Originally Posted by jyasaki
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!
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.