My wife asked me to put this question to you BITOG members.
Why do boats not have transmissions with several forward gears?
Thanks guys.
Why do boats not have transmissions with several forward gears?
Thanks guys.
Wife question.
- Thread starter callbay
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Because at a certain point more prop speed does not equal more forward speed.
The prop dictates required torque and engine speed.
Edited to say that cavitation was the word that eluded me! Exactly.
The prop dictates required torque and engine speed.
Edited to say that cavitation was the word that eluded me! Exactly.
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The prop is the equivalent of a cars torque converter without a lock up. Ed
Years ago there was a prop that had 2 pitches, so it would increase pitch at a high rpm.
Probably space and reliability issues, in addition to the fact that boats are trying to overcome a lot more drag than a car. Adding in a set of gears that would have to withstand the torque necessary to move a boat at a high rate of speed would likely add un-necessary weight and complexity while not noticeably improving fuel burn.
There are one or two 2 speed boat transmission out there. But a boats propeller is really like a final drive gear and has it's own ratios based on diameter, pitch of the blades and number of blades.
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gathermewool
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Originally Posted By: turtlevette
Years ago there was a prop that had 2 pitches, so it would increase pitch at a high rpm.
There are ships that run a constant shaft RPM and vary the pitch to change speed.
Years ago there was a prop that had 2 pitches, so it would increase pitch at a high rpm.
There are ships that run a constant shaft RPM and vary the pitch to change speed.
The lower unit of an outboard or an inboard is a transmission-it switches between forward & reverse, has gears & gear oil, it just normally only has one speed.
Originally Posted By: bullwinkle
The lower unit of an outboard or an inboard is a transmission-it switches between forward & reverse, has gears & gear oil, it just normally only has one speed.
On a Merc Bravo the clutch is in the upper- the lower determines final ratio.
The lower unit of an outboard or an inboard is a transmission-it switches between forward & reverse, has gears & gear oil, it just normally only has one speed.
On a Merc Bravo the clutch is in the upper- the lower determines final ratio.
Originally Posted By: callbay
My wife asked me to put this question to you BITOG members.
Why do boats not have transmissions with several forward gears?
Thanks guys.
GO ahead and buy her the boat, you'll be happy you did. ( just being a wise guy since the answer is already posted)
My wife asked me to put this question to you BITOG members.
Why do boats not have transmissions with several forward gears?
Thanks guys.
GO ahead and buy her the boat, you'll be happy you did. ( just being a wise guy since the answer is already posted)
None of the answers here have gotten to the root reason. Engines like a constant torque on them and a prop does a pretty good job. It's not perfect but close enough for your typical recreational boat. Ideally the pitch of the prop would change as the rpms of the engine climbed. A CVT on a car is set up to provide a consistent torque load on the engine. Cars, due to the large swings in torque requirements need a transmission more than a boat would. If there was a simple, low cost method to do this on a boat it would be done but given that a boat must be low cost, simple and does not have such torque swings, it's just not worth the cost.
Originally Posted By: Wolf359
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
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friction is the answer. If boats went the same speed as vehicles go and needed good fuel economy I bet they would have one. But boats are limited in speed "to the average buyer" I have worked on 500k speedboats but most people don't buy those.
A boat engine has an optimal RPM for a given speed. If a boat running at 4800RPM WOT at 55MPH drops RPM via a taller gear it will slow down and not be able to maintain speed. There is a helluva lot more resistance in the water than on the road.
So if the 55MPH did have a second gear, you would lose speed and efficiency as it shifted into the next gear. Unless the boat had some absurdly high torque engine, it is not feasible.
Many boaters have tried to go up just one number on prop pitch in an attempt to gain top speed and lost 8+MPH on the top end. Also they would only get 3500RPM or less at WOT. For 99% of all recreational boat engines, the laws of physics do not permit multi ratio gear boxes.
Picture your vehicle pulling a 3000 pound trailer up a 6% grade all the time. That is the amount of load a boat engine constantly experiences.
So if the 55MPH did have a second gear, you would lose speed and efficiency as it shifted into the next gear. Unless the boat had some absurdly high torque engine, it is not feasible.
Many boaters have tried to go up just one number on prop pitch in an attempt to gain top speed and lost 8+MPH on the top end. Also they would only get 3500RPM or less at WOT. For 99% of all recreational boat engines, the laws of physics do not permit multi ratio gear boxes.
Picture your vehicle pulling a 3000 pound trailer up a 6% grade all the time. That is the amount of load a boat engine constantly experiences.
Also boat engines have cam profiles that make more and more torque as they head up to redline. Cars are way more flat for general purpose use.
The drag of the water on the boat is such that this is the only way it's remotely efficient at multiple speeds. Plus going near flat out lets the boat "plane out" if the powertrain matches up properly and gain some efficiency that way too.
The cam profile is why an idling lobsterboat sounds like a hot rod, it's got terrible vacuum down low like a car with a race cam.
The drag of the water on the boat is such that this is the only way it's remotely efficient at multiple speeds. Plus going near flat out lets the boat "plane out" if the powertrain matches up properly and gain some efficiency that way too.
The cam profile is why an idling lobsterboat sounds like a hot rod, it's got terrible vacuum down low like a car with a race cam.
Originally Posted By: UncleDave
Originally Posted By: Wolf359
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
Actually in an airplane, the max speed of a proppeler is ruled by tip speed. Some airplanes with direct drive prop can run about 2,800 rpm for example, since if it gets to 3,000 + rpms you could get near ultrasonic speeds at the tip and the resultant shockwaves can ruin a prop tip in a no time, appart from the horrid sounds it would make and also the efficiency loss from flow disturbance. It depends on the lenght from tip to tip and its rpm. You can do the math from a 85inches diameter turning at 3.3k rpm and see what Im talking about.
Originally Posted By: Wolf359
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
Actually in an airplane, the max speed of a proppeler is ruled by tip speed. Some airplanes with direct drive prop can run about 2,800 rpm for example, since if it gets to 3,000 + rpms you could get near ultrasonic speeds at the tip and the resultant shockwaves can ruin a prop tip in a no time, appart from the horrid sounds it would make and also the efficiency loss from flow disturbance. It depends on the lenght from tip to tip and its rpm. You can do the math from a 85inches diameter turning at 3.3k rpm and see what Im talking about.
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Originally Posted By: Pontual
Originally Posted By: UncleDave
Originally Posted By: Wolf359
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
Actually in an airplane, the max speed of a proppeler is ruled by tip speed. Some airplanes with direct drive prop can run about 2,800 rpm for example, since if it gets to 3,000 + rpms you could get near ultrasonic speeds at the tip and the resultant shockwaves can ruin a prop tip in a no time, appart from the horrid sounds it would make and also the efficiency loss from flow disturbance. It depends on the lenght from tip to tip and its rpm. You can do the math from a 85inches diameter turning at 3.3k rpm and see what Im talking about.
An airplane 'constant speed' propeller acts somewhat like a CVT.
Originally Posted By: UncleDave
Originally Posted By: Wolf359
Airplanes don't have transmissions either. It's the start/stop nature of cars. You don't really have that as much in a boat or airplane so it doesn't really need it.
Not always- a ski boat starts and stops more than a new york taxi.
Boats are well served by transmissions, but they are extremely expensive and space consuming in the boat - pretty much a race only feature and budget these days.
The boat gets away with 1 fixed gear because of prop slippage- the prop never really operates without slippage which is the reason is can get going at all when propped for high speed running .
Actually in an airplane, the max speed of a proppeler is ruled by tip speed. Some airplanes with direct drive prop can run about 2,800 rpm for example, since if it gets to 3,000 + rpms you could get near ultrasonic speeds at the tip and the resultant shockwaves can ruin a prop tip in a no time, appart from the horrid sounds it would make and also the efficiency loss from flow disturbance. It depends on the lenght from tip to tip and its rpm. You can do the math from a 85inches diameter turning at 3.3k rpm and see what Im talking about.
An airplane 'constant speed' propeller acts somewhat like a CVT.
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