Does Deadrise=Planing degree?

Discussion in 'Boat Design' started by willfishforbeer, Jun 9, 2006.

  1. willfishforbeer
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    willfishforbeer Junior Member

    Im wondering if the deadrise of the boat pretty much defines the angle where it will plane? (consider the engine is properly installed) So a completely flat bottom will plane completely flat and one with a say 15 degree deadrise will plane on that plane with the bow slightly higher than the one with the flatbottom.
     
  2. tom28571
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    tom28571 Senior Member

    No and no.
     
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  3. willfishforbeer
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    willfishforbeer Junior Member

  4. SolomonGrundy
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    SolomonGrundy I'm not crazy...

    Deadrise..

    Deadrise is the angle or rise (measured in degrees) from the bottom of the boat to the turn of the blige. It is measured athwart at the midship, so to say it is synonymous with "planing angle" is not correct.
    Planing angle is measured fore and aft and indicates the angle of the hull in relation to the surface of the water at planing speed.
    SG
     
  5. jehardiman
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    jehardiman Senior Member

    1) Deadrise angle is the angle the bottom makes with the centerline and the chine, side to side

    2) Planing (or trim) angle is the angle the fore and aft keel centerline makes with the still waterline at speed.

    3) Deadrise angle and trim angle are not closely linked.

    4) Deadrise angle has more to do with reducing slamming motions than with the trim angle that the hull runs at.

    5) Trim angle has more to do with area and weight distribution of the hull than with transverse hull shape
     
  6. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    If anybody is interested, using Savitsky method I apply this deadrise correction factor for spray strips....

    I take a corrected deadrise angle as follows:

    Deadrise correction factor = (b1 + b2 + b3)/(B/2) > percent of flat length to half boat breadth

    Effective deadrise = deadrise x deadrise correction factor

    example:

    Deadrise = 22 deg.
    B/2 = 2 m
    b = 0.05 m ( angle = 0 deg.)
    Total number of b = 3

    Deadrise correction factor = (0.05 x 3)/2 = 0.075 or 7.5%
    Effective deadrise = 22 - 7.5% = 20.35 deg.
    Attached Thumbnails
     

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  7. tom28571
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    tom28571 Senior Member

    Ranchi,

    Interesting idea and I'm sure Savitski knows what he means, but I am a bit confused that the answer is a true equivalent deadrise. Is he saying that the result in this particular situation is that the effect of the hull is the same as if the deadrise were 20.35 degrees instead of 22 degrees? Somehow that doesn't seem right. If it were, who would bother with strips?
     
  8. dougfrolich
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    dougfrolich Senior Member

    It makes sence. The more deadrise you have the less lift it will produce, all other things being equal, add strips you get more lift, Effectively reducing the deadrise angle when calculating how much lift the bottom will produce.
     
  9. marshmat
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    marshmat Senior Member

    Essentially what that correction says is that with those strakes, the 22deg hull produces the same amount of lift as a 20.35deg hull without strakes. The flat surface of the strake produces more lift than the angled hull bottom- the flatter the deadrise the more lift you make, and a strake has about 0deg or even negative deadrise. But strakes serve a slightly different purpose as well; they allow you to keep a sharp deadrise to reduce slamming in waves, while still getting the lift of a lower deadrise, and reducing the chine-walking inherent to deep V forms. In some boats they also help straight-ahead tracking and keep the hull tracking properly through turns.
     
  10. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    This correction is not from Savitsky but is mine...

    I have to confess you that I never use Savitsky to evaluate resistance for planing hulls.

    I had the opportunity and the fortune to carry out a lot of tank test in model basin for such boats.

    If I recalculate the results with Savistky the bare hull resistance is underestimated in all the cases. Designing military craft, where the contractual speed is the most important thing, use Savitsky method is dangerous for me and the shipyard.

    This is only my personal opinion based on my own experience but I say thanks Mr. D.Savitsky for his huge research work that he has done..

    Until yet, surely for fortune, my boats have reached always the contractual speed.

    Only at the end of the evaluations I use Savitsky together with many resistance vs speed curves and I apply this correction for the spray strips.

    Spray strips have an effect on resistance for sure but the tested Savitsky's models have'nt such things probably because the scale effect.

    That's all, it was only an idea to share with you....
     
  11. yipster
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    yipster designer

    thanks for sharing Ranchi
    deadrise and planning angle may not be directly related both do form the running pad
     
  12. tom28571
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    tom28571 Senior Member

    Thanks a lot Ranchi and Marshmat. I was interested in clearing up my thoughts on this matter. I thought it would be something like the explanation you gave. Does the lift found in tests fit close to Marshmats answer?
     
  13. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    I have carried out some tank tests with a model equipped with and without spray strips.

    All appendages (like spray strips) don't run at the same Reynold number like the model and for this reason, if you calculate for each appendage (shaft, rudder, strut palm, palm...) their resistance you will remark that the measured resistance have to be reduced by 50% abt. in order to obtain the same calculated figures.

    For this reason it would be better to calculate the spray strips effect separately and to apply a correction on the deadrise angle.

    This is only my opinion...hydrodynamic is not an exact science....
     
  14. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    This is a model scale 1 : 10 (2.6 m long!) tested at speeds up to 45 knots.
    Resistance with and without appendages have been carried out as well self propulsion tests.

    Craft data:
    Length > 26.15 m
    Displacement > 73 t
    Contract speed > 45 knots
     

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  15. CORMERAN
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    CORMERAN Junior Member

    Thank you RANCHI OTTO,

    for stating that " ....Hydrodynamics IS NOT an exact science....."

    May I quote you - to the next person - that demands precise answers ?

    - Nice boat model, by the way. Elegent to me - in both engineering - and
    aesthetic terms.
     
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