Propeller shopping

jakeetaylor

I have a 1991 rinker 260 fiesta vee and wanted to upgrade my prop and am looking for advice. It has the 7.4 mpi with a bravo one stern drive. I'm looking to improve on fuel economy first and performance second. Any advice or recommendations or experiences would be appreciated. Tks Jake

Black_Diamond

Beware changing pitch for economy. You can really load up your engine. If you know your engine WOT rpm per mercury, then run your boat at WOT and see if the rpms match. 1" of pitch = 200 rpm as you increase/decrease.

212rowboat

to butt into this, RY, the simple answer is using a prop pitched outside of the design intent shifts the balance of effort to either end (engine or drive) and away from harmony between them.

too much bite, as represented by a higher pitch, puts too much stress on the engine and it can't spin it to speed.. too little a bite allows the engine to spin far too fast and above both the balance rating of the engine as well as spin outside the most effective range of the camshaft. 

imagine trying to drive a screw into wood with a low pitched (high thread count) machine screw.. imagine trying to turn a wood screw into a block of metal... the wood screw (if you could pilot the hole) would be pitched far too much and you wouldn't be able to turn it... the machine screw you could turn as fast as you want, but it won't bite.. you, (the constant, here) being the motor, would be ineffective.. if you made the screw the constant you could find the material that allows you to apply full force and bite that doesn't slip too much or resist too much.. if the material is the constant (water, in this case) the load shifts back to both the engine (power/torque) and the leverage- and the screw becomes the final leverage device... the balance for the effort should be evenly spread using the outdrive's reduction and the engines power- too low a pitch (low pitch number, say 15 forward inches for every revolution) and the engine overpowers the bite (traction), and you'll spin with more slip than bite.. too high a pitch (say 24 inches for every revolution) and the engine doesn't have the power to turn it up to speed.

the right pitch allows the engine to take the best advantage of the reduction and maintain bite and a good effort (RPM) to reward (forward movement) ratio. 

Capt Ron

You didn't mention the type of prop you have now. If it is aluminum, go to a stainless of the same pitch as a starting point. The flex of an aluminum prop is considerable. The stainless doesn't flex at all (which can be disastrous if you hit something) which will improve out of the hole performance dramatically. My advise is to deal with a dealer that will allow you to test difference pitches and etc before you purchase.

212rowboat

to further complicate- I was astounded at the difference between a ventilated prop and a non-vented prop.  the vents are found at the base (entry point) of each blade, and allow an engine/outdrive that exhausts through the hub to exhaust in part across the face of the blade instead (controlled cavitation) until the boat reaches a forward speed that the water moving over those vents 'seal' the vents by making the pressure greater to overcome than the exhaust pulse just going on out the end of the hub.... this allows a prop pitched, say, 23p overall, to behave like a prop pitched 21~22p during a hole shot, but to resume the characteristics of a 23p while on plane... (so far as engine RPM's/load are concerned)..

212rowboat

the added blade will drop your RPM's to the tune of 150ish RPM's.. the added blade also, though, will bite the water with more force and reduce slipping..

assuming your current prop allows the engine to achieve it's proper RPM's, dropping a pitch while adding a blade is an attempt to retain that operational range mathematically or on paper, and is translating what 'should' happen when altering props.

because you will have greater 'grip' on the water and theoretically less slippage with adding a blade, you will make more out of the RPM's you produce (translated to forward speed).  If you drop too much pitch, you'll be able to turn the prop faster than it can bite and MAKE slippage.. If you add pitch, you'll have TOO MUCH bite, and stress the engines ability to twist it to the top of it's intended range.

212rowboat

My opinion, FWIW, is that you may be able to twist that higher pitched prop with all that excess torque those diesels leave on the table, but your maneuverability at low speeds would suffer, making your rig not to fun to dock in a tight marina. I'm willing to bet your engines are governed and reach that engine speed easily, huh? Do they struggle when your loaded with people and gear to reach 3700rpm?

Black_Diamond

Good point Drew. I went from a 24P to a 22P and docking became much easier. Too much bite at low speed and tight quarters is a pain.

212rowboat

those things ought to be able to torque through just about anything reasonable... They rev a lot slower than gas, though, unless your turbos can make up the difference...... without a turbo, or a turbo big enough, poor ol' diesels have issues off the line or out of the hole.. if the fuel can't keep up that poses another issue altogether.. how tired are your fuel pumps? How many hours do you have on those turbos? are they original equipment?

jakeetaylor

Thanks for all the comments. I've currently got a SS 3 blade w/ 19" p. all the research I've done tells me there's to much out there to make a dry choice. I'll wait until I can get to the lake and test until happy. Anybody know of a place that let's u test props near Salt Lake City? Found a couple places but looking closer to home. Tks again fellas