502 power VS torque

Automotive powered airboat discussion.
nolaboat
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502 power VS torque

Postby nolaboat » Tue Feb 20, 2018 11:56 am

Question for the group. I have a 21ft dixie airboat. I m planning on putting a chevy 502 in it. what kind of power do I need to push this size boat?

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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 12:23 pm

Assuming you run a properly sized propller and a hull that doesn't have problems, the formula below will result in a reasonable performing Airboat across various conditions.

(Total loaded boat weight) / 8 = (required minimum hp)

In terms of thrust to total weight then,

(Maximum static thrust)/(Total loaded boat weight) >= 0.5

How much thrust you get from a given unit of hp will vary greatly from setup to setup due to propeller and rigging choices.

Lastly, exactly how much thrust you need per unit of weight will also vary based on setup due to thrust center line, shape of hull, choice of hull bottom surface, etc.

So, the above two formulas are reasonable minimum targets. The higher the ratio of thrust to weight the better.

You have a large boat. So, don't skimp on power if you have the option not to.

Good luck with your build!
-Russ
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The only thing stopping you is FEAR
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Re: 502 power VS torque

Postby ladyblackwater » Tue Feb 20, 2018 12:30 pm

It's all going to depend on how you set it up and what you want to do with the boat. You will definitely need to run a 2.37 ratio or smaller and a good Superwide prop. If you are looking to do a lot of dry ground running and with a load you are looking at needing 700+ hp and close to 800 torque.
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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 12:37 pm

Don't worry about torque, make hp.
-Russ
-----------------------------------
The only thing stopping you is FEAR
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nolaboat
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Re: 502 power VS torque

Postby nolaboat » Tue Feb 20, 2018 1:03 pm

I havent purchased a prop or reduction gears as of yet. So Im open to suggestion on that as well.

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Re: 502 power VS torque

Postby SWAMPHUNTER45 » Tue Feb 20, 2018 5:27 pm

You should focus on a big inch engine that is purpose built by a builder who has a reputation for reliability on an airboat platform.

Don't buy in to a big horsepower number on a dyno but look for a balanced hp/torque curve that gives you big torque and big horsepower together.

To buy an engine that makes only a big horsepower number up top will be a pig on an airboat when your trying to launch or cruise in an efficient range. Your not going to want to run it at 5,500 rpm all day.

I would suggest you consider a bigger displacement long stroke engine if running some dry or heavy loads. No disrespect to the 502 but a 540 or 632 may prove a better choice if within your budget. For me with a 542 Cadillac the 2.37 ratio mated to 4 Sensenich S blades has been a great asset but look around at other boats, talk to the engine builder and match the engine, prop and drive. The marriage of the three is critical for a great result.\

If you already are committed to the 502 then you only need to talk with the prop companies because your limited to what that engines output.

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Re: 502 power VS torque

Postby CarMotorBarge » Tue Feb 20, 2018 8:00 pm

nolaboat wrote:I havent purchased a prop or reduction gears as of yet. So Im open to suggestion on that as well.


Here is what to do:

1. Choose the engine and the maximum RPMs you plan to run at WOT. For the sake of the discussion below, let's assume a 502 BBC running at 5400 RPMs.

2. Because you have a heavy boat, you will want a wide or super wide prop such as an R, S, NGR, JR, JX, etc. If you make enough HSP, a 4 blade will push better down low than a 3 blade. You'll want to spin the prop at 2000 to 2100 RPMs. A pitch between 2 and 2.5 is a good place to be.

3. The gear ratio is simply the engine RPM divided by the prop RPM. In this case, 5400 engine RPMs divided by 2000 prop RPM equals 2.7. A 2.68 ratio would work good for this combo.

4. To maximize the amount of prop you can turn, you need to setup the engine (cam, heads, etc.) to make as much HSP as you can at 5400 RPMs. Don't worry about the torque numbers. They are what they are.

If you want more snap, you will probably have to give up some HSP at 5400 to make more torque down low. Reducing HSP at 5400 means you will turn less prop. Ultimately this trade off between HSP (bigger prop) and torque (snap) has to be made by every engine combo. On a bigger boat I personally would want a bigger prop instead of the snap. Hope this helps.
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Re: 502 power VS torque

Postby ladyblackwater » Tue Feb 20, 2018 8:43 pm

Swamphunter You explained it perfectly Sir. Even though I would go for a block that is already a 540 bore he can always make the 502 bore into a 540 bore. I run a 2.55 on my boat instead of the 2.37 because it saves the life of my motor and let's it work less. If I went to a 2.37 it would be a bulldozer. I chose my combination because of what I do with my boat. I know many people that have a stock 502 with a 2.1 that will walk the dog. Now with him being 21' that set up won't work the same. It can be done with a 502 but you will be better served going to a big bore.
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Re: 502 power VS torque

Postby ladyblackwater » Tue Feb 20, 2018 8:51 pm

OneBCF HP. doesn't move a boat. Torque does, horse power sustains the torque. The best motor you can get is one that pretty much goes square around the Rpm you want to run. This meaning the HP and torque are about the same numbers at let's say 4800rpm (if that's the range you are looking for) If you have a motor that builds HP and hardly any torque or vice versa than you will have a useless motor for an Airboat.
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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 9:38 pm

ladyblackwater wrote:OneBCF HP. doesn't move a boat. Torque does, horse power sustains the torque. The best motor you can get is one that pretty much goes square around the Rpm you want to run. This meaning the HP and torque are about the same numbers at let's say 4800rpm (if that's the range you are looking for) If you have a motor that builds HP and hardly any torque or vice versa than you will have a useless motor for an Airboat.


Where to start....

Well, quite simply, your wrong here and some math will explain. I'm doing this not to put anyone in their place, etc, but rather to help keep a common belief that is compeltely wrong from spreading through misunderstanding how things actually work.

Engine A has 600 lbft of torque between 1500 and 3000 RPM and spins a maximum of 3000 RPM
Engine B has 400 lbft of torque between 2500 and 6000 RPM and spins a maximum of 6000 RPM

Which engine will produce more thrust to move "a boat?"

Given that Hp(power) = (Torque x RPM)/5252

Torque is a Force
To turn a shaft at a given speed requires Power.

(Force x RPM) = Power

To get to the standard unit of "Horsepower" we divide by 5252 as a constant.

If you want to understand how Horsepower is derived see herehttps://en.wikipedia.org/wiki/Horsepower

So, the answer to the above question of "Which engine will produce more thrust to move a boat" is the engine that makes the most POWER, which is engine B. Engine A is roughly 343hp. Engine B is roughly 457hp.

Engine B will move a boat better because it will make more thrust from a propeller.

Torque by itself means nothing. You can generate 10,000 lbft of torque at 10 RPM and it's still less than 20hp.

So, I challenge you, or anyone else for that matter, to explain, exactly, how any engine with more torque but less power will "move a boat" better than an engine with more power and less torque.

I look forward to the response!
-Russ
-----------------------------------
The only thing stopping you is FEAR
400+hp Ecotec, 12x7.6 DBDO, 80" 3B Maximus, 2.3 OX,85+mph, water = purely optional
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Re: 502 power VS torque

Postby SWAMPHUNTER45 » Tue Feb 20, 2018 10:47 pm

Really only 3 blades to consider in my opinion!

21 ft boat is gonna need big push to have any chance on the ground.

Sensenich "S"
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 12:53 am

Swamphunter you are correct and which one of those blades he picks depends on what gear ratio he picks.
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 12:59 am

Sorry double posted
Last edited by ladyblackwater on Wed Feb 21, 2018 2:58 am, edited 1 time in total.
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 2:55 am

OneBFC if you go back and read what I said before you would see that I said the torque and HP need to be pretty even. I never said more torque and less HP. Please explain to me how an electric motor can do more work and carry a heavier load with less HP than an equivalent gas motor? That answer explains itself. You like to compare Apple's to Oranges a lot. Also the one with more torque is going to live much longer than the motor that is working it's butt because it has HP but not enough torque. It's called the big HP motor with little torque will be crusing and running at a much higher rpm and will have to run more rpm to stay with the motor that has good HP but equivalent or higher torque to the HP.
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Re: 502 power VS torque

Postby OneBFC » Wed Feb 21, 2018 8:41 am

Well, a lot of assertions from your side ladyblackwater, but you have only presented opinion and not any facts or physical proof of some kind. Let's step through your last post a bit.

Please explain to me how an electric motor can do more work and carry a heavier load with less HP than an equivalent gas motor?


Well, this one is easy because I don't think you realize that HP is actually work. Therefore, any electric motor is providing the same power as any other type of engine to accomplish the same "work" and "carry a load". If you can point to a specific example of an electric motor requiring less power to perform the exact same task as another engine, I would like to see it.

Also the one with more torque is going to live much longer than the motor that is working it's butt because it has HP but not enough torque. It's called the big HP motor with little torque will be crusing and running at a much higher rpm and will have to run more rpm to stay with the motor that has good HP but equivalent or higher torque to the HP.


Well, again, this is a common fallacy. For any engine a higher torque equates to a higher cylinder pressure. Cylinder pressure is a major contributor to fatigue and component stress as well as overall wear. This is why you don't "lug" engines at low RPM and high load. It's why trucks down shift to go up hills. The cylinder pressure is reduced and the engine turns more RPM to make the required power.

It's better to run an engine outside of the area where it makes peak torque so the cylinder pressure is lower if you want to maximize longevity. If you fully load the engine where it stalls at peak torque the potential for component failure increases.

RPM is not the measure of how much stress or load an engine is under.

Lastly, the amount of power required for most boats is very low compared to the maximum output their engines can produce. "Cruising and running" along at 30 mph will only require 50 to 100 hp in most cases. Any engine, even tiny ones, can produce enough torque at a reasonably low 3000 or so RPM to sustain the amount of required power and have enough in reserve to still accelerate briskly to higher RPM when needed. 100hp is less than 30% of the total available power of most boats. Not really a high load, right?

So, please, give us some examples and perhaps a bit of math to support your opinions. I can't keep typing up detailed examples to counter opinions. If all you can provide is anecdotes and opinions, then we will just have to disagree.
-Russ
-----------------------------------
The only thing stopping you is FEAR
400+hp Ecotec, 12x7.6 DBDO, 80" 3B Maximus, 2.3 OX,85+mph, water = purely optional
Life begins at 2 BAR, Just a good ole boy

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Re: 502 power VS torque

Postby SWAMPHUNTER45 » Wed Feb 21, 2018 9:00 am

We are back to the same circle jerk on whether it is severe duty, percentage of power and HP vs Torque.

The big or heavy boat guys see it one way the go fast or mini's see it another. Let's help this guy it's why I'm on SA.

Glass half empty vs Glass half full

Let's agree this fellow needs a big powerful engine with torque and horse power and a big azz wide prop or are we so far apart we can't do that ?

650 torque with 700 real hp would probably get him his best chance to fulfil his needs.

I'm not sure about a average 502 build but I'm concerned it may come up short. Probably be ok for pushing a in water displacement load but doubt it will move well at all on dry.

On another note can one of you fuzzy math geniuses educate me on the 5250 dynamic?

Why does horsepower "always" eclipse torque at an rpm above 5250 and as as such would torque not then be more relevant to usable power and therefore of more value in an airboat?

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Re: 502 power VS torque

Postby loudmouse » Wed Feb 21, 2018 9:37 am

If you want torque u need BIG cubic In or boost. Boost is the cheapest way on pump gas and can be dialed in where you want it. A 500+ CID with 10psi on pump gas will make 800+ pounds of torque down low in the rpm band. Just depends on when u set the boost to come in.
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Re: 502 power VS torque

Postby digginfool » Wed Feb 21, 2018 10:22 am

SWAMPHUNTER45 wrote:On another note can one of you fuzzy math geniuses educate me on the 5250 dynamic?

Why does horsepower "always" eclipse torque at an rpm above 5250 and as as such would torque not then be more relevant to usable power and therefore of more value in an airboat?


I'm not going to go into the derivation of the equations but there is a direct relationship between torque and horsepower.

Torque = HP x RPM/5252
HP = Torque x 5252/RPM

If you look at these two equations, you will note that there is only one point that Torque = HP and that will always be 5,252 RPM. That's why the two lines always cross at that point. 5252 is the product of all the constants that describe each of the variables at work. Torque is a measure of rotational force measured in units of distance of application and force applied. Once that force goes into motion, it becomes work, which is measured by the amount of work and the distance. HP is a measure of the rate of work applied. All of these have units such as ft-lbs, feet, seconds, etc. Once you put all the constants into the same set of units and calculate accordingly, you end up with 5,252, leaving just the variables (RPM, HP, Torque). Very simplistic explanation but that is basically how it comes about.
Last edited by digginfool on Wed Feb 21, 2018 10:23 am, edited 2 times in total.
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Re: 502 power VS torque

Postby RJC5778 » Wed Feb 21, 2018 10:22 am

With this size boat, I like to think that he would need a big inch motor with a high gear ratio and as long as a blade as possible DIA wise. this seems to be more of a work application rather than speed. he needs torque and more torque, essentially a large area under the curve, more thrust is to be had from more dia than from pitch. I would not recommend a crate motor especially the 502, the valve geometry is not the greatest from the factor where these engines live rpm wise in an AB application. purpose built (if in budget) is the way to go and focus on big torque. high ratio reduction and big, wide blades will make this boat get up and move. my $.02

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Re: 502 power VS torque

Postby digginfool » Wed Feb 21, 2018 10:25 am

Probably one of the easiest ways to reconcile the difference between HP and Torque is torque accelerates, HP holds.
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 12:48 pm

RJC what you are saying is true. I'm not sure if you are saying high ratio meaning like 2.1 or you are meaning like 2.6. Big boats dont like anything over 2.5 and even 2.5 is close to being to much gear for a boat over about 17'. If he wants a boat that will run dry ground great than his best option would be a 540 cubic inch or bigger with a 2.3 ratio.
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 12:57 pm

OneBFC I have one question for you because I'm done going back and forth with ignorance. How many airboats have you own and built that were 18' or bigger? Maybe his best bet is to build a 800hp Ecotech for his boat LMAO. Explain to me how an AC motor works with HP and Torque. Yelp they are torque motors a 300hp O540 puts out around 700+ torque. This should explain it very well.
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Re: 502 power VS torque

Postby OneBFC » Wed Feb 21, 2018 1:08 pm

That explains nothing. You are not willing to learn so I will let it go. No hard feelings.

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-Russ
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The only thing stopping you is FEAR
400+hp Ecotec, 12x7.6 DBDO, 80" 3B Maximus, 2.3 OX,85+mph, water = purely optional
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Re: 502 power VS torque

Postby CarMotorBarge » Wed Feb 21, 2018 1:17 pm

ladyblackwater wrote:OneBFC I have one question for you because I'm done going back and forth with ignorance. How many airboats have you own and built that were 18' or bigger? Maybe his best bet is to build a 800hp Ecotech for his boat LMAO. Explain to me how an AC motor works with HP and Torque. Yelp they are torque motors a 300hp O540 puts out around 700+ torque. This should explain it very well.


An A/C motor makes lots of torque and turns half the prop as a 600 HSP car motor. Should you put an A/C motor on a 21 foot boat?
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Re: 502 power VS torque

Postby ladyblackwater » Wed Feb 21, 2018 2:00 pm

Carmotor some people will say yes and I say no. I've seen AC motors on Big boats that run great but I would never do it. Just like Dave (Waterthunder) says anything over 16' needs a big block. He says a small block will work but will not be the same and as good as a big block. It goes for the same as an Ac motor. They all have their place and it depends on what you are doing with The boat. The only reason we run big props is because of the gear we are using. Put a gear box on an AC motor and it will spin a much bigger prop also. Take a direct drive boat and do nothing but add a gear box and voila you need a bigger prop. Always remember you have to compare Apples to Apples not Apples to Oranges. Bottom line you do need horse power but that isn't the main key. You have to have both to have a proper motor for an everyday ride boat. Build a motor with 600hp and 250 torque you will be burning fuel and killing a motor running very high Rpm's. Torque is what gets the prop spinning not HP. HP holds the prop there.
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