Originally Posted by MattD
I get power to weight ratios and that but my little SIB with 20hp only seems to go about 1mph slower per person. Surely if you have a gert big engine like that with those sorts of weights the differences in that chart would have little impact on performance?
Power and speed isn't everything, power to weight ratio, though important isn't everything either. The reason your SIB only goes 1mph slower as you add a person is due to the efficiency of the plane..If you keep adding people then there will come a point where if you add one more person then the boat will not plane and so that one person will drop you 5 or 6mph and make you use alot more fuel than without them.
I see from your profile that you have a tohatsu, I am going to imagine it is a two stroke for arguements sake, therefore pretty light. If you were to swap that engine for a four stroke (or any much heavier outboard), for example a honda, of the same power then the number of people you had to put onboard before the boat wouldn't plane would be reduced. The weight difference between the two engines would not be the weight of a whole new person: the reason why it falls off the plane sooner is because the weight is at the back, just where you don't want it to be when trying to plane..
It is the same with these larger engines, and becomes even more complex when dealing with rough conditions. The weight of my hull dry, is "supposedly" around 600kg, with the engine being 230kg ish, it is a significant proportion of the total weight, and anything we can do to reduce that will reap major rewards. Not just in the performance stakes, but more importantly in the handling and "feel" of the boat.
My last boat was a 4.2m rib, but it had a very heavy fourstroke 50hp on the back and so it was very rear-heavy and this caused a very unpleasant ride in choppy conditions, reducing the weight of that engine would have had massive consequences, infact another member on here specifically decided to go for a 40hp two stroke engine over the 50 from the same manufacturer because the weight difference was so large that the extra power would not only have been offset by the increased weight, but the handling would have been ruined too!
But I'm probably spouting bollocks again...
Originally Posted by MattD
If not what tends to drive the need to save such relatively small amounts of weight, does it have a significant impact somewhere else like towing or is it a hobby specifc benchmark?
Reducing weight is always a good thing, car manufacturers nowadays are constantly trying to shed weight from the bloated machines they now churn out. Light weight means better fuel consumption, better acceleration and better top speed. And you can have all these things combined with the fuel consumption, low noise and smoothness so often associated with a fourstroke...in a two stroke...only with Evinrude Etec