Riveted Hull

Discussion in 'Jet Drives' started by drev500, Aug 31, 2015.

  1. drev500
    Joined: Jul 2015
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    drev500 Junior Member

    Hey All,

    How can I go about smoothing the bottom side hull of a Starcraft Holiday that has rivets running from bow to stern? This is for a project that I am trying to design a twin jet drive set up in an aluminum hull boat. I chose the Starcraft as it is a V style hull and not a flat bottom which will help with cavitation but I am worried that the rivets in the hull will have undesired effects on the bottom.

    Do I need to worry about the rivets? This is my first time venturing down this path way so any and all advice would be appreciated!

    I attached a picture to show you what I am talking about. I forgot to take a picture of the hull but it is showing the same rivets I have on my boat.
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt the rivets constitute a problem. I'd be more concerned that the overall hull shape be suitable for jet drive. Are there any others around with jets, and particularly twins ? Sounds like a fair bit of weight involved with two, as well.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

  4. drev500
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    drev500 Junior Member

    My apologies. I should have noted that I am using jet ski motors. More specifically, motors and pumps from Yamaha Waveraiders 1100 (3 cylinder 2 strokes). The boat I am using was fitted with an O/B but was also available in an I/O.

    I have yet to weight the pumps and motors from the skis but I am willing to bet they are of similar weight of the I/O originally offered. If not, I am sure they will weight less (I hope).
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    It would be a shame to adapt your hull to twin jet drives, which with all the attendant structural alterations would be no simple job, only to find there was a snag at the end of it all. If there is some kind of external keel/skeg along the centreline, I would be very leery about its adaptability to jets, for two reasons, firstly because it might interfere with the jet intakes in hard turns, but more particularly it might indicate the hull needs a little help with directional stability, even with outboard or sterndrive, which would only increase with jets.
     
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I have never seen a jet installed in a riveted hull. Normally boats are riveted when using thin aluminum material which do not really lend themselves to a jet.
    Often, riveted hulls have a tape between the material to seal the hull from leaks

    Do you know how thick the bottom is?

    On the intake for the pumps, was there a fibreglass block formed into the waverider to which the pump attached? If so you will have to manufacture something to join the hull to the pump
     
  7. drev500
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    drev500 Junior Member

    I have yet to confirm hull thickness but a hit on it has nice solid thud leading me to believe it is moderately think. This boat is a 1973 date but it is in excellent condition. Last registration was in 1975 and it has sat since. No dings what so ever on it.

    The reason I chose the Waveraiders is the pumps actually come out separately from the hull unlike most pump intakes that are molded into the hull.

    I have planned to make an aluminum flange to which would be attached to the hull on the inside and would have threaded holes to bolt the intake to it. This flange would be mechanical fastened and chemically sealed to the hull.

    I should provide some background to the project. It is for a school project. I am a senior mechanical engineering student. The principle subject of the boat is to design a boat using jet pumps. We live on the Ohio River where logs are a very big hazard. Jet pumps underway would come out unscathed unlike a prop boat in the event of a log strike. This has been done before with Jon Boats but I haven't seen it done with a v style Starcraft and is why I proposed the project. The project was approved so I am in my research stage. I have the skis and the boat.
     

  8. Petros
    Joined: Oct 2007
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    Petros Senior Member

    How do you see the rivets interfering with the jet pumps? typically rivet heads are within the boundary layer, and though they do increase the drag of the hull, it should not affect other aspects of the hull's behavior. Are you thinking there might be too much turbulence at the intake? I doubt that will have any noticeable effect becasue the size of the rivet head relative to the other things going on at the inlet.

    If you are concerned about it you can drill the rivets out and install flush rivets (you would have to counter sink the heads, install them "wet" with sealant in the holes so they are water tight). No need to replace all of the rivets, but perhaps just a few rows up stream of your inlet.

    OTOH, they actually might help prevent flow separation by putting energy into the boundary layer. IT helps keep the flow attached at curved areas where flow separation might occur. I would think however that the design of your inlet is the really critical issue. If you have a lot of big unstable turbulence and flow separation, unstable vortex, etc. in the inlet stream it may damage the turbine, certainly will reduced the max available thrust. I doubt the rivet heads up stream of the inlet will contribute to that, that has more do do with the inlet design (curvature of the surfaces within the inlet).

    I am familiar with the inlet problem from my experiance. Years ago I used to work in aerodynamics working for a military contractor, the turbine inlets were always something that took a lot of manpower, computational studies, and model testing before they actually built any flying hardware because of the consequence of a compressor stall. A compressor stall (caused by the turbine "swallowing" a vortex or other unsteady flow) That would almost certainly results in the loss of the aircraft, if not the crew.

    though I do not have any experiance with boat jet pumps, I would think the principles are similar. In your case I think you could trash the turbine since hydraulic forces are so much larger, which would damage the hull and possible taking on water.

    Keep the inlet length to diameter (or equivalent dia.) ratio to six to one (6:1) should solve that problem, thought that would take up a lot of hull length and interior space. You also must have smooth, very large radius curves inside the inlet.

    Good luck.
     
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