"V" bottom runabout with rounded sections

Discussion in 'Boat Design' started by Wayne Grabow, Aug 29, 2017.

  1. Wayne Grabow
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    Wayne Grabow Senior Member

    An acquaintance, who restores boats for a living and has designed some of his own, tells me that in the design of a new runabout I should put some convex "bow" in the aft frames so that the two halves of the "V" bottom are not flat surfaces but slightly convex. He says that even 5/8" bow/camber will make a difference. He says it will have a softer motion and will make an 18' boat feel like a 22' boat. I can understand that the changed shape may have less tendency to "slap" against waves. A small amount of convexity should be easy to incorporate. The design I am considering would be about 18', hard chine, with speeds of 35-38 mph. Anybody else have experience with adding some convexity to the frames to improve performance?
     
  2. jorgepease
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    jorgepease Senior Member

    I don't see 5/8 being much improvement in pounding but it will be stronger.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Agreed, 5/8" of bow along the typical panel size found on an 18' boat wouldn't be much of a help. Convex entries make for softer rides in a chop, while on the planing patch additional deadrise is more beneficial, with a well rounded bilge turn. Can you post some lines so we can see the hull form?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt convex sections help ride quality, in fact I think they are generally inferior to straight vee sections. But there is a good case, imo, for working convexity into the aft sections, to improve other characteristics, though. It is just a matter of how, a reduction in deadrise toward the centreline aft has advantages, as that is the least likely part of the boat to be leaving the water, and slamming.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Convexity does improve slamming and impact characteristics, of course depending on how much, where it's located and other hull form considerations. Flattening out the centerline aft, commonly in the form of a pad doesn't effect slamming and impact, but does improve the speed in which the boat can achieve full plane mode and reduce wetted area at higher speeds, as the boat climbs up. There's a point of diminishing returns in this regard, meaning longitudinal and athwart stability issues will rise up and cause issues. In fact convexity, particularly in the forward sections is one reason the warped bottom hull (typical 1950's proportions) fell in favor of the monohedron hull forms we see today.
     
  6. HJS
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    HJS Member

  7. Wayne Grabow
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    Wayne Grabow Senior Member

    Thanks for all the comments. Better to have a variety of input than depend on one anecdotal suggestion. I can better visualize the influence of such a feature. The design I am considering (sorry, no good pictures at this point) has a convex bottom for the first 7-8' from the bow then becomes a straight 14 degree deadrise "V" for the aft 10' of the bottom with chine flats. I plan to sheath the bottom with planking in a herringbone pattern which should easily accept some convexity in the aft run. My friend said that even 5/8" camber would be enough to make some difference, but, of course, I can increase that number.
     
  8. messabout
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    messabout Senior Member

    When you introduce a bit of camber in a beam, It tends to shift the characteristics of the load from bending mode toward compression mode. Wood has more force resisting capacity in compression than it does in bending. Think of a bottom section as a bunch of beams lying side by side. When a cambered part is exposed to to an impact,( slamming), it does not flex as much as a flat part would, and so there is less vibration and resulting sound at that site. Whether the ride is softened or not, the perception is that it is softened by the cambered sections. Which reminds me of an occasionally stated philosophical position: Perception is reality.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Slightly rounded sectional shapes don't help much, but significant rounding does, if only by offering any impact load a larger surface to dissipate it. Curved surfaces are inherently stiffer and stronger and in this application, also delay impact loads slightly, of course (again) depending on camber amount. Observationally, flat and concave sections, which were once pretty common in hull forms decidedly slammed harder, than similarly shaped hull forms with convex sectional shapes.

    I completely disagree in that reality "is" perception . . .
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Most of the best riding glass hulls. are basically straight-sectioned throughout. I may have mentioned before on this site, building two models of hard-chine planing boats (20* deadrise at transom) The difference was one had straight sections, the other was a developable bottom, with the convexity forward that comes with that. Topsides both developable, the chine and centreline/keel were in the same position, These models were around 4 feet long, and towed behind a boat on slightly choppy water, it was possible to compare their behaviour. The developable bottom jumped and bucked appreciably more than the other, enough to convince me there is quite a difference. OK, just scale models, but I suspect still indicative. I believe this is the reason that people find the plate alloy hulls so popular in this part of the world, an inferior ride compared to the "old school" glass hulls, despite there being little difference in hull weight.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    "Sonny" Levi incorporated convex sections into his double-diagonal planked planing hull designs, on the basis it gave a stiffer structure, that is probably where the real advantage lies in it.
     
  12. messabout
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    messabout Senior Member

    Par, I quoted that old saw....Perception is reality, not the other way around. That is purely a psychological concept. It is pretty reliable. But let us not bother here with religion, politics or psychology.

    One day I would like to come to Eustis.....with a supply of beer of course. There we can argue the merits of boats, and the perceived merits of whatever else. may be at hand.

    Cheers
     

  13. PAR
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    PAR Yacht Designer/Builder

    I'd be glad to have you by and my comments were intended as tongue and cheek, though I'll let you decide which set of cheeks I was referring too ;). You're only in Lakeland, so . . .
     
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