Tohatsu MFS9.8A3 UL 9.8HP 4STR UL (£2,141.41p) in westerly centaur with dead engine

dylanwinter

dylanwinter

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www.keepturningleft.co.uk
Is anyone out there capable of making a few educated guesses at the physics

here are the dimensions of the Centaur

http://myweb.tiscali.co.uk/tonyandgi/Technical/Centaur26-brochure-p6.jpg

with this engine it is an ultra long shaft Tohatsu saildrive 9.8

having lived with 6hp on Katie L then I am reasonably confident that 9.8 hp applied in exactly the right place will shift a Centaur

http://www.tohatsu-uk.net/products/photo/images/mfs9.8-2009.jpg

where does the hole need to be?

how deep does the trunking have to be

I want to vector this engine 45 degrees port and stb

no need to tilt - it will stay in the water all the time I am aboard

no need for remote controls - it will be right at my feet in the back end of the cocpit I assume

It will be noisy of course

now what about the maths of taking out the old engine and replacing it with some balast - less of it and further forward I assume

the well will create some loss of bouyancy and it would be great if the sea did not come up the hole

Dylan

PS this is the page of the bloke who did the Pageant

http://www.keepturningleft.co.uk/blogs/bloody-nora-westerly-pageant/

I promise not to copy his paint job
 
I would be very surprised if ten horse power were enough to power a Centaur.I had a ten horse power Mercury on a Kestrel that I used for years & it struggled at times.It's not just the horse power but the prop size & the low down grunt that a diesel produces that seems to make the difference.(I have a 10 HP Bukh in my Macwester 27 & it seems much more powerful).
 
It is not the engine, or the HP that drives the boat but the propeller. The outboard does not have a big enough propeller to move the weight at anywhere near displacement speed.

As others have said in great detail on your other thread, outboards are not practical on a boat of this size.

Afraid you are letting your one atypical personal experience override the majority view that an inboard is far and away the better alternative if you want to have a boat of the size and type of a Centaur. Not difficult to buy one with a good engine.

BTW helped my mate with his neglected 1977 Centaur with the original Volvo the other day. Had been sitting 9 months unused in the water. Fitted a new battery and it started first time and tuff-tuffed away happily as it has always done!
 
In my view, Tranona is stating absolute sense. The idea of putting an outboard well in a Centaur is a bit pants. I think the outboard might just push you along ok but would use a lot of fuel for such a heavy boat, and to me, the weight is the main factor.
PBO's contributor, Ken Endean's boat (a Sabre 27?) is pushed along by an outboard mounted on the transom and he seems quite happy with it.
My old boat was a Pegasus 800 with the old Volvo (Honda) 7.5 hp petrol "saildrive" and that motored along very well, but was 2 tonnes compared to a much heavier Centaur.
All in all, if you want a bigger and more pertinently, a heavier boat, get one with an inboard and budget for maintenance.
 
For 2k you're going to be basically able to completely rebuild the engine which is already in the boat!

Possibly less, leaving more money for niceties, ancillary parts, etc.

Just doesn't make sense Dylan.
 
causeway said:
For 2k you're going to be basically able to completely rebuild the engine which is already in the boat!

Possibly less, leaving more money for niceties, ancillary parts, etc.

Just doesn't make sense Dylan.
Click to expand...

I poured money into the slug engine

and it just asked for more and more as it invented even more bits to break down

fuel costs are just not a factor

so what if I used nine gallons between the Humber and Scotland

nothing compared to the fuel costs for the micra

economically it makes sense to me

I am more concerned about the physics

if the 9.8 would provide enough drive to get me going at five knots


D
 
I don't think it would, and do you really want a project boat at this stage ?

Katie L struck me as the perfect boat for you the moment you got her; if stuck in some Northern harbour getting p'd off in grim weather, go ashore to a B&B for a day or two, you can plug in your 240v chargers for free and it'll be infinitely cheaper and easier than a project Centaur.
 
dylanwinter said:
I poured money into the slug engine

and it just asked for more and more as it invented even more bits to break down

fuel costs are just not a factor

so what if I used nine gallons between the Humber and Scotland

nothing compared to the fuel costs for the micra

economically it makes sense to me

I am more concerned about the physics

if the 9.8 would provide enough drive to get me going at five knots


D
Click to expand...

Stop all this shillyshallying, just buy a knackered Centaur, chop a hole in it, stick in the egg-whisk and tell us what happens.

If you want to recoup your costs, film it with Robert Redford holding the steering stick and shouting "SOS" into a mic.
 
outboards generally have their torque towards the top end of the rev range to match the pitch of the propeller. They are whizzy things.

A heavyish displacement boat such as a Centaur needs a bigger prop with more blade area and pitch than the outboard one, to provide more 'shove' against the hull pushing lots of water aside.

Who would you rather have push a car to start it ? A wing forward, or a second row ?

Sure, you will be able to make an outboard run in a well, but in any form of seaway against waves, an outboard and its prop will not be anywhere near as 'powerful' as a small diesel inboard running to a decent saily prop. I'd guess you would struggle to make 5 kts except in a flat calm.

Put aside your experience of the slug's engine; bite the bullet, get the Centaur's proper engine sorted (it ought to be tax deductible in your case anyway :) ) Well a decent percentage for the next 5 years anyway as a capital item.
 
dylanwinter said:
I poured money into the slug engine

and it just asked for more and more as it invented even more bits to break down

fuel costs are just not a factor

so what if I used nine gallons between the Humber and Scotland

nothing compared to the fuel costs for the micra

economically it makes sense to me

I am more concerned about the physics

if the 9.8 would provide enough drive to get me going at five knots


D
Click to expand...

You might get up to five knots Dylan but with the wind & tide against you? I would'nt have liked trying to tackle Portsmouth Harbour entrance with the wind & tide against me in my old Kestrel which by the way was 22' foot long & I remember struggling with rounding the needles lighthouse & channel in benign conditions,you would'nt have wanted to be out there when the odds were stacked heavily against you!
 
Habebty said:
The idea of putting an outboard well in a Centaur is a bit pants..
Click to expand...

Possibly the understatement of the century!

dylanwinter said:
so what if I used nine gallons between the Humber and Scotland
Click to expand...

Except ..

It might not be just 9 gallons. Katie L only used 3 gallons during that trip in the condition you met

A Centaur is a bigger, heavier boat and for all the advantages that brings it also brings drawbacks and one of them is that you'd almost certainly find yourself motoring in conditions where Katie L ghosts along

Also, a Centaur with a knackered engine has probably got knackered sails as well

And in any case, if you treble the fuel consumption its not just the cost - where are you going to stow 50 litres of petrol? How are you going to cart it around between the boat and the nearest filling station? And so on. Not saying it can't be done but it's just a carp idea!

dylanwinter said:
if the 9.8 would provide enough drive to get me going at five knots
Click to expand...

In absolutely flat calm water with not a breath of wind maybe, at a pinch, screaming its poor little nadgers off, a 9.8 might just manage to get a Centaur to that sort of speed - the Suzi 6 in Brigantia ran out of steam at 5 knots or so, the previous 8 would just about take her to 6

Realistically, it'd be a docking and manoeuvring engine and if that's all it's going to be good for why not stick it on a bracket on the transom and tip it up or even remove it and stow it away when underway by sail.

If you can sail everywhere and never have to motor due to no wind or adverse conditions, you're a better man than I Gunga Din and I take my hat off to you

But if you cut a hole in a Centaur and stick a 9.8hp outboard down it and can honestly claim the exercise is a success I shall eat said hat without salt!
 
dylanwinter said:
Is anyone out there capable of making a few educated guesses at the physics...
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I had a 10hp inboard diesel in my 10m 3 tonne X99. It was unreliable, blew head gaskets, and eventually had to be replaced at a cost of £5k. It was weedy and underpowered for the size of boat.

Subsequently, I have a 10hp Yamaha outboard in a 9.2m 2 tonne DF920. It has never gone wrong or refused to start. It seems to have sufficient torque and power at all revs. It struggles at 25 knots of headwind, but that's because of the windage from 3 hulls. That's good sailing weather anyway. I get 22 Nm per gallon in slight weather (which is the only time I motor any distance) at 5.5 kts with throttle at about a third open. It peaks at 7 kts. The prop matches the engine and type of boat because it is made as a sailing boat engine. Not run out of electricity in a decade, even running a small fridge. The biggest downside is the availability of waterside petrol, but you have that issue already.
 
I am sure the outboard will push the Westerly well in smooth water, into a headwind and a chop, forget it. We used to have a Hunter Delta with an outboard in a well ( the reason we changed boats ). The time the arrangement caused most embarrassment was in Holland, a strong breeze up the chuff as we approached a lock with a gaggle of waiting yots. Engage reverse to slow down, try more throttle... and more..... but still surging forward under bare poles but now with a screaming outboard aerating the Ijsselmere as we eyed up the possibility of a U turn in the restricted waters without ramming one of the beautifully tidy Dutch vessels.
Seriously it is a real issue as the exhaust gases feed straight back onto the prop in reverse. Still if you must go and change you can always use your beloved mud banks to effect a halt.
 
Every hull has it's own needs for propulsion but my experience with a similar length fin keel was as follows. The main engine was a very reliable MD5 rated at 9hp which on a good day got us up to about 5 kts going downhill. One very nasty night motorsailing home I was making about 4 kts into the wind, as an experiment I tried the outboard which I carried on a normal bracket on the stern and stopped the diesel, the 20 year old 4hp 2stroke managed under the same circumstances a very respectable 2.5 kts in 30kts wind and a lumpy head sea. Most Centaurs were over engined so perhaps 9.9 may enough. The best way to find out is to experiment
 
TQA said:
Forget the Tohatsu, for this you need a bit of proper British iron. You need the Seagull 102 with the 13 inch prop gearbox. AKA The barge pusher.

Dylan likes Seagulls.

The Centaur is a bit of a barge. [ Dons flame proof suit ]

It is a match made in heaven!
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It should get a lot of hits what with the Centaur lovers and the Seagull enthusiasts

I assume that by the time I finish KTL then the price of knacked Centaurs will have fallen even further

might be a good retirement project

D
 
Dylan
Yamaha make a good engine for this sort of thing, its a 15 IIRC and it has a larger prop then all the 10's, comes with a lower speed gear ratio and long shaft. My experience of Yamaha's are that they are one of the best Outboards and usually a bit more expensive. Down side is that its a 2 stroke in a weight that can actually be lifted!
 
Dylan,
You are letting your experiences of an old knackered inboard cloud your judgement. I played with hanging a 6HP new Suzuki on the aux bracket and it moved the boat (Swin Ranger) well but was very inconvenient. The new Beta 20 is fantastic, smooth, quieter than the old single pot Yanmar. I get no diesel smells, starts first time. All I looked for in a new outboard, but a bit more difficult to nick!
So in your thought experiments the comparison needs to be new with new not old inboard with new outboard. Of course cost is is a consideration.
 
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