Old design, modern power?

Discussion in 'Boat Design' started by graywolf, Aug 7, 2017.

  1. graywolf
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    graywolf Junior Member

    I was looking at some of the Atkins Seabright skiff designs. They all call for old time low power flathead engines.

    I was wondering what changes would have to be made to use modern high power engines in them? Would heavier engine stringers do the job? Or would the whole hull have to be built heavier?

    Since he says that the propeller runs partially in air, would one best use a modern surface piercing propeller? Or would an anti-cavitating propeller be the way to go?
     
  2. jorgepease
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    jorgepease Senior Member

    You would have to see if a high power engine would make any difference, some boats work best in slow and easy mode. As far as the prop, I run really shallow, prop up in the air type of thing, with aftermarket cupping on a regular Yamaha prop.
     
  3. PAR
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    PAR Yacht Designer/Builder

    If you over power a SeaBrite style of hull, the boat is just going to squat and nose up, but not go any faster. They do well within their operational range, but not outside of it. Simply put, if you can live with the high teens and low 20 knot speed ranges, these do well. If you want faster, you need to select a different hull form.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The attraction of those boats appears to be very low draft, if that isn't the priority, you'd not go there. Doesn't look that simple a build, either.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Nope, these puppies are traditional carvel builds, though a few are plywood, still over frames and floors. The attraction is shoal draft, low plane speeds and efficiency if not over driven. You can get a 20' boat to putter along in a few inches of water on a 8 HP inboard, just don't expect to go much faster than the high teens. The hull form hits a progressively larger drag wall if you try to push them further than their intended envelop.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    They belong to a time when outboards were still pretty small, sterndrives unheard of. I wonder if a small diesel would work.
     
  7. graywolf
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    graywolf Junior Member

    Well, they are beachable inboards, not just shallow draft.

    But, I am not sure we are talking about the same thing. The New Jersey Seabright Skiff was lapstake, not carvel. The NJ folks used to use them for rum running, with really high power engines. They were the same skiffs they used for fishing, with the banger replaced by a V12 aircraft engine. Run out to the 2-mile limit (yes, at one time it was only 2-miles) pick up a load of whiskey from a freighter and run like hell for home. Drive the boat up on the beach and pull a truck up next to it to unload. Those guys had it down to a science. I imagine, since a load or two would pay for the boat they were not too worried about it falling apart from the pounding.

    You have to see the underwater lines to understand the damn things, no real way of explaining them in words that I can think of. They looked like a traditional lapstrake sea skiff above the water but had a short pointed end flat bottom under water with the prop sticking straight out of the end of it, while the topside rolled down around the prop. Sort of like a tunnel stern, but quite different in design.

    Since they would do that 16-18 mph with only 40-45 horses they were quite efficient runners. Yes, I think the squatted going over the hump and needed a lot of horsepower for that, but then they would go like hell. I also think that they did not have modern specialized props back in those days and a surface piercing propeller would help keep the stern from being sucked down by air trapped above the prop. Of course, that may be wishful thinking on my part.
     
  8. Rurudyne
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    Rurudyne Senior Member

    Yep, they are actually fairly fast boats.

    Old powerboat designs sometimes seem to fall into two camps: those for engines ordinary people could actually obtain and those for engines out of aircraft. They did remarkably well with little power, could even could ski behind some.

    Now, what are your thoughts on stretching this sort of hull to help boost the speed at low power aspects?
     
  9. PAR
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    PAR Yacht Designer/Builder

    [​IMG]
    This is a typical 17' SeaBrite. The basic advantage was the lower engine/shaft placement. The reversed V and its related "hook" dramatically limit top speed potential and no they weren't all lapstrake. The Jersey versions tended toward lapstrake, but this hull form has been built in just about every build method. The one shown is carvel.

    Forward, they have a typical V bottom double wedge style of entry, aft the inverted V and box keel deadwood. Yes, they were overpowered occasionally, but they were not particularly fast and if overpowered they'd drag a huge wall of water behind them and become very unstable. The only way to make these puppies go "fast" is to reshape the hook, which is speed specific, so you need a target and some experence with the hull form. I own an Atkins box keel design (37', 1.5" over oak, carvel) and it's efficient, but purely displacement mode. I can run at about a 1.5 S/L ratio on a 70HP diesel.

    Stretching one is the way to make them faster, but again, the hook is the limiting factor and you'll run into a drag wall anyway, compaired to other more conventional shapes. The above boat is designed to operate in the high teens, low 20 MPH range. At 20 MPH, she'll have considerable bow rise, but you can get there with 30HP.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I rather like the look of this one, "Everhope". If a diesel inboard of enough power, and not too much weight, can be slotted in to what is a narrowish, semi-displacement boat, you'd have a very economical boat with a nice turn of speed, and good efficiency at troll speeds as well. I just wonder how the prop can be kept in the water when pitching.
    Everhope-2.gif
     
  11. jorgepease
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    jorgepease Senior Member

  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously there are many variations on the theme , and there may be a niche for them, after a long period of dormancy. But they are certainly not boats for big power, at all, the attraction would be shallow draft, handy speed and good mpg in a range where other boats are not at all fuel efficient, and reasonable ride comfort. Offshore, I don't know, the shallow running prop seems a possible problem. What the running angle looks like at speed, would factor in to that.
     
  13. philSweet
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    philSweet Senior Member

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    The potential is with the diesel, back in the day when these things were designed, it would not have been an option. I can see a decent boat would be possible with excellent fuel economy, combined with the ability to enter shallows. And not be a slow-coach.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not grasping how the hook goes hand-in-hand with bow rise ? o_O
     
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