DesignerThinking.com : Big Boat Bow Making

I am giving the bow of the big boat it's own page, like i did the little boat's bow. I'll omit the links to the little boat from now on, since you know where it is now. The small hull bows are mostly the same as the little boat's bows, only ten times heavier, 11ft long, and 4ft diameter, and dozens of parts instead of just three. And it's going to take more time and be somewhat harder to make them. And they'll be stronger, tougher, and i can (and will be) betting my life on them in the middle of an ocean.

Here's the humble beginning. I've made a jig (always with the jigs!!) to bend the edging steel the same way i did the little boat. This 3/4 inch diameter round stock will form the leading, top, and bottom of the bow, giving me something substantial to weld the edges of the hull plate to as they join up at a variable range of angles, as well as be inherently a nice clean rounded leading edge to the boat. The top 8inch radius on the edge steel is bent (red) hot, the bottom 2ft radius is bent cold.

Coincidently, the top piece of plate this jig is laying on is one of the bow hull plates. You can see of the outline, the top leading round is, well, rounded, to an 8 inch radius, and the bottom is the leading edge at a 2ft radius. The bottom of this plate does not complete the radius because this makes it easier on me. Each plate doesn't need to join up to more than one complete curve. In this way, i won't need to shave the bottom of this plate to fit to the bottom (because it doesn't reach the bottom) once it's in the rotating bow jig. Sheesh, i will have literally tons of jigs when i am done with this boat!

Here's the edge form when i am done with it. I'll be dismantling it later so i can use the rib as intended. The bits and pieces are only tacked on, and i can nick them with an abrasive cutting disk to break them loose. I needed two edge rounds bent to the bow shape, and this is for the 4ft bow. The 8ft bow is yet another jig.

For the hull ribs themselves, i built the ribs inside this form . For a horizontal axis turntable to hold the bow of the boat, the "rib" needs to be outside the hull. You'll see how/why later in pics below here. This outer edge will be 2 inches outside the bow top and bottom edges, ride on bearings, be fastened to the bow, and allow me to rotate the bow about it's long horizontal axis for easier fabrication.

The oversized "rib" off the form. The pointy end of the bow sits inside this, since the pointy end is flat in one dimension, it won't want to roll over unless i use this to make it feel rounder. Some of you may note it's symetrical outside-to-outside thru the center, even tho it's made of scrap, so it's balanced on the rollers.

This is how i aligned the normal interior rib axis to the outer rib axis. The two pipe clamps at the bottom adjust the one roller in, elevating the large "rib" until it's center is on center with the interior rib behind it, signified by the level. I don't haveto lift it, and i don't haveto shim it, i just turn the clamp handle.

Three pics to show where the frame will be for the bow. I know some of you will say it's not worth building if i use pink string, but i swear i'll be removing that and installing steel. These three pics are hotlinks, click on them to see them displayed larger in a new browser tab (it works in Firefox).

Making a tool be another tool. I added the 45inch length of 1inch angle such that the grinder is 90° to it, and the angle pivots at the far end. It has a cutting disk in it, not a grinding disk. The strake there is one of 14 that need to be radiused for this bow, then there's the other two bows to do also. Each strake comes into that rib at a slightly different angle, so cutting them prior to welding isn't possible, each cut is different and is referenced to the rib.

It's used to turn this....

....into this. I had mentioned the abrupt change in angle where the bow meets the straight sides of the pontoons on the little boat, and how the rest of the boat structure hides it. This boat will have nothing to hide the transition from straight side to bow, so i have a 4ft radiused curve here to help smooth it. It's aesthetic as well as hydrodynamic.

Details of how the strakes are radiused by the disk-grinder jig:

Here's this day's work on the bow. I know it's not much, but i spent the morning on the roof trying to get the heat off me. The top of the forward rib is in shape, the next one back is started. The top of the bow is clamped as straight as the proverbial arrow (different bow, no pun intended).

The forward rib is the 'A' just behind the roller, only it's top is done. The next rib i have started is about 1/2 way between the front roller and the rear large round rib.

I'll complete the tops of both ribs, do some front-to-back and up-and-down bracing, then roll the bow over and do the bottoms of the non-rounded ribs. Then comes more bow-to-stern ribs, then the hull plating.

The heat of welding distorts things that are not extremely well held in position. Of course, i straightened that out.

Here it is upside down, after adding internal bracing. This way i can weld the undersides of things without crawling up into it and doing overhead welding.

Here's this day's work on the bow. Hmm, deja vu. Well, here's a few strakes in place.

A down-the-throat down look at the bow upside down.

And right side up.

It's looking rather busy up inside there now. The ground is on the left because the bow is sideways, and i wanted to look at it as if it was right side up, so i turned the picture.

I'll be removing the diamond-shaped rig soon, and the closest vertical bit there. Also to be done is placing the flanges on the inside of the stepped ribs, so when this frame is skinned in hull plating, the ribs will act as I beams, like the round rib in the foreground. The closest horizontal bit is actually a vertical roof post, not part of the boat.

I may be overbuilding this frame a bit, but what bothers me is what i hear of material fatigue in boat bows, and that includes million dollar racing yachts. I'll be adding steel to the 3/4 inch round stock, to back it up, but for the plating between the ribs, especially as it gets flatter toward the prow, the leading edge of the bow, it's pretty much left to it's own as flat plate, and that still bothers me.

I quote from a ship design book, a section where usa navy ship design is mentioned: The basic design criterion is to assume a pressure of 24 kilopascal (500 pounds per square foot) for any area that is prone to “green water” (wave slap). Most navy vessels are designed for at least 71.9 kilopascal (1500 pounds per square foot), and some unique parts of a structure, such as the sponsons on an aircraft carrier, are designed for as high as 359 kilopascal (7,500 pounds per square foot).

So you see, i am concerned about a flat 6 inch span covered in of 0.187 inch 30kpsi steel!

There's 54 fore-aft ribs in 3 to 4 ft sections, with at least 3 bandsaw cuts on each end. Each oval rib is 18 segments with unique angles on the ends of each segment. Each rib or segment is welded both sides on both ends. I fit each side's bits, and cut two of each bit i measure, to be sure the opposite side is identical. Because i want it to look nice when it's done.

All the red string has been replaced by steel in these pics, and i am adding the details, like a hole thru the bow for towing or drogue lines, and setting internal braces for a catwalk out to far end. I also want a method of deploying the drogue lines from inside the cabin, the catwalk retractable, and lighting. For larger images (800x600), click the pic, they open in a new tab. For stereoscopic pics of the first two, see here .

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More later!