My new build for the Ford 408

Longest running airboat discussion on the internet.
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

My new build for the Ford 408

Post by hdsadey »

So I'm finishing up the final punch list for my new 12 x 8 Circle H hull build with the 408 Windsor DD from the last boat. Swinging a 74 WW Sig at 84.5 degrees. Got it choked down at 2750 on trailer for hopefully 2900 on the water. Here is the specs so far.

Boat weight half full of fuel 1840lbs.
Engine makes almost 300 hp & 500 tq @ 3000
Thrust measured at 2750 rpm is 550 lbs. via crane scale
Measured dry weight pull 1050 lbs. on ground with winch.

Does ny of this sound out of line? Having some issues with too much flex in the engine mounts that need addressing. But so far those are the numbers I have. Any input would be appreciated thanks.
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
OneBFC
Southern Airboat Member
Posts: 664
Joined: Tue Sep 11, 2012 1:04 am

Re: My new build for the Ford 408

Post by OneBFC »

550lbs of thrust tells me one of the following.

1. Your engine makes maybe 175hp at 2750 rpm
2. You have a restrictive cage/rigging and its really hurting your numbers
3. The prop needs more than 5.5 degrees at 2750 to work correctly.
4. Your crane scale is wrong (hope for this)
5. You tested incorrectly (second best option)

Solutions are:

1. Forced induction or N20
2. Re-rig (I would pull cage off and re test first)
3. Smaller diameter prop or one with less airfoil area (skinnier blades)
4. Test / calibrate scale with known weights in the region you wish to use it
5. Verify with others how you tested accounted for everything and you aren't leaving out some additional thrust.
6. Get a gear box and forget about everything

Good luck with it!
-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
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Engine dynoed at 291 hp at 3000 and 507 tq.
20181129_071351_Film1.jpg
2. Pancake cage with 4" square
20210103_112310.jpg
3. I measured the prop all the way out at the tip so I might have an incorrect reading. I zero the hub and the put the digital level vertical at the very end. Not sure if that's how it's normally done.

4. Crane scale is Chineseum so that could be a problem.

5. I tied it to a palm tree with the trailer in the grass so that could have been an issue. Don't have a decent pull point on a concrete surface. I was able to pick up 50 lbs. when I discovered the trailer jack wheel was sideways and straightened it out.

I have a significant amount of movement in the engine mounts which is affecting the thrust angle, takin steps to remedy this.

I was able to run dry on the last setup. 13' Big O with mushy stringers and a Sig Series prop that was 2" shorter. Do not know what the other boat weighed. Thinking I might need to repitch the prop to allow it spin up to 3100. I might be way off base but I do have quite a bit of seat time with the same powerplant. I did have a conversation with a guy at Precision Turbo about putting 12 lbs. of boost and achieving roughly 400 hp at 3000. Lots of boost is not an option considering I'm close to 10.5 to 1 compression. But for what that would cost I could bought a gearbox just not the blade.
Last edited by hdsadey on Sun May 02, 2021 8:57 am, edited 1 time in total.
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
keys2pines
Southern Airboat Member
Posts: 129
Joined: Thu Jun 28, 2018 10:55 am

Re: My new build for the Ford 408

Post by keys2pines »

I don't think that thrust test is close to accurate. You have to take into account that before the crane scale can read 1lb of force you have to overcome the friction of the trailer wheels and weight of the boat. Without putting it on a real thrust tester, you're comparing apples to oranges.
12x6'8" Frank Barnes deckover, 383 DD, 72" Sensenich JM
User avatar
OneBFC
Southern Airboat Member
Posts: 664
Joined: Tue Sep 11, 2012 1:04 am

Re: My new build for the Ford 408

Post by OneBFC »

Definitely should perform another thrust test. As already stated, you haven't taken into account the static resistance in your last test.

It could be significant.

I know you have a dyno sheet. However, as is often said here, the proof is in the prop. If you aren't making around 1000lb ft of thrust then you are not getting near 300hp.

We often use 4lbs thrust per hp as a rough guide. In practice with rigging impacting the thrust number a lot, 3 to 3.5lbs is more common. As power goes up, thrust per hp goes down due to the limited ability to keep the prop disc area in the correct size. That is, we can't run big enough props.

Find a tree close enough to tie the boat to while its floating in water. Use something that will not stretch or you will lose thrust reading there too.

You have leads to follow at least!
-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
User avatar
kwanjangnihm
Site Supporter - IV
Site Supporter - IV
Posts: 2212
Joined: Sat Mar 30, 2013 8:23 pm
Location: Bartow FL

Re: My new build for the Ford 408

Post by kwanjangnihm »

Good info from OneBFC!
Most 290 HP AV 540’s are capable of 900-1000 lbs of thrust. You could always take your boat to Simpson and put it on his thrust trailer that’s setup specially for airboat thrust testing!
I have a medium hub if you need to test the sigs at 72” just let me know!

http://www.simpsonqd.com/index.html
He'll cut your throat, baby, stick you in the back, drive off in your Cadillac.
He's more trouble than you think, he'll kill your sugar, leave you in the drink.
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Static resistance is in how much it force it takes to overcome the boat at rest? If so then no I didn't think about that. I plan on rerunning the test in a different configuration today. Thanks for the info guys...... Always learning on here.
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 1145
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: My new build for the Ford 408

Post by Slidin Gator »

The trailer jack is a big part of your error. Hang the tongue from an engine hoist with a good length of chain or strap so it can swing a big arc. Overfill the trailer tires and setup on concrete or hard ground. Use the crane scale to pull trailer to see what it takes to move with no thrust.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Roger that!
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
glades cat
Site Supporter - III
Site Supporter - III
Posts: 3071
Joined: Mon Jan 22, 2007 2:36 pm

Re: My new build for the Ford 408

Post by glades cat »

Yea, take all of the friction out of the equation. Float it, rig a wire rope harness to the rear and anchor to an immovable object, put a Bluetooth load cell in between and get a good static reading.
12' x 7' sled built by Scorpion, Cont. O-470-K, 72" Whisper Tip
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

16199080773553276597769527565639.jpg
Ok so tested again! On concrete tied to a Kubota. Repitched to spin 2900 on the trailer. 780 lbs. of thrust on the scale.

Sound like she's in the ballpark now?
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
OneBFC
Southern Airboat Member
Posts: 664
Joined: Tue Sep 11, 2012 1:04 am

Re: My new build for the Ford 408

Post by OneBFC »

Better test setup.

780 is about 200hp give or take.

I would be interested to know fuel flow rate and air fuel ratio. Probably not something you are set up to measure though.
-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
User avatar
Deano
Site Supporter - V
Site Supporter - V
Posts: 4615
Joined: Thu Jan 06, 2005 11:16 am
Location: Inverness, FL

Re: My new build for the Ford 408

Post by Deano »

Your total thrust number is still not reflecting the overhead of the thrust
required to initially move the rig as it sits on the ground (trailer or not).
ie. If you bowed up to a oak tree and it could not move, you could measure NO thrust using this method.

As was suggested, if you measured the force required to pull the rig as it sat with everything the same
(ie. trailer wheel straight, tire pressure the same, surface the same, etc), you would have that overhead number.
For case of example, if it hypothetically took 120 lbs to pull it, adding that overhead to your 780 would
then give you 900 lbs thrust and would be more in the neighborhood of what would likely be expected.

For what it's worth, the 4lb thrust benchmark was generated with a 3 blade 72" setup.
Because of the inherent slip (cavitation, etc.) involved with an air screw, when properly sized, a three blade is simply
capable of more thrust than is a two blade for that reason (given the power to turn it beyond the overhead increase).

Given that you know you're nearing 300 hp, the longer hub and the 74" length out to be the correct choice (w/ 2 blades).
In my experience, turning them past 3k quickly becomes counter-productive unless your hp is building very rapidly
right around that number and/or you like to wear ear plugs or miss your warp drive. :lol:

In the end and on the water, I think your thrust will likely be a little better with the pitch you took out put back in.
Those blades seem to be most effective/productive around 2750-2900 depending on what you want.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Yeah I was so focused on getting the setup on a hard surface that I didn't get a lb. rating for what it takes to move the trailer as well. I will be getting that info here today. I'm wondering if the long hub and the extra 2 inches are actually hurting performance.
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Takes a total of 30 lbs. to move it on the trailer. I assume this gets added to the 780 for a total of 810? So I have gone down the rabbit hole a little deeper. The flat area under the boat measures roughly 64" by 84" which equates to 37.3 square feet. Now if I take the total weight which is 1840 and divide it by surface area I get just under 50 lbs. per square ft. Theoretically a bigger contact patch vs. small yields less pounds per square ft at the same weight. But a bigger hull weighs more by way of extra material, so the question is in the quest to keep the boat as lite as possible in reference to size did I inadvertently increase the psf to the point it's too much for it move.

Below is the link to a video same engine, same 2 barrel injection, same exact blades with medium hub -2 inches from 74 now. Fiberglass hull with polymer. Is the extra 2 inches that much of a difference?

https://youtube.com/shorts/_OhtmnPUovw?feature=share

https://youtube.com/shorts/mZvU4jdHzGo?feature=share
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
OneBFC
Southern Airboat Member
Posts: 664
Joined: Tue Sep 11, 2012 1:04 am

Re: My new build for the Ford 408

Post by OneBFC »

Honestly, 800lbs of thrust for a 1850lb boat is marginal for dry running.

Some types of dry it will be totally fine. Others it just won't be enough.

I have seen a good continental 360 / 210hp rig put out 900+lbs of thrust at Simpsons place on his test rig. That boat also had an all up weight under 1000lbs. Needless to say it went wherever you wanted it to go.

I still believe you are down on power compared to what your dyno sheet says. The prop just won't make thrust unless the power is there.

May sound like a dumb question, are you sure all 8 are firing? I have seen them run on 7 and you almost couldn't tell it was missing 1.
-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
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

BFC I believe your correct about the power. I think all 8 are firing. Thermal gun is pretty much even across the header tubes. The 2 rigs must be just that much different in weight. Conduit rigging & fiberglass (old) vs. stainless steel & aluminum. So next question is stroked 460 BBF????? UGH!!!!
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
User avatar
OneBFC
Southern Airboat Member
Posts: 664
Joined: Tue Sep 11, 2012 1:04 am

Re: My new build for the Ford 408

Post by OneBFC »

If you otherwise don't need to run miles of dry all the time then your situation is pretty ideal for N20 imo.

A 150 to 200hp wet setup with e85 as the fuel for it will probably let you add pitch to choke it down to 2400 to 2500 for better cruise and then when you need it you will have the extra on reserve to make that 1000+ lbs of thrust.

150 to 200 shot won't make 150 to 2000 @ 3000 rpm, but it will help you a whole bunch.

Lot cheaper than all the other options and a 10lb bottle will run for 2:30 seconds at 150hp levels.

Think about how long you actually need full power every time you run. Filling your own n20 bottles is easy too.

Hope you find a solution you like!
-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
User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 1145
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: My new build for the Ford 408

Post by Slidin Gator »

HD,
You still are not accounting for the thrust pushing down on the nose. In your test setup this is reacted by the crappy tongue jack wheel, and we all know how well they roll with more load. Hence the engine hoist swing set suggestion to eliminate that all together.

Measure static and then WOT while Kubota pulls backwards. Average the two.

Onto your other question, friction calculations do not change with contact area, at least for hard materials such as UHMW vs steel etc. Friction drag is normal force (force pushing down) times friction coefficient. Same answer regardless of area.

What this does not account for is microscopic interaction between say UHMW and grass. I would describe it as a weak Velcro interaction. In that case, 2x as much Velcro has 2x the friction force.

So less hull represents less surface area for interaction, meaning the narrow hull at the same weight nets less friction drag.

Finally, your number to pull the hull on dry is a static friction value, which is where the Velcro effect comes in. A couple turns of the rudder to break ground and you are into dynamic friction with no Velcro.

In the meantime, finish her up and go run it. We have a 250 Hp boat that runs ground just fine fully loaded to 2,400 lb or more.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
User avatar
Slidin Gator
Site Supporter - IV
Site Supporter - IV
Posts: 1145
Joined: Sat Feb 03, 2018 11:33 pm
Location: Jupiter Farms, Florida

Re: My new build for the Ford 408

Post by Slidin Gator »

On the power side of the equation, when was the last time you checked/adjusted the valve lash? There’s a reason I stick with hydraulic lifters.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
SWAMPHUNTER45
Site Supporter - III
Site Supporter - III
Posts: 3089
Joined: Thu Nov 21, 2013 11:38 am
Location: Naturecoast, Florida

Re: My new build for the Ford 408

Post by SWAMPHUNTER45 »

Borrow a 72 NGQ or 74 Q prop pitch at the two mark see what you get rpm / thrust wise.
SWAMPHUNTER45
Site Supporter - III
Site Supporter - III
Posts: 3089
Joined: Thu Nov 21, 2013 11:38 am
Location: Naturecoast, Florida

Re: My new build for the Ford 408

Post by SWAMPHUNTER45 »

I have looked at that dyno sheet and am a little puzzled.

The engine is making its peak hp up at 4800rpm.

My experience from the limited dyno runs Branch pulled were his DD engines would tend to go flat around 3600-3800rpm. That data you posted looks like a perfect paring for a 2:12 gear drive.

My earlier comment about borrowing a prop (NGQ / Q) is based on personal experience having used them a lot on car motors and knowing how they match up on our stock and modified V8 platforms. If you put a 74Q on your engine and can not spin up 2800 - 2900 at the 2 mark you do not have 500 plus pounds of torque.

BFC / Gator or anyone else out there can you correlate his reported degree setting to any personal experience in known engine platforms ?

Two comments

I am curious if you had selected a cam with less duration if your usable DD power would be more in the lower rpm range.

Dave always commented with airboat engines to judge your power based on the props setting and performance. There has to be a explanation for this. Based on your testing and data from the dyno that engine looks like a gear drive candidate.
User avatar
kwanjangnihm
Site Supporter - IV
Site Supporter - IV
Posts: 2212
Joined: Sat Mar 30, 2013 8:23 pm
Location: Bartow FL

Re: My new build for the Ford 408

Post by kwanjangnihm »

HD could you test a smaller diameter prop that would get you up in the 35-3600 rpm range, making 360hp & 526tq as a test. (see info below) :scratch:
Davie GSO480 wrote:
Tue Jun 25, 2019 5:25 pm
Hi all.....been a while sense ive been on this board.

I also had Dave build me a DD LS.
The motor is a LS3 block
EFI holley system
430 CI, Chevrolet cnc heads.

Dave (Waterthunder) has a thrust tester, it will measure thrust output.....it makes 1100 pounds + At 3600 with the Alpha 66
Most straight valve 0-540 make 940 =/-, angle valve 1000 +if its 10-to 1
I was at the last Thruman outting......Black full deck,,,,,,,,Midnight Rider..

The prop is a 3 blade Sensenich Alpha,,,,,,it makes thrust up to 3600......at 66 inches
Right now Im running it at 68 Inches and turning it 3200.
It will run with most 0-540 boats......
Ill post up a couple pictures and race videos...
He'll cut your throat, baby, stick you in the back, drive off in your Cadillac.
He's more trouble than you think, he'll kill your sugar, leave you in the drink.
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Swamp I built the new boat with the intention of keeping the DD setup. I didn't realize how much heavier the new setup would be. I just can't justify spending a minimum of an extra 6k on a box and blade for something I use 2x a month maybe. I'm not a financially well off human where I can splurge on such luxuries without remorse not to mention I'm stubborn as a mule and determined to make this work.

Since I can do all my own work including building a new power plant I am contemplating a 460 stroked to 557. I would have to change to mounting and build new headers/minor mods here and there. Bare Windsor block weighs 162 and a bare 460 is 212. We happen to have a 460 at the machine shop that the customer abandoned hence the weight reference. Aluminum heads and intake gain a large advantage. All in may weigh 200 more than now with 150 extra inches. So that's an option. Just sell the Windsor.

I'm gonna put in a 2" carb spacer to see if there is any gain from increased intake velocity. Made one out of maple to try before spending money on a real one, hokey maybe but should tell me if it's viable!
16202184245827642155219339976324.jpg
Right now the plan is to just finish up the wiring and get the damn thing on the water to see how it performs, specially since its been ALMOST 2 YEARS SINCE I'VE BEEN OUT! Maybe Saturday will be the day. TIRED OF THE ANALYSIS PARALYSIS AND NEED TO HAVE SOME FUN!!!!!
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
hdsadey
Southern Airboat Member
Posts: 320
Joined: Sun Jul 01, 2012 7:42 pm
Location: venice fl.

Re: My new build for the Ford 408

Post by hdsadey »

Kwan that certainly is an option. I may have made a mistake with the long hub.
02 Predator 12' 0320 Lycoming
2019 Circle H 408 Windsor DD Sig Series 74"
99 Donzi 16 Classic 350 Vortec
Post Reply

Return to “Airboat Talk”