Odd one out - complicated porpoising

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

  1. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    I just compared weights but apparently the heavy diesels aren't that heavy relatively speaking after all. Reported dry weight on the MD704LI (that's the VM motori name before mercruiser calls them 2.8 D-tronic) is 325kg. For the 5.7L V8 mercruisers I see some variations reported between 500 and 900lbs depending on which accessories are removed. These boats have been supplied with those v8s and with v6 engines.
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,290
    Likes: 993, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    325kg isn't much, do you have any pics of others of these boats in the water at rest ? Certainly would be interesting to compare with the dirt line showing on your boat picture, for comparison.
     
  3. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,290
    Likes: 993, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Right, well that boat is trimming flatter, look at where the waterline intersects the outer strake forward compared to your boat.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,298
    Likes: 263, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Ok, in addition to my previous note, here is a graph from the classic paper on planing hulls by Savitsky; "Hydrodynamic Design of Planing Hulls", published in Marine Technology, october 1964.

    Cl as shown in previous note. Of course there is a tolerance band along each deadrise, but I have found it very useful. Assuming a chine beam of 2.4 m and a realistic weight of 3,5 ton and instability starting at 24 knot (~12 m/s), the sqr(Cl/2) is ~0.2 and the maximum stable trim with 20 degrees rise is 5.8 degrees. At wot you have ~0.13 and a max stable trim angle of 3.5 degrees.

    Trying to increase the trim by trimming out the drive legs will immediately cause instability. In addition, if there is (even) a slight tendency for aeration, the propellr thrust curve has a hysteresis; higher thrust when increasing speed and lower on deceleration. This effect can be enough to trig the instability if running "on the border". To that end, it is not clear from your pics how the drive legs are positioned re the inner spray rails, I do suspect that there may be a vortex trail leaving the rail and ending up in the vicinity of the propeller inflow.
    Porpoising limits.jpg
     
    bajansailor likes this.
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,290
    Likes: 993, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    A bit unusual to have those full length strakes, near the drives, but there they are and not easily changed. Trimming the drives has to be part of the process here, and downward more likely to improve matters
     
    Last edited: May 25, 2021
  7. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    Update on the props:

    48 13702 A41 21P
    48 13703 A41 21P
     
  8. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    And regarding the weight; I think the 5 tonnes measured by the mobile crane could actually be feasible if I look at the truck mounted palfinger diagrams at:
    https://www.grucomcranes.com/Images/File/Catalogo Palfinger PK27000.pdf here's the Palfinger data of the different models of the PK27000 crane, and their load diagrams.

    And this is what happens when stubborn truck drivers/crane operators don't listen...
    [​IMG]
    In my defense; the boat was not yet our property so I did not commission the transport... this wouldn't happen under my ownership...

    But it shows the weight not being super concentrated in the back
     
  9. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    There's something else I could experiment with, but no idea if useful; swap L/R props and reverse rotation?
    Currently they are 'outwards' rotating and from my understanding that gives most prop lift. Maybe I can keep the bow high/help it raise up by having the props lift the stern a little less?

    Sorry, I'm still unsure if it's desirable to have the boat a little more likely to raise the front, and if that could help prevent the bounce/porpoising, and if the available power is even enough to realize that with this boat.
     
  10. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    Also, I've been analyzing the photo of the V8 and the dark antifoul paint is deceiving.
    Judging from the equal distance above and below where that strake tip ends, the V8's which are apparently heavier, indeed push the stern down more in the water causing the nose to rise a bit, than in ours, where there's less distance under the strake tip ending to the waterline.
    What do you think?
    [​IMG]
     
  11. baeckmo
    Joined: Jun 2009
    Posts: 1,298
    Likes: 263, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    What makes you think you should "raise the bow" or "keep the bow high"? Your problem is that you are running with too high angle of attack ("high bow") for the hull configuration, not too low! Take a good look at the graph showing the porpoising limits, post no 35.

    The optimum angle of attack for maximum lift/drag at high speed is generally higher than the allowable aoa for non-porpoising operation. This is the designer's challenge and will call for various solutions, but these are out of scope here. You said you have experience from driving outboards and trimming out the rig to achieve max speed; forget about that with this boat, it behaves differently. Two hulls of identical shape and CoM, but different size have different optimum aoa. The total drag is mainly the sum of viscous and pressure drags. The bigger hull has a proportionally lower viscous drag, leading to a slightly lower optimum trim.

    To the crane issues: the yoke used by this crane operator is directly dangerous and should be scrapped. The hook eye acts as a swivel point at the same level as the sling attachements, letting the slings swing like a parallellogram. There is no self-stabilizing function. Someone using this kind of equipment can not be regarded professional, which says something about the quality of his weight measurements.......
     
    bajansailor likes this.
  12. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    I'm trying to make sense still of what you sent (diagram) and what you just now wrote.
    I've started to try and figure out what the deadrise angle is, but all I have is a shitty scan, and in it no perspective on pitch, so it's only a rough estimate but better than nothing:
    [​IMG]

    But to answer you on why I'm (also) considering the option to help the bow stay up, versus trying to keep it down, is first of all because I don't yet fully grasp the mechanics in play with our boat (else I wouldn't be posting here hehehe)
    But besides that it's because it is just so weird that in our boat it doesn't matter what you do: after it has gone into plane, the nose stays fully down. Well, untill it starts porpoising under the wrong conditions. I see all these other varying (also bigger) boats that, when trimmed wrong, also when they drive at cruising speed, look like they are making a long wheelie. There's absolutely no way we could even cause that with this boat. So many guides that explain to trim the nose down to have better handling in rougher sea states, so that the V of the bow can do it's job. Well in our boat there's only bow down, or pushing it down excessively - there's no "up".
    For this reason I'm (probably falsely) guessing that our boat just needs a little help to stay up in the 'high' part of the porpoising so we have control over pitch angle ourselves.
    I very much appreciate your technical and in depth replies, and already blindly trust your experience in this field; I'm just sharing my rationale.

    So if I understand it correctly, based on the less than ideal data I provided, it's concluded that this hull shape, with it's mass and particular weight distribution is destined to drive very flat on the water and all attempts should be made to prevent the bow from lifting rather than to help it have some more ability to remain in an increased pitch attitude?

    Re; crane; I fully agree with you and found it questionable too when I saw it but "it is the way it's done here" was the answer. Not (yet) my boat at that time. The weight measurement however came from the crane we ordered ourselves, and a much more professional crew with better rigging:
    [​IMG]
     
  13. DaanDM
    Joined: Apr 2021
    Posts: 23
    Likes: 0, Points: 1
    Location: NL

    DaanDM Junior Member

    update; Baeckmo I've now played around a bit with your formula and graph, and it all makes much more sense now.
     
  14. SolGato
    Joined: May 2019
    Posts: 115
    Likes: 59, Points: 28
    Location: Kauai

    SolGato Senior Member

    If the dual V8 fitted boats have heavier power plants (perhaps even fitted with a bigger fuel tank?) and the photo shows one sitting lower in the water at the stern causing the bow to rise, and the crane lift suggested your boat was bow heavy, then maybe you just don’t have enough weight near the stern and your boat is teetering. And then adding the anchor setup created even more imbalance.

    If it were me, I would get some ballast and take the boat out for testing to see if the handling can be improved by shifting weight.

    If you can improve the behavior with ballast, they make ballast bags for ski boats with fast pumps allowing for quick filling and draining that are reasonably priced.

    I think you’re on the right track to try all the easy stuff first.

    One of my sailboats actually sails better with more passengers than single hand because it was designed to have more weight in the cockpit. Without the additional weight, the bow tends to plow a bit in swell.

    This has always made me curious about small bow riding boats and how they are designed to carry a lot of passengers up front and how they handle without any. I imagine they must have trim equipment to make up it, but they must not be as efficient.
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,290
    Likes: 993, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Getting a bit confusing now, if the bow is always 'down' underway, that could only happen with aggressive application of tabs, and with trimmed down drives, and I see no way it would get to 40 knots, or anywhere close, thus treated, if that "dirt line" is the guide. More attention is needed to quantify trim angle, in-trim, and degree of application of the tabs. The weight remains a matter of question, but it could be reasonably guesstimated from that "dirt line", as could the COG. But porpoising should be minimised by low trim angle, the boat is going to oscillate in waves whatever the case, I think what is lacking is quantification of a lot of things, but bouncy porpoising is almost always worse with the bow "high".
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.