Outboards

[Port-Na-Storm]

Hi,
I've just picked up a new Mercury outboard for our club.

The vendor, AutoMarine Services at Botley Hampshire has two brand new Mariner outboards for sale at cost price plus VAT. These are end of season clearances before he gets next years' stock in.

He has a Mariner, 3.5hp 4 stroke short-shaft at £550. inc VAT.
and
a Mariner 4hp 4 stroke short-shaft at £705. inc VAT.

Contact Trevor on http://www.autoandmarineservices.co.uk/

Cheers Graham.

[GregHBBR]

On the subject of engines generally... how much do you trust HP figures as a measurement of useful power for a boat. What other specifications are important?

The brochure for my SeaHawk used to recommend a 4hp long shaft as ideal. However, the builders supplied my father with a Yamaha 3.5hp two-stroke when he bought his new. That was back in 1973.

A good number of current Broads-based owners report that a modern 2.5hp four stroke is fine - and the tides can be mighty strong hereabouts, although I accept that large waves are not normally an issue except, perhaps, on Breydon.

One report from an experienced sailor says of the 1.5hp inboard engine who had just spent three weeks on Scottish Lochs says:

-----------------------
I have not got a negative word to say about the seahawk the stuart turner engine pushes her at 5 knots in calm weather when there no wind to sail and get thirty miles to the gallon its very economical than my outboard I carry in case of an emergency great boat.
-----------------------

See the Full Report.

More on the engine Stuart Turner engine concerned at:
http://www.seahawk17.plus.com/description/engines.htm

If a 1.5hp engine will push you along at maximum hull speed then what does any additional power do, except perhaps run under less stress. But I can't believe that there really is an issue of stressing a modern engine when it's only pushing along a small 17ft yacht unless, perhaps, you were going to run then engine for many many hours.

Someone, please explain...

[Anders]

If its blowing 30 knots and you just want to get in there as fast as you can, then you´ll want some more power.
There´s a huge difference between pushing a boat in a calm with flat water and pushing the same boat, especially if it has a high rig and lots of stays, against 30 knots of wind and a steep building chop. Then max hull speed is suddenly something very theoretical.
It all depends on your boat. 20+ years ago I had a Nordic Folboat (not a copy) and since it beats against whatever wind and in general sails in a very safe way when it really blows, I was happy with a 3,5hp 2 stroke Volvo Penta outboard. On the Albin Vega that I´m selling now, I would prefer to have 12+hp inboard engine because it really needs a big foresail in order to sail and sometimes thats to tough when you have to beat in strong winds especially when sailing alone.

So mariner makes both a 3,5hp and a 4hp 4 stroke???

[Port-Na-Storm]

I agree with Anders,

Our club's little Sailfish 18 will be used in The Solent and environs. Cee Dubbs and I can vouch for the short sharp chop seen there on occasions when the wind whips up. Our mooring at the club is very tidal, you really don't want to be late getting home on a falling tide. If the wind is in the east and the ebb tide has really started to run out of Hurst Narrows you are really going to wish you'd bought the bigger engine.

For the record we have just bought a 4hp Mercury Saildrive and replaced the outboard bracket for one rated at twice the engines weight. The "Saildrive" bit means it has a different prop more suited to pushing a displacement hull.

The club has two old 2 stroke 4 hp outboards for sale if anyone is interested. One Tomas, and one Mercury, both are runners and sold as seen, make us an offer and we'll bite your hand off.

Anders,  yes Mariner make both a 3.5 and 4hp.  They are exactly the same as Mercury and I think Thomaso, engines. I'm also told that the Mercury/ Mariner 4-5-6hps are all the same engine with different carbs.  

Graham

 

[Chris Waite]

I'm with those two

'Tit Willow' would do hull speed, about five knots, on flat water and no wind with an Evinrude 3.5 two stroke, really without maximum revs.  On the reach below Dell Quay in Chichester Harbour, which apart from some tide, equates to perhaps Barton Broad in dimensions, with a force six and gusting ahead of her, sails furled, she almost stopped with the engine flat out in some of the stronger puffs.

There wasn't so much of a fetch and I was quite surprised at how much way the wind could take off her
 
CW  

[Jeremy]

Most small boats only need a small amount of power to reach hull speed in calm water, much less than 1hp in all probability. The 18ft Winsome hull will cruise at 4kts on around 100W, or about 0.13hp.  Windage adds to the power requirement, particularly for sailing boats with lots of gubbins hanging around, but not to a massive degree, because air is around 1000 times less dense than water, so the drag from a given velocity of wind past the boat will correspondingly be quite modest.  The real killer is the increased wave making drag that the wind induces.  Any sort of a chop forces the propulsion system to lift the boat up over every wave, a quite considerable additional power requirement.

Most small outboards are woefully inefficient, so they don't deliver anything like their rated power into the water as useful propulsive force.  The "sailing" versions are better for low speed small boats, but even they won't have a prop or gear ratio that is a good match for every boat, they will probably be optimised for the heaviest boat the manufacturers recommend, which means they will be sub-optimal for pretty much anything else.

A chap in the US, Denny Wolfe, did some experiments to see just how the prop on a small electric outboard could be improved on, following a lengthy discussion on a long running thread I started over on the Boat Design forum a year or two ago.  His experiments, powering a boat somewhat like the sort of boat we're familiar with (this one: http://im1.shutterfly.com/media/47a0d935b3127ccefac8b569e12900000030O00AZM2rdm3ct2IPbz4C/cC/f%3D0/ps%3D50/r%3D0/rx%3D550/ry%3D400/) were interesting and detailed here: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-17.html#post305142

He managed to get an efficiency improvement (in terms of input power for a given boat speed) that was pretty significant.  He ended up getting an 88% range improvement, using the same outboard motor, just by changing the prop, adding a fairing around the leg and changing the outboard controller.  Most of the gain came from changing the prop  to one that better suited his boat.

Jeremy

[Port-Na-Storm]

I used to own a Renault 5, not the sporty one, the basic 850cc gutless wonder with the umberella gearstick poking out of the dashboard.
It would reach hull speed, eventually, of just over 70mph on the flat preferably with a tail wind.
That was when it was empty, if you put four people in her, or found any kind of incline, she would just give up and pretty soon you were in third gear.

It was all down to a lack of chevaux.

A larger engine can spin a bigger prop, not necessarily at higher revs, which in turn can move more water counteracting the effect of wind waves and tide.

That's why tugs can move huge ships, they have massive props.

Graham.  

[alopenboat]

On 27 Sep 2011 at 10:24, Port-Na-Storm [via UK HBBR Forum] wrote:

>
> The club has two old 2 stroke 4 hp outboards for sale if anyone is
> interested. One Tomas, and one Mercury, both are runners and sold as
> seen, make us an offer and we'll bite your hand off.
>

I am tempted, but I think they are bigger than I need. What I really
want is a 2hp 2 stroke.

--
Hoping for calm nights

Alastair Law,      
Yeovil, England.
<http://www.little.jim.freeuk.com>          

[GregHBBR]

Thanks for the explanations.

So the long and short of it is that while I stay inland and only use a motor to get out of the dyke where I moor and pass under bridges there's little point in changing my electric outboard that has served me well so far.

However, the idea behind a petrol outboard is to be able to cope with some coastal work when I visit my niece in Falmouth, and to cope with that, there's little point in going for the 2.5hp motor. For safety's sake I really should go for the recommended 4hp long shaft.

[Paul H (admin)]

Greg,

Yes always go for a long shaft because pitching in the waves easily pulls the prop out of the water on a short shaft. Adjustable outboard brackets are the dogs doo-dahs in the lumpy stuff.

Like Al I found my 2.5hp Tohatsu ok for MilliBee in the Solent...in fact it also towed Al in Little Jim up the Medina through the nasty motorboat chop outside the harbour.

Cue Cee Dubs for a RIB ribbing!

-Paul