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Gear Reduction ?

A

Anonymous

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If I understand correctly, automotive engines require reduction units to reduce prop rpms at higher motor rpms since automotive engines reach peak output at 5000 rpms +. I'm assuming that an automotive motor turning 5000 rpms with a 2-1 reduction unit would be spinning the prop 2500 rpms. Is this correct?

Secondly, I'm a little confused because if a prop is turning 2500 rpms why does it matter how many rpms the motor is turning. Would the effect not be the same if the motor was turning 1000 or 4000 rpms. I'm guessing that it has something to do with the increase horsepower of the engine at higher rpms but if the prop is turning the same speed what's the difference?

Now for my real question. I've heard that aircraft engines generate their horsepower at lower rpms than automotive engines and because of this reduction units are not a necessity to maintain a slower prop speed. What's to be gained then by running a reduction gear on a small aircraft engine because I know folks are doing it with success.
 
If I am correct, I believe it is the torque that is more important than the HP.

But yes, a 2 to 1 reduction spins the blade half the RPMs as the motor is turning.

Without reduction gears, most automotive engines do not have the torque to spin up a propeller to the most efficient operation RPMs. And if they do, they usually will not have the HP to get it there quite as fast as they do with a reduction gear. There are few motors that could turn 1000 RPMs and have a propeller turning at 2500 or 2900. If they did, it would take them forever.

As for aircraft engines are they running the big bore strokes or the smaller ones like rotex. The big bores like Lycoming and Continetal produce lots of torque and were designed that way. But it would be hard to turn them up over 3400 RPMs without probably sending pieces everywhere. Those rotex are made to turn high RPMs under most instances. They don't ever reach their peak horsepower without a reduction drive attached.
 
On a car motor you need the rpm up to get the engine in the torque curve to make the power I was told to throw horse power out the window
 
I have learned everybody is obsessed about RPM’s. Most people don’t realize Horse power is derived from measuring torque the formula is (horsepower = rpm x torque / 5252) The gear box allows you to do two things. First is you can turn the motor at a higher RPM. Where a car motor makes it’s power because of it’s small cubic inches size and most of all the gear box is a torque multiplier. If you make 400 ft pounds of torque and run a 2.68 gear box your prop will see 1,072 foot pound of torque there is some loss of torque due to gear box inefficiency. I attack airboat car motors from a different angle than most people I don’t go for RPM’s I try to make 550 Ft pound of torque around 4,000 RPM and the let the gear box due the work. I argue this point at least twice a week. Don’t build a car motor to swing a prop more RPM’s build a car motor that doesn’t choke down when you add more pitch. That’s why I like the 2.68 to one. 99% of everybody doesn’t like this high ratio because most people think they must turn more RPM’s with a 2.68 box. Well I run a 2.68 and just keep adding the pitch most of my motors run so much pitch they turn a max of 5,800 RPM’s with a 2.68. I use the gear box as a torque multiplier not an rpm increaser. Most boats with this much pitch cruise the water from 1,800 to 2,100 RPM’s. A 2.02 to one box must spin the motor around 2,800 to 3,000 Rpm to develop the same thrust in order to cruise at the same speed. I have tested a 2.02 and a 2.68 box on the same exact boat to prove this point.
 
Waterthunder,

You are operating at a very low prop rpm. I don't doubt that it works. Props are sort of black magic. One field test is worth 1000 theorys.

I am curious to know if your pitch settings are within those recommended by PowerShift?

Jim
 
Waterthunder - What you are saying makes sense. It is also the same thing a hot rod shop said to a buddy. They used to do a weekend AM talkshow of car problems. When my buddy called they said forget the RPMs and get someone to build a motor to produce torque.

Never heard the idea about using the higher gear reduction drive, but that makes sense. Of course, very few people are looking at motors that turn up what you are talking about. Lower RPMs at cruise is always better in my book.

Thanks for being willing to share your thoughts. I have learned at lot (and sure will not recommend another ZZ4).

pat
 
Hey Marshmaster What did you not like about zz4? I ask because i just bought one with new 650 dp mechanic is installing it next week . I have heard good praise about zz4. Any thing thing i should be aware of? Thanks Dean
 
Waterthunder,
All you stated above make darn good sense to me, and I don't doubt what you say.
I always thought that an automotive engine would need to be reducted, in order to let the engine achieve it's inteded power / tourque, with out overspeeding the propeller.

I don't think I want be standing near a dirct drive prop turning 5 grand.

A couple of weeks ago I saw a nice rig getting towed back to the landing.
It was a direct drive auto-boat, and it had exploded a brand new sensenich wood stick prop. Nobody got hurt, but the owner was sure shook up about his close call, and the damage to his boat. He said he could not figure out what happend, it just let go when he got on the gas.

I was wondering to myself if he had left a tool on top of the engine, or if he had hammered that direct drive prop up to 5k or more, and slung the tips off.
 
Dean - They are fine if you don't get them hot.

Cowboy - If they turned the RPMs up too high, you are right, the wooden props will not hold together. But a tool on the motor or under the gas tank will sure spook you when it hits the prop.
 
The ZZ-4 is a good motor but don’t get them hot G.M. makes their aluminum head castings too thin on their pick up a G.M. aluminum head then pick up an aftermarket aluminum head the after market weighs substantially more. Don’t worry you will not have any problems as long as you keep it cool. A good piece of advice I can give is if you ever do pop a head gasket pull the heads of and sell them to a car guy because when they pop a gasket they always warp .020 to .040 and must be resurfaced. They are still fine for a car or truck but not for an airboat. Once you resurface them on an airboat you will start having problems because they are thinner and blow even quicker. I have seen this at least 8 different times.A buddy of mine had a wood paddle prop on an Apache hull with a belt drive. He changed his rudder linkage and a few days later took me for a ride. When he was grinding it hard on the hill he turned hard and mister prop met mister steering cable. WOW his prop was at max RPM. The idiot didn’t know why his boat quit moving and floored it again. There was kindling everywhere and the harmonics of a shattered prop made the engine stand collapse. I was in the back seat taking any cover I could find. It was a wild and scary ride. The worst part was towing a 16ft barge back in on a dry river had to pull it over a dam too.
 
Should I run zz4 without thermostat or just the restrictor type with the center removed. I dont know what thermostat comes in it. I would guess a 180. It gets pretty warm in the summer in Texas. My motor I am taking out we run without the thermostat and it would run 150 to 170 on hot day. Thanks for the advise .I will try to avoid over heating as I had cracked head last summer and was given 305 heads and they worked great. My mechanic said I got all the good out of it . Also I have 2 to 1 belt with 80 inch powershift .I sure want to get this right. Thanks for your input Dean
 
Dean

I am running the very same set up I have a 14 ft boat 7 1/2 wide and a ZZ4 with a 2 to 1 and a 80 in Power shift prop Waterthunder told me to run a 165 degree thermastat in my motor and it works great it gets cold here in Georgia,but it also gets hot and so far the motor never gest over 175 degrees no matter what the air temp is.I don't know what kind of RPM you are turning but with my current prop setting and the few mod's I have done to the motor my max RPM is 5100 and the boat does every thing I want it to do.
 
If you are located where it never get’s cold then just run a restrictor disk. That is the safest route because all thermostats fail eventually. If you are in an area that get’s cold like snipper is, run a lower temp thermostat so it will open sooner. The worst thing to do is run a thermostat that keeps the engine around 180 to 200 because by the time it opens up you may have already overheated your motor. These boats are not like cars they can go from cool to hot rapidly so you want to keep your base line temp a little lower so your cooling system can handle an immediate demand.
 
Thanks guys i really appreciate the help as this is pretty big expence for me I cant wait to run my boat again Happy Trails to all
 
Here are some facts: aluminum and cast iron expand and contract at different rates due to temperature change. A belief that I have: Cycling an engine temperature wise may be a bad thing. A thermostat helps hold the temperature more constantly .
 
ON my 502 x502 I run a a plate in the summer and temp go to 200. In the winter the same set up temp only 185. Winter are 32degree to minus 5. Summer it's arond 85. Temp seem to go up fast when running 4000 rpm or better . One thing is for sure that 502 burn alot of gas, i think about 15 gal. a hour. Next boat will be a 8x15 with a ls1 motor and a gear drive. Hunting season is over , some use the boats to ice fish with in Utah.
 
I run a old t stat that I cut the center out and use the restrictor to slow the water down and my boat runs 150 all the time in the 90 to 99 deg in the summer and runs the same in the cold wether. no matter how hard I run it
 
I have built and ran several 500HP to 600HP airboat motors that utilized Brodix, Dart Pro Action and Edelbrock aluminum heads. Unfortunately for me just about every one of my customers is a psychopath and thoroughly enjoys running their boats to the ground. The average hull life seams to be three or four years and the motors live much longer than the hulls. I have yet to blow a head gasket on any of my motor combinations so I’m not concerned about average heat cycling now if the heat cycles become drastic I have a big concern. I prefer running aluminum heads because of their ability to dissipate heat more efficiently. Iron heads will always make more power in the same configuration but their more prone to cooling issues and high combustion chamber temperatures. I have personally found if you run your boat hard and I don’t mean for five minutes I mean the guy that’s so far back in the woods you can’t see him but hear him for what seams forever and everybody else wishes they could run with him. You must keep your base temp around 170 everybody knows the conventional Chevy likes to run a water temp of 180 plus but if you set your boat up to run 180 you are limiting your hill running capabilities. I have seen several boats that run 190 to 200 for a year without a problem but the first time they tow someone in or get really stuck on a mud tussle or in black mud. Pop there goes a head gasket or motor. I set my boat up for hard running in Florida where it’s always summer. Just remember each environment has different needs. CCHARDT's set up is perfect for cases where he has to run his boat real hard for a long period of time. He should never have a head gasket or over heating problem running at 150. He wouln't make max power at that temp but he will make more power than anyone with an over heating boat. If his boat ran at 190 he couldn't run half as hard for as half long.
 
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