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  1. #1
    Join Date
    Jun 2016
    Location
    Manchester uk
    Posts
    114

    Default cavity plate 3" below hull.

    I am thinking of putting a longshaft 25hp Johnson on my boat.At the moment I have an 1964 shortshaft 18hp Evinrude on it.
    I raised the transom on my boat 3" in order to line up the cavity plate on the 18hp with the bottom of the hull.The s/shaft is 15"-16" the longshaft is 20".
    This would put the 25hp 2"-3" below the hull.
    I will be using the boat for inshore boat fishing.(sea)
    I don't need speed just power.
    Would the drag be a problem,or would the boat be usable?
    I expect I would use slightly more fuel.Any advice would be appreciated.
    (The boat has a splashwell)

  2. #2
    Join Date
    Nov 2011
    Posts
    2,823

    Default Re: cavity plate 3" below hull.

    You don't say what year the 25 is, but motors of that vintage ('64-ish) were designed to run on wood boats with keels. As such, the boat bottom would come somewhere between the cavitation plate and water pump bulge. With today's fiberglass boats, you can raise them up a bit, till cavitation becomes too annoying. Why don't you simply convert the 25 to short shaft (assuming the year/model allows it)?

  3. #3
    Join Date
    Jun 2016
    Location
    Manchester uk
    Posts
    114

    Default Re: cavity plate 3" below hull.

    Sorry,forgot to mention the year of the 25hp,it's a 1999.(J25RLEEB)
    The leg,as you probably know,can't be shortened or extended.
    The boat is fibreglass,and is strongly built.With a pointed bow,with a hull that is almost flat at the stern.(14ft Broome Gemini speedboat,1980's GB)

  4. #4
    Join Date
    Nov 2011
    Posts
    2,823

    Default Re: cavity plate 3" below hull.

    OK, your '99 has the modern gearcase and can tolerate running higher than the old-school ones. Running it deeper won't really hurt anything, other than speed due to drag. If too deep, it will also put up a lot of spray. And of course it will take deeper water because is sticks down a couple more inches.

  5. #5
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    5,049

    Default Re: cavity plate 3" below hull.

    If your looking for fast get a 80s era 25XS merc and make it fit your transom!

  6. #6
    Join Date
    Jun 2016
    Location
    Manchester uk
    Posts
    114

    Default Re: cavity plate 3" below hull.

    Quote Originally Posted by kimcrwbr1 View Post
    If your looking for fast get a 80s era 25XS merc and make it fit your transom!
    Haha,my "lead boot" days are over,getting on the plane and low end torque is my aim these days.
    Cruising at a low speed on the plane suits my "get there and back" needs for sea fishing.
    Plus,I prefer Evinrudes and Johnsons.

  7. #7
    Join Date
    Jun 2016
    Location
    Manchester uk
    Posts
    114

    Default Re: cavity plate 3" below hull.

    Quote Originally Posted by fdrgator View Post
    OK, your '99 has the modern gearcase and can tolerate running higher than the old-school ones. Running it deeper won't really hurt anything, other than speed due to drag. If too deep, it will also put up a lot of spray. And of course it will take deeper water because is sticks down a couple more inches.
    Bit of what I wanted to hear,thanks.
    The boat is set up with twin levers for the '64 Evinrude,I have the single lever control for the 25hp Johnson,but it's operation is baffling me somewhat.
    The steering comes into the spashwell thro' a fixed plate,so I will need to have a free cable to connect to the swivel tube on the 25hp Johnson.
    I made a bracket for the 18hp to compensate,but the tilt is restricted when launching.
    With adding 3" to the transom,the fixed steering cable is too low.

  8. #8
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    5,049

    Default Re: cavity plate 3" below hull.

    Next question if you had to lift the short shaft up what is the reason is long shaft for? Unless you trying to increase draft at the stern for big water? A deep draft smoker craft makes for nice outings and easy to load and tow. Just wonder what you all over goal is

  9. #9
    Join Date
    Jun 2016
    Location
    Manchester uk
    Posts
    114

    Default Re: cavity plate 3" below hull.

    Haha,I was a newbie to having my own boat,it's been an enjoyable learning curve.Unfortunately,I jumped in with both feet,especially when I bought the 25hp.
    It was a real bargain.I'm pretty handy and thought I could make it fit.
    I have mates that used to fish with,but at 73yrs I was cramping their style,so I decided to get my own boat.
    My goal is to use it for inshore sea fishing.

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