Under geared/propped?

[AdamB]

New airboat owner here. 18x8 heavy Red River hull with LSX 454. 2.3-1 century belt drive with 3 blade 80” whirlwind prop.

Not sure what prop is set at, but hits 5200 on trailer. Haven’t opened her up entirely off trailer, but should be around 5800 on the water I’m guessing?

Don’t have a lot of hours on it yet, but likes to cruise at 4000 rpm’s at 33 mph. Much above that and it starts to porpoise, which is an issue to address at a later date I think.

Just wondering if I’d be better with a 2.52 and 3 blade 82 jx sensnich prop? Or possibly something different? Thanks in advance for anyfeedback.

 

[12ftgso480]

Not sure what your goals are exactly? If anything might start with changing your pitch some ? R u wanting a lower cruise rpm, is she taking off ok? Think you need to post your goals and some of the gurus will chime in.

 

[AdamB]

Not sure what your goals are exactly? If anything might start with changing your pitch some ? R u wanting a lower cruise rpm, is she taking off ok? Think you need to post your goals and some of the gurus will chime in.

Thanks for the reply. Yes, I’d like to lower rpms at cruise speed and maximize thrust on dry ground. We’ve run it dry a little but we run way up river where you might not see another boat for miles. Just want to be able to get out of any sticky situation if the need arises.

 

[AdamB]

The more I think about it, I think I’d like to keep the 2.3 and just get a new prop to maximize low end thrust. I’m guessing I have the hp to go pretty long in a 3 blade set up. Also have heard maybe just adding a 4th blade and new hub might be a good option.

 

[Rick McC.]

I’ve been running a three blade stump puller Whirlwind on a 425 HP 454 on an 18’X8’ aluminum hull. It cruises great at 3K RPM, and will stay on plane all day long, deep or shallow, at 2400 RPM.
I wouldn’t change anything on mine. No poly, either.

EDIT: I should have noted that the boat has a 2:1 Century belt drive.

 

[SWAMPHUNTER45]

If your running an LS engine that produces 627hp @ 6,300 rpm and running a ratio of 2.3 to 1 and only getting 5,200 on the trailer your leaving a lot of power out of the mix.

You need gear and rpm to get up in your peak power. You have meat on the bone but ride quality will suffer.

That boat would be best served in my heavy boat opinion by running a purpose built 540 inch big block and a 2.55 ratio drive spinning 4 big Sensenich JX blades.

 

[AdamB]

If your running an LS engine that produces 627hp @ 6,300 rpm and running a ratio of 2.3 to 1 and only getting 5,200 on the trailer your leaving a lot of power out of the mix.

You need gear and rpm to get up in your peak power. You have meat on the bone but ride quality will suffer.

That boat would be best served in my heavy boat opinion by running a purpose built 540 inch big block and a 2.55 ratio drive spinning 4 big Sensenich JX blades.

Thanks. Yeah not taking a $20k motor off the boat at this point. Just looking to do the best I can with what I have at this point. Don’t mind spending some money on a prop, possibly a new belt drive. But the motor is near new

 

[nebraskaairboater]

Just my $.02 but, you actually might want to start with tuning the hull to get rid of the hopping problem. It could make a difference in your cruise rpm. Start by leveling the engine with the stringers. Adjust from there to get the ride right. Then try adding some pitch to the blades to see if you can get your cruise rpm down but once you get the boat to ride right. I know your engine makes it's power at higher rpm so, it might be a fact that your cruise will be up in the 4k rpm area. You can take some pitch out to get into the torque curve of your cam. Sometimes you can take just a small amount of pitch out and it will make a big jump in RPM. This is because you have made it into your torque curve. If your ride isn't correct on the water, the engine will be working harder to do the same job. This will be true on the ground as well. And these things won't cost you any money. Do one thing at a time so you now what made the difference.

 

[SWAMPHUNTER45]

With big boats you either follow the recipe or settle.

Free shit

Change pitch

Work the bottom

Change ratio


BEAST MODE

Big inch power

2.55

4 super wide blades

 

[nebraskaairboater]

He probably just spent over $60k on a new boat with a power plant that makes max torque (586) at 5100 rpm. For him to buy a stroked aftermarket big block, new drive unit, and prop it would probably be another$30K investment. My 496 makes probably 600 / 600 and I have a 2.38 box and 3 blade S prop. I have mine choked down to 4500 rpm max and can run anywhere I want with ease. My cruise rpm is 3K or less no matter how many passengers in my boat. Skinny water and just me I'm cruising at 2100 rpm. And this is with a heavy antique fiberglass hull that is 16' x 8'. My thought and maybe I'm wrong is, that the prop and drive don't care if it has a stroked big block, an LS, or even an electric motor bolted to it. His engine has more power than my stroked big block even if it makes the HP at higher RPM. I just thought he could work with what he has before he tries to "re-invent the wheel" so to speak. It looks like he has the super wide Whirlwind blades. If anything I would check with Century Drives and see if you can upgrade to a 2.5 ratio in the housing you have but I think that would take the CH4 housing. I would get the boat to ride right then go from there. You are going to want to do that at some point anyway.

 

[AdamB]

Thanks for the replies. According to Gm, my motor makes
627 HP @ 6300 RPM
586 ft/lbs TQ @ 5100 RPM
Max Rpms 6500
It was a new repower in 2021. Only has around 30-40 hours on new motor.
I Needs to find out what prop is currently pitched at. Will do that Friday when I run up the my camp on the Red River. It’s a 3 blade WW 13” ex 80” prop. Talking to Shelley at WW, it sounds like I can add a 4th blade and new hub for around $2k. That might just be the ticket.

May possibly be interested in changing the belt housing if that isn’t too big a deal. Someone mentioned price… Feel like we got a good deal at $48k for rig, so don’t mind throwing a little more at it to get her dialed in perfectly.

 

[AdamB]

Nebraska, boat runs great unless it gets about 34-35 mph which honestly I don’t do very much. Would be nice to get up to upper 30s on occasion but it’s not gonna be often. What are thoughts on the trim tab lenco makes that’s specifically made for airboats?

Other aluminum boats I’ve had poroised too. I just figure that was having most the weight in the back with not much weight in the front with plenty of ponies to make it hop. We would just bend the trim tabs down some or add weight up front. But shimming the motor I’m sure is the best way to make a permanent fix.

 

[Slidin Gator]

Thanks for the replies. According to Gm, my motor makes
627 HP @ 6300 RPM
586 ft/lbs TQ @ 5100 RPM
Max Rpms 6500

I got one of them, 416 stroker, made 600-613 Hp at 6,300 and 575 at 5,000. Difference is I'm putting it on an aviation 2 seat hull.

Adam, With a tank like you got, you gotta look at what you gain (30-50 Hp) for adding another 1,000 RPM to your whole day? First gear is certainly fun for a minute, but it get's old quick.

Assume you make 550 Hp around 5,000 RPM and maybe 575 Hp around 5,500 RPM. Tune the prop on trailer accordingly. Yep, that barge will need 4 K RPM to stay on plane, put everyone on the bow.


Looks Like I'ma gonna be a Caddy Guy - Now What? Stroker L92 LS3 Build

 

[AdamB]

Prop is currently pitched at 13* and hitting 5200 rpms on the trailer and about the same on the water. I’m not sure it can handle any more prop, but would adding a WW 2” extended hub give me a little more thrust on the bottom end? If so, that’s a fairly inexpensive, easy swap.