Adding strakes

Discussion in 'Boat Design' started by LowelandSystem, Dec 25, 2016.

  1. LowelandSystem
    Joined: Nov 2015
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    LowelandSystem Junior Member

    Hi good fellows,
    Sorry for again asking you guys, below picture of the boat is my first boat that I have built. due time constraint and ignorance, I did not put any hull strakes below. Hull deadrise is 20 degree. If I now add strakes now on the hull will the performance improve. Please your wise advise can be help me a lot. Kind regards.
     

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  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    No, strakes ("longitudinal steps") are only working in the region where spray is generated. From your picture we can see that your boat is operating with a high trim angle at a fairly low speed. There is little spray generated in the bow region, meaning that the spray drag is a negligible part of the total drag.

    In this case, adding strakes will add more wetted area without reducing the (already low) spray drag. The end result would be increased total drag.
     
  3. LowelandSystem
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    LowelandSystem Junior Member

    Hi Baeckmo,
    Thanks for the valuable answer. Is there any possible remedy?
     
  4. ondarvr
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    ondarvr Senior Member

    From the pic it shows the motor possibly tilted up too high, which will make it harder to plane, but even with the correct trim angle it may not plane with that load and that motor.

    The strakes can help, but it's more of a fine tuning method for higher speeds, at low speed the effect may be minimal on a boat like that.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The 20* deadrise is certainly no help to plane readily, and as mentioned, the trim setting of the outboard is working against it as well. also it is a fairly narrow boat ( I think, from what you have posted elsewhere) which also makes it a little harder to pop on to the plane.
     
  6. Angélique
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    Angélique aka Angel (only by name)

    -
    Trim Your Boat
    Yamaha: Understanding Engine Trim/Tilt (and Trim Tabs)

    [​IMG] Boat Trim
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Loweland System should experiment first with the boat as light as possible, driver only. If after fiddling with the trim setting it still can't be induced to get up and run, clearly adding weight cannot improve the situation.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Actually, in Angelique's picture of the white Stacer boat, the trim angle described as being too much bow-down would be just right for that boat heading into some chop. Trim out running with it a little. If you expose the flattish mid and after-body, you will get hammered in a chop.
     
  9. Angélique
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    Angélique aka Angel (only by name)

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    Below some links and pics about the Stacer's trim from Sound System Design*

    * Sound System Design is a Russian Software Site I believe . . :confused:

    Sound System Design ---> Test Range ---> Simulation Results
     
  10. Angélique
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    Angélique aka Angel (only by name)

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    Right for heading into some chop I think, but I'll guess they're utilizing an electric trim control there and change the trim when the circumstances change, so for the shown flat water conditions in this example it looks like the outboard is trimmed just too far in as they said I believe.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Yes, in flat water, get the boat on plane by having the engine trimmed in, once on plane trim out till it starts to bounce a little. The rpm at a given throttle setting will tell you which trim position is least "draggy", variable trim was a great innovation in outboards. Obviously Loweland does not have the option, so trimming the leg in to no more than one hole left, seems a good starting point.
     
  12. Barry
    Joined: Mar 2002
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    Barry Senior Member

    A design paper which I could not cust and past the table, of sets the Optimum Trim angle
    for various deadrise angles.

    0 degrees deadrise 4.5 degrees trim
    5 - 4.75
    10 - 5
    15 - 5.5
    20 - 6.5
    25- 7.7

    From the picture, and granted I assumed the keel is parallel to the gunwhale, I get about an 8 degree bow up attitude which really is not far from the 6.5 optimum value
    Often, not on this thread, but others, there has been much discussion that if the bow is too high up, that this is an inefficient attitude. But from the table, the deeper the V, then the higher the optimum trim angle

    Baekmo,
    I would like some more confirmation as to why lift strakes are not effective unless they are in the spray.
    Our aluminum jets, most around 20-24 feet had 4 lift strakes and the outperformed similar boats with no lift strakes.
    The purpose of a lift strake is to gain "free" lift, a controversial term but we have generated a mass flow rate from the keel out to the chine. If we change the direction of the water flow, ie from parallel to the deadrise angle, to a down angle, we get lift that would be otherwise be lost
     
  13. ondarvr
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    ondarvr Senior Member

    That angle of trim "might” be correct for a high powered hull going at a high rate of speed, not one trying to get on plane with marginal power. Even with maximum HP you don't have the motor trimmed at the same point when first getting on plane and when going WOT
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    A wide chine flat, preferably with some reverse angle, would help this boat, imo.
     

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  15. Angélique
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    Angélique aka Angel (only by name)

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    The given deadrise of 20° doesn't say much if we don't know where it is measured, I'll guess at the transom, but if so then this doesn't say anything about the deadrise development over the bottom as this rarely is the same over the whole boat length, but this for us unknown deadrise allocation over the boat does definitely influence her behavior. For the given boat in the picture can be seen that the deadrise is much more than 20° near the bow, but the picture doesn't make clear where it starts increasing, maybe in this case the deadrise remains about the same from the transom until near station #1, it looks a bit like that at the end of LWL ± below the wind screen.

    But for the present boat and motor I think LowelandSystem first need to fiddle with the outboard trim to see if he can find improvement and an optimum for the circumstances at that moment, and from there it can be viewed whether further improvements are needed, and if so what it might be.

     
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