1980 Merc 115 WOT RPM? And speed estimate?

Hello all,
Just got my tachometer installed on my merc 115hp 1980 OB and took it for a test run today. Put a new 19 pitch three blade prop on as well.

First question what is the RPM at WOT for the tower of power 115hp?

Next question I only get max 36 MPH with this motor on a 15ft fiberglass tri-hull, I tried all the different trim angles I could get and it was a relatively calm day. I'm thinking this MPH is way too low, am I wrong to assume this? If so what should I be looking for? Funny noises? Loss of rpm at WOT?

It has a stingray fin on it, maybe that is affecting top end? It jumps out of the water, top end is way low.

Everything is running well and sounding good.

Thanks for the help.

FL

Re: 1980 Merc 115 WOT RPM? And speed estimate?

From what I am reading the motor should run between 5500-5700 rpm at wot. I remember on my dads boat he put that fin on and we hated it because it dropped speed and he didn't like the way it made the boat handle. It was a 200 merc though so not sure if it would be that big of a difference from the 115.

I have the same motor on my 17' witchcraft and I am reading with a 19p prop it will do around 48mph. For a tri hull I would think you should be in the 40mph range.

Sorry if I am not much of a help I am still new to these outboards and still learning about the 115 I have also

Re: 1980 Merc 115 WOT RPM? And speed estimate?

I had a heavy 18' tri hull with a 125 johnny and ran 39 with a prop that I used for water skiing. I think it was a 17P. Could have put the numbers in the 40's with more pitch but no need to. Had a 115 Merc a few years later than yours on a 17' mono hull pad boat and it ran in the high 40's and 6000 rpm with a 19p which corresponds to Mr. AD's numbers.

Seems your numbers are a bit low.

Mark

Re: 1980 Merc 115 WOT RPM? And speed estimate?

GPS or water pressure gauge ?? Lose the fin. I am thinking you may be over pitched,what was on it before and how did it go then? What does the boat weigh? is it waterlogged ?You should be close to 6 grand on the tach,if you have one, which would be great help in this case.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Numbers from the book for 1980 115:

Models 115 & 1150 5000-5500

Gerry

Re: 1980 Merc 115 WOT RPM? And speed estimate?

A.D said:

From what I am reading the motor should run between 5500-5700 rpm at wot. I remember on my dads boat he put that fin on and we hated it because it dropped speed and he didn't like the way it made the boat handle. It was a 200 merc though so not sure if it would be that big of a difference from the 115.

I have the same motor on my 17' witchcraft and I am reading with a 19p prop it will do around 48mph. For a tri hull I would think you should be in the 40mph range.

Sorry if I am not much of a help I am still new to these outboards and still learning about the 115 I have also

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That does help, I am thinking the fin could lose me a little top end too.

Texasmark said:

I had a heavy 18' tri hull with a 125 johnny and ran 39 with a prop that I used for water skiing. I think it was a 17P. Could have put the numbers in the 40's with more pitch but no need to. Had a 115 Merc a few years later than yours on a 17' mono hull pad boat and it ran in the high 40's and 6000 rpm with a 19p which corresponds to Mr. AD's numbers.

Seems your numbers are a bit low.

Mark

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Another reason I think something might be up.

Mr 88 said:

GPS or water pressure gauge ?? Lose the fin. I am thinking you may be over pitched,what was on it before and how did it go then? What does the boat weigh? is it waterlogged ?You should be close to 6 grand on the tach,if you have one, which would be great help in this case.

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GPS MPH from my Humminbird 365i, I trust it mostly. I think the fin has to go as well, hopefully I can get it out this week and find out.

Before it had a prop that I don't know what the pitch was, there is no marking on the hub and nothing but what the boat shop mechanic wrote on it (90 HP mercury) I do remember the motor sounding like it was working really hard with that prop on it. With the new one everything sounded really good. The new 19 pitch has a much more aggressive look than the old one, the old one looked like it was a crusing prop. I wish I had the pitch but there is nothing on it. The boat went about 5 mph slower with the old prop at WOT.

I would estimate the boat weighs around 2100 lbs or so with boat, motor, people and gear, maybe a slight over estimate.

Pulled the plug after 10 hours of boating and got about 2 gallons out so I don't think it is waterlogged when I launch.

I installed the tach before I took it out, around 4000 rpm it would hit 28 mph or so and then at 4500 it would go to 31ish, at 5000+ it would top out at 36 and that was it. The motor also would only hit about 5300-5400 rpm's at full throttle.

carholme said:

Numbers from the book for 1980 115:

Models 115 & 1150 5000-5500

Gerry

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Thanks Gerry, I appreciate it at least I know to keep it around 5200 without worrying about blowing the thing up.



I think with all the responses I'm getting the first step is to take the fin off, after that I think maybe carb clean? Not to sure, it's a rough starting motor, but i've heard all the old towers were pretty hard to start.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Hey,
I have no answers but am usen the same engine, my tach thinks 3000 is zero but maxes out at 5500 at wot. It pushes my 1975 16' American to 43mph (phone gps)with an aluminum 19 pitch prop. I've had 3 other adults in and it doesn't seem to effect top speed or wot rpms.

Good luck!

Maybe check that the carb flaps are opened all the way with full throttle?

Re: 1980 Merc 115 WOT RPM? And speed estimate?

I have a 1979 1/2 merc 115, no distb. I get 35mph at 5200rpm with a 15 pitch 4 blade on a very heavy (3600lb) 20' deep V hull. There is something not right with your setup. You should get a lot more than you are with that little boat. By waterlogged he meant the flotation foam in the boat, not loose water. I would get that rig weighed and see what your weight really is.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Joeshow said:

Hey,
I have no answers but am usen the same engine, my tach thinks 3000 is zero but maxes out at 5500 at wot. It pushes my 1975 16' American to 43mph (phone gps)with an aluminum 19 pitch prop. I've had 3 other adults in and it doesn't seem to effect top speed or wot rpms.

Good luck!

Maybe check that the carb flaps are opened all the way with full throttle?

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Thanks Joe, I'll check on the carbs, i'm no expert on the tachometer situation but I just got mine installed. I just ran another wire from the rectifier (grey wire) in the motor case to the tach and hard wired the other two tach wires to the battery, any other tach should work in case yours is blown. I have read that a tachometer is bad if it does not rest at around 800-1000 and zero out when the motor starts, you could probably just switch the tach out on the wires you already have (maybe I dunno). I hope that helps.

Georgesalmon said:

I have a 1979 1/2 merc 115, no distb. I get 35mph at 5200rpm with a 15 pitch 4 blade on a very heavy (3600lb) 20' deep V hull. There is something not right with your setup. You should get a lot more than you are with that little boat. By waterlogged he meant the flotation foam in the boat, not loose water. I would get that rig weighed and see what your weight really is.

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As always, thanks GS for the reply. I wish I could tell the condition of the foam in the hull, but the floor is pretty solid and there is a good carpeting job done on the boat so I don't know how I would be able to tell. The specs for the hull suggest that it only weights 950 lbs but I don't know it could be way over that. I will try and get it weighed the next time I have a chance, maybe i'll swing but the recycling center or something next time I am near by.

Now that I know a 20' will get 35 mph I'm even more concerned.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

You said you drained about two gallons of water after 10 hours of boating. Unless it's raining /rough out and it's splashing in or your water sporting that's way to much water to be taking in. If you have a foam in the hull it's waterlogged by now.How does the hull look ? Any apparent cracks or poor looking caulking?

Re: 1980 Merc 115 WOT RPM? And speed estimate?

mr 88 said:

You said you drained about two gallons of water after 10 hours of boating. Unless it's raining /rough out and it's splashing in or your water sporting that's way to much water to be taking in. If you have a foam in the hull it's waterlogged by now.How does the hull look ? Any apparent cracks or poor looking caulking?

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I think that might have been a bit of an exaggeration, I have not actually measured the water than comes out each trip. I am going to take it out this weekend and i'll try to measure it this time.

When I bought it 2 weeks ago I did not see anything that I would consider other than surface scratches. I will try and examine them a little harder. I don't think the boat is a crazy weight, it's heavy but it's not crazy heavy. I can push it up my driveway by hand on the trailer with a 3% grade or so. I wish I had a weight on it to know better.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

My friend has a 83 115 on a 17 ft Bayliner and it goes 52 MPH at 5500 RPM with a fresh wax job in cool water.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

How are the compression numbers ??

Re: 1980 Merc 115 WOT RPM? And speed estimate?

I would think that 15 footer should hit the high 40s with that amount of power. I ran one on my 16 foot speedboat and would get ~60MPH. That was a high performance hull, however.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

mr 88 said:

How are the compression numbers ??

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Compression looks pretty good just checked it. Cyl 1 = 120, Cyl 2 = 120, Cyl 3= 120, Cyl 4= 115ish, Cyl 5= 120 and Cyl 6= 120. 6 was a huge pain in the arse to check.

I'm not too sure what is going on but I am leaning towards the water logged suggestion. I'm going to switch out the plugs today and run it the next few days hopefully that will do something.

Thanks for all the suggestions folks I really appreciate it.

Elliott

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Plugs probably will not do anything unless it's missing and you would feel that.When you floor her is the carb opening up all the way ? Compression is average for year of motor so you can eliminate that as a your issue.I would think the boat motor and trailer would be in the 1500 lb range.Motor 300 ish ,trailer 300- 400 depending on construction, boat 900 lbs. Maybe someone can chime in with more exact numbers.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Dump the whale-tail. With that power, you will not need it. Drag can really slow you down.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Plugs helped with the rough start, now just pressing the ball hard will get it started easily in one or two choked cranks. I took the stingray fin off and it made the boat rock up and down (cavitation?) pretty hard with the same speed 36ish. Did not help top end very much. I might try and move the pin down so it is more of a full tilt and see if that helps push the bow down and get the stern up. New plugs in it and the WOT is at 4700ish now. I wonder if I don't have all cylinders firing. I can't really see the carbs well but I will look again to see if they are opening all the way up.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

if the boat rocks up and down, it is likely porpoising, from over trim. Trimming down will eliminate that bounce, but could cost you some speed. Are you using a GPS to measure speed? Check spark to all cylinders. Check condition of spark plugs. Any that look like new may indicate water infiltration.

Re: 1980 Merc 115 WOT RPM? And speed estimate?

Chris1956 said:

if the boat rocks up and down, it is likely porpoising, from over trim. Trimming down will eliminate that bounce, but could cost you some speed. Are you using a GPS to measure speed? Check spark to all cylinders. Check condition of spark plugs. Any that look like new may indicate water infiltration.

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Thanks Chris for the reply, I will try to trim down, I will have to move the trim rod down it is all the way trimmed at the moment. I replaced the plugs before my last trip, all of them looked a little bit fowled. I still have to check the spark to the cylinders, if not would it be the wires or the coils? Probably coils would be my guess.

I am measuring speed with a GPS.

Elliott