mercury 115 speed questions

[fishdog4449]

hello all,
Now that I have time to tweak since the motor is off the boat for transom rebuild, I have a couple speed questions.
From about 3500rpm (low to mid thirties, don't have an accurate GPS yet) to WOT, I don't seem to gain much speed. The motor spools on up to 5700-5800fine and the prop isn't spun, just doesn't seem to speed up much.
Setup is the '76 merc 1150 spinning a 19P quicksilver SS (cupped I think) on a small 17' Glassmaster closed bow.
I'm wondering if there is something wrong with my setup, maybe a performance prop would help me get more speed or do I need to raise the motor? Currently, cav plate is flush or a tad bit higher (less than 1/2 inch) than the bottom of the hull.
thanks guys!!!

 

[Chris1956]

Re: mercury 115 speed questions

Fish, Speed in that RPM range will depend on trim angle and type of hull. The intent is to lift the bow out of the water to create less drag and more speed.

My 16' speedboat with an inline six Merc (150 HP) would do 45MPH at WOT trimmed in. Trim it out and the speed was more like 65MPH. The drag of the boat in the water slowed me down.

I had a 16' Winner, sorta like your boat. it had a constant vee in the bottom from the bow to the stern. As the boat ent faster, it raised up in the water, causing ventilation in the turns at high speed. It was also very resistant to raising the bow under trim. It liked to ride flat in the water.

Do you have PTT on your motor? The prop pitch sounds good, and a high performance prop won't buy you much speed, i'll bet. If you do not have PTT, raise the motor up a 1/2" and try it. You will need to get a GPS to help dial it in.

 

[tashasdaddy]

Re: mercury 115 speed questions

change the tilt pin out one hole if you do not have trim & tilt. it will pick up the bow, you may loose just a little getting up to plane.

 

[Frank Acampora]

Re: mercury 115 speed questions

Typically with outboard engines, the carbs only open about 1/4 of the way while the timing advances all the way at 3/4 throttle. The last 1/4 throttle makes a lot more noise, uses a lot more fuel, and only goes a bit faster.

However, speed on the water is very deceiving. BEFORE you do anything, run it with the tach and GPS and compare the two. Without the hard numbers to compare, you really don't know where you are.

Think about this: turn the flywheel by hand while in forward gear. Then figure out the lower unit gear ratio by comparing flywheel turns to prop turns. Then, multiply your full throttle RPM by prop pitch, divide by 12, divide by lower unit gear ratio, and divide by 88. 88 is feet per second at 60 MPH and since you would divide by 60 to get RPS and multiply by 60 for MPH, they cancel, so you only need to plug in the 88. So you will get a THEORETICAL top speed. Multiply by about .90 (10% prop slip) and you should have a close approximation of your actual top speed.

Example: 5500 RPM divided by 2 (2 to 1 ratio) = 2750. X 19 pitch=55250 inches per minute. divide by 12=4354 feet per minute. divide by 88=49.5 MPH. X .90=44.5 MPH.

Even on a planing hull, as rpm rises, speed does not rise equally BUT while not linear, you would expect them to be fairly close. So compare your speed at say, 4000 and 5000 to see if there is a problem or not. So, for example: You would not expect a 20% increase in speed but, depending upon hull characteristics, 15% might be acceptable. Only a 5% increase would indicate your perception is correct.

If your GPS reading is in the same range, no changes are necessary. If the two are radically different, then your perception is correct.

 

[fishdog4449]

Re: mercury 115 speed questions

thanks for the help guys!
The motor does have PT&T, but my control is missing the trim up button (I can only trailer full up and full down). A new set of controls is in the budget soon so ill try trimming her out and see where that puts me. After I rebuild the transom, Ill see where the motor is an try to mount it with cav plate about 1/2" to 1" above the bottom of the hull.

Frank, hopefully I will get one more run in this season after I fix the transom, ill take a friends GPS and double check this whole thing.
Thanks again!

 

[Chris1956]

Re: mercury 115 speed questions

Hey Fish, On that motor there is no difference between trimming and tilting the motor. it is the same pump, cylinders etc. The only difference is that when trimming on a fully functoning system, the electricity flows thru a trim limit switch which opens at a preset point, stopping the motor. On your setup, you will likely ventilate before you would have reached the trim limit switch setting anyway. In other words, your system is fully functional as it is.

To trim your boat properly, accelerate to put the boat on plane. Hit the "trailer" button and the motor will trim out. Continue to trim out until the prop ventilates, or the hull starts to bounce, independently of the waves. This is called porpoising. Optimum trim in smooth water is just short of the ventilate or porpoise point. Greater speed allows more trim, which in turn gives greater speed.

Max trim at WOT gives max speed. This will likely not be the most ecomomical setting, however.

If you trim to near max at just above plaining speed, it is usually the most economical. In rough water trim in to cut the waves better.

 

[fishdog4449]

Re: mercury 115 speed questions

Thanks Chris,
I do tilt the motor up with the trailer button when beaching or going in shallow water, but I didn't know if it would work while underway. Sounds like I can save that 150+$ instead of buying new controls !!
Makes sense thought, the boat is very light and small to begin with, but she is a very well built closed bow and my only seats are towards the front so I guess she does ride fairly bow down.
I hadn't worried about trim much because mostly I've used the motor this summer has been tubing and kneeboarding at low speed. As it gets colder my usage changes from watersports to fishing and cruising; thats when I try to tweak my speed issues.

 

[jay_merrill]

Re: mercury 115 speed questions

You also have to remember that the relationship between power and speed is not linear. The faster you go, the larger the increase in power required to do so.