mini airboat aluminum hull-building questions 101

Since my introduction thread has branched off into a "question and answer" thread...:lol: I figured I'd move this next line of questions to the correct forum...And by all means if anyone has their own questions or more input/corrections on this please feel free to chime in..

I'm building my own aluminum mini hull and as anyone else would do I've read{and re-read} the builds on here and keep loading up on any info I can find...


So far we've briefly discussed the different chines and the advantages/disadvantages with each type..
Attachment 3452 The radiused or rounded chine on a flatbottom mini would slide into the corners and be the worst handling without some type of aid...
Attachment 3451 This combo of a radiused/hard chine{not sure of the correct term for this design} seems to be popular with the manufactured boats and has good cornering at a cost of loss flotation compared to this last type.. Attachment 3453 The "hard chine" with a flat bottom gives superior flotation/planing but slides into the corners more than the "modified" chine in pic 2....But still corners better than the "radiused" chine in pic 1...


Now onto another area of the hull ...The transom.... I've seen transoms installed "straight up" 90* from the bottom and some installed with a rake or angle...Other than displacement{like the sides} is there any advantage to this one way or the other on an airboat??? Does the angled transom keep some water from splashing over the back when coming off plane ??? Maybe lift the back of the boat as the wave hits ???

You are correct about the lift but it isn't needed on a mini if it is designed right. A mini won't come off plane as hard as a big boat or even a jet or prop boat. The wake really doesn't hit the back of a mini because they glide a bit before stopping. There are some mini hull designs that will fall off faster but as said if the hull us designed right it will glide. This is where the length of boat comes into play, a shorter boat will fall off plane faster. Also a longer boat will hold plane at a lower rpm.

Thanks for the info Papee...I see the backwash tray is popular and will add that to the 90* transom... Speaking of backwash trays... I see most add a "dip" in the center...What purpose does that serve ??? Is it simply to lower the engine,prop and center of gravity ??? Last question on the transom..Setting the transom forward of the floor creating a lip seems to be what everyone is doing..Is there a formula for the correct set-back from the trailing edge ??? Heck you could essentially create your own trim tab back there if you set it back far enough..:lol:

Moving on to the bow area...Being a novice at boatbuilding one of the first things I've noticed during my scale{Im using 1inch=1ft} mock-ups is I need to actually design the total side including front rake first then measure the rake{along the perimeter} to get the total floor length...For example one 14ft boat ended up with an approximate 14 ft 6 inch bottom length with the designed rake used...{This is bringing the rake to a point where it meets the deck no flat nose area}
Finding the perfect rake between too steep,plowing the water off plane and too sleek, taking away valuable stability is a mystery for me...Sure would be nice to have a program to run simulating what each hull does in the water...:-P I guess that's where the real world trial and error comes into play for most...

I'll do this in two posts with my memory I forget all the questions and I can't look back on my phone. The "dip" in the transom is to keep the prop and engine as low as possible. I did it on the new boat just in case he decided to go with a redrive and longer prop.


The lip at the transom is what keeps the water flowing past the back of the transom and not causing a swirling effect which slows the boat, I'm having brain freeze and can't think of the correct term right now. I keep mine at an inch. I didn't want something too long back there that would get bent on the rocks and hurt the handling of the boat.

My hull design is full width up to where the rake starts. From there on it gets more narrow as you get to the front of the boat. I am very happy with the rake on my new design, I really wanted my boat cut like did the new boat but it ended up a bit sharper. With mine it will splash pretty good going through rapids. The new boat doesn't splash near as much because of the slower arch.

A fishing boat will have a little more of an arch because of fishing off the bow platform. If you have a longer arch the bottom of the boat may not be In the water and this causes the back of the boat to lift when only one person is in the boat. This figure really depends on the size of the boat. On a longer boat you can make a lessor radius and it won't affect the displacement. On a shorter boat if you make too long of a radius you are taking away from the surface area for flotation. The hull will sit lower in the water.

Excellent...About the only subject pertaining to the actual hull at this point would be the gunwales/gunnels {the tops of the sides in laymans terms}...I've seen some use round pipe and cut a notch in it....I saw you {papee} used 1 1/2 angle for yours and then there's the actual formed airboat pieces from eastern which I like....The formed gunnel weighs in at .85 lbs per foot and the 1 1/2 X 1 1/2 X 1/4 inch{thick} structural angle comes in at .83 so that's very close to each other...Only question for the DIY builder is hopefully the formed pieces bend without too much of a fight...

Speaking of weight I can't help but wonder how much weight could be saved without sacrificing structural integrity by adding holes{or ovals} to the 1X2 structural channel used for the bottom{Im not walking on T bars}...Besides the weight savings it would make it easier to get to any dents that might occur...But at .57lb per foot I'm not sure what could be gained....
The downside of that would be A} from a fishermans point of view there'd be holes for all the tackle to fall into....:-? And B} from the maintenence end it would make for some tough cleaning ... I might experiment with some just to see when the time comes...

Using less channel is the way to go and use the rigging as part of your structural rigidity. Putting holes in would weaken them. I only used three channel in my uncles boat and you feel no flex at all even when pulling on land.

Absolutely no flex is amazing when you look at the size compared to the amount of structural channels on that boat...Looks like you've got it pretty much figured out and it seems so easy to over-engineer the hull and end up with way too much unneeded weight...Great job by the way..;thumb;..Which leads to the next set of questions with the actual aluminum sheets and the different grades available...

All things being equal in the build...Do you see a big difference in structural integrity between the 5052 and the 6061 ??? Obviously the 6061 would take a hit better but is it a big difference ??? Is the 6061 more resistant to the elements over time ??

Speaking of the elements and weathering in general most the older airboats I see in the water all season are completely painted...But it seems the newer boats are bare with a few decals on them what changed ??? The grade/temper ??? Are they using a sacrificial annode ??? I'm asking because I too generally leave mine in all season so I can go out after work during the week{seeing how it's right in my backyard literally}...

And also I'm thinking about having a vinyl decal/wrap made up for the sides and rudders instead of painting the whole thing Which would leave some areas bare...The key word is thinking....I do have a paintbooth available for use but the scheme I'm thinking about is'nt just a one color shoot it's more like 3 colors...Speaking of decals I saw Mark's {saturn surplus} smaller airboat the other day and it had a mini airboat association decal on the rudder...Very cool !! Are you going to make another batch of them for sale ??? :D

I used 5052. I think the guys do the polished thing to start with and realize how much work it is to keep the up so they end up painting them. I'll have to check on the decals, I know I have some stickers left but not sure on the decals.