Firefly Stepped hull vacuum under step

Discussion in 'Hydrodynamics and Aerodynamics' started by Avanti, Aug 20, 2019.

  1. Avanti
    Joined: Aug 2019
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    Location: Melbourne, Australia

    Avanti New Member

    Hi All,

    As you may tell I've just signed up to this forum with little knowledge on boat dynamics with the intent on learning as much as I can so please bear with me!

    I built and completed the 15' Firefly runabout powered by a 1978 Johnson 2 cyl 50hp about 3-4 years ago from the plans I found online for free which were dated 1948. This is my first build and as a carpenter by trade I've learned a lot about how timber can be manipulated even further, but it wasn't until I completed the boat and the itch to get the most out of it has made me realize the kind of boat I had completed, the concept behind it and the importance of the level of detail that should be undertaken.

    My "problem" is I believe there is a vacuum being created at the step as it isn't ventilated very well, as you can see in the photo's the chine is solid with no point coming above the waterline, you'll also notice the terrible paint job which I believe to also play a part in this (this was a steep learning curve for me where I learned rolling with a nap is a terrible idea, tipping is a 2 person job and aerodynamics and hydrodynamics are two very different things)

    Would I be able to fix/vastly improve this issue with redoing the paint?
    Should I add lift strakes to the front and/or rear?
    Would creating ventilation through the step be a worth while idea?

    I'll also be re-painting the boat within the coming months anyway as i'm sick of the terrible job I did but I will post the performance results so we can see how 'golf ball effect' paint work affects the top speed and any thing else I happen to find

    Top speed was 29.4 knots (33mph) with 2 people on board measured with GPS

    Any and all help is greatly appreciated!
    Thanks in advance
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Pretty good speed for an old motor. I doubt anything will give you noticeably more. Rough paintwork likely won't slow it as much as you imagine, and as for "suction" at the step, that also probably won't be as great a handicap as suspected, either. Not with that engine, anyhow.
     
  3. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Since the step disappears as you go to the chines, it isn't going to let any air get in and ventilate the step if those chines are wet. We can't see where the step is relative to the side view, but if the hull were riding so that some of the step was above water it might work. You have to ventilate the step for it to work. They don't build them like that anymore because it didn't work very well. This is obviously an old design and it's got a lot of design "clich├ęs" that were thought to be good ideas at the time. the step and the extension behind the transom are just two of them. In those days outboard power was very limited, 20 hp was a big motor at the time. Adding area aft of the transom was necessary to add lift, but with 50 hp you don't need that at all, it's just adding drag. Strakes ahead of the step would help IF you get to the point where the area outboard of the strakes is above the surface. Otherwise there's no benefit. I would think about seeing if you could put a couple of holes in the step and put in a couple of shop vac hoses to an area above the waterline inside the boat. That would let the step ventilate and probably help it a lot. If you've got it upside down adding a strake on each side ahead of the step won't hurt. One thing you could do is make the strakes out of metal and make the hollow so that the leading edge is out of the water and leave that open so air gets in and would bring air into the step. That is easier than adding tubes since you wouldn't have to seal the tubes into the boat.
     
  4. Avanti
    Joined: Aug 2019
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    Location: Melbourne, Australia

    Avanti New Member

    Thanks for your responses guys! A hollow strake is actually a really good idea as air would be forced into it also, but how would I calculate how big to make them and where to put them? I've been doing some substantial search but haven't had much luck! Any info on this would be absolutely fantastic :)
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Do you notice any cyclical drop off/increase in speed as the boat bounces along ? If not, I'd be surprised that little step is making much difference. Was there no provision in the boat plans to ventilate the step ? Surely they would, if it was going to improve performance to any worthwhile degree.
     
  6. Avanti
    Joined: Aug 2019
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    Location: Melbourne, Australia

    Avanti New Member

    No there wasn't, what you see is what was on the plans, they also state using a 50hp 4 cyl which is "conservativly rated" according to the plans. I do notice a drop off yeah
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Wouldn't take much to ventilate the step, as mentioned by yellowjacket.
     

  8. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    The amount of area needed to ventilate the step is determined by the speed you are going and the height of the step. A short step like this doesn't take as much air as s bigger step. What adding a vent will do is break the "suction" of the water on the step. If you vent the step with hoses you will be able to cover a hose with your hand and feel whether or not it has much suction on it. What happens with an unvented step is that you get drag from the step and also negative lift from the suction, and the flow reattaches sooner so there is more drag from the surface. That's why a step that isn't working is a lot worse than no step at all. Strakes to vent the step would probably need to be pretty big relative to what we usually see as strakes. You would need a couple of square inches each to do much good.
     
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