Repowering with outboards- how much HP?

Discussion in 'Boat Design' started by skypoke, Aug 27, 2010.

  1. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    I'm trying to help a friend retrieve his houseboat which is presently stranded in a shipyard a couple hundred miles away. Not my boat..

    This thing has a 200 hp. diesel with sterndrive, 30 years old and not repairable. It has been removed and the stern sealed. The boat is 12' at the waterline, 43' length at wl, average immersion is 12". Based on this I'm guesstimmating 30K lbs displacement. It's pretty much a standard design, house on deck, uncovered upper helm area. It is a very low angle V monohull, maybe 5 deg. vee.

    Since this boat operates on the ICW it is occasionally having to fight its windage...difficult but doable with the single screw. I'm thinking a pair of widely spaced outboards will help overcome this problem. When it was running wide full open with prior power it would achieve 9 knots, it would be nice to have a 7-8 knot fast cruise.

    Naturally, talking with engine dealers they are saying "ya gotta have at least twin 150's, 200's would be better." I'm saying that a pair of 50's would certainly push it. What no one is thinking of, other than me, is that props are going to be a key element. Largest diameter, probably 4 blade, lowest pitch obtainable is what I'd be looking for. The BIG question, though is what horsepower would be required to maintain controllability in a modest wind situation. (Anything overr 15 knot winds will call for the hook.)

    Engines will be mounted on setback brackets fabricated from .5" alum alloy. I'll build these.

    I'd certainly appreciate any advice. I see Yamaha makes a "high thrust" 60 hp engine with somewhat lower gear ratios. What about a pair of these? Really appealing facet of this repower is that we can prefab the brackets, fit the engines, use standard wiring and gauge harnesses, reuse the hydraulic helms and lines, with a little luck a day after it hits the hard we can be very close to having it ready to go.

    Thanks for the help,

    Chuck
     
    Last edited: Aug 29, 2010
  2. apex1

    apex1 Guest

    The HT Yamahas will most probably give you the required 7-8 kn, when the 200 hp provided 9. May as well happen that the outboards give the same speed, due to better efficiency in open water (not sitting behind the "keel").

    A picture of the underwater section is not available? Nor are plans, I fear.

    Controllability should be achieved with less hp already, when you mount them as planned.
     
  3. WickedGood

    WickedGood Guest

    Twelve feet at the waterline with a 43 ft overall!

    Your gonna need a lot of power to push that puppy.



    I would reccomend about 2,000 HP or more.

    Hang on! Its gonna be a tippy ride

    [​IMG]
     
  4. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    If you are going with four-stroke motors (most new ones are now four-stroke), you need to be sure to have the engine operating in the upper range. Many modern four-strokes are designed to be run at upper rpms, otherwise they are famous for making oil (rings not seating, fuel blow-by in the pistons); therefore, I would probably go on the smaller side (the 60HT motors should be sufficient) and run them at the upper end of their operating range whenever possible. Believe it or not, they will last longer than if you run them at lower rpms.
     
  5. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    Absolutely agree with V.I., the four strokes need to be kept wrapped up. My Honda 150's have a much higher redline than prior two strokes did. They're real happy at 4500 rpm or more.

    No, plans are long gone. I can describe the hull at stern as being 12' at the wl, sides are immersed 8", keel is 14" deep.

    If it was mine, I'd go with the pair of 60 hp. If controllability remains a problem then I'd most likely redneck engineer some kind of bow thruster unit, possibly a remote steered high output trolling motor mounted in a well. Taking a clue from the push boats on the icw, many of them have an auxiliary propulsion steering barge at the front of the "tow."

    Anyone else see it differently, that higher hp are called for? Would it be legitimate to use the standard speed/hp calculation for displacement hulls with this odd duck?

    Tentative plans are to mount the engines on approx 8' centers. I'm leaning on mounting them somewhat deeper...anticavitation plates more deeply immersed...than you'd normally see with outboards, within limitations of engine of course.

    I appreciate your insights.

    Chuck
     
  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If twin 50-60HTs are in the picture, I see no reason to put them 8' apart. Just stick them on a standard dual bracket. the motor mfg will provide a min separation for twins. I would try to add a few inches to this number, but that should be good enough. Now if you end up looking at twin 20hp twostrokes, different story. I feel that having the motors separated by 8 feet is more trouble than benefit, even while docking. Much easier to bang them into things out there on the corners and it just generally clutters up the back of the boat. Good luck and - um - don't take anything too valuable. I know where three sunken houseboats are that have the same story as this one. Strongly recommend more than one short test excursion prior to the big move. I would like to hear from others on the separation issue
     
  7. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    The reason for the separation lies in the desire for controllability in wind, manuvering in tight quarters and so forth. Owning a powercat with widely separated engines has spoiled me perhaps, it's easy to rapidly spin the boat in its own length with no forward motion. It seems that countering a breeze from abeam would also be aided by wider separation, just modulate one throttle a bit, the lever arm is multiplied by separation. Wider separation also allows for the powerheads to be higher above wl. That's my thinking but I've been wrong before!

    In the interest of learning from others experiences, I'd be keen to know the story behind the three sunken houseboats.

    Chuck
     
  8. philSweet
    Joined: May 2008
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    philSweet Senior Member

    So we come at this from opposite ends. The houseboats I have driven were 40' rentals with single 80HP IOs. I'd rather churn up some foam when its windy than bump in and out of gear all the time on mild days while docking. I do believe there is a real benefit to keeping the arrangement as small and tidy as possible consistent with the needed maneuverability. Hopefully others will chime in. You might also try the houseboat forums. The one thing the three sinkings had in common was a certain amount of desperation. Two sank while being towed. There was the need to remove them from their previous home. The other was under power for the first time in many, many years.
     
  9. Submarine Tom

    Submarine Tom Previous Member

    I have a 25 foot (23.5 LWL), 10 foot beam, 12 000 pound (5 500 kg), rectangular (barge) houseboat that makes 5 knots with an 8 hp 4-stroke WOT.

    It'll do 6.3 knots with a 25 hp, Merc Bigfoot WOT.

    And 6 knots with twin 9.9hp, 4-stroke Merc's on six foot centres in line with the two skeggs (flat bottom). Skeggs are 5" wide and run about 17' fore and aft with a tear-drop finish at the transom.

    I would consider twin, 25 hp BigFoots on 6 to 8 foot centres for your application.

    Good luck.

    -Tom
     
  10. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    I suspect the numbers by Submarine Tom are true but, you probably want more insurance than that for water and wind conditions that can catch you, even on lakes and the ICW. Its a sick feeling to find that you need more thrust and find that there is none there. Docking or entering a slip in a crosswind can be a bear on a boat that has little water grab and lots of windage. Wider spaced props can be a big help with that.

    Both Yamaha HT and Merc Big Foot (same basic engine) show best operating RPM as 4400. With 14" x 11" props (the largest stock props available), they have a lot of grunt, even at the low end. Choose 25" legs and operate as low in the water as you draft limitation will allow while keeping the tilt mechanism out of the water. Motor drag is not an issue with your speed and rig so depth will not hurt you there. I've heard that Honda also has a new 60HT out now. Suzuki engines are bigger (more displacement) and heavier than the competition of equal HP rating. Etec claims more low end torque from their 2 strokes. Those might translate into more low end grunt also. Too bad motor manufacturers are so stingy with their data.
     
  11. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    I want to thank everyone for taking the time to give so many well thought out responses to this request. I'm feeling a high level of confidence in the success of this endeavor. We'll go with a pair of long shaft outboards, 60 hp min., ea, mounted as deeply as feasible. I'm finding some low pitch props in the right diameter, at this point the best I've found are 9" pitch, lower pitch would require cusom tweaking by a prop shop. No idea what kind of slip factors to use in prop calc.

    Will follow up with results when they happen. My bud's motor shopping as we speak.

    Thanks again,

    Chuck
     
  12. Village_Idiot
    Joined: Oct 2007
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    Village_Idiot Senior Member

    I suggest checking out the Yamaha DualThrust props (14x11p). I tested one a couple years ago and they pull HARD in reverse, almost as hard as going forward. I think they would be ideal for push-pull maneuvering.
     

  13. apex1

    apex1 Guest

    That sounds reasonable.
     
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