Turning a planing hull into full displacement

Discussion in 'Boat Design' started by fiftysixer, Oct 23, 2009.

  1. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Of course. All the planing performance will be gone .. but so will the astronomical fuel consumption. And of course it would take just as much time and money to convert back assuming he didn't repower. And the original vessel would be gone. And the new vessel would not be comparable to a boat designed to be a FD boat to begin with like a Krogen of similar size. Big planing boats are cheap now and good FD boats are not. The FD conversion would only make sense if fiftysixer HAD to reduce the fuel burn and could NOT afford to go to the FD hull by buying another boat. Fiftysixer wanted to know if a planing hull could be converted to a FD hull and I offered a way to do it. Only he would know his situation well enough to know if the FD conversion would be cost effective. Quite likely if fuel went down to $.84 a gallon fiftysixer would probably be back on the forum asking if a 3rd engine could be installed in his boat. I am willing to enter into a discussion about the advantages and disadvantages of the conversion. It sounded like fiftysixer was ready to buck up to the bank high enough to put a bulbus bow on his boat and I doubt if the bulbus bow would cost much if any more than a stern sex change.

    Easy Rider
  2. apex1

    apex1 Guest

    Well, dear biker pal,
    in post #10 I provided a link to the first thread to make clear what the intended use of this vessel is.

    At trolling speeds around 3kn there is no "astronomical fuel consumption" no matter which sort of engine there is installed. The required power remains the same, bulbous bow or not!
    So, the first point where fifty would like to save some fuel is shot.
    At displacement speed, the installation of Diesel engines would save some fuel compared to petrol engines, regardless of any hull "modifications", but that would be not a big saving. BUT:
    The Diesels would not stand the trolling mode for long!
    Converting a pair of new Diesels into scrap in short time can not be what he wanted.
    So, the second idea is shot.
    Paying a NA to draw some plans for a conversion of the planing hull into a efficient displacement hull would be wasted money, the result will always be a bad compromise in the best case.
    So, that idea is shot!

    Leaving it as it is, is not only very cheap, it is the only serious advice we can offer.

    At trolling speed his hull is´nt worse than a true displacement hull. The petrol engines do´nt consume noticeable more than Diesels would. So, his trolling setup is sufficient to good already.

    At displacement speeds, say up to 8kn (what a true displ. vessel of that size could do), his hull form is not as comfortable, not as efficient as a Diesel trawler would be. But how much more fuel will he burn? 1 liter? 2?
    Is it worth a investment of many, many thousands of $$$ to save a cup of fuel every hour?
    Of course not!

    Now we come to the astronomic consumption: who is holding a gun against his head and asks for 25kn?

    At trolling we have seen: no difference
    At displ. speed we have seen: marginal disadvantage
    At top speed, YEAH thats it! The screwed up hull will sit at maybe 10 -12 kn and suck fuel like hell. The present setup is still within a reasonable consumption at any speed below 15 or 17kn!
    And he has the benefit to go relatively fast when it comes to escape a weather front, or when there is a need for speed. At express train rates of course.

    And, once the hull is "converted" the boat has lost it´s value completely!

    So, selling the boat and buying a small trawler is one solution.
    Leaving it as it is, the cheaper one.

  3. fiftysixer
    Joined: Oct 2009
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    Location: cleveland

    fiftysixer Junior Member


    I did not think to check the crane guage.

    I must have mis-remembered about the 28 knots, Thats why I wanted to make one last run to get accurate numbers.

    Do you still feel my best fuel economy will be at the rpm that takes the boat from displacement to plane? Or is it better to run at 6-8 knots with that hull for the 100 mile round trip I want to take next summer?

    One other note. The previous owner told me he always trimmed the boat out with nose of the boat way up, almost to the point you could not see very well over the nose. His theory was that less of the boat in the water the better economy. I have found by trimming the nose down in the water I acually picked up speed at the same rpm sometimes by 1.5 mph. Is that a result of that hull design?
  4. apex1

    apex1 Guest

    Hope that helps.
  5. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member


    What do you think this is .. a debate club? You seem to have a mission to shoot my idea down and I have no problem with your thinking it's a bad idea but a little objectivity wouldn't hurt. To say the displacement conversion would only "save a cup of fuel every hour" is ridiculous. Any fool knows a FD boat will burn about 1/2 to 1/3 as much fuel as a planing boat a bit below hull speed. And "Easy Rider" has nothing to do w bikes .. it's the name of the prototype of my boat design that I actually built. I felt it was fitting here. I named the boat before the movie. I can see and excuse the fact that you jumped on the bike bit.

    Easy Rider
    1 person likes this.
  6. apex1

    apex1 Guest

    Are we a bit pi55ed yeah?
    May I ask where in my post #17 I was offensive?

    So, may I ask why are you? What do you think, this is a debate club? There is a member asking for experts advice, not for your amateurs opinion.
    Oh yeah, you have it easy, it is not your money that will be spoilt by a completely senseless conversion. No, it is not mine either, therefore I am careful in which sort of idiotic adventure I talk my peers here.

    Why should I want to shoot a mad idea down? You are missing the objective when you compare a true displacement hull and a more or less functional "conversion". Which btw is´nt possible in a efficient manner.
    What benefit should a bulbous bow deliver in your opinion? Did you notice the boat size? How would you make the performance much better by twisting the quarter beam buttock line?

    So, when it comes to: "any fool knows", lets start with your foolish ideas.;)
    That much to objectivity.

    And read the related thread to understand what we are talking, before you make the next mistake.

  7. Zappi
    Joined: Oct 2008
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    Location: Puget Sound

    Zappi Senior Member

    Originally posted by Apex1>> "Are we a bit pi55ed yeah?
    May I ask where in my post #17 I was offensive?"
    Could it possibly be your snide little comment at the beginning of post 17????
    You're obviously a smart guy when it comes to boats. Not so sure about when it comes to people. This forum is not solely based on "PROFESSIONALS". Even "AMATEURS" can add great insight and thoughts to all of us. Have a nice day Dick.
    1 person likes this.

  8. apex1

    apex1 Guest

    Thank you Dick for that comment.
    Snide little comment?

    Yes of course, amateurs can and should add their thoughts and insights!

    AFTER being familiar with the topic!

    Have a nice day too.
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