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1982 70hp prop size and brand

A

Andy_1602

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I have the 70hp Mercury running that is on a 1982 15' Skeeter SS-1 Sport fisherman. It has a Mercury 15" pitch Stainless Steel cupped prop. With two people in boat it gets on plane but slow. It is on a manual jack plate setting 4-1/2" from bottom of hull to top of anti-cavitation plate and 6" off back of boat. Prop does slip right when boat drops to plane so I dropped engine 1" for starters. I can't trim to high because prop was slipping with just a little trim. I have not been back on water yet but I am looking at buying a prop with less pitch for water sports and I did buy a hydofoil for it. I have a prop adapter that has 13 splines with two dimples on it so I can use lock nut setup I have now. I was told that a 40-60 small gear case Yamaha prop would work by a store that rebuilds props. They made the adapter for me. They don't carry older props for these engine because they can be bought on craigslist and ebay cheaper than they can repair them so as I was told. Would a 12",11" pitch or 10" pitch prop work for me?
Thanks, Andy 1982 Skeeter bass boat.jpg
 
Off the top of my head, 15 pitch sounds about right on your set-up.

What doesn't sound right is the distance between the cav plate and the bottom of the hull.

"Normally" you will mount with the cav plate on the same plane (to maybe 1/2" high) of the keel line.

Since you are running a 6" jack you could raise it an extra inch - so would expect that the cav should be no more than 1 1/2" above a horizontal line projected from the keel out to where the motor is sitting.

If you are 4 1/2" from the horizontal plane it's no wonder she's slipping - you are running way too high.

I just checked Merc's "extremes of known successful outboard mounting height dimensions" and they are as follows:

If your top speed is 40 mph - 1/2" above keel
If your top speed is 50 mph - 1 1/4" above keel
If your top speed is 60 mph - 2 1/2" above the keel.

So even with the extra inch the jack allows you, you are still out of whack.

Adjust your motor and give her another try. Then determine "what prop" based on your wide open rpms. A 70 Merc should top out at 5250 rpms (and probably 200-300 less at this point in the summer).

So if you are getting her up to 5000 or so, your hole shot is not a fault of the prop.

If you can't make 5000, then maybe you do need less pitch. Every 1" decrease in pitch will increase your rpms by about 200 (and going from cupped to normal will add about another 50 rpms) - the opposite is true if you find you can hit 5000 and still have some throttle left....
 
I will have to agree with you. I think it is to high. My tack needle broke off the other day so I am going to put on of those tiny tachs on that a coil of wire goes around the spark plug to get a reading. But before tach broke after boat planed out I seen over 6,000rpm's WOT because of prop ventilation running about 44mph on gps 2 people full gas tank and gear. Pull back to 3/4 throttle run's mid 30's at 4,300 rpms which is fine with me. The Skeeter 15' bass boat has a fast hull design but has a heavy stern so planes slow. I dropped the engine 1-1/2" and going to test tomorrow. Water pressure gauge did read over 15psi so I never had problem with cooling engine. I only paid $500.00 for the boat, Engine and Trailer and I have a little over $800.00 and a lot of work in it now. All I need is a fish/depth finder and I will be set.
Thank,s I will keep you posted, Andy 483161_10150899108365881_798376233_n.jpg
 
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Since I run those triples exclusively, let me warn you NOT to get anywhere near 6,000 rpms! THey have these long, floppy reeds that will break off and blow the power head up. Trust me on this. Red line on them is 5,800 or kapow!

On the TIny Tach, they work okay but there's a trick to it: Pick one ignition wire at a spot away from the others, then wrap the other wires in the vicinity with aluminum foil. Use a good, thick layer of it. If not, the tach will see various inputs and get 'confused'.

Jeff
 
Jeff I know this is to many rpm's and pull back on throttle as soon as I seen it up that far. That is why I need to replace the tach. It was a Fariss brand I think. Needle broke where it sticks on shaft. You and others here help me get the engine running don't want to rune it now. By they way cold starting this engine with the fuel enrichment valve works as follows on my engine. Prime fuel bulb tight, push center button to advance fast idle starting to the first hard spot. This moves the spark advance but not the butterfly's in carbs. Push in choke button until engine starts. Then I can move throttle up to make it run faster if needed. So the throttle plates/butterfly's act as choke plates. That works great for my engine. Bypassed fuel pump like you told me. Water pressure went up to 10lbs at idle and water stream is stronger.
Engine runs great just need it to get to plane faster. Skeeter bass boats are stern heavy so I bought a dol-fin hydrofoil real cheap ($12.50)to try. I may go with automatic trim tabs when I save up a little money. I know they work better. Other than that boat preforms great and everything works.
Thank,s Andy
 
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Well have not tested hydrofoil yet but I know how to start the engine now with no problem. Dropping to engine on jack plate 1 -1/2 inches helped a bunch as far as the rpm's go and getting on plane. I have not installed the tach but I have no prop slip so engine does not rev high anymore. I ran it about 3/4 throttle for about 10 miles and used 1.9 gallons of gas with a lot of stopping and starting. Not to bad. Still takes 30 seconds to get to plane so I think it being stern heavy the hydrofoil may help.
I found if I pumped the prime bulb up tight and pushed button in on handle to cold start but not far enough to open throttle plates the so call enrichment valve will work as a choke. This engine is very cold natured so I had to turn out idle jets needles to about one and half turns to run a little rich. That helped with the engine trying to die when throttled up slow also. I have no interfered gun to test plugs so I did the best I could. I set the idle speed in gear as slow as I could get it without dieing. I will use tach to set when I get it installed.
Thanks very much for the help. I think the engine is fine now with your help.
Thanks, Andy
 
You have to hold choke / enricher button "on " while cranking.---And throttle needs to be advanced for cold starts.---------No such thing as a " cold natured outboard " they all start easy when in good condition and with the correct starting procedure.---------Many folks are used to the cars of today where you just " turn the key "--An outboard requires some knowledge of how it all works and then they are easy to start !
 
I just wanted to add a note do not disconnect the fuel lines or let engine run dry of gas to carbs between visits to lake. I makes it a pain in the rear to restart. My fuel tank has eight feet of fuel line because the tank is on the port side of boat (74 inches wide at stern)and I don't want to make a hole in the port side for fuel line. I may do without disconnect and use seafoam if its is going to be a while to get to lake or winter storage.
Thanks, Andy
 
I fail to understand why that should be a problem! All you do is pump the bulb until the carbs fill and fire it up. Done all the time.

Jeff
 
My be just me. I have broke my back in 7 places at work and have Rheumatoid arthritis in my hands real bad and in my elbows and back so it makes it better for me to leave connected. That is why I sold my Bass Tracker 6 years ago. Sold my guns,RV but still have a 2000 1600cc Yamaha Road Star that I had to put automotive type cruise control on it to ride it. I have two son's that where in the military and the oldest got out in March. He was renting a boat ($60.00 a day for a john boat)to go fishing so I bought this boat for us to go out on. Not much money invested and I still have my bass tournament gear. I can't fish to long it is about being with my son. It still works better for me to not disconnect the fuel line. And can you tell me a way to remove the fuel line connector from the fuel tank. I can't get mine out very easy. It has the old Merc connector twist and pull.
Sorry it is hard for me to work with my hands sometimes, Thanks, Andy
 
Here's what you can do: Since I store my boat on a 45 degree angle, hanging sideways from davits on my big boat, I have to drain the carbs or gas gets in the water. Since I too don't want to mess with the hose every time, I installed a ball valve in the line--just reach over and turn the fuel off.

Jeff

PS: I used a 1/4 inch fully ported ball valve.
 
Just wanted to post an update. The hydrofoil I put on engine works great. Boat planes in a few seconds now. I have never used a hydrofoil but I would recommend to anyone now. I don't need to change props. Engine runs great. I have not hooked up tach yet so I will post rpm's when I do.
Thanks for all the help, Andy
 
I just wanted to post an update on the engine props and rpm's. Jeff I took your advise and bought a prop on ebay ($17.50)that had a small piece out of one blade and trimmed it down about 1/2 inch on all three blade. The prop was a 15 pitch the same has the SS prop I have on there now. It works perfect boat jumps out of the hole fast but wot is about 5400 rpms. So I am going to use it for water sports. The Stainless Steel cupped prop I have on there now runs at 5000 rpms wot which is great. There is only about 4 miles per hour between the two and I am in no hurry so about 3/4 throttle I can run right at 32 mph + with no problem. About 42 mph wot trimmed out but I don't like to run the engine that hard. So I am content on the performance of the 70hp Mercury.
Thanks for the help, Andy
 
Andy:

Assuming your tach is accurate, you can run all day at 5,400 rpms without damaging your motor.

Glad to hear it's working out so well!


Jeff
 
Thanks Jeff engine is a little cold natured. Those carbs have to be set a little rich on the fuel mix screw other than that engine runs great. I know 5400rpms won't hurt it but I can save a lot of fuel buy pulling the throttle back a little. The tach is a Faria NOS in box and I bought the speedometer to match it on Ebay for $34.00. They are 4" square but fits in the round holes.
Thanks a bunch for the help, Andy
 
Sounds like you're all set.

One thing I STRONGLY recommend for keeping triples going: Plug the water hose going to the fuel pump (which is unnecessary). A few Mercs did this for ??? dopey reason, but triples need all the cooling water they can get (since the impeller is a bit undersized).

This hose comes from the lower, center, right side of the adapter just below the power head to the fuel pump. Remove the hose, plug it, then the fitting, and thread ina 1/8 " pipe plug. Trust me: ths is important!

Jeff

Note: Later triples already have this hose removed, and some dealers did it as well. Yours might be one of them.
 
Jeff I did that when I first got it. You told me that back a few post earlier about bypassing the fuel pump water cooling outlets. So I think engine is set also.
Thanks a bunch, Andy
 
Ah-hah! Maybe THAT'S why she's running so well!

Jeff (AKA Mr Conceited)

PS: Merc went to a slightly richer main jet in the top carb in the later motors to reduce top cylinder failures: a 78.5 jet. They retained the 74 in the lower carb. I run this combo on mine and it needs it--the stock jetting bogs slightly when I slam the throttle open.
 
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A few Mercs did this for ??? dopey reason,
Click to expand...
This was done to cool fuel and help prevent vapor lock and there is a Mercury SB on it from the early 1980's, I always leave this in place but just restrict it by installing a yellow butt connector in the line.
 
I've never run that cooling line and have yet to fry a top piston--a common occurence with these motors. (Of the dozen or so to come through my shop, only one was okay.) It IS possible that one could experience vapor lock--I'll give you that (and I DID have a weird, similar problem once)--but keeping that top cylinder from burning up is much more important!

Jeff
 
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My 70 hp runs cool since there is no thermostat in the engine. Jeff I should have checked if the top carb has a larger main jet. Mine does fine as long as you don't just punch it. If I move throttle up slow it does fine. I may try a lager main in the top carb. If I want to punch it I just tap the choke on the switch and then punch it. That would work for water skiing or being. When I go to the prop that I cut down pre your instruction it does a little better. I am surprised they never put a secondary stage in the carbs. I have done a bunch of motorcycle carb conversions. Mostly two carbs to one and t4 carbs to two. I few 4 cylinder to one carb. I am sure it would work on these outboards as well if there was space to make the intake conversion if under the case. Ha a carb is a carb and they have one function get fuel to engine. It would be nice if those carbs had a spray jet on them it would take care of that problem. I'll try the larger jet in the top first.
Thanks, Andy
 
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