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Old 11 May 2022, 18:23   #1
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Modifying T type dinghy trailer for SIB use

I've decided a trailer would make my life easier - or at least, remove a lot of the launch/retrieve faff associated with a few hours on the water. I think the cheapest option is going to be to modify an existing non-SIB trailer, simply because so few specialist SIB trailers come on to the used market (I missed a local one last week on eBay, having said that, which was annoying!).

I enjoyed this thread from @Steve509926 and want to do something similar: 2 x bunks, a rest at the front, maybe a winch & post. Nothing more required IMO, I will be running a max of 15 minutes to the ramp.

A cheap possibility has come up locally, but it is a T type "base" trailer not an A frame. This seems inherently less strong/prone to twist to me, but I guess it can't be that much of an issue if they tow dingy's with them? Or does the dolly that slots inside provide some of the structural integrity? If not, and it would be safe to modify, does anyone have any ideas on how to attach a cross member at the front which could then attach to the front of the bunks.

I hope that makes enough sense - if not, I can try and draw a picture!
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Old 11 May 2022, 18:31   #2
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Depending on its design you can possibly modify it to a square frame using some one inch galvanised box and threaded U bolts. Most dinghy trailers are fabricated in this manner these days.
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Old 11 May 2022, 19:04   #3
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Here's a pic (not a very good one):



The A is the dolly on top, the base is just a T, without any sort of diagonal reinforcing struts at all.

The square frame is exactly what I am thinking of doing, but how would one attach 2 x box sections at 90 degrees with U bolts? I'm finding it hard to picture - I imagine some sort of plate is required to help hold everything together?

Edit to add: I suppose one option might be to cut the rear of the dolly off (axle and wheels) and then to somehow attach (bolt?) the dolly to the base frame. That would probably give me what I need to attach bunk supports towards the front..
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Old 11 May 2022, 20:04   #4
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I think with that I'd consider a cross member over the front of the A and just drill and bolt it to the end of the diagonals around 6 inches back from where they meet to get a front cross member then drill and bolt uprights in line with the existing rear ones and then join them with 4x2 (that you then cover) using L brackets drill and bolted to the top of the uprights.

Done up tight enough to start buckling the square tube and they'd be very unlikely to ever budge.
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Old 11 May 2022, 20:45   #5
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Hi Matt, what size is your SIB?

I don't understand why you are not thinking about using the launching trailer?
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Old 11 May 2022, 20:46   #6
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Thanks Tim, that's roughly what I'm thinking but bear in mind the A is not attached at this time to the base.
I think I could probably put a U bolt on the diagonal to join the rear struts of the A frame to the axle, then get the angle grinder out and just cut the rear of these struts about 6 inches behind the axle (losing the axle for the dolly wheels, which has some rust on it anyway in the corner). Not quite sure how I'd attach the front of the A to the centre of the T section, but open to suggestions (not sure a U bolt would work here). I worry a little about drilling the T centre and losing strength?

I am going for a look at the trailer tomorrow morning so will take more close up pics and maybe get a better idea.
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Old 11 May 2022, 20:50   #7
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Hi Matt, what size is your SIB?

I don't understand why you are not thinking about using the launching trailer?
I've got transom wheels already, so figured I'd just manhandle off the trailer and wheel in as I normally do. The dolly also has some rust issues, whereas the base looks fine (will inspect more closely tomorrow). You're right though, it might be something worth considering if it fits well. I have an Elling 350 which I think might actually be 3.6m. The trailer in question is 11ft long, but I think a bit of overhang at the back will be fine.
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Old 11 May 2022, 21:06   #8
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If the rust isn't to bad and is treatable, I'd be thinking about attaching the bunks on the launching trailer. Couple of reasons, far less work and you have a purpose built launching trailer, which will make launching your kb350 so much easier.

Because I wanted a trailer that I could easily dismantle to fit in the caravan, I didn't want or need the launching trailer, plus the guy who sold the road base wanted to keep it. If I wasn't dismantling the trailer, I definitely would have looked for a roadbase with a launching trailer.
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Old 11 May 2022, 21:38   #9
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I think the rust issue is fairly bad in one spot at the corner of the dolly axle - I'll know more tomorrow. However, the idea of simply leaving the dolly on top and attaching the bunks to it is an attractive one, even if I don't end up using the trolley for launching (tbh, transom wheels are fine for the final few yards). I'll try and post up some more pics tomorrow and get a plan going. As an aside, I would quite like to have this so that I can dismantle at the end of the season and put it away somewhere, I don't have enough drive space to leave it out year round.
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Old 12 May 2022, 05:11   #10
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If you can convert the dolly then have a way to fix it to the road trailer then that could be useful. Is that what's being suggested?

If just wanting to use the T road trailer (sorry my eyes and brain hadn't realised that picture showed the T trailer with the launching trailer on top) then you can turn a T into an A by cutting a pair of outriggers in box section and hammering the ends flat to make tabs through which to bolt front and rear.

If there are concerns re weakening the spine by drilling a hole through it then you could U bolt too and bottom plates to it and drill and fix the outriggers between those.

Much is going to depend on what tools you have easily available. Being able to bend tube would allow for the best solution but an A frame that extends far enough forward to allow a cross member to sit across it, be bolted and hold the front of the bunks would be perfectly good. Your only real issue would be that of tripping more easily or catching something with that design. Being able to braise it all together would be great but U bolts and plates would be fine.
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Old 12 May 2022, 09:49   #11
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OK, just been to see the trailer. First the bad news - the trailer itself didn't make it to the meeting point as a wheel fell off en-route. The better news is that the frame etc seems in decent knick, and he is now willing to let me have it for 50 delivered which means I can swap in new suspension and hubs, which might be more or less all that is required mechanically.

A frame attaches with a bolt to the spine of the trailer, so that's easy. A U bolt on each cross over of the A and the main axle should give me the support I need for some bunk struts, and then maybe a winch post and done.

Some pics - suspension units are completely rusted out, frame seems fine apart from one area at the end of the dolly axle, which I don't think will be an issue for me:




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Old 12 May 2022, 09:50   #12
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If the rust is to bad, another option would be to use the two long pieces of box section from the launching trolley as the angled pieces for the A frame on the road trailer.
Either make, or take it all to a fabricator, a delta shaped plate for the front of the spine and two triangle fillets to go onto the axle. Personally I would get the fabricator to weld (especially if its galvanised, gives off a toxic gas) the delta and fillets in place. Then I would drill and bolt the 2 long sections to the front delta and axle fillets to make the "A" frame.
Struts and bunks are easy enough to make and attach to the "A" frame of the trailer.

Didn't see your last post when I did this one.

For the cost and money involved, I think I'd be tempted to wait until another one comes along. It took me 6 month, and a 200 miles, to get exactly what I wanted, but it was worth waiting.
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Old 12 May 2022, 09:59   #13
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For the cost and money involved, I think I'd be tempted to wait until another one comes along. It took me 6 month, and a 200 miles, to get exactly what I wanted, but it was worth waiting.
Maybe, but if I wait 6 months I've missed another season!

To be honest, very happy with what I saw - perhaps even happy that the wheel fell off when it did, because if I end up with this frame for 50 I think that all it needs is new suspension and hub kit (125) and it is as good as new - no significant rust anywhere that I will be keeping.

Plan is to bolt the A frame to the base trailer using existing bolt plate/U bolts at rear, then do as you suggest with bunks etc. Should be on the road for 200 or so all up with new hardware where it matters..
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Old 12 May 2022, 19:47   #14
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Maybe, but if I wait 6 months I've missed another season!

To be honest, very happy with what I saw - perhaps even happy that the wheel fell off when it did, because if I end up with this frame for 50 I think that all it needs is new suspension and hub kit (125) and it is as good as new - no significant rust anywhere that I will be keeping.

Plan is to bolt the A frame to the base trailer using existing bolt plate/U bolts at rear, then do as you suggest with bunks etc. Should be on the road for 200 or so all up with new hardware where it matters..
Sounds like a plan
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Old 15 May 2022, 20:05   #15
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Well I got her on Friday evening and spent a few hours this afternoon taking off the rusted out suspension units and brackets etc, and what's left is in a remarkably good state - even the galvanised mudguards are pretty much perfect. There are a few surface rust marks and some white bloom on parts of the galv which I'll have a go at soon, but as far as the build goes I think just a full suspension/wheel kit, new U bolts and bolt her back together. Then to work out the bunk arrangement - oh, and get a tow-bar fitted to my car!

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Old 23 May 2022, 20:26   #16
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New running gear (suspension, hubs and wheels) as well as new hardware (C bolts etc) has now arrived, and I have started putting this trailer back together. All I have at the moment is the basic T (pics below). To me this design never really looked strong enough, but having watched the video of the folding trailer, which uses the same basic layout, and seen a few other similar dinghy trailers I guess it must be good enough?

Anyway, now to bunk layout. First thought was to more or less copy Steve's layout, with two parallel lengthways bunks at the rear and a bow support. I've recently seen a few examples of SIB trailers using cross-wise bunks instead, i.e. perpendicular to the length of the boat. The idea is these support the bottom of the tubes I guess, which is probably enough. I'm still inclined to go the more traditional lengthways but interested in opinions. The Elling has a more or less flat bottom, with the sausage keel in the middle. Plan would be for bunks to go at the edge of this bottom, more or less up against the tubes but all of the weight taken by the bottom, not the tubes themselves. I worry a bit that this is a join, but having bunks here would at least stop any lateral movement. Open to suggestions, I am hoping to have the bunks on this week and maybe even have it running by the weekend.. (if the various bits arrive in time). Worth noting that I will almost certainly not leave the engine on, so it will just be boat/seats/wheels - i.e. very light, so maybe I am over thinking things as usual?



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Old 23 May 2022, 20:51   #17
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Personally I would always go bunks along the length to support the tubes, long enough to support the transom.
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Old 23 May 2022, 20:56   #18
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Personally I would always go bunks along the length to support the tubes, long enough to support the transom.
Yeah, that's probably what I'll do, although if the bunks are going to support the transom then they are probably not going to lend much support directly to the tubes - they'll be where the red rectangles are, extending forwards alongside the tubes and backwards just enough to be underneath the transom too. Just a little worried that this means all boat support is along a seam, and tubes are effectively unsupported - but given transom is supported, and front will be too, I guess this will be fine?

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Old 23 May 2022, 21:08   #19
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Not quite what I mean. The best place for the bunks to be is under the tubes as close to where the "hull" joins the tubes (seam) and to extend beyond where the transom is fixed to the tubes to prevent over flexing.
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Old 23 May 2022, 21:21   #20
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Thanks Steve. I think I get what you are saying, but not sure how possible/easy that is with a cat style hull like the Elling - whereas with a more standard V/deeper transom, the angle between bottom of the transom and the adjacent tube bottom is less extreme, and a single bunk can probably span both.

I was previously thinking about something like this - with another bunk attached to the horizontal ones at about 135 angle to support the tubes. Set up properly, this would really help distribute the load between tubes/transom, and should be quite easy to implement using galv brackets they sell for roofs/rafters. Overkill?

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