Hull Deflection on 76' Daysailer II

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Moderator: GreenLake

Re: Hull Deflection on 76' Daysailer II

Postby GreenLake » Tue Oct 04, 2016 5:39 pm

I see your thinking. You approximate the bunk as two edges over which the plate is bent. In the ideal case, the hull over the middle of the bunk wouldn't even touch. Then each "edge" acts independently and their separation doesn't change the picture much.

On a DS1, the seats provide a support about 1' or so from the centerline. If your bunks are wide enough to reach that point then the hull is no longer an ideal plate.

The other way you can get away from that ideal model (even for a DS2) is if you assume that the bunks are padded and that the hull plate is not totally flat. Both may allow a more gradual transfer of load and that may well be different for wider bunks.

Finally, bunks should be made both longer and wider. That puts more of the support closer to the edge of the hull panel, which might mean it's closer to a support (e.g. transom) and acts less like an ideal plate.

Also, if your bunk is more like a square than like a beam (and the end of a wide bunk would look like a square) that itself would increase the area of the plate over which the load is distributed and change the load from linear to something more 2D (which may also have an effect).

I'd be interested to see what conclusion you would draw from the theory of plates, so I'm hoping you can find your copy of Timoshenko.
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Re: Hull Deflection on 76' Daysailer II

Postby Zinger88 » Fri Oct 07, 2016 11:04 pm

I, along with John (jeadstx), inspected and sailed the boat on Tuesday and I'm excited to say that I'm now the new owner of Daysailer II Sail# 7920! :D The deflection didn't have any cracks around it and John and I felt that it wasn't enough of an issue to pass on the boat. I'm really excited on being a new 1976 Daysailer II owner and looking forward to giving a vintage craft the TLC it's been waiting for. It's been a number of years since the previous owner had put the boat in the water so there's a few upgrades/refurbishments needed. Thankfully this forum is such a valuable resource of information!!

Jim
Jim H
'76 Daysailer II, Sail #7920, Windsong
Burton, TX
~~ _/) ~
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Re: Hull Deflection on 76' Daysailer II

Postby jeadstx » Sat Oct 08, 2016 12:08 am

It was good meeting you Jim and I enjoyed sailing the boat with you.

John
1976 Day Sailer II, #8075 - Completed the 2011, 2012, and 2013 Texas 200
1952 Beetle Boat Swan Catboat
Early Rhodes 19
1973 Mariner 2+2, #2607 - Completed 2014, 2015 and 2016 Texas 200
1969 Day Sailer I, #3229
Fleet 135; Canyon Lake, Texas
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Re: Hull Deflection on 76' Daysailer II

Postby GreenLake » Sat Oct 08, 2016 1:44 am

Zinger88 wrote:I, along with John (jeadstx), inspected and sailed the boat on Tuesday and I'm excited to say that I'm now the new owner of Daysailer II Sail# 7920! :D The deflection didn't have any cracks around it and John and I felt that it wasn't enough of an issue to pass on the boat. I'm really excited on being a new 1976 Daysailer II owner and looking forward to giving a vintage craft the TLC it's been waiting for. It's been a number of years since the previous owner had put the boat in the water so there's a few upgrades/refurbishments needed. Thankfully this forum is such a valuable resource of information!!

Jim


Congratulations, and add wider/longer bunks to that trailer :)
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Re: Hull Deflection on 76' Daysailer II

Postby willyhays » Sat Oct 08, 2016 2:28 pm

Another option is to make transverse bunks. This configuration essentially eliminates hull deflection and is now common on all sorts of boats, from little dinghies to monster ocean racers.

http://1.bp.blogspot.com/-GsiO4O5J_lI/V ... 104120.jpg

http://pressure-drop.us/imagehost/image ... 225542.jpg
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Re: Hull Deflection on 76' Daysailer II

Postby talbot » Mon Oct 10, 2016 12:12 am

The common recommendation is to use wider bunks, but the main thing may be weight distribution.

When we got a fairly new 1-ton keelboat this summer, it came with a custom trailer supplied by the builder and designed for the boat.
I was surprised to see that it had 2x4 bunks, just like on the 575-lb Day Sailer. The boat is a fat-butt cruiser, and like the Day Sailer, the aft section of the hull is nearly flat. I don't see any obvious hull deflection, and no one on the boat's user forum has commented on the need for wider bunks. It appears that the trailer is adjusted to put the weight on the keel, with bunks as outriggers, and the builder seems to think that's fine.

For what it's worth, I found the DS trailered better when I switched to wider keel rollers. I replaced the little 6" black neoprene rollers with 12" polyurethane. Less marking on the hull, apparently less friction, and a broader target when floating the boat onto the trailer.
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Re: Hull Deflection on 76' Daysailer II

Postby GreenLake » Mon Oct 10, 2016 12:28 am

Keelboats are a bit different.

For one, much of the weight is in the keel, obviously, so you need to support that directly.

Then, the loads from the keel, because of its weight, are more than those from a CB; this leads (or should lead) to a stronger structure where keel/hull connect. That structure, while designed to keep the keel with the boat, also supports the hull when resting on the keel.

For all of those reasons, the bunks on a trailer for keel boats have the same function as the supports used when storing a keelboat on the hard: mainly to keep it upright, not to take the majority of the weight.

Many boats with flat hulls may have significant internal frames or boxes where the hull is connected to the keel. I wonder whether that is the case for your keelboat as well.

I heartily concur with upgrading any keelrollers from the old rubber to polyurethane. I did that many years ago and I like it. One thing I did at the same time is to get an extra wide roller at the back, but with a notch in it. By catching the bow in that notch, I can line up the boat, and have an almost perfect retrieval every time.

I found that resting the hull on the bunks produced a better weight distribution than just resting it on the rollers. This weight distribution matters when retrieving the boat. If the bunks lift the boat just a bit off the rollers at the back (aft of the CB trunk) I find that the boat loads more evenly. In particular, I drop the last roller by 2-3" - all the others ones end up supporting the keel or the CB trunk (and CB) in areas where the hull is reinforced on the centerline.
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