Does a hull need to be specially designed for surface drive.

Discussion in 'Surface Drives' started by tom kane, Aug 4, 2015.

  1. powerabout
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    powerabout Senior Member

    Thats what I mean Tom hard to think of many hulls that are designed tested and modified over the years for Surface drives only other than Magnums
     
  2. powerabout
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    powerabout Senior Member

    can you shift the prop shaft vertically without changing the angle so when its buried the angle is such that the water coming from the prop will be directed under the boat when in reverse?
    Thats the catch, a Sterndrive propshaft axis is under the boat whereas any surface drive is above the hull bottom, might be less drag and might go faster but everything else is a compromise
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    Hull one is a very classic deep V planning hull with a also very classic Levy drive. These drives work very well on 10-12 degrees V hulls.
    The second one is a very light skimming boat who needs desperately some grip in the water or it will slide on the side like a piece of soap in a bath tub and a big rudder to counteract the torque induced by the propeller. That must be funny to drive.
    The third one seems to be a classic Arneson drive completely up. The normal position is something like 8-10 degrees down from the horizontal as the propellers are only half in the water made by the "wake wave" of the boat. You can see that the designer made a lot to give water to the "wake wave" with even a "tunnel" in the center line. These drives use homocinetic joints so the angle is not very limited.
    Does a hull need to be specifically designed for a surface propeller? Yes and no. If it's a 10/12 degrees V planning hull (monohedrons are very well suited) no problem. If it's a deep V that can become delicate.
    Does a surface propeller need to be complicated? Yes and no. I have seen boats designed for simple fixed surface drives working very well, as well semi displacement boats as well fast boats.
    The boat of the captain of harbor of Sete was well known with its 2 enormous hand made 6 blades propellers at the end of shafts just protruding from the stern, and it was used as tug (yes!) boat, pilot boat, servitude boat, light house boat, rescue boat and fishing boat the Sunday. But the designer was a master of surface propellers on semi displacement and planning boats...he made more than 30 with fixed surface drives from 25 to 100 feet. The secret was in the variable aeration system of the propellers and the shape of the stern. The propellers were protected by a platform for evident reasons, as they were giant meat grinders. There was a system of asymmetric twin rudders with ackerman that worked flawlessly.
     
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  4. tom kane
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    tom kane Senior Member

    does a hull need to be specially designed for surface drive

    Surface drives with retractable shaft ( Pivotal Drive)can be lowered so that the thrust in reverse is directed under the boat.Just like a fixed standard 20 degree shaft drive,the angle can go from horizontal down to 20 degrees shaft angle (or more) if you want it.

    To run surface drive on a stern drive so the prop directs all the thrust under the boat in reverse you would need a jack able stern drive.
     
  5. powerabout
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    powerabout Senior Member

    if you lower the shaft the prop wash is pointing at the transom as that is where the the propshaft axis comes from
    on sterndrive you trim it out and the prop wash goes under the boat
    just make a drawing of the 2 prop shafts and their locations and you will see one of the achilles heel of a surface drive.
    way back in the 80's several boat builders in OZ fitted surface drives in sterndrive boats, yes they did go faster but all other areas of handling were so bad they knew customers would not accept it nor have since if you look at production boats with them.
     
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  6. tom kane
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    tom kane Senior Member

    does a hull need to be specially designed for surface drive

    If I was looking for a hull that would cut through the water and displace that water and give a softer ride that hull would suit fine but it would not be an easy planning hull and require more HP than a moderate v to get dynamic lift. There is a formula in David Gerr`s book Propeller giving information on weather a hull is capable of attaining plane attitude.
     
  7. tom kane
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    tom kane Senior Member

    does a hull need to be specially designed for surface drive

    If you trim a stern drive out in reverse that is run in S/P mode and set for S/P operation lifting the stern drive would bring the prop out and up to the surface.
    You would still need a jack able stern drive to lower the stern drive deeper so as to get better thrust.
     
  8. ChrisN67
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    ChrisN67 Senior Member

    I have found that the benefits of a surface drive start at 30 knots, thus whatever the hull form, the power and design speed are the determining factors on whether a surface drive would be of benefit. Obviously a full displacement vessel is not a good candidate as they generally have significantly lower speeds than 30 knots.

    Installation angle is very important. Most surface drives require an angel of 7 degrees on the transom AS WELL AS be mounted as low as possible to be parallel to the water line.

    You can deviate from the optimal installation requirements but then you are changing the angle of attack of the propeller blades.

    I have seen installations that we completely dorked up but worked fine; marine engineering is one of those disciplines where two wrongs can make a right.
     
  9. powerabout
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    powerabout Senior Member

    plenty of go fast stern drive installs have the propshaft level with the hull so all the advantages and none of issues surface drives coming out of the transom create
    thrust is for tug boats
     
  10. Rik
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    Rik Senior Member

    Yes but your assuming that a conventional I/O is has some sort of magic ability when in reality when you are at the surface, either with a paddle wheel or a Bravo you are at the surface and you cannot lower a "Z" drive down to get deeper to reverse as it has a fixed center distance between centers. There's not sufficient negative trim in order to get deeper into the water. So what advantages are perceived are not there in reality as those running the I/O at the surface have the same exact complaints as those running any other surface drive. I'm not here to critique the I/O but just because of it's sheer volume doesn't equate to it having any advantages other than economies of scale and cost.

    To answer the original question, does a hull have to be specifically designed for a surface drive? No. Or at least from my experiences the answer is no.

    Mind you, there are some boats that are just not a good candidate and those also are not a good candidate for a speed boat in general. While their short comings might be masked by having a deeply buried I/O attempting to leverage the stern down to get the bow up this is not an efficient method of propulsion.

    If you have a good performing boat, it will not perform worst with an surface drive (please note not all surface drives are created equally) and if you have a poorly performing boat it's troubles will not magically go away with the use of a surface drive.
     
  11. powerabout
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    powerabout Senior Member

    draw a side profile of stern drive with the pivot in the gimbal and then the propshaft showing the arc the propshaft creates versus the water plane
    now do the same for a surface drive
    you are going to get a different result when in reverse even if the stern drive was propshaft level with hull ( which is already lower than a surface drive can do as the shaft must exist from the transom)
    so in the above in a marina you trim out the sterndrive so the prop wash goes under the boat, doing the same with a surface drive and the prop is out of the water
    trim the surface drive right down and now the prop wash is aimed at the transom give or take?
    its one clear advantage the sterndrive has for the average consumer. ( not that surface drives dont have other advantages)
    Bow lift if you need it is another or move CoG back to get same trim as Stern drive.

    I can see when we are talking hulls so big and heavy that a sterndrive is not an alternative therefore the prop diameter is way bigger its less of an issue.
     
  12. tom kane
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    tom kane Senior Member

    With a Pivotal drive there is a slot in the transom and the bottom of the boat which allows the shaft to lower down 20 degrees or more if designed that way, like a fixed shaft drive so reverse is as good as a fixed drive.

    For reverse you lower the pivotal drive fully and lift the drive (from the helm) to any shaft angle you desire for forward drive and even lift the prop right out of the water.

    Modern Surface Drives are designed and installed different to Hickman`s sea sled which used large diameter props and different blade design and were fixed in the transom and very close to the transom too so only the lower part of the prop was below the water plane level.
     
  13. BMcF
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    BMcF Senior Member

    Then how would you explain the superlative results achieved by converting a vintage Donzi (arguably one of the best performing out-drive/stern drive boats of the last few decades) to an Arneson surface drive?

    If I've learned one thing that is absolute in this business...it's that nothing is absolute. I'm currently converting a vintage Charley Champ 17 speed hull (1972) to Berkley jet. It started life with a Volvo out drive behind a Holman Moody V8..then I put a warmed up 235 Evinrude V-6 (~275HP) on the back, attached to a Land&Sea hydraulic jack plate....

    Point being...some hulls are "happy" with just about anything pushing them. There are no absolutes.
     
  14. ChrisN67
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    ChrisN67 Senior Member

    Agreed, but they are usually deep V planing hulls.

    But those high dead rise, narrow beam boats at rest are horrible in swells and any sea state above calm.

    It is always a compromise; generally trying to maximize one engineering aspect results in lower usability or satisfaction to the general public.
     

  15. tom kane
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    tom kane Senior Member

    I agree with you BMcF although Surface Drives are for speed so some hulls would not take advantage of S/P propulsion.
     
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