Propeller identification - 1998 Mercury Mariner 8 (8hp)

[Cintx]

Hello,

I have a aluminium boat which is approx 12 feet by 4 feet. I am powering it with an 8 hp Mercury Mariner as shown in the photos.The transom height is about 15 or 16" (the motor was in the way to get a perfect measurement). I weight 265 lbs and on the boat I have safety equipment and a 25 litre fuel tank. I am having trouble getting on plane.

I know that the boat has an easier time when I move forward to the middle bench which is why I put the fuel cell at the front of the boat. I took the boat out for a rip today and measured a top speed of 12.2 mph with my GPS. The boat typically seems to run around 10 mph just cruising at WOT. With only me in the back the bow is in the air enough that it obstructs my view, when I add a passenger to the front of the boat the boat is a bit slower (9 mph) but the bow levels off. When the boat manages to get on or near plane the bow is lower and everything is not bad. This is where the 12.2 mph appeared.

Yesterday, I felt like the boat was a little faster. When I was putting the fuel cell away today, I noticed that I had left the fuel vent closed during all of this so I will have to repeat it all tomorrow (darn!). I suspect this could effect things and also explains when the motor did not start first pull like it normally does.

Anyway, my main question after all of that is about this propeller. It looks pretty well worn out or at least it has a lot of rough surfaces and you can see that the black coating is bubbling off on one blade. Should I have this propeller serviced or replaced? Will I get better performance from repairing it as opposed to leaving it as it? Can anyone identify the part number stamped on it? I can't seem to find anything about it apart from it is designed for a Mercury motor and that the propeller pitch appears to be 8". The propeller listed in the manual for this motor is 48-828154A12.

Any help would be great.

 

[Sea Rider]

What's the max rated HP engine for that boat lenght ?
Need to know exactly which pitch is that prop to go from there
If it's just a cosmetic prop issue and prop is 100% operative should perform as expected.
Need to tach engine to know max wot rpm range when at full plane.
Need a pitch that revs fast to take boat out of the hole and lowers bow fast
Can maximize prop to some extend, but a 8 HP is not that wow performer compared to 9.9-15 HP larger OB..

Happy Boating

 

[Cintx]

What's the max rated HP engine for that boat lenght ?

Not sure. I couldn't find any kind of manufacturer identification on the boat.

Need to know exactly which pitch is that prop to go from there

The pitch is 8" as marked on the side of the prop.

If it's just a cosmetic prop issue and prop is 100% operative should perform as expected.

What I am seeing is rough edges like when you cut a pipe but before you file it. I would think that it doesn't cut through the water as well as a new one would.

Need to tach engine to know max wot rpm range when at full plane.

I ordered a tiny tach type thing from HK but who knows when it will arrive. My understanding is that it should be good enough to determine RPM for this sort of thing.

Need a pitch that revs fast to take boat out of the hole and lowers bow fast

I was thinking about the 9" 9" pitch Mercury prop - 48-828156A12. The manual says that prop would be good up to 1200 lbs and up to 16' at 12 - 22 mph.

Can maximize prop to some extend, but a 8 HP is not that wow performer compared to 9.9-15 HP larger OB.

I know. This is what my dad said. I am going to make a tiller extension with some PVC pipe and try sitting in the middle seat. I think with my weight that should help get it on plane.

Happy Boating

Thanks! I haven't owned a boat in a long time so it is nice to get out on one again.

Any idea on who makes that prop? Google isn't coming up with anything.

 

[steelespike]

You need to buy or make a tiller extension so you can be in the middle seat.
The hp is rated at 5500 rpm. I assume it shares Merc gear ratio 2.00.
Doing a lot of guessing calculator says 12.5 mph is at about 4000 rpm.
Guessing doesn't tell us much except it proves we're struggling a little.
​ You definitely don't want to increase pitch.
A light boat and load at 5500 rpm would make about 18 mph.

 

[Cintx]

Well you are right about the seating position. Moving forward gets me going better. I need a 54" extension which I can buy an adjustable one for $40 or I can make one from PVC.

I was looking at the motor yesterday after work and I noticed that when the tiller throttle control reached its end of travel the butterfly valve wasn't fully open. I tried reverse and it did open the butterfly valve fully.

I have adjusted the cable and the valve opens fully or just about at full now.

You are correct about the gear ratio being 2.00.

I did some maintenance today on the motor. Mostly applied a lot of 2-4-C to parts I wanted moving better. I also changed the gear box oil. I was right to do it because what came out was pretty dark in colour but no metal.

So on today's test ride I got on plane but sitting at the back dropped the speed to about 12 mph. I saw a top speed of 15.1 mph with me sitting in the middle. This is up from 10 mph and 12.3 mph respectively.

Adjusting the throttle made a difference for sure. When the tach arrives I'll know more about the RPMs.

Why wouldn't I want to go to a 9" pitch? I don't know much about propellers.

Thanks for your input so far. Can and one ID that prop?

 

[Sea Rider]

You have tested the current prop, right ? Wait till new tach arrives and test same way as second time to check max achieved wot rpm to know if near 5500 wot revs if that's the max revs for that engine. If close stay with that prop or buy same new if cosmetic appearance is an isssue for you.

Happy Boating

 

[Cintx]

I have not tested anything with regard to engine RPM. I only just purchased this motor. My understanding is that I should put the tach on and run it WOT. If the RPM is around 5500 then that's that. If its lower then change the pitch. I think I have it now.

 

[steelespike]

Boat motors are working very hard, Imagine a car towing a trailer up a never ending hill.Increasing pitch is like putting a car in a higher gear.
higher pitch is harder to turn if the boat was running easy with lots of rpm a higher pitch might be needed and could add some speed.
But as pitch is increased it slows hole shot and fades more as you add weight.
Your struggling to plane if any changes in pitch are done it would be to decrease pitch.I couldn't find any Mariner wot rpm range numbers.
But the Merc and Yamaha 8 are 4500 to 5500 rpm a reasonable number.
At 15.1 mph guessing slip of 15t% I get about 4700 rpm Guessing numbers isn't as good idea but 4700 seems reasonable considering the setup.
But a tach is the only way to be accurate. More to come.

 

[Sea Rider]

This is not rocket science, lots of variables to deal with. If you tach current prop and achieves 5500 wot rpm with you alone and lightly loaded boat, don't expect same rpm reading when adding extra load or more passengers. Ideal would be to dial a prop to achieve middle to max wot rpm for that engine with the amount of load you'll be boating with. Is not the same 5500 rpm with you alone than with 3-4 passengers. In both cases you'll be revving at both sides of the wot range.

Happy Boating

 

[Cintx]

Fair enough. I will update when I get the tachometer. Obviously it is all conjecture until then. Thanks for your help so far!

 

[Cintx]

So a little update. The tach haant arrived yet but I decided to take a look at the timing. I noted that adjusting the idle knob the fast idle position wasnt changing the idle speed. This got me thinking that the timing setting might be off as this motor has had the trigger and CDI replaced by the previous owner. The shop that did the work couldn't get any spark so the engine just went into storage. I fixed the spark issue in about ten minutes as I noted that the lanyard kill switch wasnt assembled correctly (off = spark, run = no spark).

Anyways, I put the timing light on it and idle is at 9? BTDC which is within spec (8? +-1?) but at WOT the timing was at 22? BTDC. This is miles off of the spec of 36? BTDC. I'll report back once I've adjusted the timing. I need to strap the boat to something to run WOT for the time it takes to adjust everything. My garbage can wasn't big enough to keep the water in at WOT for more than a few seconds.

 

[fishin98]

Please do not take this the WRONG way..it is not my intention...1st of all you have a narrow hull....takes more HP to put on plane and keep it there. You are a BIG person, which puts the boat very heavy on the stern, especially on a narrow hull . The current prop is probably a all around General purpose prop, getting 12mph out of it seems normal. Years ago I had a Smokercraft 14ft Alaskan with a 9.5 Evinrude...I didn't go much faster.

 

[Cintx]

Nothing to take the wrong way. I understand weight at the back and a narrow hull. No worries.

 

[fishin98]

Nothing to take the wrong way. I understand weight at the back and a narrow hull. No worries.

I just picked up a 1999 Merc 15hp 2str 6.5 pitch on a 3blade. I'm probably going to step up to a 9pitch later after I get my project back together

 

[Cintx]

Well, I couldn't figure out why the timing didn't work out. I was getting only 18? BTDC at WOT, which was very confusing.

I had the motor on the stand today so I played around with, adjusted and lubricated all the throttle and shift linkages. I started wondering if the linkage between the throttle cam and trigger was the wrong part. I disconnected the linkage and found that the cam could move all the way to the timing screw without issue.

I started moving the trigger manually and I found that there was a lot of resistance in its movement. I noted the complete lack of lubrication so I started trying to get some grease on it. I moved to the other side of the motor and that's when I noticed the wire from the trigger was not allowing it to move. Turns out that the mechanic who replaced the trigger had removed all the slack between the trigger sensor and the rubber clamp the holds the trigger wire then he zip-tied the wire to another wire. I cut that zip-tie and gave the wire enough slack so the trigger had full movement but not enough to be hit by the flywheel.

Now at WOT the timing cam does come in contact with the maximum timing stop screw. I got the boat on the water with my handy dandy timing light. Found that it was now at 38? BTDC so I adjusted the screw in to 36? BTDC. The engine is running like a top and great too.

Even better than all of that I got the boat up to 18.5 mph.

When the tach arrives, I'll get some RPM numbers but for now, I'm happy as a clam.

 

[steelespike]

The 15 is rated for wot of 5000-6000 rpm. Guessing 12% slip with a 6.5" prop 18.5 gets 6800 rpm.
The 6.5 prop is for a kicker setup or a very heavy boat. I think I would stay away from wot until you get a tach and 9" prop.

 

[fishin98]

The 15 is rated for wot of 5000-6000 rpm. Guessing 12% slip with a 6.5" prop 18.5 gets 6800 rpm.
The 6.5 prop is for a kicker setup or a very heavy boat. I think I would stay away from wot until you get a tach and 9" prop.

My 15hp came off the back of a sailboat...thus the 6.5 pitch, when I mount it on my 14 Alumacraft going to go with a 9p

 

[Cintx]

The 15 is rated for wot of 5000-6000 rpm. Guessing 12% slip with a 6.5" prop 18.5 gets 6800 rpm.
The 6.5 prop is for a kicker setup or a very heavy boat. I think I would stay away from wot until you get a tach and 9" prop.

There are two different prop questions here now. The 6.5" prop belongs to another fella who just posted here. Mine has the 8" pitch.