Using inverted M bracket as lifting strake, pros and cons

Discussion in 'Boat Design' started by JT2017, Apr 6, 2020.

  1. JT2017
    Joined: Feb 2017
    Posts: 3
    Likes: 0, Points: 11, Legacy Rep: 10
    Location: Troy,VA

    JT2017 New Member

    A warm hello to all of you, hoping you are staying safe. I will start out by saying that I know this is a really upscale forum and when it comes to tritoons or pontoons they don't necessarily get a lot of respect here, but I have read this forum many times and there are a lot of capable, intelligent people here so I will try to make it quick and easy. I only ask this question because it involves a little different Twist on a pontoon lifting strake, and it may involve some different dynamics that I think are worth questioning. Otherwise I would not ask a question at all.
    My question involves using an m bracket, which is usually used to attach cross members of a pontoon to the logs. Just so happens that I bought a few extra of these when I was building this tritoon boat project from scratch. I might make some of you happy to see the design that I've come up with for the boat but I won't bore you with all the details. Biggest difference in design is that I've actually extended the center log 7 ft beyond the two side logs. Center log is 26 and 1/2 ft long and the two side ones are 17 and a half feet long. It's a long story, but I am actually building a much more involved tritoon and I thought I would put this one together to tie me over so I would not be out of the water for so long. Needless to say, these particular pontoons did not have lifting strakes built on to them. and furthermore, I committed the ultimate sin with adding a bunch of weight such as the center Cabin and heavy doors. Like I said this was just supposed to be a tie over boat, but after putting in a 150 horsepower motor on it, we're looking at 22 and a half miles per hour top speed.

    Definitely disappointed with the speed, but extremely happy with the boat. The boat was built for striper fishing as well as other types of fishing. I do not regret my decision to put the cabin on or the heavy doors , they are worth their weight in gold as far as keeping me out of the weather and I can fish all winter long as well as spend the night on the pull out bench. Not the usual pontoon boat full of furniture. But you can almost feel the boat wants to get out of the water, so to speak. So my goal here is to at least pick up a few extra miles per hour and I know you guys have covered this quite a bit. I am in somewhat of a unique situation that I do have a Honda 225 horsepower, as well as a Evinrude 250 horsepower on standby, even though they need some work. My idea is to start out with adding two of these M brackets to the center log the entire length and see what kind of Speed gains I get, if any of all.

    I want to use the im bracket simply because I have these on hand. I actually have three of them that are 16 feet long and several that are 4 feet long. They look like they would make perfect lifting strakes with the surface of about 5 in. However, as you can see in the pictures, when I place them the way that they would work, they would actually have a one and a half inch fin pointing downwards and this is the whole reason for me coming on your forum. Just wondering if there are some dynamics that would be negative or positive created by this one and a half in fin that would Point directly downwards. In my mind, the only negative that I can see with the boat underway is that the fins would make it harder to turn left or right, because they would dig into the water. I'm just wondering about weather the Water Dynamics somehow would be positive or negative with the water not being able to escape out of the sides of these brackets. Are we looking at neutral forces involved or something else? That's why I'm talking to you guys. I can cut those fins off with a table saw but rather not do the work if possible.


    So I will sum it up; what effect will these 1 and 1/2 in Fins have on performance? I thank you in advance for your interest and answers and I will post results after I install the first 2 m brackets on the center log. Next Step would be to install another set on the outside logs, weather it just be on the inside of the logs or both on the inside and outside. Thanks again 20200405_123305.jpg 20200405_123319.jpg 20200405_123326.jpg
    Screenshot_20200405-210646_Gallery.jpg Screenshot_20200405-210607_Gallery.jpg Screenshot_20200405-181258_Gallery.jpg Screenshot_20200405-181323_Gallery.jpg 20200405_180622.jpg 20200405_175919.jpg 20200405_175911.jpg 20200405_123326.jpg 20200405_123319.jpg 20200405_123305.jpg
     
  2. Joe Montelongo
    Joined: Sep 2020
    Posts: 1
    Likes: 0, Points: 1
    Location: Texas

    Joe Montelongo New Member

    I’m curious how this turned out. Did it improve anything?
     
  3. JT2017
    Joined: Feb 2017
    Posts: 3
    Likes: 0, Points: 11, Legacy Rep: 10
    Location: Troy,VA

    JT2017 New Member

    Hello joe, unfortunately I never did it, even though I may still do it in the future . Pretty amazing how I got zero replies. Seems like you mention the word pontoon on this forum, and people start laughing. I disagree with that because of the number of pontoon boats that are out there currently . I thought it was a valid question, and I will let you know any results so that the next guy who is building a pontoon boat can have some more information at hand.
     
  4. messabout
    Joined: Jan 2006
    Posts: 3,065
    Likes: 242, Points: 63, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    The stakes might help a little, I suspect very little in this case.

    I submit that the real problem for achieving more speed lies with the superstructure. The box like cabin is aerodynamically horrid. So are the rest of the parts that are painted blue. That structure will soak up a huge amount of propulsion power. Aero drag varies as the square of velocity therefore even a small increase in speed will need a disproportionate amount of power to drive it.

    It is easy to presume that 20 MPH will not generate enough aero drag (resistance) to matter. It does matter. To get a feel for the idea, try riding a bicycle against a modest headwind. Just a small increase in speed requires a lot more pedal effort. For an interesting visual, attach some bits of knitting yarn or ribbon to several places on the cabin and other parts. Observe what they do at various speeds.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,832
    Likes: 575, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Well, so far as aerodynamic drag is concerned, comparing a run directly upwind, for speed, at the same throttle setting, with the reverse direction, even in 10 knots of breeze, would give a good indication. Easy with GPS.
     

  6. JSL
    Joined: Nov 2012
    Posts: 792
    Likes: 53, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    Agreed; wind resistance is a function of speed squared. At 20 mph would be about 1.5 lb per sq. ft.... at 25 mph about 2.5 lb.
    Also - I hope those windows are safety glass (laminated, tempered, etc) of some type. Plate glass would be very unwise.
     
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