More than two outboards for sailing catamaran?

Discussion in 'Multihulls' started by Markusik, Mar 1, 2018.

  1. Markusik
    Joined: Jun 2017
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    Markusik Junior Member

    I'm planning a sailing (decent-performance cruising, not racing) catamaran, S/V Hypothetical. Construction is reinforced plastic, cored above waterline. Secondary propulsion will be gasoline outboards in wells.

    Hypothetical has a LOA of 12.99m (42.65'), LWL of 12.8m (42'), BOA of 7m (22.96'), and cruising displacement no greater than 8000kg (17600#). With boards and rudders up, draft will be around 0.46m, 18".

    Given the displacement/LWL, and a slenderness ratio of 12:1 or finer, it should not require a great deal of horsepower (despite windage). Conventional wisdom suggests as little as 30hp could be adequate, presuming the outboards are well propped (relatively large diameter, low pitch), and geared to high thrust. Conventional wisdom also suggests Hypothetical is close to the upper size limit of boats that can effectively use outboards for secondary propulsion.

    Engine "transom height" is a major consideration. I'm looking to maintain a 30" bridgedeck clearance with minimal protrusions. After-market modifications offer a 5" extension for many outboards, but this still limits me to XL (25") shaft models.

    The choices in smaller high-thrust outboard engines with (or dependably modifiable to) a 30" drive leg seem limited. Ideally, the mothership engines could be interchanged (with modification) with the tender. The well-known Yamaha 9.9 high thrust comes to mind. Others make a suitable 9.9, but when looking at anything bigger, the next best option seems a Honda BF15. Yamaha makes a 25, but it's a 2-stroke, which causes regulatory issues, aside from being too heavy for the tender. The Honda BF50 seems like overkill (although the weight is still reasonable at 101kg, 223#, not counting extension), and it hardly qualifies for powering the tender.

    So, I wonder, why not use more than 2 outboard engines? 3, or even 4? Yes, the extra wells will add some cost, complexity, and weight. This could be offset by the increased redundancy, power options, and interchangeability with the tender engine. Using the lateral engines for maneuvering (more flow over rudders) and medial engine(s) for pure propulsion (less flow over wetted surface) could improve performance. For my purposes, alternator output is not a significant consideration.

    4 9.9s? 3 15s?Alternately, two such outboards for maneuvering and lower speeds, with one central BF50? That would eliminate concerns about being underpowered, but reduces redundancy and requires more spare parts.

    Why would 3 (or 4) outboards be impractical or otherwise undesirable for a catamaran, relative to the typical 2? Any other flaws in my reasoning? Thanks in advance.
     
    Last edited: Mar 1, 2018
  2. jorgepease
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    jorgepease Senior Member

  3. Markusik
    Joined: Jun 2017
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    Markusik Junior Member

    Thanks Jorge. I've had my eye on the Neander, just waiting on some more real-world feedback regarding it.

    With that said, I don't know why Yanmar claims it is lighter than a petrol engine; it's 75kg heavier than the 50HP high thrust Honda. One Neander weighs 20kg more than three 15hp high thrust Hondas, which could be all the power needed.

    On lesser points; I'd like to know if a drive shaft extension is available. It isn't any use for interchanging with the tender, and since the tender will be using petrol anyway I wouldn't mind having only one fuel type. I don't anticipate we'll see any new tender-sized diesel outboards soon.

    I've kept an eye on the electric propulsion threads. Interesting stuff, but I'm not ready to get on board with it yet; if caught trying to stay off a windward shore, I'd want to know I have multiple stand-alone sources of propulsion. Hypothetical currently has plans for a 12v system with 1200 watts of solar and 800a/h of LiFePO4 batteries. Maybe 24v, I have time to decide; anyway, plenty of weight and expense, not enough to supplement an electric propulsion system though.

    I like that petrol outboards are relatively easy to repair in much of the world. The Neander, and electric propulsion systems, not so much.

    Input greatly appreciated.
     
  4. jorgepease
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    jorgepease Senior Member

    ok, good luck with whatever you decide ))
     
  5. Rumars
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    Rumars Senior Member

  6. catsketcher
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    catsketcher Senior Member

    Yamaha make a nice 25 four stroke high thrust and a larger one as well. Probably twin 25s would be okay. Put them on pivoting nacelles. It is not weight that means you need power but windage - a cat that big will need at least twin 25s. Also you wont use them on the dinghy - totally different requirements for dinghies and large cats.
     
  7. hump101
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    hump101 Senior Member

    Here are my thoughts, for what it is worth. Sorry in advance for the long post.

    My 12m cat has a single 9.9 high thrust Yamaha centrally mounted that turns with the rudders. My boat is 8m wide with an 18m mast, so plenty of windage even though it is light (1800kg). I keep it on a mooring in a tidal estuary with up to 8 knots of tide. In calm conditions and flat water the boat will just do 8kts with the engine on full throttle, I typically do 5 kts with about 1/3rd throttle. This is rpm limited, not thrust limited. Consequently, I can only rely on the engine by being careful with the tide. Wind isn't so bad if you put a daggerboard down half a metre to give some steerage, but in any waves higher than 1m or so the prop will start to suck air in occasionally, even though the motor is mounted about 1.2m inboard of the transom (off the back beam).

    The next issue with this setup is that the Yamaha mounted onto the tender is too slow with two in, because there isn't enough pitch in the prop. Consequently, it isn't a suitable arrangement as a combined tender/boat engine.

    However, I also have a Honda BF10 on the tender, which then planes easily with two in at 14 kts. If I use the tender to pull the cat, it will do 5kts flat out, but isn't happy, struggling to rev. If I use both the Yamaha and the Honda whilst towing the cat, then it will do 11 kts with the Yamaha revving out on about 2/3rd throttle and the Honda perfectly happy at full throttle, but with a bit of prop slip.

    My conclusions with this, which I've confirmed by calculation, is:
    1. The Yamaha is underpropped for the cat in calm water, but about right when there is waves and windage to contend with.
    2. The Honda is overpropped for the cat, and about perfect for the tender.
    3. If I had two Hondas, they would be propped OK for the cat in calm water, but not in waves/wind, and either would work well on the tender.
    4. If I had three Hondas, they would be propped OK for the cat in calm water and in waves/wind, and any would work well on the tender.

    However, to work reliably in waves, two of the three engines would need to be mounted nearer to amidships to give reliable thrust, and that would require that they be mounted low, so they would need some proper protection from flooding. The third would be on the existing mount on the beam, which would need to be a long leg version, or the mount lowered, etc. I'd keep the third at this location because steerage is absolutely awful without the thrust close to the stern and steerable.

    Once I have three engines, all of which would need to be electric start (my BF10 isn't), I would have close to 300kg in engines, tanks, piping, control cables, control boxes, mounts, nacelles, etc., and at a cost of around €10000, for 45hp (I'd use BF15's as they are the same weight). For this money and weight there are other more attractive options for me, even electric is in the ball park.
     
  8. Markusik
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    Markusik Junior Member

    Thanks Rumars, the sled is an interesting idea, if I understand it properly. The bridgedeck is carried pretty far aft on this though, would it not increase cavitation in a seaway? If I picture the sled properly, then it would interfere with lowering the tender on the davits.

    Thanks Catsketcher. I still have fond memories of a PDQ 36 I knew, used 2 Yamaha 9.9 high thrusts for propulsion, and the tender used one too (just with a different prop). This is a different beast though, I suppose I should forget about trying to interchange engines with the tender. I'm surprised I overlooked a 25hp Yamaha 4 stroke high thrust, I'll have to go back and look again.

    Thanks Hump101, that helps put it into context. I'm guessing your cat is open bridgedeck, that long and that light! Changing props isn't too inconvenient, but I agree; the increased cost, weight and complexity is not worth whatever peace of mind would come from knowing I have a spare engine for either the boat or tender.

    I appreciate the ideas, and I'm abandoning the thought of using interchangeable outboards for the cat and tender.
     
    Last edited: Mar 2, 2018
  9. jorgepease
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    jorgepease Senior Member

    hump you might want to try just adding some cup to the prop, have you checked the slippage? That might kill two birds with one stone, venting and improve speed.

    I saw some images where they scalloped the hull to mount the outboard low and still be protected, if you don't mind doing major reconstruction, looked like a good solution.
     
  10. hump101
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    hump101 Senior Member

    Yes, an old F40.

    If I had a cheap source of outboards I would consider it, as for my boat the same prop setup with three engines would work on both tender and cat, but yours is much heavier, so your speed will be lower, and hence much more difficult to achieve compatibility with a single prop size.
     
  11. hump101
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    hump101 Senior Member

    In my case I can't do that, as I need the current pitch when there are wind and waves. This is the problem with such a large, light boat. The calm water resistance is a much smaller percentage of the rough water/wind resistance compared to a heavier boat.
     
  12. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Great project, great predicament, easy solutions.
    IMHO, I'd sink two 9.9's down into wells, 1/4 - 1/3 forward of the stern.
    Standard shaft lengths if you can.
    And put a 9.9 or larger on the tender.
    Make the wells big enough for 15's or 25's as per the weather/conditions it's going to operate in.
    Easy. Good manoeuvrability, redundancy, flexibility.
    Don't forget your tender is a tug boat should you ever get in a situation where you're under-powered.

    I'd forget electric drive unless you can cover your deck space with panels and eliminate batteries.
    Day power only. Motors are small and nicely shaped for cat hulls.
    Flex panels are much lighter than rigid.
    In combination with mounting a 25hp centre on your wrack perhaps.

    Lots of input here but only you have the info to make this decision.
    There is no bad advice, just inappropriate.
    Good luck, pictures please!
     
    Last edited: Mar 2, 2018
  13. kapnD
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    kapnD Senior Member

    Maybe your dingy is the solution. My dad kept the Boston Whaler out of the sun under the bridge deck.
    It doubled as auxiliary propulsion with a four point tie off.
    IMO, adding more outboards is adding more headaches.
     
  14. Markusik
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    Markusik Junior Member

    I should mention that I have nothing against using different props for the mothership and the tender, and keeping spares of both. If, in the course of a passage, one outboard became inoperable, I'd like the ability to maintain full performance, and changing a propeller is not a barrier to that IMO.

    I've figured out how I missed that 4-stroke high-thrust Yamaha 25 outboard; it isn't available in North America. Looking at it, and the bigger T60 model, makes me wonder; would I sacrifice 56kg of weight (the T25 is 28kg lighter than the T60, x2) for an extra 70 hp? I don't see the prices, but from a power to weight perspective, it seems a real bargain. Of course, since a fair amount of motoring will be in sheltered or otherwise calm water, a T60 would spend a lot of time just barely above idling. I'll have to crunch some numbers regarding fuel efficiency, and find out how I'd get these over here.

    Tender as tugboat is a valid point, BlueBell. It would not be ideal for a short crew in a seaway though. My wells are 13' fore of the transom, pretty much guaranteed good contact with the water. I agree wholeheartedly about the flexible solar panels, and that this is a nice problem to have.

    kapnD, the dinghy secured under bridgedeck as accessory propulsion is an new concept for me, but couldn't it take bridgedeck slamming to a whole new level? In my experience, outboard headaches are much like inboard headaches, only much less expensive. Your mileage may vary. On a cat with two engines, problems with one would be a 50% pain. On a cat with 3 engines, it would be a 33.3% ache.

    Thanks to all, I'm learning and getting my thoughts in order.
     

  15. catsketcher
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    catsketcher Senior Member

    I don't get the tender idea - you want the tender ready to go at a moment's notice - to pick up a hat, to run someone ashore, to go to a party. The tender should be ready within 30 seconds most of the time. It can be on a cat which is fab. I found a nacelle that was modelled on a kayak to be too voluminous and slammed - a dinghy underneath would be insufferable for me.
    You also will sometimes want the dinghy and the cat to be mobile at the same time - conning your way into a new anchorage whilst your partner circles around outside - we do this at Middle Percy - a quick trip into the lagoon whilst Deb circles outside. Or the other day at Cygnet when I saw Kankama dragging in 35 knots. I had to hightail it back to the dinghy, race out to the boat and start the boat's engines immediately to keep her off the shore. A delay of a minute could have led to damage to the boat. To nullify the mothership's ability to move when one person heads ashore is very unseamanlike in my eyes. Especially to save $400 for a secondhand outboard.
    As for the Yammie 25 hi thrust being unavailable, I don't get it. Maybe you could talk to a dealer, or try the Honda 20s.
     
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