72 merc 115 mph

[d.shoe]

i have a 76 18ft baja (1500lbs) with a 72 115hp merc. compression was 110-115 cold, fresh out of storage. Running 93 gas and qiucksilver prem. oil. at 4600 rpms my gps states im doing 30-31mph this doesnt seem right. does anyone have any rough idea of what i should be going? Is it spent? prop is 19pitch pretty sure.

 

[monk-monk]

Re: 72 merc 115 mph

Well, first off, i can't help you...i too have a '72 1150 but mine is on a 1972 Glastron v-166 Bowrider...my new speedo reads about the same as what you are saying...BUT i know im going faster than that...i don't have GPS and boat speedometer set-ups are very questionable to me...So im very interested in the replies to your thread...But now i have to poke a little fun at your subject title...72 merc 115 (mph)...NOW THAT GOT MY ATTENTION...LOL..!! I was like...OH, I GOT TO READ THIS..115 MPH...?...Good question though...How can we really determine the correct speed...??

 

[Skiwi]

Re: 72 merc 115 mph

The GPS's I've had seemed more accurate than most of the speedometers I've owned, so I got a police friend to check, and it was, so, yes the speed the GPS is showing is probably correct.

It is difficult to know if 30mph is the "right" speed for that combo unless someone here has an identical set-up.

A clue to if things are running right would be to compare what speed it was doing with that motor/rpm/prop/weight before you stored it?

To give you another comparison, my 16'7" Ski-Bite with a 75hp Chrysler gets 40kph at 4000rpm and 50kph @ 5000

 

[trendsetter240]

Re: 72 merc 115 mph

i have a 76 18ft baja (1500lbs) with a 72 115hp merc. compression was 110-115 cold, fresh out of storage. Running 93 gas and qiucksilver prem. oil. at 4600 rpms my gps states im doing 30-31mph this doesnt seem right. does anyone have any rough idea of what i should be going? Is it spent? prop is 19pitch pretty sure.

Is the 4600rpm at full throttle? If so you need a lower pitch prop to bring it up to 5000RPM. Right now, with a 19p prop at 4600rpm you should be able to reach 35-37 mph at 4600rpm.

If you drop to a 17p solas amita prop you will gain a few hundred RPM and will likely see a speed improvement as prop slip goes down.

 

[d.shoe]

Re: 72 merc 115 mph

thanks i will have to look into a new prop. will i benefit from getting a stainless? i may try to buy a newer one and put this on my pontoon. My baja is stampted for 150hp max. The transom is solid. Is it safe to put a bigger one than that on like a 175? i know it seems like alot but i really hate getting passed by similiar boats. oh and yes it is wot.

 

[J Hartman]

Re: 72 merc 115 mph

thanks i will have to look into a new prop. will i benefit from getting a stainless? i may try to buy a newer one and put this on my pontoon. My baja is stampted for 150hp max. The transom is solid. Is it safe to put a bigger one than that on like a 175? i know it seems like alot but i really hate getting passed by similiar boats. oh and yes it is wot.

You've opened up a can of worms now... IMO i feel that a little overpowering is ok... some people think the max rating is the max rating, but, i've never had anything with "too much motor."

Your engine is too big if you smash the throttle and the engine passes the boat.

 

[jerryjerry05]

Re: 72 merc 115 mph

25 more horses won't make that much difference.Neither will a new prop.
If your boat is rated for 150 then a 175 will get you a ticket.
You need to determine if your motor is operating correctly.
Check the compression.Then a spark test.Then check the fuel system. Ethanol is messing with a lot of motors.Check your pump and carbs for clogs.
That motor at WOT should do about 5300-5500rpm.Overloading the boat can hurt.
Do the tests and go for a ride and check the WOT and RPMs.Then think about another prop.
Post your results.J

 

[Chris1956]

Re: 72 merc 115 mph

Do you have PTT on that motor? 31MPH is a bit slower than I expect, but w/o PTT it is hard to maximize speed.

If you do not have PTT, move the trim rod to the third hole from the transom. Retest, and continue to trim the motor up. If she ventilates move it back down, unti it stops ventilating.

You also may be able to raise the motor up on the transom for additional performance.

 

[d.shoe]

Re: 72 merc 115 mph

Thanks for the info I'm gonna check everything and will let every one know

 

[Dave1027]

Re: 72 merc 115 mph

There must be something wrong like maybe a banged up prop? I ran your numbers through the calculator and it came to a 25% prop slip. My boat combo is at 6%. You also may want to check that you have the motor mounted at the correct height.

 

[jimmbo]

Re: 72 merc 115 mph

A 1972 115hp has a 1.78 : 1 gear ratio, the ratio was changed to 2 : 1 in 1973. 4600 rpm with this gear ratio, 19 in prop 10% slippage should be around 41 - 42 mph. With 2:1 gearing.. about 37 mph

 

[starcraft1982]

Re: 72 merc 115 mph

Although I dont have one,I thought sure those motors liked to rev.Even 6000rpms.Anyway,an 18ft+ fiberglass boat is a lot for a 115.Especially if that isnt prop rated 115(not sure on that).But only turning 4600,that really doesnt seem far off to me.JMO.As for the poster talking about a 175 on a 150 rated boat.I will say two things.1,its illegal.2, I just got inspected on saturday.First time ever.I passed.but that WAS checked.Have it right on the copy of the inspection sheet I was given.Not worth it hp wise,money wise,or any other wise.

 

[jimmbo]

Re: 72 merc 115 mph

I know here in Canada it is not illegal to exceed the horsepower rating on a boat. Hence I have seen a Glastron GT 150, rated for 90hp, with a 225 mercury bolted on it.

Myself, I had a prop rated 115 on a 15 ft, earlier the same boat had had a crank rated 140 on it.

 

[d.shoe]

Re: 72 merc 115 mph

i messed around with the ptt and got it to ride good at around 4900rpms and hit 35mph. dont seem like i can get much more out of it. the motor can go any lower, and the prop doesnt look bad, but will have to check it when i pull it out this week. I may just try to find a newer 150 and put this on my pontoon boat. Thanks to everyone that replied thats why im on here you all were alot of help