Gearbox power reduction question

Longest running airboat discussion on the internet.
Post Reply
Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Gearbox power reduction question

Post by Rich Andrews » Thu Sep 05, 2019 12:05 pm

I know lots of tests have been conducted but on average what's the consensus on power loss with a CM engine and a 2.68 box turning 3 blades
Big Ole Deckover in the making...

CarMotorBarge
Site Supporter - I
Site Supporter - I
Posts: 959
Joined: Thu Jul 14, 2011 8:32 pm
Location: Lake Harney Woods

Re: Gearbox power reduction question

Post by CarMotorBarge » Thu Sep 05, 2019 1:26 pm

Power loss or thrust loss? Are you comparing a 3 blade to a 4 blade using a 2.68 or are you comparing different ratios to a 2.68? Trying to understand what you are asking.
14x7.5 Al David hull with 14 inch transom
419 CI Horsepower Barn LS3 with 2.88 Ox Box swinging 4 blade 83.5" R
GTO Rigging and B&S Tilt Trailer

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Re: Gearbox power reduction question

Post by Rich Andrews » Thu Sep 05, 2019 1:36 pm

Thrust loss only...w that specified combo, but would gladly like to understand the loss differences from a 2.68 to a 2.55..2.38
Big Ole Deckover in the making...

SWAMPHUNTER45
Site Supporter - III
Site Supporter - III
Posts: 2904
Joined: Thu Nov 21, 2013 11:38 am
Location: Naturecoast, Florida

Re: Gearbox power reduction question

Post by SWAMPHUNTER45 » Thu Sep 05, 2019 4:54 pm

Rich not sure anyone can provide you the data you are looking for without actually doing test cell runs.

That said you can do simple power calculations. I will put a few below based on 500hp or ft pounds of torque.

2.38 = 1,190
2.55 = 1,275
2.68 = 1,340

As a SWAG you could deduct 10% from those yields to factor parasitic drag.

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Re: Gearbox power reduction question

Post by Rich Andrews » Thu Sep 05, 2019 5:47 pm

Ya this LS stuff is a bit challenging with so many tunes and cam grinds available. Im just trying to figure out how many blades I can spin with x given torque/hp
Big Ole Deckover in the making...

SWAMPHUNTER45
Site Supporter - III
Site Supporter - III
Posts: 2904
Joined: Thu Nov 21, 2013 11:38 am
Location: Naturecoast, Florida

Re: Gearbox power reduction question

Post by SWAMPHUNTER45 » Thu Sep 05, 2019 6:08 pm

If you call Darrin at Sensenich he has a very good understanding of the LS gear ratio conundrum.
He has tested and tuned with Dave on top of working with all the major manufacturers and DIY builders.

CarMotorBarge
Site Supporter - I
Site Supporter - I
Posts: 959
Joined: Thu Jul 14, 2011 8:32 pm
Location: Lake Harney Woods

Re: Gearbox power reduction question

Post by CarMotorBarge » Thu Sep 05, 2019 8:10 pm

Rich Andrews wrote:Ya this LS stuff is a bit challenging with so many tunes and cam grinds available. Im just trying to figure out how many blades I can spin with x given torque/hp
Based on my experience, roughly 550 HSP will spin a 4 blade 83.5" R. This doesn't mean build an engine that makes 550 HSP at 6200 RPMs and wonder why it doesn't spin a 4 blade R at 5400 RPMs with a 2.68. The engine needs to make roughly 550 HSP at the RPM that you are planning to turn at WOT. A stock LS will turn a 3 blade R, but a properly built LS making enough HSP will definitely turn a 4 blade R with good pitch.

So LS motors aren't really any different from SBC, BBC, or a cast iron A/C motor (aka Cadillac :shock: ). What RPM do you plan to turn the engine at WOT? This RPM will help decide gear ratio, cam specs, type of intake, etc. All of this will ultimately decide what prop.

Have you decided what the budget is for the build? The budget will tell you roughly how much HSP you can afford and how many stock parts you need to use. After deciding the budget, the next choice will be which block and stroke? Are you using an aluminum or cast iron block? Hope this helps.
14x7.5 Al David hull with 14 inch transom
419 CI Horsepower Barn LS3 with 2.88 Ox Box swinging 4 blade 83.5" R
GTO Rigging and B&S Tilt Trailer

User avatar
Prototype
Southern Airboat Member
Posts: 787
Joined: Sat Jan 26, 2013 2:28 pm

Re: Gearbox power reduction question

Post by Prototype » Thu Sep 05, 2019 8:42 pm

I don't think any tests have been done! I for one would like to see it per brand also!
It should only cost about every drive and gear ratio your asking and massive custom dyno time!
Most just buy or build a known combo instead of inventing a new wheel? Of course ford did advertise at one point a square tire built to show the ride. Yep, way off base also!

With all the known builds out there, your asking power loss per gear ratio in a 3 blade config? KUDOS to you and squeezing every half penny in making stone soup.

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Re: Gearbox power reduction question

Post by Rich Andrews » Fri Sep 06, 2019 8:55 am

This Alum block motor has good parts.. makes over 400 ft lbs of torque and HP ( at the wheel) lil over 430 Hp(at the wheel)

I know quite a few hrs have been done w thrust testing of props but the correlation of HP and torque has me interested since it appears lots of HP claims are made at the crank!

So hp at the wheel on a dyno would prob mean a close to 20 % HP loss then at the crank. So do all these 500 hp motors advertised by people actually make that power at the wheel or at the crank?

So can i spin 4 blades? Much rather have 4 than 3 and be able to go 80 inches in diameter.

PS..prototype I hate soup but love to learn how to make it!
Big Ole Deckover in the making...

User avatar
OneBFC
Southern Airboat Member
Posts: 633
Joined: Tue Sep 11, 2012 1:04 am

Re: Gearbox power reduction question

Post by OneBFC » Fri Sep 06, 2019 2:21 pm

With the 450 or so hp you think the engine is making (conservatively) the a 4 blade 80" R should work fine as long as its combined with the proper gear ratio to ensure your engine reaches the RPM it actually makes it's best power at.

This is what JR is saying earlier.

Likely your engine makes that power a little north of 6000

An R blade at 2100 RPM will need a 2.88 gear to have the engine at 6050 rpm or so.

A 2.68 would let you spin it 2250 RPM at the same engine RPM. It will require less pitch since the hp remains the same.

If your engine makes its power higher than low 6000 then you should consider 2.88 or settle for less power out of your engine and run it at less RPM.

Dont be scared of RPM. It's not like you will run it at 6000 all day long anyway. At 400hp you are sucking down 35 to 40 gallons per hour of fuel. Noone runs like that.

Good luck Rich.
-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

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Re: Gearbox power reduction question

Post by Rich Andrews » Fri Sep 06, 2019 10:54 pm

I'm planning to show that crank HP and Chassis HP are 2 totally diff animals.

Sure if I was a marketing agency I'd much rather promote crank HP vs Wheel HP because the crank numbers are much sexier!

But when I looked deep into thrust testing those "550hp motors" were actually much less HP and torque as compared to numbers off a chassis dyno.

Therefore without knowing how each and every gearbox and prop parasitically robs HP I figured it would be much better served IMO to build a motor that made good HP on a chassis dyno and assume this motor would exceed without a doubt the numbers off a crank dyno no matter which gearbox ratio or prop chosen.

Again i'm a learner on all of this LS stuff...but dammmit the old SBC FELBER 397 made some bad ass power
Big Ole Deckover in the making...

User avatar
digginfool
Site Supporter - III
Site Supporter - III
Posts: 2735
Joined: Wed Jun 05, 2013 3:00 pm
Location: South Florida

Re: Gearbox power reduction question

Post by digginfool » Fri Sep 06, 2019 11:31 pm

So what you're saying is you believe that a gear box has more parasitic losses than a transmission, at least 2 u-joints, a differential, axle bearings and tire/surface interface? Or are you saying 500 whp is more than 500 crank hp? If the latter, duh. Do you think so? Image

Sent from my SM-G950U using Southern Airboat mobile app powered by Tapatalk
“When plunder becomes a way of life for a group of men living together in society, they create for themselves in the course of time a legal system that authorizes it and a moral code that glorifies it” - Bastiat

SWAMPHUNTER45
Site Supporter - III
Site Supporter - III
Posts: 2904
Joined: Thu Nov 21, 2013 11:38 am
Location: Naturecoast, Florida

Re: Gearbox power reduction question

Post by SWAMPHUNTER45 » Sat Sep 07, 2019 8:32 am

There is one "rule" in engine building and racing and I feel it may translate to the gear drive as well.

"LOOSE IS FAST"

I'm sure you have all heard the phrase that "that engine never ran as good as it did right before it blew up!" When I hear people brag or comment that there tired engine is running stronger than it was I have learned the end is near!

Rich Andrews
Site Supporter - IV
Site Supporter - IV
Posts: 6854
Joined: Wed Jun 15, 2005 9:20 pm

Re: Gearbox power reduction question

Post by Rich Andrews » Sat Sep 07, 2019 10:25 am

digginfool wrote:So what you're saying is you believe that a gear box has more parasitic losses than a transmission, at least 2 u-joints, a differential, axle bearings and tire/surface interface? Or are you saying 500 whp is more than 500 crank hp? If the latter, duh. Do you think so? Image

Sent from my SM-G950U using Southern Airboat mobile app powered by Tapatalk

Glad U got me pegged there diggin... i'm gonna try real hard not to make the same mistakes U made on your build.
Big Ole Deckover in the making...

Post Reply

Return to “Airboat Talk”