prop testing problem

[throttlejunky]

1984 Wellcraft 210 Elite.
3000 lbs.
Mercruiser 5.0 228hp. I think that means alpha 1.6 ratio
Aluminum 3 blade 14x16 prop. Possibly cupped, can?t tell bye eye.
Manufacturer recommended rpm range= 4200-4600 WOT

Need to get a new prop. Having trouble figuring what pitch because of an issue. Right now, the current prop has cavitation burn, 2 small tears that are dime size. I know this will rob MPH?..but what about RPM? Right now with a 16? prop I can only turn 4200 rpm for 31 mph. Feel like I should turn a 19? pitch at 4200 hitting low to mid 40?s, unless I am just delusional. Boat launches hard, motor strong. So is my damaged prop not only slipping more and affecting MPH, but also keeping my engine a good 600 rpm low? Feel the 16? pitch should be hitting beyond upper rpm range, possibly 4800 if I let it wind. What do you think? Dont want to throw a 19 on her if it won't work. You guys see rpm's drop from prop dings/bends? Thanks to all who chime in!

 

[hwsiii]

Re: prop testing problem

Throttle, you have a 16" prop that won't turn but 4,200 RPM and you want to increase the pitch by 3" which theoretically will bring your RPM down about another 650 RPM below what it is now. Unless you are doing some MAJOR diameter and blade geometry changes and since the prop you have now is aluminum you don't stand a chance of that working. You don't mention your speed but I agree with you it should be able to turn the 16" to the maximum RPM of your motor. But your prop is not the problem.

Please fill this form out.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H

 

[throttlejunky]

Re: prop testing problem

I will get the info together once i am off work. I know exactly what increasing pitch will do to the rpm. Maybe i should have been more direct. My question was...The prop is damaged, will this just reduce mph, or also limit the RPM's because of it's behaivor under water? Is a damaged prop more difficult to turn, or just less efficient? thanks.

 

[throttlejunky]

Re: prop testing problem

Sorry, only got to 31mph, meaning a significant amount of slip from playing with calculators.

 

[180shabah]

Re: prop testing problem

A damaged prop is useless as a starting point. Do you have any data from before the prop was damaged?

Do you have a prop exchange dealer in your area? You could try a few different designs/pitches.

 

[junior1113]

Re: prop testing problem

is this 16 all you ever had did it always turn 4200. before 2 dimes. yea damaged prop can throw reading either way. they can also damage lower and power head if vibration is bad. running with damaged prop is like driving your car with a flat tire. eventualy more issues will arise. a 17 sounds like a good starting point.

 

[throttlejunky]

Re: prop testing problem

Bought the boat this year, no previous data avail.
Put a 19 on it, pulled 3950 rpm at 36mph. That's about a 20% slip. I can't get any better numbers with trim. Is the RPM range supposed to be reached on glass, or typical conitions? I'm on lake erie, it was a nice lake yesterday, but not glass. boat is still bouncing across the lake.

 

[sho3boater]

Re: prop testing problem

You need to see your top rpm with little weight in the boat, maybe a little over like 4700-4800 with one person and no gear in slight chop, just perfect conditions (cooler temps) and a perfect prop and even synthetic gear oil and of course a clean bottom. Most boats are fastest in small chop and little slower on glass. If you can't pull the rpm compared to other boats then might be a motor issue. A long shot is your tach is not right but that is not common. Try to look up similar boats for speeds/props, try to find a used prop a dealer here has piles of them I have got them there before....and he will let me trade them back to find the right one. If the boat is heavy with gear or waterlogged it will be slower. If you run it all the time with a bunch of stuff in it, then prop it for that. Note that the speedo is useless, I hope you are using gps. Every speedo I've had read about 5mph faster than the boat even brand new ones, though an expensive one should be closer.

A friend of mine had the same boat years back, think it had a 350 SBC in it I forget the HP. It did have thru hull exhaust and aftermarket risers. It would show 45mph or better on the speedo all the time. Before a certain year you can get vortec heads off a newer chevy truck and they are said to run really nice on the old Mercruisers and give more top end.

In my experience any dent or bend in the prop affects it much more than anything else appearance related. Burns I've seen didn't mess up the metal really. If it is cavitating do you have to trim it way up all the time? If so you need a prop that lifts the bow better, as typical on those hulls. A prop needs to be nearly perfect in shape and the edges to perform properly. One little dent the size of a fingernail can chop 5mph like nothing.

 

[throttlejunky]

Re: prop testing problem

Cool. I am throwing plugs, and both fuel filters in it today. The seperator is old enough to be rusted, have no idea how ugly the plugs are, or even the cap and rotor. Don't have much experience with different weights. All my tests have been 3-4 people in boat, 3/4 tank of gas, cooler of beer, and supplies, typical day for me. By myself no cooler, who knows what I would pull. I will keep playing. The 19" I just thew on is cupped, so I guess that acts like a 20". I also need to get gps and a different tach to verify. If the tach reads 10% off, I well into the range. I still wonder why my slip is so far off.

 

[sho3boater]

Re: prop testing problem

Cupped props often don't work as well as originals (slip), but can be better in some cases. Back then one of the best all around props was the Mercury, Mirage I think it was. The prop needs to be perfect it is worth getting it done once you find the right one. The prop is made correctly when new, every time you change it with cup or something it is actually screwing it up and can slow it down. Now if your setup really needs it changed in some way fine, but often if you put a modded prop on a different boat it runs poorly. Really those changes should also be within what the next size prop would give you as well, but I've seen much more than that done to them.

Usually the cap/rotor hold up well, we built 500hp BBC motors and once a 400ci SBC with a 305 duration cam (!) and used stock Merc ignition and never changed it. You do want the timing right, IIRC we ran 38 degrees total advance and that is the only way we set it. You need a degree tape or adjustable gun to set it. Watch for water in the oil, strange looking plug colors, stuff like that. The aftermarket risers seemed to help without buying headers, if you have thru-hull exhaust. Prop exhaust will slow them down. And always change the LU oil often plus synthetic can help though we ran Merc only in outdrives back then. Since I been running OB motors and it helps in those, in fact not sure they had good synthetic back then that was well over 10yr ago. Make sure the throttle opens all the way, the choke is open, bottom of boat is clean, etc. We even ran the power steering belt loose sometimes to race people, lol. Now some are running 4 blades, they bite harder in corners and holeshots. We never ran them because back then they had a slower top speed. A SS prop is always 1p more than an alum if you didn't know, a SS 20p is about like a 21p alum. We never ran alum props on those I/O boats, in fact I don't on my outboards either.

We got way into building motors, a good upgrade combo is better heads and larger Merc cam, and better exhaust if you go in that direction. But you might as well just get a larger motor first.

If your prop is really messed up, then I'd try a larger SS prop. Borrow or whatever to try one. If it is not that bad I'd try a SS the same size. SS props are faster and include a little more pitch to make up for it. Not going to be a huge difference though, but will if yours is damaged.

 

[throttlejunky]

Re: prop testing problem

4 people on boat, cooler, gear, and a accurate gps speedo.........19' pitch, 4000rpm, 42 mph. At least i'm in the ballpark now. Speedo was off.

 

[sho3boater]

Re: prop testing problem

Good, try sharpening that prop up, or have it done.