Hull Design and Porpoising

Hey Kevin

Your information as been extremely valuable. I am now in the process of
instituting your comments. Believe it our not I have been taking notes. I
am starting to understand boat design. I can't express how much I appreciate
it. I will get back to you and fill you in on my progress. When I am done
with the boat, maybe if your in Anchorage, you could check it out.

Thank you, Jeff

 

Ideas for Design Considerations of Set Net Skiff

houndie,
I've had a minute to sketch on your project to hopefully give some shape to my attempt in words to discuss the hull I'm suggesting as part of your thinking for this set net skiff.

I've attached several images two show the bottom without topsides or transom, two show the hull more completely. First, these are about 22' x 8'6" nearly 3'6" deep and I realize the full deck doesn't have the various deck levels you show in your model. Next the images are here not as plans but to help give image to the words I've written in previous posts SO that means I'm not trying to design so much as illustrate the concepts advocated before.

I've given a 'surf board' a rounded bow and very small or short topsides so that you might consider this building method? The cone for the bow, topsides and transom will all add upward simply and with much less effort than if you work inside the 3' topsides to install the framing and deck. Regardless of your eventual picking well, or bow deck or transom motor 'slop tray' -all those can be accounted for in this method- you and the crew will not have to work over the relatively tall sides, on your knees, trapped by the overall hull's shape when you work.

These images show a V bottom using the reverse chine to gain waterplane (read: packing ability) change. The beam here, changes 12" in one 1" of waterline immersion. This transitions the displacement by nearly 800-1,000 lb. in an inch of immersion, depending on the V's deadrise and overall chine beam. The reason to show this is to call your attention to this group of facts.

The reverse chine helps in construction, gives a nice load carrying aid when immersed, helps with planing, increases roll stability (chine beam) and by adding the small lower topsides allows you to work on the deck beams, decking, welding and fitting all these things without crawling in and out over the 3' topsides. Outside corners are the simplest seam in aluminum MIG.

The topsides are shown with the stem raked forward. Some net fishermen want to land the buoy over bow stem and that is made more difficult if the buoy is behind or under the stem as it leans forward. I'd ask them (you?) to consider hauling the buoy over the side more amidships then walk the lines to the bow, and either lift the line over the roller upright or build your roller with a tipping upright.

What I'm trying to advocate is the hull shape should come first in its design considerations and fishing should adapt. Otherwise you end up with a boat that may, for a few seconds, work well to retrieve the buoy over the bow; but will be wet 99% of the rest of the time.

I show a cone but didn't bother to try to fair the bow to a finished hull, and the proportions are very rounded at the sheer to point out the two areas of the hull- and their relative function. The sharper, deep entry beneath the chine runs into a chop well and will keep the helm from slewing in on the face of a steep swell. Above, the huge conic volume of the bow -shown for illustration's sake not as your design- will lift the bow man/men and a large flurry of 5-6lb reds very quickly. The increased rate of pitch may give a 'whoooa' ride up and down but they'll be dry and you won't be bailing a half a tonne of Shelikof's Straight back over the sides.

This shape will carry a larger load into a head sea than a more plumb sided shape. The waterplane transition is much more radical than we'd bother to design for a boat that only carries a fixed load of beer, rods and gas. Not may classes of boats are designed to load twice or three times their wt without full decks at the sheer.

The colors chosen are intended to make the various surfaces more obvious- aluminum photographs poorly even in CAD. I've only loosely adhered to your proportions but I hope to have given an EXAMPLE of the shapes we've been discussing.

I'd frame here almost exclusively longitudinally from the transom to a point 1/4 or 1/5 LOA aft the stem. There, I'd use a full transverse bulkhead from the deck to the hull. I'd want a full vertical keel of 3/16" or 1/4" 5086 from the hull to the deck - if you're conscious of weight you could cut lightening holes but I've never bothered unless they allowed conduits to pass from side to side. Next, outboard about 1/4-1/3 buttock lines I'd use another full height buttock longitudinal and then I'd use shorter verticals to break up the inside panels.

I'd tend to notch the tops of the tallest longs, while in a stack, with a jig saw and either roll some angle to camber the deck or bend them cold -simply done, if you could clamp to the reverse chine's short 'topsides' as shown. If you were working at this joinst as an outside corner you'd then lay the deck over all of the framing clamp it - mark it and cut it laying on the tacked up and partly welded bottom. Keyhole each of pocket welds and tack the deck on the fully welded bottom- you'd be ready to complete the deck while the hull was only 14-16" deep.

Once this hollow 'surf board' were "built"- that is welded and air tested -I'd tack on the topsides and finish all the other work - sure wouldn't any jigs or set-up framing for that phase as the rigid base/complete bottom, would give full support and clean seam at the 'upper chine' which is the topsides-to-deck intersection line.

The well deck would have to be cut out and finished during the bottom building work, and it would sure be easier to repair the [inevitable] inside corner air pressure leak at the bottom of the inner chine-to-bulkhead weld compared to doing that work with the sides ON!!! IN one case you'd kneel outside the hull working at the waist level leaning inside the shortened hull. In the latter case you'd laying on you stomach or crouching in the well area reaching below your knees to weld at arms length in a squat!

I'm sure everyone reading this topic would enjoy watching you firm up your design, and progress as you build. I'd suggest you designate one of the able bodied crew as the designated keeper of the log- pictures, progress, records of work. They'll snivel, of course, that they don't get enough share of the site to warrant this off-season intrusion into their time ashore, but it would sure help the rest of us see your work and they'll end up glad you asked them to take on the extra watch.

Cheers,
kmorin

 

Hey Kevin

I am currently trying to work out the change in the chine. It seems that I need to start thinking about construction. I have listed my steps below. I know that this is not similar to the steps you sited. I just thought I would lay them out there to see if anyone else has tried a similar process.

1. Have the cutters in Seattle cut my pieces out of 20 gauge sheet metal. This will include all pieces but a selected few. I will have small holes cut into the template pieces to mark the intersections of my various adjoining pieces. For example were transverse pieces butts into the bottom plate, the line in the bottom plate where the transverse piece joins will be marked by a series of 8 holes which I will punch with a good sharp punch tool during the cutting stage.
2. Make a female jig.
3. Add a temporary ¼ stripe of wood to the top of each transverse jig member.
4. Fit the bottom plate templates into the jig a check for accuracy.
5. Use duck tape to secure the bottom plate templates to the aluminum plate material. Use the bottom plate templates to scribe a nice clean line onto the aluminum. Punch the holes to identify location of adjoining transverse frame. Cut out both bottom plates. Place bottom plates into jig after removing 1/4 spacers.
6. Do the same thing as #5 to every other transverse piece. Do not include transom.
7. Start tacking the bottom plates together using transverse pieces as a guide to make accurate. Tack in transverse pieces as the process moves from stern to bow.
8. Tack weld bottom of boat at keel.
9. Grind down the tack welds on inside keel
10. Slide in ¼ X 4” keel support from the stern through notches in transverse sections as far forward as possible and as one piece. Piece in the rest of the keel in the bow and tack weld.
11. Position and tack weld the remaining transverse pieces except for transom. Use templates to make sure all fits are accurate.
12. Use the template to make sure chine is accurate.
13. Cut chine and tack weld into place.
14. Use the template to make sure side plate templates are accurate.
15. Cut and mark side plates tack weld into place
16. Assemble and tack in transom and as many associated pieces as possible.
17. Tack weld as much pipe and 6” gunnel rail as possible. .
18. Tack weld in 2” X 3/16” longitude support pieces at about 16” on center.
19. Tack weld deck support structures.
20. Begin welding inside of boat. Use a predetermined welding sequence?
21. Fit in deck and weld.
22. Flip boat over and complete welding.
23. Air test chambers.

 

Build Procedure

Houndie, the main problems I see with the overall method you've posted is the sheet metal patterns.

They will not lay to the shapes like the aluminum will being different stiffness, will not be easily held on the boat to confirm shape as you go.

This may not seem like much but I'd estimate your method will increase build time and rework by factor of five to ten times the man hours. Using the NC cut hull- which I don't think the modeling will support in direct export- I'd estimate a few hundred hours to build. Using your interposing patterns I'd estimate a thousand hours.

I'd encourage you to make the skiff as a 3' model with matting board (thick poster paper and a hot glue gun) or from plywood first. Then using your corrected outlines (if necessary) send the NC files to be cut to final shape.

When that gets to your shop start by adding the lowest piece of metal and continue upward adding each piece in sequence. I assure you; no one can slide the keel bar into grooves cut in the transverse members unless each slot has more tolerance than you'd want for a good fit. I'd add everything bottom to top, and skip the patterns -they're no gain to your build- unless they represented the stiffness and outline of the real hull panels which they don't.

I'd say the 3' or 4' model will be the most help with the assembly tutorial, and I'd use the rule of thumb that if I can't do it 3' long? then 22' is going to be 7 times as difficult.

Overall you'd be investing in cuts that you describe as questionable in every step. Confirm and then... so I take it the modeling and NC work is not known to be accurate -what use will the patterns be if a 20 long chine strake's outside curve is 3/8" out of line- that results in a 12' long gap to the bottom? What possible gain is the pattern material when its already bought but not confirmed for shape. As Pres. Reagan said "Trust, But Verify!" So I'd want to know the time, value and effort in the patterns were worth all the trust your work sequence description puts in them?

Maybe others who've built welded aluminum boats in this class have had different experiences they can report on?

Cheers,
kmorin