26'/5900lbs/255hp/Volvo outdrive/13mph tops?!

Discussion in 'Props' started by IronRoamer, Sep 29, 2009.

  1. IronRoamer
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    IronRoamer Junior Member

    Hi all,

    I have a 1978 Fiberform Baja 26' Command bridge, the dry weight of the boat is approx 5,900lbs + .5 tank of gas (85 gallon tank) and some furniture + me, say 6400lbs.

    So I have a 6400lb boat with a 255hp GMV8, I believe the outdrive is a Volvo 280?

    This boat runs a top speed of between 12-13 mph on the gps WOT @ 4k rpm.

    I'm not expert, but I feel that this motor should be able to push this boat into the mid to high 20's.

    My last boat was a Sea Ray SRV220 with a Mercruiser 260, it weighed 3900lbs and ran about 48mph WOT@4100 RPM on the GPS.

    I believe the motor runs good, it fires right up, I'm hoping I just have the wrong prop. The previous owner mentioned that he chewed up the previous prop and found a used one at a cheap price.

    Please help! Thanks guys.

    P.S. I did do a search before posting, with no corresponding results. :D
     
  2. IronRoamer
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    IronRoamer Junior Member

    I should add that the outdrive has no trim, it's either "up" for a haul out, or down for operation as it was designed.

    Yes the boat does have trim tabs.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    ....and the bottom and drive leg are clean, no barnacles, no rope around propeller hub aso? Can you check the numbers on the prop hub (on its back surface facing the propeller nut)

    And, yes you should have something like 25 knots, provided everything is ok. Now, I guess you don't know the gear ratio of that leg, so first you have to figure this out. Unfortunately, the numbers stamped to the unit are often unreadable. Instead, raise the leg with fwd gear engaged (be sure the ignition is off!!!!), so you can make a mark on one blade tip when in top position. Also, before moving anything, mark a position on the vee-belt pulley on the engine's crankshaft.

    Rotate the crankshaft by hand clockwise (looking aft) until propeller shaft has swept exactly one turn (blade mark is back on top). You will probably find that the engine shaft had covered one of following alternatives: ~1.6, ~1.8, ~2.2 turns (there are more alternatives, but less frequent).

    Then come back and tell us, and we may point you in the right direction for the prop!
     
  4. IronRoamer
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    IronRoamer Junior Member

    baeckmo,
    thanks for the insight, I'll take a look at the leg stamps tomorrow, and if they are in fact unreadable, I'll compare my timing hash mark on the harmonic balancer with the blade rotation.

    Hopefully I won't have to resort to the latter.
     
  5. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

    Do you have a model number for the engine? I think you're a little underpowered and over propped for that boat. Your WOT if it's a similar model engine to mine is 4400 RPM. "This boat runs a top speed of between 12-13 mph on the gps WOT @ 4k rpm." (likely cavitating like crazy (prop spinning in its own bubbles)).

    My '87 VP AQ271A (275 HP) & 290 DP outdrive with B5 props (struggles a little to get up on plane) drive my 4500 lb 25' express cruiser to 45 kts (4200 RPM) and 40 kts with the B4 props (and it just leaps out of the water and up on plane) (WOT 4400 RPM). Borrow a set of test props if you can to see which ones work best but you should be able to hit the WOT spec with the correct prop.

    Boats gain weight as they age. Look for extra (but un-needed) equipment that can be removed. (I found two very old and dead deep cell batteries in my boat under a settee that had obviously been there for quite awhile). Move as much of the other weight you can as far forward as possible. Take a couple of people with you and have them go forward as you try to get it up on plane and see if she'll tip up!

    Start off with your trim tabs about half way down. As the boat accelerates ease them up and see if she gains any speed. Good luck.
     
  6. IronRoamer
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    IronRoamer Junior Member

    Knotty,
    Thanks, I absolutely agree. The correct WOT operating RPM on my other 350ci was 4400 as you said before. I feel like this motor is working wayyyy to hard when it's pushing @ 12mph. I've confirmed that the outdrive is a 280, I'm still trying to find if its a 1.61:1 or a 1.34:1 reduction.

    Also, about the weight issue, I've gutted the entire interior and tossed about 400lbs worth of saturated 3/4in plywood. I will not be rebuilding the old/heavy modular style "r/v" furniture, I'm installing 3/8in hardwood in the salon, and Ikea furniture. I feel that despite the installation of the hardwood, it will STILL be lighter than the wet, heavy as hell plywood furniture.

    I got carried away with some interior projects, and didn't get a chance to lift the drive and check those numbers.:p
     
  7. IronRoamer
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    IronRoamer Junior Member

    long time no posting.... Just as a follow up, I did a haulout this week, and found out that the leg is fine (it is a 1.61 ratio). The prop however, is a 2.5 blade, as a half of one of the prop fins is gone explaining my vibration and "slipping". I appreciate all of the responses. I've removed the prop, I'm having it rebuilt.

    Thanks again.
     
  8. IronRoamer
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    IronRoamer Junior Member

    [​IMG]

    Pictured above is what my mangled prop looked like, I've pulled it, and will have it rebuilt as a spare.

    Does anybody have a suggestion on what pitch it is? I would like to order a new one for this season. I'm also open to recommendations on a pitch that may work better.

    I would like to cruise @ 18/19 and top out at around 27/28?

    The boat:

    Single Volvo 255hp (350GM), Volvo 280 leg (1.61 reduction), 26.5 feet, 6400 lbs, 10' Beam.

    Thanks again guys!
     
  9. Jango
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    Jango Senior Enthusiast

    16.5 Dia x 13 P, 3 blade w. 55% D.A.R. min. should give you max of 29 mph @ 4400 rpm
     
  10. IronRoamer
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    IronRoamer Junior Member

    Jango,
    Thanks so much! May I ask what "55% D.A.R." is? I definitely understand the 16.5 x 13 pitch, already shopping for one!
     
  11. Jango
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    Jango Senior Enthusiast

    D.A.R. = Diameter - Area - Ratio .

    Total surface area of the Blades should not be less than 55% of the AREA of the circle created by the revolving Prop. This is a common term used by Prop Manufacturers.
     
  12. IronRoamer
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    IronRoamer Junior Member

    Ok, here's what I've found: Volvo props seems to max out in diameter at 16", leaving me the option of a 16 x 13 or 16 x 15. I think the leg is setup for a LH prop, but being that I'm replacing the u-joint bellow, I could just as easily switch it to RH anyway by reversing the linkage.

    This leaves 16 x 13 or 16 x 15. I wonder, will the 16 x 15 be too aggressive for the modest powerplant on my boat? Will the 16 x 13 be too shallow, making cruising at 20 mph or so uneconomical.

    I regularly enjoy entertaining 5-6 people on board, so I have to take that added weight into account.

    Again, I appreciate all of the help, I'm used to the Merc props, which all seemed to be smaller and spin faster, these volvo's are foreign to me.
     
  13. Jango
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    Jango Senior Enthusiast

    Increasing Pitch while also increasing load is NOT recommended.- increased slippage.
    I would go with the 16 x 13. If a 4 blade 16 x 13 is available, I would also give it a try.
     
  14. IronRoamer
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    IronRoamer Junior Member


  15. piperca
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    piperca Junior Member

    Update?
     
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