Twin Inboards

Discussion in 'Inboards' started by mike Bonwill, Oct 29, 2008.

  1. mike Bonwill
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    mike Bonwill Junior Member

    Can anyone tell me if there is a standard calculation used for the distance between shafts on an inboard boat. I have been working on a 233 Formula and want to repower with two inboards. I would like to use inline 4's or v6's. I would like to space shafts 44" to 46" this would give me enough room to walk between both engines. All the twin I/O's that I have look at online look like they are about 36" apart. All thoughts and comments are greatly appeciated. Thanks Mike Bonwill.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Generally, no, there is no standard calculation or distance between engines. It is best to keep them as close together as possible for center of gravity reasons, consistent with serviceability. If you are working on an already built boat, then you are confined to the spacing of the engine bed stringers that are already there. That is, unless you want to add or change the structure to support different engines that have a wider stance. That requires rebuilding work, of course.

    Eric
     
  3. daiquiri
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    daiquiri Engineering and Design

    Just try to leave enough space, if anyhow possible, for a person who is not a Yoga practitioner to perform maintainance work between the engines.
    Otherwise the costs for the overhauls could become much higher than you could expect.
     
  4. mike Bonwill
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    mike Bonwill Junior Member

    Mike B

    Because I am converting from I/O to Inboards I will have to do major alterations any how. I can drop the engines lower in the bilge by putting them close together. But this causes a problem with raising the deck above the engines which also raises my center of gravity. By having a on center spacing of 46" on v6 4.3's I can create a 16" walk though between both engine boxes (+ or -). Then I can place my seating on top of engine boxes. I did not know if there was a rule of thumb that calculated beam of boat/prop shaft spacing. Has of yet I can not find any info on this subject. I am putting alot of time and effort in to this boat and don't want to mess it up by engine placement, but I guess when you take a boat that had run good for years with I/O's and put straight inboards or outboards on it, it's a chance you take. Any info or Articles that anyone knows of would be greatly appreciated. Has anyone ever taken a Formula 233 and installed inboards? Thank You Mike Bonwill.
     
  5. daiquiri
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    daiquiri Engineering and Design

    What was the original engine model and did you decide about what model you would like to install?
     
  6. mike Bonwill
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    mike Bonwill Junior Member

    Original engine was a 260HP Merc. I would either go with merc or volvo 4's 3.0 (140HP) or v6's 4.3 (190 to 225HP). I like the 4's because of their size, but I think it would be under powered. Your thoughts? Have you ever seen this done to a 233 Forumla or alike boat? Thanks MB
     
  7. daiquiri
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    daiquiri Engineering and Design

    Can't find info about Volvo 3.0 engine - do you have some boat and engines weight data?
    You have to be very careful because you're removing a single engine from the stern, replacing it with two engines much more towards the bow.
    Here on the forum you can find some guys who are still trying to lift the bow up from the water after modifying their boat's center of gravity in such a radical way.
     
  8. Eric Sponberg
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    Eric Sponberg Senior Member


    Mike, what you have discovered are two things: 1) There is no guidance on the placement of twin engines in relation to each other, and 2) Like all things in yacht design, major factors are a matter of compromise.

    You know that the engines should be as close to the centerline as practical, but clearance between and all around the engines has to be considered, including what's on top and the resulting vertical center of gravity. The best place for the engines is where they fit the best, all things considered. If you have to make a choice of shifting the engines outboard a bit or inboard a bit, it is generally better to shift them inboard. Do not fret over it too much. There is no ideal or preferred position. The best position is what works, all things considered.

    Eric
     
  9. rasorinc
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    rasorinc Senior Member

    Go to G M Marine Engines. The 3.0 is there with all the data and weight.
    Volvo used this G M block.
     
    Last edited: Oct 31, 2008
  10. mike Bonwill
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    mike Bonwill Junior Member

    Thank You for your reply eric I have not written anything on this board but have been doing a lot of reading and always enjoy your comments. I am going to re-draw with possible less space to walk though to the bow of the boat, then make up my mind and go with it. What is your feelings on 4 inlines or V6's?
     
  11. daiquiri
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    daiquiri Engineering and Design

    Do you have some drawing about how you intend to modify the boat? Even a preliminary one, where the boat section with the engine position is shown?
    And also - what is your boat's actual weight, with (or without) the engines?
     
  12. mike Bonwill
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    mike Bonwill Junior Member

    I do have some drawing, but I do not know the weight. I will try to search web for this.
     

  13. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    I'm asking you about the weight because a planing hull is very sensitive to where boat's longitudinal center of gravity (LCG) is placed.

    Ideally, the planing hull's LCG is somewhere between 33% to 42% of the static LWL, depending on the maximum speed desired (by the way - what is your maximum speed now?). Placing the LCG more aft gives you a boat with more potential for attaining high speeds, but also a rougher ride and the increased possibility of porpoising.
    Vice versa, the moving the LCG forward will decrease the maximum speed but will give you a softer and more comfortable ride.
    But... if you place the LCG too far forward, you might experience some handling problems with the boat, like difficulty in steering or tendency to broach in following seas.

    Now, Eric is perfectly right when he says that a more inboard engine position is preferred, but only if you stay within the limits on a global LCG.

    If you have a light weight hull, like this Edencraft 233 formula model:
    http://www.edencraft.com.au/formula.html
    (or even lighter), then moving some 600-700 lbs of engine+transmission from the stern to the center of the boat could mean shifting the LCG by as far as 1 ft, which is some 5% of the LWL.
    So, in order to avoid doing things which will later be very difficult to correct, I think it would be better to analyze them a bit further before putting them in action.

    There are some internet forums for Formula owners, it wouldn't be a bad idea to ask over there if someone has already done it and with what results.
     
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