Prop Question

[kart74]

I have a 1983 Mercruiser 3.0l with the Alpha1 gen1 drive on a 17ft Sea Sprite ski boat. The prop that is on it right now is a 15x15 and is pretty beat up. I just got this boat and took it out yesterday. It seems like it comes out of the hole pretty good and planes out quickly. The max rpm at WOT was 3800-400 according to the on board tach and topped out at 32.5 mph on gps. Does this seem correct for that engine RPM wise? I am looking to replace the prop and if going to a different pitch would be better then I would do that. I am mainly going to be using it for skiing and tubing. Thanks!

 

[UncleWillie]

Re: Prop Question

That sounds a little slow and the RPMs are low. Should be 4500-4800.
It could be the Prop, an Engine issue, Wrong Drive Ratio, Water in Foam, etc. The fun begins!
How beat up is the prop? Pictures? hoto:

 

[eavega]

Re: Prop Question

You might want to post the serial number on the engine/outdrive to get a more accurate answer, but for my 1988 Mercruiser 3.0l, the recommended WOT for that motor is 4200-4600 RPM. Your WOT RPM readings are a bit low. Definitely could be due to a bad prop, but I would think that you are under-propped at 15P and your engine would be revving much higher than your indicated 3800 - 4000 RPM. You may want to start with verifying the motor is running correctly (compression, spark, fuel) and that your boat is not waterlogged or otherwise carrying any additional weight or drag. I believe the "standard" prop for that engine on a small runabout is 19P. From there its about getting your rig to run in its correct WOT range. Stepping up the pitch will decrease RPM, stepping it down will increase it. For watersports you may want to consider going to a 4-blade prop which should get you out of the hole better (while sacrificing top-end speed).

After you verify that your motor is running correctly and that your boat is shipshape, I would work with a prop shop to find a good match for your setup. You may pay a bit more over ordering something online, but usually they will let you try out different props until you find one that works to your satisfaction.

Rgds

Eric

 

[kart74]

Re: Prop Question

Thanks for the replies! So I bought this boat knowing that it had some engine issues. It had low compression (90lbs) on 3 of the cylinders. I pulled the head and had the valves done. I am now showing 120 on 3 and 110 on one which I think should be ok. I put all new ignition parts in and cleaned the carb. One thing I need to adjust is the mixture screws. PO had them at 3.5 turns out and it didn't want to idle real smooth. I don't think that should change the WOT mixture though. The prop is bad. the tips of all blades are broke off and small chuncks missing all over. I will take a pic later. I think the 15 diameter must be close to the max for that drive. Doesn't seem like a larger one would fit. How do I find out if the foam has water in it?

 

[JimS123]

Re: Prop Question

Your rpms and speed are seriously low. With that pitch I would expect you to over-rev, not under-rev. My guess would be that a 19" P prop would be what you need if the engine were running Ok and the boat wasn't waterlogged.

 

[UncleWillie]

Re: Prop Question

The recommendations is that the engine would use a Prop with a 19" Pitch, Not a 19" diameter. A 14x19 Prop.
Waterlogged Foam will make the boat very heavy.
You will need to weight it and compare it to the specs. Boat+Trailer = 2500# ???

A 15P prop with missing Blade Parts would cause the boat to be Slow, but the RPMs to be High.

The correct Alpha 1 Drive ratio should be a 2.00:1 for a 3.0L engine. What Drive Ratio is yours?

 

[kart74]

Re: Prop Question

Ok that all makes sense. There was 2 adults and 2 kids in the boat when I got these numbers. There is no other way to tell if it is water logged other than weigh it? I don't have any water in the bilge... Im sorry I don't know what the drive ratio is. How can I figure it out? It seems like the engine is running good other than the rough idle which I sure is because of the mixture screws. I also didn't play with the trim at all while doing this so all of these number are with the drive all the way down. I think trimming it up some would give me more speed and rpm right? The boat is a sea sprite sea raven 780XL. I have tried to find info on the model but haven't came up with much. I can get a pic of the prop and boat tonight.

 

[eavega]

Re: Prop Question

There is no other way to tell if it is water logged other than weigh it?

If its waterlogged, it will sit lower in the water than it normally should, but you would need to have a "reference boat" to verify that. Another way would be to drill several inspection holes in the deck with a drill. Check for wet shavings or wet foam coming back with the bit. re-seal the holes when the inspection is completed.

Rgds

Eric

 

[kart74]

Re: Prop Question

here are a couple pictures of the prop and one of the boat. It does sit pretty high in the water, but I will try to get to the foam and see if it has water in it. Being a 15" diameter does that possibly have anything to do with the lower rpms? Seems like the largest diameter for that drive... Let me know. Thanks for all the help.

 

[Mi duckdown]

Re: Prop Question

Prop needs to be repaired.

 

[krakatoa]

Re: Prop Question

Prop needs to be repaired.

correction....., he needs a new prop!

 

[jimbo_jwc]

Re: Prop Question

Yep prop is shot mine came w 3 blade/14.25x17 its 3.0L 130HP 1987 and tachs at 5000rpm WOT .I have Solo's 4 blade 18" still 5000rpm and over winter bought MERCURY 3 blade 14.25x19 Alum and Stainless 3 blade 14X19 with big square holes and can't wait to try.

 

[steelespike]

Re: Prop Question

Typical 17 ft I/O setup for speed only would usually have a 21" pitch and wot speed about 40. the 21" pitch would tend to be a little sluggish out of the hole and not great for skiing. Typically if you were going to ski with a boat load you would run a 17" prop and have to be careful not to over rev with a light load.
Hopefully your motor issue will be something simple.Throttle not opening all the way, timing set wrong and not advancing as it should.
Perhaps one cylinder not firing.

 

[kart74]

Re: Prop Question

so should I check everything over, fix the carb, check timing and try it again as is to see if anything changes? Or should I be looking at getting a 14-17 prop right now? I went out and did some looking tonight and as far as I can tell the only places there could be foam is on the outsides on the hull in the wide area. There is like a box made of fiberglass that runs the length of the boat. I am guessing this is what I would need to drill into to check the foam? It is completely sealed in this fiberglass box. If it was wet how would you even get it out? cut the whole thing open?

 

[Maclin]

Re: Prop Question

In my opinion, any performance numbers you get with that prop will not be a good baseline to work from. I would get that prop re-furbished or get a new one just like it and run it. Record WOT RPM's and speed, and that will be your baseline to see if any change in pitch is needed. Go out with moderate load, half tank of fuel, etc.

 

[Mi duckdown]

Re: Prop Question

What Maclin posted. rebuild prop+ good tach and GPS speed. A spare prop is always a good idea, no matter what.??

 

[steelespike]

Re: Prop Question

If you had saturated foam it would be slow out of the hole and would tend to drop off plane easily.
You may be able to use a hole saw to get a peek into that "box"Just drill into the top and draw out some foam
or just feel around with your fingers.
I would start with another compression check with a different gauge check compression then put a couple of squirts of oil in each cylinder and check.
Note the condition of the plugs as you remove them.
Check and set the timing and be sure it advances as it should.Be sure the throttle opens all the way.

 

[kart74]

Re: Prop Question

ok so I had a little time tonight to check a few things. I cut a hole in the rear of one of the "boxes". I figured that would be where water would be the worst if there was any. I used a pip and pulled out a core sample. It was about 6 inches deep and was able to pull it out. The foam wasn't heavy feeling and didn't feel wet. If I squeezed it as hard as I could I did notice some wetness that came out of it then. Looking down the hole it made I used a screw driver to remove the bottom later of foam to get to the fiberglass. At the very bottom there was a bit of wetness. I cant imagine that would be considered waterlogged but I guess I don't know. What do you think? I also checked the throttle and it is opening all the way. Tomorrow hopefully I can check the timing and do another compression test. Any thoughts are welcome. Thanks!

 

[airshot]

Re: Prop Question

Your compression sounds low even after the head rebuild. I have a 3.0 140 hp that is a 1983, my compression avgs from 135 to 150.
I run a 14 x 19 pitch prop at 4450 rpm at do 34 mph by GPS. I have a 22' aluminum boat. Sounds like your motor needs some work, and a new prop, if the motor were running correctly that 15 pitch would definitely over rev. Get the model/serial number off the drive and post it so one of the good folks on here can tell if you have the correct drive for your engine.

 

[kart74]

Re: Prop Question

ok so I had a little time tonight to check a few things. I cut a hole in the rear of one of the "boxes". I figured that would be where water would be the worst if there was any. I used a pip and pulled out a core sample. It was about 6 inches deep and was able to pull it out. The foam wasn't heavy feeling and didn't feel wet. If I squeezed it as hard as I could I did notice some wetness that came out of it then. Looking down the hole it made I used a screw driver to remove the bottom later of foam to get to the fiberglass. At the very bottom there was a bit of wetness. I cant imagine that would be considered waterlogged but I guess I don't know. What do you think? I also checked the throttle and it is opening all the way. Tomorrow hopefully I can check the timing and do another compression test. Any thoughts are welcome. Thanks!