Advice re boat

[Jonathan]

Hi Nick,
The speed which you're achieving with the aux is probably due to what is known as the hull speed - every boat is limited to a certain hull speed according to the length of its waterline and to exceed this hull speed you need sufficient power to push the boat onto a plane and no way will a 9.8hp have sufficient power to plane a boat weighing over 1 ton. The hull speed is calculated by the formula: 1.34 x ( the square root of the waterline length in feet). If you assume your LWL at being somewhere along 16ft, you get a hull speed of 5.36knots which is pretty much the speed which you mentioned at flat out throttle using the auxiliary.

[fishfinder]

when I've asked my seller if my 115 four stroke works with low revs for trolling, he replied that all 4 strokes are capable to work for a long period of time in such conditions without doing any damage. But when i've asked for the 2 stroke he replied to me that 2 stoke engines over 25hp will not work properly, they consume much more and there's a big probability that they over heat.

what Jonathan said it's true but with an 8hp and a 9.9hp on a 15' boat you will surely get 5.5knots with half trottle. then there's the issue of the propeller as Skip said, a 4 bladed can give better results on aux.

[skip]

Jonathan, didn't think about the hull speed in fact that sounds about right then. Hopefully what it means is that if in pretty good conditions I manage a reasonable speed at half throttle, if the current and wind was high in an emergency I should still be able to move at the same speed.

I would still like to try a four bladed high thrust prop to lessen the stress on the engine, just not sure whether to stick with the same pitch or try something different.

In general what your seller told you about 2's vs 4's is correct, but Direct Injection ECU engines should be different and should be able to adapt from what I've read. Could be that the engine is still new and therefore wearing in but will be an interesting one to watch.

The 4 stroke is using quite a bit I must say.