Hull Design and Porpoising

Discussion in 'Boat Design' started by Houndie, Dec 30, 2007.

  1. Houndie
    Joined: Dec 2007
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    Houndie Junior Member

    Hello

    I am a rookie boat builder and need to build a setnet skiff. I am getting close to a hull design. My concern is that I have a little bit of bow in the aft section of my hull. It is not much, however it sounds like it does not take much to cause a porpoising problem. I am hoping for a little input on the topic. I have read all the post I could find on the topic. I have attached a profile picture of my hull, the boat is 22' long. A straight line drawn between points 1 and 3 show a curvature between my keel and the straight line of 3/4 of an inch. A line drawn between points 2 and 3 show a curvature of 1/4 inch. The buttocks show a progressively more but slight curvature as I move towards the chine. Any help would be greatly appreciated. I am hoping the numbers are low enough and will not have an effect on porpoising. I guess is what I am looking for is do evening small curves in the aft section as I have create porpoising.

    Thank you
     
  2. tom28571
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    tom28571 Senior Member

    Houndie,

    A general rule of thumb for a planing boat is that there should be no curvature at all in the buttocks from a point 60% aft of the stem waterline to the transom. The curvature you have is not a lot but why have any at all? Even a little near the stern is bad for performance. Some curvature in the forward sections is necessary in a warped plane hull.
     
  3. Houndie
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    Houndie Junior Member

    Hey Tom

    Thanks for the information. The problem I am having is I can not get a surface to loft when I straighten the keel out when using the computer program. Between that and my lack of experience I can not put it together. So either I have to except the current surface or go back to the drawing board and try to mess around with the loft more. I think the aluminum plate will form to a straight keel. The computer program just does not think so. Nevertheless, thanks for the data, it is very useful.

    Thanks, Jeff
     
  4. tom28571
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    tom28571 Senior Member

    Jeff,

    I personally would never have this issue because I don't trust a computer to do everything right. There is absolutely no reason why the buttock lines cannot be straight in aft sections with sheet material like aluminum or plywood. I'm a dodo when it comes to CAD but you may be trying to make the buttock lines by pulling a line to conform to a few selected points which will cause some curvature everywhere along the line. That is the extent of my knowledge of the subject but there are some CAD guru's on the forum that can talk much more intelligently about this. I draw the lines on paper based on some experience with what wood will do and not do.
     
  5. kmorin
    Joined: Apr 2005
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    kmorin Senior Member

    Set Net Skiff Design

    houndie,
    Welded aluminum set net skiff bottoms are often built of a single plate, some folks insist on slitting that sheet to form a V then re-welding with a VKB of some type.

    'The Cook Inlet net skiffs I built, as Kasilof Boat Works in the 70's and 80's, were single sheet chine to chine. Instead of working for a V, which would ground a few boat lengths farther out on the slowly sloping Cook Inlet beaches, I always cupped the bottoms several inches.

    Building in 50", 72" and 84" bottoms that cup would help the skiff to lay beam ends to the surf and rock with each newly landed swell and move up the beach. So that design element was retained in order to give some shape to the bottom and for the skiff to behave in a reliably when broached in the surf.

    When you cup a sheet of 5086- I'd advise avoiding the softer less expensive 5052- the butt lines and keel will be straight until you heat/weld the inside, that heat will allow the tension to bulge the bottom in a very undesirable way. As Tom has mentioned, the bottom's after butts should all be as straight as you can get them.

    I'm not sure what software you're using but it should allow a cylindrical section - which is what I'm describing- to be developed within some limits? If you're planning on a V fully fore and aft (?) then it could be the mount of change in deadrise, shown in your profile attachment, is creating the 'undevelopable' surface.

    The amount of V forward is a matter for discussion. If you run to outside gear into a head sea then its nice to have some V forward, but if you're on the inside in Kodiak or somewhere sheltered, its not really worth the shape unless you use the skiff when you're not fishing.

    If you get three or four tons in the skiff the V and sharp plan view bow, which is so helpful at speed in a chop becomes a loss when you clip one of those swells and add another half ton of water to the load.

    I usually tried to look at the loads versus the beach to decide how full the bow should be for set net skiffs.

    In order to shorten the waterline, when running empty, or with just the bow man a few shackles of gear, I'd recommend that you taper the chines in plan. [Keeping the butts straight as you can] This will give a slightly bow up attitude, bringing the running waterline aft, and improve turning. Parallel chine skiffs are much harder to round up on a buoy, especially in a swell, than tapered chine skiffs the same size and chine width.

    On the other hand, this taper should be done very subtly or the you'll have "squat", the beginning of porpoising. This is the same phenomenon that happens with a bulged convex bottom within the last few feet of the bottom. The gain from tapering the chines can be loss- if the chine at the transom is too narrow. I'd use the figure of 1" taper in 12' of run. So if you're 22 LOA its likely you'll have 15-18' aft the master section and 1.25 to 1.365" of taper will improve the skiff's running and turning. If you use an angle extrusion (?) to cover the outer chine and it looks down a few degrees, then as much as 1.5" taper per side is probably livable.

    I left the entire bottom fully together, cut the 'orange-peel' from the bow for the V, then cut the centerline two feet aft straight through and the next two to three feet were scored half through. This will fold into a very nicely transitioning curvature that is fair, easy to build, and stiff.

    Power also has an effect on 'porpoising' even with straight butts, because the larger outboards are, as a general rule, much larger percentage of the overall displacement on a welded skiff. A 400lb. 22' hull with a 175lb. 55hp kicker is one thing- the same hull with a 400 lb. 90hp is a whole separate set of numbers.

    The 50 may not make the hull lope but it may do so with the 90 -exact same bottom and with butts as straight as can be built. The hp/wt. ratio and the moment arm of the thrust are so different that the larger engine can cause performance which is wildly different.

    A 25 hp won't make the curved butts you show porpoise because wide open it won't get the hull's waterline leading edge far enough aft. She might squat a bit but once down the balance would stay. But the 50 might get some pitch swing, if you were empty and the curves were 3/4" in the aft 4'. But even a flat buttock line bottom might lope with the 90hp because of the wt aft, the total force or thrust, and the resulting changing waterplane shape while running.

    Once you get a net, crew, maybe some anchors or other gear in the skiff, you'd naturally move it forward if she bounced (?) or loped. And when you pick, porpoising isn't an issue back to the beach or wherever you deliver. So all my remarks are concentrated on the light running conditions.

    Cheers,
    kmorin
     
  6. tom28571
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    tom28571 Senior Member

    Hi Kevin,

    I was looking for your email address and see that you have already answered Jeff's inquiry. He could not find a better source with your experience in the very area he is seeking.

    Almost 70 degrees here today. Global warming does have its plusses.

    Since you have done so many aluminum boats, have you calculated the weight difference between a plywood boat and an aluminum one of the same design?

    Tom
     
  7. Houndie
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    Houndie Junior Member

    Kevin

    Thanks for you informative reply. It is nice to hear from a fellow Alaskan.

    We fish the west side of Kodiak were we see alot of chop, big swells and
    sometimes just plan old crappy weather. We have a holding skiff so we
    rarely travel with more than 2000 pounds of fish onboard. Our beach is
    usually protected and has a good incline so the influences of the beach have
    little effect on my design. We have about a 1 1/2 mile run to our nets.
    We frequently travel in a sizeable day breeze chop. It's a long ride
    sometimes in our current vessel. We also frequently find ourselves picking
    and travelling in a sizeable swell that makes use go "whoo" so the boat
    needs to have a limited tendency to roll.

    I am still studying your response and trying to put it all together. I
    would love to get my computer document to you so you can see where I am at
    with this design. You don't happen to have Rhino do you?

    Do you still live in Alaska?


    Once again thank you for taking the time to share your experience with a
    green horn.

    jbassett@alaska.com

    Jeff
     
  8. TollyWally
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    TollyWally Senior Member

    I've spent some time In Cook Inlet and down around Kodiak, fishing in the good old days. Best wishes for a Happy New Year from Puget Sound. ( A small, tame, anemic, over crowded, pathetic little bit of water that has lost most of it's goodie.)
     
  9. kmorin
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    kmorin Senior Member

    Set Net Skiff

    Houndie,
    Fishing in the Shelikov is clear enough for me (!) - say no more- about the conditions I know what it can be like, although I’ve never fished beach gear there, I’ve built beach seiners for the area and hunted in Larson and on Unganik in the early 70’s. I’m glad to hear you don’t land the fish that makes life much easier in the design work. Where are you from Tombstone Rocks to Raspberry Island?

    For your fishing conditions I’d recommend that you built a double bottom skiff with a full welded deck both for stiffness and safety. The V bottom you show in your concept sketch works great with a full deck with lots of flam to the topsides, good rake to the stem and a very full sheer line in plan view. I’d plan on a full length reverse chine both to give more beam at the chine to help make her a little more stiff when working gear, and it’s a fine way to increase her waterplane area so she packs more with less draft.

    The reverse chine also helps the construction process greatly since you get to build the bottom with a plan view former tacked at the chine. This simplifies the tack-up by adding a accurate plan view outline of the chines without needing transverse framing- a big labor and set up savings.

    Do you clean the gear across the gunwale’s or do you hoist it alongside “Kodiak” style? The reason for asking is the details of the gunwale construction can be heavily influenced by the corks being passed over the skiff in swell. Some combinations of extrusions in cross section make that very hard on the crew, others make it fairly simple. The construction methods in this matter vary greatly but it all hinges on how you pick the gear on your site?

    I know that reading someone’s remarks can be slow going as my references will be to experiences past for me and in the future for you. The person writing is trying to use words to describe what the other person reading will see in the future based on what the writer has already seen. I’ve been involved in many discussions with new builders in Europe, Australia, and all over the US but I’ve only been of very limited help. I’d be happy to exchange sketches and lines in order to help you TRY to avoid the countless mistakes I made while learning to design and build this class of boat.

    I have Rhino, so I’d be happy to look at models, and I’ve lived in Kenai since ’59, except for summers in Seldovia, making me more or less a local. It would help to understand your welding machine(s), cutting tools, shop conditions, and aluminum welding experience to see how far you’ll need to go?

    Tom, regarding relative weights of the two materials in this class of boat; I’ve only built small plywood skiffs and never had the chance to compare my work larger welded skiffs with a like model in plywood. The few wooden skiffs of about the same size and shape in wood are all heavier – from twice to four times because of the need to be open without decks. Wooden commercial fishing skiffs need some dramatic rib systems to resist the transverse compressive loads of becoming displacement vessels with a ton or two of fish. This load change moves the waterline down a bit therefore the side-to-side or transverse compressive element becomes high- quickly. Aluminum skiffs usually have a welded element at the gunwale like a 3” 6061 pipe, or a small shelf type deck with extrusions inside and outside. This composite welded assembly can be viewed as a beam in all three views. Wood composited gunwale structures, unless glued and glassed, are no where near as stiff in for the given weight. Once glassed and glued they’re heavier, mainly because of material use.

    The transverse ribbing/framing usually used in wooden commercial net skiffs is much heavier than the same in aluminum since they’re most often hollow in metal and solid in wood. So a 4" wide 6" deep inverted 'U' of 0.125" aluminum is welded to the bottom and sides of a skiff at 5-10 lb. the same relative strength in solid wood is 50 to 70lb. If the transverse elements in wooden skiffs were hollow and the gunwale structures were laminated of stripped or planked longitudinals so they too could be hollow; then the wooden skiff would be lighter wt or at least AS light. But in general practice the labor to incorporate these well known wood epoxy techniques into this class of skiff has not been done here- that I know about. Hollow aluminum extrusions offer a strength to wt benefit not available in wood without dozens of times the labor to include in a skiff.

    Also contributing to the lack of higher level design innovations for wooden work skiffs is the general fragility of wood. Jeff’s description of the working conditions hardly do justice to what it would be like to offload a ton of fish to another boat after pulling them over the side in southern blow up the Shelikov. Just one full broad-sided slam of the wooden boat, full of fish, against a metal hulled tender or holding skiff may well ‘start the seams’ requiring the boat to re-glassed or repaired. On the other hand that’s just noisy for aluminum skiffs.

    iisco@acsalaska.net

    Cheers,
    kmorin
     
  10. Houndie
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    Houndie Junior Member

    Hey Kevin

    We are just heading down to Aleyska and the kids are making a solid effort to pull me out of here. I just thought I would let you know I will get back to you in a few days with an appropriate reply to the questions you wrote. I have attached my Rhino file. I will also attach a picture of our current picking skiff which shows a bow roller. We get under the net a drive thought it with the outboard.

    I am out the door talk to you later. Once again thank you very much.

    Jeff
     
  11. Houndie
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    Houndie Junior Member

    Try again.
     

    Attached Files:

  12. Houndie
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    Houndie Junior Member

    Photos
     

    Attached Files:

  13. Houndie
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    Houndie Junior Member

    Kevin

    We fish about 7 miles NE of Larson Bay, between Spiridon and Uyak Bay.

    Your second paragraph lays out nicely some changes I can make to improve performance. I need to design a reverse chine. My intentions are to taper it from about 5” in the stern to 2” at the bow. I think this will make construction of a bow cone a little easier. A cone will work nicely in making a smooth surface for my net to slide past when pulling over the roller. I also think I can pull the sheer out in plan view and get a good surface, which will in-turn create more flare to the topside. The only thing I am a little fuzzy on is a “good rake to the stem”? Does this mean a steep angle to the forefoot in the profile view?

    My construction plans at this point are to export my Rhino pieces to AutoCAD. I will then have a company in Seattle cut the pieces via water jet cutter. Alternatively, I am considering having the patterns cut out in 20 gauge sheet metal so I can see what they look like. I will use the sheet metal pieces as a template to cut the aluminum. I will build a jig which I will design in Rhino. My hope is I can lay the bottom plates onto the jig and use the stations to get the same exact shape of my Rhino model. I will tack weld everything I possible can except for the deck and begin welding. I expect some heat distortion but hopefully with everything tacked together it will come out OK. I have limited welding experience but plan to move slow and perhaps acquire a Tig welder in addition to my current welder a Millermatic Mig 251. I also have some friends with good welding skills. I anticipate doing a lot of welding on scrap metal to get the right settings

    With relation to your last paragraph, the most dangerous part of the fishing job is tying up to the tender in a bad SW blow, you really have to keep your whit’s. My tie downs have to be built with that in mind.

    There is one thing about my current design that troubles me. In the plan view my chine begins to make its curve towards the bow at about midships not 15’ to 18’ from the stern as you eluded to in a previous message. The sheer does the same thing but I can pull it out when I make the sheer more full (I think).

    Once again thank you very much.
    Jeff
     
  14. Houndie
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    Houndie Junior Member

    Sorry that is NW of Larson Bay not NE.
     

  15. kmorin
    Joined: Apr 2005
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    kmorin Senior Member

    Reply to Post -Shape discussion

    OK, I figure you’re on the point in the middle of the Uyak opening, with the little lagoon behind a spit, maybe on the Spiridon (N) side? There are more flats on the south side near the cabin on that spit but net fishing there has to be serious work, no matter where you based.

    Raked stem: That term refers to the incline in the profile view of the line of the stem. More rake is a greater angle or more leaned forward where a plumb (vertical) stem is 90 to the water and a cigarette boat has a lean of 60deg forward or 30 above horizontal. If the stem is raked in any hard chine shape the chines are naturally farther aft than the sheer line as each intersects the keel plane. This naturally overhangs the topsides outward from the chines- this is called flam on single panel metal boats as the topsides are most often convex and flair on the boats where the body plan sections are hollow or concave. Increasing the rake of the stem increases the plan view offset of the curves: chines inside and aft- topsides sheer outside and forward. The more rake the more and given set of curves gives increased flam- most often called flair. In the body plan this is seen as the sides leaning outward more, but remember that buttock lines leaning forward also dry out the boat.


    My construction plans at this point are to export my Rhino pieces to AutoCAD.
    If this is your first design and you don’t realize how to do the conic projection of the bow and bottom camber (?) there is some risk of the transverse shapes being a poor fit without distortion to the bottom panels. There is a fair discussion of NC shape development on Bruce Robert’s metal boat site. More info is available in other quarters- the issue being that model to developed sheet to NC files isn’t quite as simple as you’ve described. Acad may be a good choice but Rhino will do the same work in all cases.

    RE NC cutting: If you’ve done this already maybe you know more about it than I? My cost replies show that I make extremely high wages cutting the outlines myself. I found the local (SEA) handling and pick-up/delivery between the vendor- NC shop cut charges and shipping and handling back to shipping crating site- were almost equal to the cost of the metal!!!! So I’ve continued to cut by hand.

    Modeling:

    I’d use the modeling confirmation stage for a first boat- instead of sending Transverse fit patterns will show up if the curvature is off much and the assembly practice helps you get ready to tack up. I’d suggest that you consider a door skin -1/16” plywood for this model- it can be assembled by you and that fine looking crew of young fisherman in the posted photos with hot glue and Super glue for wood. The gain is the smaller size, cheapo materials and the ‘throw-away’ level of risk for thousands of dollars in aluminum that has outline curvature issues.

    The gain is to confirm the curves being clean and edges matching evenly - all you need to learn, for this stage of work, is to use battens and to (correct) fair the curves of the outlines.

    I agree, a female support jig is helpful but on the after few stations are really needed the reverse chine and a Vertical Keel Bar will do the rest. A full length supporting cradle wasted as there’s not risk of shape problems using a VKB and reverse chine plates.

    I’ll make a suggestion that may sound foolish until you sketch with it for a while. I think you should consider making the bottom and deck(s) in one unit. Then air test and add the topsides and full transom LAST.

    Not many people will build this way but adding a deck is true pain in the back and the inside butt fillet is prone to a few problems by comparison to the outside (full-fusion) corner fillet. Once the hollow ‘surf board’ is built then adding the topsides is simpler than adding the deck and framing inside after the topsides are on. I realize that since many folks (including me in the past) haven’t done this method that is will seem too radical an approach, but having built a few decked skiffs- that’s how I’d do it today in this size.

    Heat Distortion of Welding Contraction: Once tacked most panel’s heat distortion comes in mid panel not as much at the edges since very often the hull has another sheet at nearly right angles to the first. This edge to edge support will help to keep the contraction in a fair line- except for middle of panel welds like rails or longs on the bottom and topsides. This is where the weld schedule keeps the results distortion free. Small length, short face, planned applications stitching is the key to minimizing mid panel distortion. There are some seriously important design considerations that help too- but you’re a few days form those considerations in your lines.

    Tie Too’s: The tie-downs or tie-too’s as some call them, are only part of the stiffness for laying along side a tender in a swell with a load. The gunwale cross section is important too- by using a box beam you can get a smooth side- skip transverse ribbing, and have the stiffest boat with stronger tie-too’s included.

    You current skiff has a 3” or so pipe full length but that isn’t near stiff enough in my opinion, I always used a half 3” outside- maybe rubber D in some seine jitneys- then a horizontal plate of 6 to 12” and another pipe or half pipe inside. Then I’d put legs to the side rub-rail inside. This surface can also be plated to create a triangular box beam with air for flotation incase of swamping it will keep the skiff upright. The combined plan view and profile view curves of this triangular section, in body plan view, beam is about as strong as can be built in and open skiff. It allow you to skip ALL transverse framing above the deck and gives the most strong base for the tie-too pipe rail.

    Chine and Sheer in Plan: Using your pictures as a starting point, that shape is boxed aft the 1/3 or maybe the ¼ station aft the stem. That makes the bow very, very full an the forward waterplane being full in plan view means the bow won’t loose much buoyancy if the guys haul a big flurry of fish in over the stem. The farther aft the max beam point is in plan view (max beam of chine is the main guide her) the more pitch you’ll have for any given moment by the bow. So a more pointed sharp bowed hull which runs into a swell the best- will ship the most water over the bow when two d crew add their weight to the 200 lb of fish and the tension of the rest of the shackle overboard on top of the bow stem at your roller. While the closer to a flat bowed landing craft will haul the most fish with the least pitch downward but is not nearly as pleasant to run into those same seas.

    That’s both ends of the spectrum- you’ve got to balance the two in your design cycle- now begin very aware of the adage that boat design is an endless series of compromises. One idea you’ve already started to explore is the sharp bottom coupled with a reverse chine allows a lower running shape that is good into a swell and its topped by a very full conic bow and much more full topsides ‘spoon’ as the bow of these skiffs is sometimes described.

    I found the conic bow very helpful to add radically increasing waterplane shapes to welded skiffs that will haul loads. By adding much more flam (topsides lean) to a sharp under body the resulting hull can pack a good load and travel head on into a chop remaining dry.

    cheers,
    kmorin
     
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