Turboprop aircraft engines run at a constant speed and vary the pitch to control speed.
Wife question.
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We had a boat once that when fully loaded had a heck of a job getting up on plane. We used to wiggle and move people around while the engine was at WoT at 2800 rpm seemingly just digging itself a hole in the water.
If we could get it to plane, it would run like a scolded cat right up to the red line.
We later changed the prop to one with a less argrssive pitch, which got the boat on plane, but cost us top end speed (and I think) economy.
If we could get it to plane, it would run like a scolded cat right up to the red line.
We later changed the prop to one with a less argrssive pitch, which got the boat on plane, but cost us top end speed (and I think) economy.
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
2. Any boat that goes faster than 50 mph is a waste of money. So is any boat that goes slower than 50 mph, but it's much less likely to kill you.
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
2. Any boat that goes faster than 50 mph is a waste of money. So is any boat that goes slower than 50 mph, but it's much less likely to kill you.
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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.
During my time at Torque Engineering Corp., we actually developed a two-speed transmission package based on the GM Powerglide that had a 1.76 low and 1:1 high. With the big props that we were using to get 100 MPH top speeds, the engines did not like spending hours idling at 900 rpm in the no-wake zones around Miami. We used low gear in the no-wake zones to keep the engine up around 1700 rpm for a boat speed of 3-4 MPH. Then when we got out on the ocean, we shifted to high for the go-fast stuff. We didn't develop an "overdrive" transmission to try to get more top speed. This is because it is the nature of the propeller law that if you have properly matched the prop pitch and gear ratio to allow the engine to run up to max power, any higher gear will cost you speed, because the engine will only make less power at the lower rpm.
It may be possible to make an "overdrive" marine transmission to allow economical cruising at low engine rpm with high torque output, but to get maximum boat speed, the transmission would still have to be shifted into a "speed" gear. But the ratio split would have to be fairly small, maybe ~15%.
There were some offshore race boats running the Lamborghini V12's that had 4-speed transmissions with very close ratio splits. The transmissions would be downshifted in the turns to keep the engine speed up because the Lambo had a narrow powerband.
My wife asked me to put this question to you BITOG members.
Why do boats not have transmissions with several forward gears?
Thanks guys.
During my time at Torque Engineering Corp., we actually developed a two-speed transmission package based on the GM Powerglide that had a 1.76 low and 1:1 high. With the big props that we were using to get 100 MPH top speeds, the engines did not like spending hours idling at 900 rpm in the no-wake zones around Miami. We used low gear in the no-wake zones to keep the engine up around 1700 rpm for a boat speed of 3-4 MPH. Then when we got out on the ocean, we shifted to high for the go-fast stuff. We didn't develop an "overdrive" transmission to try to get more top speed. This is because it is the nature of the propeller law that if you have properly matched the prop pitch and gear ratio to allow the engine to run up to max power, any higher gear will cost you speed, because the engine will only make less power at the lower rpm.
It may be possible to make an "overdrive" marine transmission to allow economical cruising at low engine rpm with high torque output, but to get maximum boat speed, the transmission would still have to be shifted into a "speed" gear. But the ratio split would have to be fairly small, maybe ~15%.
There were some offshore race boats running the Lamborghini V12's that had 4-speed transmissions with very close ratio splits. The transmissions would be downshifted in the turns to keep the engine speed up because the Lambo had a narrow powerband.
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Originally Posted By: NHGUY
If the prop spins too fast,it will just cavitate,not providing any further forward thrust.
Cavitation - that is what happens to 85w140 gear oil when it's -10 out!
If the prop spins too fast,it will just cavitate,not providing any further forward thrust.
Cavitation - that is what happens to 85w140 gear oil when it's -10 out!
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Originally Posted By: Miller88
Originally Posted By: NHGUY
If the prop spins too fast,it will just cavitate,not providing any further forward thrust.
Cavitation - that is what happens to 85w140 gear oil when it's -10 out!
I know you're joking, but I'll clarify, for others:
What the poster above is most likely talking about, is aeration of the gear oil.
At colder temperatures, oil is further away from its vapor pressure. For cavitation to occur, the suction pressure, on the inlet of the oil pump (or the trailing edge of the gear tooth, if no pump is present) would need to be lower than or equal to the vapor pressure of the oil at the pumping temperature.
For those who don't know, cavitation, in basic terms, can be described similarly to boiling. Instead of raising temperature at a constant pressure (atmospheric,) temperature remains constant and the pressure is lowered to the point where the liquid vaporizes.
In practical application, and in context with the OP, the point at which pressure lowers below the vapor pressure, and cavitation occurs, is on the trailing edge (back edge) of the propeller. The leading edge (front edge) of the prop "pushes" the water and experiences a higher pressure than the pressure of the water at that depth; however, there is a corresponding lowering of pressure on the trailing edge that.
Based on certain conditions (e.g., temperature and gas content of the water,) cavitation (phase change from liquid to gas) will occur as the prop spins faster, because the faster the prop spins the lower the pressure will be on the trailing edge.
Slippage. Since the prop will shear the water, especially at high prop speeds relative to forward velocity (think, initially giving it a lot of gas while trolling along,) there will be a difference between how far the prop should have theoretically "screwed" through the water and how far the boat actual moves; the difference is slippage. As slippage increases, so too does cavitation. As the boat comes up on plane and the throttle is leveled off, slippage will still occur (you can't literally screw a prop into water; there will always be slippage,) but it will be minimal, and cavitation may lower.
If the prop is lower in the water (e.g., on a submarine,) the chances of cavitation are lower to non-existent (practically speaking.)
The problem with cavitation, isn't necessarily in the formation of the bubbles, but in their collapse. As the pressure that created them rapidly rises, the bubble collapses on the prop, creating a little shock wave. Over time, these formation-and-collapse events, and their associated shock waves, will erode material away.
Originally Posted By: NHGUY
If the prop spins too fast,it will just cavitate,not providing any further forward thrust.
Cavitation - that is what happens to 85w140 gear oil when it's -10 out!
I know you're joking, but I'll clarify, for others:
What the poster above is most likely talking about, is aeration of the gear oil.
At colder temperatures, oil is further away from its vapor pressure. For cavitation to occur, the suction pressure, on the inlet of the oil pump (or the trailing edge of the gear tooth, if no pump is present) would need to be lower than or equal to the vapor pressure of the oil at the pumping temperature.
For those who don't know, cavitation, in basic terms, can be described similarly to boiling. Instead of raising temperature at a constant pressure (atmospheric,) temperature remains constant and the pressure is lowered to the point where the liquid vaporizes.
In practical application, and in context with the OP, the point at which pressure lowers below the vapor pressure, and cavitation occurs, is on the trailing edge (back edge) of the propeller. The leading edge (front edge) of the prop "pushes" the water and experiences a higher pressure than the pressure of the water at that depth; however, there is a corresponding lowering of pressure on the trailing edge that.
Based on certain conditions (e.g., temperature and gas content of the water,) cavitation (phase change from liquid to gas) will occur as the prop spins faster, because the faster the prop spins the lower the pressure will be on the trailing edge.
Slippage. Since the prop will shear the water, especially at high prop speeds relative to forward velocity (think, initially giving it a lot of gas while trolling along,) there will be a difference between how far the prop should have theoretically "screwed" through the water and how far the boat actual moves; the difference is slippage. As slippage increases, so too does cavitation. As the boat comes up on plane and the throttle is leveled off, slippage will still occur (you can't literally screw a prop into water; there will always be slippage,) but it will be minimal, and cavitation may lower.
If the prop is lower in the water (e.g., on a submarine,) the chances of cavitation are lower to non-existent (practically speaking.)
The problem with cavitation, isn't necessarily in the formation of the bubbles, but in their collapse. As the pressure that created them rapidly rises, the bubble collapses on the prop, creating a little shock wave. Over time, these formation-and-collapse events, and their associated shock waves, will erode material away.
I can't believe people answered without first requesting photos of the boat and the wife.
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.
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.
Agreed on the pict of wife and boat!
Some of these responses are hilarious....
Fast V drive Boats sometimes use a TH400 with first locked out or removed altogether.
The "Lenco" 2 speed was also fairly popular.
There really isn't anything available for the "regular guy" with an outdrive boat.
A switch pitch type prop is the closest remotely affordable thing- but this stops working at high HP (500+) I tried one and sent it back in a box with all the springs hanging out it and the rollers bent.
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There were 2 of these variable pitch deals on the market last I looked.
This one - with 2 fixed positions.
http://aerostarmarine.aerostaraircraft.com/ppp.html
And another one called the "Torque shift" from Land and Sea- that was infinitely variable between an upper and lower pitch range.
http://www.torqueshift.aeromarine.com
This one - with 2 fixed positions.
http://aerostarmarine.aerostaraircraft.com/ppp.html
And another one called the "Torque shift" from Land and Sea- that was infinitely variable between an upper and lower pitch range.
http://www.torqueshift.aeromarine.com
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Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
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Originally Posted By: Doog
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
If I count my canoe 40 years a boater here....
I actually love my boats and dont want too sell, but wish I had time to use them. Probably because I dont slip them and have to pay huge sums every month to do nothing.
Pretty sure those transmission are single speed N-forward and reverse. Or do they have 2nd forward gear? No twin diesel I was on had that, but that'd be super uber cool.
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
If I count my canoe 40 years a boater here....
I actually love my boats and dont want too sell, but wish I had time to use them. Probably because I dont slip them and have to pay huge sums every month to do nothing.
Pretty sure those transmission are single speed N-forward and reverse. Or do they have 2nd forward gear? No twin diesel I was on had that, but that'd be super uber cool.
Originally Posted By: totegoat
My guess is little rudders.............like I said, just a guess.
IF you are talking to me Im not sure I understand the question?
As in my boat are little - or - I had powerboats vs sailboats ?
(Im the rare boater than likes both)
My guess is little rudders.............like I said, just a guess.
IF you are talking to me Im not sure I understand the question?
As in my boat are little - or - I had powerboats vs sailboats ?
(Im the rare boater than likes both)
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.
Actually a variable pitch propeller in an airplane sort of acts as a transmission. At low airspeeds the pitch can be set very narrow so it doesn't grab as much air compared to the pitch being open. That allows easier/more efficient climb kind of like lower gears in a car.
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.
Actually a variable pitch propeller in an airplane sort of acts as a transmission. At low airspeeds the pitch can be set very narrow so it doesn't grab as much air compared to the pitch being open. That allows easier/more efficient climb kind of like lower gears in a car.
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Originally Posted By: Doog
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
In the last 45 years our family has had antique wooden boats, sail boats, stern drive fibreglass boats, outboard driven Boston Whalers, you name it.
But the most fun and utility we have had from a boat was a 12' Aluminium with a 9.9 Evenrude.
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
In the last 45 years our family has had antique wooden boats, sail boats, stern drive fibreglass boats, outboard driven Boston Whalers, you name it.
But the most fun and utility we have had from a boat was a 12' Aluminium with a 9.9 Evenrude.
Originally Posted By: expat
Originally Posted By: Doog
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
In the last 45 years our family has had antique wooden boats, sail boats, stern drive fibreglass boats, outboard driven Boston Whalers, you name it.
But the most fun and utility we have had from a boat was a 12' Aluminium with a 9.9 Evenrude.
See how my Observation #2 about boats that go faster than 50 mph applies?
Originally Posted By: Doog
Originally Posted By: A_Harman
I developed two fairly churlish observations during my years of working on high performance boat engines:
1. The two happiest days in the life of a boat owner are the day he buys it, and the day he sells it.
After 30 years of boat ownership I can say that is 100% true.
My Sea Ray boats all had inboard transmissions made by either Hurth, Borg Warner or ZF. The Zf boxes were by far the absolute best.
They were ZF63a
http://www.tadiesels.com/transmissions/ZF_Hurth/ZF-63A.htm
In the last 45 years our family has had antique wooden boats, sail boats, stern drive fibreglass boats, outboard driven Boston Whalers, you name it.
But the most fun and utility we have had from a boat was a 12' Aluminium with a 9.9 Evenrude.
See how my Observation #2 about boats that go faster than 50 mph applies?
A car motor doesn't have the RPMs and power needed to move a car at highway speeds without a tall gear.
And conversely, it can not get a car moving from a dead stop in a tall gear, hence the need for different gears.
And conversely, it can not get a car moving from a dead stop in a tall gear, hence the need for different gears.
I am surprised a woman would think to ask a question like that.
Originally Posted By: Taildragger
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.
Sure, I meant DIRECT DRIVE, I'm affraid
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.
Sure, I meant DIRECT DRIVE, I'm affraid
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