Odd one out - complicated porpoising

Discussion in 'Boat Design' started by DaanDM, May 23, 2021.

  1. DaanDM
    Joined: Apr 2021
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    DaanDM Junior Member

    Hi all,

    We own a very rare 27ft deep v "speedcruiser" that has twin 165hp diesels with bravo 1's.
    Of the approximately 9 built, we probably have the only one with (very heavy) dual diesel engines.
    Due to this, it's hard to find relevant information, and I'm not helped with the countless topics and replies on porpoising where everyone shouts "Reprop! Add tabs! Add xyz!"

    The boat is equipped with instatrim tabs and weighs slightly over 5000kg all fueled and loaded up according to a crane operator. It seems very heavy compared to the factory values, but the wood core is dry and hard as far as we can tell (I drilled into the transom only to check).

    We manage to barely reach 40kts and the engines just about fail to reach their rpm limiter of 3800 so I'd say its propped really well.

    When buying the boat we did a testdrive and the conditions were almost glass mirror like. It had a malfunctioning tab and one leg didn't trim well. However we had a comfortable and non porpoising top speed identical to now. The big difference was that the boat only had some 30 meters of very thin chain, and a tiny anchor. Probably a combined weight of 30-35kg. Unfortunately we had these briliant conditions and only a quarter full tank of diesel, else we'd have had a bit better reference.

    We bought it, boat went on the dry, and we did a complete refit and fixed/upgraded all systems. One of the first relevant changes was a new and beefier anchor chain of 70kg (plus the 10ish kg weight of the old anchor), which is in the normal position up front.

    When all was done, we took the boat out and over the course of a few weeks we have accumulated perhaps 10-15 hours of driving it.
    IIRC At around 19kts the boat will plane. I can't recall if tabs are required for planing at that speed, tabs are generally not required to get it out of the hole due to the massive torque of the turbodiesels.

    Above 23kts the issue starts and the faster we go the more easily it occurs; any larger disturbance (wave/wake) will push the boat into porpoising.
    I understand outdrive trim and trim tab operation very well and there's a lot I can do to mitigate the issue but A: we can't really drive the boat at WOT without it eventually happening regardless of the involved and careful continuous/active control of leg trim and B: I feel I can't trim the leg out/up as much as we should? I'm used to outboards that you trim outwards quite a bit and gain a lot of speed and RPMs that way, but trimming out the drives will just destabilize the boat and make it more likely to start hopping.

    The only way to stop the porpoising is to ease on the throttle. No trim setting or tab adjustment is able to dampen the bunnyhops once it started.

    As a remark; I don't believe the trim setting actually is able to push the nose of the boat upwards as I'm used to with other boats, but it already didn't do that during the testdrive with less weight in the anchor chain locker and a near empty tank.

    For some reason, the boat is very 'nose heavy' in the hoist, (already with the old chain) even though the engines are heavy and far back. There's no evidence of lead balast added by the mfgr.

    This summer I have some to do's: Measure and mark neutral trim on gauges, verify there's no hook or something in the hull (which i'm pretty confident about there is none) and make some fly-by video's to see where the contact patch is at full speed and different trim settings. Finally, but very difficult, is to take the anchor and chain out for a testdrive.

    I've added a picture that sort-of shows where approximately the center of mass could be, based on the position the straps had during lifting. The boat actually fell out forward the first time the crane operator tried to hoist it with the straps on conventional positions.

    After this wall of text, the questions to the experienced people on here are; when a boat runs really well and extremely stable for it's size in slightly rough conditions, and is comparitively a little underpowered (there are versions of our boat with twin 5.7L V8's) but runs very fast for its weight, but starts porpoising to no end during anything but perfect conditions if we want to gun it; have we really made the issue much worse with the 'extra person on the bow' for anchor chain weight? Or is this more to do with the design of the boat? Anything we can learn or good feedback is welcome!
    Amongst things I'm considering maybe upgrading to k-plane 280's to have a longer virtual water line and a sort of 'wheelie bar' effect with them fully up to keep the nose DOWN. Is this a bad idea and is the trick to see how we can keep the nose UP??

    As I understand it, most planing hulls lift the bow rather high, minimize contact patch, center of lift moves aft, and to compensate you trim the drive or motor out. Our boat does not really behave that way, so I'm lost.

    [​IMG]
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the beam of this boat ? Might seem like a strange question, but may be pertinent.
     
  3. DaanDM
    Joined: Apr 2021
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    DaanDM Junior Member

    IIRC it's 2.56m or 8.4ft at the bump/fender strip.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Wow, then you have a very heavy boat for a narrow beam 27 footer, too heavy I would think. Does it really hit 40 knots ? That is surprising to say the least. The first thing I would be thinking would be much larger trim tabs, but there can't be a lot of beam to play with to install them, with twin outdrives
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    It might be worthwhile removing all the additional chain from the anchor locker, and going out on another test run to see how much effect it has?
    How much additional weight has been added to the boat during the re-fit?

    How often do you need to go flat out at 40 knots? That is an impressive speed for a 27' boat with 2 x 165 hp engines.

    What is a typical weight as quoted by the factory? As Mr E notes above, It does sound rather heavy at 5 tonnes.
    Is the whole hull cored, or just the transom?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The picture of the boat shows a "high water" mark that appears to betray a stern heavy boat, I would think the anchor chain is "not guilty"
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You would rarely see a boat that size with twin diesel sterndrives, on the narrow beam, it seems likely to set up an unhappy relationship of too much weight to too little bottom area, and lowish power to weight ratio, I feel a powerful compulsion to extend the stern rearward. But jumbo tabs may help, if you can squeeze them in there
     
  8. DaanDM
    Joined: Apr 2021
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    DaanDM Junior Member

    It does yes, on GPS. I'm not certain how precise the crane measurement was, but the boat is indeed heavy for it's size. For tabs, I'd rather not install them due to access limitations on the inside of the transom and the huge work involved to get to the bolts (engines out), but I could hypothetically fit the longer K-tabs. I'm just not too sure if that is a good or bad idea.

    That's a test that I certainly want to do this summer. I think the chain (and later heavier anchor but we didn't drive the boat with the new anchor fitted) has added most, in the most influential location. All other refit work has been relatively light (bimini, electronics, repairs, aesthetical things).
    The issue is more that it happens rather soon already, at slightly over half-speed it can already begin.
    The whole hull is indeed cored, it's a super high quality built boat.
    Attached a picture of the toilet through-hull, and a core I drilled out of the transom.

    [​IMG]

    In the operator manual the following is listed regarding weights: displacement 3.5 tonnes (7718lbs), maximum payload 1158kgs (2553lbs)
    But it also has a page where this is listed:

    [​IMG]
     
  9. mc_rash
    Joined: Aug 2020
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    Location: Netherlands

    mc_rash Junior Member

    Do you know the manufacturer?
     
  10. DaanDM
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    DaanDM Junior Member

    It's Shakespeare, a business from the UK that is no longer active.
     
  11. DaanDM
    Joined: Apr 2021
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    DaanDM Junior Member

    Okay, it indeed droops with the stern when laying still, but in the crane it does need the straps counter-intuitively far to the front.

    Yes indeed, that's why I ended up posting in this forum, I think it has the highest level of skill/experience for this oddball situation.
    Extending the stern is out of the options, and so is removing considerable weight, hence I was already looking at advertisements of used k-planes. But would that help as a sort of wheely bar, to keep the nose down, or should we actually do things to try to keep the nose high? I'm still surprised the boat wants to drive so 'flat' and doesn't noticeably climb a lot. It's not a sluggish boat by the way, the diesels really push it from standstil.
     
  12. mc_rash
    Joined: Aug 2020
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    mc_rash Junior Member

    If you search for the Shakespear 830 on the internet you find that it's weight is 3,5 ton. According to what your crane operator said your boat weights 1,5 ton more than usual. And I only can find 830's with (also twin) but min. 300 hp engines. Your boat is missing some power while it does weight more.
    Do you know if earlier owners changed something?
     
    Last edited: May 23, 2021
  13. DaanDM
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    DaanDM Junior Member

    Indeed, that's what is spec'd. If the spec is correct is open to debate, even by the words of the original dealer. This one was actually bought (via broker, coincidentally the same person that was the original dealer) from the family that owned the brand/boatyard. It has not been modified, and was built with these engines and was private property of the family.
     
  14. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor


  15. DaanDM
    Joined: Apr 2021
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    DaanDM Junior Member

    That one is from Mahon, Menorca, and was being refit with new powerplants during the same time we bought ours, indeed the one from your second link, in Mallorca.
     
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