36 foot, $300,000 boat porpoises to no end

Discussion in 'Boat Design' started by gofastguy, Mar 26, 2019.

  1. Barry
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    Barry Senior Member

    Are hydrofoils similar to Doelfin, Stingray and others an option. On small over horsepowered inflatables, this is an easy fix and one of the sites showed the foils that attach to the ventilation
    plate up to 500 hp.

    The 12 foot long fuel tank. Can you get a drawing for this. I would be interested in looking at the baffle location and size of opening between.

    Re the trim tabs
    Just an addition: I am discussing the hinge axis about which the tab rotates. Not a trim angle of the tabs.
    Currently the hinge access is at 23 degrees. At 0 degrees of hinge axis, horizontal, you will create some drag but the majority of lift. As the hinge axis increases, up to your 23 degrees of deadrise, there will be a significant increase in drag with a corresponding decrease in lift, the component that you might be wanting to maximize/optimize.

    Obviously at 90 degrees of hinge axis, ie vertical, you would not get any lift only drag,

    Firstly, because it is cheap, I would build a set of trim tabs that were longer and as wide as possible with the PRESENT hinge configuration that you have, turn the longitudinal edges down, to maximise lift, I don't have your measurements but say 18 inches wide (chine to chine), 24 inches long (longitudinal) , with a 2 inch turn down on each edge. Maximize the width

    And check what happens on the ocean

    If it does not get you what you need, then reconfigure the mounting of the tabs, horizontal axis, but with as long and as wide as you can fit. This will give the most stern lift for the least
    amount of drag.

    I see that the pod angle is 16 degrees. I could not find the rigging information for the 350 but thought that it was closer to 12 degrees. Certainly with trimable outboards
    this should not cause a problem but it may limit the trim up range (which normally makes porpoising worse) ie you have 4 degrees more of tuck and 4 degrees less of trim u p
     
    Last edited: Mar 29, 2019
  2. Angélique
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    Angélique aka Angel (only by name)

    Hobbles on animals legs are to limit movements, or to improve movements by limiting or directing, in the latter sense some pacers need hobbles on all mid upper legs¹, to help to maintain pacing gait, those have not a flawless gait naturally and need to be forced to a good gait, just like the boats that are in need of hobbles.

    ¹ ‘‘ Springfield Globe (± 1938 pic) by Globe Derby was foaled in Tasmania and later won the Inter Dominion. He was an outstanding sire that produced 229 winners. ’’

    ‘‘ Harness Racing (Pacers) at the 2007 Interdominion Championships held at Globe Derby Park in South Australia (#2 Make Me Smile, #4 Braeside Seel Star (number not visible), #5 Dee Dees Dream, #1 Tactical Dreamer). ’’ (some aptronyms in the pic, Make Me Smile in front, Tactical Dreamer in the back.)

    [​IMG]
    (large)

    Note the one in front, #2 Make Me Smile, has rather loose hobbles* (aka hopples) on all mid upper legs, connecting the front and back legs, to improve the gait, #5 Dee Dees Dream has the same hobbles on in blue.

    * a correcting horse race variant of the given meaning 2: ‘‘ Tie or strap together (the legs of a horse or other animal) to prevent it from straying.’’
     
    Last edited: Mar 28, 2019
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    You never fail to deliver a plausible argument. Hobbles or hopples are a restriction, through. I would be curious as to how the boat version actually got the name. In the case of the pacing horses, the hopples do restrict the horses from breaking into a full gallop, which would be a faster gait than pacing. Horse do frequently "gallop" in these races, but the hopples makes the galloping slower than the pacing, such that a horse that "gallops", loses ground by so doing. But enough of those old trotting races, which I don't bet on. They are known to many as the "red hots" ( a play on the word trots), some are even unkind enough to refer to the drivers as "cheats on seats" ! The traffic problems associated with horses pulling carts, does lend itself to horses being "unlucky" in running, but the whole game is probably not much more corrupt than any racing.
     
  4. DCockey
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    DCockey Senior Member

    A trim tab at 90 degrees would cause significant lift on the hull, behaving like a very deep interceptor. While there would not be any lift directly on the trim tab surface the pressure on the hull ahead of the trim tab would increase. The pressure on the hull surface in front of the center of the trim tab would be close to stagnation pressure.
     
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  5. fallguy
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    fallguy Senior Member

    Yamaha F350 are 12 degrees at spec.

    Gf-have you ever measured the real angle of the bracket to the waterline?

    Despite the incongruity between less tuck and porpoising, I am estimating you are at 18 degrees angle less the 2.5 or 15.5 against a designed 12.
     
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  6. gofastguy
    Joined: Mar 2019
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    gofastguy Junior Member

    I have considered lighter engines for sure; I am conflicted about repowering with the Yamaha 300. Would save 400 pounds on the stern which is significant. But we would lose 100 hp and 80kW of torque combined. As far as the design water line measurements. If this is a theoretical computation maybe their naval architect would have it. Otherwise the builder doesn't have that data for a unencumbered version of my hull since this is the only one they have built. (Except Ribcraft UK which has built quite a few)


    No venting in a hard turn. Our cave plates right now are visible, but covered in a good amount of spray. it could probably tolerate coming up more more hole, which is 3/4" on a Yamaha. I just don't see 3/4" of engine mounting height being the determining performance factor here, but it doesn't hurt much to try.


    Thank you for your work on the diagrams. I am open to testing modifications. I'm still having a hard time figuring out where exactly you would mount suggested cheekpads on the chines of the pod? What color did you use for that? Thanks.

    I don't have the ability at the moment to measure the chine width (you mean the chine flat, specifically... or the hull's total width from chine to chine)? The "internal beam" is quoted at 7' 11", which seems AWFULLY narrow, I suspect the actual width from chine to chine is more like 9 feet, but not much wider than that.


    Screen Shot 2019-03-28 at 10.22.02 PM.png
    There's the drawings for the fuel tank. And here's a pic I diagrammed showing approx location of the fuel tank.

    Screen Shot 2018-08-29 at 10.53.37 AM.png

    I have not measured the real angle of the bracket to the waterline.

    I think, as suggested here, I need to aggressively get Armstrong involved on this. Considering, once again, the fact that this model of boat operates successfully in the UK with aft installed inboard diesels (which frankly weigh a LOT) and with outboard mounted directly on the transom, to no complaints I'm aware of. Or... does it? Here's a video from the Thames... doesn't look like that smooth of a ride for being in a river:
    skip to middle of video



    One thing I do know from experience....my Zodiac brand RHIBs have a outboard bracket which conforms to the hull and they perform flawlessly, despite possessing similar relative dimensions and loads.

    Here's a picture of the aft of my 9 meter:
    IMG_7643.jpg
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    By chine width, I meant the breadth of the bottom at the stern measured from chine-to-chine. Looking at the Thames video, it does seem to be that kind of hull, it bounces a fair bit, I'd like to see a lines drawing of the boat, but I doubt that would be available. Surely it does not have rocker. But really, as Barry said, and I mentioned, tabs should be the way around it, they just need to be the right size.
     
  8. rwatson
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    rwatson Senior Member

    I tossed together a couple of illustrations.
    The first one shows a potentially really easy fix. Aluminium extrusions on the "cheeks" of the Pod, to break up any Laminar flow adhering to the Pod.

    The second one, is a detailed view of a complete "add on" to change the underwater shape of the whole Pod, AND provide extra Buoyancy if they were made of solid foam or similar. You could even add the "interceptors" onto those add on pieces to remove as much laminar flow ( suction) as possible.

    Note - on the Photo of your Aluminium Boat, see how the Pod's lines are parallel with the main Keel ?

    PorpBoat21.png PorpBoat20.png
     
  9. Barry
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    Barry Senior Member

    From your picture that shows the fuel tank located far forward.

    Do your other RHIBs have the fuel tank in this location?

    I assumed that the tanks were at the back.

    Just floating an idea

    Many of the comments have been focused on the pod as the issue but perhaps it is the center of gravity. With the heavy passenger loading and the 4000 pounds of fuel tank located so far forward, the COG is also far forward.

    A planing hull (assume a normal non porpoising boat) underway has a wetted surface and if you were to find the center of lift within this wetted surface is often occurs about 1/3 back from the leading wet edge. The center of lift ( COL) is where you could balance the boat with a single force, for simplicities sake. (omitting many more variables, thrust angle, deadrise, drag, just to keep it simple)

    This COL changes location with different speeds and is obviously an upward force.

    As you power up, the COL is far forward and this results in a bow up attitude as the COG say it is at the midpoint of the length of the boat. So an upward force near the front of the boat and a downward at the middle of the boat. As the speed increases the COL moves rearward, the bow drops, not as much of a moment arm, as the COL is getting closer to the COG. Then with more speed, the COL moves behind the COG slightly. The bow drops, the COL moves forward as the wetted surface moves forward, which causes the bow to rise. Now there is some angular momentum in the hull (on a per cycle basis) and when the bow begins to rise, the COL moves rearward but slightly further back than the original position, the distance between the COL and COG is greater, and the bow drops again but with a little more speed, and as this continues on, eventually the porpoising gets dramatic.

    So as the speed increases the couple/moment caused by the COL which is moving longitudinally and COG which is fixed, creates the oscillation.

    We had a similar situation with an outboard jet mounted on a shorter inflatable. The designer had a built in a bow tank, right at the front, and because the boat was fast and
    extremely light, the boat porpoised easily. (there were some other tunnel issues adding to the problem) We emptied the front tank, temporarily hooked up a day tank and
    while we could still get the boat to porpoise at high speeds, we could eliminate the situation by trimming the outboard and adjusting the trim tabs.

    I would check your other RIBs and see where the tanks are located.

    On our 21 foot jets we mounted fuel tanks right at the back. The tanks with fuel were about 1000 pounds and the engine package maybe another 1100 pounds. And this is in a boat that
    might weigh 3500 pounds with a couple of guys. At speeds above 42-45 knots, on smooth water, the boat would just begin to porpoise, a drop back to say 40 knots and the problem just went away.
     
    Last edited: Mar 29, 2019
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  10. Angélique
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    Angélique aka Angel (only by name)

    I suppose it's local speech in and around the Albemarle Sound area, perhaps the greater Outer Banks region, maybe the Manager of the Roanoke Island Maritime Museum can tell something about the etymology.

    Back to (anti) porpoising of the topic boat, which term in fact also comes from the animal kingdom, but has a more clear etymology.
     
    Last edited: Mar 31, 2019
  11. Manfred.pech
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    Manfred.pech Senior Member

  12. Yellowjacket
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    Yellowjacket Senior Member

    Your pictures at 30 knots show nice separated flow at the step and you've indicated that it wasn't porpoiseing at that condition. What is likely happening at lower speeds is that the hull is running at a higher trim angle and the flow off of the step is reattaching to pod. Because of the angle of the pod this generates a huge amount of negative lift (downforce) on the pod. This is also why you have poor lower speed efficiency, the negative lift aft of the transom means you need additional lift that you have to generate to keep it on a plane. You don't have a lot of a step here and that is the crux of the problem. If the step were larger the flow wouldn't be reattaching and at least the boat would be responding like a normal hull to changes to cg and motor tilt. It isn't now because you really have two boats, one that is operating normally over 30 knots and a hull with the reattached flow at lower speeds. What you might try would be to extend the pad that you have at the transom back under the pod. This would generate lift aft and would reduce the trim angle at lower speeds so that the flow doesn't reattach over the majority of the pod. At higher speeds the trim angle is lower and this won't do much. You can take some pics at lower speeds and see if it is indeed reattaching and if it is then you can then think about what will work, but my best guess is that this is the real issue here.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    If it was re-attaching to the pod to any degree, that should be obvious as a rooster tail-like display behind the boat. Also, if the water was attaching to the pod faces, it would shoot water against the outboard legs, above the splash plates, creating a fountain effect, that is not evident in the video where it was porpoising. Also, factor in that the pod vee is very close in deadrise to that of the hull, and the wake flattens behind the hull, making re-attachment more unlikely. But if there is no visible effect, I don't think much of the kind is happening.This boat may even have rocker in the bottom, like the early Cigarette race boats, for all I know. But I doubt it. It is heavy and narrow with a 23* deadrise, all of which tends to help porpoising, as might aspects of the boat's lines, were they available to study.
     
    Last edited: Mar 30, 2019
  14. rwatson
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    rwatson Senior Member

    Not at all. A tall Rooster tail is a function of excess power. We are only talking about a moderate re-attach, that is truncated by the final transom, with the flow occupying the void behind the pod.

    In the video, the underwater shots show a clean break after the Pod, but a really laminar flow alongside the Pod "cheeks", as Yellowjacket pointed out.
    Between Barry's advice about COB, and Yellowjackets comments about re-attach at certain angles, these seem to be the most obvious factors without actually being at the site.


    Thought Flash : It would be relatively easy to determine IF there is any serious "downsuck". Film some telltales, some on the Pod cheeks, some freeflowing to indicate the hull angle.

    Something like this.

    Flow.png


     
    Last edited: Mar 30, 2019
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  15. Barry
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    Barry Senior Member

    H
    I did not mean the angle of the trim tab, but rather the angle that the axis of the hinge points make with respect to a horizontal zero degree.
    The best position for max lift least drag is if the trim tab axis angle is zero, meaning that it is mounted horizontal. The higher the deadrise, the more drag and less lift
    That was my point. The OP said that when his tabs are fully deployed, he can somewhat control the porpoising but fuel consumption is bad
     
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