Reply
 
Thread Tools Search this Thread
 
Old 07 November 2013, 19:48   #1
Member
 
Country: UK - England
Town: Gloucester
Boat name: pain
Make: c craft
Length: 4m +
Engine: mariner, seagull, to
Join Date: Feb 2012
Posts: 214
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?
__________________
Stubber is offline   Reply With Quote
Old 07 November 2013, 20:27   #2
Member
 
spartacus's Avatar
 
Country: UK - Scotland
Town: Aberdeenshire
Boat name: Sula
Make: Ribcraft 4.8m
Length: 4m +
Engine: Tohatsu 70hp + aux
MMSI: 235087213
Join Date: Jun 2007
Posts: 4,521
RIBase
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.
__________________
Is that with or without VAT?
spartacus is offline   Reply With Quote
Old 07 November 2013, 20:51   #3
Member
 
Country: UK - England
Length: 3m +
Join Date: Sep 2008
Posts: 2,758
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?
__________________
ShinyShoe is offline   Reply With Quote
Old 07 November 2013, 21:40   #4
Member
 
Country: UK - England
Town: Leicester
Length: 5m +
Engine: 135hp Mercury
Join Date: Sep 2013
Posts: 1,408
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.
__________________
paintman is offline   Reply With Quote
Old 07 November 2013, 22:40   #5
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,622
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.
__________________
Poly is offline   Reply With Quote
Old 08 November 2013, 07:11   #6
Member
 
Country: UK - England
Town: Gloucester
Boat name: pain
Make: c craft
Length: 4m +
Engine: mariner, seagull, to
Join Date: Feb 2012
Posts: 214
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
__________________
Stubber is offline   Reply With Quote
Old 08 November 2013, 12:34   #7
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,667
Quote:
Originally Posted by ShinyShoe View Post
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......


Quote:
Originally Posted by Poly View Post
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.

Quote:
Originally Posted by Stubber View Post
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.
__________________
9D280 is offline   Reply With Quote
Old 08 November 2013, 15:00   #8
Member
 
Locozodiac's Avatar
 
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
Join Date: Sep 2007
Posts: 1,998
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
__________________
Locozodiac is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




All times are GMT. The time now is 14:12.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.