Over weight boat?

Discussion in 'Powerboats' started by Bruce Hess, Jul 24, 2008.

  1. Bruce Hess
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    Bruce Hess Junior Member

    I posted this in the stability forum but this may be a more appropriate place. Can anyone give me some advice on a 32 downeast yacht I am considering making a offer on?
    It is a BHM 32 hull designed by Spencer Lincoln powered with a 260 HP 3208 cat. My concern is that this hull and power configuration normally has a 14 knot cruise and normally weighs about 14000#. This is based on conversations I've had with a current manufacturer of this design, Atlantic Boat Company.
    The boat I am considering was laid up extra thick and has some other characteristics that have increased the weight to 20,000# and has a cruise of 9 knots at 5 gph.
    Is there anyway to know whether this extra weight has caused detrimental s effects or if repowering the boat with a more modern lighter weight diesel could achieve a moderate increase in speed without a considerable increase in fuel consumption? It seems, at present, that the weight may have negated some of the positives usually achieved by the semi-displacement aspects of its downeast design.
    How expensive would it be to have a NA run some calculations and are there any recommendations of someone with downeast hull experience that could do this for me?
     
  2. PAR
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    PAR Yacht Designer/Builder

    A downeast hull that is built and equipped 30% over the weight of her contemporaries, suggest the transom is well immersed and you'll save little by repowering.
     
  3. Bruce Hess
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    Bruce Hess Junior Member

    Here is a photo of the boat and I've been on her. She doesn't seem to sit extremely low in the water. However she does daft a bit more than her contemporaries
     

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  4. Jango
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    Jango Senior Enthusiast

    You would need to increase power by about 100 hp to 360 hp, in order to get similar performance as a 14000 lb Boat. Seems to me, weight reduction should be attempted first and then perhaps a lighter weight more effecient motor.
     
  5. Bruce Hess
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    Bruce Hess Junior Member

    This same hull seems to preform in the 15 to 17 knot range with similar power. I'm suspecting a prop issue. Can the wrong prop slow you down 6 knots?
     
  6. Jango
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    Jango Senior Enthusiast

    Yes, the wrong Prop could slow you down 6 knots, but unlikely. It would have to be extremely Low Pitched. You would see high rpm,s. unless it was also excessively large Dia.

    Using 3600 rpm as max and a recommended gear reduction of 3:1. w. 260 hp

    Proper prop would be:

    For a 20000 lb Tot Disp. 27D. x 20 P , providing 13.8 knots max

    For a 14000 lb Tot Disp. 27D. x 22 P. providing 16.2 Knots max

    Cruise speed should be about 2 knots less
     
  7. Bruce Hess
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    Bruce Hess Junior Member

    My understanding is the prop is a 24x24 4 blade. I'm unsure of the pitch. The 260 cat 3208 is rated 2900 RPM. I've been told the reduction gear is 2.5:1.

    In preliminary discussions with Tad Roberts even at 20,000# the boat should achieve 13 to 14 knots and at 16000# should be 15knots. I'm not sure if that's wide open or cruise
     
  8. Jango
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    Jango Senior Enthusiast

    My figures agree with Tad Roberts, 13-14 knots (13.8). These would be wide open numbers.

    I suspect the reason for the 24 x 24, 4 blade is Space considerations. My calculations for a 27 x 20 (3 blade) Prop are ideal for a 3600 rpm max motor.

    With 2900 rpm max w. a 2.5 gear, the Ideal Prop would be a 28 x 20, 3 blade (55% D.A.R.). If 24 dia is max usable, additional blade area is required. The 4 blade prop should have a min. D.A.R. OF 74%. Theoretically, a 20 pitch is ideal. The 24 pitch may being used to help compensate for the smaller dia and could be slipping excessively, accounting for the slower speeds.

    Hope this Helps,
    Jango
     
    Last edited: Jul 30, 2008
  9. Bruce Hess
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    Bruce Hess Junior Member

    Jango,

    What you're saying is a little above my boat understanding but I'm gathering the ideal prop would be the 28x20 (55% D.A.R). What does (D.A.R) mean? I'm attaching a couple of pictures that are from the last survey. It looks as if there is room for a larger prop.
     

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  10. Jango
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    Jango Senior Enthusiast

    Yes, according to my calculations a 28 x 20 is ideal for:

    1. 260 HP @ 2900 rpm
    2. 2.50 gear
    3. 20,000 lb

    D.A.R. = Diameter Area Ratio, i.e.

    Total Blade area
    -------------------------------- equals 55%
    Area of circle created by Prop Dia

    In order to use a 28" D. Prop, you would need a MIN. of 5 inches (6 is better) between the Blade tip of your 24" Prop and closest point on Boat. This of course would decrease to 3" (4") with the 28" prop. Min clearance is 10 - 15% of Prop Dia.
    If 28" is not posible, I would go to the largest posible w.74% D.A.R.( 4 blade) and a 20" Pitch.

    I believe my calculations are correct, but you may want to confirm with a reputable Prop Man.

    John G.

    i.e. ( a 24 x 24 will slip alot less with a 14000 lb boat)
     
  11. Bruce Hess
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    Bruce Hess Junior Member

    Jango,

    I'd like to post again when the weight has been verified. Would the ideal prop change much if say the weight came in at 16000#? What affect does weight distribution have on speed? If more weight is aft or forward does this have a lot of effect?
     
  12. Jango
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    Jango Senior Enthusiast

    With Total Displacement of 16000 lb, 2.5 gear, 2900 max rpm, prop pitch could increase by 2 in. Recommended prop 28 x 22.
    Also calculated slip will decrease from 29% to 27% - slightly more efficient.
    Calculated max speed will increase from 13.8 with a 28 x 20 prop to 15.8 knots.
    Weight distribution on this type of craft (relatively slow speed) should have little effect unless perhaps a huge amount of weight is in the Bow. You photo does NOT indicate this situation.

    Sounds like you're on the right track,

    John G.
     
  13. hemidodge
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    hemidodge Junior Member

    31 BHM ,Fly point .altlantic boat

    Hi Bruce, There are a few of those boats in my harbor. 2 of them are owned by family. Both are powered by John Deere 225hp. One is 2.5 to 1 and swings a 26 X 28 wheel and wot is 19 mph with a clean botton. The other is a 2 to 1 and is getting a 24 x 24. That boat is being retrofited with the replacement engine and wheel so I can't tell you if that is the correct combo for it. Another boat just got repowered with ( I believe) a 260 perkins with wot is 17 mph. Another one is a John Deere 225 keel cooled and has a 26 x 28 and does 18 wot. Everyone of those boats I've seen handles weight well but I'm positive the boats aren't 20,000 lbs. I also know of another boat(not a BHM) that must weight that much with a 225 3208 and tries to swing a 26 X 26 and is about 14 wot. So I think you should do at least 14 wot with the right wheel. Those boats have to get the bow up so you might try the weight thing as suggested or at least try to move some aft. gl
     
  14. hemidodge
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    hemidodge Junior Member

    OT Jango

    I'm getting a boat built and am still wondering about prop size. I think I'm under wheeled with what they told me to buy for it. I got a 330 hp d6 volvo with a ZF 2.478 to 1 . The D6 turns 3500 with a peek torque of almost 600 ft lbs , Boat expected weight of 17000lb. What ya think I should have for a wheel? The hull is supposed to be very easy to push. Expecting low 20s wot. tia
     

  15. Bruce Hess
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    Bruce Hess Junior Member

    hemidodge,

    You're just the guy I need to hear from. The boat was weighed today and came in at 19000#. The wheel is a 24x24 and they are getting 12.4 knots at 2850RPM. I'm really hoping that a different wheel could boast the top end to at least 15. Do you think this is likely?
     
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