True grit getting started

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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True grit getting started

Postby Notsure » Fri Mar 24, 2017 6:58 pm

Well having just received the plan I have some questions... going to be building out of steel and don't quite understand the frame floors (layout and location) and also wanted to reduce from 10ga to 11ga on the bottom with 12 on the sides. Just a thought. Any advice would help

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Gayle Brantuk
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Location: Bellflower, CA

Re: True grit getting started

Postby Gayle Brantuk » Tue Mar 28, 2017 3:49 pm


We wouldn't recommend reducing the material on the bottom, but the sides will be okay with 12 ga. What is your question about the frame floors?

Kevin Morin
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Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: True grit getting started

Postby Kevin Morin » Wed Mar 29, 2017 2:15 pm

notsure, I'm not replying directly to your questions as I don't know the plans well enough but I'd like to make a note about welding. I'd guess (?) you know how to weld or have access to someone that does- otherwise a welded steel boat could be a real challenge!

If you do know about welding then my remarks may be redundant- but in general - boat welding is somewhat different that automotive welding where very thin materials are often welded with MIG then sanded fair-leaving enough strength in the body panel to hold. I'm just noting that thinning any material in a structural sense skin or frames in a boat design; not only makes it harder to weld - the welds become a little more 'critical' with each few thousandths removed by thinning scantlings.

So if the bottom is designed in 10ga. (0.1345") and you reduce that to 0.1196" (11 ga.) that small amount makes a big difference in the section modulus of the resulting framing and plating strength. I"m not saying your boat won't float- just urging caution because the few thousandths implied in this substitution has impact on hull strength WAY out of proportion to a few thousandths thickness.

Next, that same thinning of materials moves the weld integrity necessity up- and makes weld print through just that much more obvious- implying more sanding (fairing/thinning/paint prep) which further hogs off another thou. or two?

I'd say, and I don't mean to be rude or disrespectful, if you're asking the question(s) you may not be well prepared to fully predict the outcome? So while it's only a little thickness- I'll suggest the best path is to use the plans as they're provided. In any steel boat - regardless of paint system- the life expectancy of steel is very related to its original thickness due to the potential to rust through thin material more quickly than thicker sections.

I'll note it is easier welding and a stronger boat to put an added few % or the all up displacement into INCReasing the scantlings? not decreasing them? I'm mainly comparing the gain of the hull stiffness- not advocating the increase in overall displacement; especially if you're planning enough power to plane this hull?

If you want to reduce the hull's overall displacement (?) and change her roll moment ("stiffen up her roll" as it's said) I'd switch to an aluminum bolt on house/cabin structure (lots of work where the two metals meet - without doubt) this would leave the initial stability (a little) higher than a steel pilot house - if that is your goal in thinning the scantlings of the hull plates?

In addition to lightening the cabin/pilot house you might select a little lighter thinner cabinetry- deck/sole material/ ceiling and other joinery - all of which can contribute to a little less roll and more initial stability for the cabin cruiser living arrangement in small boats.

an example; By choosing 3/8" higher quality ply (if that is called for?) over a lesser quality 1/2" ply- for example- there could be a wt savings overall without compromising the build. YES that will cost more money but the idea stands. By carefully examining the joinery- a not insignificant wt in a cabin cruiser type of design- you'd save wt up high in the design and reducing the all up displacement a few hundred pounds means less fuel to push same shape if you're planing.

One other note or question for you to consider
: how to the plans show the hull longitudinal bars/angle/"longs" at the bow below the chine? I've noticed several threads here and on other forums where the new builder has had some difficulty conforming to the plans' design elements in this area.

I've seen several solutions to this conic developed shape's interior framing and advise you to look at the other threads about the type of work involved, both layout and forming or tack up, AND the final shape of these longs relationship to the hull bottom panels. Maybe the trickiest area of work for welded metal hull builders their first few times?

Just my couple of cents, not looking at the plans, haven't built this design, (and no trolling intended). Wishing your build the best- and looking for to your build thread here in the Metal sub-Forum. Always enjoy watching small welded metal boats go together.

Kevin Morin
Kenai, AK
Kevin Morin

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