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Old 07 November 2013, 15:48   #1
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Will it push it?

Got hold of 15 foot speedboat (I know its not a rib so a bit so a bit rude but thought I'd ask anyhow) and have a 3.5 hp Tohatsu short shaft that I can use as a backup engine and wondering if it will push boat ok. The boat weighs 450 kg's all up plus will have 2 or 3 people on board when used.
This engine pushed my inflatable well but know I'm asking for a lot more here. Has anyone had any experience of using this engine for a back up motor?
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Old 07 November 2013, 16:27   #2
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Used to have a Tohatsu 3.5hp long shaft on a Zodiac Pro but never had to use it in 2 years of ownership. I think you might be asking a bit much from it given the boat weight and the likely numbers on board. One thing to consider will it remain sufficiently in the water given its a standard shaft? How's it mounted, adjustable transom bracket?

I used to strap mine in the tilt position as the Tohatsu tilt pin and saddle are weak points given the vibration when using the main engine at WOT.
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Old 07 November 2013, 16:51   #3
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There was an article in Yachting Monthly in September (I guess October edition) about moving a yacht with a Sib tender with a 2HP. Done to test the principle of what you do if your engine fails. It seemed to be perfectly possible. They obviously had the engine properly in the water...

For a further comparison - consider something like drascombe weight 400-600kg depending on the version. Seen 4hp move them quite easily with 5 or 6 people onboard.

SO - pop it on and give it a go?
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Old 07 November 2013, 17:40   #4
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I've got a 17' Dory Sportsman & the back-up is a 4hp Suzuki 2-stroke.
Boat & main engine weigh around 550kgs plus the weight of 2x25litre tanks, 1x5litre tank, the Suzuki, fishing kit and usually just me, but can be up to four people.
Pushes it along without any problems. Mainly used for trolling for bass but has had to get me back in once - only about 2 miles but against wind & tide - when an oil line on the main engine split.
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Old 07 November 2013, 18:40   #5
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Quote:
Originally Posted by Stubber View Post
Got hold of 15 foot speedboat (I know its not a rib so a bit so a bit rude but thought I'd ask anyhow) and have a 3.5 hp Tohatsu short shaft that I can use as a backup engine and wondering if it will push boat ok. The boat weighs 450 kg's all up plus will have 2 or 3 people on board when used.
This engine pushed my inflatable well but know I'm asking for a lot more here. Has anyone had any experience of using this engine for a back up motor?
the ribnet rule of thumb for an aux is 1hp per m of rib length (ie. 5HP would be preferred) - but in reality many of us are constrained to smaller engines, eg to fit on the transom beside the main etc. It is a compromise and it means in tough conditions you may not have the choice which way to head, but can probably hold position at the least.

To get the most out of the engine though you need to try and 'prop' it sensibly - otherwise it just makes a lot of noise and froth. You probably want to stick the smallest prop you can find (4.5"?) on it and it might give you about 4-5 knots through the water.
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Old 08 November 2013, 03:11   #6
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Think i'm going to give it a go and see if it's any good as a back up as little motor has a fair bit of go for size, had a 3.5 mariner 4 stroke a few years ago which was rubbish no go and noisy so got rid. My problem is where to put a back up as no room on transom, will have to come up with a crafty idea
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Old 08 November 2013, 08:34   #7
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For a further comparison - consider something like drascombe weight 400-600kg depending on the version. Seen 4hp move them quite easily with 5 or 6 people onboard.
Aye, but that's a streamlined hull at displacement speed - anything designed for an outboard will have a transom = LOTS of drag......


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the ribnet rule of thumb for an aux is
<snip>
on it and it might give you about 4-5 knots through the water.
+1.
My rib is about 420Kg, and will toodle along at about 3.5 knots with a decrepit (1974) Johnson 4 (so actually nearer 3.5 @ prop). I can get an extra knot out of it by moving the weight (me)forward & lifting the transom out the water a bit.

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Think i'm going to give it a go
Go for it. There's only 2 outcomes: 1) it works. 2) you have a have a "better than nothing" backp until Plan B comes together.
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Old 08 November 2013, 11:00   #8
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We use 3.5 HP outboards as aux to push a J-24 fleet that weights around 1 ton empty with no issues at slow displacement speeds of 4-5 knots, that's on calm flat no wind waters, windy conditions and strong currents is other story.

As a aux a hort shaft would be an issue when on choppy rocking seas, you could optimize prop pitch to achieve a better pushing performance, would need to install an induction tach to check max rpm achieved at wot so to play with pitches.

Happy Boating
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