Crouch Formula - Repower / Depower Sea Ray 32?

Discussion in 'Powerboats' started by techyjeff, Dec 13, 2007.

  1. techyjeff
    Joined: Dec 2007
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    Location: Alton, NH

    techyjeff Jeff K

    Hello folks,

    We have a 2006 Sea Ray 320 Sundancer, twin 350 Mag w/ Bravo III.

    If I were to consider re-powering with diesels at some point in the future would the Croucher formula be a reliable means to estimate expected speed based on stated HP?

    I used it to calculate speed based on my current power package and it was right on. I used a 'C' value of 150, disp. of 14,000 lbs. and played with different SHP values that I believe roughly correspond to how I operate the boat and the given speed results matched what I actually experience with the boat.

    Where I'm going with this is seeking an answer to the question whether it would be viable to convert my twin 300 hp gas engines to 100 or 150 hp diesels and utilize the existing Bravo III outdrives.

    My speed expectations would be to have a cruising speed of around 20 knots; I'm not really concerned with reducing my top end speed because I don't use it today other than for the occasional romp.

    Using the Croucher formula I estimate that twin 150 hp diesels, running at 80% power would result in approx. 20 knots speed.

    My wife loves the boat, we use it a lot, and I'm trying to find a way to make it a little more viable from a fuel consumption perspective in the long term.

    Any thoughts?

    Thanks in advance.

    Jeff
     
  2. Willallison
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    Willallison Senior Member

    How many hours do you do in a year? Most recreational boaters manage less than 50. Few do more than 100.
    How much for a pair of diesels? 25K each? Plus the cost of fitting them, rebuilding your fuel system etc etc.

    My point is that you are VERY unlikely to ever recoup the cost of your additional investment in fuel savings.

    Crouch's formula is a reasonable means of estimating performance - albeit a rather basic one. If you know the hp required to give a number of planing speeds, it's a simple rearrangement of the formula to give an average value for the constant (C). I say this because it can vary a little across the speed range.
    But you must also take into account the hp required to get the boat onto the plane in the 1st place and also consider what might be required with varying load's and sea-state's.
     
  3. techyjeff
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    techyjeff Jeff K

    Hi Will,

    Thank you for your reply.

    We do 200+ hours per year, much of it at 5 - 10 kts but plenty of on plane running as well.

    The decision to re-power is not imminent rather out in the future somewhat. Obviously we have a fair amount of money tied up in the boat, though that is a relative figure I realize (it is a fair amount of money to us). I enjoy using the boat and though I like a reasonable turn of speed to get from point A - B 20 kts is fast enough, and I prefer 10 or less just for ambling along.

    In the long term, 10 years, 24 gallons / hr. for a 32' boat is probably not the most viable solution though I would love to wrong. My wife loves the boat, and I do too, but am concerned that it will not meet our needs over the long haul and if that is the case, the longer we keep it the more more money we loose. We're not getting out of boating so it's more a question of what solution works for us, not whether we're going to spend the money.

    I am a student of boat design and spend countless hours studying, researching all manner of design and mission so I'm very familiar with what we're getting in a 32' production sport cruiser. That being said, my wife loves the amenities and I've been very happy with how the boat performs for coastal New England cruising.

    I would like more of a sea boat, she likes the living space, and we like being on the water together. Twin 350 gas engines are not intended for extended cruising in this day in age (in my opinion) and I like to run the boat in all weather, April - late November. I have to say though, that 24 gallons / hr or 1 mile / gallon, in a 32' boat, assaults my senses and makes me question the long term viability of this type of boat. If my concerns are valid, I'm better off dumping her in the next few years and licking my wounds or planning for a repower at some point in time and keeping her given that trading boats is always a money loosing proposition.

    It's not the next five years that really matter, it what makes sense over the next 10 - 20, and when you have six figures tied up in a completely discretionary and depreciating asset such as a boat, I at least, think of ways to minimize my downside because I know I'm still going to do it.

    Thanks in advance for any and all unvarnished thoughts, opinions, and insight.

    Cheers
    Jeff
     
  4. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Jeff - glad to see you use your boat! Nothing wrong with a Sea Ray IMHO - I owned a 270 Sundancer very happily for a number of years. Yes, they're a high-volume production boat, but there's advantages as well as disadvantages to be had from that. They do have their shortcomings however and being quite heavy is one of them...

    Anyway - let's work with your 200hrs a year. At 20 knots and and 1 nmpg, that's 4000 gal per year. If you (optimistically) halve your fuel usage by going to diesels, that'd be 2000 gal per year. At...what.... US$1.50 per gal you'd save $3000 each year. In ten years you would have saved the cost of one of your engines.....

    If it were me, I'd either relax and enjoy the boat you've got, or sell it now and buy something "more appropriate".
    Also, whilst resale with diesel's will be higher, you certainly won't recoup anything like the additional cost of their installation
     
  5. techyjeff
    Joined: Dec 2007
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    techyjeff Jeff K

    Hi Will,

    Yes, I do see your point. One thing is for sure, we're definitely going to continue to enjoy the boat. We have extended warranties on the power package and will probably look to sell her while she still has at least 2 years of coverage left.

    I'm just completing a 26'x40' workshop in which to build a boat. I have actually purchased the plans for WHIO from Peter Sewell and have had several good exchanges about building her.

    One thing I miss greatly about not having a trailerable cruiser is the ability to take her from the lake to the ocean whenever I want. We've taken the 32 to the ocean once so far and had a terrific 500 mile run between Portsmouth, NH and Norwich, CT, hitting all the best spots along the way. This is something we used to do regularly with the 27.

    Thanks for your input.

    Jeff
     
  6. FAST FRED
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    FAST FRED Senior Member

    24 gallons / hr. for a 32' boat

    At 12gph for one engine you are using about 120hp.

    This would equate to a 150 or so diesel, being run efficiently.

    While the downside of a Marine diesel is the insane cost , an auto takeout could be made to fit the drives at FAR LESS cost.

    Got a garage to play in , great , head out to the junk yard!

    I would look first at the GM V8 diesels. When brought out they were a modified gas engine and had ZERO reliability. GM then cast special blocks , used forged cranks and all the rest. While not popular in cars (no turbo so slow pickup) the engines are fine as diesels now.

    You may find them hard to locate in the better years as the racers would convert them to gas engines for the fantastic reliability all those truely HD parts.

    At 120 hp they will last "forever" and are hardly expensive.



    FF
     
  7. Willallison
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    Willallison Senior Member

    Jeff- Whio is a vastly different boat to your Sea Ray.... I hope your wife is into 'minimalist boating'....;)

    Converted truck engines would be a way of minimising the changeover cost - but, given that this is a relatively new boat, it would also be a way of maximising the depreciation.....
     
  8. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    crouch formula-re power

    to Fast Fred, what year early GM diesels or Chevy should I look for? Have no idea when they changed over to HD. I plan on using 2 so they will not be run hard. Really would appreciate your knowledge on this. Thanks much, Stan
     
  9. FAST FRED
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    FAST FRED Senior Member

    I believe it was in the last 3 years or so of diesel production.

    Google the hot rod , drag racer sites and see what gear is made/sold for these engines 350ci diesel.

    FF
     

  10. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

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