Higher Cp at higher speeds equates to less resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Dec 1, 2020.

  1. DogCavalry
    Joined: Sep 2019
    Posts: 1,568
    Likes: 838, Points: 113
    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Trust Ad Hoc to steer the conversation back on topic.
     
    TANSL and BlueBell like this.
  2. BlueBell
    Joined: May 2017
    Posts: 1,747
    Likes: 485, Points: 83
    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Dusty,
    Also add to Ad Hoc's query:
    water line length and water line beam.

    EDIT: 24' x 1' wow 24:1 that's good!
     
  3. bajansailor
    Joined: Oct 2007
    Posts: 2,208
    Likes: 745, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    You could have what might be referred to as a 'pontoon boat' re your 24' waterline and 1' waterline beam - this would have a pretty high prismatic coefficient, perhaps too high if you are looking for best efficiency.
    So you take some volume out of the ends, and reduce the prismatic - but if you keep the 24' LWL and 1' beam then you will lose displacement, and hence carrying capacity.
    For relatively slow speeds, you might get the most overall 'bang for your buck' from something like a pontoon boat - hence one reason why they are so popular . Another reason is that they are relatively easy to build, with minimal amounts of welding required, hence should be a bit 'cheaper' to buy initially.
    But if you make the hull shape more like a conventional boat (assuming that you can happily lose a bit of displacement for the same length and beam) then you will need less power for the same speed and be more efficient (I am generalising here, dangerous I know).

    The 49' power cat in my avatar was initially optimised for sedate speeds with minimal power (the initial power choice was going to be 2 x 40 hp O/B's).
    Halfway during the build, it was decided by the owner that 2 x 70 hp O/B's would be fitted instead. She achieved 16 knots on initial sea trials with about 10 people on board.
    A bit later on she was fitted with 2 x 115 hp O/B's, and then a few years later (she is now 20 years old, and was still working happily as a passenger tour boat before Corona caused her to be laid up) she was fitted with 2 x 150 hp O/B's - these gave a max speed of 26 knots with 10 people on board. Her owner wanted to fit 2 x 200 hp O/B's thinking they could go faster still, but there was no point. Even at the transoms the deadrise is still about 25 degrees, with a lot of rocker in the hulls. And with a L/B ratio of about 15 at the waterline she doesn't have an awful lot of planing surface re lift.

    Her owners also have a stereotypical pontoon boat (built in North America) for carrying passengers - same beam (16'), slightly narrower hulls, same power but a bit longer (60', rather than 49'). Both carry the same number of people, but at typical cruising speed the 49' vessel is almost twice as efficient re fuel consumption, as the hull shape has less total drag.
    And the pontoon boat can only dream wistfully about speeds of 15 knots, never mind 25 knots.
    This pontoon boat was ordered about a year after the one in my avatar - her owner's main concern at the time was initial purchase cost, and this boat was cheaper to buy than a custom built vessel like the 49' boat. However I hate to think how much more petrol the pontoon boat has burned over the years in comparison.

    Some pontoon boat builders now add planing wedges to give some extra lift - and some pontoon boats can go very fast indeed.
    All kinds of 'go fast' additions are available - eg the Bennington boats.
    High-performance Features | Bennington Marine https://www.benningtonmarine.com/en-us/performance-packages/
    But probably only in fairly calm water. Everything is always a compromise in boat design.
     
    BlueBell likes this.
  4. BlueBell
    Joined: May 2017
    Posts: 1,747
    Likes: 485, Points: 83
    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Dustman.
    Are you in the dust already, have you abandon your thread?
     
  5. dustman
    Joined: Jun 2019
    Posts: 109
    Likes: 11, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    Working my butt off, will be back soon!
     
  6. BlueBell
    Joined: May 2017
    Posts: 1,747
    Likes: 485, Points: 83
    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Lol, what is your target speed and displacement.
     
  7. dustman
    Joined: Jun 2019
    Posts: 109
    Likes: 11, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    Hoping to cruise around 7 kts in 10 knots of wind and 9 kts in 15 kts. 1200 lb displacement more or less. The form of the hull would be pretty similar to a double scull, just a little pudgier and shorter. The reason I asked the original question was because I'm trying to decide my entry and exit angles how gradual I should make the transition to the parallel midbody.

    Ad Hoc, that would be the approximate waterline beam and length, 24x1, probably need to add a few feet in length or a couple inches in width to achieve the 1200 lb displacement while maintaining the waterline at the right height for minimal wetted surface area. I am still a ways from a final design, still a lot to learn and figure out. Sailboat design is way more complicated then I ever imagined.
     
  8. dustman
    Joined: Jun 2019
    Posts: 109
    Likes: 11, Points: 18
    Location: Tucson, AZ

    dustman Senior Member

    bajansailor, Thanks for the input. I guess I am, in a way, building a pontoon boat with a much smoother entry and exit. I like to think of what I have in mind more like a double scull hull shape.
     
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 7,047
    Likes: 975, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Ok.. based on your figures, you ave a very good LD ratio...almost 9.0
    Your Fn is just over on the prismatic hump at 9knots..but beyond is ok.
    However, being a good LD ratio...and L/B....the amount of trim will be minimal.

    Basically in plan view, take a straight line from the bow...to the widest section, in beam, roughly midships. That will be your angle of entry.
    Then do a similar, in profile view, from midships, at the deepest draft, to the transom, but leaving around 60% of the draft at the transom.

    Your target LCB will be circa 8%.

    Do that, and your hull will be like a magic carpet for you
     
    BlueBell, DogCavalry and bajansailor like this.
  10. DCockey
    Joined: Oct 2009
    Posts: 4,995
    Likes: 487, Points: 83, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    LCB measured from where? I have seen LCB specified as
    - distance from stern
    - distance aft of mid-ships
    - distance forward of mid-ships
     
  11. TANSL
    Joined: Sep 2011
    Posts: 6,734
    Likes: 491, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    LOL. How can someone press "Like"?.
     
  12. Eric ruttan
    Joined: Jul 2018
    Posts: 190
    Likes: 28, Points: 28
    Location: usa

    Eric ruttan Senior Member

    For TANSL.jpg
     
    Ad Hoc, DogCavalry and BlueBell like this.
  13. TANSL
    Joined: Sep 2011
    Posts: 6,734
    Likes: 491, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  14. DogCavalry
    Joined: Sep 2019
    Posts: 1,568
    Likes: 838, Points: 113
    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Well, that's getting on the extreme end. Very little wave making resistance, but very large wetted surface drag, and no cargo capacity to speak of. And the stiffness of a component is proportional to the 3rd power of the dimension normal to the axis it's being measured on.
     

  15. Ad Hoc
    Joined: Oct 2008
    Posts: 7,047
    Likes: 975, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    This one:
    On vessels that are of "reasonable" size, i.e not small run abouts or small monohulls, it is customary to use midships as the datum.
    And in general, most LCB's are measured aft of this location.... but there are always exceptions to this rule.
     
Loading...
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.