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  1. #1

    Default Suggestions for design of Inboard setup for Kohler 25hp VTwin

    I am seeking almost any kind of 11 or 12 ft long hull to install a 25HP Kohler VTwin into via
    the use of an inboard prop shaft etc. What I am effectively wanting is a cheaper to run
    ( and acquire) substitute for a Personal Watercraft. I am strongly influenced by the Racing Crab Skiffs
    up in New England whereby they are using 20 ft long skiff hulls that are 42 inch at their widest and
    using Honda and Briggs 20 HP engines via an inboard setup.

    Hopefully we can find affordable components used by some of the other commercial inboard products
    and adapt to the recreational use Kohler 25 hp PWC project.
    Or better yet someone might have some ideas of how we can take off the shelf components from
    some other application ( like Go-Kart Rear Axles and sprockets etc ) to build up a workable and
    reliable solution.
    here are some photos of the racing CrabSkiffsClick image for larger version. 

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    Especially also please suggest other locations on the internet where I can learn more about inboard propulsion designs and setup in
    the 10 to 35 hp range.
    Last edited by preventec47; 08-05-2016 at 03:03 AM.

  2. #2
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
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    5,142

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    Ayuh,..... Air cooled motors don't make much sense, when yer surrounded by water,.....
    Any Grease is Better,... Than No grease at All....

  3. #3
    Join Date
    Dec 2015
    Location
    maplevalley WA
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    3,469

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    https://www.generalpropeller.com/
    Sounds like a fun project maybe look into sailboat parts for the strut, shaft, seal and coupler you can adapt it to any motor finding the best prop would be a challenge.

  4. #4

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    Quote Originally Posted by bondo View Post
    Ayuh,..... Air cooled motors don't make much sense, when yer surrounded by water,.....
    Maybe you didn't notice those photos in my message. Those are "race" boats. Those lawn mower motors are plentiful and
    cheap and ALWAYS easy to find cheap repair parts. The great thing is you dont have to worry about muck, sludge, chemicals
    and salt inside your precious engine. Maybe you also didnt notice.... that you are ALSO surrounded by AIR. Take advantage
    of it and cool your motor. Google "Crab Skiff" races and check out the videos.... The other thing you may not have realized
    is the VTwin industrial motors of today are one fourth of the price of same power outboard engine.

    PS I'm into this project because my motor was free ! I plan to use it. With all your hundreds of posts.... do you have anything
    useful to add to the conversation ?

  5. #5
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,469

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    Lawn mower motors are not designed to handle the extreme loads put on a boat motor. They may have the same HP but it is like climbing a steep grade hill full time no coasting, When the motor stops the boat stops. Once you connect a driveshaft/propshaft direct to the crank with no gear reduction you are greatly decreasing the life of the motor.
    Last edited by kimcrwbr1; 08-06-2016 at 09:38 AM.

  6. #6

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    Maybe I have tainted your opinion by referring to the powerplants as lawn mower engines as in truth these engines I am
    considering are only found on the most expensive commercial lawn tractors..... which by the way operate all day everyday
    at full throttle.
    I agree In general it is true that marine constant use at hi throttle settings... is far more demanding certainly than any
    automotive engines are designed for but......
    The top of the line Briggs, Kohler, and Honda industrial engines have come a long long way. Many of these participate
    in racing programs that subject them to extreme abuse and they have the reputations to be able to take the abuse.
    The beauty is that these motors are made by the millions and therefore repair parts are cheap and plentiful and they
    are easy to work on AND CHEAP by comparison to anything specified for Marine use.
    These engines have proven themselves in Airboats, airplanes, GoKarts and pressure washers etc where constant full
    throttle is the norm. I'd like not to focus on how appropriate others feel but how to implement their utility in the best possible
    way so that more of the masses can enjoy the great boating pastime.

    Do me a favor and check out this "Class" race all using 20hp VTwin industrial engines without modification.

    https://www.youtube.com/watch?v=PtcgXa4XIqg

    BTW Kim- thanks much for the Sailboat source idea as they seem to be the main users in the lower power inboard realm....
    Its just that the pricing of the components are beyond justifiable for weekend toys etc. It is good to see how
    the various mfgrs accomplish their goals and especially neat to see the use of diesel fuel engines.

    We have to be more creative with our solutions and think outside the box. Automotive sourcing is likely the secret
    to making this work. Junkyard Driveshafts maybe with ten dollar u-joints etc. Maybe even Go Kart centrifugal
    clutches and chain drive sprockets ... Go kart rear axles and bearings and wheen hubs.... how do we stick em through
    the hull and keep things watertight ?

  7. #7
    Join Date
    Dec 2015
    Location
    maplevalley WA
    Posts
    3,469

    Default Re: Suggestions for design of Inboard setup for Kohler 25hp VTwin

    They used to use iron wood for the keel bearing. Once you get all the mechanical stuff ironed out then you need to size the prop for proper WOT rpms. With no gear reduction the props are probably pretty small compared to boat motors.

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