Hull Layup Question for the Pros
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Hull Layup Question for the Pros

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    Senior Member djmadmatt's Avatar
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    Default Hull Layup Question for the Pros

    Just curious why I really havnt seen any step hulls on jets. Maybe they are out there and I just havn't seen any.

    I realize that a properly set up drag jet doesnt really have any hull in the water per say but, what about a nice cruising speed efficiency?

    Wouldnt a step hull be more efficient?

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    Senior Member OkieDave's Avatar
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    Default mine does

    my cheyenne copy has steps 2' forward of the transom on the outside sponsons. about 3/8". at around 60mph they are not in the water. at around 70, the sponsons are not throwing any spray, only the keel is in the water.

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    Owner/Crew Chief 1/4 Miler's Avatar
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    The basic reason that you don't see 'Stepped Hulls' used in jet boats is that the design goes against the basic principle of how jet boats move primarily on top of the water, i.e.: Jet boats are designed to get up and move on top of the water 'On Plane' (partially using the W.I.G. Effect ... Wing In Ground Effect) ..... With a Stepped Hull, the depth (draft) of the hull gets shallower with each step as you go aft which has the effect of lower the stern more into the water ..... in the case where a jet pump is used for propulsion, this is not desireable as it would create more drag by putting the deeper, forward portion of the boat more into the water increasing drag ..... the reason it works with prop propulsion is that the aft of the boat is lifted by the prop freeing up the back of the boat thereby 'airing out' the hull and reducing drag ..... if you were to try to lift the back of a jet boat by using alot of down nozzle, the amount needed to lift the hull enough to 'air it out' would diminish the forward thust so much that you would move forward very quickly at all ..... that's basically why no one designing and building jet boats builds step hulls for jets.

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    Senior Member djmadmatt's Avatar
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    Quote Originally Posted by 1/4 Miler View Post
    The basic reason that you don't see 'Stepped Hulls' used in jet boats is that the design goes against the basic principle of how jet boats move primarily on top of the water, i.e.: Jet boats are designed to get up and move on top of the water 'On Plane' (partially using the W.I.G. Effect ... Wing In Ground Effect) ..... With a Stepped Hull, the depth (draft) of the hull gets shallower with each step as you go aft which has the effect of lower the stern more into the water ..... in the case where a jet pump is used for propulsion, this is not desireable as it would create more drag by putting the deeper, forward portion of the boat more into the water increasing drag ..... the reason it works with prop propulsion is that the aft of the boat is lifted by the prop freeing up the back of the boat thereby 'airing out' the hull and reducing drag ..... if you were to try to lift the back of a jet boat by using alot of down nozzle, the amount needed to lift the hull enough to 'air it out' would diminish the forward thust so much that you would move forward very quickly at all ..... that's basically why no one designing and building jet boats builds step hulls for jets.
    Thanks 1/4 Miller. That makes a lot of sence and puts it into perspective for me.

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    Amber Racing Services BUSBY's Avatar
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    Quote Originally Posted by 1/4 Miler View Post
    The basic reason that you don't see 'Stepped Hulls' used in jet boats is that the design goes against the basic principle of how jet boats move primarily on top of the water, i.e.: Jet boats are designed to get up and move on top of the water 'On Plane' (partially using the W.I.G. Effect ... Wing In Ground Effect) ..... With a Stepped Hull, the depth (draft) of the hull gets shallower with each step as you go aft which has the effect of lower the stern more into the water ..... in the case where a jet pump is used for propulsion, this is not desireable as it would create more drag by putting the deeper, forward portion of the boat more into the water increasing drag ..... the reason it works with prop propulsion is that the aft of the boat is lifted by the prop freeing up the back of the boat thereby 'airing out' the hull and reducing drag ..... if you were to try to lift the back of a jet boat by using alot of down nozzle, the amount needed to lift the hull enough to 'air it out' would diminish the forward thust so much that you would move forward very quickly at all ..... that's basically why no one designing and building jet boats builds step hulls for jets.

    I was taught differently ... that hull steps are transverse breaks in the bottom plane. That they create a clean break in the planing surface for reduced skin friction. More like a transom, that a step ends a planing surface. And the theory is that it causes the water to miss contact with forward portion of the following plane and will assist in allowing lift sooner.

    That substantial reduction in wetted surface under relatively smooth water conditions can be achieved. Of course, there must be a clean break of the water as it leaves the step; with air behind it. If not that the step will create drag and even suction.

    The reason jets are not using stepped hulls is because of their size, that they usually are too short to have the need for steps to get on plane quickly. That the use of lifting streaks have been more than sufficient in most cases.

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    B1 Racing cs19's Avatar
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    As of last race Im running a step in the bottom of my center sponson. So far so good we may do more for next season.

    B1Racing.net

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    Senior Member djmadmatt's Avatar
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    Hmmmmmm ... I think this string may get very interesting.

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    TRG
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    Quote Originally Posted by cs19 View Post
    As of last race Im running a step in the bottom of my center sponson. So far so good we may do more for next season.

    ...and im sure you went with the "Finnegan dimpling effect" too!

  11. #9
    "It's HONDO, honey" Kyle's Avatar
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    Keel, keel, keel!!! Its in the keel..... and the straights,
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    "Chine walkin: (verb) - to show another brother that you will stay in to win. (syn.) Holy *hit!!!!!! Don't Lift!!!!!"

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    Owner/Crew Chief 1/4 Miler's Avatar
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    While I'm not going to 'debate' hull design with you, I think that you yourself have stated very clearly why 'Stepped Hulls' are not used with jet pump propulsion hulls ... Note what you posted, factor in what I posted and, then go back to basics, what everyone who deals with this aspect of jet boats knows (ask Bennett, ask Shoemaker, for that matter, ask Ron Edhe) ... water, and for that matter air, does not like abrupt changes as it causes disruption in the overall flow of water into the pump system ... disruption of the flow of water, either by infusion of air or the breaking of the contact of the keel (immediately prior to the jet pump intake) with the water, resulting in air injection, diminishes the volume of flow of water into the pumps intake thereby reducing forward thrust .....

    Stepped Hulls rely on air/water interface disruption (as you pointed out) to free up the hull and diminish drag ... this is contrary to how a jet boat works as a jet needs a constant flow of water into the pump to function properly.

    Quote Originally Posted by BUSBY View Post
    I was taught differently ... that hull steps are transverse breaks in the bottom plane. That they create a clean break in the planing surface for reduced skin friction. More like a transom, that a step ends a planing surface. And the theory is that it causes the water to miss contact with forward portion of the following plane and will assist in allowing lift sooner.

    That substantial reduction in wetted surface under relatively smooth water conditions can be achieved. Of course, there must be a clean break of the water as it leaves the step; with air behind it. If not that the step will create drag and even suction.


    The reason jets are not using stepped hulls is because of their size, that they usually are too short to have the need for steps to get on plane quickly. That the use of lifting streaks have been more than sufficient in most cases.

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    Amber Racing Services BUSBY's Avatar
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    Quote Originally Posted by 1/4 Miler View Post
    While I'm not going to 'debate' hull design with you, .
    No 'debate' here Jak ...

    Quote Originally Posted by 1/4 Miler View Post
    what everyone who deals with this aspect of jet boats knows (ask Bennett, ask Shoemaker, for that matter, ask Ron Edhe) .
    I'm good thanks ... no need to ask anyone, been dealing with jet boats for a while ...


    I was just throwing out a different angle, stating what I was told/learned ... and I'm not pointing out any flaws with what you posted or the things I saw wrong with your post ... just answering his question with my version

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    Owner/Crew Chief 1/4 Miler's Avatar
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    Good here too.

  15. #13
    Spiral out MikeF's Avatar
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    Jeff put steps on my Liberty hull in 1977/78.

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    Quote Originally Posted by MikeF View Post
    Jeff put steps on my Liberty hull in 1977/78.
    And???

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