New Tilting A-Frame for Osprey Vipermax

Richard Selman · 13 October 2010, 17:27

Just arrived from Mark at Marine Fabrications in Staffordshire.

Started this nearly 18 months ago. It needed to be able to tilt forward so that I could still get the RIB into the garage and also it needed to curve out over the transom to leave space for the aux. Since I would not be able to get the boat to the fab shop I knew I would have to get all the curves and angles right (hence the 18 months!).

I first got Mark to make me up the lower support arms so that I could fit them to the boat and then have four points on a plane to draw up the rest.

Had to remember how to do trigonometry calcs again.

Also made a prototype with waste pipe.........

Richard Selman · 13 October 2010, 17:30

the finished article is somewhat nicer to look at.

Along the top I got mark to weld on two Shakespear type aerial mount ratchets, a threaded socket for a reflector, a flag pole holder and a threaded GPS mount. No clamps.

Also two cleats built in. There are two nice light holders for LED lights that came separately.

Richard Selman · 13 October 2010, 17:32

Closer view of mounts.

Forgot to ask for mouse wires to be left in. Have some work to do to run the cables.

Downhilldai · 13 October 2010, 17:33

Looks good. How much?

Richard Selman · 13 October 2010, 17:35

Rear attachment arm.

There are two hidden M12 bolts into the tube as well as the M12 U bolts

Richard Selman · 13 October 2010, 17:36

try again

Richard Selman · 13 October 2010, 17:42

Front attachment arm. No hidden bolts this time as I needed to flex a little to reduce the load on the transom boxes. Osprey provides a nice chunk of ply in the front of the boxes to mount onto.

The radius had exactly the VMs tube diameter to look right. Mark did not have the exact formers to do this so it was finished in the rolling machine. This indented the tube exactly where the flanges on the transom boxes were and made it a uniform clearance around the rube and makes it look spot on.

Richard Selman · 13 October 2010, 17:44

A frame in place....

Richard Selman · 13 October 2010, 17:47

A frame tilted forward

The frame height was set to just clear the seat backs. The aerial attachment points are set so that they clear the backs as well so the aerials can point forwards between the seats and tubes.

Richard Selman · 13 October 2010, 17:48

moment of truth........

Richard Selman · 13 October 2010, 17:56

jobs a good un.

drawings attached if anyone needs them

Richard Selman · 13 October 2010, 17:59

Downhilldai - £lots but quite fair for the amount of detail work involved

If I say it out load it makes it real and the wife can read!

One reason for the A-frame was to protect the aerials when towing dinghies. Any one notice the deliberate mistake?

Richard

Downhilldai · 13 October 2010, 18:08

Quote:

Originally Posted by Richard Selman

Any one notice the deliberate mistake?

You built your garage too low?

martini · 13 October 2010, 18:18

Very nice! There's a lot of very awkward detail gone into that, I won't say what I'd charge for it

Couple of Q's, were the ratchet mounts modified from an off the shelf jobbie then welded on? If so any problems welding or special procedures etc used? I had considered this on my a-frame but as the mounts are made from cast stainless of unknown properties I figured there was a limited chance of success.

Also the 14 TPI gps mount, is that made from an off the shelf unit as well?

Looks great though, proves my theory that form comes with function

Richard Selman · 13 October 2010, 19:04

Garage too low. Nearly!

When I designed the fixed frame for the northcraft I measured the clearance above the stern with the jockey wheel extended (i.e. bow up). of course when its on the ball the bow is down. I ended up with only a few mm to spare........

Hence the tilting one the this time......

The mistake was chopping off the aerial with the a-frame. Seconds after I first mounted it on the rear supports, I turned around to get some rope to hold it in place. It fell backwards and sliced through the aerial which was resting on the gutter. Doh.

Martini - The aerial fittings were off the shelf but they're not taking much load. The ones carrying the radar reflector (heavy), flag and GPS are fabbed. No idea about the welding questions. Mark asked about the aerial fittings and I sent them down for him to check them out. Ask again in a year's time!

Richard

BogMonster · 13 October 2010, 21:58

That's one thing I forgot about on mine ... some bits and pieces tacked on to tie stuff on like ensigns and flare containers. Still, I ain't sending it back now

What size aux are you using? pretty big prop on it!

Richard Selman · 14 October 2010, 04:08

the aux is the high thrust (i.e. lower prop speed / higher torque) version of the Yam 9.9 (FT9.9). Designed for being a small yacht engine. Much better suited for being an RIB aux than a standard version imho since we are doing about the same speed.

Richard

BogMonster · 14 October 2010, 10:05

Quote:

Originally Posted by Richard Selman

the aux is the high thrust (i.e. lower prop speed / higher torque) version of the Yam 9.9 (FT9.9). Designed for being a small yacht engine. Much better suited for being an RIB aux than a standard version imho since we are doing about the same speed.

What sort of speed does a beefy aux give you - do you get through the "hull speed" or is there still a cap on it at about 5 kts? I got the Saildrive version of the Merchatsu 4hp for the same reason.

Is the A frame completely rigid once it is all rigged or do you get any rattles? It's a great bit of design

Richard Selman · 14 October 2010, 19:30

Still about 4.5 to 5 knots but it does it into waves and wind. Just feels right when going along - its difficult to describe.

Frame seems rock solid so far. Not had it on the water yet though. No rattles and very little flex at the top.

The idea is that the all four bolts are nipped up once it is in place to make it secure. Only takes a few seconds.

Richard

Maximus · 15 October 2010, 13:35

Quote:

Originally Posted by Richard Selman

Downhilldai - £lots but quite fair for the amount of detail work involved

If I say it out load it makes it real and the wife can read!

One reason for the A-frame was to protect the aerials when towing dinghies. Any one notice the deliberate mistake?

Richard

Nice neat job! The trouble with ''one off'' commisions is always the price,not SO bad when you DO get what you want though!