What needs to be changed?

Hi everyone.

New guy here. I just finished some sketches of a fishing boat.

Here are the rough specs

17' long

90" beam

10 degree deadrise

reverse chine

This boat would be used primarily for fishing (maybe a little bit of duck hunting), both inland and Great Lakes (notoriously hard on boats), so it needs to handle bigger water.

It would be made of steel, I have experience with both steel and aluminum boats and steel suits my needs better.

I have downloaded freeship, but I need to learn the program before I get too far into and I thought I should get some advice on any major design flaws I may have incorporated into it (stability or planing issues?).

Without further adieu....

 

B-N, welcome aboard

Your sketches indicate a hull that is amenable to planing and amenable to steel or aluminum construction. It needs some cleaning up and fairing, as I'm sure you know, but it's a reasonable starting point that is fairly close to other, successful craft of this size.

Before going any farther, you'll have to do a rough weights and moments calculation. In other words: put together a spreadsheet listing every single piece of the boat, the weight and position of each piece, and thus calculate the total weight and the overall centre of gravity.

This will make it possible to get some idea how the boat will balance, and whether it will be light enough to plane well. As a starting point, I'd suggest trying to get the centre of gravity ahead of the transom by about 40% of the waterline length, and the weight such that there is no more than 200 kg of (boat + motor + crew) for every square metre of below-waterline area. Anything much heavier than that tends to have a hard time with the transition to planing speed.

I know you said you like steel, but light weight counts for a lot in planing hulls- and steel invariably ends up thicker than ideal in small boats in order to leave enough of a margin for corrosion. Aluminum has, generally, been far more successful than steel under about 30 feet or so.

 

Matt

Thanks for the reply.

Obviously this is all in the early stages of planning, so I need to work out all the details.

For starters I do realize the boat is wider than what is ideal for boats of this length, but I figured if Lowe, Lund, Starcraft... etc get away with beamy fishing boats I should be able to pull it off.

The biggest thing I am interested in is if I have any major design flaws. Are there any lines that should be moved for a better/drier/smoother ride? Is the reverse chine wide enough? Is the vee steep enough to handle some chop? Will it be stable at rest (or trolling). I realize that everything is interrelated, so here is a rough idea of what sort of numbers we will be looking at.


If it was made out of 10 gauge mild sheet metal just the shell would weigh about 1290 lbs (590 kgs ish). I'm hoping I can get the weight down by using some thinner metals. If 11 gauge (3 mm) was used it would trim it down to 1140 lbs (520 kgs). My best friend is a bodyman by trade, and he has lots of experience welding thin sheet metal (18-20 gauge), so I have some good resources to draw from. I think around 1000 lbs for the shell might be possible if caution was used. I would also consider using heavier metal on the floor and the transom and lighter stuff on the sides and gunnels.
Some aluminum boats are 0.070”, why can’t a steel boat be made out of 14 gauge (0.075”)? that would be like a 700 lb shell.

As far as leaving room for corrosion protection, I would plan on using epoxy paints for the hull inside and out, so hopefully corrosion wouldn't be too much of a factor, which would help with the weight a little bit.

One thing to keep in mind with these weights is that they are just for the sheet metal, I haven't figured out what kind of bracing that would be necessary, I do have a steel boat kicking around to look at and see what used there, but again this is more in the brain storming stage than anything else. I also know of a local boat builder that I could likely talk to and get some advice as far as that goes, but I would like to have a good grasp on what I am doing before I approach him.

How much can you change the center of gravity when rigging a boat. I would think moving infloor gas tanks and trolling motor batteries should help fine tune the weight distribution somewhat.

I forgot to mention one thing, this boat would be powered by a 70 hp motor, that is what I own. So maybe building a 16' boat might be a better choice.

 

The lines need to be faired up, but the general shape is pretty similar to many boats that have been reasonably successful. Wide is generally a good thing for planing hulls, to a point.

Without some numbers and an at-rest waterline, it's hard to say much more. Comparing against existing, similar boats- lots of which are out on blocks and trailers right now- is essential.

Thin steel needs a lot of stringers and ribs to keep it stiff. And it's often assumed that something like a quarter of the plate thickness will rust away over time. If you build in steel, this is going to be a very heavy boat. That might be what you want- but there's a good reason why the only small steel boats you see on a regular basis are relatively slow working craft, scows, barges, etc.

I'd suggest reading Dave Gerr's "Elements of Boat Strength" if you want to understand more about how to design a hull structure at this level of complexity.

If it's just a boat you want, of course, your best bet is usually to buy a used one that's sort-of similar and fix it up. If you really want to build from scratch, buying a set of proven plans (only about a hundred bucks in this size range) is a lot easier and cheaper than starting from scratch. I'm operating here on the assumption that you really want to try your own hand at designing, and really want to test your own skills.

 

Matt

I think your probably right, using pre-existing plans is likely the way to go.

The problem is that most plans in this size are for aluminum, and from what I understand you can't just take an aluminum plan and make it out of steel (or can you?).

The Glen L slither definitely catches my eye, it seems to fit the bill nicely, although I was hoping for something with a reverse chine.

I think I might be better reading up on steel boat construction and using someone else's design.

What are the recommended readings on small steel boat construction?

Thanks for your help

Ben

p.s. Where in Ontario are you from?

 

Once you calc. the weight, figure out where you are going to put the floation blocks and seal them in or at least attach so they will not float out. syryofoam is compatable with epoxy.

 

Again, I ask: Why steel? In this size, it won't have much (if any) cost advantage over aluminum, once the fancy paint system is included, and it'll certainly be heavier.

If you want the challenge of drawing up your own design from scratch, by all means go for it- but be aware that it's a long, tedious process. If sitting over a computer with a heap of papers and textbooks doesn't appeal to you, just buy a set of stock plans and put the time you save into more enjoyable efforts. Like fishing.

The Dave Gerr book I mentioned earlier (Elements of Boat Strength) is a good introduction to boat structure in the common materials, definitely worth the $35 or so. Perhaps some other members can chime in with recommended books on steel/Al construction?

 

I prefer steel because I want a boat I can kick the crap out of and not worry about it too much, punch a hole in a steel hull and it is easier to find the tools to fix it than it is aluminum.

I am also on the school of thought that all other things being equal a heavier boat will have a smoother ride than a lighter boat.

At this point in time I think I am sold on using existing plans, I just need to find the appropriate plans.