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This is probably a question for the engineers in the group. I don’t fully understand how the engine RPM’s are the same, the transmission is the same and the prop is the same. When I am turning 1600 RPM’s I am going what feels like a lot faster(I have to watch my wake when passing the marina) and when I plow through the mud, which is necessary when leaving my slip, I have more power at a lower RPM. Why is that? The same machanical device is creating the force: the prop. So how does the HP/ torque project to the water differently at the same RPM’s. Granted the difference in speed is all perception not hard facts, I have not had the GPS on or even the speedo paddle wheel in the water. I am judging from the bow wave, wake and the way the boat seems to settle down in the water as the speed increases. Maybe I’m just over compensating for the added vibration at idle.
Linus
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Hi Linus,
The difference between horsepower and torque is this: Horsepower is a function of work over time (770 ft/lbs/sec or the power needed to lift 770 lbs 1 foot in one second) whereas torque is a measurement of angular forces (ft-lbs – for instance 1 ft-lb of torque is the force on a bar 1 foot long when one end is fixed to something and the other has the force of 1 lb applied to it). Torque has no speed component. That's why diesel engines are rated at so low a horsepower but can still tow an 80,000lb truck and trailer. Most semi diesels are 400 hp or less, yet they provide up to 700+ ft-lbs of torque at road speeds – remember, they don't spin very fast. Contrast that to a gasoline engine where the horsepower and torque curves are similar – you can get 400 hp and 400 ft-lbs torque but the engine is spinning at 8-10,000 rpm. Most gasoline engines depend on speed and gearing for torque multiplication.
The long and the short of it is that diesel engines typically have low horsepower and gobs of torque and gasoline engines typically have high horsepower and much lower torque.
Bob
On Mon, Sep 7, 2009 at 5:47 PM, sumocean wrote:
This is probably a question for the engineers in the group. I don't fully understand how the engine RPM's are the same, the transmission is the same and the prop is the same. When I am turning 1600 RPM's I am going what feels like a lot faster(I have to watch my wake when passing the marina) and when I plow through the mud, which is necessary when leaving my slip, I have more power at a lower RPM. Why is that? The same machanical device is creating the force: the prop. So how does the HP/ torque project to the water differently at the same RPM's. Granted the difference in speed is all perception not hard facts, I have not had the GPS on or even the speedo paddle wheel in the water. I am judging from the bow wave, wake and the way the boat seems to settle down in the water as the speed increases. Maybe I'm just over compensating for the added vibration at idle.
Linus
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Bob Fine
s/v Pelican
Hull #8Post generated from Pearson424 Forum using Mail2Forum
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Yes. HP is proportional to torque times RPM. Gas engines don’t produce their rated torque over their full RPM range whereas diesels typically do. A gas engine may not have enough low-end torque to get a heavy load moving. Once they get up to speed, 400 hp is 400 hp regardless of engine.
No matter which engine you have, your boat should go the same speed with the shaft turning the same rate.
Regards
Rich
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