That Spinning thingy on the back

[tdanzer]

Hey guys,<br />I was hoping someone could help me with these numbers. I stood in 73 degree water this weekend to find them. I don't have a trailer.<br />I want to replace the prop because it is old and crusty, plus I want to know what is on there. Previous owners have no clue, Maintence shop has no clue. So I got in the water and this is what I read.<br /><br />48 78118 A ?6? ?0? ?1? I put the question marks around the last numbers because I can't be totally sure thats what they are. Could not clean it off enough<br /><br />I read the prop finder booklet but it said the second number was supposed to be pitch. The way I read this, it is a 48 inch diameter prop, Pitch 18, but what is the 781? and do I need to worry about the last numbers? Thanks Guys,<br /><br />Oh and it's on 1980 22 ft Bayliner cuddy,<br />Mercruiser 898 Stern Drive, 198 HP<br />Currently runs about 3900 rpm with a recommended window of 3800-4200.

 

[stevens]

Re: That Spinning thingy on the back

IF these numbers are correct, I believe the first part of the code (48 78118 A) refers to a 14.5 diameter x 17 pitch three-blade aluminium Black Max propeller for Mercury/Mercruiser (according to Michigan Wheel's cross reference guide). The diameter and pitch numbers would be given in that last part of the number that you couldn't read.<br /><br />Again, IF these numbers are correct, I would have replaced this prop with maybe:<br /> <br />a Mercury Marine Black Max 48-78118A40 alu 17 pitch 3 blade<br />a Mercury Alpha 4 alu prop 48-834850A45 16 pitch 4 blade<br />a Mercury Vengeance steel prop part number 48-16314A46 17 pitch<br />or perhaps<br />a Precision Props Stiletto Adv II part number 10317 17 pitch<br /><br />But, please don't trust my word alone on this as I am far from certain that this is right. I think you had better wait until Walleyehed or Dhadley or someone else comes along that would know what they are talking about.

 

[tdanzer]

Re: That Spinning thingy on the back

Thanks for the info. I would have to think you are pretty much on the money. Thanks for the Michigan Wheel hint. I went there and filled in their Propeller search page with the information about the boat and motor which I am sure is 98% accurate. It came back with either a 15 3/4 X 17 or a 14-1/4 x 17 in stainless steel. So your analysis seems to be accurate. Saved me a boat load of money because I was ready to buy an 18 pitch which probably would have bogged the motor down.<br /><br />I did read somewhere in here that you should match the prop to the highest recommended WOT rpm. Which I guess means I should drop down to a 16 P. I am only running 3900 out of 4200. But I am not sure I want to do that. I finally figured out how to work the trim, and trimmed out at WOT I can hit 35 MPH. Running between 3400 and 3600 it hits about 30 to 31 mph. That's me alone, about 200 lbs of gear, and a full tank of gas. Which I think is a respectable speed for a 25 year old boat. I am afraid that a 16 P will allow me to over Rev the engine. Plus a friend of mine said that I would get better performance with a stainless steel prop so I am hoping that the increase in performance with a stainless will be better than dropping to a 16 P. <br /><br />What should I be concerned about when deciding between a 15 3/4 and a 14 1/4? And is there really any benefit to going to a 4 blade prop?

 

[stevens]

Re: That Spinning thingy on the back

A larger diameter prop will allow you to push a heavier load without the propeller starting to cavitate. A larger diameter will also slow your top speed down a bit, and run at slightly lower RPM than a similar pitched but smaller diameter prop.<br /><br />Same with a 4-blade. More blade area means better ability to push a heavy load, a better grip, better holeshot, but also a slightly lower top speed.<br /><br />I don't know about your outdrive, but newer Alpha drives will not take propellers larger than 15" diameter.<br /><br />Yes, you probably could drop down an inch in pitch, but I don't think it is critical if you do. At least in theory, the increased RPM will compensate for the drop in pitch, giving a resulting unchanged top end speed.<br /><br />If you go to a steel prop, then maybe you should drop an inch of pitch, as a steel prop will often have a more aggressive design and will thus rob a few RPMs because of that. Then again, 16P is not a very common pitch for a 3-blade, so I'd probably try a 17P first.<br /><br />Don't count on steel being a lot faster than aluminum, though. The differences are usually marginal, at least for ordinary non-racing type boats with sterndrives.<br /><br />Have a look on Ebay. Lots of very affordable yet spotless props there these days. How about this one: item no. 4584836962, or these here on iboats: alu steel <br /><br />But again, please don't act on my word alone. Best to wait until you get a second or third opinion...

 

[tdanzer]

Re: That Spinning thingy on the back

??

 

[Dunaruna]

Re: That Spinning thingy on the back

2nd opinion. Much the same as the first but...........<br /><br />Before you prop, make sure the tune is good. If you're not making every available horse then your not making WOT RPM's.<br /><br />Load the boat with an average load, then go for a spin and see what WOT RPM's your current prop is giving you.<br /><br />Propping to the upper limit of your motors' RPM's is important, you want to aim for 4200. Propping for longevity is not the same as propping for speed.<br /><br />Aldo

 

[tdanzer]

Re: That Spinning thingy on the back

Good advice,<br />I have been trying for two months to get the overpaid and under worked mechanics to change all the fluids, engine, transom, out drive. Tune it up, etc. They always seem to have more work then they can handle. I might have to buy a trailer just so I can get the normal maintenance done. The engine sounds really strong. So strong in fact that if something goes wrong it's going to be major. I went 50 miles, north of my dock, all the way to Hoover dam and back to my dock and it never even stuttered. A tune up may give me another 25 to 50 rpm. but I think it's probably running close to max. So my thought is to buy a 14.5 X 17 pitch in stainless steel. If that bogs down the engine to much then I will get another one at 16 pitch. At least I will have a spare if I bottom out and break it.<br /><br />Thanks for the advice guys! I hope as I learn about boating I can return the Favor. <br /><br />Smooth Sailing, ERR Boating!<br /><br />CPT Ted

 

[tdanzer]

Re: That Spinning thingy on the back

Oh a couple more questions.<br /><br />On the Michigan wheel prop finder which boat length do you select. I was looking again and I selected one boat length and it gave me a different prop than before.<br /><br />Do you select the length where the lowest number in the range corresponds to your actual length? My boat is a 21.5 cuddy. If I select 19 to 23 it gives me a totally different prop than if I select 21 to 24. <br /><br />Also what is the procedure to replace a prop? Do you basically unscrew the nut and pull one off and put the other one on? should I replace the nut that is on there or use the old one?<br /><br />Thanks

 

[propline]

Re: That Spinning thingy on the back

Replacement of your propeller is fairly easy to do, just follow the guide on the following website: http://www.propline.com/Propeller-General-Information/Propeller_Installation.htm