Behaviour of a stepped hull

Discussion in 'Boat Design' started by erik818, Nov 8, 2008.

  1. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    I’ve built a small boat with a stepped a hull. The boat is 5.7 m long (5.5 m at waterline) and 1.9 m wide (1.6 m at waterline). It was finnished enough for a few test trips before winter.

    The first 2 m of the bottom from the fore is V shaped with a deadrise of 17º and ends in a step. The attack angle is 5º when the boat is at the design attitude. After the step, the bottom is flat with an attack angle of 2º at the design attitude. The central 1 m is flat all the way from step to transom, but at the sides the bottom twists to a negative deadrise at the transom. The idea is to channel the air flow from the step for lubrication and to get some grip in the water.

    The step is ventilated with a 110 mm PVC pipe. So far I’ve not tested forced ventilation but hope for natural suction to do the job.

    The boat is powered with a used 40 hp outboard from 1976, with slightly damaged propeller and several peculiarities that I’m discovering one at a time.

    I estimate the weight of the empty hull to be approximately 300 kg, so with the outboard, fuel, driver and some small stuff the weight should be 500 kg. Maximum design gross weight is 1 ton.

    I choose to make a double skin hull (plywood/glass/epoxy) to get stiffness without bulkheads.

    The boat behaves rather well at speeds up to 20 kn, but at 25 kn in a small chop and no passengers the propeller tends to beat air every now and then. At 20 kn with 3 extra passengers I didn’t have that problem (my wife didn’t allow me to go faster with her on board.) The behavior above 20 kn is not a real problem as the intended cruising speed is 15 – 20 kn.

    A behaviour that annoys me and I would like comments on is that the boat doesn’t bank when turning. It stays horisontal and this feels awkward. Is there anything to be done about this?

    I include some pictures.

    Erik
     

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  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    I am not surprised that the boat does not bank in turns. I am also surprised that it does not trip and capsize in high speed turns if you have tried that. Flat bottoms aft do not tend toward banking inward on turns and usually slide a lot and don't turn well. Your chines do prevent sliding but also create the possibility of tripping and turning the boat over. I would think the pipe inlet is sufficient to provide air to the step.

    It's an interesting concept but I wonder how you arrived at the unique design elements in both forward and aft sections.
     
  3. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    I'm not sure if I make the step out correctly, but it looks like at some speed air gets fed under the hull in the form of bubbles from the venturi you create there.

    This is the same as when you're in the surf. The foam doesn't provide bite for the prop and the prop just spins but nothing much happens.

    What is the intent of the step and the pipe ?

    Final answer. It is a speed limit endorsed by the wife :D
     
  4. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Well, me neither understand fully why did you choose to make a stepped hull for 20 kts max speed...
     
  5. erik818
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    erik818 Senior Member

    The idea with the step and the pipe is to create a flow of air for lubricating the rest of the hull, where most of the lifting forces act. With the step I also get a lifting force in the forward section, which allows the CoG to be further forward. The planing surfaces are calculated for a CoG 2 m forward of the transom.

    I want the boat to travel well on plane also at low speed, which is the reason for the large flat bottom (low weight per square meter). The bottom profile at the rear is mostly for channeling the air flow from the step for lubrication, but also to provide better grip to prevent the boat from sliding sideways. Air lubrication is for better fuel economy.

    I don’t know if the features really are so unique. Stepped hulls are not new, and neither are inverted V bottoms nor flat bottoms. Air lubrication isn’t much tested, but I’m not the first one to try it. Using all features at once (and on aslow boat) might prove to not be optimal however.

    The aspects I have considered seem to work as I expect, though the positive effect of the air lubrication is not proven. The problem is all the things I did not consider.

    I didn’t try to turn rapidly at 25 kn, and I don’t think I’m going to. I’ll learn what I can from the boat and then find a safe way to handle it. If my purpose was only to get a boat I would just have bought one or built a proven design.

    I do appreciate help with understanding why the boat behaves as it does.

    Erik
     
  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    The prop is slipping, as Fanie pointed out, most probably because it is aerated by the flow from the pipe. You could try to resolve it by using a long shaft outboard, or by changing the position of the outboard, in order to place the prop out of the airstream.
    And the boat is turning flat because it was designed to turn flat - though inadvertently, as it seems. :)
     
  7. Fanie
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    Fanie Fanie

    I've seen steps on hulls before, but they were mostly near the transom to improve the steerability with a usually big outboard. I'm talking of about 300 / 400mm aft. PersonallyI don't like steps, they reduce booyancy and the motors usually 'hangs' off the transom. Some of the bass fishing guys like the image :D

    I don't think a long shaft outboard is going to cure it, the cavitation plate has to run on top of the water, long shaft short shaft the same.

    I'm sure you will see a slight performance increase if you block the pipe. Will improve even more if you can get the step out.

    Flat bottom boats does skid over the water sideways if you turn hard at speed, and it works, but only on flat water, a single wake even from your own boat may cause the hull to stick and you eiether get throwed out or the boat capsize. Neither is fun.

    You can add a few finns if possible in the aft 1m of the hull. They should be spaced around the hull rounding so at least one is always biting when the boat banks. Remember at speed water is a solid, if you turn the boat turns like a car.

    Good luck.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I wouldn't advice him to block the pipe completely. Some amount of ventilation is needed to prevent a vortex formation behind the step, which would create a big increase in drag.

    You could mount a valve at the air intake of the pipe, and play a bit with the valve aperture until you find the correct ammount of air which will not block the step and will not aerate the prop.
    Though it means you can say goodbye to the idea of air lubrication.
     
  9. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    When spring comes and it get’s warmer I’ll check the problem with aeration further with someone to help me in the boat. Alone in the boat it was not possible to hang over the transom and watch the water and propeller. I will also rule out the influence of the damaged propeller by mounting a new. Experimenting with the air flow will be interesting; I will also try a fan to increase the air flow to see if I can notice any effect of lubrication. At the end I’ll adjust the air flow for best performance at 15 – 20 kn, and it is unlikely that I will complicate things by using a fan other than for a test.

    I understand now that I have to live with the way the boat turns. It’s not a big deal; I don’t mind that the boat isn’t forgiving to every sort of mistreatment. Even if I had understood it before I built the boat I still would have built a flat bottom, as I prioritise the low speed planing performance.

    I still believe in the step far forward. According to my calculations the step should allow the CoG to be more forward than otherwise and also be more insensitive to a lengthwise shift. I’ve not yet seen anything that contradicts this, and taking the step out is not worth the effort anyway. When I use the Savitsky equations for calculating drag I get the result that a stepped hull, which I treat as two hulls in tandem, has lower drag than a normal hull. On the other hand, the effect on the rear part of the wake caused by the first part isn't considered. It would have been interesting to have the same boat, but without the step, for comparision.

    Erik
     
  10. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    I'd suggest changing out to a better prop, a nice stainless steel with double-cupping to handle the aerated water. Four-blade will prolly work better than a 3-blade prop, unless you get a really well-designed 3-blade. The extra cupping will give the prop more bite, or traction.

    I've seen small steps at the transom for smaller boats, with the idea being that the step would allow upwards flow of water behind the transom more quickly, thus allowing the motor to be bolted up higher onto the transom and gaining some shallow-water advantage.

    I've a flat bottom (w/ pocket tunnel) boat that slides in the turns. It is kinda like racing a snomobile on ice - once you figure out how she handles the turns, it is no problem and can be kinda fun.
     
  11. DaleG
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    DaleG New Member

    It amazes me at what I consider the continued incorrect usage of caviation, when I believe the correct term here is ventilation --- (caviation being when the water is boiled due to low pressure on a prop blade surface) -- a different phenomena then air at the hull boundary later interface is getting into the prop stream -- that to me is ventilation ---

    The "plate" on an outboard lower unit is more correctly called an "anti-ventilation" plate -- it prevents the air at the water surface from entering into the prop stream --- (whether due to the boat turning and leaning - or lower unit trimmed up -- or whatever) ----

    The plate is where it is becase a designer felt that was the best compromise between the lower unit drag and having the prop deep enough to get a good bite -- (and also the risks associated with a prop father below the boat and it hitting objects)

    In this case I would think the prop needs to be lower -- and out of the stream of bubbles -- the anti-ventilation plate would then probably be unneeded as the prop is now lower in the water and away from the water surface -- a longer shaft (and then removed plate) or a lowered engine postion on the transom might accomplish the desired effect -- and prevent "blow-out" which is a term used for ventilation of the prop.
     

  12. DriesLaas
    Joined: Aug 2009
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    DriesLaas Weekend Warrior

    Hi Erik,

    Did you ever go further with the development of this hull design?
     
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