Converting from twin I/O to twin outboards

Discussion in 'Powerboats' started by u4ea32, May 4, 2010.

  1. u4ea32
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    u4ea32 Senior Member

    I own a boat that I really enjoy, except for the powertrain. I really have come to hate twin Mercruiser I/O. And and hour ago I discovered that both engines are toast. Or more accurately, rust.

    Its a Fountain, so very fast -- 70 mph top end. But we go boating on the Pacific, which means 22 knots is the cruising speed. I've exceeded 30 knots for a few minutes at a time, perhaps 2 hours total in the past many years. The Pacific sets the speed out here, not the throttle! At 22 knots, I burn 20 gallons per hour, which on gas engines suggests I actually only use 100 HP per engine at cruise.

    So, working through all the possible options, I come up with the following:

    1) Just sell the boat as a project: as-is.

    2) Rebuild the engines and sell the boat.

    3) Install small twin diesels, such as twin Volvo 190 DPs or small Yanmars with Bravos. This is expensive, two bids have come in for about $50K.

    4) Install a single gas engine. Requires transom work, move stringers. Single sisterships are reported to work really well. Fiberglass work is cheap, a single engine could use one of my existing outdrives (I'd have a spare).

    5) Install a single diesel engine, anything from a Volvo 190 DPs to a Volvo or Yanmar 300 HP, with I/O. Same fiberglass work as above. Probably need a new fuel tank for diesel. Diesels are substantially more expensive than a gas engine and used outdrive! One quote came in for about $35K.

    6) Install twin outboards, about 150 to 200 HP each, just so they are not working hard putting out 100 HP each at cruise. Cheap installation!! Fiberglass work would be a new cockpit sole to make up for the current engine box, and a new transom. The much bigger cockpit would be a welcome improvement!

    I'm leaning toward the outboards. I think the huge reduction in weight -- about 2500 lbs, or about 25% total fully loaded displacement -- will make a difference for fuel economy. I think the far simpler installation and service access will save me a lot of money. And I really like the idea of plugging all those holes under the waterline!!!

    But diesel may make an even bigger difference in fuel costs. It seems going to diesel can reduce fuel costs by 30%. But the maintenance costs of diesel are still daunting: turbos, intercoolers, outdrives, ...

    For those of you not boating in Southern California, we really burn a lot of fuel out here because the distances are so great. On a typical weekend I burn at least 100 gallons (100 miles). On a typical four day weekend, 250 gallons is not unusual, and I've burned through 500 gallons a few times. At my typical 200 hours per year, and today's $3.50 a gallon, about $14,000 per year. By far the largest expense of owning the boat!
     
  2. rasorinc
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    rasorinc Senior Member

    You have not supplied weights, length, beam, or type of bottom (deep V etc.)
    so everything is a guess at this time. My love of speed in cars, airplanes, and boats has not ended but the costs of same has required changes in my life, not to add age also. I would consider a single engine, gas or diesel for under $6,000.00, thats 400hp plus some extras and I can supply sellers of same, and a high thrust 10-25 hp Kicker engine to putz around with when speed in not important. Don't know you age or expected usage plus data needed above
    which I would need to give you a firm opinion. What are your existing engines and hp? Save everything that bolts up to them.
     
  3. Stumble
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    Stumble Senior Member

    Just off hand without knowing what you are running it sounds like you are probably better off just selling this and buying a boat that can either operate in your conditions but at a faster speed, or get something that is more fuel efficiant, but recommending a boat is pretty hard without knowing what type of accomidations you are looking for, and how willing you are to trade top end speed for useable speed.

    My 24' Twin-Vee for instance burns about 7.5 gallons per hour at 24kn total (on twin 115hp), but has no accomidations. However I have run it at that speed in all types of weather, and routinely pass up more powerful boats in the Gulf of Mexico that can't take the beating, while we are barely rocking.

    Conversly I refuse to look at the fuel bill on my 60' poweryacht since I don't want to know how much fuel we burn... (honestly it is a pretty fuel efficient 32gallons/hr at 16kn).

    Either way from what I see you have a boat that is capable of massive speeds, but the sea conditions won't allow you to ever run that fast. My advice would be to find a boat that will cruise at the speeds you actually want to run at, and work from there.
     
  4. u4ea32
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    u4ea32 Senior Member

    The current engines are Mercruiser 454 Magnums, 385 HP each.

    The boat weighs 8500 lbs "lightship" (boat, engines), but about 10000 lbs loaded (gear, food, water, fuel, people).

    Its a stepped deep V. Its 32 overall, including that famous Fountain "beak" and swim step, so the bottom length is similar to a 29 foot typical deep V. Its 8.3 wide.

    The reason I like the boat so much is because the accommodations are adequate for two for 2 to 4 days, the cockpit is large, the hard top and enclosures are great for California off shore (generally cold water, so cold air when underway), yet the enclosure can be removed for warm weather (like when we are on a lake, or at anchor).

    The efficiency -- generally about 1 nmpg at most reasonable speeds -- is so low because I need a lot of trim tab to smooth out the ride. When the water is smooth, like on a river, I can get 1.5 nmpg. But we burn the vast majority of fuel at sea, so that's 1 or 1.1 nmpg.

    We are most strongly tempted to go with the outboards, as that will actually improve everything we already like about the boat, and fix most of the things we really dislike about the boat.

    To go with outboards, we will need to extend the hull to the end of the swim step. The swim step is an integral part of the deck moulding. Adding a bit over 2 feet to the running length of the hull, combined with getting rid of nearly a ton of iron and aluminum, should allow the boat to cruise with much less trim tab, so less drag.

    But boy, that sure sounds like a lot of work to this boat...
     
  5. u4ea32
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    u4ea32 Senior Member

    Oh, to be complete, we also have the option of cutting off the swim step. But the fiberglass work will be similar, as instead of adding to the hull, we'll have to reconstruct the aft part of the deck.
     
  6. Stumble
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    Stumble Senior Member

    David,

    I don't mean to harp on this, but I from what I am hearing you own a boat that is really optimized to run in the 70kn range but never actually get to do so because of the sea conditions you tend to operate in. Just taking a quick look at some other options for you, staying at around the same length of 30' you could trade what you have in on a number of corvette options any of which would run a top speed of around 40kn, at the same rough mpg you are currently burning, but give you a lot more accomodations compared to the Fountain. Or you could actually trade down to keep the same accomodations, keep a top end speed of around 40kn, and have a smaller boat but be more efficient to operate.

    As for the options you suggested, I can see the desirability of going to outboards, but I am worried about a few things.

    1) You are reducing the total weight, but moving all of that weight pretty far aft compared to the internal engines. This could really change the trim of the boat drastically, and might have some really undesirable consequences. I would definatly try to identify the effect of this move before doing the work, either by finding a boat on which this has been done, or having a NA do the calculations

    2) I would also be worried about what this modification would do to the resale value of the boat. Major changes like this could dramatically reduce its desirability to the next owner.

    3) The existing transom may or may not be strong enough to handle the weight of the outboards. I would definatly look into this before doing anything.
     
  7. Easy Rider
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    Easy Rider Senior Member

    David,
    Looks like your'e on the right track to me. Sounds like you're OK w paying for the fuel. Stumble has a very good point about how you can't use the boat like it's intended and I thought also about the aft move of CG. At least your'e not thinking of mounting the outboards aft of the transom. I think we should try very hard to have boats that have their engines, fuel, batts and other heavy equipment a bit aft of center of boat but that may not work well w your kind of boat. I would think the CG should be not much different than w the IOs since the OBs weigh so much less. Just do it.

    Easy Rider
     
  8. Willallison
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    Willallison Senior Member

    Personally, I'd go for option 1.

    There are so many good 2nd hand boats about these days that I can't for a moment imagine that you couldn't find one that better suits your needs. Sure, you'll do some $ in selling a boat that's no a go-er, but you'll sure as hell save a stack by avoiding what will almost certainly be an expensive alteration. You will also be back on the water in a metter of weeks, rather than months - or even years, depending on who's going to do the work.

    The 2nd 'best' alternative IMHO opinion would be to put a pair of new sterndrives in - whether diesel or petrol is up to you. Personally, I'd probably opt for the petrol as even at your 200 hrs per year it'll be several years before you recoup the difference between the two in terms of fuel usage.

    I would also urge caution in selecting engine size - going smaller doesn't always equate to using less fuel - often quite the opposite. And lightly stressed engines will also last longer.
     
  9. Easy Rider
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    Easy Rider Senior Member

    Will,
    Steve DeAntonio of Passagemaker Magazine says underloaded engines die early from lack of heat.
    I, personally would go for option 1 as well and all that fuel saved would mean more for us that burn 1gph.
    But Will, he calls himself "Rocket Scientist" .. and he's from LA. He may need that stuff.

    Easy
     
  10. u4ea32
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    u4ea32 Senior Member

    Thanks, guys. All very good points. A couple of friends in the industry are also suggesting option 1, or a much cheaper option 2 than I've found so far. A few more people looking at the boat, and maybe I'll have sufficient information to make a decision.
     
  11. Willallison
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    Willallison Senior Member

    It's true that underloaded diesels can suffer from problems like glazing of the cylinder walls. Though this was much more of a problem in older engines than newer electronically controlled ones. Different manufacturers recommend different things to avoid this - though most suggest that the odd burst at higher revs will avoid this.
    It's not a problem I've heard of in petrol engines however... I'm yet to come across one that doesn't run at temp at lower revs.
    And of course there's a difference between underloaded and lightly loaded...
     
  12. Easy Rider
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    Easy Rider Senior Member

    Will,
    Yea ..gas engines are exempt from light running woes because the temp of the burning gasses in the cylinder are always the same so the heat losses through the cyl walls is always high. Conversely you can't even fry an egg on the exhaust manifold of a diesel at idle. It's fuel mixture is about 60-1 at an idle and 15-1 (don't remember exactly) at WOT .. about the same for gas.
    It's probably a good idea not to talk about the underloading issue as it tends to launch big arguments. I will say though that the percentage of load is determined by the % of WOT fuel burn. If your engine burns 10gph at WOT and you cruise at 6gph your'e at 60% load.

    Easy
     
  13. Willallison
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    Willallison Senior Member

    :D Yes... hard to see consensis when even the engine manufacturers differ in their advice!
     

  14. u4ea32
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    u4ea32 Senior Member

    All very good points, Thanks.

    Regarding (1), I totally agree. I'm using this as an excuse to really act like a real NA and do the math, as well as looking at other similar configurations to ensure I'm not computing myself into the weeds. There do seem to be a lot of boats that are configured like my boat would need to be, so its no longer all that exotic. But there are horror stories as well as successes.

    Regarding (2), I think the resale with big blocks is pretty close to zero. The resale with outboards will be somewhat more than zero. But financially, its pretty bleak.

    Regarding (3), I am certain the structure will need to be changed to support outboards. The I/O is mostly carried by the engine mounts, some by the transom, and of course will outboards its all transom, and if on a bracket, the force is acting on a lever!
     
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