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  #11  
Old 02-28-2018, 08:10 AM
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randydupree randydupree is offline
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I have built a few alum airboat hulls,we used a "T" rail in the bottom for stiffeners,5 total,front to back.
The main thing we had to do was make sure ther was not a "hook" in the bottom,make sure the bottom was as flat as it could be.
I have a big steal table we clamped it all down on.

A totally different boat from this one,but i think the bottom is still very important.
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  #12  
Old 02-28-2018, 11:41 AM
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LKeithR LKeithR is offline
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Originally Posted by bigfun View Post
Not much of a plan, winging it mostly. Pics. and info.off internet and local boats. Learning as i go.
A good way to learn. Looks like you're on the right track and have a pretty good idea of what you need.

Quote:
Hp will be in the 65- 85 at jet range.
Through hull? Or outboard? Only reason I ask is because it makes a difference on how you build the transom. An 85 HP motor is definitely not the biggest motor you could run on an 18 footer but you'll still have lots of get up and go, especially when you're sittin' up on top of the water like that. Getting the jet set up right makes a big difference too.

Quote:
The type of boats Lkiethr is referring to are the NW big river class 1,2,3, 4.... rapids.
Yeah some of the water they run out here is pretty rough with a lot of hard spots. We've got some pretty tough water here in B.C. and some good boats to run it but the real classics--and the real origin of the river running concept--are the boats they use on rivers like the Rogue in Oregon and the Snake in Idaho.

Quote:
...All the local custom sleds have 1" angle welded with the v down I'm toying with the idea of welding them in the flat L position?
I'd keep them in a laydown position. I've seen them welded with one leg pointing down and they tend to get pretty beat up and broken off. If you want a little more "cut" in the water I'd suggest you step up to a 1-1/2" angle--two on each side will make a big difference in the handling and they'll also stiffen the bottom up quite a bit...
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  #13  
Old 02-28-2018, 05:01 PM
bigfun bigfun is offline
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Outboard.
Transom is my biggest worry. Trailering a heavy 4 stroke is hard on a transom. Want it bullet proof without adding unnecessary weight.

Got the rub rails fitted up and ready to weld today, fishing tomorrow.
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  #14  
Old 02-28-2018, 05:29 PM
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Quote:
Originally Posted by bigfun View Post
Outboard.
Transom is my biggest worry. Trailering a heavy 4 stroke is hard on a transom. Want it bullet proof without adding unnecessary weight.

Got the rub rails fitted up and ready to weld today, fishing tomorrow.
I had an aluminum boat with a 90hp prop and I always used a transom brace. It really did stop the bouncing. Pretty easy to make you own too.

We will expect some video when you get it in the water.
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  #15  
Old 03-01-2018, 06:54 PM
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Wolfram Wolfram is offline
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I'm with Milomilo on the brace. Fatigue is a biotch, mo'specially when the motor decides it wants to come inside the boat with you at WFOT. DAMHIKT.
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  #16  
Old 03-01-2018, 10:52 PM
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Originally Posted by bigfun View Post
...Transom is my biggest worry. Trailering a heavy 4 stroke is hard on a transom. Want it bullet proof without adding unnecessary weight...
I'd suggest that running a motor that size puts a lot of stress on the transom too--more so than trailering. Keeping weight down is always imortant but it pays to put it in the right place when you need to. I've seen too many boats that start to stress crack and come apart prematurely because they weren't properly reinforced.

The attached drawing is just something I threw together to show the construction of a typical transom on something we'd build...
Attached Files
File Type: pdf Transom Knee.pdf (2.5 KB, 120 views)
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  #17  
Old 03-02-2018, 11:03 AM
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Wolfram Wolfram is offline
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More stress, but almost always in the same direction.

The sine wave cyclic whipping of that motor out on a cantilever when trailering will put more stress-strain cycles on the transom in 30 miles than 30 years of running it in the water...something to pay attention to, particularly with alum boats...they love to get their fatigue cracks...
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  #18  
Old 03-02-2018, 12:41 PM
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Quote:
Originally Posted by Wolfram View Post
...The sine wave cyclic whipping of that motor out on a cantilever when trailering will put more stress-strain cycles on the transom in 30 miles than 30 years of running it in the water...something to pay attention to, particularly with alum boats...they love to get their fatigue cracks...
Afraid I have to respectfully disagree. I've seen more than one boat that never spent an hour on a trailer that was riddled with stress cracks. A properly constructed hull shouldn't develop issues just because it spends a lot of time on a trailer. Running with a motor imparts a torque stress on the transom but it is the high frequency vibration from the running motor that causes most of the problems.

I will agree that if it's aluminum it's gonna crack sooner or later--a boat built with heavy enough material, good engineering and proper welding technique will almost always provide a longer service life than one that is under built...
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  #19  
Old 03-02-2018, 01:07 PM
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LW Hiway LW Hiway is offline
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I can speak from two different areas of manufacture using AL. One is from the Crawfish boats off shore crew boats and builds, pleasure craft etc and the other from 26 years of Aviation Structures work etc.


Al does not like to be worked, flexed, bent etc etc etc. It can be, but it will over time develop problems. But do to it's weight to strengths ratio's, it beats the shit out of steel. We form intricate bends etc using softer alloys and hardness's but then go to long controlled lengths to heat treat and age said piece before it see's life installed on the plane.


From my side of that fence I have to agree with all mentioned by others of using the transom to foot brace, removable, but used when trailering.


With our boats in the fields we have hydraulically controlled wheels, one for steering and the other for pulling or pushing the boat using a mud wheel in contact with the ground. Some will have different setups different from the other, but there is considerable weight hanging off the nose and the stern that if it were trailered like a typical boat and not placed on a full stem to stern flatbed trailer, things would fall off from cracks quickly.


I can almost guarantee that if a boat build using Al is not built using plans drawn up by a Marine Engineer or suitably experienced Boat builder, there will also be found to develop stress cracks and in some cases corrosion to develop from stress's when in use.


Not to poo poo on anyone's work, it's just the nature of the beast when it comes to making anything out of AL unless it's a simple tank or furniture.


In aviation we develop replacement parts with respect to the end use and of the alloy and hardness makeup of the Al itself.


In our case, usually Boeing will have blueprints approved with specific info's for manf of the part. If Boeing has no allowable support data, we will take readings from/of the part to spec the material type and makeups.


With boats, it's about the same deal, but seldom will someone go to lengths to do so.


Even if due care is exercised with the boats inner structural elements like ribs, gussets and frame work in general, you can/should be looking for cracks after the first time out and after every use there after with respect to a small pleasure craft.


For years we used a dozen or so tightly spaced pieces of half round on the exterior of the hull to be seen as rub strips. Over the years, some folk are now adding sheets of a hard type plastic riveted to the exterior of the hull subject to rubbing actions of the mud or when crossing levee's etc. Air boat hulls are seeing the same type materials due to their possible terrain in use.


I do want to point out your build has moved along smartly and looks sea worthy in a big way. Loads of fun and props when one makes their own boat etc. Well done.
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Last edited by LW Hiway; 03-02-2018 at 01:17 PM.
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  #20  
Old 03-02-2018, 01:12 PM
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I'd like to point out that I have in my little shop of materials, a good thousand pounds of "O" hard Al in sheet and all matter of extrusions. It is with regret that each time I have a need for an Al part to be made, I have to talk myself out of using it as I have no way to control it's heat treatment or aging of it's intended structural ability.


It's easy to bend and work with, but worthless to be used in a needed structural way for durability etc. So most of it see's use as furniture pieces or bling in a bling setting. lol
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