Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

  1. Pjitty
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    Pjitty Junior Member

    Guido:

    The link does not work for me..,.

    Joe D
     
  2. FAST FRED
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    FAST FRED Senior Member

    "Having a tunnel stern reduces planing efficiency -- ask Dave Gerr if you don't believe me."

    Sure plaining is done at SL 3 and above.A ski boat would not be a candidate

    The concept with the box keel and reverse deadrise is in the area below SL 3.

    Usually below SL 2.8 for cruising boats that want better efficiency than the usual big wake ,wallowing "semi displacement" cruiser.

    AS the semi displacement boats are so poor at fuel burn at speed most seem to be run as a displacement boat 95%+ of the time.

    A plaining or semi plaining boat hull will give up a considerable amount of comfort in motion and poorer fuel burn in displacement mode , for the chance at 5% operation at higher speed.

    The historical records do not show this for the Atkin boats .They also seem to be more seaworthy than a near flat bottom plainer.
     
  3. brian eiland
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    brian eiland Senior Member

    Hi Ken,
    Do you have any photos, illustrations, reference sites for Dave Gerr's 'tunnel sterns'?? I'm just now doing a little research on displacement powerboat hull forms.
    Brian
     
  4. Easy Rider
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    Easy Rider Senior Member

    Peregrine was featured in Wooden Boat Magazine.
     
  5. FAST FRED
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    FAST FRED Senior Member

    I think the big problem here is the confusion of any sort of tunnel drive with a boat with box keel and REVERSE DEADRISE.

    It seems the tunnel does reduce draft , and perhaps if well constructed increase thrust like a Kort ring might.

    Reverse dead rise is across the entire stern and is an area of captured accelerated water flow.

    Much of the water that was accelerated by skin drag from the front of the boat is channeled at the stern with the prop adding more energy .

    The USE of the accelerated water is probably why the Atkin gets the claim of higher efficiency at modest SL..
     
  6. Easy Rider
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    Easy Rider Senior Member

    I think the reason these hulls are thought to be efficient is that they are ... relatively. And the reason that they are has next to nothing to do w the hull shape and almost all to do w their light displacement.

    If you redrew the lines of this boat into a conventional planing hull it would be considerably more efficient.
     
  7. brian eiland
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    brian eiland Senior Member

    Gerr's Tunnel Hull or Shannon Boats' SRD technology

    These are all good qualities if the hull shape lives up to its billing. I have just become a lot more interested in this powerboat hull shape subject as I look for possibilties for a canal trawler design.

    I saw an article in the May/June issue of Passagemaker mag titled "Skinny Water Queen" featuring Gerr's tunnel hulls.

    And over on this trawler forum, there was a pic of a certain vessel with an unusual bottom design. Could this have been inspired by either Gerr's tunnel hull concepts or Shannon Boats' SRD Technology?
    http://www.yachtworld.com/core/listing/boatMergedDetails.jsp?boat_id=2596716&checked_boats=2596716&ybw=&units=Feet&currency=USD&access=Public&listing_id=1709&url=&imc=pg-fs
     
  8. brian eiland
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    brian eiland Senior Member

    Other Threads?

    Would you have some specific links to those other threads?
     
  9. DennisRB
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    DennisRB Senior Member

    Interesting thread.

    I just want to point out my 40 foot sailing boat (Hunter 40.5) does 11.3 NM per gallon at 1700rpm with a yanmar 50 and 2 blade prop at 5 knots, 0.44gph, loaded for trans oceanic cruising. Will do 8K at full speed, but I have no idea on the fuel consumption. But it would be far less than some of the semi displacement 30' trawlers doing 8k mentioned in this thread.

    I'm interested in the Bluejacket designs. The whole point of this design appears to be a slow speed, full planing boat. IE, it will fully plane at slower speeds where other boats of the same size will be in semi displacement mode. With the most obvious reason being a low bottom loading. What happens when you try to go at real displacement speeds, say 5k with this design? What would the fuel burn be then?

    My tender is an achilles 10.5 foot with wood floor and inflatable keel with 8hp Nissan 2 stroke + foil. With just me on board 210lbs, I can accelerate from a standstill and transition smoothly onto the plane with no noticeable hump and no giant wake, with minimum throttle. I have not measured the speeds, but I can "plane" at very low speeds.

    As soon as I add an extra person (much lighter than me) the behaviour changes drastically. I can no longer smoothly transition to a plane. The bow lifts and the stern digs in creating a huge wave train. Moving weight to the front assists to get it over the hump, but the wave train is still huge no matter what. Once up and planning the weight must come back to prevent the nose pushing down, and the speed can be reduced to maintain a full plane, but the speed has to be kept a lot higher than just with me aboard. The fuel burn at "trolling speed" is subsequently massively higher with the extra person.

    I just thought this is a good real world example that many of us can experiment with, or have already probably had experience with, to illustrate the pronounced effect that bottom loading has on the transition to planing and power required to stay planing. It shows the concept of "semi displacement" is highly related to weight and not just SLratio. At a given SLratio between planing and displacement, the same boat can either be planing or plowing (semi displacement) with the only difference being weight.

    As a side note, I can plane 3 people with my 8hp on this dingy if everything is right, which has been disputed a number of times when I have mentioned it in the past. The hull shape effectively has "hook" as the hypalon floor conforms to the flat wood floor behind the inflatable keel, it sort of acts like trim tabs trimmed down. That explains why top speed (which is pretty impressive) is achieved with all the weight as far back as possible. Actually when you think of it, the hull is a little like a seabright but with a flat instead of a tunnel, when you consider the way the hypalon floor conforms to the flat floor at the back with the inflatable keel bulging through at the front.
     
  10. FAST FRED
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    FAST FRED Senior Member

    <Would you have some specific links to those other threads?>

    Rescue Minor - Robb White

    www.robbwhite.com/rescue.minor.html‎ ,,,,,, Plenty more info on Google

    I am not sure the MPG of the Rescue Minor has ever been topped .
     
  11. tom28571
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    tom28571 Senior Member

    Maybe, but first, you have to believe the claims. I am a skeptic when such claims are made and not supported by independent verification. Rescue Minor has had more than its share of such claims, especially by Robb, who was known to be a bit enthusiastic at times. The threads mentioned above were filled with magical deductions to support unusual performance reported of RM. Some boats were built during these discussions but have disappeared with no following data that proved any of the magic. We are left to wonder why and make our own conclusions?? I suspect Robb's boat was much modified including lighter weight and a more slender hull form if he actually got the performance he credited it with.

    I had an opportunity to drive a very nice RM once and was unfortunately underwhelmed by its performance relative to many other boats. The speed reached with a proper diesel was way less than anticipated based on glowing claims. The seaworthyness was also suspect and we did not want to venture into an inlet that I would readily enter with several other similarly sized skiffs.

    In its element of extreme shallows, it meets a real need but otherwise, I am not impressed with its general utility.
     
  12. Willallison
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    Willallison Senior Member

    What a bizarre modification on an otherwise sound vessel!:eek: The bulb in particular would cause me some concern.... The shape appears to have been derived on the basis that it would possibly provide some lift... but the flat shape of the top would do its very best to stop the bow from rising to meet a wave.... It'd be interesting to sea trial it...

    I agree with Tom re the claims made about RM and her miserly fuel consumption.... I think that any improvements over 'the norm' are more likely down to low weight and narrow shape.
     
  13. daiquiri
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    daiquiri Engineering and Design

    How often do you guys beach a 50-ft vessel?
     
  14. Tad
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    Tad Boat Designer

    I'll ad a third voice to the "I don't really believe it" chorus.....Rob's claims were so vague that they became really questionable. Few people understand how difficult it is to build a speedy powerboat with a D/L of approximately 40, and almost no one understands how difficult it is to keep it there.

    Here are some guesses.....

    MPGtable.JPG
     

  15. brian eiland
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    brian eiland Senior Member

    Water slap at sterns?

    Thanks Tad, I've got to get my copier working correctly so I can enlarge that card and study it.

    Wave slap at stern?
    I don't know if it has been mentioned in this subject thread, but I do remember seeing a reference to the reverse deadrise hulls of the SRD hulls 'slapping' at the water surface when their sterns were rolled out of the water on either side.

    Anyone experienced this firsthand??
     
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