mercruiser 3.0 alfa 1 prop recommendation

[jmarines]

Okay guys.. need a little help with my prop. I just purchased a 95 searay 175 . Took it out a few weeks ago to make sure it was working correctly. My WOT was at 4000 rpms but from the reading I've done it should be closer to 4600. I just checked my prop (which is pretty beat up) and read the numbers.. it's a stiletto m1419 which is a ss 14 1/4 with a 19 pitch 3 blade. My top speed was at 29 mph via gps. Is this normal for this prop and my motor or am I loosing top end speed somewhere? I've contacted my local marine shop to see about refurbishing my current prop which will run between 150 to 250 range. My question is how do I get the most out of my motor.. the boat will see mostly pulling a tube with kids/adults. I've read about people going 4 blade which will improve top end and hole shot... is this correct? What recommendations would you give on switching to different prop which will put me in the optimal rpm range for my motor.

 

[steelespike]

Well the news isn't good but hopefully not too serious.A 3.0 in a 17 ft. with a 19" prop
should make 38 + And it might turn a 21" up to 40+.
Your ss prop may be worth a repair though for $250 you might be able to come close to
getting 2 new aluminum props.One for best speed and the other for water sports with a boat load.
Read up on the stickys at the top of the prop forum. Prop selection can be really confusing so don't hesitate to ask questions.
Some things to check/do.
Do a compression test. Note plug condition and location.
Check that the throttle opens all the way.
Be sure the motor is in excellent tune,be sure the timing is set right and advancing as it should.
Is the bottom clean and smooth? Do you raise the trim for the best speed?

 

[jmarines]

I'm very new to boating so please excuse my noob questions. I will be replacing the spark plugs tomorrow and plan on the compression test. How would I go about checking timing.. is there a sticky on that? I will definitely check tk make sure the throttle is opening up all the way.

 

[fishrdan]

SS gave some good advice as that Searay is a reasonably fast hull, most likely capable of running a 21P aluminum prop (or that 19P stainless steel) and hitting 40MPH. That engine should turn 4800RPM WOT RPM (wide open throttle) with a light load and with the engine trimmed. Only hitting 4000RPM and 29MPH tells us something is wrong, poor engine tune, totally hosed prop, excess weight in boat. How many people were in the boat when you took it out, or more important, how much did everyone weigh when combined... 300# or 1000#?

When I mentioned excess weight in the boat this could come from extra gear, or waterlogged foam due to a leaky deck. It may be worth removing all gear from the boat and taking it through the scales at t a truck stop to check the boat's weight.

What elevation are you running the boat at?

For timing, look through the Mercruiser Forum as there are write-ups on timing a 3.0L, I'm guessing your engine has the EST ignition that requires a special procedure to get into base timing mode. You should also find an online manual in that forum for your engine. You will need a timing light to check the timing.

 

[eavega]

I have a 17.5' Stingray with the Merc 3.0L engine. I run two props depending on what I am doing on the lake; I have a 20" SST 3-blade for cruising/Fishing. I have a 17P 4-Blade aluminum for water sports. The 20P blade will get me close to 40 MPH @ 4400 RPM. The 17P 4-blade gets me a top speed of 38 MPH @ 4600 RPM. The 4-blade will give you a better hole shot at the expense of top end speed. I don't need to go much about 25 MPH when doing water sports, so I'm willing to sacrifice speed for the ability to get someone out of the water easily on skis/board.

You definitely have another issue going on if you can't turn the 19 P aluminum at least 4200 RPM. See if you can get a loaner prop to see if you get the same results.

If you are still not turning the prop at a good speed, then its time to investigate your setup to see if the engine might have an issue or if the hull has saturated foam.

Rgds

E

 

[steelespike]

Prop sizes are written diameter X pitch. Your 14 1/4" X 19". Key number is the pitch. 19" is the theoretical distance it moves forward in one revolution.
Prop is working in a liquid so it doesn't actually move 19" but somewhat less,this is called slip and is measured in %.Typical slip is in the low double digits.
A big heavy boat will have higher slip than your your little runabout. As the prop slows slip will go up.
There are 5 key numbers used to evaluate performance. rpm,gear ratio,prop pitch,speed and slip.Slip is the trickiest to find and critical in evaluating a setup.
There are calculators that using 4 of the numbers to figure the 5th.
Gear ratio on a 3.0 is typically about 1.98 or 2.00 when I calculate I use 2.00 as close enough.
2.00 means the motor turns twice to the props one.
If we use the calculator with your numbers 4000, 2.00,19",29 mph we get 19% slip, reasonable on a boat at 4000 rpm.
So initially we can assume your known figures are accurate. Oft times slip will calculate higher than expected sometimes it is because the tach is off high(fairly common) or the speed is off low.So the gps speed is good to have. The calculation is only as accurate as the input.
If you do a search using "prop calculator" I think the one I use may come up, a green page.

 

[jmarines]

I'm definitely taking it out this coming weekend and I'll see about removing all the extra weight. As far a weight goes on my last trip out.. I would estimate 800 to 900 # total. That's with personnel and extra gear. I will definitely check timing in the next day or 2 along with new spark plugs. I did notice that when I removed one of the screws that holds on the deptfinder sensor that water started streaming put for a good while. I would assume that extra weight from water wouldn't help the top end speed. Do I need to worry about that as being a issue that water was coming out? Also the wot was with the trim all the way down and NOT trimmed out

 

[steelespike]

Trimming up will help the top end.Trim up as you gain speed. Water coming out of the transom doesn't sound good to me.
Rap on the transom with your knuckles I think a transom would have a hard crisp sound and if a problem a dull thud.
Sometimes if there is water in the foam under the floor/deck if you prop up the bow and leave the plug out water may continue to drip for sometime.
If there is a problem the floor/deck may have some soft spots.
I think wet foam is usually toward the back of the boat and is most noticeable on hole shot.

 

[jmarines]

No Title

Here's the prop in question. Should I just go ahead and replace it?

 

[fishrdan]

How does the prop run, is it smooth or do you feel vibration? It looks a little chunked to me and I'd most likely rebuild or replace it. But if it's running smooth, I'd hold off on replacing until you can figure out why the boat is running slow, then re-prop once you have some good performance numbers.

The water draining out of the transducer hole isn't good, I'd prop the bow up and let it drain as SS suggested, to see how much comes out and if it keeps dripping. When foam gets soaked it won't dry out and you could be dragging a huge amount of extra weight if everything is waterlogged, that 800-900# could turn into 1300-1500#.

Run the boat to WOT with just 1 person, to see how it responds to less weight. I can tell a big difference between 2 and 4 people in my boat.

I'm running a 18P Alpha4 prop (4 blade) and love it. I'm not suggesting the 18P as my boat is different, but I really like the 4 blade holeshot.

 

[steelespike]

Here's the prop in question. Should I just go ahead and replace it?

Doesn't look that bad. As suggested if it seems reasonably smooth see how it tests out.
If you boat where hard contact is likely (rocks) Your ss prop could become an expensive" prop"osition.
If soft contract is likely(mud or sand the SS will hold up better than aluminum though will cost more when repair is necessary.,

 

[Mule Laker]

As far a weight goes on my last trip out.. I would estimate 800 to 900 # total. That's with personnel and extra gear.

Are you talking about everything from your anchor, gas, battery, etc? If not, and you mean 800-900 pounds of people, that's a lot of weight in a small boat with a 3.0.

 

[jmarines]

Ok guys... I'm not going to lie.. I was a bit worried that something major was going on so I went out and purchased and timing light and compression gauge. First thing I did was warm up engine to operating temp.. then tested all cylinders
#1 150
#2 160
#3 160
#4 155
That took a load off my back knowing the compression was good. Next thing I did was put it in base timing mode which it cut off the engine. I read that this problem was happening to someone else because there timing was so off. So what I did was set timing at idle at 12 degrees btsc which I hope I did correctly. I tried to get a close to 12 as I could but the plug coming off the distributor was against the hose to the power steering pump. I was able to get it to 10 degrees btdc. I then put it in base mode which the motor stayed running but I could tell the rpm's dropped soni k ew it worked. It was at around 2 degrees after tdc. I then throttled uo to 2800 rpms because a thread I read stated it should go up the 28 degrees btdc.. mine only went to 12 degrees btdc. Am I doing something wronge?

 

[jmarines]

Are you talking about everything from your anchor, gas, battery, etc? If not, and you mean 800-900 pounds of people, that's a lot of weight in a small boat with a 3.0.

No.. just the weight of the personnel that was on the boat.

 

[fishrdan]

If not, and you mean 800-900 pounds of people, that's a lot of weight in a small boat with a 3.0.

Yup.... And this could be why the boat won't perform well, too much pitch and too much weight. I mentioned trying the boat solo to see how it reacts, and think this is something that still needs to be tried. If it's still a dog and under-performing with only 1 person, then being overloaded isn't causing the poor performance. If the boat responds and performs nicely with only 1 person, engine reaching 4600-4800 RPM and upper 30's MPH, then the prop pitch needs to be reduced for heavy loads.

It's very possbile that the boat's last owner only took his skinny wife and rat-dog out with him, both adding up to 120# soaking wet..... And the prop that worked great for him won't work at all for this load.

 

[steelespike]

Can't help with your procedures but you should post your timing issues in the I/O service/repair forum.Should get more help there.
if the distributor Is against the power steering hose something is amiss.Either the hose in the wrong spot or the distributor assembly has an issue.
Perhaps inserted wrong. Could be as simple as rerouting the hose.