Planing catamaran vs planing mono hull vs displacement cat fuel 8kts

Discussion in 'Powerboats' started by johnnythefish, Feb 23, 2024.

  1. johnnythefish
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    johnnythefish Junior Member

    For a given boat length - let us say 26 foot - and all else the same in terms of the boats payload, will a planning cat be more or less fuel efficient than an “average” planing mono hull at non-planning speed? Let us say 8kts.

    In the same vein how much more efficient is a displacement or semi-displacement catamaran (basically a catamaran not designed for very high speeds) compared to its planning catamaran counterpart at these speeds (8kts).

    I know that it is very hard to compare apple with apples here but I want to understand whether “the equivalent” planning catamaran is better than a mono hull in terms of fuel efficiency at these speeds; or if real efficiency gains at these speeds only happen with a displacement cat hull.
     
  2. calevi
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    calevi Junior Member

    I would think, CAT is going to be much more efficient, given it's LWL is double vs monohull. How much more, I don't know.
    At those speeds it probably doesn't make much different, where higher speeds (when other is planning, difference could be HUGE).
     
    Last edited: Feb 24, 2024
  3. philSweet
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    philSweet Senior Member

    What load? You need to be very specific. The relative performance is very sensitive to weight. Catamarans always have a lot more wetted surface area, which is bad 100 percent of the time. But they make less waves at some speeds, and can be tuned to have small wave losses at a narrow band of speeds. So they can be better at least some of the time for some displacements. Note that cats have a considerably lower payload percentage. If you are trying to build a boat around a specific load, the monohull is at a significant advantage because the craft will be lighter overall. But 8 knots is a really awkward speed for any 26'er with any weight to her.
     
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  4. fallguy
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    fallguy Senior Member

    I started a reply and lost it. The problem with the question is the premise is false. All else cannot be equal.

    The planing cat, offers more stability with wider beam, for a theoretical similar displacement and waterplane.

    The problem for you is you have left too much detail out of the query.

    If you want a high speed trolling platform; running at 6-8 kts is not efficient in many monohulls. The flipside is the monohulls allow for lotsa passengers and lotsa gear, but they are going to be doing poorly at that speed.

    The best way to fight this issue is a cat that is bigger than 26'. Of course, this is a classic OPM by me.

    @DogCavalry would tell you to build a 26' Seasled, but he still can't get into 15' of beam.

    DM me please as I have some private issues to mention.

    Let the group know a bit more about the plan and remember, in boats, ceteris parabis is mythical.
     
  5. DogCavalry
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    DogCavalry Senior Member

    @johnnythefish , I'm going to echo the other fellows. Not enough information here to answer your question. The biggest thing missing is how much weight you want to carry. Then what sort of conditions you'd like to be safe in.

    @fallguy has obliquely hinted that I might like sea sleds. This is my 26' sea sled named Serenity. To save fuel I typically cruise at 5 knots, which requires 12 hp. But that's with 800# of people and beer and snax on board. I've done 15 knots with 3500# of cargo aboard, at 150hp. Or 32 knots with #700 at 250hp. I expect to seriously improve that last number, now that I have the prop @baeckmo recommended. As far as lumpy water goes, I've been out drinking a coffee from a regular cup, and been interrupted by a phone call from a watcher on shore to let me know that if I sank they'd have to watch, because their boat couldn't leave the dock under those conditions. I'm not sure where @fallguy got 15' beam from. I stopped at 10' but she's already stable enough. @Ad Hoc was kind enough to run the hydrostatics for me. Posted on my thread back in 21/08/07. More stable than a flat bottom barge.

    This is almost certainly TMI, but the question was raised. And it's an opportunity to praise the fine gentlemen who generously help folks on here who are willing to be helped. IMG-20231022-WA0056.jpg IMG-20231022-WA0047.jpg IMG_20230813_155847909_HDR.jpg
     
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  6. johnnythefish
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    johnnythefish Junior Member

    Firstly - thank you for all the responses.

    @DogCavalry nice looking sled - where can I find more info on this concept.

    How does the sled do at 8kts?

    Let me try and define my parameters more specifically and let’s do aside with the comparison between a mono hull and a catamaran altogether and ask this instead….

    At 8 Kts, how much more efficient will a “displacement” catamaran of 26 foot in length be than a planing catamaran if the beam are the same (can be more than trailerable width and no limit on how wide).

    The boat must carry 6 adult men, fishing tackle and enough food, fuel and water for an overnight; accommodation can be sparse and minimalistic; basically boat does not need the kitchen sink. More space obviously is preferable but not paramount. Weather conditions are warm and balmy. No chance of rain. Sleeping accommodation can be a mattress on deck that rolls up. Shade is important.

    The vessel will need to cover 80 miles as fast as possible given it’s design limitations. Faster is better but not to the detriment of the overall fuel economy so long as a minimum speed between of 16 mph can be maintained. The vessel must then travel 160 miles at 8kts.

    Sea conditions can range from flat calm to large swell and/ and or wind driven chop. Water is tropical.
     
  7. fallguy
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    fallguy Senior Member

    Your issue with any boat, is most planing boats at 26' are not "out of the hole" at 6-8 kts.

    What you are really asking is what designs are more efficient at speeds of 6-8 kts.

    There are semi-displacement monohulls and cats. The cat can offer a more stable fishing platform, but make no mistake, building a cat is more difficult. Two hulls, beams and a bdeck, engines on each.

    It all depends on how much wirk you are in for for a more stable platform. A 30' cat can probably get into 15' beam which would be an advantage over the Sea Sled. Maybe 13.5 for a 26'er. But everything gets spendy, too. More cables, two start batteries, long cables to the helm.

    So, I recommend you find some semi-displacement monohulls that will do well at 7 kts (not in a hole), and wide enough for a nice fishing platform.

    The reason for semi-planing vs displacement is the speed desired and ability to take a 6 pak a farther distance than displacement.
     
  8. fallguy
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    fallguy Senior Member

    I noticed you had the same type of thread going awhile back.

    I picked a boat for you. The Bluejacket 28. But make the cabin smaller and the cockpit bigger. This will shift lcg for'd some, so you'll need to accomodate the change somehow.

    Bluejacket 28 – Bluejacket Boats https://bluejacketboats.com/bluejacket-28/

    The Bluejacket 27 is a better design for fishing; not sure if it has a head onboard.

    On further, probably not big enough in available displacement for a 6 pack, but close.

    Gonna be hard to find a 26' cat to accomodate a 6 pack as well...
     
    Last edited: Feb 27, 2024
  9. DogCavalry
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    DogCavalry Senior Member

    I have a thread called
    Sea Sled Information
    full of general info. I also have the build thread for Serenity called
    Sea Sled Madness: It's in my Brain!

    The hull form has a very minimal planing transition resistance hump, so it does fine at 8 knots. She'd need about 30hp for that. If all you need is a sleeping bag on a floor she'd sleep... ten? without crowding. That front deck is seating for four couples dining. The interior is ten by thirteen. But with proper seating she'd seat 23 comfortably and still get on the plane. Transport Canada limits a boat like this to twelve passengers plus an arbitrary number of crew. Say fourteen in excellent comfort plus luggage.

    I like my boat. Usually she's how I make my living.

    In terms of fuel efficiency no other boat of her dimensions could compare. She has large wetted surface drag though, so if you're content to carry ½-⅓ as much, you could burn less fuel within a 26'*10' envelope.
     
  10. fallguy
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    fallguy Senior Member

    The one issue with the Sea Sled, is a massive fishing cockpit might mean lcg is too far forward. I'll let John remark more.
     
  11. DogCavalry
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    DogCavalry Senior Member

    Like yours? My LCG is too far forward because I built the bow super massive to carry an actual ton of material in the forward cockpit, or to push a ten ton barge with the bow. Both of those things are job related, so I'm fine with them. With the throttle at 15%, the bow pops up, and she's planing at about 3° with respect to the twin keels, or 6° with respect to the apex of the tunnel. So that's fine. Her drag is too high from huge wetted surface, if I was always running light. It's not the most efficient form. But I carry heavy stuff, and for that I think it's as good as there is.
     
  12. fallguy
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    fallguy Senior Member

    I would argue lcb is too far aft on mine. I digress.

    I think the Sea Sled would be a fine fishing platform, so I asked!
     
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  13. DogCavalry
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    DogCavalry Senior Member

    I built the sides of the forward cockpit much too high for convenient fishing. I honestly never thought of it. I was thinking of keeping kids inside and water out, but folks regularly ask me about fishing, so I thinking I pooched that one. I just prioritized wrong.
     
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