martiniboat

I'd just like to hear some opinions and thoughts on automatic transmissions and how they could be utilized in a boat. I know from a (boat) production standpoint they're not used although there are some two speed trannys out there. Why are the multiple gear versions (3 or 4 speed car versions) not being utilized? Weight? Cost? Space?

Don't get me wrong, there's nothing that produces wood like the sound of a high performance, open exhaust marine engine at full song but...with todays gas prices worked into the equation, could worthwhile benefits be realized through an auto transmission?

Let's hear it

Brian

rchevelle71

iTrader: (1)

Brian,

GOOOOOD Question, I have thought about this many times, and cannott come up with a logical answer??? other than maybe weight.

Reed Jensen

ZF makes a two speed.... Weismann makes multiple speed transmissions for marine use... It's probably because of the cost and the space considerations that keep boat builders from installing them... I know Power Glide two speeds are available that will take all the horsepower a blown engine will put out. My understanding is the lower gears are just used to taxi around the docks and wakeless areas and possibly getting up on plane. I've never been in a high speed boat that needed the lower gear to get up on plane... usually there is enough torque to easily spin the props in the water. The main reason for multiple gears in a car is to keep the engine within it's torque "band" during different load conditions. I've never had to boat up a steep hill at a slow speed.... and the waves I've encountered may have been tall... but not high enough to make me feel like I'm trying to climb surf on the North Shore of Hawaii... Possibly during race conditions you might need different gears to keep a " race engine " up in it's torque range coming around a buoy... but for pleasure boating... not really needed..

glassdave

iTrader: (1)

boats resitance to motion is somewhat linear. in other words it requires continuously increasing torque as speed increases. in a car with multiple speed transmissions they can take advantage of the fact that once it is moving, friction is fairly low and higher (lower numerically) gear can be utilized. i believe there are some diesel packages that do use multiple gear trans's , they have the avaliable torque though.
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in other words . . boats dont coast to well

nitro

Back when we raced flat bottom circle boats, we built several 20' v-drive ski race boats with turbo 400 automatics. 1000 hp blower motor with reverse. We never had any problems, but the weight and space thing is a problem with offshore stern drive applications. Put an AT on in between motor and drive, you shove the motor forward 3+ feet in tends to mess with COG.

Next project was a 40 offshore boat with a 2 speed. hassle vs benefit wasn't good. Gave up went to fast cats, lots more fun.

Friend tried a Lenco 2 speed air shift out of his drag car. Smaller boat, works OK, not a lot of benefit for the hassle

Airpacker

Dynamic relationships, or how an engine is designed to make power. The two speed transmissions that are out there were really designed to allow engines with big camshaft profiles to reach their designed torque generating rpm range while turning large props and getting on plane. Once a boat is on plane, reducing the gear ratio, ie up shifting to a 1 to 1 ratio allows the engine to stay within its designed rpm range as the boat speed increases. Trying to lower the ratio further with another gear would simply "LUG" the engine thusly making it very inefficient. Unlike a motor vehicle on a roadway, the amount of torque required to maintain a given hull speed cannot be reduced as the physics involved with a planing hull are not the same as a tire acting against a road. The torque required to accelerate a road vehicle from a standing position is higher than the torque necessary to maintain a given speed once it is reached up until the point where areodynamics come into play, thusly a lower gear ratio can be utilized lowering engine rpm and saving fuel. A boat on the other hand requires the same amount of torque to maintain that given speed. Lowering the gear ratio and thusly the engine rpm would require more throttle to be applied to maintain that torque output thusly raising fuel consumption. Unfortunately, the laws of hydrodynamics wont let a boat coast without coming to a stop relatively quickly. Figure out how to do that and you can cruise at 100 mph in an old resin bucket at 1/4 throttle at 2500 rpm

ragtop409

Seams to me it is all about tork and resistance in a boat unlike a car. Mabey if you had a BIG motor you could get away with a overdriven gear but then why not just run a larger pitch prop? Rag's

glassdave

iTrader: (1)

hey Airpacker . . .you broke the OSO record for the amount of times the word "thusly" is used in a single post

JUSTONCE

you'd need a close ratio trans and 2 speeds would be the most. Still due to drag the trans would be smoked in no time cause of the strain through the shift, otherwise it would fall off plane before the rpm could get back up.

Airpacker

"Thusly" , Steve should send me a free OSO prize for setting the record shouldn't he