converting 34ft twin inboards to outboards

Discussion in 'Boat Design' started by naturewaterboy, Sep 20, 2006.

  1. Bob S.
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    Bob S. Junior Member

    I'll vouch for that. In my area we slap an 8-10 hp 4 stroke on fairly beefy fishing boats to troll for salmon. At WOT going with wind & current you can make 5-6 knots even with that little power. Likely that 50 hp would provide a similar speed for the 34 Silverton even into the wind. I wouldn't want to be in any real snot with those small engines though, at least not in a big, empty planning hull. You'd be sideways to the trough in a hurry and with not enough power to bring her around the bottom of your boat will likely be on the evening news.
     
  2. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    So it sounds like I definitely don't want to put a pair of 50 hp on the boat then take it out in rough seas. If it had 2 150s on it though, wouldn't that be enough power to keep it from being pushed around - enough to keep it going where it was pointed? The boat could run at 6 knots with the engines loafing - unless things got sloppy, then crank the engines up and plow thru. Would this work?
     
  3. Bob S.
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    Bob S. Junior Member

    Nobody intentionally goes out in rough seas (usually), problem is they get nasty once you are out there. The bigger power would provide a means to make safe harbor at a reasonable clip, to keep your nose in the direction you want when bad things overtake you or safely in the slot when you need to run an inlet. No law says you can't shut one of them down and putt along at 1500 rpm on the other one when conditions allow. I'd set it up to run as a properly powered boat. Even the biggest 4 strokes hardly use any gas at displacement speeds but the power will be there when you need it.
     
  4. Frosty

    Frosty Previous Member

    I agree with Bob you dont want to be in a trough and not be able to get the stern round, and in a bit of wind you wont and thats frightening stuff. I think a pair of 150's with 17 pitch might get it onto the plane but you will have power to save your life.

    Here in Thailand they have huge speed boats probably a similar shape made from ply. They are about 30 feet and carry 60 pax to islands such as PiPi ( where they made the Beach movie) these monsters use 3 or 4 Yamaha 200 hp and handle rought stuff ,--infact they are called upon occasionally to go out to rescue.
     
  5. Willallison
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    Willallison Senior Member

    Unless we know the boats approx weight (preferably withoiut the old diesels) then there's no way of knowing if it would get on the plane with a pair of 150's. If it's not too heavy, it might, just - though remember it originally had a pair of 260hp inboards, which were considered underpowered....

    To suggest that you need 300hp to simply 'bring the stern around' of a displacement boat is ridiculous. If it needs 30hp to putt along with, then 100hp ought to be more than enough to push it 'around'. The problem though - regardless of engine size - will be ensuring that the props remain under water.
     
  6. Frosty

    Frosty Previous Member

    Yes Will the boats wieght would be of great assistance to move forward on this. However I dont think any one was saying that 300 hp was required just to push the nose around, but if you got it then you got it. On the other hand I would not under estimate the amount of power required to quickly get the nose around. If you had the misfortune to find yourself side on to the sea and you had 3-4 seconds before the next wave I would gas those 150's

    In a wind and with that boats wind resistance the bow will be all over the place and if left to drift would probably present her stern to the sea. If for no other reason that the props would offer drag.

    For such conditions as these I carry a sea anchor, but doubt and hope I will never use it as I genarally island hop for the reasons stated above . My power cat has 500 HP and at zero speed I could not bring her round in 5 seconds
     
  7. Bob S.
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    Bob S. Junior Member

    Its the same problem we all wrestle with . . . trying to make one boat do a variety of contradictory things. In this case the owner needs the attributes of a lake-bound shallow water houseboat along with the ability to range far offshore and be safe in poor conditions. To complicate things instead of starting from a blank sheet of paper these characteristics are to be coaxed out of an old beamy powerboat hull. Makes for great conversation . . .
     
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  8. Pericles
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    Pericles Senior Member

  9. naturewaterboy
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    Location: key largo, florida, usa

    naturewaterboy Steel Drum Tuner

    Yes, I've talked to Armstrong and a couple of other manufacturers at the boat shows about building a custom bracket for the boat - probably cost about $5k. Thought about finding a good welder and building my own, but probably just let Armstrong or Gill or Stainless build one. I think it was Armstrong that was the most willing to build one for this boat - but none of them could tell me much about any brackets they built for similar boats.
     
  10. Bob S.
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    Bob S. Junior Member

    Your best bracket for that boat would be a Porta type. It would give you up to 24" of vertical lift which would provide the flexibility you'll need. Way up for shallow running and way down for offshore bite. Its not a buoyancy type of bracket which I don't think you'll benefit from considering all the weight that has been removed from the aft end of the boat. You are going to be light back there anyway IMO. Easier install than an Armstrong type as well.

    http://www.portaproducts.com/bracket.html

    Ask for Scott, they are in Edgewater

    Put a twin engine, 26" setback Porta Bracket on that thing with a pair of 175 Suzuki 4 strokes and you will be happy
     
  11. naturewaterboy
    Joined: Sep 2006
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    naturewaterboy Steel Drum Tuner

    performance table

    Here is some more info on the boat - I believe the boat weighs about 10,000 to 12,000 lbs. without engines. As you can see from the picture in an earlier post, the hull sticks out of the water and will catch a lot of wind. I have a rpm speed table from another 34C Silverton with inboard gas engines - I'll paste it below - it has 7 columns and it's hard to read when pasted, I'll attach an excel spreadsheet also:

    1978 34' Silverton Sedan w/ 1986 270 hp Crusaders spinning 17"x19.5" 3 blade props
    Full fuel, 1 person onbard with 100 pounds of gear
    INTO THE WIND AT 10 MPH

    RPM's gph - port gph starboard Total Speed
    Knots Miles per hr Miles per gallons
    1000 1.5 1.75 3.25 4.6 5.2 1.6
    1500 2.5 3 5.5 6.4 7.3 1.3
    1600 3 3.5 6.5 6.9 7.9 1.2
    1800 3.25 4 7.25 7.1 8.1 1.1
    2000 4 5 9 7.1 8.1 0.9
    2200 5 5.5 10.5 7.6 8.7 0.8
    2400 6 7 13 8.9 10.1 0.8
    2600 6.5 8 14.5 10.4 11.9 0.8
    2800 8 9.5 17.5 11.4 13.0 0.7
    3000 9 10 19 13 14.8 0.8
    3200 10 11 21 14.4 16.4 0.8
    3300 11 13 24 15.5 17.7 0.7
    3400 11 15 26 16.7 19.0 0.7
    3600 12 18 30 17.8 20.3 0.7
    3800 15 18 33 20 22.8 0.7
    4000 16 20 36 22 25.1 0.7
    4600/4400 20 22 42 24 27.4 0.7

    WITH THE WIND, AGAINST THE CURRENT
    3200 9 11 20 15.8 18.0 0.9
    3400 11 14 25 17.1 19.5 0.8
     

    Attached Files:

  12. Bob S.
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    Bob S. Junior Member

    see post #40
     
  13. naturewaterboy
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    naturewaterboy Steel Drum Tuner

    I've decided for now that I'm going to put on a bracket and hang a pair of 250 or 300hp 30" shaft outboards on it. I am going to look for a pair of used engines, and have one of the manufacturers design a bracket with flotation. I can then experiment with fuel and water tank placement to balance the boat properly.
     
  14. Willallison
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    Willallison Senior Member

    I think you should do a simple weights and moments calc to establish just how far out of balance you're likely to be.
    Take the weight of the inboard engines and gearboxes and multiply by the distance aft from the bow. This gives you the moments.
    Do the same for the outboards, including the bracket. It'll only be rough because you are ignoring the shafts etc, but it will give you a rough idea of what you're up against.

    Trying to ballance the boat using tankage is best avoided if you can, as these (quite obviously) change depending on how full they are. Batteries and other permanent things are better if they're sufficient.
     

  15. Frosty

    Frosty Previous Member

    You should have no problem balancing the wieght after all you now have an empty engine room . Store your stuff there ropes ,spare anchors, beer, jerry cans, whatever, Paint tins, and beer. You could also put in a fresh water tank , thats handy!!!
     
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