calling prop guru's

Discussion in 'Downeast Boat Builders (25 Ft+)' started by markfromaine, Aug 3, 2019.

  1. markfromaine

    markfromaine Member

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    31' BHM 3208 CAT
    So I recently blew up my old CAT 3208 despite its extremely low hours in my 31' BHM. With the lobsters just now hitting I need a quick turn around, and everyone with a re-manned 3208 thinks they're made of gold. So I have made the decision to do a total re-power with a new CAT C7.1 in the 400 HP model. I'm running a twin Disc 506, and currently have a 28x26 prop and 2" shaft, that with the old 355HP 3208 provided a lackluster 11 knot comfortable cruise and WOT of about 14 with the wheel cage on. I assume with the new engine 50 more HP but the same gear and prop I will not see a great increase in speed, but should be able to get a new prop (when funds permit, Engines ain't cheap) and see a few more knots. What do I need to optimize efficiency/speed? Thoughs?
     
  2. djmarchand

    djmarchand Captain

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    What is the rated maximum hp rpm of your old and new engines? What rpm would the old engine reach at wot?

    David
     
  3. powderpro

    powderpro Captain

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    Was your old 3208 able to hit the 2800 rpm redline? I’m assuming you have 2.5:1 reduction? Quickest way to gain efficiency and speed is to lose weight, put the boat on a diet if you can. Lifting rails would help you a little too. Are you able to hit the 2,900 rpm redline with your new engine?
     
  4. markfromaine

    markfromaine Member

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    With the old 3208 it wouldn’t quite make it to max rpm. Close but It was more like 2680. I never ran it that hard because it seemed like the extra rpms it took to get from 11 knots to 14 was a waste of fuel and hard on equipment for 3 knots difference. I haven’t gotten the new engine in yet. Hopefully it’ll be here Monday. The new engine is lighter and a few more hp. The boat is heavy.
     
  5. Brooksie

    Brooksie Captain

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    "The boat is heavy" Wow I guess heavy. I know the BHM 31 is a 35' boat in disguise. What is the weight? You need to know your fighting weight and gear ratio to come up with any answers. My Bruno 35 makes those same numbers w/ 4-5 POB and only 150-160 HP.
     
  6. markfromaine

    markfromaine Member

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    So new engine showed up today (see below, she is pretty) and I got the gear off the old engine today it’s a twin disk 507 with a 2.54:1 ratio. The prop is actually a 28x26” Leftie 2” shaft. The new engine will be lighter and more power. Like I said I couldn’t achieve the full 2800 rated rpm of the 3208. The new engine is 400hp and turns 2700 I think.
     

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    Last edited: Aug 6, 2019
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  7. hardcore1

    hardcore1 Captain

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    Sounds like you have the right prop for your application already. Maybe an inch of pitch will be able to be added unless you run the boat without the cage
     
  8. markfromaine

    markfromaine Member

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    guess i'll see when we do sea trial, hopefully it wont be much longer, got the new engine set in today and we're starting to hook things back up. I know i would gain a lot taking the cage off, but I don't think its worth it.
     
  9. powderpro

    powderpro Captain

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    It’s 2,900 rpm for your new engine. So if you couldn’t achieve 2,800 with your old engine, you likely will not hit 2,900 with the new engine without some pitch reduction. The engine looks nice, post some pictures and performance numbers when you’re done with the repower.
     
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  10. ShaftMan

    ShaftMan Member

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    Call Mark at Nautilus Marine in Trenton, Me
    He'll be able to answer all your prop questions
    207-667-1119
     
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  11. markfromaine

    markfromaine Member

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    I likely will call the guys at nautilus. We put the boat in Thursday (shout-out to Dennis welding and marine in beals Maine for a 10 day turnaround for full re-power) and haven’t done the full sea trial yet with CAT. But we need more prop as we easily over turned the prop. The engine would not go over the rated RPM due to the electronic governing but after a certain amount of throttle it just stopped pushing any harder and the laptop said we were overturning. I will likely have better info once the cat guy comes for the full official sea trial. But we were able to make 18.6 knots before running out of prop, and a comfortable cruise at about 14 knots at 2300.
     
  12. eyecatcher

    eyecatcher Senior Member

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    difficult to say without more information but your probably very close since there shouldnt be much change. Were you fully loaded with fuel and gear etc? If you were you can try a 28X28 4 blade be good to see where that lines up, but prob only gain a knot. Your better off probably just keeping what you have and be a little under if your looking to stack the deck with lobstas
     
    Last edited: Aug 19, 2019 at 10:44 AM
  13. hardcore1

    hardcore1 Captain

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    Can't imagine a speed increase like that with only changing the engine. Something else had to change. Why not put up better specs otherwise it's hard for anyone to steer you in the right direction. Is the cage still on the boat? If the Cage is off now and you intend to put it back you will most likely be overloaded. Cage makes a big difference
     
  14. BillD

    BillD Captain

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    when you did the 1st sea trial, did you notice the "% engine load" on the computer at WOT when the electronic governor did its job and ya ran out of prop ?
     

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