Planing boat speed prediction app

Discussion in 'Software' started by Ben Land, May 17, 2021.

  1. Ben Land
    Joined: Mar 2015
    Posts: 16
    Likes: 3, Points: 3, Legacy Rep: 10
    Location: Finland

    Ben Land Junior Member

    Dear boat designers,

    Over the last few years I have been doing a bit of research on the mathematical models to predict resistance and thrust for planing boats. I thought it would be a nice way to keep myself motivated and to give something back to the community by making part of my work actually useful to the public.

    This made me start a personal project Home https://www.boatspeedcalculator.com/ which is a rather simple, but free online calculator to predict the speed of a planing monohedron boat using a modified Savitsky and a mathematical model based on AEW-series propeller regression data.

    Despite of its simplicity, I have found the calculator to yield relatively accurate results in the speed region of 25-40kn. Currently the calculator is limited to outboards and stern drives only, but I am aiming to keep developing it further. I have attached some of the validation that has been made so far in form of screenshots.

    I hope the app would help a regular boat owner to understand his/her boat and its driveline better and maybe even serve as a preliminary tool for a boat designer or a surveyor to evaluate performance. I want to keep the app simple with minimal inputs as I am aware of how much "friction" this would add to the usability. The aim is to find the correct balance where the inputs are limited and non-trivial while the results are accurate enough for the practical boat owner or professional.

    I would be very grateful to hear your feedback and comments on the app.

    Best regards
    Ben
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  2. TANSL
    Joined: Sep 2011
    Posts: 6,625
    Likes: 446, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Your attempt is very commendable, I wish we had indeed such a simple tool for such complicated calculations. There are three things I would like to comment on your post:
    - Savitsky's studies did not take into account whether it was an outboard, a waterjet, or whatever.
    - I assume that by "engine shaft speed" you mean shaft revolutions. In that case, at some point the performance of the axle line would have to be taken into account, which can vary from boat to boat. I honestly do not see the correspondence R.P.M./boat speed very correct nor generalizable to any boat. Why not use HP / boat speed like everyone else
    - I find it difficult to understand or explain that the curves are so equal, between the theoretical study and the sea trials. It is impossible that the conditions of both are the same, unless you have a very complicated and sophisticated way of simulating the environmental conditions at the time of the tests.
    Thanks in advance for your answers.
     
  3. Ben Land
    Joined: Mar 2015
    Posts: 16
    Likes: 3, Points: 3, Legacy Rep: 10
    Location: Finland

    Ben Land Junior Member

    Thank you TANSL very much for the comments. I will try to answer them here in order:

    -You are very correct. Actually the propulsor thrust line and resistance is taken into account by the calculator in the moment balance as per Hadler's work. To keep these inputs minimal and to have better control at this stage I decided to limit the calculator to stern drives and outboards.
    -By engine shaft speed I mean crankshaft speed. The term I used is indeed a bit confusing. I have used some rules of thumb to include typical mechanical losses for outboards and sterndrives.
    -I really would prefer to use power/Speed plot to compare results, as you suggest, but to be fair I have had hard time to get my hands on "reliable" and complete seatrial data. Most of the data I have been able to access has been in the form of rpm/speed, maybe due to lack of torque sensor in the propulsion line. The calculator considers calm water conditions only.
    -I will keep looking for more data to validate the results. Currently, the few validation cases I have made are rather similar to each other and it might be just the case that the calculator works well for these types of craft at these speed ranges. My own experience is that sometimes these semi-empirical and regression data-based mathematical models work surprisingly well as long as their boundaries are respected. Maybe next validation attempt will result to be not so accurate :)

    I hope I managed to answer your questions. I will keep validating and developing the app and hopefully this turns out to be a good project in the end.

    Thanks
    Ben
     
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  4. TANSL
    Joined: Sep 2011
    Posts: 6,625
    Likes: 446, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Thanks for your responses, Ben. I think they confirm the rigor of your studies. Continue with the validations and I hope that sooner or later you can offer us a tool as useful as this one promises to be, more generic and refined.
    Greetings
     
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