Dory in aluminium

Discussion in 'Boat Design' started by pafurijaz, Mar 14, 2018.

  1. pafurijaz
    Joined: Dec 2016
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    pafurijaz Junior Member

    Hi, I'm still here, nobody can tell me how to put the Suzuki 25hp outboard motor, I tried to find information about it and I understood that, the engine should be in this position as seen from the images below. I hope it's positioned in right place, compared to the hull and the anti-cavitation plate.If needed I can also provide the model if it can help to provide the corrections.
    Dory_Skiff_outbard_placement1.jpg
    Dory_Skiff_outbard_placement1.jpg
     

    Attached Files:

  2. messabout
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    messabout Senior Member

    A standard short shaft outboard typically has about 17" (43cm) distance between bottom of clamp bracket and cavitation plate. . On planing boats, the cavitation plate is usually set about 2 to 4 cm below the bottom of the boat. On a 22 inch long shaft (56cm) the plate should be positioned at he same distance below the bottom as in 2 to 4 cm. That will be the determinant for the transom height. Larger outboards often have a hydraulic "jack Plate" that allows the operator to control the relative vertical position of the cavitation plate. The cavitation plate is often raised so that it is even with the bottom of the boat when running in fairly smooth water and the best speed is sought. If the plate is lifted too high the prop my break out in a sharp turn so that it is ventilating. That is not what you want to have happen if you can avoid it.

    The outboard will have some trim adustment. That is it will have some provision for changing the vertical orientation of the shaft, therefore the propeller will either tilt slightly upward from the horizontal or slightly downward depending on the adjustment. Once again big motors (spell that big dollars), may have a remote controlled system for changing the tilt while underway. If the prop tilts a little bit up then it tends to lift the bow of the boat. If tilted down it tends to lower the bow. There are plenty of variables here. One of the variables is how the boat is loaded. If it is front heavy then the prop can help lift the front if it is trimmed to do so. ..................... The best positions can only be determined with sea trials.

    The tilt factor is what I had in mind when I suggested 14 degree transom inclination, in a previous post. Smaller Outboards, like the Suzuki 25, are typically engineered for that transom angle. If your buyer insists on 12 degrees, so be it. He may know something that I do not know. He can adjust tilt if necessary by using wedges with the clamp bracket. Also if the cavitation plate is a bit too low for best performance, the height can be adjusted with what are commonly called "rev sticks". That is simply a wooden spacer under the clamp bracket that results in raising the plate a little bit.

    All the comments above are subject to an examination of the actual configuration of the motor including its clamp assembly and the actual angular tilt measurements. That is simple enough to do and will be well worth your time to do so. If a buyer insists on some measurement or feature and that ultimately fails to not work as well as it might have, then it will be your fault for building the boat wrong. I have been there and done that.

    Best of luck to you.
     
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  3. pafurijaz
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    pafurijaz Junior Member

    Thank you very much, you were very kind and you gave me a lot of useful information, now I can draw the boat correctly, which among other things I'm designing for free. Thanks again..
     
  4. gonzo
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    gonzo Senior Member

    Messabout: that should be 2 to 4 mm ;)
     
  5. pafurijaz
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    pafurijaz Junior Member

    Now I am confused, for now I have made a correction and I have scaled the engine, the anti-cavitation plate is 25mm from the bottom of the boat but if you think this must be 4mm maximum, then I'll have to correct the position.
    In the weekend when I have time I try to do other parts of the project by going into the details of the metal sheets design.
    Dory_skiff_side.png
    [​IMG]
     
  6. messabout
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    messabout Senior Member

    Gonzo the two to four millimeter distance does apply to certain kinds of boat. A racing hydro for example, but not to every boat. The height of the cav plate is a matter of delicate adjustment when top speed and reliable turning ability is the the goal.

    Back in the dark ages, when I was a hydro racer, we would adjust transom height in increments of a quarter inch more or less. The adjustment was tailored to the race course layout and condition of the water. If we had the motor jacked up too high on a course that had tight turns or choppy water the prop could ventilate/cavitate which could realistically cause connecting rods to ventilate the crankcase because of over revving. Such an event would spoil a perfectly good day.

    The OP is building a fishing boat not a hot rod. True enough that if the plate is run excessively low, the shaft housing above the plate will plow unnecessarily. If the boat's bottom runners terminate near the prop they may cause a problem if the plate is too high..........and a whole gaggle of other variables come into play. I am confident that you are well aware of most of these variables and their probable influence.
     
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  7. messabout
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    messabout Senior Member

    Parfurijaz, The detail of your most recent post is good. Let the plate be 2.5 cm below the bottom as in the detail. That is a safe place to put it and that location conforms to standard practice.

    Refer to my conversation with Gonzo, above, for additional commentary.
     
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  8. gonzo
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    gonzo Senior Member

    The drawing indicates 0 to 1 inch. The top of the anti-ventilation (Not an anti-cavitation) plate should ideally be dry and not submerged when the boat is up on plane. Outboards have adjustment slots for fine tuning.
     
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  9. TANSL
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    TANSL Senior Member

    How are they different from each other? What mission does each one fulfill? Is it possible to carry the two plates on the same outboard?. Thanks in advance for your, always well-reasoned, explanations.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Gesss...this is tiresome.
    You're constant attempt to try and trap Gonzo on every single thread that he posts, just highlights, endlessly, your complete lack of knowledge on the subject at hand, yet continue to post as if an authority on the subject!

    Gonzo is correct.

    You clearly do not know the difference between cavitation (boiling of water owing to a high velocity) and ventilation (loss of lift owing to an air path from the lifting surface to the atmosphere.).
    If you did, you would not have questioned his comment....your petty attempts at trying to discredit Gonzo merely highlights your own ignorance on each subject you riposte.
     
    Last edited: Mar 21, 2018
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  11. TANSL
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    TANSL Senior Member

    Dear Ad Hoc :), leaving aside my attitude with Gonzo which, by the way, is the same as yours with me and which, therefore, merely "highlights your own ignorance", I will answer you with pleasure. Yes I know what ventilation is and what is cavitation. I also see that people use one or another name for that same plate. That's why I'm intrigued by the fact that a great expert, such as Gonzo, knows that the anti-cavitation plate is something different from the anti-ventilation plate and, therefore, I ask to learn a little more. If your answer is that I am ignorant, then thank you, you have already answered me with one of your scientific, contemptuous and abundant, answers and you have already confirmed my idea about you two.
     
  12. Angélique
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    Angélique aka Angel (only by name)

    An anti-cavitation plate is a non existing item, but it's an often used misnomer for an anti-ventilation plate by people who don't know the difference between cavitation and ventilation, or they're just being sloppy in the used names, and so unintentionally cause confusion about what's really going on, and what to do about the negative effects that are caused by each of the different problems of cavitation and ventilation.

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    The below is in response to my buddy TANSL - ---> - rant warning to the more sensitive types - ---> - option to skip rant
    By far not true buddy, you often come up with nonsense in general, and specific after Gonzo's posts like above.

    While Ad Hoc sometimes takes a bit of his time to correct your nonsense with good information, which is something quite different from the nonsense you often post on these forums.

    It's only caused by your pure ignorance about matters at hand, and your almost unimaginable rudeness, that you don't recognize and appreciate when being rightly corrected.

    And clearly you also lack the basic knowledge and also the will to learn from from whatever right corrections that you get, which are all kindly given to you here.
    No you clearly don't know anything about it, otherwise you wouldn't have put up the below questions that testify to your pure ignorance about the matter at hand.
    Or you did put it all up for evil polemics sake, like you often do, and so only embarrass yourself, and also intentionally distract the thread, which often leads to your very infamous thread hijacks on these otherwise fine forums.

    An anti-cavitation plate is a non existing item, but it's an often used misnomer for an anti-ventilation plate by people who don't know the difference between cavitation and ventilation, or they're just being sloppy in the used names, and so unintentionally cause confusion about what's really going on, and what to do about the negative effects that are caused by each of the different problems of cavitation and ventilation.

    Any way, it's just clearly explained to you by Ad Hoc, which you didn't appreciate, and so you came up with more rudeness and nonsense, shame on you buddy . . :(

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    Last edited: Mar 23, 2018
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  13. pafurijaz
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    pafurijaz Junior Member

    I also had a doubt about it, but I kept drawing like the example ... But after you made me notice the obvious difference, I did a quick search, and I found a link with some clarification, I also downloaded the manual of the engine manufacturer, and in the manual that is the anti-cavitation plate .. as you can see .. I must however warn the guy that engine is too powerful to that kind of boat ..

    Suzuki_25hp_outboard_02.png Suzuki_25hp_outboard_01.png
     
  14. messabout
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    messabout Senior Member

    Gentlemen, be kind to one another. We are all in this together and we are not being helpful to the OP by generating a pissing contest.

    For my own part, I rest my case. Let us suggest that Parfurijaz defer to the manufacturers recommendation. (2.5 cm) .....Suzuki, Honda, Mercury, Evinrude, Tohatsu, and the rest, recommend pretty much the same dimensions. Special purpose racing lower units ....like a Quicksilver or other brand of special purpose unit are a different breed of cat. That is not what we have here and the conventions are different for the two types. The standard lower unit, as on the subject Suzuki, is more than adequate for general use. Why would one not do what the manufacturer says to do?
     
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  15. Angélique
    Joined: Feb 2009
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    Angélique aka Angel (only by name)

    Excellent info Paolo, thanks . . :)
    Thanks for your friendly call for decency, which I'll gladly meet.
    100% true . . !
     
    Last edited: Mar 22, 2018
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