80's Merc 50 hp on a 15 ft v hull Speed question

[jcac27]

Hi,<br />I am new to the boating thing and am trying to figure this all out. My boat seems to be underperforming speed wise(At least what I would expect from it).After reading hundreds of posts about this, I have decided to post a question. I realize that I’m not providing enough info, ie. years, rpms, etc. but I will get that info when I get to my boat next.(its at the lake house) <br />Anyway…<br />I bought a 15 ft ski boat made by Bonito in 1989. It is rated for 4 people. Our lake has a 50 hp max, so I replaced the motor on the boat with a mid eighties mercury 50 I bought on ebay. I mounted it myself, (somehow) and learned a great deal about boating in the process. There is no hydrofoil on the motor(buying one this year). <br />With the boat at full throttle and me and my girlfriend in it(combined weight of 320) and a tank of gas and flat water, it will hit 23 mph. If any of my 200 lb friends get in it will only go 17-19 after that, with a fourth it wont plane and only goes 11mph. <br /><br />Does that sound reasonable? <br /><br />I just realized that the pitch or diameter might not be right for my boat size, weight etc (as I bought it separately) and would like to try a different prop. Could that make a noticeable difference? Basically I’m looking for top speed, as I use the boat on a large lake sometimes and would like to get around a bit faster.<br />Any suggestions?

 

[rickdb1boat]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

jcac27 <br />If your engine is preforming properly, I would say you are overpropped. What pitch are you running now? You should be somewhere in the area of 10-1/8 X 12" Pitch.

 

[jcac27]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

Rick,<br />Thanks for the quick response. I just realized that the prop size and specs could be the problem. I will be at the boat this weekend and write it all down. I am just trying to get a feel for whether this could be the problem(or if there is one)<br />Thanks for the dimensions, those will be very helpful to see if I am in the right area.<br /><br />Thanks again,<br />James

 

[ledgefinder]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

RickDB is dead-on with the 12" pitch. I have run a Merc 50 with almost exactly same boat (and same weight of passengers!), and 12" pitch sounds just right, including for skiing work. I think those typically shipped with a 13" pitch prop (last few characters of the model number on the prop are "A40 13P"). By the way, correctly-propped, a healthy Merc 50 is plenty of motor for that boat.<br /><br />The 12 pitch recommendation assumes your motor has a 2-to-1 gearbox ratio. The flywheel should go 2 turns for every turn of the prop (Turn the flywheel only in the direction the engine runs, clockwise, or you may mess up your impeller. Make sure the engine can't start (e.g., plug wires off), or you may mess up your fingers.) <br /><br />If your pitch and gearbox are OK, check compression (the lowest plug is a bear to test) and timing. When I first put the 50 on, it ran just like you describe. Turned out someone had set the max timing way down, I adjusted it correctly & she ran great.

 

[snapperbait]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

If it's proped and tuned right your engine should be able to reach 4800-5500 RPM (depending on model) at full throttle with a light load..<br /><br />I suspect you should be able to run at least 26-28 MPH when it's all straightened out..

 

[jcac27]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

Hey,<br />Thanks alot for all of the responses.<br /><br />The numbers stamped on the prop read:<br /><br />48 73136 A40 13P<br /><br />So as you suspected ledgefinder it is a 13 pitch.<br /><br />I forgot to measure the diameter of the prop. Can you tell if it's a 10-1/8 by that number? I should not change the diameter right?<br /><br />So just to get this straight, I should try a 10-1/8 x 12" prop?<br /><br />Thanks again for all of your help.<br /><br />James

 

[ledgefinder]

Re: 80's Merc 50 hp on a 15 ft v hull Speed question

I wouldn't bother re-propping just to go down one inch in pitch. Rick's 12P recommendation probably assumes a healthy motor, and a 2/1 gearbox. However, since you're already at 13P I'd take a look at other factors on the boat to make sure she's healthy first.<br /><br />First of all, I'd check the gearbox ratio. If someone's swapped a 65/70/75hp gearbox on, you're at something like 1.66/1, instead of 2/1.<br /><br />OK, if it turns out you do have 2/1 ratio, then unfortunately we've got to look at the motor & the boat to see what's wrong or mis-adjusted. <br /><br />A compression check would help a lot in diagnosing. Again, the lowest of the 4 cylinders is a bear to get at. If you don't have a gauge, you can get a real rough idea of the compression by pulling the motor over with a rope. (There's a place for an emergency starter rope to be used on the flywheel. Make sure you've got a handle on the rope so you can give it a good pull.) What you're after is whether all four cylinders resist and 'pop' the same. Do the test 'dry' - that is, don't spray oil in the cylinders beforehand. If the compression in one or more is low, you've probably got a sick motor, but it's at least possible that a decarb would help.<br /><br />You also might check the timing (at least at WOT), and whether you're running on all four cylinders. <br /><br />Finally, it's possible the boat is waterlogged - does she trailer like she weighs a lot compared to other boats you've pulled? On the trailer & unstrapped, you should be able to lift the bow a few inches off the trailer yourself (given it's 15-footer).