Roberts Spray 36 only does 3K with new 50hp volvo. Is this right?

Discussion in 'Propulsion' started by DennisRB, Apr 11, 2010.

  1. DennisRB
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    DennisRB Senior Member

    Mt friend just acquired a heavy and beamy steel Roberts Spray 36 foot. It has a near new volvo 50hp. The manual says max RPM for this engine is 3000. With the current prop it only does 2400rpm. Would less pitch on the prop get us some more speed so the engine can rev more? Or would this just lead to more noise and fuel consumption for minimal gain in speed?

    Anyhow going through boat classifies it seems that many steel yachts of similar or greater length have less powerful motors. 3K under power is very disappointing. What sort of speeds should a steel yacht of this size range be doing and with what HP engine?

    Thanks. Dennis.
     
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  2. Gilbert
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    Gilbert Senior Member

    You would probably receive a lot of help if you mention the details, such as the gear reduction ratio and the prop diameter and pitch.
    Right off the top of my head I will venture a guess that you should be able to get 6 knots easy with this boat and engine. I would also guess that you should figure you are going to have about 50 percent prop slippage with this type of boat.
     
  3. pistnbroke
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    pistnbroke I try

    if it has a 2 :1 gearbox then a reduction in pitch of 2 inches should raise the rpm by 400 //// I dont think you will increase the speed of the vessel as the lower pitch turning faster will cancel out each other ..if its doing hull speed thats it .
     
  4. DennisRB
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    DennisRB Senior Member

    I would love to be able to provide details on here about the prop but we have no idea. I guess the diameter can be measured, but I am not sure on how to find the pitch. I don't know about the gear ratio either. We have the user manual for the engine and box, but I wont be able to find anything out for a while as the boat is a 4 hour drive away. Also if there are variations with this motor I guess it would be pretty hard to find the ratio without stripping it?

    Wouldn't hull speed of a 36 foot yacht be around 11K? I have seen the same boat for sale but in 45 foot and it is advertised to do 9K under power with 100hp. So if a 45' can do 9K with 100hp, surely a 35 with 50 should be able to do more like 7k?

    I am a newbie to this and don't know that much, but something just seems off.

    For a general question. Assuming correct gearing, props and staying below hull speed, what percentage in speed will you gain for doubling the power roughly?
     
  5. Gilbert
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    Gilbert Senior Member

    The 'hull speed' for a 36 foot spray would be 8 knots. Another useful piece of information would be the displacement of the boat which I expect would be about 30,000 pounds or so. If the bottom is not clean you can lose a lot of speed also. Your suggestion of 7 knots with this boat and engine is not out of line but getting the perfect combo of rpm, gear ratio and prop pitch and diameter can take a little bit of trial and error sometimes. But 3 knots sounds like you may have a ball of pot line wrapped on your wheel. Or no bolts in your shaft coupling?????he-he! You should have no need of a larger engine.
     
  6. DennisRB
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    DennisRB Senior Member

    haha. This was the first time we have gone on the boat and we didn't have snorkeling gear. I am hoping the prop is full of growth or garbage or something easy like that. I would have thought 50hp would be more than enough for this boat to reach reasonable cruise speeds.
     
  7. daiquiri
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    daiquiri Engineering and Design

    I shouldn't assume anything, but I'll take that risk... So, assuming that you have a Volvo Penta D2-55 (which corresponds to your description in terms of max HP and RPM) the tech specs sheet can be found at the Volvo Penta site:
    http://vppneuapps.volvo.com/ww/PIE/...=ProductBulletins&s=712203&id=2605&lang=en-GB
    From the SHP curve you can see that at 3000 rpm the shaft gives 53 HP, whilst at 2400 it is somewhere around 43 HP - so you have 19% loss of available power due to overpitched (and under-revved) prop right now.

    The fuel consumption at 3000 RPM is around 12.8 l/h (3.4 gal/h), compared to 10.5 l/h (2.8 gal/h) at 2400 rpm.
    It means that your boat now makes around 1.1 mpg at 3 kts @ 2400 rpm. If you could gain even just 2 more knots (for a total of 5 kts at 3000 RPM) with a correct propeller, you would have an immediate benefit of moving much faster, and at 1.5 mpg - which is not excellent but makes it well worth the effort.

    Try to discover the diameter/pitch of the actual propeller and the gear ratio of the transmission installed. And, of course, check out if the engine is really a D2-55.
    Then get back here with the data and we'll see what would be the best thing to do. ;)
     
  8. terhohalme
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    terhohalme BEng Boat Technology

    Gear box ratio? Propelled dimensions? (see blade root)
     
  9. DennisRB
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    DennisRB Senior Member

    Thanks for that. I will try to do all this. Diameter is easy, but how do I check the pitch? Can this be measured? The engine is a MD-22
     
  10. Landlubber
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    Landlubber Senior Member

    The diameter and pitch of a prop are usually stamped into it at the boss, such as 24 x 18. The 24 is the diameter and the 18 is the pitch. The gearbox has a plate attached that has the ratio on it.
     
  11. DennisRB
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    DennisRB Senior Member

    I will try to have a look. I hope the Clarance River water will be clear enough to read it.
     
  12. FAST FRED
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    FAST FRED Senior Member

    The most common setup for a boat like this would be 2-1 tranny and a 20 inch diameter prop with a 12 inch pitch.

    She should run 6K on 3/4 a gal to 1 gal an hour ,7K at 2 to 2.5 and at 8K will demand about 4 gph.

    Sounds like a dirty prop to me,

    FF
     
  13. DennisRB
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    DennisRB Senior Member

    Thanks Fred. When you say 2-1, does that mean a reduction in shaft speed or an increase in shaft speed compared to engine RPM?
     
  14. Landlubber
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    Landlubber Senior Member

    The engine is doing twice the shaft speed
     

  15. baeckmo
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    baeckmo Hydrodynamics

    You say it is a Volvo MD 22. This is basically a Perkins engine, painted green in Gothenburg to make it a Volvo Penta. Its nominal shaft power is 42 kW (~57 hp) at 4000 rpm (not 3000 rpm!!!). Normally it came with Volvo transmission MS2B, ratio 2.4:1. Also available with MS2B 3.0:1. Ratios are stamped on a label on top of the gearhousing.

    With 2400 rpm on the shaft, the engine is working far too hard and producing far too little hp's; something is certainly wrong, and the engine is not going to take this for long. First rough estimate of propeller dia should be something like 17" for the 2.4:1 ratio, pitch depending on speed and wake factor.

    So, on with the snorkel and goggles and remove the crab pot with its rope and else......
     
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