DD thrust

Automotive powered airboat discussion.
Norris
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DD thrust

Postby Norris » Thu Feb 16, 2017 8:54 pm

Hi everyone, long time no post! But I have a question that only the southern airboat folks can answer. What kind of thrust would a 500 caddy direct drive engine create?
Seems I've heard of gear box engines creating 12 to 14 hundred pounds of thrust, but never heard what a big cubic inch dd might do.
Thanks Norris

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Rick McC.
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Re: DD thrust

Postby Rick McC. » Thu Feb 16, 2017 10:42 pm

Norris wrote:Hi everyone, long time no post! But I have a question that only the southern airboat folks can answer. What kind of thrust would a 500 caddy direct drive engine create?
Seems I've heard of gear box engines creating 12 to 14 hundred pounds of thrust, but never heard what a big cubic inch dd might do.
Thanks Norris


:shock: :scratch:
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SWAMPHUNTER45
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Re: DD thrust

Postby SWAMPHUNTER45 » Fri Feb 17, 2017 9:58 am

What are you looking to accomplish?

I ran one of the stronger DD Cadillac engines for 2 years on a bigger boat, it was spinning a 76 NGQ pitched on 3 mark.

You can figure a stock take out 500 Cadillac should spin a 74 Q pitched around 1.5 to 1.75 so use that as a guide and call Sensenich and see if they can provide thrust data off my numbers.

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WhirlWind Propellers
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Re: DD thrust

Postby WhirlWind Propellers » Fri Feb 17, 2017 4:52 pm

The real question is what HP is your Caddy making at 3000 RPM. This is what the propeller (if correctly sized) will make Thrust out of for your boat. The magic is in the HP your motor makes at your max RPM, which is 3000 for a direct-drive set-up.

If the prop is sized correctly for your engine's power output and operating RPM, you should get in the range of 3 to 4 lbs of thrust per HP at max power (3000 RPM). So let's assume your DD Caddy makes 250 HP at 3000 RPM, then your thrust will end up in the range of 750 to 1000 lbs of thrust at 3000 RPM.

Sincerely,
Patti
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Re: DD thrust

Postby SWAMPHUNTER45 » Fri Feb 17, 2017 8:46 pm

It's torque in my world <5250

Your objective for the information (intended purpose) would be paramount.

Data is subject to enormous variables but the blessing here is you can tap years of real applications and experience and calculate or form your own analysis.


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jimbo_jwc
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Re: DD thrust

Postby jimbo_jwc » Fri Feb 17, 2017 9:14 pm

WhirlWind Propellers wrote:The real question is what HP is your Caddy making at 3000 RPM. This is what the propeller (if correctly sized) will make Thrust out of for your boat. The magic is in the HP your motor makes at your max RPM, which is 3000 for a direct-drive set-up.

If the prop is sized correctly for your engine's power output and operating RPM, you should get in the range of 3 to 4 lbs of thrust per HP at max power (3000 RPM). So let's assume your DD Caddy makes 250 HP at 3000 RPM, then your thrust will end up in the range of 750 to 1000 lbs of thrust at 3000 RPM.

Sincerely,
Patti
:proud:


MY dd CADDY will just spin a 74X34 wood paddle prop a little over 3000 but don't know how much hp or thrust but its a lot on 16' Rivermaster fiberglass 4 seater w/rod and frt lockers couple on board and full fuel 47mph.

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Deano
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Re: DD thrust

Postby Deano » Sat Feb 18, 2017 8:56 am

Norris wrote:Hi everyone, long time no post! But I have a question that only the southern airboat folks can answer. What kind of thrust would a 500 caddy direct drive engine create?
Seems I've heard of gear box engines creating 12 to 14 hundred pounds of thrust, but never heard what a big cubic inch dd might do. Thanks Norris

Hey Norris, good to see ya . . .

I can't give you a definitive answer as I haven't done any thrust testing with my junk. We probably can arrive at a reasonable quesstimate though, by comparing a few other similar scenarios that are less abstract.

First off, good sized, and healthy automotive engines with the proper gearing and prop choices are capable of, and routinely generate 2000 or so ft/lbs of thrust. Dave reports there is a proverbial wall to be overcome at around 2200 ft/lbs where the power requirement increases dramatically. This is quite obviously beyond the realm of your question, I mention this only to put your geared guesstimate more in perspective. If I'm not mistaken, I believe Russ has reported his geared Ecotec to producing around 1250-1450 ft/lbs depending on all kinds of variables (tuning, gear, props, blade count) that he has been changing over time, while fine tuning his ride. His is a story unto its own, but does fall in the same neighborhood as did your numbers.

Getting back to the question at hand: In the beginning (well almost), when Water Walker did some of the first documented thrust testing, they used an angle valve O-540, said to produce 300 hp, to record 1200 ft/lbs of thrust. That was with a 72" 3 blade Signature and as far as I know, this is where the now proverbial 4 ft/lbs per HP benchmark came from.

It has been reported that an 220 ground power generated 800 ft/lbs thrust. This calculates backwards to 3.6 ft/lbs per HP. The caveat there is that we don't know what prop was used to produce that number. Be aware that where generating thrust is concerned, after the HP produced, the type prop used is the single biggest variable. The prop efficiency can be nearly as relevant as the HP produced when posing this question.

If you were to quesstimate that 'in most cases' or 'typically' a healthy 500 would outrun a 220 ground power and that a healthy AV O-540 would out run the 500, it would be fair to deduce that the 500 were producing somewhere between 220 and 300 HP and 800 and 1200 lbs thrust. Take the means there, and you have 260 hp producing 988 ft/lbs of thrust. I would perceive this to be a reasonably accurate guesstimation based on the number of, and the associated weights of the variables involved.

Needless to say, this only goes to show that what Patti said is correct, which wasn't my intent as there was no need. I started this post yesterday before she replied, but didn't finish it till now. I just left what I already had there as it might help put things in a little more useable perspective where thrust expectations are concerned.
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but it is not the path to knowledge; it has no place in the endeavor of science."
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glades cat
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Re: DD thrust

Postby glades cat » Sat Feb 18, 2017 1:22 pm

Good one Deano.
One thing left to factor in a practical aspect is, how much power/thrust is needed to break the hull free from its terrain from a stopped position. Prop thrust tests are usually static and different hulls have their own friction peculiarities, which will allow one hull to break free easily while another is more difficult to overcome inertia.
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OneBFC
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Re: DD thrust

Postby OneBFC » Sat Feb 18, 2017 2:10 pm

I dont know about other public test results than these:

Real Time Thrust Tests with Complete Boats - Page 4 - Southern Airboat
http://www.southernairboat.com/phpBB3/viewtopic.php?f=85&t=42159&start=75#p409827

So, I have yet to see someone produce a measured on the boat thrust test sheet or video or even a basic forum post that shows 2000 lbs of static thrust was made.

I know Dave says on his bench test rig where he tests and tunes his engines he sees 2000+, but that is only source and its not on a boat.

Would be nice to have another thrust test done with a wider range of boats attending. However, there are so many variables that play into what a complete boat makes for thrust that it makes it really difficult to do a "fair" compairson.

Case in point, in that thread i linked to the Big block boat put down a little over 1400 lbs. Thats what I know mine does too....and I most certainly do not make 500, let alone 600hp.

So, as has been said many times over the years here, "setup is king". I believe that to be totally true.

That, and the water walker props push like mad!
-Russ
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Norris
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Re: DD thrust

Postby Norris » Sun Feb 19, 2017 8:49 am

Thanks for the replies, where it was really headed on this was thrust bearings and how much thrust they can take. This all helps.
I've heard that the big caddys sometimes tear up the thrust bearing, so wondering how much it takes to do that.


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