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My new build for the Ford 408

H

hdsadey

Well-known member
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.
 
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!
 
Engine dynoed at 291 hp at 3000 and 507 tq.20181129_071351_Film1.jpg

2. Pancake cage with 4" square20210103_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.
 
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.
 
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!
 
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
 
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.
 
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.
 
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.
 
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?
 
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.
 
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.
 
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.
 
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
 
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.
 
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!!!!
 
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!
 
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.
 
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.
 
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