Planing Hull de-powered for use at displacement speed

Discussion in 'Hydrodynamics and Aerodynamics' started by AndySGray, May 15, 2015.

  1. AndySGray
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    AndySGray Senior Member

    The corollary to this post was to be about a 30 knot boat which a friend was looking at, the big V8 gassers were shot so he was considering a couple of tiny 4cyl diesels, about 20-25% the power, looking for something to chug around efficiently at 8-10.

    The boat itself was cheap but basically too many problems - massive negative equity - would have taken many thousands of dollars and would still have been worthless


    The post-mortem drinks session raised some interesting questions so I figured it was worth posting for discussion in case others have a similar idea.:rolleyes: THIS boat was not a possible but could the concept have worked out with a more solid example...

    The general consensus was that the planing hull would be less efficient, though at those speeds the difference would only be a small loss, comparable to e.g. the loss from not having a clean hull, or being badly trimmed. (No science used, this was alcohol fueled mancave logic)

    The flip side was that the stability and seakeeping was a much bigger area for concern.
    There was also a debate the planing hulls tend to be built with lighter weight in mind but that is less of a concern for a displacement hull.

    We also went off on a tangent about how hull efficiency has evolved over the years - I think that is a separate thread though...

    Crack open a bottle and feel free to join the fray...

  2. waikikin
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    waikikin Senior Member

    I'm all for it. Sometimes sportscruisers are pretty cheap with engines & drives out of action with other fitout quite serviceable, for inshore use an add on pod to smooth some flow & hold some tankage plus carry a couple of outboards would be relatively cheap, some extra tankage or ballast could be used to trim, & would still make a cool boat to hang out on.... I don't even need a bottle......

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  3. tom28571
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    tom28571 Senior Member

    I saw a WW2 PT boat that had been converted to a cruiser and repowered with diesel. Don't know any specifics but it trimmed high in the stern compared to the originals. I suspected that it would work Ok for coastal cruising and certainly offered huge interior volume. Because of the great weight difference, twins would be sensible for handling and windage. I also remember wandering through a PT Boat graveyard in Pearl Harbor and looking at some with three huge Packards in the engine room.

    On the other hand it was docked in Naussau at the time.
  4. daiquiri
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    daiquiri Engineering and Design

    In a sense that the stern was lighter, or else?
    If so, it was almost surely beneficial for the resistance, because it decreased the transom immersion and the relative drag.
  5. tom28571
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    tom28571 Senior Member

    Yes, that is my take on the probability of much lower resistance due to smaller transom immersion.

    This was when before I started any serious study of powerboat design so my observations were pretty superficial. That the stern was a lot higher as well as the rest of the waterline was obvious though. Operational PT boats were not only heavily loaded with engines, fuel, armament, munitions, etc, but were generally overloaded with more than the design called for in those. I wish I had looked at the boat more closely but was busy with in-port work in the sailboat I was on.

    Like many others, I wish I'd had the foresight to acquire surplus planes and other stuff when it could be had for a song after WW2 and Korea. I guess the operational requirements dictated most of it, but the waste of just shoving new or perfectly serviceable planes and equipment overboard as well as abandoning it overseas seems so wrong now.
  6. Barry
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    Barry Senior Member

    Depending on the hull deadrise, length, weight, there will be different efficiencies but I can give three examples that I have had specific experience with and perhaps by chance they will be useful, perhaps not

    1981 Silverton, 350 V8, 260 hp, twin Crusaders, 18,000 pounds wet, 34 feet, 22 degrees deadrise
    At 20 knots, .75 nautical miles per US Gallon when running both engines,
    At 8 knots with one engine running 1.2 nm per US gallon

    2006, Aluminum, twin 350 hp Volvo diesels, IO, duoprop, 30,000 pounds wet, about 24 degrees deadrise,
    From 15 knots to 24 knots, both engines running, pretty close to 1 nm per US Gallon
    At 9 knots, one engine running, 1.7 nm per US gallon
    At 9 knots, both engines running 1.3nm/USgal

    A buddy owns a 42 bayliner, round chines, twin direct diesels
    At 16 knots he says he gets around .6 nm/USgal
    At 10 knots, about 1.1nm/USgal

    All planning hulls

    I would expect that if any of these boats were properly designed displacement hulls, that they could should produce in the 2-3 nm/US Gallons

    Boating magazine used to print and maybe they still do, fuel consumption tables for boats that they test so it would be easy to check fuel consumption per mile to get a feel for fuel usage. They also test displacement hulls, so it would be easy to find out the differences between the two hull types. Note that there are quite a few variables.
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  7. pogo
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    pogo ingenious dilletante

    You got it.
    Gallons, or better liters per nautical mile.

    Regarding speed , speed and consumption ratio , one motor is the better option.
    a second motor delivers max. 20% more speed.
    Safety ?
    How often does the one and only motor of your car failures ?
    The only advantage of twin enginges is The superiour manouverabilty of the boat.
    But, , With today's bow and stern Thrusters a boat w. single engine...

    An example
    My father owns a Princess 30 with a 165HP Volvo Diesel. Max. Speed 22knots.
    Cruising Speed in planing mode 16 Knots.
    The same Boat with two of these engines , doubled consumption, reaches a topspeed of 25 knots , cruising speed in planing mode is the same. Weight of second engine and it's fuel limits so much !

  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The most efficient speed range for a planing hull like those is circa 5 knots, go faster than that, and it just gets worse. Plenty aspire to push them to the, dare I say it, displacement "hull speed", but in that area they won't do much better in nmpg terms as they did as planing hulls. If you are happy to go 5 knots, it will work in fuel usage terms, I would expect roughly twice the mileage you will get at 8-10 knots, which is attractive, but don't kid yourself you can nudge the speed up any further than the ~5 knots and not pay.
  9. Rastapop
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    Rastapop Naval Architect

    Not an ideal situation.

    You're going to have a submerged transom, chines, and presumably high warp also - these are all added resistance, compared to a displacement hull.

    I'd guess the effect from the transom in particular would be very significant.
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    No-one wants to go as slow as you need to go, to bring the fuel consumption to the point where it becomes attractive, basically a slow trolling speed.
  11. whitepointer23

    whitepointer23 Previous Member

    I new someone who removed 2 318 chryslers from a 28 savage. ( very heavy deep v). He fitted 2 new 50 hp isuzu diesels and now cruises at 7 knots but it cost him 40,000 which would have bought an awful lot of petrol. He thought the engines were the biggest cost until he realized he had to change all the stern gear as well to suit the reduction gear torque. I guess if you got good used engines and running gear it might be viable.
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Doesn't sound too clever to me, the difference in fuel used at 7 knots betwen one of the 318's and the two diesels couldn't justify that exercise, and with the small engines, no option to power up on to the plane if the need arises, in, for example, running a barred inlet.

  13. whitepointer23

    whitepointer23 Previous Member

    I bought both 318, s in perfect condition plus a heap of extra stuff for 2000. I saw the boat advertised a couple of years later for 40000. It owed him over 70.
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