Catamaran Conversion

Discussion in 'Boat Design' started by Deering, Jul 18, 2011.

  1. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Hello All,

    I have a 40' LOA aluminum power cat that I use for cruising/fishing/toy hauling here in Southeast Alaska. It's powered by Volvo D-6 370's with DPH outdrives. Yes, it's a bit of a fuel hog, but I've learned this amazing thing...if I slow down, I don't burn so much fuel! The addition of an autopilot this year made a huge difference in tolerating displacement speed running.

    Last week I experienced my second major Volvo outdrive failure. The first failure was addressed under warranty, but this one is three months past the warranty period. Reason for failure is as yet unknown until the boat is hauled and the outdrive removed. A new replacement drive is...$11,000!

    I'm reaching the conclusion that the Volvo outdrives can't handle the power/torque of their diesel engines. I'm reluctant to install another $11K unit if it'll fail after 400 hours. Instead, I'm thinking about converting my cat to a canoe stern displacement hull, similar to the Malcolm Tennant design. I think it may be feasible, though I understand that I won't see quite the same efficiency that a purpose-built displacement hull would provide. Still, it would be more efficient than running at displacement speeds with outdrives on a flat transom. And I would avoid the reliability and maintenance headaches that the outdrives are giving me.

    I'd consider buying a new boat but my budget won't allow, and I have this one customized exactly to my liking - I love the boat other than the outdrives and fuel economy.

    Thoughts? Experiences? AM I nuts? Am looking for technical references to the canoe stern design concept. Any links you can send my way?

    Thanks in advance...
     
  2. rasorinc
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    rasorinc Senior Member

    Not addressing your questions but I would put Volvo on notice of design failure. their outdrive cannot handle the power from their engine and you wish to know how many outdrives have failed with this engine combo or any combo. And, of course, you do not really want this issue to end up in court. Also, $11,000 for an outdrive is absurd unless you are running 1,000 HP.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Until the reason for the failure is established, do nowt.

    You simply can't chop and change without knowing why you're doing it and what you're aiming at too.

    Just hasssle Volvo more, write emails etc and post them publically to ensure they take you seriously. :p
     
  4. keysdisease
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    keysdisease Senior Member

    I know this may sound simplistic but what about just spinning the engines around, bolting a V drive to them and hooking that to a conventional shaft strut drive.

    This doesn't address the hull mods you want but if you were getting good efficiency with the outdrives I would bet gearing down and turning larger conventional props would make efficiency even better. Why do you think you need a canoe stern to be efficient at displacement speeds?


    You have to do something. You've already decided not to replace the outdrives. I would think of all your options, jets. surface drives, different brand outdrive, pod, that V-drive with conventional prop is cheapest, proven and reliable.


    Steve
     
  5. rasorinc
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    rasorinc Senior Member

    Ad Hoc and I agree--hassle Volvo first.
     
  6. Deering
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    Location: Juneau, Alaska

    Deering Senior Member

    Ad Hoc, I follow what you're saying, but I do know why I'm doing it and what I'm aiming to do. I am repurposing the boat from a high speed planing hull to an efficient displacement/semidisplacement hull.

    Even if the outdrives were functioning perfectly, the boat is not meeting my expectations. I was expecting far better fuel economy than I'm getting. This expectation came from the builder (now defunct) based on similar models, but it turns out that my cruising configuration (large battery bank, water tanks, fuel tanks, real galley...) is much heavier than the models he was estimating from, and the fuel consumption is correspondingly higher.

    Additionally, my usage has changed and running at slower speeds is acceptable now. The Volvo outdrive system is not the appropriate displacement speed drive system - props too small, gear ratio not optimal, too expensive, complicated, and maintenance heavy for that application.

    But yes, I intend to hassle Volvo in a big way.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Can you post an image of the lines plan, or sketch what it looks like and picture of boat/hull too?

    Also, what is the lightship and full load displacement.

    Would aid in providing some guidance/thoughts for you...
     
  8. Deering
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    Deering Senior Member

    Keydisease, you make a good point. But I think that by the time I made the necessary hull mods for a vee-drive (stuffing boxes, rudders, struts...) I wouldn't be that far ahead. Not even sure it's feasible with a cat hull, or other cat builders would be doing it now.

    Besides, if I'm going to make the mods, I'm going to do them right. With a vee-drive solution like you're suggesting, I'll get mediocre performance - the flat transoms will suck efficiency, and I'll be limited to around 8 kts. The canoe stern design is well proven, and is used on many new commercial and pleasure displacement cats. It's not the same design as you might be thinking for a monohull boat. With that design I'd have far better efficiency (I'm estimating 25% or better) and possibly be able to cruise at 12 to 15 kts. It also provides for a flatter prop shaft angle, coming straight out the canoe stern. And it provides great protection for the running gear, which is a big deal up here where the water's clogged with big floating logs and big swimming whales.

    Malcolm Tennant is the guy who developed this design. Here's an article he wrote that describes it pretty well. http://www.catamarans.com/news/2006/04/catcomparison.asp
     
  9. Deering
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    Deering Senior Member

  10. Ad Hoc
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    Ad Hoc Naval Architect

    Deering

    You are best suited to running slower with the current hull and weight you have for your boat. Your max displacement circa 13.2tonne, on that hull length gives a very heavy boat, its length displacement ratio is not good, roughly 4.8, that is low. Once you go above 12 knots you will struggle. To go from some 12 knots to 17-8knots you would just about double the power requirement. Simply because it is short and very heavy for its length. It probably trims a lot too as it tries to get up on the plan.

    That delta addition will just add drag, I would remove it.

    If you want fuel economy, for the displacement you have aim for 10 knots…going higher just needs more power for very little gain.
     
  11. keysdisease
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    keysdisease Senior Member

    Nice looking boat, looks like she would be right at home in Alaska.

    So putting aside the hull mods for a minute, what kind of drive train are you thinking about if you junk the outdrives?

    Steve


     
  12. Deering
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    Deering Senior Member

    Ad Hoc, the L/B ratio is more like 10:1 for each hull. That's why displacement cats are able to exceed Freude's Law of 1.34* WL squared. You are assuming that the overall beam is what drives the math. If the hulls were spaced twice as far apart, would the drag increase correspondingly? No.

    As far as trim getting on plane, actually not. Stays quite level.

    You're a NA. Doesn't sound like you've done much work with multis.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Deering

    Im not totally sure I understand what you’re talking about…what is this 1.34 law you’re on about, care to explain please.

    You said your boat weighs 29,000 lbs in normal language this is around 13.2 tonne.

    (29,000/2.205 = 13151kg = 13.2 tonne)

    Your LOA is 40”, so Lwl is roughly 37”, in normal language this is 11.3m

    Your length displacement ratio (L/D) = 11.3/(13.2^(1/3)) = 4.8

    Where do I say that?

    You appear to have designed more than I then…as you seem to know so much.
     
  14. Deering
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    Deering Senior Member

    Thanks. It fits the need pretty well up here.

    The MaineCat 47 is a roughly comparable boat in size and displacement and it runs a pair of 190 hp Volvo D3's. It's top speed is 24 kts, and it has the canoe stern hull I've been talking about. Its economy at 15 kts is better than mine at 8 kts. http://www.mecat.com/power/powerspecs.htm

    Since I have all of the Volvo appurtenances (gauges, shifters, NMEA 2000 bridge...) already, I'd consider shifting to that motor. I've been happy with the D-6, it's a great engine. I hear the rest of the D series are similar. Not sure about prop sizing yet - that would probably be defined by the hull dimension constraints.

    I'd also save about 1,500 lbs, which will need to be addressed in weight distribution during the conversion.

    BTW, I considered other drive options you suggested early in the process. Jets are commonly used for cats, but only at high speed. And they give up a 35% performance penalty. In the photos I sent, during the sea trial the other boat had Arneson surface drives. Its performance has been a disaster. And again, I understand that surface drives are for high speed travel.

    The most proven solution for my needs is the Tennant canoe stern design, if I can get there from here.


     

  15. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Apologies Ad Hoc, I was thinking about L/B ratio and you were talking L/D ratio. I missed that shift. My fault. And you are correct - it's a heavy boat for its size so it doesn't plane w/o lots of power (and stress on the running gear). Planing cats, having less bottom surface area than monohulls, require higher speed to get on plane (i.e. more power). I think that's been empirically established with my boat.

    And as a NA you certainly are aware that as a traditional displacement hull approaches 1.34 * the WL (feet) squared the power required to push it begins to go asymptotic...but you're the NA, not me, so correct me if I'm wrong about this. And explain why that rule doesn't seem to apply to CS-hulled cats? The MaineCat 47, a not too dissimilar boat from mine in size and displacement, achieves 24 knots with a pair of 190 hp engines, and operates very efficiently at 15 kts.

    Would the delta pad add anything at 15 kts? The Tennant design calls for fairly flat deadrise near the stern and I'm thinking an extension of the pad might accomplish that, but not sure if the drag would negate the benefits.

    No, I have not designed a lot of boats, but I am an engineer and have studied catamaran design (as an enthusiast, not a designer) for many years. Your insights are appreciated.



     
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