Last step on a stepped hull question

Discussion in 'Boat Design' started by jiggerpro, Sep 27, 2007.

  1. jiggerpro
    Joined: Sep 2007
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    jiggerpro Senior Member

    Being a person that always think about the whys and why nots, I wonder about the justification for the rearmost step that most builders of large center consoles are so prone to introduce in their designs for example Invincible boats or Yellowfin just to to name my favourites.

    Some builders even use this feature in non stepped hulls ...

    It seems to me that this last step aerates the water just where the props should bite solid undisturbed water but the feature is so widely adopted that there must be some advantage to it can any charitative soul throw some light about this apparent contradiction ??

    thanks in advance from the Canary Islands (Spain) Jose
     
  2. Gilbert
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    Gilbert Senior Member

    I'm not familiar with the boats you are mentioning but I will assume (perhaps wrongly) that you are reffering to stepped hulls whose steps are more like ventilated slots than what most of us would call a stepped hull. The idea of these is mostly to introduce air under the planning surface to reduce skin friction at higher speeds. As you probably know the resistance caused by skin friction increases exponentially as speed increases.
    So I believe the whole Idea is get as much air introduced to reduce drag as much as possible. Air has a very strong tendency to float on water, and so will not get very far down in the water, at least most of it. So since production boats are using these features I believe it is safe to say they are not causing serious problems or else they would not do it.
    It would be nice to hear from someone who has actually done research about what you are asking. But, if nothing else my post will put you up at the top of the list again for a little while.
     
  3. tom28571
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    tom28571 Senior Member

    I think what he is referring to is not really a step in the normal sense of the term. Lots of boats are now using this feature. It looks kind of like a pod that is part of the hull. I don't know much about these and don't have a high opinion of pods anyway.

    In the way of introducing air under the hull, many experts say that this is not advantageous but actually increases drag. I have not actually seen data that offers proof one way or the other. My thought is that laminar flow solid water is best for minimum drag, better than turbulent water or water mixed with air.
     
  4. jiggerpro
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    jiggerpro Senior Member

    Hello Gilbert, thank you for your time and effort, but what I am asking about is coincidental with what Tom28571 points, what I am asking about is this sort of step to call it in some way which, appers to be there just to to rise the waterline near the propellers, used even in non stepped hulls (to my surprise) if steps do aerate water this design feature should do the same but it will be aerating the water that propellers should bite ???¿¿¿ somethings clearly not logical, but there in many outboard powered boats , in relation to gilbert ´s comment: " So since production boats are using these features I believe it is safe to say they are not causing serious problems or else they would not do it." I have serious doubts since I have seen many manufacturers doing things that are only marketing gimmicks just to give a product a more technically sophisticated aura which should raise sales the feature being effective or not. The tipe of boat that most commonly has them are sportfishing center consoles.
     
  5. RatliffFranklin

    RatliffFranklin Previous Member

    Stepped hulls

    Aren't a lot of the big offshore tunnel hulls surface drives anyway?

    Even almost all classic three-point hydroplanes are also prop-riders rather than using a submerged prop.
     
  6. Gilbert
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    Gilbert Senior Member

    You also may be referring to a tunnel or cutout such as I have heard are used on flatsboats or others to operate the outboard motors on jackplates in very shallow water. I have no experience with these or motors mounted on pods or transom extensions. But I have heard from people I consider reliable that these setups can work very well, sometimes even better than a motor on the transom.
    So we are operating with some conjecture about just what we are discussing.
    Some pictures or sketches might straighten some of this out.
     
  7. tom28571
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    tom28571 Senior Member

    Just google yellowfin boats to see what he is talking about. Not a tunnel, not surface drives or anything like that.
     
  8. jiggerpro
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    jiggerpro Senior Member

    I never thought my english was so poor, It seems umbelivable that I am not being understood, the last step I am talking about is adecuately named it looks like a step but is used even in non stepped hulls please check for example the bahamas 31 boat in this link: http://www.bahama31.com/buildingabahama31.html

    please observe those pictures ( of stepped hulls) where you can see the stern section try to look under the hull you will find a sort of step relatively close to the transom, and which usually has the trim tabs inside it.

    Many other stepped hulls also have this last step and so have the two steps plus this so called by me last step which seems ilogicall to use and have unless we want to aerate the water that the propellors should bite which is what appers ilogicall, not to mention that this last step reduces flotation just where the heaviest area part of the boat is, which is the motor area. which off course needs some extra flotation volume.

    This is why for me remains a mystery the reasoning that lead the designers to implement this "last Step".

    Hope to have clarified my question now and to get a "decent " explanation if there is one to introduce in the design of those boats such an aparent contradictory feature.
     

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  9. SamSam
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    SamSam Senior Member

    That's not a step. Thats more of a built in engine bracket. At planing speed that extension would be nowhere near the water and would not aerate it. At displacement speed, I would guess it would supply extra flotation, just when needed.[​IMG]
     
  10. tom28571
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    tom28571 Senior Member

    jiggerpro, I think I understood your point completely although it required looking at some photos to confirm it.

    The proponents of these brackets, pods or built in motor extension mounts claim performance advantages. I have never seen any tests data that confirms this claim.

    It does make a nice swim or boarding platform and access to the motor is improved. Some pods also include a built in fuel tank which places it outside the main hull. At speed, it does not contact the water so any performance "advantage" has to be related to the prop operating in water further removed from the boat hull. One disadvantage that I noted while driving one in waves was a much greater tendency to ventilate with pitching of the hull which was a nuisance and required chopping the throttle to regain control..

    Another claimed advantage is more cockpit room but that is only true for a boat retrofitted with a pod. In new design, for a given over all length, the cockpit is always larger with the motor mounted on the transom. If the boat was properly designed for a motor on the transom, adding a bracket or pod is bound to upset the balance. Until I see some evidence otherwise, I still think it is most likely a fad.
     
  11. mydauphin
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    mydauphin Senior Member

    The last two post are basically correct. But here is the deal.
    If you get a 25' boat and make it longer with a platform and call it a 29' footer then you can ask for more money. Because of extra flotation you can add extra horsepower. More money. The surface area under plane remains same. Boat is bigger, and may be faster at plane.

    I have fixed a couple of open fishermans by adding brackets with floatation and make them run correctly. They were setup incorrectly to being with, too much weight in rear.

    If done right all these can add performance for reasons mention above, but it usually out of amateurs hands to design and make it work. You need tanks and several trail and errors boats to get it right. But basic idea of reducing surface area and weight to just what is needed gives you best performance. Rough water may be a different thing, hopefully the designer made a compromise between both.
     
  12. Gilbert
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    Gilbert Senior Member

    jiggerpro, there is nothing wrong with your English. Those of us who speak English all the time have difficulties like this. We call them semantic problems.
    I agree with you completely that these pods or extensions are counterintuitive and I don't understand WHY they would be an advantage. But a local aluminum builder built two identical boats, one had a transom mounted motor and one had the same make and model motor on a bracket behind the transom a ways (not sure how far) and the report was that the one with the bracket accelerated quicker and maneuvered better on plane. I don't have a clue why it should be better.
    Tom has already pointed out a rather serious disadvantage.
    Myself, I always want as much planing surface as I can get on a boat; I just have never thought that you could have too much.
     
  13. rwatson
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    rwatson Senior Member

    An intuitive guess is that the 'overhang' may trap the usual 'roostertail' of water that usually results from high speed on water.
    This could conceivably provide lift and trap a bit more horsepower.
    I await more info with interest.
     
  14. RatliffFranklin

    RatliffFranklin Previous Member

    Roostertail

    Spray drag may cost horsepower but lifting the stern may reduce drag so whether it's a negative, a wash, or a gain is the question.
     

  15. tuantom
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    tuantom Senior Member

    I think Jiggerpro is still a little misunderstood here. Pretend the boat in the pictures doesn't have that engine bracket on it - poof - it's gone. If you look, especially at picture #1, there is a step in the keel starting ~ 3 or 4 feet in front of the transom.
    You would think this would disturb the water right in front of the prop. Why not?
     
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