Why did the bunk breach my hull?

SteveNaz

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Aug 23, 2015
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I have a Force 5 sailboat that weighs about 160 and is supposed to sit on two bunks cut out of 2x10s according the manufacturer?s template. These bunks go across the hull, fore and aft, and were carpeted with fairly thin indoor-outdoor carpet. This rear bunk has little sides that curve upward, so maybe I was jamming the boat too tight into the bunk. After less than 200 miles I am faced with a troublesome fiberglass repair and stress cracks are starting on two other corners. Why did the bunk breach my hull, and how can I prevent a reoccurrence? Some possibilities include: carpet too thin, boat located too far forward or too far back, straps too tight, straps too loose, tires too hard, springs too hard. Just for starters I am ready to cover the carpeting with foam sheets cut out from ?? anti-fatigue floor mats, possibly even doubling them to make them an inch thick if that's what the readers recommend.078 Hull Breach.png .
 

pckeen

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Jun 20, 2012
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There's a variety of possibilities - ranging from a rotten interior structure, to strapping it down waaaaay too tight. However......that photograph l ooks like an impact to me. You can see the paint is pushed in there.
 

smokeonthewater

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Dec 3, 2009
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sounds like you have covered the likely causes except for thin carpet... the carpet wouldn't cause it..

generally assuming it's not rot in the hull (and I'm guessing there's nothing to rot in it) this would be caused by not properly supporting the hull.... as you said jamming in.... and or strapping much too tight.

if padding is needed closed cell pipe insulation would work well..

I hauled a 20+ year old sunfish on the roof rack of my jeep 1000 miles at 70-80 mph with only some pipe insulation and ratchet straps without issue.... dunno how the structure of your boat compares to a sunfish tho.
 

bruceb58

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Mar 5, 2006
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dunno how the structure of your boat compares to a sunfish tho.
Made by the same company so likely similar.

To the OP, did you cut the boards to the exact contour of the hull? Show these sides you mentioned.
 
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tpenfield

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FWIW - a Force 5 is similar to a Sunfish or a Laser. Anyway, no internal structure of the hull to speak of, just 3/16" (ish) fiberglass.

The hull puncture was probably caused by a pressure point, presumably at the end of the bunk and the hull bounced up and down on that point until it weakened and fractured. This can be caused by strapping the boat from the bow and stern, which would be beyond the length of the bunks, making the end of the bunks a pressure point.

Usually, sailboat bunks are made to curve with the shape of the hull in order to avoid this sort of thing. When I strap my Sunfish to a bunk trailer, I usually strap it near the middle of its length as well as at the bow and stern so that I am not replying on too much holding force at the ends of the boat.

Can you post a few pictures of the boat/trailer setup and how you fasten it down?
 

gddavid

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Jan 4, 2010
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What weight is the trailer rated for? If the trailer wasn't made for a dinghy, the springs may be so stiff that it effectively has no suspension at all. I have seen unloaded trailers bounce up and down violently and could imagine a the damage it would do to a delicate hull resting on it.

Many of the laser trailers support the boat at the gunnels instead of the hull. I don't have one of the fancy purpose built trailers so I haul my laser upside down on its deck is laying across two flat bunks the same way you would support it on roof racks. The biggest downside to this set up is not being able to put the spars on top of the boat.

Your damage does look like some road debris may have hit it, was this area resting on your bunk or does the boat sit on the trailer just like it is in your photo?

You are probably doing to need to put an access port above the hull puncture so you can patch it from both sides.
 

SteveNaz

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Aug 23, 2015
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7
Thank you for all the great feedback. I will try to start at the top. Yes pclkeen there have been some impacts, if you consider hard bounces the like the time my wife gunned it pulling off the ramp and bounced the whole trailer in the air over a bump she forgot about. I sailed one time after that incident with no problem, but the damage may have already started. As for really tight straps, it?s a possibility. I am seriously hoping someone can post some guidance on that topic. My rule of thumb for next time may be to actually put my thumb under each strap and tighten until it starts to feel uncomfortable.

The photo above shows another view of the damage from a few feet back. You can see a loosened strap and the boat is pushed aft from the bunk to reveal the damage. (The mast pieces normally ride on a wooden cutout, not directly on the deck!). It also shows a 2-piece leaf spring. The wheels are 12?.

Yes, the Force 5 structure is pretty close to a sunfish, almost identical to a Laser. That?s a deck overtop of the hull with lots of volume to take on many gallons of water in-between. My boat is a 1978 by AMF, before ALCORT, and long before Weeks Yachts bought the brand.

The other pic is a stock photo that happens to show the little wings rising from the aft bunk. Those wings dictate where the boat should sit, and apparently can make pressure points if you get it wrong. Today I cut the little suckers off (good riddance!) and put an inch of rubber mat over both bunks. I followed the template which was supposed to be a perfect match to the hull, but I probably trailered the boat one or two inches too far forward so a slightly fatter part of the hull sat between the wings.

I can post a better picture of the trailer setup tomorrow morning, But you will see the bunks are now covered with fat cushioning, not just carpet. Tpenfield, I wonder about the dynamic loading, since the boat is a few inches wider than the trailer frame and the bunks. I figured the best support would be to put a strap right over each bunk and an extra one in the middle. Granted, they all may have been too tight.

Mr. gddavid, I worry that you may be right about the springs being so stiff that I have no suspension. That?s exactly what it feels like sometimes. I have to wonder if it would be advisable (or possible) to remove the shorter spring leaf from each side. And yes, I did slide the boat back to show the damage that occurred over the bunk. Of course, the most frightening thing by far is the suggestion that an access port may be necessary to patch the hole from both sides. Forgive my ignorance, but what happens if I just try to build up the 3/16 inch of fiberglass from the outside and then resolve to never place the repair directly over a bunk? I saw a huge hole getting patched on a laser where they put a board with a rope inside the hole and tied it to a tree. If necessary, I suppose I could put something through the hole and pull the piece back closer to where it belongs.

Thanks again for all the thoughtful replies.
 

tpenfield

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Photos are not showing up for some reason. I think you need to have a few more posts to be able to add photos.
 

SteveNaz

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I finally have a camera icon in front of me. Anything you saw previously came from the attachment button. Here are all 3 photos (captioned) plus more.
 

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Scott Danforth

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The impact on the keel does not look like it came from the trailer. However I could be wrong
 

SteveNaz

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Aug 23, 2015
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Tonight that trailer is parked at Interstate Spring & Alignment waiting for a full evaluation. It's in good company surrounded by 18-wheeler cabs, enormous dump-trucks, etc. I explained that the trailer carries less than 200 lbs and I want the springs to move when I step on the trailer. The guy understood the predicament and he will be checking for frozen bushings as well as evaluating the strength of the springs themselves.
 

HotTommy

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If the breach is anywhere near one of the sharp corners at the outer ends of your carpeted supports, I'd guess the boat bounced into the air, moved a little sideways and landed on the corner of the support.
 

NYBo

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If the breach is anywhere near one of the sharp corners at the outer ends of your carpeted supports, I'd guess the boat bounced into the air, moved a little sideways and landed on the corner of the support.

Yep, tha'ts what it looks like to me. Is that the standard setup for bunks for those boats? Bunks running parallel to the hull, like those on a traditional drive-on trailer, would spread the load across a much larger area.
 

SteveNaz

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Aug 23, 2015
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Yes, HotTommy you may be on to something there.The rear bunk did the worst damage, but the front had problems, too. Here are 4 new pictures after I started working on the edges. Bondo Fiberglass resin calls for sanding off all the color for 3 inches all around each defect. That sounds excessive, but I am inclined to follow their advice. I already know what their answer will be if I call the 800 number. "We recommend extending a 3-in radius around the repair if you want the strongest, longest-lasting repair."
 

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smokeonthewater

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Repair from the inside with a non stick backer on the outside if you can and don't use anything that says Bondo on it

If you can do the repair from the inside then you only need to make the outside look pretty... IE sand and paint if you want.

Start a thread in the restoration section and the guys will walk you through step by step.
 

SteveNaz

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Aug 23, 2015
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Yes, these bunks were made from templates supplied by the manufacturer. They specifically warn against using flat bunks along the length of the boat.
 
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