aluminium small jet boat concept

[rembrandt]

I have been pondering a while with the idea of building a small boat of aluminium. I have never build a boat before, and have slight welding and fabricating skills. The idea for now is purely conceptual.

My idea was to make a jet boat. It seemed however that jets are quite expensive, so most probably it will become an outboard until the budget has grown big enough to be able to afford a jet. It might even stay an outboard.
(the jet idea would be interesting for me in combination with a salvaged motorcycle engine)

The boat would be used mainly in inland waters, small canals, and occasionally in saltwater (possible mild to medium waves and very shallow waters). It would have to be trailerable.

I found this model in particular quite attractive:
16' Jet Boat - Ultimate River Boat - Aluminum Boat by Silver Streak Boats https://www.silverstreakboats.com/boats/16-jet-boat/

The reasons for this hull shape are:
- seems easiest to fabricate
- easy to get in plane
- lots of room inside
- small, therefore cheap
The delta pad im not so sure about, as it is not decided when and even if there will ever be a jet installed. Might it be better to just make a keel and adjust it later, in case of jet installation?

Here are some attempts to mimic the hull form in DelftShip:


I do have a few questions already:
- Which aluminium alloy would be preferable?
Our local metal supplier can deliver 1050 H24 and 5754 H111. The 5754 might be better suited? Or is there another alloy that absolutely would be better? If so i could consider other metal shops.

- Can hull construction be done with multiple sheets?
The dimensions I had in mind are: +/- 4,5 m length, +/- 2m beam.
This might be best done out of 1 sheet, so that there are no vertically welded seems in the hull and sides. But is there anybody that has done this with multiple sheets?

Any input and thoughts would be greatly appreciated!

kind regards,
Rembrandt

 

[Ad Hoc]

Welcome to the forum Rembrandt,

....Our local metal supplier can deliver 1050 H24 and 5754 H111. The 5754 might be better suited? Or is there another alloy that absolutely would be better?...

I would select 5083 alloy, rather than 5754.
Even though you're attempting to do this in one sheet.. if you hull has any appreciable shape to it, the strain hazarded temper will make this job much more difficult.
Thus using an O temper would be better in that sense. But, if you're ok that there is not much curvature, thus easier to form/bend the plate... then use the H21 temper.

But my first choice would be 5083 - O/H111

 

[TANSL]

This might be best done out of 1 sheet, so that there are no vertically welded seems in the hull and sides. But is there anybody that has done this with multiple sheets?

Sure, it can be done in all the pieces you need, but anything that avoids welding and heat input to the sheets will result in higher quality and reduced assembly hours.
You should always avoid accumulation of welds in a certain area, weld crossings and some other standard in parts definition and the assembly process, but, everything can be done, if done correctly.
If you are as an artist as your name implies, you will surely be able to do what you need.

 

[rembrandt]

But my first choice would be 5083 - O/H111

I think I will check anyway at other metal shops. One is 30 km's over the border in belgium. The shop specializes in non-ferro metals so it might have more alloys.Maybe the prices are better there too.
Are there any reasons you would choose 5083 over 5754?
As i have read, the 5083 has a much higher magnesium content (4 - 4,9%) than 5754 (2,6 - 3,6%) source: 5083 aluminium alloy - Wikipedia https://en.wikipedia.org/wiki/5083_aluminium_alloy, wouldn't that make it more prone to salt water corrosion?

but anything that avoids welding and heat input to the sheets will result in higher quality and reduced assembly hours.

That was what I was thinking. I will try to search for 5 m length plate, even if it will cost a bit more.
Aluminium already is difficult to weld, and i have only a TIG machine. So the least welds, the better.

 

[Mr Efficiency]

Where are you intending to use the boat, river or lake, or on more open sea ? Low deadrise will just kill you with slamming, if you have ambitions to venture into choppy water.

 

[rembrandt]

Mostly on river and lakes, occasionally on coastal waters/estuary ( the 'Westerschelde', if you are interested), if the weather is nice.
No open seas, as i think 4,5 m length might be a little small for that.

Low deadrise will just kill you with slamming, if you have ambitions to venture into choppy water.

I would like to keep the possibility to make it a jet boat in the future, so that's why I am clamping on to the low deadrise, flat bottom, delta pad hull. But I am, however, more and more considering changing the hull into something with a little more deadrise.

 

[Ad Hoc]

...As i have read, the 5083 has a much higher magnesium content (4 - 4,9%) than 5754 (2,6 - 3,6%)... .

Yes it does.
But do you know what the magnesium does to the aluminium?

It increases its strength.

Strain hardened 5474 has the similar yield strength to 5083 in the annealed condition; 5083 having slighter higher values. That's a major plus...so regardless whether you weld it or not.. it is better.
But the O/H111 temper is significantly easier to work too... so plus plus.

.... wouldn't that make it more prone to salt water corrosion?....

No.

 

[Barry]

Most of the aluminum boat builders in the Pacific NW of the US and Canada use 5086 H116 for the hull material, and often 5052H32 for the deck and cabin. Google a specification sheet off Coastal Craft for aluminum alloys used
5083 H116 would also be an option. (or the 0/ H111 that Adhoc suggested)
I would not use Tig for the welding due to amount of distortion that can occur in some situations where the area close to the bead is not constrained for distortion by say the profile or proximity to stiffeners. Ie not all Tig beads will cause distortion in all situations but long welds in some areas will.
If the boat will serve its purpose in the waters that you want to use it in an outboard configuration, I would not consider an inboard jet.
Jets have some disadvantages, ingestion of weeds into the intake requiring the ability to clean the intake on the water, noise within an engine enclosure (without an often intensive sound attenuation process) plus the amount of
inside space that the engine/jet uses up.

If an important goal is to run shallow rivers and the jet configuration is more than just a wish, you could also consider an outboard jet to maximize inside space.

If you are intending to run rivers where impact with rocks at high speeds is a real consideration, I would use 1/4 inch bottom plate. Without knowing the area of the bottom of the boat, but lets say it is 80 square feet,
you would only be adding about 60 pounds to the total package. The sides can be 1/8 inch.

Joining sheets is not a problem but the cost per pound of aluminum is not based on sheet size. Small aluminum suppliers may only stock shorter lengths but that is for convenience. We purchased from a large distributor that had a coil
line and could supply technically any length that you might need.

You will see on the picture of the Silver Streak that you provided above that there appears to be an overhang at the gunnel and bow. Without a better view, I suspect that they took a piece of 1 1/4 square tubing and manually bent this
to form a fair curve for the bow. It also provides a leveling member to run your sides into.

 

[rembrandt]

It increases its strength.

I think i get it. So if i understand correctly: because it will be welded, the heat affected zone will always return to a fully annealed condition. Therefore an alloy with higher yield strength in annealed condition is desirable.

I would not use Tig for the welding due to amount of distortion that can occur.

Have been worrying about that too.
Would it be somehow manageable by dividing it in stitches, welding 20 - 30 cm and then the same length gap, letting it cool in between? I do not have much experience welding long welds between plates like this, but what i do know is that aluminium dissipates heat with the speed of light, or so it seems.

 

[rembrandt]

by the way, thank you all for all the info so far!

 

[Ad Hoc]

I think i get it. So if i understand correctly: because it will be welded, the heat affected zone will always return to a fully annealed condition. Therefore an alloy with higher yield strength in annealed condition is desirable....

Kind of....

Typical yield strength (0.2% proof stress) of 5747 is 80MPa. If you strain harden an alloy, it increases its mechanical strength, H32 temper increases it to 130MPa.
Noted here:

Whereas O temper 5083 starts at 125MPa.

As soon as you weld any strain hardened alloy the strength of the alloy in the HAZ reduces to its base annealed strength, in the case of 5754 is back down to 80MPa.

But when you welded 5083 - O/H111 etc... before welding it is 125MPa and after welding, it is still 125MPa. And because it is O tempoer, it is much easier to roll and form than any strain hardened temper.

 

[rembrandt]

100% clear!

 

[Mr Efficiency]

16 feet would be the very bottom of the size range where plate would be used to make an alloy boat. You are basically depending on the thickness of the material to maintain the shape, without "dishing", whereas swaged sheets of relatively light gauge aluminium do the same job, just much lighter. Plate alloy has become a bit of a cottage industry, because it does not require hugely expensive plant to form metal, I really don't think it worthwhile to fabricate in plate at that size, unless you have some need that requires a specific hull design not available in the ranges of mass production "tin" boat makers, and I guess a hull tailored to jet drives does fit that description, but if you don't specifically need jet, as in conditions like shallow rivers strewn with sand-bars, it is wiser not to opt for that kind of hull, anyway.

 

[rembrandt]

but if you don't specifically need jet, as in conditions like shallow rivers strewn with sand-bars, it is wiser not to opt for that kind of hull, anyway.

I am going to go with this advice.
Slowly i'm realizing that the extra cost and lower efficiency at slow speeds will become a bigger disadvantage than the decrease in draft and higher efficiency at high speeds will be an advantage.

So Outboard it will be.

Since the jet is no longer necessary, I think i will postpone the aluminium boat project, or wait and see if a good offer comes on my path. In any way it wont be in the near future.

Thanks again for everyone's input!!!