outboard mounting on transom? how high to mount the motor?

dkonrai

Senior Chief Petty Officer
Joined
May 9, 2009
Messages
719
it seems that i have been hearing and reading alot about outboard installations. i have a 86 14 foot bayliner. the force blew a piston, so now im repowering with a 72 merc 402.
the question, is do i mount the motor, so the big cavitation plate is 1" ABOVE the bottom of the boat? the original installation, was too low (i think) because the large cav plate was buried under the transom. at full throttle the plate was all the way in the water.
so is it true that the cav plate creates unnecessary drag? and bringing the plate up 1" above the transom is better? also i have read that for every 10mph, u should raise the motor 1" for ever 10mph, so if i can get 30mph, i should raise the motor 3"?????
help!
dino
 

trendsetter240

Lieutenant
Joined
Jun 22, 2009
Messages
1,458
Re: outboard mounting on transom? how high to mount the motor?

Not sure what a merc 402 is but if you're talking about an outboard then start by mounting it with the cavitation plate even with the bottom of your hull. See how the boat performs with that then you can raise it up one notch at a time until you start having problems with the prop blowing out in turns.

Generally the higher up the motor is mounted the more speed you will get. Usually the best setup is even with or slightly above the bottom of the hull.
 

dkonrai

Senior Chief Petty Officer
Joined
May 9, 2009
Messages
719
Re: outboard mounting on transom? how high to mount the motor?

Not sure what a merc 402 is but if you're talking about an outboard then start by mounting it with the cavitation plate even with the bottom of your hull. See how the boat performs with that then you can raise it up one notch at a time until you start having problems with the prop blowing out in turns.

Generally the higher up the motor is mounted the more speed you will get. Usually the best setup is even with or slightly above the bottom of the hull.

sorry, i have a 1972 mercury 40 hp outboard (for some reason they call it a 402).
 

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
Joined
May 19, 2001
Messages
26,019
Re: outboard mounting on transom? how high to mount the motor?

You are on the right track. The plate needs to be even or slightly (up to an inch) higher than the running keel. ;)
 

dkonrai

Senior Chief Petty Officer
Joined
May 9, 2009
Messages
719
Re: outboard mounting on transom? how high to mount the motor?

You are on the right track. The plate needs to be even or slightly (up to an inch) higher than the running keel.

the cav plate will be up another 3/4", bc im too stupid to figure out how the lu goes back on. had to pull it to change the impeller. now i cant put the thing back on.... and i cant install the shift/control till the lu is back on.
hope i can get this thing squared away before next weekend.
heres some picts of the work in progress.
dino
P1000397.jpg

raised the motor by 1". got some plastic from tap plastic.
P1000398.jpg

new holes
P1000399.jpg

straight edge off the bottom of the boat. the lower unit will be up a little higher (lu not all the way in )
P1000402.jpg

another view
P1000403.jpg

P1000404.jpg
 

62larkowner

Seaman
Joined
Jul 29, 2009
Messages
64
Re: outboard mounting on transom? how high to mount the motor?

most of these have to be in revers to drop LU , is it still? try rotating the prop while pushing up to make sure splines slip togeather
 

tmcalavy

Rear Admiral
Joined
Aug 29, 2001
Messages
4,005
Re: outboard mounting on transom? how high to mount the motor?

Looks ok to me once you get the LU seated correctly. On my 73 Merc 500 50 hp, the manual said to pull and re-install the lower unit in N, but it was hard to deal with that way. I used to put it in D or R and it slipped back together easier/better...just make sure the lower unit is in the same gear as your control when you put the LU back on, and spin the prop or flywheel a little as you do it to get the splines aligned.
 

dkonrai

Senior Chief Petty Officer
Joined
May 9, 2009
Messages
719
Re: outboard mounting on transom? how high to mount the motor?

Looks ok to me once you get the LU seated correctly. On my 73 Merc 500 50 hp, the manual said to pull and re-install the lower unit in N, but it was hard to deal with that way. I used to put it in D or R and it slipped back together easier/better...just make sure the lower unit is in the same gear as your control when you put the LU back on, and spin the prop or flywheel a little as you do it to get the splines aligned.

that thing was a bear to put back on.... somethings that johnny/evi engineered better....
 

tmcalavy

Rear Admiral
Joined
Aug 29, 2001
Messages
4,005
Re: outboard mounting on transom? how high to mount the motor?

Agreed, OMC's from that period were easier to maintain. But now you know how the beastie works, it will get easier every time you have to service the LU and water pump. Like riding a bicycle...
 
Top