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Old 08 March 2010, 12:38   #11
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Generally, if you take a motor of some hp rating, and add another similar motor, you get a max of about 75% of the total output in useable hp (twice the LU drag, added weight, etc.) at the props.

So your plan of using a pair of 2 hp motors would yield 3 hp or so at the props, which I suspect will not push a great deal faster.

Second the idea of upgrading to a more powerful single.

jky
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Old 04 April 2010, 17:31   #12
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You will not gain anything worth noticing by fitting another engine the same.
If your getting 5mph now then adding a second might give you 6 but i doubt it.

A 4hp mercury will give you about 14 mph and a 6hp about 15mph but you could carry more people.

Ive got a 4hp mercury on a 9ft 6 meastral that will do 14mph flat out on the sea with one person onboard.

If you have a round transom (inflatable all the way round)you wont get as much performance as one with a solid transom .(points on the back)

Personaly i think you need a bigger engine, 2.2hp is not realy worth having unless your on a pond.

4 horse power mercury is good ,light, economical and very easy to get spares.
can pick them up reasonably cheap now as well.

or maybe a 5hp honda, heavy but reliable.


phill
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Old 05 April 2010, 07:43   #13
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Quote:
Originally Posted by jyasaki View Post
Generally, if you take a motor of some hp rating, and add another similar motor, you get a max of about 75% of the total output in useable hp (twice the LU drag, added weight, etc.) at the props.

So your plan of using a pair of 2 hp motors would yield 3 hp or so at the props,
Errr 2 x 2 hp will give you 4Hp at the props!

Drag argument:
As a rule of thumb, Drag through water goes up approx at the square of your speed, and the cube of the cross sectional area of whatever you're dragging through the water. I would suggest that at 2-4Hp end of the scale and the size of gearboxes we're talking here, the "75% argument" will fall flat. I'll bet the 2hp has no F/N/R, which the 4 probably will, therefore give a substantial increase in cross section, and probably not be too far off the twin "slim" 2s. Then realise you're not going to pull 30+knots with either setup,and I bet if you do the maths it's nothing like double the drag at the sort of speeds you'll be pulling with 4Hp.

Weight, as shown, you can get a lot more HP for the mass, I won't re-iterate my usual reply, just search for other twin engine argum- er I mean discussions , but suffice to say you'd be surprised at how lardy a single can be, esp. when you take an aux into consideration.


Having said that, if you have a reliable 2Hp, and can find a suitable "big" lump, There's a good argument for keeping the 2Hp for trolling / aux & get an 6/8hp-ish for your main engine. The main thing to remember is that the arguments that apply at the 200+ horsepower end of the scale won't usually translate linerarly down to the "sub 20 Knot" end of the scale.


One thing I think we are all agreed on - rowing an inflatable any distance is just going to kill you.
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Old 05 April 2010, 08:17   #14
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Engine

If you are going to get the 4hp Mercury, why not go for the 6hp one? It looks identical, weighs the same, and has 50% more power. The only difference is that it has an external fuel tank, but I would say this is an advantage in most cases as it holds 10 litres, which lasts absolutely ages....no need to carry a fuel can in the boat for refilling.
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Old 06 April 2010, 01:04   #15
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Another option (though pricey) if you want a backup motor is a Torqueedo electric outboard. It has it's own integral lithium battery on the head and folds up into a small bag.
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