small steel boat.

For a long time now there I've had the itch to build a small boat, even started a glass over ply project that I ended up having to cut up and dispose of for various reasons (one of which is I'm a terrible wood worker).

Since I have a welder and rather enjoy working in steel, I often pondered how realistic a small steel boat would be.

I'm mostly throwing this on here because there isn't a lot of info on small steel boat building when I was looking, but I think its a very easy and quick way to go if you are already proficient in steel fab.

Then I stumbled on this thread:
http://www.boatdesign.net/forums/metal-boat-building/my-one-sheet-steel-boat-project-28485.html

All I needed was some indication I wasn't totally out to lunch and some baseline for material thickness's, and that thread provided both.

Steel also solves the suitable space problem, I can't glass in my basement with out making the house uninhabitable for about a week, but I can weld to my hearts content without so much as boo out of my wife about smells.

So spent some time with hulls, spat the final version out as dxf, exchanged some beers with the cad guy at work for some drawings of the chines split and nested onto 4x8 sheets. Sent them off to another guy with a laser, and a few days later I had a flat pack kit. All in its over 5 sheets, could fit on 4, but wanted the extra for bits here and there if I needed it (decks etc). The cost of the cutting was very reasonable, based on the price I would have paid for 5 uncut sheets, and the discount the laser cutting guy gets for the material, the cutting was almost free.

Due to the accuracy of the cuts, the hull went together amazingly easily, everything matched up bang on.

I'll throw up progress as it happens, the aim is to have her mostly built come spring. Its a sideline project that is lower priorities than other things....

 

Now that's cool

 

Great stuff !!

 

What are you going to do when you have to tell the misses on how your getting it out of the basement ?

 

very good! essenmein

 

haha a fair question. The hull was dimensioned to fit out (hopefully). It has to go up one floor through a 6x8 shaft (that used to be stairs) and out a 32" door. The shaft is high enough to fit the boat bow up and then shimmy the transom out the door. There is a chain falls there to help.

The highest point of the hull (bow) should just get out the door without removing the actual door.

My soulless co workers that know about this have all offered to come watch me try.

 

Seems there is basic boat info missing on the first post.
LOA = 12ft
Beam = 5.5ft
Estimated hull weight ~400lbs
Material 16GA (1.6mm) hot rolled steel

Freeboard of about 12" loaded with 1300lb (incl hull weight), total submersion displacement was almost 3000lbs.

Intention is to be pure displacement hull 5-7kn for leisurly exploring lakes and sheltered coast, power is an old 1965 evinrude fishermen 5.5 I found for next to nothing, needs some paint but ticks like a clock.

Highly curious to see how this works out, first ever boat I designed, or built (or owned for that matter)... Part of me wants to throw it in the water before lakes are frozen.

 

steel skiff

essenmein, the skiff looks a lot like the 'Dynamite' Payson and P.Bolger "Diablo" planing skiff because of the chine strake (or bilge plank) combined with the 'dory shoe bottom'; but at 400 lb for a 12' boat (!!) its not quite in the same class of hp/wt ratio!!

What will you do to coat the steel? That is always the issue with thinner steel and moves most people to aluminum in this range of welded skiff because most often the coating process brings costs exceeding the lighter metal.

Will you be adding pipe or a flat bar shape to the gunwales? How about the interior framing, is that planned?

Thanks for posting, looks interesting and will be very educational to learn what you'll do about coating and finish? I vote MORE PICTURES!

Cheers,
Kevin Morin
Kenai, AK

 

Keep in mind this is more of a proof of concept to get an idea of how hard it is to build something like this before tackling something a bit bigger.

Basically longevity is not a primary concern, so I won't be spending a lot of time/money on paint or finish systems to give me decades of life. The plan is to clean/abrade with flap wheel, prime and paint and touch up as needed.

The 400lb is the upper estimate of where it might end up fully finished. At the moment I've got ~80% utilization of the 4 sheets that went in, (I have 2 half sheets sitting there of the 5), at 2.5lb/ft2, right now shes weighing in around 250lbs.

I picked up 2 lengths of 1x1 square tube for the gunwales, 1.35lb/ft, so there is another ~35lbs give or take. there probably won't be a lot of other material going in, so maybe 350lb is more realistic. Either way I'm not too worried about weight, in someways the heavier it is the smoother the ride will be.

The one thing that you can't see from the photos is her bow, which looks more like an ice breaker than a traditional dory... Basically a plumb bow to the first chine, then rakes back at almost 45deg from there.

The boat I want to build (and this skiff), and you might appreciate this being from AK, I'd like them to have mild ice capability and not to sink at the first sign of a rock.

 

Steel coatings

essenmein,
most of us who've built in steel do the mill scale removal while the sheets are flat, doing it after the shape is tacked or welded vastly increases the effort. In fact in most locations buying wheel-abraded, primed steel is considered worth the small added costs do to the reduction in labor.

Mild steel's mill scale, intact in your photos, actually promotes rusting in any dampness so getting it off is considered by most steel builders as paramount. I'd sand blast a small skiff like this personally, in order to get to a white metal state for even the cheapest primer and topcoat.

I do appreciate metal boats' rock and ice resistance, but all of mine are welded aluminum and lasting lifetimes was one of their main reasons for being built. We have no small steel boats here at all, well I think there were some galvanized military surplus lifeboats around some decades ago, but I've not seen any for a while?

Small boats sinking is more often water over the sheer not holing the hull material. Most often a plywood or plastic skiff of this size would just slide up on ice, rocks or even the beach if hard aground on any of those surfaces. The hull material being steel means to keep from sinking she'll need flotation voids of foam or air, 99% of the risk of sinking is being awash from waves, not from impact holes.

Do you plan voids and reserve interior flotation to keep her at the surface if awash?

Skiffs don't scale to larger boats unless the modeling displacement ratios were observed; so, while a interesting exercise in skiff building she has no proportional relevance to a larger boat's level of effort or even the methods of building.

Looking forward to more pictures of the build as she comes along.

Cheers,
Kevin Morin
Kenai, AK

 

@Kevin

definitely appreciate the input.

I'd like to build in Al in the future, however for this one at least, the main reason I didn't is I don't have a spool gun or gas for my welder, so steel was the "path of least resistance" as it were for tools, building location etc. Didn't have to buy anything other than the cut sheets and start welding.

Then joint preparation in Al is critical, how does that work for the hobby builder that might weld for a night or two then spend a week hanging dry wall before getting back to it? Flux core on the other hand burns through almost anything and gives you a good penetrated weld.

The other thing that puts me off on ally is working in engineering, I know the stuff has serious implications if not designed properly and will fail catastrophically over time if you are not careful. Flexing and vibrations in a steel hull are fine unless you exceed the yield strength, then it bends a bit, however in Al it is not. I still need to work out how to feel comfortable sizing Al frames etc to not spend the entire time wondering when they are going to fatigue on me. Most likely will involve more beer, and Ansys.

Regarding mill scale, firstly it actually seems really thin on this material, just a mild touch with the flap disk and its down to bare metal, not like hot rolled 1/4 flat bar I have, where its really thick, then i have to constantly ask what my co workers said when I contemplated cold rolled sheet for an extra ~$20/ea... "How long do I really plan to keep it".

re flotation, the front and rear bulk heads will be sealed, and I like my buddies idea of inflating used car tire tubes in the cavities instead of styrofoam, just deflate and take out for drying out etc.

There will be some seating etc going in as well as some sort of way of steering/throttling the motor, so I'll incorporate more flotation there, and I think in the floor there will be ~2-3" of flotation as well.

 

Metal Surface Prep for Welding

essenmein,
It seems you'll need to put in a little self ed' time on metals' surface prep so I'll just make a quickie summary of the two different materials not trying to give a full review.

Steel, hot roll needs lots of cleaning to get off mill scale or no weld is 'acceptable' by standards, but cold roll and various other grades of prep/milling may be nearly cleaned. ALL welds** should be power brushed before MIG or TIG, just a RoT (rule of thumb) to get the parent metal as clean as possible before welding, why not get the best weld you can.

** AL takes special considerations here.

Steel is much more forgiving of junk/surface contaminants almost exclusively due to the rate of puddle freeze, the slow heat dissipation compared to Alum. (about 7:1) allows the puddle to 'gas out' so it lifts the gas out of the molten metal for a longer period of time than fast freezing Alum.

Aluminum is shipped with a shiny mill scale that should be but is often not removed. This is embarrassing- an otherwise high quality builder not cleaning the metal before welding(!) and putting out highly porous welds!

Aluminum can be cleaned of mill scale before welding by simply brushing with SS brushes that are not used on steel to avoid leaving ferrous particulate to bubble up in the alum weld. (Alum.) mill scale retains water in that it traps and holds water vapor so it adds to the hydrogen bubbling in MIG welding where TIG usually is slow enough to allow this to 'gas out'.

But.... weld joints that are clean of mill scale can be left for years, even outdoors, then simply brushed with a SS toothbrush (small hand brush) just before the weld. This removes some oxide and scarifies the remaining oxide film (0.001 to 0.004") which forms in a 2-4 seconds when bare aluminum is exposed to air. By grooving the oxide film it will break up in tiny platelets and be blown off by the near plasma heated argon flooding the weld puddle. Oxide melts at much higher temp than Alum metals, so you can actually lay a weld on a 'barrier' of aluminum oxide not welding at all! This is done more often than I'd like to admit... but the break bend tests are the only way to insure you're getting good welds in aluminum.

So aluminum welding, of mill scale cleaned metal, can be done anytime within a few seconds of a hand brush scrubbing the weld area, and it's common practice to keep one in the welder's pocket/belt just to do this exact job. Or if you're old and need help, like me.... I wear one on a length of wire on my welding arm so I can just flip it up into my hand to scrub, drop the brush and pick up the torch and weld where I've cleaned. Most of the guys I work with claim this is an 'idiot brush' like the children's winter mittens of the past....

Alum welding is typically done at 3-10x the speed of steel welds so it does take a bit of practice.

As to aluminum design, that's another subject entirely. But good news is that Dave Gerr has a good book on boat strength that gives a good set of RoT regarding design of aluminum boats. I've used his simple equations to design and build boats, not always following his framing methods, but using the book(s) as a guide from which to base design decisions.

Interesting idea to use inner tubes to fill voids. I personally use air to test the weld integrity of all my voids. Since this weld test method confirms the voids weld seal integrity to a few inches of water column; I then vent the voids with Schrader valves (like those in inner tubes!) mounted directly (tapped) into the voids' side wall in recessed cups, then the voids can be vented to allow air in or out as the temperature varies. I don't use foam but I think its required in regulations?

cheers,
Kevin Morin
Kenai, AK

 

I will precede this by saying I'm not a pro welder, just some dude at home with a little Lincoln that likes making stuff. So I did a bunch of experimenting with it when I got it to figure out settings, what I should/shouldn't do etc. And spent a lot of time reading welding web et all sites.

Re welding through mill scale, not quite what I meant! I agree though you shouldn't do it if you want a clean weld, I did some experiments on some hot rolled angle, and yeah the part of the joint where the scale wasn't removed (T joint for example), lots or porosity and generally a crappy weld. So based on that I grind those clean if i'm doing that kind of joint.

For most applications though Ive welded to cut/prepped (beveled appropriately depending on mat thickness etc) tube/plate so the weld fillet goes in on clean metal. I was more talking about mild surface rust that appears if you leave it for a while and come back to weld it a few weeks later. Again for experiment to see how far you can push it, I took some rusty old painted angle and just welded it, as long as the wire hit something to start the arc, it put in a weld, not the prettiest, but it held (angle bent before weld failed in my crude test).

As far as ally weld, I've never done it, so I imagine there will be a bit of a learning curve there.

Didn't know ally also has mill scale, but alumina is great stuff, we use it a lot in power modules

 

Made a little bit more progress.

Bent up the gunwales and welded them on. I was a bit worried I wouldn't be able to get a clean kink free radius using a manual hydraulic pipe bender, let alone two identical ones. But with a bit of patience and being systematic it turned out near perfect.

Also welded on 1" x 1/8 flat bar each side of the bow to reinforce it a bit, I didn't want to leave that just two welded 16ga sheets.

Way stiffer now!

 

Looking like Progress

essenmein, small cross section shapes like the gunwales would bend on fair without forming, what we usually do is put a few sticks of wood between the gunwales about amidships or 1/3's LOA and then pull the ends tight. Then by tapping the spreaders a bit fore and aft as you tack from one end to the other, you can center the shape or control the topsides to shape alignment pretty easy.

The few sticks will spring that small section material uniformly and then you can tack both sides with a cold bend, helps to keep shapes this small fair.

As to stiffening the bow, by tacking the bar(s) behind the topsides at the stem, transverse, you'd get lots more stiffness without any external doublers. The triangular section resulting from a transverse bar behind the sheet edges is an order of magnitude more strength with less weld and heat.

I hadn't realized before the bow stem/forefoot was knuckled, is this kink for a purpose or just the result of this design's profile look? A curved stem in profile is somewhat more attractive to my eye and every bit as strong; surely?

I've never tried this size boat in steel so it's interesting to compare it to an aluminum build of similar size and relatively similar shape.

Cheers,
Kevin Morin