Need help gear ratio

Discussion in 'Props' started by Roberto Certain, Apr 24, 2021.

  1. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Hi everyone:

    I bought a Silverton 330 boat totally wrecked the only thing working is the hull. I stripped the boat completely. It used to have two volvo penta engines.

    I searched the web and was able to find two New diesel marine engines 120hp, im not sure of what gear ratio to use because the boat normally the boat has 550-600 hp and im only installing 240hp. I really dont care for speed, I would be happy at 10 knots. The transmission does 3000rpm, I would not like to force engine or transmission so I need a gear ratio and propellor that works.

    My boat dry weight is about 15000 lbs.
    Total length: 10,7 meters
    Beam: 3,76 meters
    Draft: 0,89 meters
    Displacement: 7115 kg

    Can some one out there help me pleeease I beg you.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Roberto.

    It looks like you accidentally clicked on the button to post your comments before you had finished writing them.
    No worries, you can simply reply in another new post to carry on from where you left off.

    Please do tell us more about this Silverton 330, and the work that you are doing on her - it sounds like a very big project, if you have stripped it back to a bare hull.
     
  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

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  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I used to work in a boatyard here many years ago, building small (27' - 37') fibreglass fishing vessels.
    We fitted John Deere engines, and we obtained our propellers from Frank & Jimmie's Propeller shop in Florida.
    They are still going strong - here is their website.
    Home: Frank & Jimmie's Propeller https://www.fjprop.com/

    Anytime that we had a query like yours, we would simply send them a fax (this was in the days prior to email) and they would promptly respond telling us what size and type of propeller to use - and we invariably found that their prediction was very accurate when we subsequently bought that propeller from them.

    Who is the manufacturer of the 120 hp diesel engines that you have found? And what type of gearbox was / is usually fitted to these engines?
    Re a reduction ratio, I am thinking you could start off with a ratio in the region of 2.5 - 3.0.
    But if this results in a propeller diameter that is too large (what is your maximum allowable diameter?), then you could reduce the ratio, depending of course on what ratios are available.

    Are you literally starting from a bare hull, or do you still have any old shafts, 'P' shaped (or 'A') brackets for supporting the shafts or stern tubes fitted?
     
  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

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  6. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    A couple of things to consider. This is a twin v-drive, correct? I've replaced the v-drive on my single Silverton. IMO Walter Gear makes the best drives in the business. For 120 HP I believe they will spec an "RV-26" either direct or remote mount. As of February 2021 the direct mount, the shaft coupling and the reverse gear coupling cost $3,200 USD. Of course you'll need two of them and additional parts so $6,400 USD would be just a starting point. Used engines can be checked out by a competent technician. I would n0t go with used v-drives. Raw water flows through the drive so there is always a concern.

    You may want to think about that whole idea of building a 10 kn0t yacht. It will probably be OK for awhile but if you are out in anything but fair weather you will find yourself wishing for more power. This is especially true if you're running inlets and in close quarters where there are strong currents. This isn't about cruising speed it's about thrust and speed when you need it.

    Unless you are sure that you'll be traveling long distances why diesels? Gas engines are much lighter, less expensive and (as long as you pay attention to your boat) just as safe.

    You say the boat was wrecked so you're probably be replacing props anyway. Make sure you have enough clearance.

    Good luck and welcome to the forums.

    MIA
     
  7. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Hi thanks for the response. The boat originally came with twin diesel engines with Stern drive. Stern is a rsally bad idea for hot climate because the marine life buildup is really fast if the boat sits in the water. This is why I want to change to direct shaft. I closed the holes where the Stern drive used to be.
     
  8. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Thanks for the quick response. I want to go with diesel engines because in Colombia diesel fuel is very cheap. I want to make a 10 knot boat because I only want to be inside the bay área and there are no huge waves or currents.

    What I dont know is what gear ratio to use or prop size. I want to guy with a direct shaft and the small engine so it can fit in my original compartment.
     
  9. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Gas is very very expensive because the government taxes gas not diesel.
     
  10. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    In view of how you have filled in the holes in the transom where the stern drives were, I wonder if it would be feasible to just install a larger single diesel engine instead of two smaller engines?
    Or have you already bought the two 120 hp engines?

    Assuming that you still have the beds for the old engines in the boat (?), they would probably still need a fair bit of modification, even if you wanted to use engines with vee drive gearboxes instead.
    If you have to build new engine beds, then it might be easier to just build beds on the centreline for a single engine?

    Can you post any photos of the boat showing it's present condition please?
     
  11. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Hi,
    I preffer 2 engines because if one fails I can still move. The engine beds would have to be modified. Im thinking of a direct shaft but I dont know if I have the space. Normally this boats come with Stern drives because space is a factor.
     

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  12. Roberto Certain
    Joined: Apr 2021
    Posts: 9
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    Location: Colombia

    Roberto Certain Junior Member

    Boat is totally stripped. No engines OR anything (only fiberglass)
     
  13. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Here is a copy of your screen shot - I saved it to my computer, as it was too small to see clearly in your post above.

    Can you post some photos of the boat as she is now?
    Including photos of the stripped out interior?


    Silverton 330 engines.jpg
     
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  14. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Looks like most of the usa domestic Silverton's had twin 7.4 liter motors at 330 hp or twin 5.4 liter motors at 290 hp turning a bit over 3k rpm. Could not ascertain what size diameter they could swing but one old thread on tht from way back indicated 19's. Stands to reason with gas engines companies seem to opt for fast rpm small diameter wheels.

    So the hull seems to work ok from an adaptability standpoint. An old listing from Australia had one powered with 4bta Cummins on v drives, it had no specifics but my guess is roughly 150 hp around 2800-3000 rpm as that was a really common arraignment.

    I've a fair bit of time on boats with 12.5 foot beams and 30 foot water line lengths, guessing you'll cruise at or around hull speed with very little effort and plow water to low teens. Not going to get on plane with 240 hp. Depending on the displacement of the diesels, guessing zf 220s are a bit big. So zf 80 iv come in 2.0 and 2.5 options. With the given curb weight the 2.0 to one with a 120 hp main turning 3000 rpm gives a prop of 19x16. But I'm a realist. A boat with that much volume, its going to gain weight like almost all boats do. If you can swing bigger go with the deeper ratio 2.5:1 and swing a 21 or 22 inch wheel with an 18 pitch prop. Gets you to 7.3 knots with ease, tops around 13 at max. Just enough power for a leisurely cruise but enough reserve to push the fairly high wind aspect ratio of the boat if needed.

    This is a cool project idea, but its worth evaluating price and being honest with your budget capacity and work capacity. This will be a fairly significant endeavor, 2x gears, props, shafts, packing, bearings, rudders, and tiller arms easily equals the cost of 2x 120 hp diesel motors. While my personal thoughts are this hull isn't a bad donor for a displacement speed trawler, it will likely be a fairly long expensive project.
     
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  15. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I would agree absolutely - I have seen so many examples of project boats being rebuilt where the owner has spent say $3X on re-fitting the boat, only for it to be effectively worth $1X or $1.5X at the end.

    What is your realistic budget for this project Roberto?
    Have you already bought the two 120 hp diesel engines?

    As you are starting from a bare hull, the cost of re-building it all, even if you do all the work yourself, is probably still going to be more than the cost of buying a second hand boat that is 'ready to go'.
    Do you have high import duties and taxes in Columbia on boats built overseas?

    Here is a rather nice looking Hatteras for sale in Columbia with an asking price of US$140,000 -
    1988 Hatteras 36 Convertible Power New and Used Boats for Sale - https://www.yachtworld.co.uk/boats/1988/hatteras-36-convertible-2975867/

    It was the only motor yacht I could find on Yachtworld in Columbia though.

    I think it would be very easy to exceed US$140,000 re the cost of refitting your Silverton 330.
     
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