• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

general questions

jpatter123

jpatter123

Well-known member
These questions are just for my general knowledge.

1. Does a gear reduction unit really multiply horsepower by a direct equation. Ie a 3 to 1 gear reduction doesn't make a 600 hp motor an 1800 hp motor does it? I have had several people say that to me about the units but for some reason that seems way over simplified to me.

2. Is a 1500 hp motor turning a prop at 1500 RPM any better then a 500 HP motor turning the same prop with the same pitch at the same RPM? Does the more horespower motor spinning the same prop better cause it can get it there faster? I understand the principle of a bigger motor able to turn a bigger prop.

3. If your motor will turn your prop at the max RPM for the prop what advantage would it be to put nitrous on it? Wouldn't you have to adjust the prop and pitch to where it max's out only when you hit the nitrous? Or is this another case of just getting the prop spinning at RPM quicker then just on motor.

4. Is there a point where you can the prop to RPM to quickly and loose traction ie thrust. example if I can spin my prop to 2000 RPM prop speed in 4 seconds but a super charged nitrous alcohol motor can do it in 1 will there always be enough air and traction to the prop?

think thats enough ?'s for now.
 
jpatter123":cgtsw9m1 said:
These questions are just for my general knowledge.

1. Does a gear reduction unit really multiply horsepower by a direct equation. Ie a 3 to 1 gear reduction doesn't make a 600 hp motor an 1800 hp motor does it? I have had several people say that to me about the units but for some reason that seems way over simplified to me.

Gear reduction multiplies torque, not power. If your engine makes 600hp and 600 lbf-ft at 5,252 rpm, with a 3:1 box it will make 600 hp and 1,800 lbf-ft at an output shaft speed of 1,750 rpm (not counting for losses in the box itself).

2. Is a 1500 hp motor turning a prop at 1500 RPM any better then a 500 HP motor turning the same prop with the same pitch at the same RPM? Does the more horespower motor spinning the same prop better cause it can get it there faster? I understand the principle of a bigger motor able to turn a bigger prop.

If it takes 500 hp to turn the prop 1,500 rpm, the 1,500 hp engine will only be producing 500 hp at the same rpm. It will be at part throttle.

3. If your motor will turn your prop at the max RPM for the prop what advantage would it be to put nitrous on it? Wouldn't you have to adjust the prop and pitch to where it max's out only when you hit the nitrous? Or is this another case of just getting the prop spinning at RPM quicker then just on motor.

You're pretty much spot on there.

4. Is there a point where you can the prop to RPM to quickly and loose traction ie thrust. example if I can spin my prop to 2000 RPM prop speed in 4 seconds but a super charged nitrous alcohol motor can do it in 1 will there always be enough air and traction to the prop?

I guess (theoretically) it could happen. I've never heard of it on an airboat or airplane.

think thats enough ?'s for now.
Click to expand...
 
thanks airduds I was talking to someone about how I could see it multilpyling torque yesterday but not HP. Think the guy got pissed cause everyone knows beyond simple maitenance I don't know my way around a motor. But everyone I have run into with a gearbox swears they are multiplying their HP.
 
It will let your engine make more power.

You're limited to how fast you can spin your prop by it's tip speed. Anything over roughly 600 mph and the tips begin creating a transonic shock-wave, vastly increasing drag (and NOISE!!) and decreasing efficiency. On a 78" prop this begins to happen (again, roughly) in the neighborhood of 2,800-2,900 rpm.

At that rpm, a stout big-block may only be making 200 or so hp. Putting a gearbox on allows the engine to spin up to where it makes its maximum power while keeping the prop speed down in the range where it's most efficient.
 
I think of prop pitch in terms of gears in a car. Less pitch equals first gear where a car engine can operate at high rpms where all the power is. The drawback is your up on a plane at about 3500 rpms and bad fuel economy. A little more pitch equals second or third gear. The plus side of this is on a plane at 2700 rpms and better fuel economy. The draw back is we cant get our engines over about 4200 rpms and can't run the hill at all. Whoever comes up with an affordable, dependable variable pitch airboat prop will give us the best of both worlds. Until then we'll keep driving around in first gear.

Because you shouldn't spin a prop above 3000 rpms because of the physics mentioned above, limits car engines with direct drive. A direct drive airboat is pretty much going to have to drive in third or forth gear.

But I think that spinning a given prop of the same size, pitch, and number of blades at the same rpms, will push a boat the same no matter if it's a 496 cubic inch Chevy or a Briggs and Stratton providing the energy, if you forget about weight.

For the life of me I don't understand how you can multiply torque with a drive unit. Torque is another way of saying power or energy and it takes the energy in gasoline to produce wind energy that moves our boats.

I would agree that drive units enable our engines to operate where they are most powerful, at high rpms, to spin a bigger and more pitched prop, which makes more wind than would otherwise be possible.

As someone else says around here, "Just My Two Cents."
 
For the life of me I don't understand how you can multiply torque with a drive unit. Torque is another way of saying power or energy and it takes the energy in gasoline to produce wind energy that moves our boats.
Click to expand...

Actually, from what you wrote in the rest of your post I think you do understand it, you just have a little problem with terminology.

Torque is just a force. It is not power. If I sit on the end of 10 ft cheater bar I can create over 2,000 lbf-ft of torque. As long as nothing moves, there has been no power made or used. Now, if I can maintain a force of 2,000 lbf-ft and turn the load at one rpm, I'm making about four-tenths of a horsepower.

The components of power are force (or torque if you're talking about rotation) and speed (rpm). A gearbox lets you divvy between the two. You can have a whole lot of speed without much force - or a whole lot of force at a slow speed.
 
in the Orlando science museum they have a neat set up to let you understand gearing and the amount of force/torque. And thats why I had a problem with it when relating it to horsepower. They have an entire room of gears. You start with a little handle that a 2 year old can turn it goes around the room and ends up turning a pole with a handle bar around it. Get 5 guys and try to stop the pole while a 2 year old turns the handle. Good luck shes gonna spin you around like a top albeit shes turning more RPM's then the pole is. I know I could make that same set up and use a small 1/10 th horespower eletric motor with all those gears and generated thousands upon thousands of pounds of torque at the final gear.
 
Airduds

I can not tell you how many time I have tried to explain to guys that big HP is not as important as big torque when it comes to turning these props. When I had my motor built I told Potter it was for an airboat and I wanted torque and I did not care about HP at all. Every engine builder I talked to always beat on his chest about how much HP they could make but when you ask them how much torque they could build into a motor down low they would always got a little fuzzy. big torque with a big reduction will spin a big prop with a bunch of pitch. I am living proof of this. Now this does not mean I have the best combo by any means. Diffrent rigs like diffrent set ups, my boat would hate a warp drive and a 2.1 where a smaller boat would love it.
 
Good topic -- You in Fl. have a big advantage over us because you have engine builders that understand airboats... But for the engine we have now -- before we would let our mechanic start he had to learn how to drive the airboat we had then so he would understand what we needed... and that a dirt dragster engine would not do the job for us.
 
Hey Bill how is the motor running I haven't heard from you in a while. This is always good. Back to gear reduction As with everything there becomes a point of diminishing returns! Folk's this will shock ya an I hope nobody starts the A/C vs car motor lame crap and sit's down and thinks about the physics. When a very strong car motor races a strong O540 the higher horsepower car motor usually barely out run's the strong O540. However if you throw 4 people in the boat and run the hill the O540 ain't got a snowballs chance! Each motor does have an area it will out perform the other. There are many many forces involved here! Just to name a few the drag coefficient of the hull to water, water displacement, the big car motor problem is torque roll, then gear reduction inefficiency, and another big one I believe is prop tip acceleration which gear reduction happens to kill! All the answers are here and it takes along time or it least it took me 11 or so years to think I have it figured out and I doubt I have . What all the chest pounding logo bashers aka Ford vs Chevy Lycoming vs Chevy don't realize is there is no magical combination . No special motor everything run's the way it should and simple physics will allow it to. There are many reasons why a very strong A/C motor can run with a very strong car motor that makes twice the horsepower. Neither one run's any better then it should but a lot of physics become involved when using torque multiplication! I have always enjoyed how two completely different types of motors the car motor that makes much less torque that has much less cubic inches but makes twice the horsepower run's neck and neck with an A/C that is huge in cubic inches with less horsepower and all torque. These motors are fast base on two different principles but they run close enough to always be competitive with each other. In the end I have been around and worked on about evrything that can be raced and I have yet to find any motor sport that is as complex as airboat racing everything is involved and there all on curves nothing is linear. It a very tough pyhsics coarse!
 
You in Fl. have a big advantage over us because you have engine builders that understand airboats... But for the engine we have now -- before we would let our mechanic start he had to learn how to drive the airboat we had then so he would understand what we needed... and that a dirt dragster engine would not do the job for us
Click to expand...

You got that right Ron,
You can't find anyone up here to work on these things.
If you try to tell them "they aren't like race cars" they just don't understand.
 
flying fish":2zd66au9 said:
crowhater -
you said your boat would hate a 2 to 1 and a warp drive. How do you know??
Click to expand...

We have a friend with an 18x8 Panther 502/502 and his motor is worked over with Bordix (sp) heads, cam , etc. He has a 2.1 K-way with a 10 or 12 blade warp drive. This boat is a trailer queen for sure, I have never met anyone who is as this anal. On dry ground he has to wiggle his rudders to break loose with a load. Once it is moving it will run dry just fine. We are talking about yard grass not mud. This is the fastest airboat I have ever been on. This is Texas and we do not have 50,000 airboat so that is not saying alot. On the top side his boat is so fast it made me nervious, we hit 90mph when I told him to let me off. If we tied are boats together I would drag him like a little kid all over the bay.
 
You must log in or register to reply here.
Back
Top