extending hull - good or bad?

Discussion in 'Boat Design' started by maybeturbo, Aug 7, 2021.

  1. maybeturbo
    Joined: Aug 2021
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    maybeturbo Junior Member

    I'm looking for a flybridge boat in the size 35-40ft. Then I met a owner of a Fairline 36 turbo. He said he was planning to sell his boat after this season. I had a look at it and I said I will buy it if he let me have it before I find something better. We agreed on the price. Hence I might be the owner of a Fairline 36 turbo in a few weeks. The first thing I'd like to improve on this boat is the swim platform.
    The owner told me that the boat does struggle to get up on plane when loaded - family of 4 going on vacation with all tanks full.
    So I got the idea - what about creating the swim platform that doubles as a hull extension? Would that be good or bad for getting a fully loaded boat up on plane? How would it affect the boat in other situations? Will a lightly loaded boat be slowed down by a longer hull? If it was a displacement hull one would expect the hull speed to increase. Does a similar change happen to plaining hulls? Will the fuel consumption go up or down at the most used speed (5-7 knots)?

    (The boat has twin 250hp diesels, shaft drive.)
     
  2. kapnD
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    kapnD Senior Member

    First you must ascertain that the current engines are running up to spec, and they are correctly propped.
    Very likely overpropped.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You really need to evaluate the situation after trialling the boat, one step at a time fashion. It is too early to be planning alterations to a boat you don't really know the actual performance of, as it stands.
     
  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    It doesn't appear that the boat is exactly over provided with power and there may have been other engine options available from the factory.You could add an extension in the manner described and not get the gain you are seeking as it would definitely increase the wetted area of the hull and consequently the drag.It might be that some experimenting with trim tabs and propellors would extract a bit more performance.So might a ruthless clearance of "stuff" in the lockers throughout the boat.I have a feeling that if you asked about the performance of similar boats on this forum Motor Boat Forum https://forums.ybw.com/index.php?forums/motor-boat-forum.14/ a few owners would pop up with real world advice.

    Edited to add: any extension that isn't a bonded on single skin might add buoyancy as well as wetted surface and could affect trim.A floodable compartment,similar to the bottom of some RIB's might be possible but might just become a haven for mussels or other marine life.Which could make a nasty smell over the annual layup period.
     
    Last edited: Aug 8, 2021
  5. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Is being fully loaded and all tanks full with the entire family on board something you will likely experience?

    Until you've had it on the water and tested it in the way you plan to use it, you won't know if needs help or not. And if it does, it may just need fine tuning as mentioned above.
     
  6. maybeturbo
    Joined: Aug 2021
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    maybeturbo Junior Member

    The use of the boat will be full family for several weeks. My family likes books, so it will be heavy loaded most of the time. When I tested the boat we were two families onboard, all thou only gear for . About 3/4 fuel, full water. We managed to get it up on plain, but it took quite a while, steering had to be turned from side to side and trim tabs had to be maxed. Once up, tabs could be raised. Sure there will be some gain if engines are fully disassembled and cleaned. Extending the hull is kind of like adding large fixed trim tabs. The size of the "fixed tabs" doesn't need to be full width. It could be made like the sides only are down in the water. It could be like center only is in the water. It could be made like the hull bottom profile in full width. It could be made round like the stern of a sailboat. Or anything in between these options. The only thing that is certain is that the swim platform will need some kind of support that will be in the water at slow speeds.

    Sure this thing will provide some more wetted surface. But it will also provide more lift. Larger trim tabs will also provide more wetted surface, and while used they are also extending down under the hull acting like brakes. Using brakes while trying to increase speed doesn't sound like the best idea either. I'm not sure what is worse.
     
  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Better get a survey. Maybe the hull is soaking wet and the issue isn't power, or maybe the boat has been loaded too heavily with stuff.

    The idea you'd extend the swim platform as a hull extension is... bonkers. Sorry.
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    In your original post, you said that the boats most used speed will be 5 - 7 knots but rest of your comments are suggesting that you want the boat to plane easier. A rounded sailboat hull form at the rear might be good for a fuel saving at the 5 - 7 knots, but may offer little else for planing efficiency. Ie if you are going to extend it for better planing performance, keep the chines sharp, Add lift strakes.

    While there will be more wetted surface, there will also be more area to generate lift and the additional lift will be more significant than the skin friction. If the planing parameter is most important, extend the transom under the swim grid, the bottom and sides.

    There is a Fairline owners forum/group. I would spend a bit of time to find them out to see what others have done. Another contributor mentioned this option but should be a main focus of your information search

    Are you then moving the engines rearward? If not the CG/CL will change. I cannot say that I have seen the prop location remain the same with with an extended transom/swim grid.

    You are correct in that the short narrow trim tabs act as brakes.
    Before assuming the expense of the extension IF YOUR ONLY GOAL IS TO GET THE BOAT ON PLANE EASIER an option would be to custom fab a full width pair of trim tabs. Which effectively give you a hull extension. Make them longer than
    the norm but full width. The longer wider tabs will provide more lift and less drag than your current tabs
     
  9. maybeturbo
    Joined: Aug 2021
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    maybeturbo Junior Member

    Thanks Barry for these toughs.
    For some reason I'm assuming that rounding the stern, like a sailboat doesn't really help with anything. The original stern is nearly 4m wide. Even with a long swim platform, like 1.2m, there simply isn't enough length to make a round stern. Hence I thought it's better to go for improved plaining than rounding that is to sharp to do anything useful. I might be wrong thou. In my mind, extending the hull in the same profile as the original hull, just longer, the stern will get more lift. This will result in easier to plane when heavily loaded. Plaining at lower speed when not loaded. The bow will require one more knot before it starts to lift. At low speed, the modification will not give any noticeable changed behavior.
    The option of rounding the stern will worsen the plaining ability, and I don't really see that it will improve low speed behavior unless the platform is made extremely long - like I could add a second aft cabin and a swim platform, and still it would be too short to do much low speed improvements.

    I did not intend to move the props. I'm aware that an extension will affect steering to the worse (unless I do something about it), but I don't think it's going to be much of a problem to leave it as is.
    I'll search to see what kind of modification other owners has made...
     
  10. kapnD
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    kapnD Senior Member

    I’d encourage you to look into the suitability of the current propeller dimensions.
    It should be sized to make maximum rated rpm at WOT with a full load. (Two families, lots of books, and full tanks)
    In its current state, it will eventually harm the motors.
    I’d get this straightened out before even thinking about hull modifications.
     
    DogCavalry, BlueBell and bajansailor like this.
  11. maybeturbo
    Joined: Aug 2021
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    maybeturbo Junior Member

    Does anyone know where to find the max rpm on old volvo engines? (Boats I've tested so far has been powered by VP tamd 60c, tamd 61b and tamd 71b)
     
  12. wet feet
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    wet feet Senior Member

    I'm a bit surprised that a poster from Sweden,of all places,is finding it necessary to ask outside his own country for information on Volvo engines! Did you sign up for the forum I linked to in post #4? There is a considerable amount of knowledge there if you ask such questions.
     
  13. kapnD
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    kapnD Senior Member

    A major factor in “testing” any diesel motor involves knowing the motors specs beforehand, and insuring that it is operating within them.
    The most common fault I’ve found on diesel powered pleasure boats is overpropping, meaning that the motor is unable to turn rated rpm when fully loaded. This will lead to many problems, and eventual demise.
    The solution could be to simply clean the bottom and running gear of marine growth, or it may involve reworking the propeller, or even work on the motors fuel and air systems.
    It should start up immediately from stone cold, and run smoothly without much smoke.
    The engine and engine space should be clean and not rusty.
    Arm yourself with facts before you go shopping!
     

  14. maybeturbo
    Joined: Aug 2021
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    maybeturbo Junior Member

    With cars, the engine specs is easily accessible. I might suck at searching, but I've not been able to find specs for the VP engines.
     
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