calculating strength

Discussion in 'Boatbuilding' started by Psycho D, Sep 2, 2003.

  1. Psycho D
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    Psycho D Junior Member

    For my next boat restoration (which will involve an ib/ob to o/b conerversion), i was thinking of making my own motor bracket. i will probably use biax and /or carbon fiber with epoxy, but am not sure how to calculate the FRP thickness for a given motor weight, torque, etc. i also would need to figure out how to make the motor mounts and bracket to transom mounts sufficiently reinforced. Any thoughts on where to begin? i have some basic FG'ing books, but are there any other liturature sources that i may inquire into. i'm in no rush as i am in the middle of restoring another boat currently, but i like to have further projects simmering in my mind. Thanks.
    d
     
  2. gonzo
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    gonzo Senior Member

    I think you need to first figure out what changing the CG so far aft will do to the boat's trim. An I/Ohas a heavy motor several feet forward of where an outboard in a bracket will be. Most likely the boat will porpoise and run with the bow pointing up.
     
  3. Psycho D
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    Psycho D Junior Member

    Hey Gonzo. That is a great point. Luckily, the boat i am doing, a Formula 233, is a common project boat that is usually converted over to an outboard using a bracket. So far i have heard of no complaints about the conversion other than the crappy backward manuevering. That said, what do you think?
    d
     
  4. gonzo
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    gonzo Senior Member

    I'm not familiar with that model. If it runs fine with outboards it may have been designed for them. This wouldn't be the first time an outboard boat gets I/O's. What makes them not maneuver in reverse?
     
  5. Willallison
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    Willallison Senior Member

    We have a 23 foot Formula fitted with 2 x 225hp Dfi Mercs on a pod. There is no problem with porpoising, but the boat does drop off the plane at anything under about 23mph.
    Our pod is integral with the hull in that it is made of f/glass and you can access the inside of the pod from within the boat. I suspect that at almost a metre in length it places the outboards further from the transom than is ideal. This (and the fact that the boat is VERY heavily overbuilt) contributes to it's high min. planing speed. Other, similarly powered, local boats fitted with aluminium bolt-on pods appear to suffer less from this problem.
    Many years ago the Formula was built here under license. The molds have been sold and re-sold many times over the years and the boat is still being built today. All of them are o/b these days.
    Ours was one of 3 or 4 built by an ablaone poacher. It was originally rigged with a pair of 200hp 2.5L Mercs. The motors were run so high that remote water pick-ups had to be installed at the base of the transom! In this guise it wouldn't do less than 30mph before the ventilating props would lose their bite and the engines would do their block (quite literally on one occaision!).

    It's pictured here (in the foreground)...christmas a couple of years back just after the motors were fitted.
     

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  6. Psycho D
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    Psycho D Junior Member

    Hey Gonzo. i think that they go poorly in reverse cuz the prop is extended about 26 inches farther away from the keel. Just my 2 cents. Thanks for the replies, but i still need to find a source to figure out glass strengths.
    d
     
  7. Willallison
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    Willallison Senior Member

    I can see no reason why a boat with a pod would handle poorly in reverse - ours is no better or worse than any other large o/b powered boats I've driven.....
    The greater distance from the transom would, if anything, improve backing....
     
  8. Psycho D
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    Psycho D Junior Member

    brackets in reverse

    i pulled this explanation off of "The Hull Truth" about why boats with brackets installed do not manuever well in reverse. It sounded good to me:

    "In reverse the water being pushed by the prop tends to hit the transome instead of going under the boat. When I reverse I tilt the engine up a couple of notches and that allows the water to flow under the hull to get reverse propulsion.

    It's a little tricky but it works.

    Hope this explains it"

    d
     
  9. Willallison
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    Willallison Senior Member

    I understand the theory - but unless the engines are mounted higher on the pod than they would otherwise be on the transom, then the thrust should still pass under the boat. In fact, given that there is extra distance to the transom, you'd think it would be better...?
    This is a problem with surface drives, where the prop thrust is directly into the back of the boat. Most boats designed for surface drives have a steeply angle transom to redirect this thrust down and under the boat.
     
  10. Psycho D
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    Psycho D Junior Member

    great observation

    Hey Willallison. The rule of thumb for outboard setup on a bracket is install them with the anti-cavitation plate 1 inch above the keel for every foot of setback. Late.
    d
     
  11. Thunderhead19
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    Thunderhead19 Senior Member

    Strength Calculation

    What will drive your calculations for the OB bracket will not likely be the torque/thrust characteristics of the OB, but the collision resistance. If you're manoeuvering in reverse, and your control is not good, forsee hitting stuff. Also, unless you're primarily going to be prop-walking you'll probably hit some kind of flotsam, sand bar, dead-head, submerged rock or cable, sooner or later. Also, you're going to use a core whether it's plywood or high density plastic or chop-strand pre-formed like Penske board. The Section Modulus is the key, not so much the actual thickness of the plys. Without knowing more about the project I can't offer more.
     
  12. gonzo
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    gonzo Senior Member

    I agree that with any projection from the hull collisions are likely.
     
  13. Willallison
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    Willallison Senior Member

    Why? I'm not suggesting that a pod shouldn't be engineered to cope with collision (I would have thought that anything capable of absorbing the weight and power of a pair of big o/b's would be able to do so regardless...) But we aren't talking about something projecting down from the hull - just back and up from the transom. True, if either engine hits something, then much of the load will be transferred to the pod so it must be capable of dealing with that....but you're doing some really weird stuff if you manage to belt something with the pod itself.....
     
  14. gonzo
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    gonzo Senior Member

    Backing a pod into a dock or a piling is a common occurrence. I have done many insurance claims about that. Usually it is the corner that hits.
     

  15. Willallison
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    Willallison Senior Member

    Hmmm..... fair enough then.....
     
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