What a difference a new prop makes.

[Cheech]

Had been looking at a new prop for ages because my 100hp merc was way over propped. The process was slowed somewhat because my existing prop was seized on.

Anyway, I ended up picking up a second hand 17pitch prop at the recent Redcliffe boaties swap meet really cheap. So spent last Saturday with an angle grinder and piece by piece disected the old one off. That was fun.

So going from a 21pitch to a 17 pitch, it is like the motor breaths a big sigh of relief, hand brake off, and off it goes. Gained an extra 500 rpm (but lost 3 or 4 kph top speed). My only concern is economy. I normally cruise at around 45kph at 4250 rpm, but need to do around 4500 rpm for the same speed. Is this about normal? Relatively speaking of course as I know normal is very dependent.

Would love to test run a 19pitch just to get a comparison.

Cheech

[blaze]

just from the rev difference you a talking about I would thing that the 21p may be a smaller dia than the 17p, I believe rule of thumb is 2" of pitch will equal 400 rpm on the same dia prop. Maybe if you had the 17p cupped it would have the desire affect, I think you will gain about an inch of pitch by cupping, still trying to get the right one for my 115 merc (keep smashing them though) Send an email to Solas- very helpful. Thats where my next one wil come from
cheers
blaze

[NQCairns]

Happy to hear you finally able to get to it cheech.
I dunno for sure but your economy might actually improve (or be the same) but it shouldn't get worse IMHO because the engine is not lugging anymore (if your WOT rpms are now high enough?) it is now spining easier at any throttle setting ie for arguments sake with old prop at 4250 the carb betterflys were 7/10ths open now with new prop at 4500 the carb butterflys are at 6/10ths so fuel consumed per hour at the new setting may be less.
Bit like going up a steep incline in a vehicle one gear to high sucking heaps of fuel, heavey on the accelerator compared to changing down a gear reving higher with less press on the accelerator and less fuel actually being used over distance or time. cheers nq