propeller size for one-off 48ft catamaran build ?

Discussion in 'Multihulls' started by tevake118, Aug 17, 2018.

  1. tevake118
    Joined: Jan 2013
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Hi everyone! After building/sailing a Wharram 35' back in the 80s I finally got around (at 69, 7 yrs ago)to building my own version of a catamaran improving on that old design> waterline 45', hulls parallel sided with 6' beam at midships, so you can walk from bow to stern inside; flush decks (5' topsides; downside: more windage when abeam) ; 3 hard chines, so shallow draft (22" fully loaded), and much weight-carrying capacity( 3-4T); modern R&D 3 stay rig with most of the power in the fully-battened main (620sq.ft.); and Maine-cat style open bridge deck with large, open-sided hardtop (10'x24'); kick-up, retractable rudders for Florida's West coast; AND a Deutz, 2 cyl. air-cooled diesel, mounted in a sound-deadening box on the bridgedeck, connected to a Thai-type "long-tail" propeller shaft (18ft), as already done by "Hans", on 48' WAKATAITEA (Wharram Builders forum)...
    Here's my question:
    (1)Hans said he used a 2 blade, 14" prop for his 28 hp Kubota engine; my engine has about the same hp, but I am at a loss to commit to a buy...A well-known source here in SoFl says I should buy a 17"x12" 3 bladed prop...?! Wow, thats quite a spread.. (Hans said he used that drivetrain for over 25k miles without problems (1" shaft).. but hes no longer reachable, so can anyone help me, please...I have read Dave Gerr's Propeller Handbook several times, but is it enuf to just double the "beam" measurement when doing the calcs?
    (2) Why cant you tow a new hull and measure the resistance at your intended cruising speed (me: 5-6 kts), if you know, for example, that the (fish scale?) is showing 200 pounds at 5 knots windspeed ( its a sailboat with a lot of time, so I only need the motor in a calm), shouldnt you be able to figure out a suitable propeller???
    Thanks for any feedback; I will acknowledge and reply, including results from using anybodys suggestions.
    dennis
     
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Measuring your drag at speed while towing is a brilliant first move.
    Use 2/3 full load: tanks, people, luggage, provisions, etc.
    Do it in both directions of travel, twice.
    Use a video camera if it proves awkward to read the numbers on the scale.
    Calibrate your scale as well.
    If you can tow from a side pole in clean water it is preferred, but not always possible.

    If you've read Gerr's prop book ( twice! ) then you're way ahead of most!
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    So the prop clears the water when under sail ? If in doubt, err on the side of slightly bigger diameter, more blades, and lower pitch, you never know when you may need to battle high winds and the like against a lee shore, keeping the anchor
    from dragging in a wild storm, etc, and that is where greater thrust will be advantageous.
     
    Last edited: Aug 17, 2018
  4. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I was going to get to that after we had some numbers from his tests.
     
  5. tevake118
    Joined: Jan 2013
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Thanks for the encouragement, BlueBell! OK! I will do the drag /tow/fish scale test, as you indicate; it may take a week or two to get things organized, but I will do it and report back.
    Re: "loading" = I have three ballast/trim/ water tanks in each hull; if I load all six = 2x300gal/hull = 600gal total = 2 1/2 tons, plus 10 or 15 friends, I should get about 3 1/2 tons load, plus present hull weight of about 5 1/4 tons, so a total of 8 3/4 tons hull inertia...
    ps: I have pictures of the boat at launch, on this computer, but I dont know how to add them to my thread?
     
  6. tevake118
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Right, Mr Efficiency; we had to use our Chrysler Sailor back in the day to get out of tight spots in turbulent conditions, so I definitely want more thrust than speed, but with only 24 hp (primary diesel power; there is also, in a well in the port hull, that 10 horse Chrysler Sailor, as an emergency backup) on a 9 ton, high windage catamaran, the wheel may actually have to be around 14 inches, as Hans stated...
     
  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I am just curious here. What is the drive on these engines? If you go too big on the prop; is it same like an ouboard where you won't get the engine rpms high enough?

    I don't see how oversizing helps here, but I am not an inboard guy.

    The bigger prop meets my gut intuition better, but it also seems too big to my gutcheck.

    Of limited value, I know, but I would call another prop guy.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Fantastic!
    You may want to consider a full run-up of speed/drag numbers at 1-knot intervals.
    Valuable now and in the future, especially at resale time.
    You can use time as your friend, run a video camera on the scale and one on the GPS speed-over-ground.
    Slowly accelerate and gather your data for later analysis.

    2/3 load is a general rule for running a test like this.
    I don't follow your tank numbers, use your discretion.
    Obviously, you're attempting to simulate a typical ( average ) day underway.

    Good luck getting pictures up.
    Members here will tell you how easy it is in just 139 convoluted steps.
    I can't do it, but I'm no spring chicken and IT is not my forte!
    Personal Message me and we can exchange emails for photos if you like.
     
  9. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    You have quite a heavy cat for a 22hp engine. I would like the prop to be as efficient as possible. I think your boat will probably top out at 6-7 knots under power in a calm. As others have stated the most important time to have a good engine is in a squall or similar when you drag. So I would use a top speed of 6 knots and the motor's power curve to find the correct sized 3 bladed prop. Two bladed seems odd to me, you really want to have as much grip as you can. If possible increase area as you are retracting the prop when sailing (I hope you are!). This means you can get the biggest low pitch prop you can. My mate has a 46ft Wharram with a retractable longtail. He also has a self made bow thruster because she is not very maneuverable. Lovely big prop and old slow revving diesel, nice setup. Let me know if you want my mate's contact details.
     
  10. tevake118
    Joined: Jan 2013
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    tevake118 Junior Member

    Right, Fallguy; I did another round, using Gerr's chp on Crouch method, of calcs for a target speed of (conservative, I know, but I dont have much diesel hp) 5 knts, and came out with a 3 blade prop of 12" diameter/pitch 8"..!? Since my outboard (see above) uses a 10 1/2" x 7" for 10 hp, and that drove my 35 ft Wharram at 7-8 knts, I dont trust my numbers here...
     
  11. tevake118
    Joined: Jan 2013
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Hi again, BlueBell! Since your last - encourageing = reply I have ordered 4 garage door extension springs, which have a rated pull of 125 ft #s, and when they arrive I will use Hooke's Law to calibrate their expansion constant... I also (see my post to Fallguy above) redid Gerr's
    chp 5 method, and was discouraged by the timid specs.. I also have obtained the local Seatow guy's assistance for when I am ready to do as you say above...
    I might qualify as grizzled chicken with a stat like 6.2.41, dont you think? Anyway, I will attempt to send you an email with attachment fotos..
    ps: I am presently working on the bridgedeck , and will get back to you when I am closer to seatrails...
     
  12. tevake118
    Joined: Jan 2013
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Hi Catsketcher! Its very encourageing to me to be able to get advice from guys like you and BlueBell., et al, as I'm basically going it alone here in Naples, Florida... I definitely will go with the 3 blade ( see above posts, please), and would love to know what size engine and prop your friend's 46' Wharram (displacement, too) uses? I know my Deutz is not really big enuf, but I've invested too much to change, and I DO have considerable experience (2 yrs liveaboard in the North Sea, Atlantic, and Caribbean) regarding the importance of a motor for emergencies.. So I will restrict myself to mostly fair weather sailing; I'm not ambitious anymore about faraway places... Its the beauty of the skyies and the habitat at anchor here in the Ten Thousand Islands that keeps me working, and I dont need a marina, as I have my own diy boatramp/dock...Shall I send you an email, too?
     
  13. tevake118
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    tevake118 Junior Member

    BlueBell; I just tried to Personal Message you, by clicking on your name, but got only "Start a New Conversation" // How do I - fellow gizzer - get my email address to you? thanks,.
     
  14. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    That's it -- click his username and click "Start a Conversation".
     

  15. tevake118
    Joined: Jan 2013
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    Location: Goodland, Fl.

    tevake118 Junior Member

    Thank you, Mr moderator!
     
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