Single engine / dual shafts?

Discussion in 'Boat Design' started by dreamer, Apr 26, 2009.

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    FAST FRED Senior Member

    "I even have higher aspect, around 15% area ratio, and can get high eighties efficiency with low speed operation around 6 to 7 knots. At higher speed, say 10 to 20 knots you can get efficiencies in the low nineties. "

    Given no structural hassles what size, shape prop would you chose for 120 HP and an 18K top speed?And a desired LRC of 12K?

  2. flyinwall
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    flyinwall Junior Member

    ok i just briefly scanned this and i didnt see (it might have been there but i didnt see it) what sort of hull this idea is going into the best way to get 2 props from 1 motor would have to be with hydrolics that way you could put the motor where you want and just run a couple of hydrolic lines to the motors and be done with it (i have seen this done with a 60' twin hull cat prawn trawler they mounted the motor (a big diesel v8 catapiller) under the sorting tray in the middle of the deck )
  3. Guest625101138

    Guest625101138 Previous Member

    It becomes purely academic if you remove structural aspects.

    Also I assume your engine rating is based on a low efficiency prop. With a prop that was much more efficient you would use much less power.

    I would design for the cruise and check structural and cavitation for higher speed.

    Taking the 12kt cruise and assuming drag of 2000N you could get 86% efficiency with a 1.2m diameter prop spinning at 328rpm. The blades would have a maximum cord of 100mm. This is the sort of size needed to handle the bending loads. Power would be 18.6HP.

    If the boat is more easily driven such that it takes 1000N at 12kts then a 1.2m prop will get 89%. The chord would be 80mm. Power is 9HP.

    Now assuming the prop can handle say the 5000N at 18kts the power will be 68HP but cavitation is coming into play. Efficiency is 87%. So more care needs to be taken with the blade shape to suit the higher speed.

    The root of the blades would need to be beefed up quite a lot to cater for the a 5000N thrust.

    You need to consider the hull rather than some assumed power. I expect you can appreciate that a 4ft diameter prop absorbing 65HP is going to have a lot less slip than a 20" prop absorbing 120HP. A 4-bladed 20" prop will absorb around 88HP to produce the same thrust as a 4ft at 68HP so this is getting closer to your nominated power. Efficiency down to 69%.

    So it can be done structurally for the prop but you have to decide if you like the idea of swinging a 4ft diameter prop under a small boat. Being two blade it would need a little over 2ft deep to float the boat or you could have some lifting system. Yachts often have much deeper keels than this of course.

  4. u4ea32
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    u4ea32 Senior Member

    On twin engine powerboats, its not all that unusual to leave the helm centered and only use differential thrust to steer, especially while docking.

    On my boat, I sometimes do this. Usually, I find the vectored thrust of the I/O drives to be extremely useful. But when getting on the trailer, docking, or driving through mooring fields, its usually better to steer with the shifters than with the wheel.

    The complaint I have heard about these splitters (take power from one engine and apply to two props) is NOISE, along with cost and weight when compared to single engine driving a single prop.

    Nevertheless, I've also been intrigued by the same idea -- single engine, two props, separate transmissions (FNR) and separate CVTs -- for all the following reasons, in no particular order: (a) the potential for shallower draft for bigger props, and (b) the resultant potential for increased efficiency. And of course, (c) to obtain twin screw maneuverability with a single engine. While single engine boats can be amazingly maneuverable in one direction, they aren't in the other, and they generally can't back down with much directional control at all. And finally, (d) to be able to put sufficient load on the diesel to actually achieve the 20 hp/gallon/hr that they usually don't achieve due to light loading at cruise settings.

    Now, I'm thinking about low power applications, where simple approaches like bicycle chains and the CVT transmissions used by millions of scooters would suffice. Low cost, low weight, cheap, and very available. The key problem with small diesels (10 to 50 hp) is that they really don't achieve, in practice, much better efficiency that gas powerplants: often only about 12 to 14 hp/gallon/hour, versus the pervasive 10 hp/gallon/hour for outboards. And outboards are just so advantageous in so many ways.

    So far, my trade studies have always resulted in choosing one outboard over even two outboards, and well ahead of the single small diesel with a custom complex power train.
    Last edited: Mar 10, 2010
  5. fpjeepy05
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    fpjeepy05 Senior Member

    Honestly I think the only benefit is that they do not make an inexpensive stern drive that can reliably handle the amount of torque that a 480hp diesel puts out. That leaves Arneson and NXT. Both are very expensive. Two rebuilt Bravo ones and a gear... your looking at around 17k. Arneson your looking at around 25k and its not going to have nearly as good of handling.
  6. Rik
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    Rik Senior Member

    The ASD8 Drive for a diesel is only 16K. Handling will be better than the NXT. Fountain Powerboats found this out when they were comparing the performance characteristics between the two.
    1 person likes this.
  7. BMcF
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    BMcF Senior Member

    We built a demonstration craft..a trimaran, long and slender main hull with all propulsion in the main hull..that had a single large V-8 turning a pair of 12JG Berkeley jets, each independently steered and, of course, with independent control of the reversing buckets.

    The splitter transmission used two 3" 'Gilmer' belts to divide the engine output to the two output shafts. Each belt/pulley combination was rated to 300HP, so 600HP total engine input power.

    The 'transmission' was not particularly noisy but it did radiate the belt noise always associated with 'blower drives'.

    Since the propulsion was via waterjet, all the benefits for maneuvering that you would expect from a twin jet installation were realized. Obviously, that simple (reliable and fairly cheap too) splitter transmission had no forward/revers/neutral capability at all, and so would not suit for prop propulsion if the whole idea was to gain the manuevering benefit.
  8. dreamer
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    dreamer Soñadora

    These were all the things I was thinking of when starting the thread. Also, with this arrangement, it would be entirely possible to have the shafts be completely horizontal while having the engine down low in a boxed keel or deep v.
  9. rasorinc
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    rasorinc Senior Member

    A while back a design of what you purpose was posted as a picture. I thought I saved it but I've looked to no avail. It was a drawing of an engine, transfer case, 2 drivelines etc. Maybe you can find here on the site.
  10. dreamer
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    dreamer Soñadora

  11. fpjeepy05
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    fpjeepy05 Senior Member

    This is accurate, that is what a ASD8 costs. However, if all you spend is $16k your not going to be going anywhere. If you want to actually drive the boat, Include about $4k for a prop, $5k for a transmission, $2,834 for a power steering kit if you don't already have one. $28k will likely be more accurate. A rebuilt Bravo has a transmission, prop, and power steering for $6k.
  12. Rik
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    Rik Senior Member

    I am sorry that things are not free and you forgot the cost of the engine, the boat and the fuel the rigging, etc.. Oh, with the twin drive single engine you have to factor in the cost of the splitter box, drive lines, couplers, etc. etc..

    Put a Bravo on an engine that has over 200+ diesel and see what that cost long term. This is why they want to split the power off to multiple drives. I've done this in the past for different reasons and there is no cheap and easy way of doing things. Those two thoughts are mutually exclusive of one another.

    Also, price out what a NEW Bravo complete, not just a leg cost and then compare apples to apples. Used items are obviously less expensive than new things. Your prices for the items are a bit high, but that will all come down to the actual application at hand. If the customer has a Cummins QSB480 and you want to use a Bravo, best of luck to you. Even Mercury/CMD will not do that even if you beg them.

    The Power Steering system does not cost $2,834 of a single engine application, especially when the engines you are referring to come with a power steering pump on them. No need to purchase things twice.

    Propellers will have to be purchased for a Bravo also so please do not forget to add their cost in also. Last I checked, nothing is free from Mercury either.
  13. fpjeepy05
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    fpjeepy05 Senior Member
    The system works. I talked to owner of the company who has the system on his personal boat, and he says it works flawlessly. Has over 1000 hours on it and hasn't broken a drive yet.

    As for price... Professional Boat Builder Issue #122. There is an article stating that the price for the three gear boxes are priced at around $5,000. Call Mastery and I'm sure they can give you a more accurate price.

    If price is the major driving factor. I see it as a viable solution.
  14. Boston

    Boston Previous Member

    at some point, this price for just the gear boxes and as has been pointed out all the other associated stuff, begins to add up

    I suppose my next question would be, would not the maneuverability gain vs cost be significantly less than say a geared Azipod system ?

    just a thought I've been working on for my own build is a Azipod without an electric drive unit at the turn but instead a simple bevel gear along with another gear at the top set up exactly like a automotive pumpkin so it can still swivle 360° and leading to an engine that instead of tilting down along the typical shaft line tilts up so the line of the drive shaft meets the top of the Azipod'

    I know its kinda junk yard wars but I'm working on a budget over here and frankly the diesel out of my truck is just right for the build.

    I can see a maneuverability increase with two props but I can see a larger one for less money, less gear connections and less to go wrong with an Azipod using off the rack components

    only thing custom in there is the Azipod housing and a few shafts and even then you could probably find shafts of the right length and diameter to go with standard bevel gears at the turn, that and a few bearings and grease seals
    or even slice and dice the business end of a standard outboard rather than do much of anything requiring a custom casting


    we now return you to your regularly scheduled programing


  15. fpjeepy05
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    fpjeepy05 Senior Member

    I look at the increased maneuverability as a byproduct. The problems that the gear actually solves is an inexpensive drive for large diesel engines, and fits design constraints that limit building to a single engine.
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