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Looking for some help. I have an 83 Forester Pro Bass 162 with a 1985 Mariner 115hp (Tower of power inline 6cly). The RPM meter when it does work flirts around 3800 rpm at full throttle. Th boat is only 16.2 feet long and even with just myself in the boat, I can only get it up to 39mph on a calm day. The prop has a few nicks in it but is in decent shape no major chips taken out or anything. My question is this. I feel like i should be getting more speed out of this boat. It does have a hydrofoil on it white helps my hole shots. I tried taking it off and couldn't even get the boat to go over 33mph. When full throttle in most boats, you can trim up the motor and find the sweet spot where it rides smoother and a little faster. With this boat, trim al the way down at hole shot, once on plane the trim doesn't help in fact actually seems to almost hurt the boats speed when trimming up. Im currently looking at a jack plate to try and get less drag out on take off and hopefully after fine tuning, a better top end speed. Does anyone have a similar set up (boat wise) as i do and if so what your numbers looking like for speed and RPM? Any and all help is appreciated! -Sam

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Step A would be to get your tachometer fixed so you get a true read on what your max rpm is.  3800 rpm seems pretty low, but if the tach is hanky - who knows.

 

The hydrofoil which is helping your hold shot is what is hurting your top end when you try to trim.  I don't know where you are at, but at most of the lakes in Missouri, running one of those hydro foil fins is asking for it.  Sooner or later you will catch something with it.   I'm not familiar with that brand of boat.  Is it one of the old "bomber" style hulls or a pad type hull.    If it is a bomber style hull, 39 mph isn't bad.

 

I don't know.   I am not an engine/boat performance expert - I'm just a guy who listens carefully when  I'm around those guys when they are talking and I'm writing to you what I've heard.  Hope it helps.

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Trees is def. correct. Not familiar with the boat and google searching hasn't helped... planing hull or old school style? If it's old school style, then disregard most of the upcoming commentary....I'm oblivious when it comes to performance on old school hulls.....

 

 

Need to be getting a proper RPM reading before you start fooling with stuff. Go with the most modest and conservative fixes first. If you do decide to get a jackplate, you will need a reliable RPM reading when tuning your jackplate into it's sweetspot. So get a proper RPM reading and if it really is only running 3800RPM then fix that problem first. 3800 is super low. If you fix the gauge and are actually running 5k+RPM's then we can start making some fixes. 

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As far as the research that i have done has concluded, the 83 mariner was one of the last years they did not have an rpm chart for pitch and performance numbers. It is a tri-haul but when on plain it cant be that much more worse then a normal v haul. i did take the Hydrofoil off a for a few runs just too see if that was the resone my top end was off and the boat not only struggled to get on plane quickly, the top end speed was noticeably slower. So i came to the conclusion that the hydrofoil was in fact helping dramatically.  not being able to trim to increase performance is the biggest thing im thinking that the motor needs to be raised up.

 

The prop has some nicks and little (edgecurls) on it that where done before i owned the boat but would that really make a difference in speed?

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Get the rev counter fixed as has been said. You can't really do anything until you know what the motor is actually doing. If the revs are what you think then you're hugely over propped and probably have the engine mounted too far down too. Find out your rpms and look at the prop you have to find out it's size (diameter and pitch).

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If you're really only running 3800 RPMs then you have WAY too big of a prop on it or you have engine issues. The fact that it's got a fin on it is a good indication as well as the fact that you stated that it was a dog out of the hole without the fin. The wide open throttle RPM range on that motor is 5000-55000 RPMs. You usually want it propped towards the upper end of the RPM range. The first things you need to do is get your tach fixed, secondly you need to be sure that your engine is running correctly. Some engines can actually drop a cylinder without any adverse effects other than the loss of power. Once you have both of those taken care of, it's time to look for the correct prop. There's about a zillion and one options out there, but on a lighter boat like that you should be looking for something along the lines of a 3 blade stainless prop. Stainless will give you the best grip on the water and allow you to trim farther in most applications. You didn't say what pitch prop you're currently running or what material, but to ballpark your RPMs for each inch in pitch you go down, you'll gain roughly 200 RPMs. If your tach is correct and your engine is running correctly as they are, then you need to go down about 8" in pitch.

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Perfect, this is the help i was looking for. All the cylinders are firing so i know thats not the issue. Im planning on cleaning out the carbs this spring but i know even that wont chance top end that much, maybe a mph or two. The hydrofoil is great for the hole shot but like i said, even with it off I couldn't get it to run nearly as fast. Just looked at the prop and it said "Quicksilver 48 7250g 45 21p" i assume the 21p means its a 21 pitch but what does the rest mean? 

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Probably just the model number. Get the tach fixed and the motor checked, then get your true readings of WOT speed and RPM. Then you can start trying to understand where the issue is.

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Probably just the model number. Get the tach fixed and the motor checked, then get your true readings of WOT speed and RPM. Then you can start trying to understand where the issue is.

this x2

 

A 21 pitch seems like it could be a little big. It depends on the gearing in your lower unit. Definitely get the tach fixed first. Are you running a stainless or an aluminum?

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Sounds good, I have a jack plate im going to be putting it on soon here in the winter months then first thing in the spring it will be trying to find out the rpm's and what its actually running at! thanks for all the help! Any advise on taking off a 115hp tank of a mariner? 

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A jack plate will put more strain on the transom, so be really certain that the transom is completely sound before you fit it. Older fibreglass boats usually had plywood transoms inside the fibreglass and if it got wet, through unsealed outboard mounting holes, cracks in the glass, leaking through hole fittings etc the plywood can rot.

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To echo Tim Kelly - be certain of the soundness of your transom prior to installing your jack plate.  Installing a jack plate on a 35 year old hull might cause more problems than it solves.

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The haul looks like its in decent shape, but i guess ill find out when i take off the old one! Is it safe to say i should be getting more speed then i am with a 16 foot boat and a 115 hp right?

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The boat/motor has issues not jack plate in the world is going to help.  I have a 1988 115 like yours on a 1987 Stratos 168V and with a 21" generic Michigan Wheel, it easily runs 52 mph at 5,800 rpm.  First thing I would do is find a place to weigh it the whole rig, launch the boat, go back and weigh it again without the boat.  That will give you the floating weight of the boat.   Now, why is this important, I've seen a number of old boats like that with saturated flotation foam and that makes them more of a barge than a boat because of hundreds of pounds of weight.

My next step would be a compression, and preferably a leak down test, to make sure the motor is not used up.  Low compression kills a motors power big time.

From there you start checking ignition.  Mercs do things a little different and have a high speed and low speed stators.  May have a problem with the high speed side.  Then you have the trigger modules.  Six cylinder motors can have a misfire and not even realize it, but will kill approx. 1,000 rpm off it. 

The prop is another easily checked item and can have a rolled blades and never look like it, but performance will totally suck.  Find someone with a good one you can try.

 

For dang sure, don't waste your money on a jackplate.  They are for tweaking performance, and will do nothing for fixing your problem.

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Way2slow ;)

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You can start with the motor or the boat. The boat is probably the easiest and least expensive place to start. If the boat is or has been stored outside. The floatation foam needs to be checked, this can be done by weighing at a feed store or truck stop that has scales. Drive on, weigh everything, go launch the boat, come back and weigh it again without the boat. That will give you the weight of the boat, see if it's several hundred pounds heavier than should be. If it has a floor drain, take the cover off and see if it just dumps on top of the foam, many do, or if it has PVC pipe. If no pipe, you can take a core sample there by pushing a short piece of 3/4" conduit about 18" or so long, all the way to the hull and pull out a core and see if it's dry. Fill the hole back with some spray foam.

By the way, the reason for launching the boat, sliding one off on dry land and getting it back on the trailer is something you will probably never want to do again if you ever do it the first time. Use at least a 4' straight edge and check the hull for a serious hook I the back half of it. Improperly adjusted bunks can cause serious hull problems. This can easily be done with it on the trailer.

If it has dry foam and no hook, time to check else where.

The other, easy to check would be try another prop, maybe you have a friend with similar size Merc prop. Even a 19" will let you know if the prop is bad, it should launch out of the hole and turn at least 6,000 rpm with a 19.

From this point, you are going to have to addresss probable motor issues. Now it can get expensive, depending on your knowledge level.

Compression would be my next test, and doing that on the bottom cylinders of that motor sucks.

From threre the free ride is over, time to go to the ignition and fuel system. Most of the time about the only real way to verify an ignition component is bad is to try another, that's where a shop with good used parts or a good friend with a like motor comes in handy. A merc guy worth his salt can trouble shoot and diagnose it to a probable problem, but don't get upset when you buy the new part and it doesn't fix it. I found a long time ago, nothing beats swap tropics with good used parts on Mercs until you find the problem, because sometimes it can be more than just one part.

Then you have the fuel delivery system, carbs, fuel pressure etc. Again, an expensive guess if you do them first and find it wasn't them, unless you know how to do them yourself. Which I find most people that think they do, really don't.

Edited by Way2slow
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