Lower unit size? Inline 4 115 - 1993

[crackedglass]

I am getting a clean used 1993 115 to use on my Starchief, I am not getting a prop with it, and need to know what size prop I can borrow to get some sort of baseline to start with. I went to the prop selector on Michigan Wheel's web site and need to know what diameter lower unit I need. The lower unit on this motor measures 4 7/16" ID where the prop hub fits into the lower unit, and about 4 1/2" overall on the outside. MI Wheel shows only a 4 3/4" lower unit, and a 4 1/4" V4 gear case for their Vortex aluminum 3 blade props. How do you measure the hub diameter of a lower unit?
I gave these measurements to a guy at a prop shop and was told they have no idea what I have. I didn't even know Mercury had a V4. All I need is to know what size hub I have so I can get something ordered so I have a prop to use when the motor gets here in two weeks. Also, what motors could I borrow a prop from for trial purposes?

 

[achris]

Re: Lower unit size? Inline 4 115 - 1993

... I didn't even know Mercury had a V4.

They don't, and never have.

All I need is to know what size hub I have so I can get something ordered so I have a prop to use when the motor gets here in two weeks. Also, what motors could I borrow a prop from for trial purposes?

Just say it's a 4 cylinder 115 hp Merc and they will be able to supply the right prop. If they still don't know... find another prop shop...

Same prop used on all 75hp to 150hp inline engines.

Chris.........

 

[crackedglass]

Re: Lower unit size? Inline 4 115 - 1993

They don't, and never have.



Just say it's a 4 cylinder 115 hp Merc and they will be able to supply the right prop. If they still don't know... find another prop shop...

Same prop used on all 75hp to 150hp inline engines.

Chris.........

What is the diameter of the prop where it meets the lower unit? The MI parts listing don't go by motor, only by prop diameter or lower unit size.
The Vortex catalog page online shows a 4 3/4" lower unit, a 4 1/4" lower unit.

http://i40.tinypic.com/34o4sax.jpg

These are all of the Vortex aluminum props that they make, they use an interchangeable hub to fit various makes. Neither of the two larger sizes match the measurements of the lower unit.
I need to know what the actual measurement should be on the prop to work with this motor, I have a shop nearby that will let me borrow a prop but they have no idea what they fit. I don't have an original prop to use as a comparison.

 

[achris]

Re: Lower unit size? Inline 4 115 - 1993

... I have a shop nearby that will let me borrow a prop but they have no idea what they fit. ....

Interesting... I just went to Michigan Wheel's website and the prop selector's first question was 'Outboard or Sterndrive?' Next question 'Manufacturer'. Then it gave a horsepower range to select. At the end of the selection process it gives a list of props.

The next thing I did was to go to download the catalog... And there in big bold letters were all the different engines (by manufacturer) AND WHICH PROP WILL FIT ON THEM!!!

It's all there.. If you don't think so, click HERE...

Or try THIS one, it's on page 20...

Chris.....
SOUNDS TO ME LIKE YOUR PROP SHOP GUY NEEDS TO GET OFF HIS GREAT FAT ARSE AND DO SOME WORK, INSTEAD OF GETTING HIS CLIENTS TO DO IT ALL!!! And I still think you should let him know (or better yet, the owner of the shop) that you will be taking your business elsewhere due to the 'wonderfully helpful' person serving you... That's what I would do...

 

[Chris1956]

Re: Lower unit size? Inline 4 115 - 1993

Achris, Isn't the 60 HP Bigfoot the 4-3/4" gearcase on an inline motor?

 

[sschefer]

Re: Lower unit size? Inline 4 115 - 1993

13-1/4 x 17 3 blade, 14x16 4 blade.

 

[achris]

Re: Lower unit size? Inline 4 115 - 1993

Achris, Isn't the 60 HP Bigfoot the 4-3/4" gearcase on an inline motor?

Same gearcase as the 75-125hp 'big' displacement engines... Never bothered measuring gearcases, if you use Merc props you don't come up with any issues.
From what I read in the MI website, it's a 4 1/4" box, their 'series B' propellers.

As I said before about the lazy prop shop assistant... Took me 3 mouse clicks to find this page...

 

[crackedglass]

Re: Lower unit size? Inline 4 115 - 1993

That's the chart that started all the confusion, how do they come to the measurement of 4 1/4" ? No point on the lower unit measures 4 1/4".
I came up with 2 MI props, one is a 992004, the other a 992115, neither measures as if it will match the 115 lower unit. I found a 1996 motor at the boat shop here, the 992004, which is listed for a 4 3/4" lower unit fits but touches the lower unit housing on the inner lip just a bit, and the 992115 looks small but fits and clears. The prop hub is noticeably smaller than the bearing housing area on the lower unit. There's enough room to get small rope between the lower unit and the prop. I just always thought that this area should be a super close tolerance fit to prevent anything from getting wrapped up in there?
MI is a well known name brand, I've used them for years on my OMC outboards. This is my first bigger Mercury. If it didn't look funny with the B series prop on there, I'd have never bothered to ask.

 

[achris]

Re: Lower unit size? Inline 4 115 - 1993

Here's a suggestion... If you are having trouble with the Michigan props, and want an ali prop, why not get a Merc prop.... You KNOW that one is going to fit...

 

[crackedglass]

Re: Lower unit size? Inline 4 115 - 1993

I got the motor last week, the seller included a used prop after all, I sent that out and had the hub replaced and a few repairs made. So far it seems ok, I've got about 5750 RPM on plane and about 31 mph. The prop is a 13 1/4" x 19P. I'd like to try a lower pitch or maybe a smaller prop but my gut feeling is that any change at all would be too drastic and put me over the max RPM. It feels like its running comfortable now.
And better yet, I've only got $45 in the prop. It too is a Michigan Wheel prop but it matches the lower unit, which is much smaller than 4", maybe about 3 3/4" or so. I guess the I/O prop I have is no good on an outboard? At least nothing I'll ever be running. That chart don't match either motor or prop I have.
The 1994, which it turns out to be, 115 lower is smaller than, not larger than the I/O lower unit housing.

 

[achris]

Re: Lower unit size? Inline 4 115 - 1993

Those numbers don't look right... Here's the read out from my prop calculator...

 

[crackedglass]

Re: Lower unit size? Inline 4 115 - 1993

A bit late getting back here but I finally got to spend some time with the boat in the water again.
The prop that came on the motor is a 13 1/4" x 19P, the boat weighs in at just over 1278lbs with fuel and motor. (I weighed it at a truck stop last week, subtracted the listed weight of the single axle galvanized roller trailer which was weighted separately at 569lbs last week). Total weight on the scale was 1847lbs.

The motor maxes out at 5700 -5800 RPM, the boat's speedo reads only 24mph, my hand held Garmin GPS reads 31 to 36 mph, depending on the wind and tide.

I do experience an occasional flare up in RPM, it sounds like the prop loosing it's grip on the water. This usually happens in rough water or over strong surf or whitecaps. It sounds as if the prop is coming out of the water even though the motor is trimmed all the way in.

As far as specs, all I have to go on is what the prop is marked, the boat's tach, and my GPS.
Where do I find the gear ratio for any particular motor?
It feels like I need a lower ratio or pitch? It feels like a car with too tall a gear in the rear to me, sort of lugging at times and slow getting to top speed. A buddy has a similar boat, but glass, with the same engine and will outrun me in top end by a mile. His boat is at least twice as heavy, his motor only a 90hp, yet it runs away from me from the start, and I have trouble keeping up even at cruise. He does much better in rough water as well.
I switched props with him just to see if that helped, his prop is a 14x10p, and on my boat gave me a holeshot you wouldn't believe but I topped out in RPM not speed at about 37 mph. My RPM was nearing 6400 when I backed off it. His boat wouldn't get over 12 mph, and wouldn't even get on plane or over 3300 rpm with my prop on his 90hp motor.
I did notice that with his prop, my boat put out a HUGE wake, sort of a rooster tail looking wake coming off the port side of the prop and the boat jumped right up on plane, it popped right up on plane instead of the normal 100' lag to get on plane. I think more of the lower unit was out the water.
I'm not sure if any of that proves anything or not, but do feel I need to go with a lower pitch, but I'm concerned about the max RPM?

 

[Dave1027]

Re: Lower unit size? Inline 4 115 - 1993

I have the 100 hp version of that motor. I run a 13.25 x 17p stainless steel Stiletto prop. I get about 5500 rpm and just over 40 mph and it's pretty torquey. My boat weighs about 1530 lbs dry and about a ton in the water with a deep v hull.

The motor gear ratio is 2.07:1

It could be that you don't have the motor mounted properly. Too high or too low on the transom maybe? The cavitation plate should be just above the bottom of the boat. As light as your boat is the 19p prop should have worked about right.

 

[crackedglass]

Re: Lower unit size? Inline 4 115 - 1993

The motor sets about 13/16" above the bottom of the boat, checked with a strait edge along the bottom of the anti-cavitation plate. That's the closest it comes to the bottom through out the trim range.

I would have certainly figured that the 13 1/4" x 19P would have worked since the boat the motor came from was also aluminum and only a few inches longer, but with no cabin. The other boat was more of a V hull, mine isn't as deep of a V at the stern as that boat was. The last owner had it on a newer 19' Starcraft Super Sport that he scrapped after the hull corroded away. I also got the whole gauge package, controls, and complete fuel system. He claims that boat did 42 in light chop, and about 39 in the river.
I've got a lot more wind resistance but probably a bit less weight and less water drag. Since the motor clamps on, with only two lower bolts, adjusting height wasn't much of an option without making some sort of spacer. I was able to reuse the two lower bolt holes from the old motor, and filled the two top holes. I was a little surprised to see such a large motor still using only upper clamps.

It feels like it needs more bite, it's like the prop is spinning but not moving the boat. Nailing the throttle from a standstill feels about the same as doing so while still tied to the dock, the boat lunges downward vs jumping up and moving forward. It sort of sinks in the stern real hard then moves forward.

I called the prop shop that did the hub and reconditioning job, they said send it back and they can try to make a few changes they feel will work.
I did mark the bushing, hub, and prop to make sure nothing is moving and all is fine.

What dictates prop diameter? Wouldn't going to the largest prop that will fit on the motor be an advantage on a light boat? Then adjust the pitch to achieve proper wot rpm?

I've also tried moving weight around in the boat, but not much helps. I've got two fuel tanks, both under the front driver/passenger seat boxes. 8 Gallons on each side. I usually only keep 6 per side. There are two group 24 batteries in the stern below the splashwell, and one gel cell up front that runs the fish finder and radio, which is also charged by the engine but controlled by a dash switch so I can take it out of the picture when no longer needed or when things like lights take priority in a way of trying not to over tax the charging system on a single outboard.

The boat sits well in the water, the deck and all under deck foam is new, so it's not full of water weight. The prop 'slippage' gets worse as I move more weight forward but only in rougher water. The motor sounds like it's starting to lug a bit at full throttle. With the the borrowed prop, I think it would scream till it blew if I pushed it. I wasn't going past the 6450 mark though by my control.

I'd like to try either a 13x17 or a 14x13 or so. I think the larger I go the more control I'll have at low speeds? It may make docking it on windy days a bit easier if I can move more water at lower rpm?