Still on about the size of my equipment...

[bobtooke]

So, what would you consider to be the range of engine size for an aux motor on a 535?
Assuming estuary and coastal use with a main engine of say, 50hp.
Is a 3hp too small?
Is a 30hp unnecessarily big?

[rbmatthews]

We have a Tohatsu 3.5 hp 2st auxilliary on the back of our Family Four, which is the same size as the 535 (although probably a bit heavier), and that is pretty good for pushing it along at 3-4 knots provided there is no current. We use the boat mostly for coastal trips, so similar to your plans.

http://shetlandf4.wordpress.com/auxiliary-engine/

The bigger it is the heavier it will be, and the more difficult to lug on and off the boat if you want to do that.

R.

[bobtooke]

Yep I will be taking it off to avoid it walking off on its own but at the same time, need it to be usable even if only for emergencies.

Your 3.5hp sounds good and I'm sure would get you home in an emergency although, I do find myself wondering how vulnerable I would feel restricted to 4 knots in anything but a calm sea. I suppose 4 knots is far better than nowt.

[Deaks]

As a veteran of about 12 hours in total on the water, and a certificated 'know-nowt', I shouldn't be commenting.
But.........
The maximum speed of any displacement hull, regardless of horsepower, can be calculated by 1.34 x the square root of L.
L being the length of the waterline in feet, and speed being in knots.
For my Black Hawk that's about 1.34 x sq root of 19, or 6 knots.
That's borne out by my vast practical experience. (see above).
Fortune cruises very happily on one-third throttle at 4-5 knots, using a Yamaha 9.9. Full throttle give loads more noise but not much more speed.
The only way to radically increase speed, assuming one has a planing hull, is to apply enough power to climb over the bow wave and go into planing mode.
So there's no point in going too big for your auxiliary motor unless you're going to go big enough to enable you to plane. I'd say more than 10hp would be a waste of money.

Deaks

[daveinglasgow]

Hi

way way to big. for low currents we use a 2.3 i.e. s the Honda as it is air cooled so does not clog up and needs v little servicing. The biggest you should ever go is 6hp and that would be for strong currents. also above this and you are getting into twin cylinders which means v v heavy if you want to take them on or off and also not needed.

Dave

[dellquay13]

... I use the Johnson variant of the tohatsu 3.5hp single cylinder, which I believe is also rebadged as mercury too. It's simple to self service, especially stripping the carb and getting the gunky old fuel cleared out when the outboard isn't getting run as often as it should, lightweight to lift on and off, easy to pull start, pushes you along at close to the max displacement speed for a 17' boat (Deaks was nearly right, the waterline length is a bit shorter than the overall length because your boat is pointy at the front, on my 17'6" F4 the waterline is about 14'6" at a guess) cheap to buy and easy to find... What more do you need, apart from a reverse gear (instead of spinning the engine through 180 degrees to stop...)
Chris

[daveinglasgow]

re spinning the engine.

the honda is the only air cooled one so does not need flushing. on big engines this is less of a probelm due to the size of the pipe going up to the power head but on small engines it is a pain and we spend far longer servicing them.

also no impeller and no reverse means a more simple engine and less to go wrong. the down side though is the honda is as noisy as 2 stroke and they are often stolen due to them being so light and having a 6 year warranty and being a silly silver colour that you can see from miles away just to make it even easier for the toe rags!

ps remember that you should always start you aux before going out as this is the engine that could save your life!