On a planning V-Bottom powerboat are chines below the waterline actually useful?

Discussion in 'Hydrodynamics and Aerodynamics' started by Ribber, Sep 16, 2020.

  1. Ribber
    Joined: Sep 2020
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    Ribber New Member

    Hi there, here's question for you experts. We have a couple of RIB boats with long relatively narrow chines running along the entire running surface of the hull. The picture below isn't the actual boat but similar and with the same hull design.

    [​IMG]

    We have the feeling that the boat is running rather "wet" and "on the nose" and with a lot of water resistance. Someone once told me that chines below the waterline have little impact and mostly just create drag in the water. The question I'm asking myself is this.. if we were to do some bottom work to the boat and sand it down and apply new epoxy and gelcoat etc. could it be of benefit to remove some of the aft chines at the same time?

    The thinking is that by removing maybe 1.5 meters of the lowest aft chines we will reduce drag and maybe also plant the aft a little lower in the water, lifting the nose a bit instead. What do you think?
     
  2. brendan gardam
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    brendan gardam Senior Member

    What do you mean lowest chine? There is only 1 chine and that is where the bottom meets the side. I think you are talking about the planing strakes. That's what we call them in Australia anyway.
     
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless the boat is of rather unusual construction, you won't easily be able to alter the boat, in the way you describe, because those strakes will very likely be an integral part of the layup. They will tend to flatten the trim angle, and do seen a redundant feature of that boat. Your best plan might be to apply more out-trim to your engine.
     
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  4. brendan gardam
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    brendan gardam Senior Member

    I am pretty sure glass bottoms would be a lot stiffer too with strakes
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Naturally they do have a stiffening effect, but there is not much reason to have them on a part of the bottom that is basically always in the water.
     
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  6. Ribber
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    Ribber New Member

    Yes, planning strakes! That's what I meant :)

    The inside of the hull is flat where the strakes are, and the stringers inside the hull are perpendicular to the keel line. Might it not be possible to "simply" grind down a part of the stringers and seal it up with epoxy? Potentially fiberglass too? Bit of work but not impossible?

    That's pretty much what I figure, they don't do much good when submerged. I'm not an expert though. Do you know of anything I can read or view on youtube to increase my understanding of this subject?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Without knowing how they laid up the hull, it is difficult to say. You say the inside looks smooth, and you can't see ( I assume) the outline of those external strakes, from the inside. Does the inside "flat" surface show a pattern of a coarse woven fabric ? That might indicate the strakes "hollow" was filled, after initial glassing with non-woven material, so that the woven material could be laid in, without the problem of trying to get it conform to the depressions. Even if that is the case, and the best case for what you are contemplating, it would still be a mild form of vandalism, and weakening the hull, to grind off the strakes. I would not do it. Whilst those strakes do provide some lift, it is not likely to make a drastic difference. As I say, the first remedy I would attempt, is greater out-trim of your motor.

    Wetness of planing boats is largely a function of the shape of the forefoot area. and the less "rake" to it, the wetter they are. It is hard to gauge from the picture provided, but it does appear they have attempted to get a fairly deep foot, and that is heading in the direction of "wet". I think you are stuck with the problem, if out-trim is not effective. If you want to maximise "cut" in choppy water, and want to remain dry, it is really a matter of operating at cross purposes. I really am a big fan of the cutaway forefoot, on balance it makes the better boat, drier, broach-free, and still rides well if there is sufficient deadrise under the mid-body.
     
  8. kapnD
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    kapnD Senior Member

    I’d like to see a picture of the hull from the transom.
    It looks like the chine may be morphed into a broad skeg-like appendage at the stern chine?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I see what you are talking about kapnD, or it could be a "reverse" chine, and that would increase stern lift. Doing an image search, it appears to be a 7 metre "Tornado" rigid inflatable boat. Might be a better view of one on the web.
     
  10. kapnD
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    kapnD Senior Member

    The website doesn’t talk about hull design, or even characteristics, and shows several different hull designs on their ribs.
    The one relevant photo has the section in question hidden by the landing gear, but does show a particularly steep vee.
    The OP needs to step up with more info if he’s interested in helpful feedback.
    235E4F0B-6914-490B-9551-53B31D770EDE.jpeg
     
  11. Ribber
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    Ribber New Member

    The specific model is the 8.5 Multi-Purpose. Here's a better example of what the hull looks like.

    [​IMG]

    There are no major chines on the sides but quite distinct strakes which protrude and run the entire length of the hull. Right now I'm not so interested in if the modification is possible in terms of plastic modification and construction, I'll probably discuss that with someone with a lot of fiberglass experience if it comes to that.

    I'm mostly wondering if you guys think this kind of design with the strakes running the entire length of the boat was a bad idea from the manufacturer to begin with... and if there could be gains in terms of rough-water handling and speed/economy by removing a part of the "always" submerged strakes at the rear. I hope that makes sense.

    We're going to sand down and epoxy the entire hull over the winter anyway, the question is just if we should attempt to modify it the same time and if it could be worthwhile :)
     
  12. gonzo
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    gonzo Senior Member

    You could end up with a boat that "chine trips" on hard turns which is very dangerous. What you describe is a boat that is not trimmed properly. How is your weight distribution? Also, try trimming the outboard up to get the bow up.
     
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  13. Barry
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    Barry Senior Member

    Your opening post "are submerged chines ( currently called lift strakes or planing strakes, ) actually useful". You suggest that someone has the opinion that they only add drag if submerged, and it sounds like you are of that opinion, but
    then suggest that you want to remove some of the aft portion to let the stern sit lower in the water. If lift strakes did not work submerged, then why remove them.

    When water comes off the hull (after the stagnation point just to keep things clean) due to speed and the dead rise, the water comes off at an angle starting at the keel and moving outward and sternward. If the bottom was
    completely smooth, the water would exit relatively parallel to the boat dead rise.

    With strakes, the water is pushed downward at the strake, which creates lift.
    So yes, lift strakes work when submerged, that is their purpose. The other purpose is that they help keep the boat from drifting/slipping in a turn.

    The first jet boat that I owned was a 21 foot, 8 degree, inboard jet, a 460 engine with a 12jx Berkeley pump WITHOUT STRAKES. At 30 miles an hour, all you had to do to have the stern buy pass the bow is crank the wheel hard. This
    was called a Victory Roll. As jets do not have a rudder or leg in the water, they are susceptible to this action.

    When we began building jets, we experimented with different strake configurations from narrow, flat through wide, flat, to finally wide with a 20 degree downturn. With the new configuration it was difficult to make the boat swap ends with normal inputs to the steering. Less drift, accurate steering, made it quite a bit easier to avoid rocks in rivers

    If you remove the aft part of the strakes the stern should sit a bit lower on step but you will lose some steering control. Ie the back will slide easier, especially if the chine is not engaged.

    Gonzo mentioned weight distribution which on a light boat is extremely important. Where are the fuel tanks, what type of loading?

    You have not indicated what happens when you trim the engines up. With the high degree of deadrise from the photos and the smaller strakes, I doubt that you plan will make a significant difference.



    The video shows a complete 360 Victory Roll,
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Old mate C Raymond Hunt didn't see any good reason to extend strakes past the stagnation point, and so far as I am aware, the laws of physics haven't changed in the interim. There was always the suggestion that with twin motors, you were inviting trouble by putting strakes immediately in front of them, and as we saw with the legendary Formula 233, that was supposedly designed around the new-fangled Volvo-Penta sterndrive, x 2, that there are no strakes at all that might interfere with them, only a pair each side, well to the side. But with Ribber's boat, and the somewhat exaggerated deadrise of it, those strakes should not be driving the bow down to any great degree, if you can't get enough out-trim to trim the nose up, drop the motor a notch. Grinding off any strakes, no way I would contemplate it, madness really.
     

  15. fallguy
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    fallguy Senior Member

    I think too many strakes is probably extra wetted area with little gain. But as Gonzo suggests; unlikely the cause of your nosedive.

    You have another issue.

    Trimming up should tell you more about the issue.
     
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