nose diving riveira

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

  1. Jango
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    Jango Senior Enthusiast

    Assuming no major problems with the Bottom. Appears to be A-OK from previous pictures.
    In my opinion, the Problem is a NO-Brainer! I went thru the same exercises many years ago with a Home Built Inboard.

    In 2004, I finished construction of a slighty longer than standard Monaco - 19'-9". I moved strut and Engine/Trans Aft from plans along with other improvements. My distance from outside of Transom to carb CL. is 79 1/4 inches. This is with an angled carb spacer which increases the Dim from less than 79 inches.

    In addition, I am using a Longer trans- 72C, instead of the "normal" Sm Block 71C Velvet Drive.
    Assuming water is resonably flat, My Monoco will NOT bounce (transom Lift) unless I instantly unleash ALL 450 HP at High Speeds.

    An alternate "Fix" is to weld a small inverted Hydrofoil - 8 to 12" wide to the bottom of the rudder.
     
  2. Jango
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    Jango Senior Enthusiast

    In case it's not obvious, You need to move the Prop (Strut) back in addition to moving the motor/trans. Currently there is way too much bottom behind the Prop acting like a large Trim Tab, holding the Bow down.
     
  3. Oyster
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    Oyster Senior Member

    The problem is that we do not yet know the angle of the shaft and the real numbers from the transom either in its present form. If its your position that the angle of the shaft is extreme too, the wheel should have more than enough clearance from the tip to the bottom, unlike what has been stated even with the existing wheel. But if you add that to the silly notion that if such a major mistake was made, what other mistakes were also made. Compared to other builds, the motor is not too far foward using the plans and other builds using the same motor. But even so we also know that he has tried 300 lbs in the transom area and the boat still plowed.
     
  4. TollyWally
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    TollyWally Senior Member

    Jango,
    If I am understanding you correctly, excessive lift is caused by the accelerated water of the propwash having more surface area to work with due to the location of the strut and wheel. That this propwash is more effective than the normal flow of water caused by the movement of the boat through the water because it is sped up by the prop.

    Your rudder fix is shaped to provide negative lift to counter balance excessive lift in the stern.

    There are certainly a lot of subtleties in hull design and dynamics as speeds increase.
     
  5. Jango
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    Jango Senior Enthusiast

    I believe one should think of the prop location as the for/aft pivot point. Any Forward motion causes positive pressure against the bottom in Front of the Prop as well as aft of the prop. If for instance, the boat had huge, lets say 5' protruding trim tabs, also creating lift, the rear portion of the boat would lift, making Bow lift next to imposible.

    Forward CG compounds the problem, since additional pressure (Force) is required to lift forward of the pivot point (Prop). It's all a question of Moments taken about the center of axis (Prop)

    Since Proper Prop location alone creates little Transom lift, forward CG Locations can be overcome by dynamic pressures (lift) from forward movement. As the CG is moved back, less upward force is necessary to create bow lift
     
  6. TollyWally
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    TollyWally Senior Member

    I see I was misunderstanding your point, thank you for the clarification. It seems as if you are speaking of a fulcrum and the relative lengths of a lever on either side so to speak.

    Was I any closer on the rudder explanation?
     
  7. Jango
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    Jango Senior Enthusiast

    Yes, Right on with the Fulcrum and lever analogy. And yes, your understanding of the rudder hydrofoil is correct - Donzi, does something similar on some of their 120 mph boats.

    And Yes Oyster, Shaft angle compounds the problem. Higher shaft angles increase the vertical component of thrust with increased transom lift.

    It's all a tradeoff, moving CG aft, while counteracting transom lift, creates larger shaft angles with more lift. This is why a V-Drive is such a great Idea in fast boats, you can get optimum CG placesment Aft with very small shaft angles - 7 to 10 deg.
     
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  8. Oyster
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    Oyster Senior Member

    Correct, I understand shaft angles. The problem in this particular build is that the stuffing box is also foward of the intended position and we have yet to establish the true position of the strut as that is also moved foward. Sure we see the backing plate. But we also do not know the shaft length from the intended position either. We think we now know that the engine is in close proximity to the plans by the gear output flange. Notice I said Think, because the gear and type is still up for interpretation with his lingo.

    But he did not use the speced out strut, instead he used a flatter angled strut. If you think that I plan on committing to the notion that the core of the problem is shaft angle, think again. I reserve judgement and will only add opinions which can be skewed without the information to conclude the existing shaft alignment.

    There can also be some grey areas in regards to the gear and its specs, lengths included. So for now I remain a skeptic that one issue and one issue only is causing this. I remain open to be proven wrong and have gone out on a limb and expressed a lot of opinions in this build so that all relevant parties both in the threads and the builder to throw mud back at me.

    But for me my thoughts and opinions are as follows. We will never know the REAL and unfiltered numbers and specs in the existing setup and boat. As I also have stated, that if you cannot isolate the problem, you cannot truely fix the problem. This is done slowly and one step at a time. You can also quite possibly compound the problem with attempts to reinvent the "wheel" so to speak by just ripping everything out, not withstanding the additional expenses.
    By the way any pictures of your build?

    edited to add, I am old enough to have installed offset drive transmissions for engines which minimized high attack angles for straight drive shafts for large diesels.
     
  9. Jango
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    Jango Senior Enthusiast

    It was brought out in the other Forum that the Dim. from the outside of the transom to the carb CL on the troubled Riviera was 94"with about 66" to the coupling. Another builder also had 66" to coupling which is why some believe the CG,s are similar. However the second builders dim. to carb CL is 84". This tells me that the CG,s are Not the same, and the troubled Riviera merely has a longer tranmission which you would have with a reduction gear.

    If I'm reading all thus correct, The Troubled Riviera CG is much further forward than specs! AND Prop is also way to far forward compounding the problem. The builder has already said his shaft angle is 15 deg.
     
  10. Oyster
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    Oyster Senior Member

    The 84" is on the Monaco which is a shorter boat, 19' according to the site.
    We need to compare the same boat with the overall length of the engine, at least to be able to compare apples to apples.


    The Rivera that is now being built with the stuffing box entering inside the boat at the frame slated on the plans using the same engine is longer than the Monaco. This setup has the same engine but has more bottom in the boat. He also enters the hull at the frame which is drawn on the plans.

    The engine and transmision tip to tip is 43 inches using the same block, IIRC. He also has a deminsion from the inside of transom to the output flange of the gear is 64.9". To the front of the engine that gives him 107.9" from the transom.

    I do not know the overall deminsion of Brian's engine to the front of the balancer and still do not know that number which is the important number.

    But if he has 94 inches to the carb, then his[Brian's] engine by comparison should have another 12" foward, the most important number to be able to compare the deadweight factor to the hull. Then if in fact someone feels that the shaft is incorrect, then go back and just redo this which is one heckava easier and less destruction. Then run the boat and see where we are with any other factors. At this point you can then have a boat that you can use without carrying balast, destroying stringers and being discouraged and frustrated. You have some concrete information in which to say even to Glen L that some additional notes needs to also be added with specific specs to the builders using simular factors and power unit too, in the interest of all the other builders coming along the way. Afterall this excersize should be constructive too as we evolve with newer and better power plants. They are indeed open to listen and their sucess depends on successfull builds too, which has been their cornerstone and interests since 1953. What have I missed?
     
  11. Jango
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    Jango Senior Enthusiast

    Oyster, The Riviera is merely a stretched version on the Monoco. Frames are exactly the same spaced at 27" compared to 26" for the Monoco. Transom to carb dim should be exactly the same, about 84" for a conventional inline Trans/Engine

    Jango.
     
  12. Oyster
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    Oyster Senior Member

    I understand that. But the contrasting gears on both engines cannot and are not according to the book 10" longer since you are using the carb measurement. That is a flawed number if you step back and think about it. We need to know the TRUE overall measurement to the front of the engine because the output flanges are almost identical from the transom.


    We know that the output flanges on the two gears are within one inch using the number from the transom. We know that Brian's gear and stuffing box has almost no clearance between the nut on the stuffing box and the shaft coupler and the stuffing box only appears to be foward of the designed position. There is no way that the engine and gear in Brian's boat is 10 inches longer using the specs and type of gear thats listed according to the books, unless I missed that imput. I would be glad to be corrected.

    edited to add remember Brian gave us the deminsion from the transom to the output flange on his gear. Thats the only number thats been provided to us to determine engine and gear placement along the bottom and keel line. Forget about the shaft angle and concentrate on deadweight placement along the keel line no matter where the frames are located. This is the number thats used for the COG and balance as long as the beam of the boat bottom is as designed.
     
  13. Oyster
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    Oyster Senior Member

    Lets look at the carb placement on his engine. Now we also know that we are also working with an overall length of around 107 " on another Rivera which is the number being used with the same boat and the same motor on another build with what is supposed to be the right placement for the shaft log thats entering the inside of the hull at the frame. If this number is 94 inches, then we have actually about 13 inches further foward of engine block and weight. So if this is too far foward, then the other boat's engine is also too far foward.

    [​IMG]
     
  14. Oyster
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    Oyster Senior Member

    Let me add one most post to this recent exchange. If you are taking the position that the engine is too far foward, then you need to know where to move the engine. To date we know that there is another package and boat being built that is almost identical in the power plant. So we must conclude that his work is also too far foward when dealing with balance if the boats are built to specs. So for some reason two folks thousands of miles apart has concluded that the engines need to be in a simular location assuming that the power plant, engine and gear is within a few inches of the overall length. We know the the flanges are anyway.

    The first step in the process throwing away the angle of the shaft and dealing with the distribution of weight is to physically measure the bottom beams and and compare using the plans. Then you create a spreadsheet showing the finished build and the recompute the location to make sure that when you begin to rip out and redo that the work is not in vain. Guess work and redoing is disappointing when the outcome is less than ideal.

    In boat building we normally do this in reverse, setting the driven train to the plans and then position the power plant in its position. But since the boat is finished, and we want to minimize the deconstruction and reconstruction, its important that we step back and see exactly where we are before just going on our own. If I was the one that has made a major mistake dealing with what we are dealing with now, I would also like to elicit others before proceeding in hopes that this time around, it will work. This project is toast at this point without some further imput from Brian.
     

  15. Jango
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    Jango Senior Enthusiast

    Oyster, I suspect that there may be errors in the Dims. In addition if a Velvet Drive reduction gear is being used it is 6.4 in longer than Direct Drive. I can not find a Velvet Drive angle Down gear. Other builders have used PCM angle down units,(1.5 in. longer than Velvet Drive) Also we already know His prop (Strut) is too far forward. This all suggests the 10" differential could be correct especially with velvet reduction gear.

    In any case His CG is further forward than inline applications with correct Strut (Prop) locations. I believe the Strut(Prop) location is the Biggest reason for His Problems. CG only aggravates the problem.

    Knowing what I know about my application, I would : First, make full sized drawings of changes, checking clearances etc.

    1. Relocate the Strut 13-14in from the Transom (check Rudder/prop clearence)
    2. Redrill Shaft hole at 15 deg.
    3. Move Engine/Trans (Mounts)
    4. Plug Holes etc.
     
    Last edited: Mar 9, 2010
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