do!
     

  13. nemier
    Joined: Jan 2004
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    nemier fascinated with distance voyaging, power or sail.

    Marcoh,
    2 x 160HP engines sound perfect. If anything, they may be on the large size - assuming Continuous Duty engines.
    The Deere 6068TFM75's would be great, and I would even consider a pair of BETA 150's.
     
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