Sizing prop and gear ratio for semi displacement speed cat?

Discussion in 'Props' started by DennisRB, Jan 15, 2020.

  1. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    Hi. I just purchased a light weight sailing cat with blown outboards. I want to finally install my hybrid system of one gasoline shaft drive engine in one hull and one shaft drive electric in the other primarily for dual engine maneuvering.

    The primary SOR of the engine system is light weight, high power and long range. The vessel is a 43 feet long performance cruising/sailing cat and weighs only 5500kg in cruising trim so is very light for its length.

    The engine I want to use is the 1NZ-FE from a Toyota Yaris. 111hp @ 6000 RPM and 140nm @4000. It only weighs 86kg. I'm happy to spec the prop and gearbox to use the full 111hp and not derate it. This engine has 4 times the power required for cruising speed so in regular operation it will not be overloaded, but I would love the extra power for emergencies.

    The yanmar KMH40A gear box is only 18kg and is available in the following ratios 1.24 1.53 2.04 2.45. Im not settled on this gearbox but I found it would do the trick.

    I plan to use a 2 blade folding prop like flexofold which is available to 20" diameter.

    What speed can I expect with 111hp in a 43 foot 5500kg cat? My old cat of the same length but around 7500kg could do 10 knots with 50hp so I imagine 15k would not be impossible?

    What gear ratio and 2B prop spec should I go for to achieve this speed? Will I be worrying about prop overload and cavitation with a 2B prop?

    Whats the smallest prop shaft I can get away with?

    Thanks, Dennis.
     

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  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    You might do 15 knots with the electric motor, but it won't last very long before you need to re-charge the batteries.

    Re '140nm @4000' - is this 140 mile range, or torque (newton metres) or ?????

    Would it not be worthwhile have an inboard diesel engine, instead of petrol?
     
  3. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    If you can get it, I'd suggest picking up a copy of Dave Gerr's book "Propeller Handbook". If you, or someone you know can use a scientific calculator this book will walk you through a detailed analysis that will provide you with enough information to order the props you need. The reason for the calculator is to solve engineering equations using decimal (fractional) exponents.

    https://www.amazon.com.au/Propeller...Gerr Propeller Handbook&qid=1579128513&sr=8-1

    Sadly I see that this book is pretty expensive in Australia. Here in the US we can get it for less than $10.00.
     
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  4. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    Thanks for the reply. The electric motor in one hull is only so the boat has a motor in each hull for maneuvering. This question is only regarding the gasoline engine. Those are the specs for the engine:

    Output is 109 hp (81 kW; 111 PS) at 6000 rpm with 141 N⋅m (104 lb⋅ft) of torque at 4200 rpm. The redline is 6400 rpm.

    A diesel engine does not meet my stated SOR due to high weight and low power.
     
  5. DennisRB
    Joined: Sep 2004
    Posts: 1,268
    Likes: 25, Points: 48, Legacy Rep: 228
    Location: Brisbane

    DennisRB Senior Member

    Thanks! I see there are a few online calculators like this. But I dont trust the results. I like to defer to knowledgeable people to sanity check the results. Right now im only in the feasibility stage.

    Vicprop - Prop calculator for Displacement and semi-displacement hulls https://www.vicprop.com/displacement_size.php
     

  6. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Dennis, Good thing that you have time.....
    I'm working with Gerr's book. I did a full restoration on a small criuser and have never been happy with the performance. She's a great boat but I never altered the running gear and feel that I can improve on it. I've been running through all of the engineering in Gerr's book. What I'm learning is that there is so much more to a prop selection than the diameter and pitch. You have to look at propeller slip, the developed area of the blades, prop loading and more. At first I thought a simple pitch change would solve my problem. As I delve into it though I realize that the prop on the boat is over loaded and I'll probably need a 4 bladed prop in order to get the blade area large enough to reduce the prop loading to a proper level. As is often the case this propeller analysis isn't as simple as it appears at first glance. I'll be checking my math (again) today.
     
    Last edited: Jan 19, 2020
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