Longshaft outboard on an inflatable

[Mhvoiceuk]

I currently have a Mercury 5hp long shaft outboard which I use with my permanently beached 10ft GRP tender. I usually leave the tender on my (swinging) mooring when I go sailing and put the 5hp engine on the outboard bracket - so that I have an emergency engine in case of main engine/tiller failure. The 5hp engine is bit heavy to shift around and I would like to get rid of it and buy a lighter 3.5hp long shaft instead. I have an inflatable in a valise on board and would like to be able to use the 3.5hp engine with that too. The long shaft works OK with my GRP tender although it does push the bow up and I have to keep the revs low. A couple of people have told that it's a bad idea to use a long shaft with an inflatable, but I wonder what thoughts folk here might have?

I also wonder whether it might be possible to adapt the transom on the inflatable to take a removeable, slot-in piece of timber which would raise the transom to accomodate the extra shaft length? I've never seen anything like this and I'm surprised that it hasn't been manufactured (or maybe it has)

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[andy_wilson]

I use an extra longshaft Johnson 4 (1989 model so 3.3hp in new money). This is fitted on a Suzumar 265 - rated upto 8hp I think. As you might expect, little problem there. With 1 or 1 and a half on board it planes (airdeck) and it can take full throttle.

The main problem is the turning moment of the engine and transmission. This is a function of power applied to the propellor, and the length of the lever (shaft). The main limiting factor is how much 'power' (turning moment) your inflatable transom can handle.

You can moderate the power to mitigate the greater turning moment by using less throttle. Unfortunately you can't reduce the turning moment except by shortening the shaft. If you raise the bracket on the transom, you will reduce the amount of turning effort on the hull because the propellor isn't as far below it as it was BUT the force at the transom will be amplified as you have mounted it on a lever, thus increasing the chance that the motor will collapse the transom, possibly ripping it off, as you apply more power.

That's why it hasn't been manufactured.

In short, if the shaft is short, use up to the rated power of the dinghy. If the shaft is long or extra long, use less than the rated power. In theory the amount of de-rating is proprtionate to the increase in shaft length.

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[Mhvoiceuk]

Thank you for a very interesting and comprehensive answer.

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[kds]

i did use a 4 1/2 hp. Seagull longshaft as you describe on an old Avon.
The mounting bracket slides up or down the drive shaft, so I slid it down 4 inches or so until the water impellor was just covered and held it there in a rope loop(you still need it to rotate to steer). It probably looked strange, so high above the transom, but as the effective drive shaft was short, there was a reduced turning moment on the rubber stern; but it worked fine.
I should admit that I only did this after trying it with the full shaft in the water, to find the propellor winding its way forward and up to the rubber bottom of the boat, Scary !!!
Ken

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[snowleopard]

you may be able to reduce the nose-up attitude by adjusting the rake. if you move the prop forward the line of thrust will pass closer to the centre of buoyancy.

a lot of long-term cruisers extend the tiller with a piece of plastic pipe so they can sit further forward.

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[geoffatstanpit]

The main problem with a long shaft ob on a rubber dinghy is that you're still in welly-filling depth when the prop hits the beach.

Geoff

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[Mhvoiceuk]

I still just about have the strength to row myself the last twenty feet

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[Peppermint]

Re: Did it for years

with a 3.5hp. It's a bit stiff to steer. Gets stiffer the faster you rev OOH! Matron! You need to tilt it earlier but it'll still take you the last few yards to the beach.

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[jkim1]

I used a mercury 4 and still use a long shaft Malta on my inflatable tender. A inflatable with a decent moulding all down the tube sides is best, but I dont have one of those. They work if you dont wind them up too much, otherwise they tend to try and fold the bottom of the tender towards the front end. I like you bought a long shaft as it could be used on the sailing boat and on the tender. The only problem is getting the heavier motor on board, what seemed light 5 years ago is getting rather heavy now. An old kicking strap rig solves this problem. This year I am going to make up an adaptor so that I can mount the longshaft on the boarding ladder. Just in case the Volvo packs up. Yes you should be able to use it but those people who have those Tohatsu 3,5 hp lightweight 2 strokes will give you some gyp.

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[joecham]

I used to use a 4hp Evinrude longshaft on a 2.6m inflatable (with a solid transom) and on a 9ft fibreglass tender. I had no problems regarding the shaft length.

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