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View Full Version : PROPER PROPELLERRPM QUESTION



arrowsniper
05-03-2006, 10:09 PM
"I have a 1984 4 Winns Santara that is 20 feet long, with a 488 3.7 liter mercruiser in it. What should the proper prop size be? i'm not sure if the prop on it is the original prop, or the correct size. Also does anyone know what the engine rpm's should be wide open? i also have a decent clicking noise at 3000 rpm's and beyond. could this be due to a prop too large and putting too much strain on the engine? It runs fine any rpm range below 3000. Any help would be appreciated."

troym
05-04-2006, 08:55 AM
"Is 3000 rpm the most you get at wide open throttle?
That's a bit low... you should see 4400-4800 rpm.
There may be too much prop. An average prop for that size boat is something like 14" diameter x 17" pitch. It's not an exact science. The factors that affect the prop size are the power of the engine, the gear ratio of the drive, the weight of the boat, and what you plan on using the boat for. If you want to go fast, but acceleration is of little concern, you'll want a bigger/steeper pitch prop. If you want accleration for pulling skiers out of the water, a smaller/less agressive pitch is the way to go.
The clicking noise is probably a different issue. Try to pin point the source with a mechanic's stethiscope."

arrowsniper
05-04-2006, 05:41 PM
"TROY,

i can get more than 3000 out of the engine, but when I heard the noise, I backed off. I have researched it a bit more, and being the boat was doormant for six months or so before I bought it (in October) the general consensus is that it is probably a stuck or collapsed lifter. The prop that was on it did not have any stamping of size whatsoever, but did have an id # on the hub. The prop that I was told matched that # was a 14X19. I replaced it with an aluminum prop with a spinout hub. I needed to do this due to shallow water here in Jamaica bay (NYC). I would rather wipe out a $120 prop instead of a $300+ stainless prop with no spinoff hub, (and a few hundred in outdrive parts) I will be using the boat for fishing, and cruising, accelaration and high top end speed not important. I actually used 3.5 gallons to go 14 miles, at around 2000 rpm. (approx 13- 15 mph). low fuel usage is probably my highest priority.

Thanks for your input and advice, maybe the additional info I supplied here will help a little more. Thanks again."

troym
05-04-2006, 08:26 PM
"A steeper and/or bigger prop is probably what you want to drop the rpm's down to save gas.
Don't over do it or you'll never get on plane... and you'll burn gas like there's no tomorrow!
Just as a data point, my boat (5000#, 22ft Searay, 5.7L merc, 14.25"x21" prop) cruises nicely at about 3200 rpm... gets me 22-24 knots, depending on the seas, and I burn between 10 and 12 gallons per hour. That's also with the trim tabs down full and bumped up as necessary to balance any list due to loading (the kids like keep me busy with this!) and I have a hydrofoil on the anticavitation plate, which does make a noticable difference.
Wise choice with going with the aluminum prop. Realize there's no guarentee you'll keep from damaging the outdrive if you do hit something, but the likelihood of at least being able to limp home under your own power is a little higher. I carry a plastic emergency prop... $50, and it came with a spare set of hardware (hub, nut and lock washer) and a floating wrench."

arrowsniper
05-04-2006, 08:50 PM
"Troy,

thanks for the input, I had this question because i was not sure if the right size was on there in the first place. Being that you said around a 14 or so put me at ease that the right one is probably on it. As far as going to a larger prop, I'm pretty satisfied with it's economy (around 3.5 mpg or so)that I'll probably leave it alone; it can't take a larger one in fear of hitting the zinc.

Thanks again"