Gear ratio questions for a diesel stern drive.

Discussion in 'Sterndrives' started by ben2go, Apr 25, 2012.

  1. ben2go
    Joined: Jul 2008
    Posts: 183
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    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    I couldn't find the answers through search.

    1)Should I gear, and prop, my stern drive to keep the engine cruising at the beginning of the torque curve or in the peak of the torque curve?The engine's torque curve is short from 1800 to 2200 rpm with a red line of 3000 rpm.Hull cruise speed is 6-7 knots.

    2)Would it be better to go with lower final drive gear(2.1) with big diameter low pitch prop or higher final drive gear(1.1) with a smaller diameter high pitch prop?

    3)I will have to make the adapters to mate the stern drive to my engine,Kubota.IS there a certain drive I should look into for a low horsepower diesel?I want something that is easy to get replacement parts for.I don't want to get stuck on the hard while I wait weeks for parts to arrive.I am familiar with Mercruise and Volvo Penta I/Os.I have repaired a few OMC I/Os but I am not very familiar with them.I know they are hard to find parts for around here.

    The boat plans only call out for a horsepower range and designed (displacement) hull speed.The boat's final weight loaded should be 3500lbs to 4500lbs.Intended use is for ICW and great loop cruise with some large local lakes thrown in for fun.
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    First: the propeller disc area (read propeller dia) is a function of the vessel's power/weight ratio and hull shape, resulting in an operating speed range.

    Second: with above knowledge, the resulting propeller dia must work within a given (small) range of rotational speed.

    Third: having done first and second steps, you select transmission ratio so that you get the required prop rpms (equals prop absorbed power) at the engine max power rpm's.

    So, until the info on engine power@rpm, total hull weight loaded and hull shape is at hand, your question cannot be answered.
  3. whitepointer23

    whitepointer23 Previous Member

    can you even get a 1-1 stern drive. older volvo's were around 11/2 -1 for v8, 1.89-1 6cly and 2.3 -1 4 cyl. i would go for a v8 ratio because i have fitted diesels to legs before and had to extensively modify an 18 x 18 inboard prop to fit. you need to allow for the lower cruise revs and higher torque of the diesel. i got 16 knots out of a 24 ft steel cabin cruiser with a 70 ford diesel hooked up to a chrysler/dana 300 series leg.
  4. ben2go
    Joined: Jul 2008
    Posts: 183
    Likes: 4, Points: 18, Legacy Rep: 13
    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    The hull is a flat bottom 30' Pacific dory cabin cruiser being used at displacement cruise speed of 6-7 knots.Draft will be between 10 and 12 inches loaded at 3500lbs.Actual water line is close to 27'.The Engine hp is 35 at 3000 rpm and max torque is 67ft-lbs at 2200.Torque starts to drop off at 2400 rpm.I think a 10 inch three blade prop will be more than enough for this boat considering it's shallow draft.Designs call for 30 to 75 built light and up to 150 for heavily built boats running at planing speeds around 25 knots.

    The stern drives I find around here are either VP or Mercruise from high rpm GM/Chevrolet 4 cylinders of roughly 130hp-140hp.I know this wouldn't be gear properly for a low reving low hp diesel.I'm not beyond stripping and re-gearing an I/O.If I buy used,I will be going through the entire unit anyway.I know there are a few units out there that will bolt up to a Kubota but they are outrageously expensive.

    Here is what I am planning to build.I will be using the plans for the 30' length and vee entry forefoot instead of the flat bottom.

    Here is the engine I am set on because I have seen it used as an inboard for small tug yachts.I discussed my intended use with the designer and this was his HP recommendations.

    I was just using the 1.1 as and example.I haven't looked at all the actual available ratios yet.I've never re-geared an I/O.I've only replaced the damaged internals with factory replacement parts.Most of the lowers were brought to me at the 4x4 shop, where I worked.
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    You are way off here. The little diesel is a pretty high end drivetrain for this boat. You will spend a lot getting it in there. Maybe $25,000. Unless you plan to amortize this over 30 years, you really have to ask yourself if it is worth it. You are basically stuck with choosing between a few options that come as a package deal. There is really little you can do to assemble a driveline yourself for this sized boat.

    No, a ten inch prop isn't any good if you are going to the trouble of putting a diesel in. If you spend the dough on the diesel, you need to pony up for a proper drive train or it just doesn't make any sense. Get a copy of Dave Gerr's Propeller Handbook for a cookbook style approach to figuring props. The outrageously expensive stuff that goes with the motor will usually be a pretty good guide as to proper sizing of things. Don't go smaller.
    A 2:1 tranny would be typical here. 2.4:1 is also pretty common. Done well, you should be able to get away with less than a 35 diesel. The three cylinder version should work ok. You aren't building a tug boat. You can get away with 1/3 the cost of what they need.

    Good luck and keep posting as you make progress on your parts hunt.

  6. Frosty

    Frosty Previous Member

    This is what propeller shops do before they make a prop to fit your boat.

    Its like asking what gear should I use to go up this hill.
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