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Old 10-02-2014, 12:28 AM   #1 (permalink)
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Multi-geared transmission in a boat?

A debate my dad and I got into earlier today. I think it's possible to grab a 302 Windsor like the one from my '91 5.0 Mustang with a 3-speed manual and put it in a boat (currently has a 4.3L V6) and get efficiency gains along with more power when needed. Who is right? I know boats are mechanically different but Yea.

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Old 10-02-2014, 05:46 PM   #2 (permalink)
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The boat propeller design plays the biggest role. You dont want it to cavitate.

You do however want to lug it in the sweet spot vs rpms.
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Old 10-02-2014, 07:03 PM   #3 (permalink)
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A propeller in water acts much like an auto anyway, just like a toque converter they have prop slip of 10-20% approx.
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Old 10-02-2014, 10:05 PM   #4 (permalink)
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The few ones Ive seen are a direct bolt or 1 to 1 ratio. I dont know if the design was just lazy or efficiency was out the window when it comes to boats anyway.

I read some books from amazon about electric boats and it said hull design was the most important for efficiency. Next up was propeller design and motor. You needed to know the graph to your motor and setup the voltage to run at that rpms.

I use to cruse the lake not too far from me in a small boat with a small gas engine. I often went half throttle tops. Then one of those guys with a power boat came through. Boy it went a shock wave through the water that took forever to go away.
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Old 10-03-2014, 12:21 AM   #5 (permalink)
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Interesting... I guess more of the question is, would a multi-speed tranny even work in a boat?
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Old 10-03-2014, 12:33 AM   #6 (permalink)
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It will work, it just might not work well.
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Old 10-03-2014, 07:04 PM   #7 (permalink)
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Being an avid fan of Whale Wars the various boats the Sea Shappards used had strange drive systems. One of the boats used 2 drive trains, 1 for slow speed, 1 for fast. They had to switch to pursuit the whalers vs just idle around looking for them.

Another boat they had to come to a dead stop to "shift" gears in order to pursuit vs cruse while the others are like a car they just give more throttle and watch the gas gauge to allow enough fuel for a return trip.

I imagine a boat is piloted more with fuel consumption in mind?
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Old 10-16-2014, 01:58 PM   #8 (permalink)
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Look in the technology tab and look at trans, gears and specs. Seems that they have 2 speed lower units for making a shift. Thing about boats is, it's impossible to shift. The water slows you down so fast that by the time you grab your next gear, you've slowed down so much that the engine is stuck in the hole again, only this time it's fighting a taller ratio + drag.
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Old 10-17-2014, 05:36 PM   #9 (permalink)
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Best BSFC seems to be at around 1/3 redline rpm at 75-80% throttle for most engines. having a 2 speed gearbox seems like a sensible idea?
- 1 gear maxes out the engine power 100% throttle tops out at (slightly above) peak power RPM.
- 1 gear thats (much) taller, putting it into the BSFC sweetspot with the throttle at 80%-ish.
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Old 10-17-2014, 05:58 PM   #10 (permalink)
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I'd rather have a variable pitch prop- if there is such a thing. Just like airplanes.

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