Why don't boats have transmissions?

Question

I know that boats do have gearboxes that change the gear ratio, but how come they don't have transmissions with more than one gear. i assume that you don't need a transmission because there isn't alot of resistance for the propeller to turn unlike rolling resistance that you get in a car, but i would think that multiple gears would be helpful in raising the rpms of the prop and lower the engine rpm while already moving in the water. also if you know of any where to learn more about marine mechanics please let me know. thank you

Answer 1

some do.  most drag boats do.  some of the faster jet boats do.  saw one last week had a GM PowerGlide 2 speed in it.

Answer 2

Those that answered about the dif between cars and boats are correct. Boat engines always have a load on them (like going uphill in a car.) So boats wouldn't really get the most use out of a tranny and the trannies weigh alot. That is why  boats that do use them only use a two speed tranny like a power glide. But there are some props that can change pitch are various rpms. They due this using springs and weights in the hub. I don't think that they can handle super high horse power though. There are also props that have a progressive pitch that changes as the pitch across the face of the blade. Read the links below for more info.

Answer 3

A friend of mine actually built a small powerboat with a complete drive train and discoverd the hard way that you can only spin a prop so fast before it becomes useless.  Based on hull designs (displacement, planing, foils etc.), certain rpms are need to most efficiently push a boat.  Above or below that range, severe inefficiencies creep in and the prop just doesn't work properly any more.  In my friends case, all the transmission really did was to add weight and extra parts that could break.

Answer 4

Ryan hit it very well. The engine is always working to keep a boat moving, like going uphill all the time. And there is a marine transmission with 2 forward speeds made by ZF that is made to take advantage of todays high horsepower diesels.

You can look into Marine Mechanics Institute for training.

Answer 5

Props are designed to be efficient only in a certain rpm range. If you turn them too fast, they will cavitate. Cavitation is the formation of vapor bubbles (when the pressure drops below the saturation pressure for the given water temp)  in low pressure areas of the prop near the root of the blade. When the bubble reaches an area of higher pressure further out on the blade, the bubble collapses. Cavitation is like a lot of little explosions happening on the surface of the blade, causing damage eventually. Cavitation also occurs in water pumps.

A great way to build a prop would be to have variable pitched blades.

Answer 6

d**n I never gave this thought! Good question! My guess is the fact that it doesn't need a drive train to keep a steady motion. Its wheel-less. and floats on water. A wheel in motion is a form of antigravity. In water these needs do not apply. and the power goes directly to the propellers. to propell the object(boat) foward.

Answer 7

Outboard motors are simpler than automobiles.  They do not need multiple gearings to change the prop rpm's.  This is done simply by raising or lowering the engine rpm's.

I don't remember the name, but there is a technical school that advertises in many of the hunting and fishing magazines that offers a course in marine mechanics.  Check the advertisements in Outdoor Life, Sports Afield, and Field and Stream.

Doc

Answer 8

I graduated from that school, I believe the school in question is MMI. (Marine Mechanics Institute)  They are part of the UTI corporation.  There is a motorcycle school, same initials, UTI, universal Technical Institue, Nascar...  Anyways, yea... I'll leave that question about the trans to those who already are answering.  Good question for the lot of them.  SAVAGES!!!!!!!!!!!!!!!!

Answer 9

Actually, it's quite the opposite.  There is tremendous force on the prop it the water.  In basic terms, you prop the boat so it can reach maximum RPM's for the Horse Power of the motor.  (diameter of the prop and pitch of the prop).  Too large and too much pitch would not allow the boat to plane and get up to speed.   Too little would cause it to over rev.  

In a car, there is momentum with wheels that glide which allow different gears to maximize economy and less wear on the engine.  

Think about it, take your foot off the pedal of a car and it will continue to coast.  On a boat, pull the throttle to Neutral and you almost immediately come out of plane.