ANOTHER PROPELLER QUESTION

"I have a mercruiser 488 4cyl,

"I have a mercruiser 488 4cyl, 3.7 liter 188 hp in a 20 foot 4 Winns. As I have said in a previous post on the same subject, I switched the stainless prop with an aluminium one. The stainless prop has no diameter or pitch denotations on it whatsoever. It does have what seems to be a part number stamped on the hub. The number is 48-73980-19. As the prop is held with these numbers at 12 0 clock, there is a stamped and circled 5 at about 9 o clock. At 6 o clock there is a slightle askew letter j that is also cicled. Other than that, there are no markings identifying the manufacturer.

My question is what do I have? I measured the diameter as best I could and come up with 14. I brought the prop to a local trusted marine supply, and they looked up that number in their catalog, and came up with a 14x19, which I purchased, and put onto the boat. My gas milage is phenominally high, almost to the point that to go 12 miles, I only used 1.5 gallons of fuel! My problem is that the boat will not go above 3500 rpm's. My thought is maybe they are the same pitch and diameter, but the aluminium one could possibly be more cupped than the stainless one, giving more load.

I don't mind the low fuel use, but on the other hand, I don't want to lug the motor down, as to blow a head gasket, or put too much strain on something that can't take it. My next move is to swap the prop back to the one that was on it, and see what happens.

Thanks in advance to those who's advice or guidance gets posted to me, it's well appreciated!!!!!"

"With the stainless steel prop

"With the stainless steel prop, you'll see even less WOT RPM and more strain on the running gear. The stainless prop will not flex as much as the aluminum one. The stainless prop is effectively more efficient in tranfering the power of the engine to foreward thrust... just like putting on a bigger prop.
Before you do anything, how pleased are you with the performance of the boat for the kind of boating you do?
If you aren't interested in going 100 mph, or not worried about yanking a 250lb skier out of the water, why mess with it?
Here's the skinny:
Less prop (smaller diameter, less pitch) will give you better acceleration but lower top end speed, and more fuel consumption for the given cruising speed.
More prop (larger diameter, more pitch) will give you better fuel economy for a given cruising speed and more top end speed TO A POINT. That point being the point where the engine doesn't have enough umph to give you anymore speed. Engine Hp, weight of the boat, and final gear ratio all having thier part in it.
All that being said, the typical wide open throttle (WOT) engine rpm is around 4400-4600 rpm for most stern drive applications.
So, if you do anything, I would give it less prop, but not a lot less... maybe shave a 1/4 to 1/2" off the tips of the prop you have or going with a 14x17 aluminum prop.
One more note on stainless steel props:
They're great for the speed junky that wants to go. If you hit something in the water, like a partially submerged log or a rock, you can pretty much count on replacing the sterndrive, maybe the engine too.
For the typical boater, aluminum does just fine and is a good bit more forgiving.... $150 prop vs. a $5000 sterndrive... what's that shiney prop worth to you?"

"Tanks Troy, I remember you an

"Tanks Troy, I remember you answering this question a while back....but a new question came to mind after that post and I got the boat in the water. The stainless prop seemed to give the engine a higher rpm (around the 4400 mark so I'm told by the previous owner) than my new aluminium one, which leads me to believe that the prop that was on it last year (the stainless one) was probably not a 14x19, like the marine store said that sold me the prop. This prop has only been in the water about 2 months or so, and only used for a total of 3 hours or so. (which doen't really mean anything) returning the prop at this point is out of the question, but i really need to find out what size the stainless one really is, so i can get the right aluminium on it. I figured that if I change back to the stainless prop, and it gives me the proper rpm range, than that is the prop I need, I just need to identify what size it actually is.

Also while going wide open with the aluminium prop (14x19) the engine got noticably hotter, and started to detonate at 3200 and above. I did not run it like that for more than 20-25 seconds or so. If you have any other ideas on id'ing my prop from the numbers i gave before, It would be greatly appreciated.
(by the way, it is not a big deal for me to change the prop, only about a 10 minute process, so if the stainless one works, then I know that is the size, i just have to id it!

Thanks Again!!"

"Tom,
Troy didn't elabora

"Tom,
Troy didn't elaborate, but any decent prop shop can remove diameter from your new prop to raise rpm, or they can re-pitch (will probably cost more). You've already bought it and can't return it, so why not modify it so that it can be of use?

Bob"

"BOB,
thanks for replying.

"BOB,
thanks for replying. youre right about that, and that is what i'll probably do, but I still need to find what diameter and pitch the stainless prop I have is, so I can get (or like you said) modify the one I already have. i gave this prop to all the knowlegeable old timers at the yacht club I belong to, and no one could come up with the size or pitch, just that part number that is on the hub. I have just gotten through searching every propeller website, to try to id the prop size with that above serial number in my first post, but all come up empty. I think the diameter is pretty close, but I think there is an issue with pitch, because the engine SHOULD be running at around 4400 or so, not 3400.

Thanks for the input and advice. any other ideas, or reccommendations, just shout!!

Thanks TOM"

"That is a quicksilver part nu

"That is a quicksilver part number. the '-19' would indicate a 19" pitch. If you're game for the science experiment, slap that stainless prop on and go for a ride.
I think you're going to find that either the previous owner exaggerated just a bit (after dropping $350 into a nice shiney prop realizing he gooned up), there is something else wrong with engine, or somewhere along the way the gear ratio in the drive was changed.
If you put that thing on it turns right up to 4400rpm, there's going to be some serious head scratching going."

"Troy,

Thanks for the info.

"Troy,

Thanks for the info. I kinda thought that the 19 meant the pitch. Today I took the aluminium prop off, and put the stainless one back on. Before I did that though, I got out my trusty tape measure and started measuring the blades and stuff. Found out something VERY interesting. Both the props were identical in diameter to the "t". Both of the pitches of both the props matched perfectly as well. Here's the fun part. the blades on the aluminium prop (even though they were EXACTLY the same pitch) had a noticable larger surface area to it. What I mean is even though they both were the same diameter, as well as pitch, the aluminium prop blade had more surface area on the blade towards the hub. There was considerably more meat to the aluminium one over the stailess one. I did put it gear at the dock at idle, and got almost 100-150 rpms MORE at idle than before. I pushed the throttle up to around 1200 for a few seconds, and the throttle position is more rearward than before, at the same rpms, which tells me the engine was seriously lugged down. I won't get to take it out on the open water until tomorrow, due to the pea thick fog that engulfed myself and Jamaica Bay (NYC). Thank's and I'll keep you posted.
Also, if anyone else has a prop problem such as mine,keep in mind, those props LOOKED identical until I got the tape out! Also I forgot to mention before that the stainless prop was on the boat for years (about 16 or so!) before my hands got on it! Thanks again!!"