British Seagull 4 Stroke Idea

Discussion in 'A Work In Progress' started by cox24711, Nov 10, 2018.

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  1. Nov 10, 2018 #1

    cox24711

    cox24711

    cox24711

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    Recently in a daydream during assembly at school I had the idea of bringing British Seagull back, with new model, a single cylinder 102cc ohv 4 stroke using mostly the same parts as the original as possible. like the gearbox, props, mounts, flywheel, water pump, carburettor and fuel tank just to name a few. Oh and it will certainly be rope start with no recoil! :)

    (they have certainly proven themselves reliable) due to their outstanding reputation, and knowing that many people are fed up with the maintenance of new small outboards from the regular scumbags (Mercury Yamaha Evinrude etc) with the stupid cowlings getting in the way, the recoil starters watch spring snapping, the annoying impellers that wear out or go hard and their unnecessary complexity. They certainly aren't built to last.

    The one trait that you want in an outboard is for it to be reliable and that is what british seagulls nail. Also the ease to work on and accessibility. The idea is to make maintenance of it as simple and easy as possible, so it isn't a chore.

    Because of making new 2 strokes is now outlawed the only option I am left with is a 4 stroke. My aim is to make the simplest design possible, with as many off the shelf components as possible, too keep the costs down.

    I am not sure yet what the bore and stroke should be to keep it in 4hp-5hp range possibly the same as the original 57mmX40mm. I know that with the original gearbox the RPMs need to be quite high ~4,000 so we are looking for peak power at High Rpms.

    Thanks Greg
     
  2. Nov 10, 2018 #2

    cox24711

    cox24711

    cox24711

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    Also I forgot to mention on the crankcase it will have one of these
    500-1001_large.jpg
    that will screw into the water jacket (with a 1 way check ball behind it) so you can connect your garden hose to it to make it easy and simple to flush out after use!
     
  3. Nov 10, 2018 #3

    XD351

    XD351

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    I think the outlawing of new two strokes is for commercial manufacturers and not something that will effect you . There are plenty of parts on ebay and i use the pistons and rings off the 50cc minibike because i can get a piston and rings for a couple of bucks . You could use parts from a briggs and stratton motor and you can pick up things like pistons fairly cheaply .
    I remeber when i was at high school someone gave me an old clinton outboard and to this day I don’t know how it ended up here in Australia as no one i talked to from just about every boat shop in sydney had ever heard of one ! I always wanted to stick a victa motor on it but that was way beyond my skills at the time . Victa also made outboards and occasionally they pop up on ebay .
     
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  4. Nov 10, 2018 #4

    cox24711

    cox24711

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    Thanks mate I was thinking of the same kind of idea using off the shelf bits and bobs. Ideally I would like to make this commercially. But first I need to design it and build a prototype and see where it goes from there.

    Greg.

    p.s that is a nice pic of an XD falcon you have got there. if I am not mistaken it looks like tru-blu, and that would make sense with your username.
     
  5. Nov 10, 2018 #5

    XD351

    XD351

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    Yes Tru blu lifelong DJR fan .
     
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  6. Nov 28, 2018 #6

    JOHN DUNCKER

    JOHN DUNCKER

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    You probably know of this site already http://www.saving-old-seagulls.co.uk/

    He is the goto guy on all things Seagull.

    There would be some interest in a roller bearing 2 st Seagull that could run on 59 to 1.
     
  7. Nov 29, 2018 #7

    cox24711

    cox24711

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    hello john,
    thanks but I am already aware S.O.S I have personally bought parts from John (the owner) and I have spoken to him about my idea, and he likes the idea and is interested with where it goes.
    Greg
     
  8. Nov 29, 2018 #8

    lennardhme

    lennardhme

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    Thanks for the thread fellas.
    I have an old seagull & you've given me the inspiration to "get into it "
    cheers,
    Lennard
     
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  9. Jan 1, 2019 #9

    cox24711

    cox24711

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    Well, with another year done and dusted am back with the basic layout of the seagull 4 stroke.
    Screen Shot 2019-01-01 at 12.04.58 pm.png Screen Shot 2019-01-01 at 12.05.11 pm.png Screen Shot 2019-01-01 at 12.05.39 pm.png Screen Shot 2019-01-01 at 12.06.00 pm.png images.jpeg

    It is single overhead cam (thereby reducing the parts count, reducing the weight and most importantly keeping it simple)
    the arrangement it is done so by utilising 3.5 flat head briggs lawn mower camshaft and (modified valves) and springs etc.

    The piston and rod and crank are your bog standard seagull ones ( I decided on using the standard piston and rod, so it won't mess up the balance of things, otherwise it will probably end up running like a wacker packer :D)

    Because of the shape of the piston, I went with a wedge combustion chamber, and I am shooting for 9.5:1 compression ratio (with a head gasket on it will probably be more like 9:1

    According to john from S.O.S the 102cc models had a compression (when new ) of 55psi that equates to a compression ratio of 3.75:1! :eek: and they even run well with 35 psi! ~ 2.4:1!

    Lets get back to topic
    the camshaft will be driven by a toothed belt (a brilliant idea that I picked up from Malcom Stride's Miniature Internal Combustion Engines Book) thereby bypassing any complicated arrangement of gears, or being worried about chains coming off. (for small 4stroke outboards toothed belts are a common sight.)

    The crankcase is bog standard seagull as well (I wish there was room to put roller bearings in but there isn't)
    and the carb will be standard villers as well

    Anyway, Happy New Year Everyone.
    Wishing you all the best with your various projects

    Greg
     
  10. Jan 1, 2019 #10

    cox24711

    cox24711

    cox24711

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    oh and I forgot to mention that the spark plug will be a 10mm, with 1" reach
     
  11. Jan 1, 2019 #11

    BobNZ

    BobNZ

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    Crankcase modification for big end, gudgeon plus some bore lubrication, along with the possibility that scavenging of oil, many be needed depending on whether you plan on a "dry" sump with pressure fed lubrication or a wet splash system. Rings will need to be modified to cope either way.
    All good fun
     

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