Aluminum outboard mount design

[Blether]

Hi. I'm hoping to make a custom outboard mount for my Open 5.70. I bought the boat a little over a year ago. It's one of the original series, with a breakage-prone twin rudder system design lifted as is from a Hobie Cat.

I've bought and fitted the revised "mark 2" rudder system (having broken one rudder blade in last winter's strong winds and one rudder cage in this winter's).

The maker's recommendation is to move the footing (four metal studs) for the existing straight engine mount, 17cm to the side. This also involves cutting an inspection hatch into the deck to get access to re-fit the mount studs firmly.

Where I live there are far more light metal fabricators than FRP shops, and I'm not strongly tempted by the thought of cutting into the plastic monocoque just to end up with an assymetric motor position.

The existing aluminum motor mount is a straight shot in 5cm x 3cm rectangular-section aluminium. I don't know the gauge - maybe 3mm like the end plates ? The original mount is rated for 3hp. I have a 3.5hp Tohatsu that I tell myself I won't use at full power.

I can draw CAD, but I'm not a mechanical engineer. If I design a motor mount that starts and ends in the same place as the original one, but takes a C curve or a bent C curve or S curve around the new rudder system, what's a good aluminum extrusion to base it on ? I have faith that I can get good aluminum welding done, but I also don't want to make it too difficult.

My measurements today suggest I've got a gap at full helm that a 40mm or 45mm section will comfortably get through without having to take too long a path sideways. Will round be best ? What section and gauge ? Or am I mad to attempt this whole thing ? There are two of these boats in the country and little prospect of more, so I'm not concerned about compliance for one-design racing.

 

[SukiSolo]

Welcome to the Forum.

I would like to confirm that the rudder is becoming a single centrally mounted unit? If so the side mount areas (old rudder fitting areas) become potentially part of a mounting solution. Also may I ask what are the 4 screws port side of the main existing mount doing? Is there heavy reinforcing (to take screws/bolts) inside that part?

Another partial support may be the track/horse rail across the transom - not enough in itself but maybe a partial steady. Initially I would try and find the strong parts of the transom that can take screws and/or bolts. At least it's only a 3.5hp.

 

[Blether]

Hi, and thanks for your reply.

The system is still twin-rudder - the previous pintles being replaced by gudgeons that use those same mountings. the difference is the single central tiller with control rods aft the transom.

The four screws are for mounting a rowlock for a sculling oar - removed by the previous owner, I guess. And the main traveller track won't support much fore/aft load.

As it is, the only strong point on the transom is the existing mount point.

 

[SukiSolo]

I've checked out the two designs of rudder attachment on that boat so I now understand the change.

You could fabricate something like the sketch below? It should give a little more clearance. If the section is close to the original, should be fine. I would guess around 45mm (1 3/4") to 50mm (2") vertical and around 40+mm widthways (1 9/16") in 3mm (SWG 10 guage) guage box, I believe it would be OK. Use the same guage as you already have for the end plates especially for the keyholed part. The shorter you can keep it, the better, partly access to motor and less leverage. If you go further out it may need upping the section on the box slightly. Material could be 6082 T4 (one of the stiffest alloys you can get off the shelf) preferably treated after welding to restore tensile strength. This can be quite important as the treatment can double the tensile strength of the same alloy. Make sure your fabricator can do this. It should be treated ideally to give the T6 condition after welding.

 

[Blether]

That's really helpful. Thank-you.

Yes, I'd forgotten about that option - easier to draw and easier to build: and I realise now I haven't taken the measurements for it. It might be harder to "unmount" the engine (on the water) using this design rather than one that comes inside the arms. But that's not a showstopper.

Overall, I'm planning to keep the mounting point in the same position relative to the hull. The existing mount projects 31cm fore and aft, and gives a rise of 22cm. (The spec weight of the engine is 19.4kg).

I don't have wide-view pictures of either rudder system, fitted. The old one had a single control arm inboard, between the tiller arm on the port rudder and the tiller arm on the starboard rudder, with the tiller extension attaching at the midpoint of the control arm (and the whole thing taking up altogether too much cockpit space).

As I understand it, the Mark 1 boat owners all broke rudder blades. So the builder made a new, strengthened rudder blade. Then everyone broke rudder cages. Then the new system was designed for the purpose.

I've almost come through the same sequence in miniature. A couple more photos: November 2012 and November 2013:

 

[SukiSolo]

Thanks for those shots. Looks like not enough prototype testing! Quite high loads right at the w/line and quite a few reasons potentially for breakage. Too thin (too slim a section), too narrow front to back, not enough glass (my own gut feel) along with too light a foam or core material in the head area. You can get light at the tip but right at the rudder neck, that is stress point central!. It is also possible the rudders slipped out of being parallel or out of any toe in as set up by the yard, resulting in extra stress. If even one tiller flexed this would allow misalignment. I note the new arrangement seeks to minimise this possibility.

Hopefully the Mk2 version that is attached at the rear of the rudder head(s) works OK. Hard to make out the detail from available shots, but assume its hunky dory if they haven't exploded.

If you draw it up, you should be able to cut a 'V' in a single box and bend and weld. It needs drawing properly because of the nature of equal angles to get nice butted edges. A light bend when still in a half hard state will not weaken it. You don't want to see little stress fractures, if you do, then two pieces is a better solution.
Equally a little light heat (local annealing) on the 'hinge' area won't hurt when persuading it.

The V angle will give you the angles to the stern mount plate and the motor bracket, within the distance/height you have defined. Worth a little time drafting, it will give you a neat resolution. Yes, you can curve this type of section with rollers and it is possible if the fabricator has a suitably large set. I suspect the V notch or butted 2 section solution will be easier and OK, even acceptable visually given the constraints of the new tiller arrangement. Might be a little weld dressing may be required, depends on the fabricators skill. If in doubt a small gusset could be welded inside the V but I doubt it will be necessary unless you up the motor weight.

 

[Blether]

Yes, another aspect is sail balance - we're fine in 15 knots, but at 25 it'd take a well-oiled crew and a braver man than me to get out the gennaker. That leaves a big powerful main, a wee jib and heavy, heavy weather helm. We maxed out at 14.3kts by GPS on the reach that broke the cage, with the hull and my tiller arm both humming like violin strings The rig is set up by the book - in fact (in between those two Novembers) I returned it to standard from the slightly more aggressive rake the previous owner had.

In the Mark 1, the owner set up the toe-in post-delivery (two-part control arm with one pole inside another; electrical tape) and it's the same with the new setup, this time by rotating the control arm on each side on the threaded attachment to the tiller. I've put in the suggested 2 degrees - which I interpreted as 1 degree per side. Haven't sailed it yet.

Looking at the second sketch again, I see I do have the measurements for the overhang of the tiller arm at dead-ahead: it's 28mm out to the pivot plus 214mm to the outboard end = 242mm. Measuring the existing mount (which I brought home), it will interfere with the arm at deck level about 65mm aft of the transom.

 

[Blether]

So I've drafted up the boat in blue, the existing mounting bracket in black, and the beginnings of the new mount in red

 

[SukiSolo]

That gives the geometry problem nicely!.

If the motor bracket is an L shape rather than flat plate + ply block, you can come up much more vertically with the rectangular tube and it will still give enough support. Imagine literally the L shape in aluminium with ply/wood block sitting agains the left side and the bottom face. I am sure a bit of notching the rectangular tube and welding to that rear plate will give enough support to the bracket. Obviously you still have a tube to tube joint still to do heading towards the transom.

Another solution is a topside brace that drops in like a rudder on pins but may need wing nuts to tighten. Not really what you want, I suspect.

It may also be possible to fold the rear mount plate if it is thin enough but be aware of the resulting radius.

 

[Blether]

Some more focussed CAD drawing and I now have this (attached). As a PDF, the lines are coming out too thick for the details to be clear.

Bringing the tube up inboard of the control arms doesn't give a better angle out to the engine mounting plate. Inboard or outboard, it ends up at about the same shallow angle. And running the tube outboard means that if there's ever a failure in the mount, it and the engine will all fall off outboard, with significantly less damage risk to the steering gear.

Off to visit some fabricators in the next few days.

 

[SukiSolo]

Personally, looking at the side elevation on the right hand side, I would have put the L bracket the other way up. Exactly as in L. That way the box section is sealed inside differently and acts as a gusset support vertically (transom side) for you. It is a more awkward chop in the box section, but doable and the weld area is longer but working over a better area for you. Then any forward twist from the motor is better resisted. More like the outline below.

Otherwise I am sure it will be OK for such a modest engine.

 

[Blether]

Yeah, that's the one part I'm not quite happy with. Because I'm still going to finish it with a block of marine ply, I'm wary of an angle at the bottom becoming a water trap. Looking at it after I finished the drawing, I think that at least I'll cut the slot in the plate only half as deep, and cut a corresponding slot in the upright to take it. Otherwise, the plate-to-upright joint is going to be weak, even with the length of weld that can be put on it.

I do want to adapt to what stock extrusions the fabricators have, though, so I'm not thinking anything's final yet.

ETA: I like these 3D drawings that you make. What software are you using ?

 

[SukiSolo]

Definitely use stock size extrusions. If you have a good fabricator he may also have access to an anodising facility, 20-25 microns would be enough, though it is not strictly necessary to anodise. I would solve the water trap problem with a couple of limber holes (as in plan view) and also ensure slight local raising of the ply so water does drain and not rot it. Epoxy coating of the ply plus a couple of local ribs in say epoxy/microfibre or filleting blend would achieve that OK.

The drawings are actually 3D models with rough rendering only, modelled in Rhino with render using Flamingo plug in. Very crude, lazy effort on my part but the 3D model is accurate and real world size to your application. It imports DWG and DXF from Autocad so you can use that data to create a model.

 

[Blether]

So this was my final overview CAD drawing:

and yesterday I picked her up at the factory looking like this:

More photos here - tomorrow I'll be attempting the first fitting.

The horizontal strut is a 3mm, 70 x 30 extrusion; the standing strut is cut from 3mm sheet and bent into a U, with a rear side cut, bent and welded in.

 

[SukiSolo]

That is looking good, hope it all works OK. Nice long weld face on the box sections so should be able to take the load fine. The chop into the box for the plate keeps good integrity too, I can see why it has got there. Should be a neat jobbie that gets you around!.