Lightweight Turbo Diesel Jet setup

Discussion in 'Jet Drives' started by groper, Jan 21, 2014.

  1. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Hi,

    Im considering installing a pair of turbo diesel engines matched to jet drives in a catamaran im building. What im looking for is a direction in which to go in terms of engine choice and jet drive choice, and do a cost vs benefits analysis.

    Without a great deal of research done at this point, the direction im looking at going is with a small marinized TD car engine, something around the 2L displacement such as the ford duratorq or VW engines. I wanted to use a full common rail EFI engine for efficiency, and high power to weight ratio. These engines are available for reasonably low prices from car salvage yards with low mileage.

    The second part of the problem is choosing a jet drive to suit, the prices from varying manufacturers seems to vary alot. Which units are worth considering for a boat which stays in the saltwater and only hauled out once every 2 years?

    The sizing of the jet is also of interest as it seems most of these pumps are designed for gasoline engines. The TDi engines im looking at develop peak torque of around 380Nm at 2800rpm. There is also around 80% of the peak HP available at this RPM also. So i think the jet should be sized to handle the HP at a lower RPM as the unit would rarely ever be run above 3500rpm.

    Cruising speed is designed to be 20kts, but with so much power available from these turbo diesels, ~160hp per side, a considerably higher speed could be maintained in calm waters. The hull requires 120hp total (combined both engines) to maintain 20kts when driven by outboard motors.


    So what combinations of engines and jet pumps are suited to this task in terms of cost, simplicity, and maintenance intervals?
     
  2. Brian@BNE
    Joined: Jan 2010
    Posts: 262
    Likes: 13, Points: 18, Legacy Rep: 151
    Location: Brisbane, Australia

    Brian@BNE Senior Member

  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Someone who knows more about jets will probably indicate the cruise speed isn't high enough for an efficient jet-drive. Your fuel consumption could turn out to be higher than you'd like when backed off. Incidentally, is there room enough to accomodate diesels in there, I once entertained the idea of a cat with diesel sterndrives, but realised the hulls would need to be a bit fatter than ideal. And you have the issue of a sizeable alteration to the balance of the boat, maybe less so with jets being further forward, but still a lot of extra kg to have 2 diesels in a boat that is a bit weight-sensitive.
     
  4. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Not true Mr E, the Jets can be sized and pitched to suit most applications - including this one. Theres even guys out there building full custom jet pumps.

    Room to accommodate them is enough - its a tight fit but it will fit. Maintenance will be a bit fiddley but i think i can manage it.

    The balance of the boat will be ok, it moves the CoG further forward which is better than sitting on the transoms, the total weight is pretty good as these diesels are very light and dont require a gearbox with the jet drives... weight including jet pumps is only a touch over 200kgs each... but we now have 160hp per side to boot.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    A fully marinized complete unit weighing 200 kg, and 160 hp ? Seems optimistic in the extreme. You would not get a petrol 4-stroke 150hp outboard weighing that little. Such a product would take the market by storm, why have we not seen it ?
     
  6. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    OK, well that is 110hp rated at the crank, a few horses would go missing in the jet unit. If you look at certain outboards that come with a jet drive option, you note the output is rated well under the propellor driven alternative. That engine I see is near enough to 2 feet wide, does that fit your boat ?
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    http://www.yamahaoutboards.com/outboards/Jet-Drive/overview

    I know this isn't strictly comparable for your application, but Yamaha indicates a 150 outboard reduces to 105 hp with a jet drive. If there weren't these kinds of losses, we'd all be getting around in over-size jetski type watercraft.
     
  9. waikikin
    Joined: Jan 2006
    Posts: 2,317
    Likes: 103, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    So where are the losses generated? is similar to props or extra surface etc in the delivery & focus of impeller/"jet"/thrust?
    Both questions as I don't know, who's got the vibe on this?

    Jeff.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    My "guess" is the very high speed of the accelerated water within the unit and its frictional interaction with the walls of it. I don't know what the typical exit speed is, compared with an external conventional propellor. I expect the improvement in efficiency at higher speeds is helped by the absence of appendage drag.
     
  11. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    The design of the yamaha outboard jet is very sub-optimal. Have a look at the nozzle size for a start, then there is the way the impellor and housing is designed to accept a vertical drive shaft. You cant simply say that "this is the efficiency loss you can expect from every jet pump" Theres alot more to it, and there is plenty of into contained within this very forum -search out BAEKMOs posts and you will see how it all works. Basically, you need to keep the flow speeds moderate and this means much larger diameters than the types of pumps you see on high revving engines like PWC`s - which are outputting well over 200hp these days...

    If efficiency losses were unacceptable, you wouldnt see so many commercial vessels using them... we have many up here, ferrying punters out to the reef most of which operate between 20-25kts cruising speeds...

    Low speed, high volume pumps suit diesel engines just fine and will get comparable efficiency to a prop... but this is not the point of starting the thread!
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,958
    Likes: 342, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Baeckmo is your man by the sounds of that, it does look like you are making a lot of work for yourself in customizing a diesel/jet arrangement to suit a boat that is basically untested with any kind of power.
     
  13. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 120, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    WOW, - one thing I found is I would like to remove the drive system from immersion in the water - hence a present preference for outboard, but the die is set and I have sail-drives that have proven to give me a very fuel efficient home on the seas... 2 miles per litre at 7 knots... Back to living on the boat on Friday 21 February - to slip or do some refit work in the water over the next 2 of more weekends, tidy up, fuel up, fill up the water tanks, finalise the food stores and go cruising again sometime February or March 2014... I may have internet access but that is notorious for its lack of coverage unless satellite is accessible (money is NOT).
     
  14. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    Found what I needed in here - Hamilton jet design guide - http://www.hamjet.co.nz/includes/files_cms/file/HJ Designers Manual LQ 2009.pdf

    Seems are Hamilton hj241 (9.5inch dia impeller) with 6.1 pitch impeller would suit a little 2.0 TDCi nicely.

    Not looking good as far as cost goes, I might be headed back in the direction of outboard motors simply for the cost factor... The Diesel engines are very cheap, but the jets are not... Second hand jets are very hard to find and the chances of finding the exact size and impeller selection just isn't going to happen. Buying new seems the only way and it'd just too expensive to justify.

    Well at least I learned something along the way... :)
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 6,184
    Likes: 404, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Its going to be dictated by your length displacement ratio and the resistance at the hump speed.
     
Loading...
Similar Threads
  1. Seabear23
    Replies:
    4
    Views:
    2,438
  2. weehenry
    Replies:
    1
    Views:
    2,643
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.