Conversion from gas inboard to diesel

Discussion in 'Boat Design' started by rattleandbang, May 19, 2015.

  1. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    I'm considering acquiring a lovely 60's vintage wooden cruiser with a planing hull powered by a single 327 inboard. I have a 55hp Westerbeke diesel I had for another project, and I'm wondering about replacing the V8 with it. I have no illusions about ever getting onto plane with the diesel, but fuel economy is far more important than speed to me. What are the downsides of running at say 7-8 knots tops in a planing hull, other than getting there slower?
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Yep, the 327 will likely have in the 200 HP range, so an engine 1/4 of this output will be pretty anemic comparatively.

    It depends on the hull you're looking at, but most from that era are warped bottoms with probably a sea skiff heritage to it. These will wallow around quite a bit, with obnoxious steering qualities and some rolling too. This would be typical of most of these hull forms at these speeds. The big skeg under it (my assumption) will help a bit, but low speed steering in one of these will be "testing" to say the least, particularly when backing down. Learn how to "back and fill" or you'll be forced to reverse in one direction only.

    So yes, it can be done and has been, but it's not well suited to these speeds. Which vintage boat are you looking at?
     
  3. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    If you have no experience with the boat there will be no problem since you have no reference for a comparison.
    I replaced 2 gas engines with small diesels in a boat I already possessed for over 20 years and kept saying to myself that I was in no hurry so 8 knots were fine, but deep inside I did feel handicapped.
    Last year I bought a RIB with a large outboard and cruise happily along the coast at a few knots, knowing it can reach a screaming 42 knots should the need ever arise.
     
  4. BMcF
    Joined: Mar 2007
    Posts: 967
    Likes: 45, Points: 28, Legacy Rep: 361
    Location: Maryland

    BMcF Senior Member

    I own an older 32' Marinette with twin 225HP Chrysler V-8s. The vessel spends 90% or more of its time underway at 8-9 knots because that is comfortable, affordable, and fairly quiet. Did I mention affordable?..;)

    I've always considered putting two smallish (50-60HP max) diesels in the boat in place of the gas engines....they would be a great match for how we use it.
     
  5. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    Its a mid 60's vintage owens cruiser. The distances I prefer to travel in this area are quite large and the idea of spending many hundreds if not thousands on gas certainly is a deal breaker -if the boat wasn't such a pretty thing going at a very good price. I've read very conflicting info about simply throttling back, that while your GPH certainly drops way off, but the gals per mile still stays in an exorbitant range, at times even higher than if you were planing.
     
  6. BMcF
    Joined: Mar 2007
    Posts: 967
    Likes: 45, Points: 28, Legacy Rep: 361
    Location: Maryland

    BMcF Senior Member

    My 32' Marinette achieves about (just under) 2 nm/gallon at 8-9 knots and almost exactly 1 nm/gallon at planing speeds of 17-19 knots. I've calculated that a low-power diesel conversion would get to about 3.6 nm/gallon at 8-9 knots.

    I've yet to see a planing hull design that used less fuel on plane than it did remaining at or below hull speed...
     
  7. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    I got this curve on another forum. Similar hull, same engine.
    [​IMG]
    Bumbling along at 14-1500 RPM shows a not too utterly outrageous fuel consumption, all things considered. But what are the downsides to going that slow? Some suggest that the boat is uncomfortable and even barely controllable in any kind of sea?
    Obviously a displacement hull would be best but in this part of the world precious few displacement hulled pleasure craft exist from that era. It seems only workboats went that route and in those cases human needs and comfort was the last on a long list of priorities. Gas was cheap and so pleasure yachts went fast.
     
  8. BMcF
    Joined: Mar 2007
    Posts: 967
    Likes: 45, Points: 28, Legacy Rep: 361
    Location: Maryland

    BMcF Senior Member

    There isn't a lot of difference in the underwater lines between that Owens and my Marinette. Same vintage planing cruisers.

    I have no issues with how my boat handles at 8-9 knots except for the expected ones...she's wet in a head sea and that requires extra thottle to bring the bow higher. And she's a pig to steer in a stern quarter or following sea. Usual stuff that all similar boats exhibit.
     
  9. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    Can I ask where you do most of your motor g and what conditions?
     
  10. BMcF
    Joined: Mar 2007
    Posts: 967
    Likes: 45, Points: 28, Legacy Rep: 361
    Location: Maryland

    BMcF Senior Member

    I boat in the lower Potomac, lower Patuxent, and Chesapeake Bay...and tributaries thereof. I avoid conditions higher than about 0.75 meter significant wave height (Sea State 2?) and even that is unpleasant.
     
  11. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    I'm wondering how it compares with a sailboat? Mine was an IOR hull that is typically squirreley as hell downwind in rough stuff.

    This was the worst I've been in the biggest ones were probably 12 feet or so, and I had to spin the wheel 360 degrees first one direction and 360 back again to keep her on course. A lot of work. Would it be similar or would it be impossible without planing?
    https://youtu.be/v7PbetV6eF8
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,708
    Likes: 267, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    As a rule , you need to go at "no wake" speeds to really get the big drop in fuel use, push them to the point where the nose starts to rise, and their is no advantage to be had. Who wants to go 4-5 knots ? I am slightly bemused by the number of people who think they can get 8 or 9 knots out of these planing boats and enjoy more ecomomical boating, it does not happen. You really can't win, if you have long distances to travel, slowing down to a crawl is a drag, if the distances are fairly short, the fuel burn isn't such an issue anyway. There is an advantage in having twin engines though, particularly if you can raise the drive out of the water, in that running one only at crawl speeds offers some fuel savings.
     
  13. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    For most boats the SQ RT of the actual LWL times 1.15 is about the best cheep to operate speed.

    Only a slender (better than 6-1 L/B ratio) or very very light boat might go a bit faster.

    They dont call it the Trawler Crawl for nothing.


    However on the Atlantic side in the ICW about 6K is ideal.

    The sail boats will usually go 6K and the bridges will only open for a gaggle ,.

    Get there 10 min before the crowd , and if the crowd is in sight , you will wait till the group is all there.

    Speed might be useful if you can clear low bridges , under 20 ft with no opening.
     
  14. BMcF
    Joined: Mar 2007
    Posts: 967
    Likes: 45, Points: 28, Legacy Rep: 361
    Location: Maryland

    BMcF Senior Member

    You can be amused as much as you like but I've been operating my Marinette 32 for many, many years and am intimately familiar with her rate of fuel consumption at any speed she can run.

    She'll go a long way at 8 knots....and not nearly as far at 16. ;)

    About 5 or 6 years ago I was seriously considering repowering with a pair of Toyota-based marine diesels rated at around the same as the gas engines..about 225 HP or so. I wanted to be able to go fast(er) and afford it. Then I calculated the "payback period" and decided to live with what I have. Will buy a diesel-powered vessel next time around..and most likely a trawler or other displacement hull too.
     

  15. rattleandbang
    Joined: Apr 2015
    Posts: 74
    Likes: 2, Points: 0, Legacy Rep: 22
    Location: Victoria BC

    rattleandbang Junior Member

    It turns out my daughter's neighbor has worked on boats all his life and has a planing hull fishing boat, and because he often gets off work late most of the time he doesn't go on plane because of low light. He travels about 30 miles to Saltspring Island doing about 8 and is quite happy with it. He also doesn't like the pounding he gets on plane. He did advise increasing the size of the rudder though.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.