Custom 19' all weather, minimalist, strip plank composite 'go fast'

Discussion in 'Boat Design' started by socalspearit, Sep 2, 2021.

  1. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    Thanks very much for sharing the photo of that hull. I really wish there was a way to look at the hulls of all the boats I've been on.

    I am going to build a scale model of my hull design out of basswood, it'll take 3 hours. Not for towing but to play around with it in a tank and look at stability.

    I understand your pencil and analogy and why the exterior tubes of a RIB provide so much stability, but experience climbing onto so many surfboards, SUPs, my inflatable, and jet-ski's tells me things are different. Perhaps I take this for granted since so much of my time is spent in the water and climbing in and out of various unorthodox (compared to 'regular' boat) contraptions but loading and weight management is something of a given on those--on a Whaler 4-5 guys can jump into the boat, throw gear anywhere they want and while it may affect planing they can pretty much sit wherever they want without affecting initial stability, and on any of those contraptions before they heel far you fall off.

    I've only ever boarded a jetski from the side. I'm not a huge jet-skier but I've put in my lifetime probably 200 miles on one (mostly two guys with two skis) in the open ocean, used for fast and cheap Catalina runs for spearing so we did loads of climbing in and out (not like a typical jetski user), and running on average about 30 kts but sometimes flogging them up 45 kts for as long as we could stand it on the way home. We have to board from the side because the back is taken up with coolers and spearguns. Jetskis are very tippy when boarded but are impossible to flip unless you catch air the wrong way at high speeds, and certainly controlling speed for passengers, loading, and sea conditions is a given on any boat. Part of why the skis are tippy vs my long skinny inflatable is certainly the V hull and high center of gravity but also that they are short and stubby so you don't get the bouyant force multiplied over the long edge. Perhaps my experience boarding from the side is also on some pretty large jet ski's; any that I've used are really designed for a two people and three if crammed on, so they may be longer than what you're referring to.

    Most of my other examples cited are flat bottom so I know the V definitely will affect things, but the hull I am designing is not a particularly deep V and has a 3.25" chines. Ordinarily that's nothing but on something this narrow I think it's meaningful especially when multiplied over the length of a 15' waterline. Most anyone's experience with a boat of this beam, if they have any, is something that's also only about 8' long, with a 6' waterline. Heeling my middle section guides on a table and looking at COG, COB, and righting arm I can see that the chine provides bouyancy when it heels (again an estimate and not a computer simulation or scale model in the water). Here are some shots showing more of my bottom:

    SpearIt_One_v2 v14_a.jpg SpearIt_One_v2 v14_b.jpg SpearIt_One_v2 v14_c.jpg SpearIt_One_v2 v14_d.jpg
     
  2. fallguy
    Joined: Dec 2016
    Posts: 5,281
    Likes: 1,015, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I don't think the boat will even sit close to level with the driver offset. Sorry.
     
  3. fallguy
    Joined: Dec 2016
    Posts: 5,281
    Likes: 1,015, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Also, make sure you understand you cannot model wave motion accurately. So your work will be only understanding initial stability. However, you perhaps could model a 200# person getting on or sitting at the helm.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,026, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The two biggest risks to small boats are breaking waves on barred inlets, and unexpected high winds, such as storm squalls. Years ago I would regularly cross a quite nasty bar, and would often see another fella cross in a tiller steered open aluminium boat only about 15 feet, which was actually a very seaworthy little boat, one of the best, and very popular with pro fishermen, but I remembered thinking it was only a matter of time before the bar got him, in such a small boat, it was away from habitation and a mishap would not likely be seen happen. He did come to grief, but not on the bar, he was well offshore when caught by a severe storm accompanying a wind change, he was never found, his boat was, upside down, and with a sea anchor out.
     
    fallguy likes this.
  5. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    Thanks, yes, that is a good point. Again, I know the pontoon vs modified V is different but I've spent so much time on my inflatable in the waves... the way a long skinny boat behaves in the waves I feel I understand very well (and love otherwise I wouldn't be doing this). Seeing initial stability on the model and extrapolating against the kaboat's behavior I think I can get somewhere. The main difference is of course the bottom shape and slightly lower COG; on the kaboat at rest two guys can sit on one tube and it heels a little but they hardly notice (I do of course because it's my boat and I'm responsible for everything). One guy on one tube with their *** hanging off (me doing number 2 at the islands) it does not heel at all.

    Thanks, yes. Our coastline and waters become very deep suddenly so we have no such thing as a barred inlet. The closest thing to that for us is a breaking wave at the beach and my long skinny inflatable if it gets turned sideways in that wave gets VERY tippy especially if someone is sitting on the downwave tube (the only time years ago when felt like it was flipping until I fell off). That beach break though is never in more than 12' of water and I'd never be diving there, plus it's technically illegal in LA County to bring a motorized vessel within 300' of the beach. We can get that type of wave on the open sea if the wind is big enough but those are storm type winds for us which are much more predictably forecast here. We typically get quite large swells but they are long period, so rough for those prone to motion sickness, hard on divers entering and exiting at the beach, but small boats, low to the water on the open sea, just scoot up and down them.
     
  6. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    I was looking at my model and I just realized some of why I am so kneejerk against high upswept bows and planing boats that squat at low speeds... 99% of my lifetime hours on boats has involved freediving/spearing. One of the biggest hazards to a swimmer is getting hit by a boat, and while I'm never behind the helm when I have students in the water, in most spearing situations we may be live boating and motoring around in a cove doing pick-ups with a few guys in the water. Anytime the bow is up you lose sight of what's at the waterline right in front of your boat. In my case that could be a diver or spearfisherman breaking the surface. Me and my buddies are very used to diving around boats and have good situational awareness, etc, but even a very experienced freediver will make stupid mistakes if they are hypoxic enough and could surface too close to a moving boat.
     
  7. sharpii2
    Joined: May 2004
    Posts: 2,092
    Likes: 231, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I look at this and see what looks like two long 'air boxes' that run the length of the boat. It also appears that they extend all the way up to the sheer line. They seem to be around 10 inches wide.

    If this is the case, and if they are water-tight all around, they make a good imitation of inflatable tubes.

    The important factor is that their buoyancy extends all the way up to the sheer, or at worst, no more than a few inches lower than that.

    This is so the boat will be stable if swamped. The water that comes aboard will the actually add to the stability. This is because it cannot slosh all the way over to the low side when the boat is tipped, and it cannot fill the boat up to the sheer line (without pouring out the transom).

    I think you may want a dramatic upward hook in the sheer line at the bow. This is to provide adequate local buoyancy there to prevent the bow from diving under.

    Going with a pram bow might have the same effect. The problem is that there is not much buoyancy at the point of a pointed bow, and the buoyancy only starts to build up at about two-thirds its length.

    Weight wise. I find your 200 lb hull estimate to be a bit optimistic. And I find your weight estimate (including the 200 lb hull) to be about 100 lbs off. 18 gallons of gasoline is going to weigh about 108 lbs.

    Adding this together, I get 608 lbs including the engine.

    With five people on board, I get a total weight of around 1670 lbs. I figured 200 lbs per person plus 15 lbs each for dive weights. The real number may be closer to 1750 lbs when you add other gear, plus an allowance for a heavier hull.

    The heavy outboard will put a lot of bending load on the hull just sitting on the transom. If you bring the transom inboard a few feet and leave the air boxes extended past it, you may solve some of this problem.

    One thing to keep in mind is that an inflatable uses air pressure as a compression member. Since air is compressible, an inflatable can flex quite a bit with no damage. With a rigid wood hull, this is far less true. You may need more structural weight to give it sufficient rigidity.

    Just a few thoughts.
     
    bajansailor likes this.
  8. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    Thank you very much for the input. I am going to build the model next week when I get time just to be sure but as you noticed I think initial stability is greater than people realize when the hull gets this long. Here's a photo of two guys standing on one tube of the inflatable (actual waterline length and beam comparable to the new design) with no counterbalancing, just a 140lbs of engine and stuff at the stern and 50lbs of stuff in crate near the bow...

    PXL_20210910_143743291.MP.jpg

    The bouyant sides make sense; I realized this about the inflatable, since the air tubes are so dramatic and take up so much internal volume, even if entirely swamped (it's happened more than a time or two, running heavy in bigger seas), any water is still contained directly over the centerline of the vessel. I did incorporate some beefy inwales for floatation but the further down they run the more stable if swamped it would be as you point out. I don't want to give up too much interior volume and am not trying to make something largely idiot proof stabilitywise in any weather and loading conditions, especially given the type of people on this boat, but I will think about this. Stuffing the bow is alarming even in my inflatable but having done it a time or two, in our seas what always happened is the inboarded water runs to the transom a few seconds later and starts to drain (SLOWLY, since the scupper on the inflatable was never intended for that sort of thing), and inertia from the engine keeps it there and g-forces from the engine can push it there. If the engine were to stall out or be inoperable things do not work so well... this kind of a design, running successfully in seas, definitely assumes a reliable and solid engine and thoughtful steering. Again, I'm not heading out in that weather and certainly not with students but I do want some worst case margin.

    Regarding weight, I did the calculations per USCG and while based displacement volume, etc, but I do not have any actual experience being on a strip plank boat or have anyone in my immediate circle who's built one. So thanks. Regarding the weight though, based on experience with my inflatable... in weather I never actually want to be anywhere close to its rated weight capacity and I'd be treating this vessel the same way...

    Yeah, transom load is something I've been thinking about--it's the weight of the engine PLUS the torque/bite of the prop so the forces on the transom are quite large (I added cable reinforcement on my inflatable connecting the tubes to the transom since the shearing/bending forces are so great with that size engine). It's not in the computer model but I am planning on during execution putting on some kind of very solid knees on the transom. Extending 'tubes' out the back (like most inflatables) is an interesting idea and something which could provide a little more ultimate protection from the prop when someone is in the water, and if foam filled they would help balance the weight of the engine if swamped. Dunno, might doodle with this.

    Structural reinforcement: When I started this I was looking at books on strip plank canoe/kayak construction. Obviously the forces are much, much greater with a powered vessel and I was planing on more reinforcement but I've realized that I'm going to really need to formally incorporate some kind of longitudinal i-beams. I'm thinking I can do this successfully with two vertical planks running the length of the boat (center of the i-beam) under the deck on either side of gas tank, with the bottom and top the of the ibeam being the deck and hull respectively, and then glass or CF the 'beam' below decks. Probably go with WRC instead of marine ply for this to keep weight down. For the forward bulkheads I'm likely going to go with maybe 1/4" marine ply (if it even comes that thin) clad in WRC to keep the weight down, and only use 3/4" thick marine ply on the transom (this also will be clad on one side with WRC).
     
  9. TANSL
    Joined: Sep 2011
    Posts: 6,819
    Likes: 501, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The inflatable boat in the photo appears to be resting in the mud. Hence its enormous stability.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,026, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    And you want a skinny boat with similar dimensions, and self-draining ? You cop a lot of water inboard in a hurry, and it will go over. There are good reasons why self-bailing is not a feature on the vast majority of small boats.
     
  11. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    What?!? Are you implying that the water in the back of a Los Angeles harbor might be dirty??
     
  12. TANSL
    Joined: Sep 2011
    Posts: 6,819
    Likes: 501, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Let's keep calm, please. I am not implying anything or assuming anything and less of the very clean "back of a Los Angeles harbor", I am just saying that it seems to me that that boat is resting in the mud. It would be very pedantic of me to suppose that I know the ports of the USA so perfectly that, from the color of the mud in the photo, I am able to tell that it is "the back of a Los Angeles harbor". No, dear member of this forum, I suppose that the water in the back of a Los Angeles harbor is very clean.
    But, back to the boat of this thread, if you don't mind, such a boat is very unlikely to have great initial stability, whether the water is clean or dirty.
     
  13. socalspearit
    Joined: Apr 2021
    Posts: 46
    Likes: 10, Points: 8
    Location: Los Angeles, CA

    socalspearit Junior Member

    I gave my inflatable a couple very thorough bow stuffings yesterday on the water to really observe what it does....

    I am thinking to combine some beefy knees at the transom with your 'air box idea', especially at the transom. I obviously never want to submerge the engine but at the same time I don't want too much rear floatation because it's a given that water will enter the boat so it needs to flow quickly to the rear and get pumped/drained as quickly as possible, so I don't want to add exterior volume behind the engine, but additional interior floatation right there near the engine, keeping water over the center of the hull will keep rear weight down in case a large amount of water inboards, and in the case of a complete swamping will provide additional insurance for level floataion and possibly allow the boat to flush and correct itself (as my inflatable does, but way too slowly since the scupper is maybe only 2" in diameter). Beefy knees that are longer than they are wide will provide good support for the transom and that interior freeboard portion of the hull at the knees can be covered and foamed.
     
  14. fallguy
    Joined: Dec 2016
    Posts: 5,281
    Likes: 1,015, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    What two long air boxes in a 4' wide vee hull?

    Doesn't anyone have a software program where you can model the obvious issues with this boat?
     

  15. TANSL
    Joined: Sep 2011
    Posts: 6,819
    Likes: 501, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Any naval CAD software is capable of studying all the problems that this boat may have. Do you think it should be a special software, why, if so?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.