Building the C19 in Aluminum-Questions before buying plans.

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WolframM
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Building the C19 in Aluminum-Questions before buying plans.

Post by WolframM »

I plan to build myself a boat over the next 5 years. Why so long? Because I need to buy a house first.

I got the job, got settled, now I gotta buy the house, THEN I can build the boat!

I am not a woodworker. I *AM* a metalworker-I teach machine tool and engineering at my local high schools, and have run my own fabrication shop, including large aluminum and steel weldments, CNC sheet/plate cutting, and CNC milling and turning, for many years.

I like the C19 design-I have read some of the build threads here and while it is a little larger than most boats in my area, it's not too big for the larger lakes-Kentucky Lake and Lake Barkley regularly have 24" seas and winds when the weather isn't perfect. Not ideal fishing conditions but I'd rather have a boat that handles that with grace than try to pilot a 16ft jonboat on that!

I'd like to get some input on converting the C19 to a welded aluminum design with a 3/8" thick keel plate dividing the two lowest hull panels. I understand, from my limited reading and research, that wooden hulls converted to aluminum don't often need more stations (the lateral side-to-side stringers, right?) but do need more longitudinal runners (Not sure what the shipbuilding word for this one is) due to the way aluminum handles the loads in a boat.

Once I get the "Yep that's doable" I'll order the plans and start the adjustments needed. I figure it will take me two to three years to make the plan adjustments, double check all the forces, and save up the money to buy the sheet, because I also have to go back to college for the next two years for my job-new state requirements and all. Oh, and buying a house...but that is already planned for!

I got laughed out of a few other boatbuilding forums already, but this forum seems a bit different, so what do you folks think? Is the C19 design soundly convertible to aluminum?

OneWayTraffic
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by OneWayTraffic »

The panels are fully developed in the plans. They will come together just fine. You will need to adjust the scantlings to work with Aluminium. There have been a few boats built in Aluminium from these plans, though I am not aware of any C19s.

I have a copy of elements of boat strength from marine architect Dave Gerr. In it he lists specs for boats of any regular size as scantling formulas. I also have a copy of the Cx19 plans, a almost identical hull to the C19.

Using them I get the following.

Length 5.85m beam 2.44m height to gunnel midships 1m.

Plate hull bottom 4.4mm (0.173")
Plate topsides (optional hull bottom for a lightly built hull 3mm (0.118"))
Longitudinal spacing 214mm, height 50mm thickness 5mm.

This is pretty close to standard NZ boats in that size. They usually have a 4mm to 5mm bottom, several longitudinals and just a few frames.
The boat as designed has some very sturdy stringers which come much higher than the 50mm and join solidly to the sole above, and according to Gerr and my reading of the CX19 plans using a 5mm bottom plate would match the given longitundal spacing for the CX19 exactly, though I might add a stiffener between the second stringer and the side as the panel is a little larger there. If using a lighter plate you definitely want to add some panel stiffeners.

I am not an engineer, or a nautical architect. I am passing on what I've read in design books, and I'd strongly recommend that you have an engineer look at it or get your own copy of Gerr before proceeding.

jonnymac
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by jonnymac »

I think the doable depends somewhat on horsepower you plan to put on it. If you are thinking at the top of listed range or maybe a little above. This makes converting the plans more risky without professional help. At the lower hp range you are slower speeds and your margins of safety increase. I don’t know you, but with that background you seem to have the ability to manage your own risk.

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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by Fuzz »

It is my believe that most or all of the plans here can be built in metal. If wood will make the bends then metal will also. I am not a welder so I have never built an aluminum boat so take all I say with a grain of salt. What I see most often is folks use heaver gauge aluminum than what might be needed not for strength but for ease of welding. Thinner plate seems to warp when welded and looks like crap. Around here there are a ton of 20 foot setnet skiffs that are around 20x8. They are worked hard and run up on the beaches every day for most of the summer. Many are made with a 1/4 bottom and 3/16 sides and hold up for 20-30 years. I am sure they could be built lighter but they are true work boats and treated as such.

WolframM
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by WolframM »

Going through some of my reference material, I had already figured what I thought should be appropriate thicknesses of aluminum for the hull plating, and I wasn't too far from what has been mentioned. By my estimation, the two lowest hull panels should be 0.187" thick, and the upper panels should be 0.125" thick plating, which is darn close to what OneWayTraffic's reference suggests. This is good, it tells me that my references aren't too far out of line. I'll try to find a copy of that book, it seems to have the information I'll need to make good choices.

As far as horsepower goes, I will likely buy a used outboard in the 60-115HP range, as they are very common in my area when people scrap out a pontoon boat. Folks around here buy pontoons, go out on them a few times, then the boat sits...and sits...and sits. Eventually they can't get the engine running even though it may have only 40-50 hours on it from new, and it'll end up on Craigslist. Ideally I'd find a 75HP 4-stroke, I think. (a non-runner due to gummed up fuel system or mouse-chewed wiring harness is not a problem for me to fix.) The bigger 21 and 24 foot pontoons usually have something on them in this size range, and you'd not believe what I've seen whole boats go for around here after they get a moderate amount of carpet mold going.

For the sole, I had planned on welding the 0.125" thick tread plate down to angle stock that had been previously leveled and welded to the stringers and ribs, but this means I cannot pour in foam before welding...the foam will burn and the pressure from the hot gases will cause considerable problems. Maybe I will have to cut foam blocks instead or pour in the foam, and cut it away from the weld locations?

I'll definitely be looking for a NA that has the time and inclination to work on a small project like this with me, but so far I have had zero luck. Thanks for your thoughts-I'll get a copy of Gerr's book and do some more reading.

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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by TomW1 »

No problem converting to aluminum, sounds like you have all the skills. The current spacing of the bulkheads are not enough so something else is needed between them. As for HP the 75 will be ok and give you a top speed in the low 30's, a 115 in the high 30's - low 40's and a 90 in the mid 30's. So, pick your poison. The speeds will all depend on your load of course. The higher the weight the slower the speed.

Tom
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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by TomTom »

I have read it stated elsewhere that an aluminium hill ends up weighing nearly twice what one of these hulls will weigh. Having built and owned a C19 with a 90 HP I would say that a 75 HP is on the low end of the spectrum - especially if the boat is heavier. I would aim for a 115 hp as I think it is usually the same block as a smaller engine so you gain power not weight. This is certainly the case with the Yamahas

Will be an interesting project to watch.

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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by OneWayTraffic »

Agreed with this. I’d even consider a 150hp. That’s not unheard of for 19’ boats.

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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by TomTom »

WolframM...


This thread brought up another "aluminum boat build question" link - and I was interested to see Jacques say that aluminum boats can weigh similar to epoxy-wood - so I guess I stand corrected on that.

However, I had read it stated elsewhere on another forum that sells epoxy-wood plans. That designer was answering a similar question about his plans in aluminum... I guess so much may depend on whether the design was originally for aluminum or not. The design is a popular pilot house design originally from/ for Alaska - of which there are many commercially built equivalents in aluminum - and yet the sales point of the amateur epoxy-composite is in part the lighter weight.

The more one learns, the more one respects it as a construction choice for one off builds/ amateurs. It is hard to beat for weight. Even the "die-hard" no wood-no rot brigade have to concede that fact.

There is a 28 foot aluminum center console that was amateur built locally (to me) that is absolutely horrible. I would say it is an absolute dog - but I like dogs and they don't deserve to be associated with this abomination of a boat ... both looks and performance wise. It will barely plane with twin 250's. I cannot help but think it would have been a decent boat in epoxy-wood. I can see nothing inherently wrong with the hull design. It must simply weigh far too much.

I am sure they made all sorts of compromises/ adjustments because the better grades of sheet aluminum were not available, welding skills are limited etc. It seems like the sheet that they used was far thicker than any commercial aluminum boats I have seen - of which there are some nice examples. They also really struggled to get fair lines on it. The owner of the aluminum "barge" as it has become known looks on with great envy at a friend's similar sized GS28 (built from plans from here in epoxy-plywood composite) as he gets over taken by a similar sized boat with a pair of 85's on the back!

When I think of commercially built aluminum boats / "tinnies" they all seem to have a lot of ridges and folds in the metal. I am assuming this is to add strength whist allowing the use of thinner plate. I suspect that this is where an amateur build / plan not designed for aluminum would trip up as well. I suspect that some of the better/ lighter aluminum designs that are commercially made must have a fair amount of them "stamped" into shape?

I personally cannot imagine bending aluminum sheet into the same fair lines that plywood takes when "stitching" these plans together. Aluminum does not "spring back" the way plywood does, so the chance of it sagging/ deforming, rather than holding its nice fair lines in tension would not happen.

I feel like you will end up spending any time you save with the CNC/ faster weld times trying to push and contort smooth lines out of a material that will want to bow and sag and then change shape the minute you apply the heat of a weld. Maybe a plan like the Garvey 17 would take the shape better.

I agree 100% with a comment made by Matt Gent that CNC adds very little time savings on an overall build time. I had my C19 panels all measured up, and cut out in 2 days! It took more time to tape it for paint!

With your skill set you will not find these epoxy/ composite a challenge as you probably have far more "engineering background skills" than 90% of us who have built these boats. I would strongly consider building it in the material it was designed for if you love the C19. If I had your metal working skills, put these to work on things like a fantastic T-top, jack plate, fuel tanks, aluminum trailer etc!

I am 100% supportive of your idea - but I am sitting here sipping a beer and thinking about my own experiences, and have realized that the materials are vastly different in the way they will behave in construction and the outcome is unpredictable at best. I also like to remind the owner of the epoxy-composite GS 28 (who also came from a metal work background) that he too wanted to build the GS28 in aluminum once - and I think he thanks me now for talking him out of it!

I am only saying all of this, because its great to see another builder on the forum and I want you to succeed, but I personally think you are borrowing trouble.

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Re: Building the C19 in Aluminum-Questions before buying plans.

Post by OneWayTraffic »

I've seen some very nice builds in Aluminium. Access to some professional equipment and real welding skill was the common factor that I could see. Not just welding machines, but presses etc, and a CNC cut jig to keep it all fair.

For me I'd love to build from Aluminium. No worries about fairing or painting, or UV or anything. Just weld and done. But I don't have the skills or the equipment. But in NZ there's a lot of small aluminium boat building companies all welding boats. Many of them started in some pretty basic shops.

Another factor is that most Aluminium friendly designs in this country have some pretty square lines. Maybe easier to design and weld together.

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