Extension of a planing boat.

Discussion in 'Boat Design' started by HJS, Jul 28, 2010.

  1. HJS
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    HJS Member

    Has someone any personal experience to extend the bottom of the boat or place the engine further forward into the boat?

    js
     

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  2. keysdisease
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    keysdisease Senior Member

    This is common on some flats boats here in South Florida and has been done on production models for years. I have seen something similar on larger boats too.

    Steve
     

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  3. HJS
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    HJS Member

    How does it work?
    Are there any drawbacks?
    Why is it not more common instead of attached platforms?

    js
     
  4. anthony goodson
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    anthony goodson Senior Member

    Worked very well on this 20ft inboard dory it planes at lower rpm and there is excellent access to the boat, and from the boat to the jet.
     

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  5. messabout
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    messabout Senior Member

    Back in the day, when ships were wood and men were steel, this concept was used often. A portion of the bottom protruded aft past the transom for a few inches. The term used to describe them was "squat boards". Even some racing hydros could be seen with them. We still use them on many boats but now they are movable and are called trim tabs.

    As the extensions grew, they morphed into closed boxes sometimes used for light storage and occasionally as built in fuel tanks. I believe that the latter use would be a bad idea. If there are any down sides to the scheme it might be that some of these arrangements cause, or at least are blamed for, cavitation during tight turns. Larger bouyant extensions make the boat a little more reassuring when stationary or at very low speed. Those big ole outboards are heavy. Moving the mass effectively inboard makes some common sense.
     
  6. Submarine Tom

    Submarine Tom Previous Member

    I agree with all of the above.

    My experience with this type of extension on 30 foot (10 meter) planing hulls (twin outboards) has been nothing but positive.

    -Tom
     
  7. gonzo
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    gonzo Senior Member

    They act like fixed trim tabs.
     
  8. keysdisease
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    keysdisease Senior Member

    In the case of the flats boat, this type boat is designed for fishing in very shallow water. When an outboard powered boat gets on plane the stern tends to "squat" until the vessel get underway. One of the things the extensions do is reduce this "squat" effect and help the vessel "pop up" onto plane quicker while reducing "squat"

    But then, I don't know "squat"

    Steve
     
  9. HJS
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    HJS Member

    .............If there are any down sides to the scheme it might be that some of these arrangements cause, or at least are blamed for, cavitation during tight turns........... QUOTE]

    That was what I am asking for. Can some of you tell me more about it?
    On what type of boat is that common?

    All of the other options are familiar to me.

    js
     
  10. Landlubber
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    Landlubber Senior Member

    ...many Bertram 25's have had pods for outboards put on the same way, they are all good.
     
  11. tom28571
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    tom28571 Senior Member

    This acts the same as what is locally called a "well boat" with the transom cut out to allow tilting the outboard. The Simmons Skiff is the most popular version. Steering is adversely affected by placing the prop in this slot. The part of the slot on the inside of turns is forced into the water by the sideways motion of the hull in a turn, creating resistance to the turn. This also means that the motor must be turned further to get the same turning radius of a motor on the transom. This can certainly lead to more prop ventilation in turns. Placing the prop further forward also reduces the steering moment arm.

    Transom brackets or pods don't have this problem but they can have other issues unless the boat is designed for them.
     
  12. u4ea32
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    u4ea32 Senior Member

    You more commonly see the OPPOSITE arrangement: e.g., outboards on brackets.

    This is due to, at high speeds, one wants the CG as far aft as possible in relation to the planing surface. There are two reasons for this: First, for efficiency: with the CG aft, this allows the angle of attack to remain more certainly within the best lift-to-drag range of about 1.5 to 3 degrees, depending on the hull deadrise (more deadrise, more AOA). Second, an aft CG (in relation to the planing surface) also suppresses porpoising, according to Savitsky models.

    Having the planing surface terminate well forward of the transom (outboard) allows the CG to be far aft relative to the planing surface, therefore one sees notched transoms on offshore V bottoms, and one sees brackets on most outboard powered boats.

    However, if one wants to go slow -- displacement or semi-displacement speeds -- then the demand for aft CG is lessened, and one may see an advantage to moving the engine forward, or extending the planing surface aft as in the original post.

    At high speeds, it is a bad idea. At low speeds, it might be a good idea.
     
  13. tom28571
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    tom28571 Senior Member

    This doesn't jibe with the data I see on planing hulls. Most show a minimum wave plus frictional drag at 4 to 5 degrees of bottom trim which agrees with my testing. I like to have a low trim angle trim to help promote low speed planing but find that speed increases at WOT if I increase the trim angle. Also thought too far aft CG was a cause of porpoising. I'm not going to argue with Savitsky though we may have a different interpretations. Porpoising can often be stopped or reduced by moving weight either fore or aft.
     
  14. scotch&water
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    scotch&water Junior Member

    extention to help lift

    We have a problem with a 2859 Bayliner that is to heavy in the back,dfficulty in getting up on plane with trim tabs all the way down and it will drop at anything under 4000 RPM, not good. So by extending the hull on ether side of the drive lift will be created ? I have caculated 65lbs. per cuft? Now let the experts tell us the problems with this.
     

  15. HJS
    Joined: Nov 2008
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    HJS Member

    Let us take the question a little simpler. Many boats have an integrated platform as standard. Is it better to extend the bottom or just have a platform above the bottom? The cost and weight should be the same.

    js
     

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