Pocket cruisers

Discussion in 'Powerboats' started by Guillermo, May 13, 2006.

  1. Easy Rider
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    Easy Rider Senior Member

    Don't mean to say your'e wrong even though you said I am but on a flat bottomed boat what you see is the rocker. Nothing more nothing less however the beam along the hull length may get involved in some way. The buttock lines are all the same.

    I have the assumptions about the performance characteristics of the three basic types of rocker that I posted in post #359. If you know a great deal about rocker please comment.
  2. PAR
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    PAR Yacht Designer/Builder

    Rocker can get to be complex subject. Cindy Lou (Egress) does seem to have a little more volume forward in her rocker than typical, but just viewed in static profile. This is because her entry is relatively fine, so I need to drag some volume forward to get her to trim up where I wanted. It's also a bit of an optical illusion. Here stem is basically plumb, so the LWL starts literally just a few inches from the forward most perpendicular, yet the exit is about 3' from the transom. The rise of the buttock in this area is shallow, giving the impression the LWL is closer to the transom, which visually makes the rocker look a bit forward.

    Underway, the stern will settle and the bow will rise, particularly as she approaches about 7 MPH. At her theoretical hull speed (8 MPH @ 1.35 S/L), her transom should be just clear with a half load and just touching the water with a full load, unless everyone is standing in the cockpit (except the skipper driving of course). The intersection of the bottom and topside planks at the bow will also be just clear of the water. In this attitude, she'll push past her theoretical hull speed limit a little, because of her fine entry and shallow buttock angles. I suspect this hull will easily reach S/L 1.5, likely through 1.6, before the wave train and drag, truly limits any more speed, without huge amounts of power. With the modest outboard power spec'd for this boat, the speeds she'll achieve, are quite respectable and fuel use will be reasonable. These are the complex set of choices made on this particular hull and you get what you get.

    I could have fattened up the entry in plan view, which would have permitted me to make the rocker shallower in the forefoot, but this also means I would have had steeper aft buttocks as a result. It should be noted that Egress (Cindy Lou) does have a fair bit of dry bearing area just forward of the transom, in a static state, that comes to play as she squats underway. This helps keep her trimmed and offers a clean exit as she leaves a pure displacement mode and transitions into low semi displacement speeds. The deepest portion of the rock "belly" is about 18" farther aft, compared to it's static location.

    To directly answer your questions Eric, having a significant amount of the rocker's belly aft will cause a great deal of turbulence, right where you'd prefer to keep flow "attached". Ideally, you want to part the water as easily as practical, both form the bottom and sides, then let it "reassemble" as neatly as possible as the boat leaves it behind. If you take Cindy Lou's profile and cock the boat with a 2 degree bow up trim, approximating the rotation around midship(ish), then look at the resulting rocker, you'll see it look much like what your use to seeing. To take it to a higher plane, the increase in forefoot slope, in this attitude, more closely matches the entry 1/2 angles, which helps even out the pressure waves along the entry and trailing aft, so eddy making is reduced to a large degree, especially if the chines are heavily radiused. I didn't bother to run this hull form up over 1.6 S/L ratio, just because I knew what would happen, how much power it would need and the resulting uncomfortable bow up trim she'd have to contend with. In order for her to adopt a 3 - 4 degree angle of incidence for full plane mode, her negative bottom/chine trim aft, would cause her to bow up several degrees (apparent trim), which just isn't practical, though possible with enough power and a skipper in a seat belted chair. Her bow up that high, would cause some aerodynamic issues though.

    In the end, when playing with rocker there are a number of things you look at, including pressure waves forming at speed, wave train generation, required volume distributions compared to desired distribution, entry angles, desired Cp and other hydrostatic and dynamic considerations, etc.

    There's no best arrangement, with each being "a convoluted collection of discontinuous compromise", made in an effort to satisfy the damn SOR restraints, as best as you can. Mostly you just cuss and pull your hair a lot, as you live with the results.
  3. tom28571
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    tom28571 Senior Member


    I'm not sure what you are referring to since my comments were not pertinent to flat bottom boats, which I did not mention. You are correct that the profile of a flat bottom boat does define the rocker well but it says very little about the action of the boat in a seaway. For that, you need to know the hull shape and all the physical particulars that affect balance.
  4. Easy Rider
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    Easy Rider Senior Member

    Thanks ever so much for the With the rocker analysis PAR. I actually got a lot out of that. I love your expression "reassemble" re the water leaving the stern. I've tried for some time to think of a word appropriate for that.

    I'm thinking Cindy Lou's higher speeds will be blessed w a lower angle of incidence, reduced drag and a smoother ride. One could go so far as saying your Cindy Lou has to a significant degree a fish form hull. Perhaps there's a relationship between rocker fwd, the fish form hull, fwd biased cg and a more stable angle of incidence and much less tendency to porpoise.

    I have a rocker or perhaps BL question about the boat "Ketewomoke", an Atkin design. It looks like nearly a pure displacement hull but 16 mph is claimed from quite modest power (24-45hp). Does anybody here think she could make that speed w a reasonable angle of incidence and drag appropriate to her power level? I feel that if she had constant deadrise aft she'd be much more suited to that speed .. and that's just mph not knots. The added volume aft would then make her suitable for OB power.

  5. PAR
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    PAR Yacht Designer/Builder

    The key to Ketewomoke is her chine, which is neural, so she can take some "set" and provide a "washed out" planing surface. This washout does reduce her speed potential, but also permits lower then semi plane speeds, with some level of efficiency and the ability to get up on plane pretty quickly.

    I seriously doubt 16 MPH is possible with 45 HP, unless the skeg is yanked off her and she's built especially light. 16 MPH is up on full plane and I suspect she'll need all of 65 - 85 HP to see that speed (S/L 2.9). This said, I do see her running at 12 MPH (2.2 S/L) with modest power and if she did have say 100 HP, you could push this puppy into the low 20's, once over the hump. I just don't think 45 HP is enough, considering the build style typical of Atkins, but 65 HP up to about 150 HP and she'll act like most second generation warped bottoms. Drag on this hull form, will max her out in the low to mid 30's, but if the target is well south of this, say 25 MPH, you could do it with a 4 cylinder. I'll bet her displacement is in the 2.5 - 3 ton range, so with 10% slip, the drag from her skeg and hanging gear, yep, I don't think I'm far off.

    Cindy Lou will have a nice ride and she'll be efficient if not over loaded or pushed too hard. Her wake will be modest, though I expect her angle of incidence will be 3 degrees, with the hammer down. I wasn't thinking about the "Cod's Head" approach to hull forms with her and as the British sorely found, in the early America's Cups, it's not the best avenue to efficiency. She'll never get near the longitudinal instability speeds, typical with porpoising and her CG is about where you'd normally find it on a displacement hull. Then again, she could be all wrong, will flop over on her side come launch day and when this issue is fixed, she'll putter along at 3 MPH at WOT on a 50 HP outboard.
  6. El_Guero

    El_Guero Previous Member

    Uh-oh ....

    All that work? Well, I, for one, hope Cindy Lou doesn't under perform.
  7. Easy Rider
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    Easy Rider Senior Member

    PAR thank you very much for post #365. And if you're up to more Atkin talk compare the 26' Tang to Ketewomoke.

    Sounds like you've taken PAR literally. No chance of her being a flop on launch day and I suspect the 50hp OB won't be necessary.

    PAR said this about Cindy Lou "because of her fine entry and shallow buttock angles" ... Chine wise I don't see a fine entry so I assume he's including the bottom in the equation. An if so .. rightfully so as the bottom is very much a part of it. Many Atkins boats have no rocker at all fwd and have (chine wise) a very narrow entry. The bottom should stay low and start pounding later but there should be lots of chine turbulence.

    Then there are the "flat iron" skiffs w no rocker at all. Flat on the bottom totally .. both transversely and longitudinally. Must be very level running and for a flat bottomed boat pound the least if it has a long bottom but very inefficient at slow speeds unless it has a very narrow stern. Eventually that thought will of course lead to a Banks Dory.
  8. El_Guero

    El_Guero Previous Member

  9. huibes
    Joined: Mar 2010
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    huibes Sittin' on the dock of the bay

    Ive read this thread with lots of interest and got many ideas from it thanks!

    Besides the discussed marex boats, there are other suitable scandinavian boats that cruise quite economical around 13 knots:

    15 kn - 48 hp
    sollux 760 or 24

    14 kn - 57 hp
    bonum 800 (albin 25)

    11 kn - 50 hp
    tresfjord 26 or 28

    13 kn - 55 hp
    master 740

    But after getting the ideas below from this thread, I havent made up my mind yet...






    Great to see nice boat designs are coming back again!
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