Hello just picked up UT style Aluminum hull that needs a repower. This is my first airboat project but not my first mechanic project. The hull was previously powered by a Lycoming O-435 but it was sold before I picked up the boat. I have been investigating putting an LS motor, 6.0 LQ4, LS2 or 6.2 L92/LS3 on it in lieu of another aircraft motor. Recently concerns were raised to me that the automotive route might be too heavy for the hull and potentially swamp the boat so I wanted to know what dimension hulls folks were running with an automotive setup and was this a serious concern. This hull is 15.5' x 7' with a 17" transom on either side of the deck over for the prop with a tapered chine. Thanks in advance for the help.
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Aircraft to Automotive Repower Concerns
- Thread starter Wildhair
- Start date
SWAMPHUNTER45
Well-known member
It’s not uncommon to put a SBC and gear drive on a 14 or 15 ft hull in the south.
I have pulled up to the public ramp and had folks gather around shaking their heads, whispering. “That is gonna roll right o er and sink” or “what were they thinking”
Yep !!!!!!
That said we are SWAMPLIFE and not running the rapids or crossing an inlet.
I have pulled up to the public ramp and had folks gather around shaking their heads, whispering. “That is gonna roll right o er and sink” or “what were they thinking”
Yep !!!!!!
That said we are SWAMPLIFE and not running the rapids or crossing an inlet.
Slidin Gator
Well-known member
I'm presently doing a full rebuild on a 14' x 7' round bottom hull (4' flat at rear) with a conversion from Lycoming O-540 260 Hp to Full aluminum LS.
I'm Building 2 Stossel Boats with High Torque Motors
I am in the process of building a stroked LS3, all aluminum 408 with Holley fuel injection that should make about 550 Hp.
Looks Like I'ma gonna be a Caddy Guy - Now What?
I expect to save weight overall but have a bit more in the engine, gear and radiator. Weight placement is key here.
The biggest issue you will have is your existing width and cage dimensions keeping you from achieving optimum prop diameter and gear ratio. I did some research on trying to force 550 Hp into a 74-76" prop but the results would be wasted fuel. In my case I have the ability to widen my hull to 7' 4" which will give me room to upsize to an 80" prop.
If you are hard stuck at 7' width, don't bother trying to get more power out of the engine, go 450 Hp LS3 with 2.37 gear and see what you can squeeze in for prop. I do have some options for prop I can advise, just not as good as adding 4" width.
Good luck!
I'm Building 2 Stossel Boats with High Torque Motors
I am in the process of building a stroked LS3, all aluminum 408 with Holley fuel injection that should make about 550 Hp.
Looks Like I'ma gonna be a Caddy Guy - Now What?
I expect to save weight overall but have a bit more in the engine, gear and radiator. Weight placement is key here.
The biggest issue you will have is your existing width and cage dimensions keeping you from achieving optimum prop diameter and gear ratio. I did some research on trying to force 550 Hp into a 74-76" prop but the results would be wasted fuel. In my case I have the ability to widen my hull to 7' 4" which will give me room to upsize to an 80" prop.
If you are hard stuck at 7' width, don't bother trying to get more power out of the engine, go 450 Hp LS3 with 2.37 gear and see what you can squeeze in for prop. I do have some options for prop I can advise, just not as good as adding 4" width.
Good luck!
Most of the running I plan on doing out here in UT is on the Great Salt Lake in the skinny water there is some deeper water, but nothing like an intercoastal waterway like on the coast.
Thanks for the feedback. I just finished reading the thread on your LS3 build, I am looking at doing something very similar. Maybe not stroked cause I may only need the 450hp, but same EFI controller, upgrades etc.
I appreciate the detail you put in the build thread, very helpful. I am stuck on the 7' width but I will need to build a new cage anyway if I go the LS route. Main concern was the increased weight going automotive would I swamp the boat.
The back of the hull is enclosed so if a corner dips in it should mostly run off and the bilge get the rest. I was able to talk with a few knowledgeable airboaters out here and they believe i should be ok but keep the build weight and location in mind.
keys2pines
Well-known member
I have a 12' x 6'8" deckover with a 383 direct drive on it, I can stand with two people at the back and not swamp it. My boat weighs 1940lbs with fuel, it's heavy. My suggestion is to put the amount of weight you're going to add on the transom, then see how it floats. I'm thinking it should be just fine. I wouldn't hesistate to add a gearbox to mine.
I think that’s my next step. Need to replace the winch on the trailer first. Also need to replace some bunks so I’ll launch it and have some buddies stand on the back and see how if floats and replace some bunks as well. Just curious what’s the transom height on your boat?
Slidin Gator
Well-known member
The O-435 weighs in 350-375 lb dry, figure 400 lb or so full of oil with cooler. Power is 180-220 Hp. Good move on deleting that engine, it's a museum piece, it was awesome in it's day, but a 4 cylinder IO-360 outperforms at 200 Hp and 100 lbs less.
Assume you are at ~ 550-650 lbs all in on an LS with gear, 550 for aluminum block and 650 for Iron.
If you don't need to cross much dry ground a direct drive 383 or 400 can be a really great solution for a narrow hull like you have, but the O-435 can outrun them if you got the parts to keep it going.
Assume you are at ~ 550-650 lbs all in on an LS with gear, 550 for aluminum block and 650 for Iron.
If you don't need to cross much dry ground a direct drive 383 or 400 can be a really great solution for a narrow hull like you have, but the O-435 can outrun them if you got the parts to keep it going.
keys2pines
Well-known member
I wanted to give that info, but I wasn't around the boat. I keep it at my parents because I have a HOA. I want to say it's between 12" and 16".
That’s helpful info, makes me more confident in the 17”transom I have with the rear deck over. Thanks for following up
Those are the estimated weights I was thinking but glad you are able to confirm with a similar estimate. I never had the O-435 it was sold before I got the boat but I don’t think I’ll be looking for another one. I have been considering some of those GO-480 that are for sale right now but unsure I want to go the aviation route still.
Have you had a chance to weigh your LS3 build yet?
Slidin Gator
Well-known member
No full up weight on the LS yet, but unfortunately a lot of pictures got lost in the last SA refresh, below is the first boat covered in one of the links above. This one is 14' x 7' with a 510 lb O-540 and a 17" transom.
Given a rig that is setup for direct drive aviation. Plus, given a rig that lives at elevation in UT. I would build a high compression 6 cylinder aviation engine, put it on the rig and get on with it.
The boat got around with that O-435, a built O-540 would sure wake her up!
Slidin I appreciate the honest feedback. I’m looking at aircraft engines now to see the options. I’m thinking i should get this boat going and log some hours on the water. I can always upgrade on the next boat.
Any experience with a continental O-470? I might have found a decent one locally that I think is actually a GPU 220. Still looking for an O-540 but we will see based on budget
Slidin Gator
Well-known member
You are on the right track here for sure, don’t put too much money into your first boat and get some stick time.
I’m biased to Lycoming, but yah a 6 cylinder Continental is a great option, 470 or 520. Regarding the unit you are looking at, the GPU has one spark plug per cylinder and a single mag. If it has dual spark plugs it may be a full aviation O-470 or it may be a GPU that has been upgraded with aviation cylinders and redundant ignition/mags.
A full aviation spec 470 is better but there are plenty of GPU’s out running the swamp. The biggest issue with the GPU is the crank shaft. The thrust bearing was not designed for driving a prop and the cranks are getting rare.
If it runs and is cheap enough I say go for it.
Im not sure what a good price is, that seems to be all over the board from the engines I can find for sale. You think 4K for a running GPU is a good deal?
Slidin Gator
Well-known member
I would not call $4K a great deal, but it is reasonable for used and running, the running part being key. Next I would look for Aviation cylinders or original GPU. Loaded aviation cylinders are $300-$600 which is a negative for the GPU if only one plug, but that GPU crank is worth up to $1K alone.
I imagine the airboat engine market in UT is a whole lot more in-breed vs. the Fla. market, so make sure the engine is solid (we can help with that). The high dollar stuff is probably aviation rated (not bad if you have the money), but $10-15K for a fully rebuilt (from junk) 540/520 is typical here in Fla.