Designing a 360cm(11ft) powerboat hull, suggestions?

Discussion in 'Hydrodynamics and Aerodynamics' started by Glattnos, Jul 14, 2018.

  1. Glattnos
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    Glattnos Junior Member

    I am planning to build a small powerboat as a hobby-project but would like some advice on the hullshape.
    It will be approx.
    360cm
    (11ft)
    long
    95cm(3ft) wide
    displacement of max 310kg(683lb)

    It will have a 9.8hp four-stoke engine and a cruising-speed(about 60% throttle) of about 14 knots I aim for.
    The boat should handle a bit waves like costal conditions on good weather days and archipelago even in windy weather conditions. But fule-economy and speed should be good on flat water.
    The hull-shape looks like this right now:
    Hullshape1.JPG
    (this is just the shape of the lower part of the hull, the interesting part)
    I will also add strakes to it but concentrate now on the overall shape of the hull bottom.
    It have a deadrise of 12 degrees at the transom and 20 degrees midship.

    I would like to have suggestions on this design :)
    Some boats have a "pad" in the rear part to create more lift, is that a good idea on this size/speed boat?
    Is the deadrise to much in the midship or in general?

    Edit: Is there any free/cheap software to test different hull designs without being to complex?
     
  2. messabout
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    messabout Senior Member

    I suspect that 95cm wide will cause the boat to be rather tender. With a 9.8 four stroke the weight of the engine will be a bit much for that width. The engine is likely to weigh 40 to 45 kilos, depending on whether it is electric start. You need plenty of aft bearing to support the engine weight without pushing the transom down too far.

    Best to re- think your dimensions. Something like 450 cm length and 125 cm width would be safer and more comfortable. The engine would easily propel the larger boat at the speed that you have mentioned and probably more than 20 knots at full throttle.
     
  3. Glattnos
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    Glattnos Junior Member

    Thanks for your thought :)

    I already have the engine and it is 37kg and I use it on a 380cm x 120cm boat right now, it cruise at 11-12 knots but that boat is not really optimised for that speed.
    The weight distribution on the new boat is not a problem I think because I have the CAD model now with the C.G. in a good spot. It is a covered and self-righting design. The width of the boat is something you can be correct about, it may be a bit narrow but if I add more width I think the midship will need even more deadrise to not give to much buoyancy that will make it jump more on waves. 310kg is the max displacement so the boat should work even with like 60-80kg less.
    I may consider increasing the dimensions a bit but not much, it should be bit smaller than my current boat.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Boat is to be constructed of what material ? Your drawing shape is not something that could be easily made, even it were desirable to do so, which I very much doubt, as you have too much warpage between the stern and amidships, and a forefoot that seems certain to want to steer the boat.
     
  5. Glattnos
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    Glattnos Junior Member

    I cut out core foam with CNC and sandwish it with epoxy and fibreglass so the shape is no problem. As stated, the picture is just the outer shape of the lower part of the hull, the deck and inside of the boat have more complicated shape.

    So you mean that there is to much deadrise(to sharp V) midship? And what do you mean with “forefoot that want to steer the boat?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, the shape certainly would be achievable with foam core, I was thinking perhaps you had the idea of using sheet plywood, for example. Your boat would be quite "tippy", as drawn. At max 300 kg, I am doubtful you would get much speed out of it at all, with just 10 hp. The bow is quite deep and slender, which in combination with the rest of the boat could easily lead to broaching tendencies. Even a 5 degree change from transom to mid-body is getting close to too much. I'd be more inclined to a vee-nose punt shape, for better stability, and more space. With at least 1.2m beam, you might bump in a little chop, but the seaworthiness is much better.
     
  7. Glattnos
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    Glattnos Junior Member

    Thanks for your thoughts :)

    I think you are right, before I had the boat drawn more flat in the bottom but was adding more and more V angle with time so I may have exceeded the sweet spot long time ago. I was backing off a bit now.
    Deadrise transom 10 degrees
    Deadrise midship 14 degrees
    Width 110cm(3ft 7in)
    I also moved the "midship" or how to say, a bit forward so the nose is "shorter" and the boat a bit more "fat".
    Hulldesign2.JPG
    I think that may be closer to a good shape.
    I was thinking if I should tune the design more towards PWCs (jetskis) hull-design? I was thinking about that in the beginning because the size of the craft is similar to what I will build, but when studying PWCs I was thinking that they are a bit to flat and when they are passing outside my hose(I live next to the water) they jump and slam quite a lot(with a passive driver). The thing is, they drive much faster then my design will do so I think now that a PWC in 14 knots maybe drives quite fine in choppy water. Strange enough I never tried a PWC but maybe someone here have some input?

    I have a competitive background with RC-boats, 140 cm 4-5hp engines. Those boats drives fine i choppy water even without much V angle difference between transom and midship so maybe that is the way to go even for a 360cm boat.

    Edit: PWCs have about 16-24 degrees deadrise so it is not fair to say that they are "flat" but most models have very small change in deadrise over its length. This last design I posted is very flat compared to a PWC.
     
  8. Glattnos
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    Glattnos Junior Member

    Another version with a pad
    Hulldesign3.JPG
    and a crazy idea
    Hulldesign4.JPG
     
  9. Yellowjacket
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    Yellowjacket Senior Member

    Your narrow beam does make it easier to self right, but a hull that narrow with that little power and that much weight is going to have a hard time making 14 knots at 6 horsepower (60% of the 9.8 rated hp).

    I have a vision problem with that... I just don't see that happening...

    You're likely going to need more beam and planing surface area to keep it from just mushing along, and if you get the weight forward it might work but I'm no sure you can get there with that little power. Have you run a Savitsky analysis on it to see where the system optimizes? You really need to do that first and set beam, length and CG parameters and then get a realistic estimate of the performance our you're just kidding yourself.

    Pads are great for improving efficiency at speed, but at the low speed you're traveling, it isn't going to improve the lift much. If you just calculated an effective deadrise based on the outer chine position relative to the centerline you'll see that since the entire beam is always immersed at the speeds you're traveling the pad only changes the average deadrise a small amount.

    I expect that to plane that weight effectively with that small hp you're going to need more beam and zero deadrise just to get any speed out of it at all. I'm going back to my background with small boats as a kid and with 6 hp and that much weight in a john boat with a 3 foot planing surface width, we'd be lucky to plane all at 600 lbs. With a 10 we could plane, but to get any reasonable speed we'd be near wide open throttle to make 15 to 20 mph...
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    360 cm is nearer 12 feet than 11, I think it feasible that a 9.9 4 stroke might propel a 12' x 4' boat with an all-up weight of 300 kg to mid-teens, mph, cruise, but not at 60% throttle. It will need a fairly flat bottom to both plane effectively, and also to be a steady platform, in a situation where the weight of one person, is a significant % of the total. Being relatively flat aft is virtually mandatory, to plane easily, and having full waterlines forward gives better stability, if you intend to take the thing into any kind of ruffled water, that extra margin is well worth having.
     
  11. Glattnos
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    Glattnos Junior Member

    Thanks for your replies, I will make some more calculations and try out a bit wider/flatter designes :)

    I should say that 310kg is the maximum weight that includes two persons and 20kg extra. The hull is calculated to have a weight of 91kg, then add to that:
    Motor 37kg
    Me 70kg
    Fule 10kg
    Some extras 10kg

    Makes a total of 218kg when I am alone and thats probably 80% of the time I will use the boat. So to be correct 220-230kg is the displacement where I want 14 knots cruising-speed. If another person will join we need to add like 80kg and land on 300-310kg(the passenger seat is located exactly on the C.G so it will not affect the C.G much) and if the boat can still cruise at like 10-11 knots at maybe 70-80% throttle it could be fine.

    That said, I am aware that more beam can make the boat more fule-efficient and faster even if it is 218kg. Seams like beam is what matters in those slow planing speeds because, as Yeallowjacket pointed out, the boat will not likely climb up so high that the beam of the wet surface will be much reduced in such slow speed as 14 knots. However, a pad in the middle is not just like the same as decreasing the deadrise. A pad takes away area that is angled(and direct water sideways) and add area that are flat(and direct water downward), even a submerged pad will increase lift.
    I am not sure but it also feels like a pad would create less turbulence and higher pressure on the water at the propeller, witch will increase the propeller-efficiency.

    I may increase the beam to 120cm, then it have the same beam as my current boat and that one planes very easy with the 9.8hp, goes up on like 50% throttle and stay on plane with like 25-30% throttle.
    It is such: RYDS 380 - Fakta, bilder, prisstatistik, annonser, mm https://www.sokbat.se/Modell/ryds/380 but mine is heavier because it was once blown onto the rocks and was sucking water into the composite for a period. The beam is also stated 153cm but that is on the widest part, the bottom chines are on 120cm.

    There are many different hull-features on both boats and PWCs, what is beneficial in 14 knots on a small boat? Is it to slow to do anything else then a low deadrise V-hull?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously, 10 hp and any kind of break-neck speed are mutually exclusive, but you should be able to maintain mid-teens mph cruise, which is a respectable rate of progress, in a 12 footer, provided you keep weight under control, and have a little more beam, and less vee, than you have been considering. Here's a local design that is similar to your size/power, I'd be nervous of going far in anything this size, but it is about all that a 9.9 will propel comfortably on plane. Looks like a 15 hp motor on this one, to my eye the transom deadrise is not quite enough, I'd take it a little deeper, but that would throw a spanner in the works with the plans. I do like the full bow shape. I reckon the front half is spot-on, should be no nose-diving into the back of waves, or excessive spray production, but maybe 2-3 degrees more deadrise at the transom could have been allowed. But keeping it less tippy, at the light weight, has been a priority. I think it is just 7 degrees.
    Magnum 12 https://bowdidgemarinedesigns.com/magnum-12/
     
    Last edited: Jul 16, 2018
  13. Glattnos
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    Glattnos Junior Member

    Thats a nice boat :) Seams to carry quite resonable load, two adults, two kids and a 50kg engine. Seems like flat and wide is the way to go :)
     
  14. Glattnos
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    Glattnos Junior Member

    I can not find a reason to not have a pad on the boat, it will take away buoyancy from the low middle resulting in increased stability at the same time as it increase the hydrodynamic lift. I think the extra pounding is not a problem when the boat anyway is so flat.
    13 degrees deadrise and a 260mm pad have the chines on the same place as a 10 degrees deadrise without pad.
    Hulldesign5.JPG
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    At shallow V angles the area outside the pad has higher V angle than pad area. If you are doing a Savitsky analysis you'll be very close by just assuming that you have a shallower v based on the depth at the center of the hull and the total rise to the chine. The flat area in the middle is indeed more efficient at producing lift than the v would be, but the problem is that as you go out to the chine the angle is higher and the angle at the chine is really important for creating efficient lift when running immersed. If you were going faster, the chines get unloaded (are running shallower), as the boat lifts up but at low speed the entire bottom is immersed and edge effects are more important. Another thing you could do is put a one inch wide reverse chine to make the hull more efficient, but I'd probably just do a flat bottom and be done with it. With a flat bottom it'll be a lot more efficient at high weights.

    While the pressure under the hull is higher as the hull passes over it, since water is incompressible it doesn't make any difference to the prop efficiency.

    Most PWC's are a lot faster, have higher deadrise and more weight and power and a much further forward cg. You don't want to look at them for inspiration. Look at small john boats, skiffs or small aluminum boats that are intended for low power and you're going to see that most of them are basically have flat bottoms. The speeds you're talking about are so low that you aren't going to see much pounding even if the bottom was entirely flat and turned up like a john boat. So long as you have a decent narrow bow and deadrise forward it will be as good as it gets.

    You need to get dingo tweedy's Savitsky spreadsheet and run it with the weights and cg's that you intend to run and see what you have. Until you do that you're just shooting in the dark.
     
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