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Author Topic:   Engine Mounting Holes for 15-footer

Binkster posted 11-16-2014 12:34 PM ET (US)
Today I'm planning on re-drilling the engine bottom mounting holes on my 15-footer. The old bottom holes, I plugged and glassed over. The top holes are already drilled 1-inch (on-center) below the top of the transom. I didn't plug them as I figured re-drilling them 1-7/8-inch (as recommended for standard layout would) just weaken the structure of the hole. Here is the diagram; [Link required registration to view. Please do not link to resources that require registration with some entity in order to view. This violates the general policy of having open information available--jimh.]
I laid out the bottom holes using the dimension 6-1/2-inch. This layout would put the bottom hole above the splash well. Now, before I drill, I should check the engine bracket just to be sure it will work. The 1999 Yamaha C60 is mounted on an engine stand, and I find the holes won't work even if I mount the engine two-holes-up. I am planning on mounting it one-hole-up as the top holes are already drilled 7/8-inch too high anyway. Even using the hole at 7-1/4-inch, it will only work mounting the engine two holes up, which as I mentioned before will be too high as the top holes are only drilled at 1-inch instead of 1-7/8-inch. Does this diagram not work on a 15-footer? I'd rather not drill holes 8-inch and use lags in the bottom holes.
Maybe I need a bracket.
jimh posted 11-16-2014 01:47 PM ET (US)
The standard spacing between the top holes and bottom holes is 8-inch. If the top holes are drilled in a non-standard location, you just need to offset the bottom holes by 8-inches.
If the location of the bottom hole is too low for a splash well, then you raise the the offset between top and bottom holes by a multiple of 0.75-inch, that is, the space between sets of holes as drilled in the engine mount
For example, if you need more clearance on the bottom hole, see if raising it 0.75 or 1.5-inch will work. Since your top hole is already 0.875-inch higher than normal, your bottom hole, even with normal 8-inch spacing, will be 0.875-inch higher, too. This means that if you move the bottom holes up by one-hole-set (0.75-inch), the net effect of that plus the higher top hole will be additive, and the bottom hole will be raised by 1.625-inch higher than normal layout.
If you raise by two-holes, the bottom hole will be 0.75-inch even higher, or 2.375-inch. Even for a very shallow splash well, I think that 2.375-inch higher for the bottom hole ought to help clear the bottom of the well. If it does not look like it will, then drill the hole on a slight angle upward from stern to bow, to get the exit hole in the splash well to clear.
To determine how high to drill the lower hole, measured the depth of the splash well from the top of the transom. Then account for the position of the top hole from the top of the transom. (In your case, the top hole are 1-inch below top of transom.) Now add 8-inches from the top hole to get the standard bottom hole location. If that hole will be below the splash well bottom, then move it up in increments of 0.75-inch.
By the way, there is not much value in following any standard, diagram, or advice if the holes in the engine mounting bracket do not follow the BIA standard. If the BIA standard holes and the holes on the particular engine you are trying to mount do not align, you will have to modify either the transom holes or the engine bracket holes. It is probably easier to modify the transom holes as the transom is usually rather uniform in the region of the engine mounting location and can accept new holes, where the engine bracket might not be so uniform and the bracket's material at a particular location might not be sufficient for mounting the engine.
jimh posted 11-16-2014 02:02 PM ET (US)
[Moved to REPAIRS/MODS for discussion.]
Binkster posted 11-17-2014 03:11 PM ET (US)
Thanks for your help. I drilled the (8") holes which places the engine one hole up. Its the lowest position I could go and I drilled the holes wth an upward slant which gave me about a 3/4" clearance from the splash well. The lower holes in the bracket are slots, not holes so if I decide to go two holes up, I just need to loosen the lower bolts, remove the top bolts and lift the engine 3/4" with my cherry picker, replace the top bolts and tighten the lower ones.
rich

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Author	Topic: Engine Mounting Holes for 15-footer
Binkster	posted 11-16-2014 12:34 PM ET (US) Today I'm planning on re-drilling the engine bottom mounting holes on my 15-footer. The old bottom holes, I plugged and glassed over. The top holes are already drilled 1-inch (on-center) below the top of the transom. I didn't plug them as I figured re-drilling them 1-7/8-inch (as recommended for standard layout would) just weaken the structure of the hole. Here is the diagram; [Link required registration to view. Please do not link to resources that require registration with some entity in order to view. This violates the general policy of having open information available--jimh.] I laid out the bottom holes using the dimension 6-1/2-inch. This layout would put the bottom hole above the splash well. Now, before I drill, I should check the engine bracket just to be sure it will work. The 1999 Yamaha C60 is mounted on an engine stand, and I find the holes won't work even if I mount the engine two-holes-up. I am planning on mounting it one-hole-up as the top holes are already drilled 7/8-inch too high anyway. Even using the hole at 7-1/4-inch, it will only work mounting the engine two holes up, which as I mentioned before will be too high as the top holes are only drilled at 1-inch instead of 1-7/8-inch. Does this diagram not work on a 15-footer? I'd rather not drill holes 8-inch and use lags in the bottom holes. Maybe I need a bracket.
jimh	posted 11-16-2014 01:47 PM ET (US) The standard spacing between the top holes and bottom holes is 8-inch. If the top holes are drilled in a non-standard location, you just need to offset the bottom holes by 8-inches. If the location of the bottom hole is too low for a splash well, then you raise the the offset between top and bottom holes by a multiple of 0.75-inch, that is, the space between sets of holes as drilled in the engine mount For example, if you need more clearance on the bottom hole, see if raising it 0.75 or 1.5-inch will work. Since your top hole is already 0.875-inch higher than normal, your bottom hole, even with normal 8-inch spacing, will be 0.875-inch higher, too. This means that if you move the bottom holes up by one-hole-set (0.75-inch), the net effect of that plus the higher top hole will be additive, and the bottom hole will be raised by 1.625-inch higher than normal layout. If you raise by two-holes, the bottom hole will be 0.75-inch even higher, or 2.375-inch. Even for a very shallow splash well, I think that 2.375-inch higher for the bottom hole ought to help clear the bottom of the well. If it does not look like it will, then drill the hole on a slight angle upward from stern to bow, to get the exit hole in the splash well to clear. To determine how high to drill the lower hole, measured the depth of the splash well from the top of the transom. Then account for the position of the top hole from the top of the transom. (In your case, the top hole are 1-inch below top of transom.) Now add 8-inches from the top hole to get the standard bottom hole location. If that hole will be below the splash well bottom, then move it up in increments of 0.75-inch. By the way, there is not much value in following any standard, diagram, or advice if the holes in the engine mounting bracket do not follow the BIA standard. If the BIA standard holes and the holes on the particular engine you are trying to mount do not align, you will have to modify either the transom holes or the engine bracket holes. It is probably easier to modify the transom holes as the transom is usually rather uniform in the region of the engine mounting location and can accept new holes, where the engine bracket might not be so uniform and the bracket's material at a particular location might not be sufficient for mounting the engine.
jimh	posted 11-16-2014 02:02 PM ET (US) [Moved to REPAIRS/MODS for discussion.]
Binkster	posted 11-17-2014 03:11 PM ET (US) Thanks for your help. I drilled the (8") holes which places the engine one hole up. Its the lowest position I could go and I drilled the holes wth an upward slant which gave me about a 3/4" clearance from the splash well. The lower holes in the bracket are slots, not holes so if I decide to go two holes up, I just need to loosen the lower bolts, remove the top bolts and lift the engine 3/4" with my cherry picker, replace the top bolts and tighten the lower ones. rich