Prop Advice Needed For Signature/115 Johnson

[steveo]

Hi, I'm a newcomer to the boating game, recently got a Signature 530C with 115 Johnson. The aluminium prop (pitch17) is not looking too good (thanks to me, lack of care at ramp) and thinking of stainless steel prop. Am I best off sticking with the same pitch in a SS or going higher? Boat gets out of the hole pretty well but I'm thinking about economy as it currently does about 26knts at 4000rpm. Any help appreciated.

[Sammy0884]

Changing the pitch depends on the boats performance at the moment... selecting the right prop for your boat is a real black art and like myself, you may need to try 2 or 3 different ones until you are happy.

Is the boat reaching its maximum rpm at wide open throttle, which is usually 5000-5500rpm, if it is then the pitch is fine, if it is lower than the max rpm then you need to go down in pitch to 18 or 17p, and if it is hitting the rev limiter at full throttle then you may need to go up in pitch to 20 or 21p.

Remember that every inch you increase in pitch you alter the revs by around 150-200rpm, one inch more lowers the revs by 200, one inch lower increases the revs by 200. You also need to bear in mind that the lower pitch prop increases acceleration 'out of the hole' performance, lowers the load on the engine and lowers top speed. A higher pitch prop will make your boat slower out of the hole, place a higher load on the engine, and increases top speed.In extreme cases, a too low pitch prop will cause your engine to over rev for the speed its travelling or possibly exceed the max rpm of the motor, and a too high pitch prop will cause tremendous loads on your engine and cause poor fuel economy The key is to find the balance between out of the hole performance, mid range economy and top speed, the best economy is when a boat is travelling the fastest with the lowest revs and throttle opening, usually around 2/3 of maximum RPM.

there are other factors in assess, such as blade cupping, which increases dynamic pitch, and rake which can improve grip in high speed cornering. You should also consider stainless and 4 bladed propeller designs. Stainless usually have a sharper leading edge which can make the propeller slightly faster through the water, and four bladed designs are great for offshore work as they give greater 'grip' and bow lift in the water, they are also great for tow sports and heavy loads.

[steveo]

Thanks Sammy. I think I need to test current performance again before going any further. I'm finding it hard to justify extra $$ for a SS prop - eg Solas for my application is $655.