Originally Posted by jyasaki
Indeed it does, but the link gives the whole discussion in two clicks!
Originally Posted by malthouse
As an example we have a 90HP four stroke on our 6.25m boat, were I to add another 90 I would expect to get about 135HP "in the water" but at the cost of a whole load of weight. I would rather upgrade to a single 135.
And "expect" is the important word here!
Comparing twin 90s to a 135 of course you're going to be overly lardy - a quick spin onto a random OEM brochure I found on the PC here - (Johnson 2008) The 135 is the same engine as the 90, but runs at a WOT of 5900 instead of 5000 rpm so of course you are going to double the weight for naff all "real" extra power.
And if we are talking weight, for a 6.5+ metre rib, just how heavy is that aux you will need with the single? In a lot of cases it tips the balance in favour of the twins assuming you actually double the HP.......
As for drag, ideally you need data for the gearbx sizes - my Clamshell for example has the same size leg/ gearbox as my Suz 25! The Yam 50 it replaced had the same gearbox as the 90 - i.e. twice as big as it needed to be! Twin Clamshells will therefore create a lot less drag than twin Yam 55s........ Then if you start playing with Mariner "bigfoot" engines.....
All I'm saying is weigh up the facts before jumping to conclusions.
I still reckon if you have a single on there already, the cost of butchering the transom / converting the engine to short shaft will probably mean upsizing is the easier / cheaper option.