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Thread: Engineer help

  1. #1
    Member akshrop's Avatar
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    Default Engineer help

    Ok last year my gunnels (both side, same spot) started ripping through 2 feet in front of my stern. I took the boat back to the shop (brand new boat) and they welded the cracks and added some support between the gunnels and stern. I was told I must have trailered the boat incorrectly (too far back). I knew I didnít, but could not say for sure. My thought was that the engine was incorrectly mounted. It has lift plates and angles away from the boat with about 15 degree outward angle. Well it did it again this weekend. I specifically check before putting the boat in the water; no cracks. When I took the boat out; cracks about an inch on both sides above the supports the shop added last year. These cracks go almost through the gunnels. I figure that when they get all the way through, the stern will rip off like a soda can. I am taking the boat to the shop tomorrow, but wanted to know if anyone has any ideas to help me out. I am not an engineer but when we put downward pressure on the engine, the cracks open up. Thoughts/experience? I have an 1860 crestliner with a 60 hp jet. The boat is rated for a 65 hp. I know it is a cheaper hull, but it should not tear open. I bought it new last year about this time. Thanks.

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    Default Power trim

    Shrop,

    Are you using your power trim alot trying to get the boat set right? I've heard of boats that had weight forward and used the power trim bottomed out to pull the bow up a bit while underway. Awesome amount of torgue on the transom and where it hook to the gunnels. Especially if you bounce across a couple wakes. I have an aluminum boat cracking out on the left side at the gunnel. I use a motor support when trailering like you I don't think it is happening on the trailer. It seems to be getting worse when the bow is loaded and I play with th trim getting her to set right. My $.02 worth.

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    Sounds like your bow-up trim is putting a lot of load on the gunnels. Is your motor on a bracket or jack plate that puts it out behind the transom a ways? That'll add a lot of torque on the transom and could be part of your problem. Does your boat have longitudinal (fore/aft running) stringers or 'beams' under the deck? If so, then the correct fix is to add supports that tie the transom to those longitudinal stringers, not beefing up the gunnels. That's just a patch job if you ask me.

    And if you don't mind explaining, is there a reason you feel a need to trim the bow up a lot? Do you have a weight distribution that is putting too much load in the bow? Can it be moved aft? Most folks will find a need to trim the bow down a bit to reduce pounding in a chop, not the other way around ...so I'm curious to hear more about your setup. Regardless though, the shop or manufacturer owes you a boat that doesn't fail so easily... that just isn't right (my 2-bits).

    Brian

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    Akshrop,
    Is it possible to attach some photos of the lift/ bracket arrangement and the break in the Gunwales?

  5. #5

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    This sounds like the designer of the boat and the collaborating engineers had a serious mathematical lapse. Under no circumstances should this be happening. First and foremost, you should not have to trim the bow of the boat up - it should be the opposite. What in the world is in the front of that light little boat that would be causing it to do that? Clearly, this is the opposite of what the laws of physics dictate it should be doing, unless you have intentionally created this by putting all kinds of stuff up there - like a large collection of Cannon downrigger balls, your batteries, your fuel tanks, your zodiak raft (just in case) and an old chevy 350 just for fun. And, all joking aside, if you have really loaded it, then you really need to redistribute some of it.

    I can see that maybe if you have a jackplate and doing some goofy stuff with trim tabs that the problem you are having could happen after taking a serious pounding at sea. But, under normal conditions? It still doesn't sound right to me.

    I am sticking to this just should not be happening. Something structural is seriously amiss. In a perfect world, there should be almost no stress on the gunwales as the bottom of the boat should be the part of the boat that is supporting it as it moves across the water forward and aft. It should be strong enough that it doesn't even need gunwales and should hold its rigidity almost all on its own. I do know though, that the gunwales do play a role in the rigidity of the structure. If this thing is flexing enough to crack the gunwales then you would think, that upon opening up the floor, that you would find that something just is not welded correctly in the bow, or in the stern as it is moving just as much as the gunwales during this period of flexing. Or, upon inspection, you may even find that something that should be there was just omitted accidentally.

    If I were you, I would take this boat back to where you got it. I'd probably even go as far as being armed with an lawyer and demand a new boat - one that isn't about to break in two.

    If they do end up repairing it, I would be out in it (with another boat nearby) in heavy seas, and see if you can get it to do it again. And, I do it until I made darned sure this problem was gone.

    I wish you the best of luck.

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    I would have to agree with TR, you would have to terribly overload the boat to put enough strees on it to crack the gunwales. In fact I have serious doubts that you could overload it to the point of breaking unless you were putting engine blocks in the boat.
    It may not be seaworthy overloaded, but I cant see it breaking apart, I have heard stories of guys taking riveted Jon style boats and cutting the seats out to lighten them out, then take em out on the Yukon and fold the sides up! But not the sides breaking apart.
    There has to be an issue in how it was built or how the motor was mounted, What model Crestliner do you have?
    I didnt see a 1860 listed on their website, sure would like to see a photo of this outfit.
    Let us know how it turns out?

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    So let me see if I am following this. The shop you bought the boat from rigged the boat, set up the trailer, and now won't back up their work or product? They say you are trailerig incorrectly and they set up the rig? They should be making this right for you!
    Spending my kids' inheritance with them, one adventure at a time.

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    Quote Originally Posted by T.R. Bauer View Post
    <snip>
    In a perfect world, there should be almost no stress on the gunwales as the bottom of the boat should be the part of the boat that is supporting it as it moves across the water forward and aft. It should be strong enough that it doesn't even need gunwales and should hold its rigidity almost all on its own. I do know though, that the gunwales do play a role in the rigidity of the structure.<snip>
    It's not really accurate to say the sheer structure does not carry a load. Framed boats must have a strong enough sheer structure to transfer load from one frame to another for example. The same applies to any boats that contain required bulkheads that tie one side of the boat to the other. My boat design (see http://www.glacierboats.com ) for example carries all hogging and oil-canning loads jointly in both a box-girder type hull (main stringers, structural deck, bottom panels together act as a 'box girder') and a stiff sheer structure. The sheer in my boat has 'shelves' or longitudinal framing under the sheer decks that are quite strong, and the sheer deck itself is structural as well. The result is a boat that is able to be designed as a semi-monocoque structure (loads shared both by internal structure and the skin) ...the advantage being a) light weight, b) strong, c) lower cost in both the hull and required motor (lower horsepower requirements.) This is not a new idea.

    In the case of this boat cracking the gunnels, it implies that the transom is flexing aft and the boat design/structure was not properly designed (or was improperly manufactured). The things to look into are 3: a) was the boat built right, as designed, b) is there something unusual making that transom flex aft (brackets/jack plates, too much horsepower, weight on transom exceeds specs, excessive bow-up trim combined with the above), or c) is the design itself inappropriate (center of gravity too far forward, weak transom, transom loads not properly transmitted to the rest of the boat, etc.) If the answer is not obvious, then the best way to find out, especially if this starts to involve attorneys and what not, is to have a licensed surveyor inspect the boat and type up his/her conclusions. Personally, I've seen so much shoddy engineering and manufacturing in the world of boats, I'm very surprised that more issues don't arise.

    Brian

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    I toook some pictures, but have not figured out how to attach them yet. The boat was not over loaded. I did give a ride to 3 very large men after their boat sank, but we went only a little above trolling speed as they were wet and cold. I am taking the boat in to the shop today. I will let everyone know how it turns out and I will get the pictures posted. Thanks for all the help.

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    Here are some pictures. I never trimed the bow up, nor was there any crazy weight in the bow. The second pic shows the support added last time and the new crack. Both sides are exactly the same.
    Attached Images Attached Images

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    Is the raised transom (for the jet) normal or is it something they add on when putting on a jet? I see that the boat has no longitudinal stringers (from what I can see) and there's nothing to tie the transom into to keep it from flexing. I think the raised motor is putting extra torque on the transom and is the load that needs some attention. How best to do that is a good question to ask them.

    Brian

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    Its not unusual to have to extend the transom for a jet installation. It looks like the motor you have could be a long shaft (25")??
    If so this is acting like a big lever on the transom, if its not bowing the transom, and it may be, it certainly is straining the next weakest spot, which appears just forward of the gusset plates that tie the transom to the gunwales.
    In any event as some one else said earlier the dealer is responsible for this setup, particuarily if he sold you a package on boat, motor and trailer.
    Did he supply the motor? was it a new motor?
    Boat and workmanship should be warrantied, many hulls carry a warranty for the orginal buyer.
    Those type of lighter duty boats can be marginal, you add a jet, raise it up and it can really torture the transom.
    Let us know what transpires...!

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    Tough to make a call without seeing additional photos and putting some sort of stress model together, but regardless, here are some observations:

    -The location of your cracking isn't all that mysterious - there is a "hard point" aka stress concentration, in this case occuring at the intersection of a strong piece of structure with a weak piece of structure at that very spot, where the gusset joins the gunwale pipe. Before they added the gusset repair, you can see the location of the previous crack's weld repair, right at the hard point between the triangular corner piece and the gunwale pipe.
    -Anytime you see cracking like that, at a hard point, you have repeated, cycled application of stress at that point. You can rack up a lot of load cycles on a bumpy road, moreso than in water, in my opinion.
    -The fact that the gunwale pipe cracked there the first time suggests either a motor/trailer setup problem or a boat design problem.
    -The repair did not address the root of the problem, which is a repeated application of load causing a large number of cycles of high stress at that location.
    -Were I a betting man, I'd bet that the answer is either adjusting your trailer so that the bunks extend PAST the transom, or structurally increasing the effective thickness of your transom, making the transom rigid enough that it does not transfer any bending load to the point of your cracks, only tensile load. This isn't easy to explain, nor would it be cheap to do...

  14. #14

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    Quote Originally Posted by tananaBrian View Post
    It's not really accurate to say the sheer structure does not carry a load. Framed boats must have a strong enough sheer structure to transfer load from one frame to another for example. The same applies to any boats that contain required bulkheads that tie one side of the boat to the other. My boat design (see http://www.glacierboats.com ) for example carries all hogging and oil-canning loads jointly in both a box-girder type hull (main stringers, structural deck, bottom panels together act as a 'box girder') and a stiff sheer structure. The sheer in my boat has 'shelves' or longitudinal framing under the sheer decks that are quite strong, and the sheer deck itself is structural as well. The result is a boat that is able to be designed as a semi-monocoque structure (loads shared both by internal structure and the skin) ...the advantage being a) light weight, b) strong, c) lower cost in both the hull and required motor (lower horsepower requirements.) This is not a new idea.

    In the case of this boat cracking the gunnels, it implies that the transom is flexing aft and the boat design/structure was not properly designed (or was improperly manufactured). The things to look into are 3: a) was the boat built right, as designed, b) is there something unusual making that transom flex aft (brackets/jack plates, too much horsepower, weight on transom exceeds specs, excessive bow-up trim combined with the above), or c) is the design itself inappropriate (center of gravity too far forward, weak transom, transom loads not properly transmitted to the rest of the boat, etc.) If the answer is not obvious, then the best way to find out, especially if this starts to involve attorneys and what not, is to have a licensed surveyor inspect the boat and type up his/her conclusions. Personally, I've seen so much shoddy engineering and manufacturing in the world of boats, I'm very surprised that more issues don't arise.

    Brian
    I think that "in a perfect world says it all," in reference the sheer structure, as there is not such thing....and we are for the most part in complete agreement. However, as you say, I say and anyone with their head in the right place, it is clear that there is way too much flexing. Without close inspection, one can only guess at the cause. Obviously, something is very wrong. It is clearly something that 30 minutes of welding isn't going to fix.

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    Quote Originally Posted by Vek View Post
    Tough to make a call without seeing additional photos and putting some sort of stress model together, but regardless, here are some observations:

    -The location of your cracking isn't all that mysterious - there is a "hard point" aka stress concentration, in this case occuring at the intersection of a strong piece of structure with a weak piece of structure at that very spot, where the gusset joins the gunwale pipe. Before they added the gusset repair, you can see the location of the previous crack's weld repair, right at the hard point between the triangular corner piece and the gunwale pipe.
    -Anytime you see cracking like that, at a hard point, you have repeated, cycled application of stress at that point. You can rack up a lot of load cycles on a bumpy road, moreso than in water, in my opinion.
    -The fact that the gunwale pipe cracked there the first time suggests either a motor/trailer setup problem or a boat design problem.
    -The repair did not address the root of the problem, which is a repeated application of load causing a large number of cycles of high stress at that location.
    -Were I a betting man, I'd bet that the answer is either adjusting your trailer so that the bunks extend PAST the transom, or structurally increasing the effective thickness of your transom, making the transom rigid enough that it does not transfer any bending load to the point of your cracks, only tensile load. This isn't easy to explain, nor would it be cheap to do...

    Stress cracking at the gunwale means your transom is moving forward and aft at the top of the transom. Trailering being the most likely cause. Especially with a that liftplate setup. I remember seeing the same type cracks on my dad's boat way back when. Put gussets in and the cracks move further forward. He wound up building a support setup that went from the top of the transom to the floor and to the rear seat.

    On the water there is a tremendous amount of pressure but not something I think would make cracks. Bend maybe. Think of the prop/jet trying to push itself under the boat and the fulcrum point being your attachment points. A motor under power rarely would vibrate to cause cracks.

    Dewey's sells many, many of these setups. So I wonder if yours is just the odd ball out.

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    The second picture tells the story; welding removes the heat treatment of the Aluminum (if it is a heat treated series), and creates stress risers in the metal. In a nutshell, those are weak points. That gusset that was added just added a new stress riser to the gunnel. The motor is adding the torque, and the riser is extending and adding to it. Probably some sideways load as well, bouncing around etc.
    I would:
    1. Add a thicker plate along the top of the gunnel from the transom forward. Taper it up at the forward end. Thats what i would do. It would involve removing the back gussets, and adding the plate along the inside of the gunnels, the putting the gussets back in. Given the thin hull material present, this would be TIG job IMHO.
    2. This would look like he!!, but you could add some braces down from the top of the motor riser to the aft seat. That would reduce the stress the gunnels see.
    Bring it to Whitehorse and i'll do it for beer + materials.
    Hope it works out for you.

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    Quote Originally Posted by Yukoner View Post
    The second picture tells the story; welding removes the heat treatment of the Aluminum (if it is a heat treated series), and creates stress risers in the metal. In a nutshell, those are weak points. That gusset that was added just added a new stress riser to the gunnel. The motor is adding the torque, and the riser is extending and adding to it. Probably some sideways load as well, bouncing around etc.

    I would:
    1. Add a thicker plate along the top of the gunnel from the transom forward. Taper it up at the forward end. Thats what i would do. It would involve removing the back gussets, and adding the plate along the inside of the gunnels, the putting the gussets back in. Given the thin hull material present, this would be TIG job IMHO.
    2. This would look like he!!, but you could add some braces down from the top of the motor riser to the aft seat. That would reduce the stress the gunnels see.
    Bring it to Whitehorse and i'll do it for beer + materials.
    Hope it works out for you.
    Spot on....what he said. bracing, and more material.

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    Just further info. The entire set-up was as I bought it new. I went to the shop today and they simply talked about fixing it (the second time). I told them I want a new boat. I want to upgrade anyway. Now they told me I would get a better deal selling it myself than they would give me on trade in. That stinks. I was told to call back tomorow to hear what the boss/owner has to say. I think I have a good case for a replacement without cost and especially that I want to upgrade. I doubt they will actually lose any money, but if they do, so be it. They should stand behind their product. I will update tomorow. Thanks for all the help. It helped my unengineering butt talk to them today.

  19. #19

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    Quote Originally Posted by akshrop View Post
    Just further info. The entire set-up was as I bought it new. I went to the shop today and they simply talked about fixing it (the second time). I told them I want a new boat. I want to upgrade anyway. Now they told me I would get a better deal selling it myself than they would give me on trade in. That stinks. I was told to call back tomorow to hear what the boss/owner has to say. I think I have a good case for a replacement without cost and especially that I want to upgrade. I doubt they will actually lose any money, but if they do, so be it. They should stand behind their product. I will update tomorow. Thanks for all the help. It helped my unengineering butt talk to them today.
    Clearly if you sold it yourself it would be a better deal for them as this makes it so their hands are entirely clean of this mess. It also shifts the responsibility (money wise) from them to you. Since they set this boat up, put the motor they selected and recommended on it, and set up the trailer, they are basically screwed. It is all them to make it right and you happy. I think they need to just be ready to "eat" this one and move on. But, from my experience, they will drag their feet, try put the blame on you in some way, and try to squeeze their way out of it any way that they can. Please, please, don't let them.

    Just so you know, in cases such as this if they ever make it to court, the consumer (you) rarely lose. They dealer knows this too. You just need to put some pressure on them. And this something that an attorney will gladly do for nominal fee in most cases.

    Best of luck and I hope that you get this all worked out.

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    Quote Originally Posted by Vek View Post
    Tough to make a call without seeing additional photos and putting some sort of stress model together, but regardless, here are some observations:

    -The location of your cracking isn't all that mysterious - there is a "hard point" aka stress concentration, in this case occuring at the intersection of a strong piece of structure with a weak piece of structure at that very spot, where the gusset joins the gunwale pipe. Before they added the gusset repair, you can see the location of the previous crack's weld repair, right at the hard point between the triangular corner piece and the gunwale pipe.
    -Anytime you see cracking like that, at a hard point, you have repeated, cycled application of stress at that point. You can rack up a lot of load cycles on a bumpy road, moreso than in water, in my opinion.
    -The fact that the gunwale pipe cracked there the first time suggests either a motor/trailer setup problem or a boat design problem.
    -The repair did not address the root of the problem, which is a repeated application of load causing a large number of cycles of high stress at that location.
    -Were I a betting man, I'd bet that the answer is either adjusting your trailer so that the bunks extend PAST the transom, or structurally increasing the effective thickness of your transom, making the transom rigid enough that it does not transfer any bending load to the point of your cracks, only tensile load. This isn't easy to explain, nor would it be cheap to do...
    That's a good analysis and good answers. How about if you not only tune the trailer and/or use longer bunks, but what if you support the motor as well. You know, the usual "transom saver" set-up where you lower the motor against a support and the support goes down to the trailer. I've seen fancy rubber gizmos all the way to chunks of 2x4 or 2x6 used for this. Might not be a bad idea.

    Brian

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