nose diving riveira

Discussion in 'Powerboats' started by aussiebrian, Feb 25, 2010.

  1. aussiebrian
    Joined: Feb 2010
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    aussiebrian Junior Member

    need some assistance from anyone.
    I am a new to this forum, so a brief background. I recently completed a 2 year project to build a glen-l riveira and apon launching we struck a problem with the operation of the boat under way.
    I followed the plans to the letter, but at 25mph to 30mph the boat nose dives and all steering control is lost. I have tryed a number of things suggested by boat builders and other specialists, but nothing is making any difference. The specs are ase follows - 6.1m hull, weight (incl. motor) 1400kg, motor PCM 5.7ltr (310hp) with 1:22 : 1 stepdown velvet drive, 13" x 15pitch 4 blade prop.
    I have tried adding balast to the transom(150kg) and tested different props, but there was no improvement, the boat gets out of the "hole" with ease, motors along on the bow wave comfortably, but as soon as the power is increased to speeds greater than 30mph, the boat "flops" on the bow and stays there till you reduce the speed below 25mph.

    I have contacted Glen-l, and they were unhelpful and unresponsive so I would greatly appreciate any suggestions.
  2. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    WOW...that certainly is a strange behavior...this will be interesting.
  3. aussiebrian
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    aussiebrian Junior Member

    any ideas?
  4. Typhoon
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    Typhoon Senior Member

    Where is the engine mounted, as per plan?
    Photos of boat?

    Regards, Andrew.
  5. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Have you posted this on the Glen-L forum? That is where you get help from members and some who have built the same boat. There is no one at headquarters with specific knowledge to assist you. See another builders pictures.
  6. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    A nose diving at high speeds may indicate that your boat is running too flat on the water. Running too flat (or even down-trimmed) can create a low pressure area in the forward aft of the hull, where it has a significant rocker. This low pressure tend to trim the boat even further down - till the point where rudder loses the steering authority and the boat starts steering by bow.

    The possible cause for that may be a wrong (too forward) CG position, some trim-down device in the stern region (a too-inclined prop shaft could play a part), or a combination of both.

    That's all that can be said without seing the boat plans and without knowing your eventual modifications of the original design.
  7. TollyWally
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    Location: Fox Island

    TollyWally Senior Member

    Per previous discussions on the mysteries of hull dynamics and planing. Is the stern lifting too much @ 25 mph? Hook perhaps.
  8. Hunter25
    Joined: Mar 2006
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    Hunter25 Senior Member

    CG too far forward and the boat is falling off or stuffing.
  9. TollyWally
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    TollyWally Senior Member

    You'd think 300# in the stern would do a little something.
  10. tunnels

    tunnels Previous Member

    Get back to basics !

    What would cause the nose to go down or the back to lift up on a power boat , remembering unlike a outboard there is not trim button !! damn !!
    Hook would be amongst the first things id be looking for! In reality slight rocker is better in a inboard but the placement of the high point of the rocker is critical ,
    Next the angle of the prop shaft !! if its steeper than it is recomended it like running a outboard with the trim hard down continuously !!and as said in the previous post your cg is to far forward . Make the back go up and the front go down !!!!

    Next look at the way the boat is loaded ,tanks ,batterys and what ever else you have in the way of weighty items.
    All these things to be aft of the motor !!,
    After all that the weight of the Engine/gearbox ??

    Something to try are bags of sand and place them in the back , take the boat for a run and keep adding sand bags right in the back as close to the transom as possible till you get to where you want it to plane once you have got the boat to plane correctly take a count of the kgs that are needed to achieve that .
    The imediate feeling i have is first the shaft angle in combitation with second the weight of the motor and gearbox is the problem :confused:

    Can you post pictures please of the boat sitting in the water and then all the inside stuff , also a side shot so can see the propshaft angle and where it comes out !! Pictures are worth a thousand words !! Plus to see the boat ! i have a great passion for that style of boat and have built modles whan i was a kid . Century and Chriscraft were my favorites . or just email direct to
  11. Hunter25
    Joined: Mar 2006
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    Hunter25 Senior Member

    Why are you yelling with red type Tunnels?




    As you can see from the upside down hull taken from Glen-L, the shaft angle is steep, but they all are, unless aussiebrian is not telling the truth about building it exactly to plan. There also does not seem to be any hook in this design. These designs have been built with small block Chevys and velvet drives before. This does not leave many other explanations.
  12. tunnels

    tunnels Previous Member

    Wow thats a steep shaft angle !
    Red to get the point across , not shouting !.
    So how does you boat go ?? the rim is ok ?? where are all your heavy bits ? tanks and battery etc
    That a nice looking boat you have !!
    As you say there could be more than we are being told maybe . :p
  13. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    I would put this post on the scream and fly forum ( as they have boats doing 100mph and know all the problems ....are you sure the bottom is flat not concave ??
  14. Ike
    Joined: Apr 2006
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    Ike Senior Member

    What you are talking about is called dynamic instability and there have been some really startling cases. The most notoriuos was the USCG 30 foot Surf Rescue boat. It did exactly what you describe. The Naval research Laboratory did a lot of test and discovered that under certain conditions negative pressures occurred near the bow and this caused the bow to nose dive. It didn't help that it was a single engine with a really big prop (lots of torque trying to twist the boat to the right) and a hook in the aft port planing surface. Also the engine was rather far forward for a planing hull moving the CG forward when planing. But the big culprit was the bow. The transverse shape of the bow was convex. This contributed signficantly to the negative pressures.

    Anyway looking at your boat that engine is almost amidships longitudinally (fore and aft). On planing hulls the CG of the boat when the boat is up on a plane should be nearly over the center of upward force That is near the center of effort of the wetted surface. That requires the engine to be a lot farther aft. This is why many planing boats appear to be riding on only the prop and the last few feet of the hull. The forward CG of this engine is holding the bow down when the boat is planing. this boat would do better with a Vee drive rather than a straight drive. That would move the CG aft and the boat would be better balanced.
    Last edited: Feb 27, 2010

  15. tunnels

    tunnels Previous Member

    What have you changed from what was on the drawing ????? :confused: :confused:
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