Making a custom outboard bracket for a boat never intended to be motored

Greenheart

Greenheart

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I've buzzed around a few times with the Tohatsu 3.5 two-stroke on the back of the Avon Redcrest. It's a lightweight engine at around 13kg, but may be a bit more than Avon recommended, because the stern squats rather dramatically, and I began to imagine at the weekend that such an old inflatable might burst a seam under the stress.

The obvious thing (to me) is to find a way to mount the engine on the back of the Osprey, for the amusement and versatility it would add.

Trouble is, the Osprey's stern is mostly thin glassfibre panels that don't bear weight and were never meant to carry an engine. The deck flexes under hand pressure, and the transom feels very light.

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The only strong points are those 3" protruding, outermost solid wood gunwales, encapsulated in glassfibre. So here's my first idea...

...I'll get a sheet of stainless, probably 2mm thick, and about 120cm (4ft) by 25cm (ten inches). The steel will be flexible enough to follow the curve of the afterdeck between the gunwales, so I can use heavy 10mm bolts to secure the sheet through the gunwales, making a stiff surface to the deck...

...then (before bolting the sheet down) I can drill holes through the steel and use countersunk 8mm bolts to attach a hardwood or ply bracket, to the steel sheet. I'm thinking that the steel will be a big enough area to spread the load of the engine (both its weight and thrust) over enough area of the thin GRP deck, that it won't harm it.

I realise the sheet of stainless will weigh nearly 5kg, plus the wood bracket, but I can't see how else to attach the motor. Plus, I really don't want to prevent the dinghy's rudder working as usual - so sheet steel seems smart because it's very stiff, but won't raise the level of the deck where the bracket needs attaching.

I await your criticism.
 
I think you will find the deck to transom joint across the stern to be fairly robust. Also one would imagine the area that the rudder pintles are attached to would be reinforced. So a block of wood suitable to mount the light weight o/b that sits in the rudder pintles but is reinforced outwards to the deck transom joint. Might be strong enough. ol'will
 
Have you thought through how the osprey rudder will operate once the bracket and outboard is mounted. I can see it being severely impeded from full movement. Or are you going to unship the rudder then mount the outboard and use them exclusively of each other. I only ask because I have a similar problem on my rib where I want to mount a small back up engine alongside the big 'un but can't due to lack of space.
 
If you're using the outboard, you don't need the rudder, just steer the OB, so could you make a bracket that would fit on the rudder pintles? Do make it sold though, I reckon 3.5 HP would get that hull planing nicely!

Of course, you don't need to use full throttle. I rarely use more than half throttle on my 3.5 because all that happens is the flubber sticks its nose higher in the air, the moored boats I pass bob around more and I don't go noticeably faster.
 
Piece of thick ply up the transom with spacers so it clears the joint, then brace it with two 'beams' dorward to the cockpit opening. Couple of small bolts at the bottom of the ply pad through the hull where the moulding is thicker (could lay up a bit more, or glass in a ply re-inforcement) The forward braces could either be clamped or bolted by the cockpit where it is more rigid. This would transmit the loads to areas well apart and reduce them. The major load is the twist on the pad and the forward braces would carry that through easily.
 
Have you considered mounting it on the side, as Wombat mentioned above?

If you Google "side mounted outboard; images" you can see many ideas.

A removable bracket mounted on one side, or capable of being mounted either side, would not stress the transom at all and be to hand for operating the controls. It would also be easier to lift it on and off.

A small electric outboard would be good.
 
Thanks for your thoughts. Actually my boss has an XOD and one of those nifty side-mounted removable brackets for his o/b. Very tidy bit of woodwork, and you'd scarcely know it fitted there when it's not in use.

I'm much less sure the Osprey's gunwales are individually up to the task of carrying 3.5hp; nor will the boat's trim respond as well as a keelboat's, with a couple of stone perched outside the gunwale.

I'd like the engine on the transom (or parked above it), far enough to one side of the rudder that a reasonable angle of steering is possible. I suppose I could attach a rod between the engine's tiller and the boat's, so the propeller's thrust is always vectored and no steering movement is wasted. I hadn't dismissed the idea of motorsailing, so I want to keep the boat's rudder in position.

The idea that the rear of the boat, as designed, is rugged enough with a little reinforcement to carry an engine, I doubt. I recall seeing the deck moulding lift slightly free of the transom during some manoeuvre ashore, a year or three ago. I have often agreed with onlookers that she's over-built compared with modern racing dinghies, but she was never meant for a localised burden like an engine.

And, critically, I enclosed the whole rear deck section in 2014 (stuffed it with empty plastic bottles for unsinkability), and carefully, permanently Sikaflexed and screwed the forward ply bulkhead into position; it would now be an unwelcome job to shift it. I confess, I used to have a photo of an Osprey with a fairly conventional-looking o/b bracket on the transom, but reinforcing the stern is a job I don't fancy unless the other way doesn't work.

Aluminium instead of steel, is a good thought, although when I tried a similar substitution for a mast-gate last year, the softness of the lighter metal was very disappointing. Aluminium seems to be terrific if it is braced by 3D curves, but my plan is much simpler.

What I want to know, about my original idea, is whether the engine's vibration and upward levering of the bracket, as the thrust from the prop tries to kick the engine's leg out of vertical, will test the stiffness of 2mm thick stainless sheet. Would I be smarter to use, for example, two much narrower strips of 3mm - or even 4mm stainless, 25cm apart, to which the fore and aft ends of the wood bracket could be bolted, gaining stiffness while saving weight over my original plan?

Is 3mm or 4mm stainless a reasonably workable material, allowing the gentle curve of my transom-top to be exactly matched?
 
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Why not make it out of wood? Much easier to shape, you are only dealing with a tiny engine, the turning forces are easily managed with wood, rubber dinghy transom are only about 25mm or so thick and they easily cope with upwards of 10hp

The easiest method would really need to get to the inside of the transom, bolting weight to the outside of it is not going to decrease the stresses, if the other side of it has not been braced accordingly.

Why not consider a bracket like the one from your Avon, with mounting on the transom and onto the top of the rear deck, that would spread the load and remove the turning forces from a very narrow transom
 
I've a simple idea - but I see a problem. Are you launching from a beach and needing the rudder blade up, or could you make do without the rudder as suggested upthread?
 
Thanks John. I reckon if I go without the Osprey's rudder, I'm into a whole different type of boating. It might be better than the Avon, but it's not quite what I was intending.

I've always launched from a beach with the rudder up.

Northwind's idea of using wood might work. If I can recreate the subtle lateral curve of the rear deck in a piece of 12mm 1088 marine ply, it could be bolted to the gunwales the same way I'd planned for the stainless strips. The 12mm 'deck-topper' wouldn't obstruct full movement of the tiller, and it wouldn't be hard to attach thicker vertical and 'buttress' sections to the ply, in a way that makes the bracket section removable without unbolting the 12mm sheet.

If 12mm marine ply is sold flat, can a degree of curvature be induced by mechanical force over time in the shed? I don't want the Osprey's gunwales to be constantly levered upward, nor the middle of the light glassfibre deck to be pressed down, by a layer of flat ply decking, screwed down into a curve to overcome its spring.
 
The transom will be quite strong where it forms an angle with the bottom of the hull and the deck.
You just need a wooden bracket which puts the forces in close to these edges.
An inch thick of ply and/or wood epoxied to the transom, with a spacer to take it clear of the deck edge.
Does the outboard have enough shaft length to have the clamp screws above the deck though?
 
I've got a Wanderer with the original outboard bracket - one of those that puts the outboard well back from the transom so it clears the rudder. It amuses me because the outboard bracket is thick aluminium and very heavy, but the GRP it is attached to is very thin and flexible.
Apart from the transom strength there are other issues:
- if the rudder isn't all the way down the prop will chew the rudder;
- the weight is all in the wrong place hung out the back
- fitting and removing the outboard is too scary to be practicable, even though it is a Torqeedo and separates into lightweight lumps
- it is hard to reach the controls. How happy is your Osprey with you sitting on the transom?

If you can't easily sit right at the back of your boat then I would seriously think about side mounting, or even (for the really keen) putting it in a well.
 
Dan, what are you planning on doing with the engine when sailing? If you have 20kg of weight dangling off the transom that will mess up your boat trim, and I suspect it will always get in the way of the tiller, and no doubt drag through the water a bit with any heel. You are going to need a very long outboard tiller extension which will only give you limited steering anyway, and I'm assuming your Tohatsu does not have reverse gear so you can forget about any tie-bar to the rudder as you won't be able to rotate the outboard. Dangling over the back tank trying to start it with the bow in the air being blown around would not be fun. The Osprey is a long boat and pitching in waves might make the prop come out of the water, unless you run a long shaft, which makes your prop drag problem bigger.

I would seriously go down the side bracket route, something you can easily drop over the gunwhale, and store the outboard in the bow tank or similar. Have the thing mounted near the pitch axis of the boat, and so you can reach it when holding the tiller extension.

I suspect your Redcrest squats due to poor weight distribution or outboard leg trim angle.
 
On the odd occasion that I have had an outboard on a dinghy transom, smaller than yours, it was not a nice experience. Bother with rudder and mainsheet really didn't help. Furthermore the weight distribution encouraged me to sit well north of the centre thwart in attempt to level things and this made the outboard well out of reach. It does work on dinghies like Tideways but they are heavy and immovable in comparison with your Osprey.
 
My Dad used to use an outboard on an Enterprise, for fishing on the sea and also for river trips.
It was a little Suzuki 2hp, two stroke of course.
The Ent has no stern tank, so it simply clamped on the transom well over to one side if memory serves.
Weight distribution is not a problem. You need a long tiller for the outboard and some means of locking it straight ahead.
Singlehanded, the helm needs to be well forwards anyway.

Side mounting would not be trivial, but would have advantages.
But also a downside, motor-sailing in lightwinds, you may have a very limited range of heel you can run with.
The engine may be hard to start if you have to move to that side, it will put the exhaust low in the water.
 
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