Mercury 115 hp power

[RAR45110]

Gentlemen,<br /><br />I have a 1997 suntracker 21 foot pontoon boat. The horsepower rating is 90. I have read a lot about the old inline 6, 115 hp motors and their power head versus propeller hp ratings. I just purchased a 1989 Mercury inline 6, 115 hp motor and would like to put it on the pontoon boat. I realize that I may need to change the stickers to 90 hp to keep out of trouble with the officials. However, my real question is the capability of the boat to safely handle a motor of this rating. I am an engineer and know that design ratings usually have a 1.5 or greater safety factor. I do not want to be dangerous or just stupid about this overpowering and would appreciate your opinions. If you think this is unsafe to do, then I will clean up the motor and resell it.<br /><br />You have helped me in the past to get another "dead" motor running and I appreciate all your help. Thanks in advance for any input you might have.<br /><br />RAR

 

[rickdb1boat]

Re: Mercury 115 hp power

One thing is that the 115 weighs in at 365#, whereas the 90 weighs around 265#. This is a considerable increase in weight and strain hanging off the back of the transom. I really don't have any experience in how well the suntrackers are built, as far as transom strength, so I cannot say if it will safely handle the added load. I would like to think it could, but they rated it for 90 HP for a reason. I would have to follow those guidelines, not only for safety reasons, but for insurance reasons as well..

 

[Texasmark]

Re: Mercury 115 hp power

Agree on the insurance although I don't know what BIA uses for rating their boats.....like to get length and width, some mfgr's don't have much hanging out; but it measures and that allows for more horsepower. Don't even thing transom bracing is part of the equation; just l and w.<br /><br />17.0' Boat I just bought and insured:<br /><br />1. Needle sharp nose and real rake on the keel (aluminum basser). Essentially nothing for the first foot and a half. Transom width is basically a piece of gunwhale tin (bottom width is 60" but transom width, at the top, is 84"). But it rates 130 BIA cert hp. Run a 90 on it and have seen 50 mph and could do more as I'm at 6250 rpm's. But enuf is enuf for me.<br /><br />2. My insurance questionnaire had a space for rated hp and actual hp, so since I was 40% below the rating I got a break.<br /><br />3. My last boat was a Ranger Fisherman 17'. Ran a '89 115 on it. Saw 55 on it "once". Loved that engine. Started fast, smooth as silk running and boy when she was hooked up with all the trim I could stand, whatta sweet ride....and sound. (I know you don't care about this with a houseboat, but I had to rant.)<br /><br />In short; insurance could be a problem, the 115 is a hot dog and you might rip out your transom. That's a big 25% power increase with that engine over your boat's rating.<br />(percentages are not calculated....rom's)<br /><br />Good luck.<br /><br />Mark

 

[bluewater19]

Re: Mercury 115 hp power

Rick, that 1997 90 weighs 305lbs Dry(had same motor on my boat), and a 1989 weighs 312lbs dry .we are talking about less than the weight of 2 gallons of fuel(these weights are from Nada). No difference in weight so to speak.<br />I would have no worries about doing it myself... but in California there are no laws saying I can't overpower my boat and the coastgaurd doesn't care as long as I don't excede my hulls weight carrying capacity.<br />I don't think that you will have any problems. Call you insurance company and see what they say.<br />Mine had no problems with me overpowering but my rate would have gone up.

 

[Clams Canino]

Re: Mercury 115 hp power

Originally posted by rickdb1boat:<br /> One thing is that the 115 weighs in at 365#, whereas the 90 weighs around 265#.

Bzzzzzt!!!! In any given year there was no weight difference between any of the inline 6's regardless of HP. Power trim or not was the only facotr.<br /><br />-W (change the stickers to be safe)

 

[rickdb1boat]

Re: Mercury 115 hp power

Bluewater and Clams, Thanks for the correction. Must be having one of those can't read the chart days...

 

[horsefly38425]

Re: Mercury 115 hp power

the 115 were/are fast but over rev one and she's a gone'r. got some in my yard. you don't have to run her wide open if you don't feel safe' but for ins. i don't know about changing stickers. we run 90's on 115's so we can bust those 150's B-tts

 

[Clams Canino]

Re: Mercury 115 hp power

So long as it's propped light enough, has the proper oil mix, timing, AND fuel flow, the 99ci inlines will rev to 6000 and stay together (assuming it's a tight motor). The 2+2 fours are much more prone to breaking crankshafts due to over-rev than the 6's were.<br /><br />-W

 

[gatorred]

Re: Mercury 115 hp power

I know the pontoon boat's i saw didn't have that big motor on them, i know a bass boat run all out where the motor on pontoon boat has a change in prop pitch to. this is a little fact that might be important in this case most people don't under stand there is a prop change to.

 

[RAR45110]

Re: Mercury 115 hp power

Thanks for all the input. I know the inline 6 is heavy since we had to lift it into the van when we bought it. However, the same engine was produced in a 90 hp with, I assume, very little difference in weight. I do not intend to be crazy with this motor, I just kind of fell in love with the older inline 6. It looks sooo cool. I will check the prop and make sure it is not reved over 5000 to 5500 like the book says. I do have a tach. I would have settled for the 90 hp version of the inline 6, but did not see one available and found this 115 instead. What with the rating at the crank for this 1982 115 being supposedly being 15% below the rating at the prop and the 1997 boat probably being rated for 90 hp at the prop, I thought this would not be a big difference. I'll take it slow and watch the transom during takeoff to see if it is flexing and I won't run it hard.<br /><br />Thanks again for all the input.<br /><br />RAR

 

[RAR45110]

Re: Mercury 115 hp power

Gentlemen,<br /><br />Upon reading my first question, I noticed a typo. The motor is a 1982, not a 1989. It was built, I think, before the change in hp rating point.<br /><br />Thanks,<br /><br />RAR

 

[Clams Canino]

Re: Mercury 115 hp power

Close call, the rating change was in 1982. As well as making sure it don;t overrev - also make sure it CAN rev to 5500 - overpropping leads to detonation.<br /><br />-W

 

[MercFan]

Re: Mercury 115 hp power

Phone Merc with your serial number and ask them if your 115 was crank or prop rated?<br /><br />But, Clams van correct me here, if it's a 115 Crank rate your main jets should be in the region of 0.062-0.068. If it's a 115 prop rated the main jets should be more in the 0.076.<br /><br />Do not re-jet!!! That's not the only difference between the motors! But it's a simple enough way to check.

 

[jimmbo]

Re: Mercury 115 hp power

serial #s below 5050763 have .072<br /><br />serial #s 5050763 and above .074

 

[Clams Canino]

Re: Mercury 115 hp power

Jimbo sounds about right. Mine had .074 before the hi-dome conversion.<br /><br />-W

 

[MercFan]

Re: Mercury 115 hp power

There we go!<br />BTT