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bunks on boat trailers

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Old 02-17-2004, 04:51 PM
  #11
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Most twin engine boats have a "center stringer" that runs all the way from the stern to the front of the cockpit... and usually have two fuel tanks.... one on each side of the stringer.On the outer sides of each of the fuel tanks are smaller stringers that correspond to the outer chines. The large tall center stringer usually ends where the door into the cabin starts... there is a big bulkhead that goes from keel to deck with a "hole" to pass through... then two smaller stringers continue up toward the bow and are usually "boxed in" .. The seats inside the cabin are over the top of these... they usually end at a keel to deck bulkhead where the front bunk starts... does this make any sense to you? In the engine compartment you will usually see smaller stringers that run under the engines. Sometimes there are small bulkheads between these smaller stringers... and even smaller ribs running at a diagonal angle... these all stiffen the outer part of the hull. The pressure of the water being displaced is greatest against the area where most of the weight is... under the engines and transmissions.... and when entering the water from coming off a wave... this is the area that usually hits first... so builders tend to strengthen this area more. l If you look into the "cabin" area of a race boat... there are more stringers and bulkheads... and sometimes a balast tank for balance... but not much room for people.Does this explain anything to you ?Chart?
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Old 02-17-2004, 05:05 PM
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One more thing.... the bunks don't always run directly under the keel... but close enough .. in the picture you posted .. the keel bunks look like they are only about 8 inches apart.. on a 40 foot boat the hull is about 1 1/2 inches thick along the keel... it's a pretty stiff area of the boat.. not only because of the thickness... but the shape of the hull... When the boat floats onto the trailer... it finds it's "center" between those inner bunks... then lays down on the outer ones when the trailer is pulled up the ramp... That might be something that you haven't considered Chart... the boat has to lower itself onto the trailer at an angle. The picture you posted is what is called a "float on.. float off" trailer... there are other types of trailers for race boats where the boat is lifted off with a crane and then lowered into the water.. the trailer is never submerged in the water.
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Old 02-17-2004, 05:22 PM
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Quote:
Originally posted by Reed Jensen
Does this explain anything to you ?Chart?

Yes, but I had to go real slow and get help with the big words

You know much more about the design of stringer systems than I do, and I appreciate the lesson. It looks like the center stringer that runs up to the front cockpit bulkhead is actually supported by the keel bunks on most trailers under the cockpit/fuel tank area, but not under the engines.

Could that be because the transom, one of the strongest parts of the boat, supports (holds) the keel stringer and inside engine weight more than adequately, when the trailer bunks support the outer stringers? This prevents the boat from deforming the "running surface" if the trailer is not set perfect for the boat? But forward of the engine compartment, the transom is too far away to offer strength to the center stringer, and so the trailer has have a keel bunk?
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Old 02-17-2004, 05:27 PM
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Reed: You posted #12 while I was writing #13. You may be on to something there about aligning the boat during launching and loading. Without keel bunks in the front of the trailer, the increasing vee of the front of the boat might hit cross members in some conditions
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Old 02-17-2004, 05:47 PM
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That is correct Chart.. on some trailers there are rollers... but if the boat doesn't hit the roller dead on... the frame can scratch the keel .. I've done it on my boat. That is the reason for the bunks near the front.
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Old 02-17-2004, 05:56 PM
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Hers a pic of my old trailer for a step bottom boat. The outer bunks were right near were the chine flattens out and the inner bunks were inside a set of strakes which also lined up with the hull stringers. The bunks are split for the step and at different heights and they ran right ti the edge of the transom. Worked good for loading and didnt put any hook in the bottom after 6 years.
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Old 02-17-2004, 06:09 PM
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One more post for you Chart... in looking more at the picture you posted... the reason the center bunks for the keel don't run as far back as the bunks for the chines at the outer part of the trailer is this... the boat has a notched transom.
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Old 02-17-2004, 06:24 PM
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Reed: That's a good point. Once again, you could be on to something.

DanB: I would think your trailer offers better overall support at the transom, but there must be a reason Myco, Manning, Eagle, etc elect not to do it that way.

We've got members that mfg trailers (Manning, ??? from West coast of FLA sorry I don't recall the name) and Myco advertises here. It'd be nice if one of them weighed in here also.
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Old 02-17-2004, 07:05 PM
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What is your concern Chart? The thing to watch out for is the trailer not supporting the hull... over time the hull will deform and then have a "hook" in the bottom.. Hence... the trailer has to be quite rigid from stem to stern. But steel tends to bend ... and since it is supported just at the axles.. they tend to bend right there.
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Old 02-17-2004, 07:18 PM
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It's not concern, but curiosity. I've wondered about this for years, and finally asked. Heck, we had a hotly debated post years ago about which freezes faster, hot or cold water. People weighed in with all kind of scientific and engineering insights as to which was right. And that was not even boating related, unless you count that it was water, and that ice is needed for the coolers.

Occasionally a picture will appear with the boat overhanging the bunks by 2-4 feet, and we know that is hard on the hull. It simply strikes me as odd that the heaviest part of the boat has the greatest span between the bunks.

Reed, you've contributed the most to this discussion, and thank you for that.
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