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Ok, well the time has come to drill the now blank transom of my redo project to reinstall my Honda BF90. Through my other posts and discussions with some of you I have decided to mount 1 hole up but I am cheating up 1/4" to make sure I hit my splashwell at the top of the drain plugs allowing me a nice landing area. This will allow for any motor in the future + more vertical movement. Now my question. Is there a proper method to determine absolute centerline on my transom? I made a beautiful jig with 3" drill guides so drilling will be easy once the jig is in place. I just want to make sure I hit the mark the best I can. I know there is a better way than just measuring 1/2 way and clamping the jig then start drilling. Just kinda want to do it once and right! Any help on this matter is appreciated. I plan on doin this tomorrow nite. Thanks in advance, Wayne

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Wayne, measure, measure, and measure to make sure.
Maybe others here can give you a little more advice.

One thing though, do take some photos for us with the jig in place before drilling the holes and after....
Thanks...

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I agree with Joe. Measure, measure, measure. There is no other way I am aware of.

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The way I have done this; take a piece of cardboard say 24 inches square ( depending on the engine size). Use some oil and spread a thin coat on the engine mounting shoe, place the clean cardboard against the mounting shoe and this will leave a print on the cardboard. This will give a template with all the holes marked; then take the cardboard to the transom and drill the holes wanted. I did this at the dealer where I bought my DF 70 Suzuki, the Tec just stood there with his mouth open as he said he never had seen such a thing like that. Very quick and easy, I might of used a darker oil like used engine oil or a dark two cycle oil next time. Clean 30 W was almost clear but it showed every hole perfectly. Just coat the shoe that goes against the transom place the cardboard against the shoe with out moving it around, if it moves around recoat the surface a use the other side of the cardboard. Then wipe the oil off the engine, that easy to make a template.

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Guts, I made a template out of steel plate with the exact pattern I need and also welded on 3" drill guides/tubes so the angle stays true. What I am trying to determine is a method if any to find true centerline. Based on manufacturing tolerances just measuring across then divide by 2 just seems like not enough. I have heard there is a method measuring from all points on the transom. I am not sure but I am of the addage "measure twice cut once". I guess I am hopeing for some proven method. If not I will stick with what Joe and sraab928 said measure, measure, measure.

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here is some information that was given to me some time back by a fellow member. I found it to be very helpful when mounting my motor on my project 15.

You can make your own template, if you have any drafting skills at all. Here are the standard boat pattern dimensions, which your Yamaha would also comply to:

Obviously, there are four holes, two at top, two at bottom.
Most brands use 1/2" diameter fine thread SS bolts with brass locknuts, but I have seen some Yamaha's mounted with smaller diameter bolts, so check on the diameter your engine requires. Dimensions given are bolt CENTERS. You should always draw the transom top and vertical centerline on your template.

From the top of the transom, both top holes are 1-7/8" down, and the bottom holes are and ADDITIONAL 8" down.

Top holes are a total of 12-7/8" apart, or 6-7/16" from transom centerline.

Bottom holes are a total of 9-7/8" apart, or 4-15/16" from transom centerline.

This is the universal bolt hole pattern, and applies to all brands of engines (assuming they adopted it), at least 1984 and later, from about 30HP on up to 250HP. None of this applies to OMC's blind hole option, which other brands did not use.

Also, bolts through the top holes should have the nuts on the outside, and through the bottom holes, on the inside.


Thanks Larry.

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All the dimensions that Larry provides above are included in this article along with a drawing:

http://www.whalercentral.com/articles...icle_id=82

Wayne -- Don't make this more complicated than it needs to be. It is impossible to be more precise in determining the absolute centerline than to measure the width and divide by two. What you use as points of reference for your measurement can make a difference though.

Ideally, you would measure across the hull from gunwale to gunwale without the rub rail in place to eliminate any variance the rub rail itself might impart. This is not practical, however, and would not amount to much if any variance, certainly not enough to worry about.

In reality, the motor could be mounted up to an inch off-center and it would have zero affect on the boat's performance. You would see it long before you felt it.

If it were me, I would measure across the hull at the stern, stretch a piece of masking tape across and mark the center. I would then use a string secured at the center of the bow and stretch it to the stern aligning it with my mark on the tape there. The string would project the centerline to the top of the transom. But I would only do this if I felt like being extra fussy ;-)

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Bake, Tom, I made my template out of steel plate with 3" drill guides (1/2" I.D. tube welded to the plate) to keep the drill strait, like the drill block on my project page that I used to drill for my new bow eye. As you suggested in one of my previous threads I am going to cheat 1/4" up from 1 7/8" down to land really nice in the splashwell. I don't want to complicate this I just don't want any adverse effects from it being off just a bit. I guess I am more concerned with the motor being tilted rather than on center. My template will sit right on the transom it should be very simple. I guess I am just being carefull. Don't want to screw up when I am getting down to the final stages of the project. I have removed and installed many outboards and rigged all my previous boats but the transoms were always punched prior to me. So if I understand correctly you are saying that a little off and the effects will be unnoticable? I like the sounds of that.

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Yes, but a little out of plumb, and it will be noticeable to the eye, though again, there would be no ill-effect on performance.

I have seen dealer rigged outboards that did not sit perfectly vertical on the transom. Just a little off, but it looked terrible.

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Tom, that is how I view it. I am a little fussy on appearence but my main concern is performance. If being off up to 1/8th of an inch is not a concern then I should be home free. I'm just pickey enough to stay within that. I will post pics of the install and of my template I made tomorrow. Thanks, Wayne

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Even though you are installing a Honda, here is some great information from Mercury that may help you out;

http://www.marinepartsman.com/install...-boat.html

The key in all of this is to get a true perpendicular (to transom top) center line drawn on the boat. You can use a framing square to check that it is truely square, assuming the top of your transom is straight and level.

To accomodate the splashwell, I assume you are using 7-1/4" vertical spacing instead of the standard 8"?

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Finnegan, yes I am using the 7 1/4" spacing and plan on mounting 1 hole up. What I did is cheated the whole pattern up 1/4" to hit the splaswell perfectly. The level transom thing is what I was worrying about. I plan on measuring everything multiple times with the boat perfectly level from gunnel to gunnel and triple checking it before I break out the drill motor. Thanks...

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FYI, the BF90 is one of those motors that is 1.1" longer than advertised shaft length. You will probably end up mounting at 3 holes for best performance and may want to start there.

Recently did a search for the Mercury drill fixtures and found them to be relatively inexpensive (in boat terms) for what they do. Less than $90.

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Wayne,
I am sure with the craftsmanship you put forth in the rest of the boat the chance of not getting you motor hung plumb is slim to none. The fact the you are so concerned speaks to the fact that you will make sure it is right. When I did my 15 footer I measured a lot and used a square and a level. All the things I know you are doing too. I Know you will get it square.

Number 9

you need a personal page with some pictures of the 18 footer running with a 50.

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Thanks for the words of encouragement! We will know soon as I will be heading out back in just a bit to get started. I will take pics of the jig I made as well as the drilled transom and the finished product. Wish me luck.

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Yeah, know I need a PP with some photos. Hopefully will get her back in the water soon and will definitely get some photos posted. It's on high may list of projects now it's a bit cooler outside.

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Update while I close the office for the day. The drilling went perfectly. The motor slips on and off with a push of a finger. Upon doing some looking before final set I think I am going to mount 2 holes up as that leaves the cav plate approx 1 inch above the bottom of the hull. I will post pics this weekend.

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Well I updated my project page to show how I did the drilling. It went so well I also had time to install the console tonight. Looks like tomorrow I can pull controls and steering. Thanks everyone for the advice. I did find out that my transom was out of level. You can see it in the pics. I leveled off the gunnels and plumbed. My jig also has centerline holes to line up with the plumb line. With motor hanging and bolts in the transom I could swing her on and off like butter. To match level and plumb I had to raise the jig on the starboard side approx 1/8". I am glad I didn't just plop it on the transom and drill. Yeah!!

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Looks like you have plenty of room in the bottom of the splashwell for the washers on the bolts.

So, you ended up using 7 1/4 inch vertical spacing for the bolts, correct?

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Yes Joe I did. After discussion with Tom and others I also cheated the whole pattern up 1/4". If I did not do that I think I would have been just fine but very low and right at the beginning of the curve at the bottom of the splashwell. By cheating up 1/4" I landed on all flat surface. If you don't want to cheat up just use long drill guides and you don't have to angle anything. They keep you right on line.

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That's good to know. I have heard this could be done with the 7 1/4" spacing but have never seen photos of the finished process. That is why those holes are colored yellow in our article so people do use caution.

For people that already have the top 2 holes drilled in the standard position, then by drilling the lower holes 7 1/4" down, there will be just enough room in the splashwell even though they may have to trim the washers on the bottom side slightly.

I prefer thru bolting the bottom holes rather than using lag bolts. Lag bolts will work but thru bolting just seems to be a better installation method.

Your project is really coming along...
When's splash day?

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Mounting nuts on inside verses mounting nuts on outside. I`ve seen both ways which is considered better way? I always put the nuts on the inside.

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I like them on the outside for looks. Plus if you need to move the engine you don't have to remove the bolts in the long slots, just crack the nuts a little loose remove the other bolts, slide the motor up lock it down and replace the top bolts.

Jeff

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Joe, that is correct. Slight trim and they should sit really good. Splash day is probably end of the month or next month. My son and I rigged the motor and controls today, installed the batteries and gave her life for the first time in 8 months. That is a really good feeling. Now just got to fininsh details/rigging, rubrail and such. Good times!!
Binkie, Spongebob, most manufacturers say nuts on inside but I agree with you about ease of motor adjustment and removal also my last boat had studs off of the swim step so what's the differnece I say.