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Mercruiser Marine 120 Installation

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captaindada

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"Hey Group - 1st time poster h

"Hey Group - 1st time poster here. I just hunted and aquired 2 of the older Mercruiser Marine 120s... They have GM motors and the outdrives... I am not sure exactly what they are.

I plan on installing both of these into a 1989 25ft Deep Vee(not modified) Glassmaster.

Can anyone tell me how to find a template ? This will be my very 1st I/O Installation and I don't want to screw things up. I have no idea of the distance to have in between the outdrives nor do I know the exact height to mount them at. Any and ALL help would be greatly appreciated.

Regards,

JohnnyO"
 
"[b]"Can anyone tell me ho

""Can anyone tell me how to find a template ?"

Ayuh,...
I always use the fiberglass that I cut out of the Donor Boat...
Of course,.. I'm usually doing a refit of an old OMC Stringer drive to Mercruiser...
Single drive...
Twins could be done the same way...

As for Spacing,...
Whatever works for that hull will be fine...
Inline motors can be closer than V-motors, but it isn't necessary, it's done to create more or less space for other things in the hull..
1 thought,...
The Wider space between the drives, the more leverage they have against each other...

Height of the drives is about the same on twins, as singles....
The anti-cav. plate should be At, or just above the height of the water passing under the hull, On Plane...

Good Luck,...."
 
"Thanks for the reply - I was

"Thanks for the reply - I was considering the maximum spacing so the center of the swim platform would be free of the outdrives.

I've got 6 boys - yes ... 6 ... This has become a fun project for me and the boys to work hand in hand together. Even the 2yr old is helping out !

I'm sure I'll have a ton of other questions. I'll load up some photos of what we've got to work with and what we've done so far later this afternoon.

Can you tell me what alignment tools or installation tools I'll need when I goto mount the outdrives and the engines ?

Also - does it matter if they are NOT counter-rotating... and what would be involved in making one of the engines or outdrives counter rotate ? Is it worth the $$ spent ?

Regards,

JohnnyO"
 
"[b]"Also - does it matter

""Also - does it matter if they are NOT counter-rotating..."

Nope,... It really doesn't matter...

"Can you tell me what alignment tools or installation tools I'll need when I goto mount the outdrives and the engines ?"

Ayuh,...

You'll need an Alignment Bar,... Same 1 used on All the outdrives..."
 
"25 foot deep vee, 7-9 people

"25 foot deep vee, 7-9 people - 120HP twins -
that's going to be very marginal, maybe inadequate power, specially if you will be pulling skiers."
 
"25ft Deep Vee - 7 people &#40

"25ft Deep Vee - 7 people (6 of which are kiddos) - 120 gallons of fuel, 40gallons of fresh water, prob 100lbs of ice/food/supplies/etc. 150lbs of fishing gear..... Yup....

25ft Deep Vee with 70000lbs of displacement sitting still in the water.... 18inch Bennet Trim Tabs....

At 5000rpm should see around 28kts with 14GPH fuel burn .... at 3500rpm should see around 17kts with 5gph fuel burn.... This is with 1.8:1 ratio on outdrives and 14" x 15pitch props....

Boat should come up on plane around 13kts with the trim tabs.....

I think we'll be fine with this power setup... If you have any actual calculations that shows it will be marginal it would be appreciated.

Thanks,

JohnnyO"
 
"[b]"This is with 1.8:1 ra

""This is with 1.8:1 ratio on outdrives"

Ayuh,...

Those Drives, Should be 1.98:1s......"
 
"the 120 are 4 cylinder GM eng

"the 120 are 4 cylinder GM engines from or used in the Chev II's from the 60 era, blocks are now cast for mercruiser in Mexico. To make them 140 hp, need to have blocks bored to accept 350 CID pistons on same rods. Ran one in a 22 ft boat pulled skiers no problem, a shipmate down in key's ran a pair in a 30 ft cuddly cabin, pulled skiers with no problem."
 
"Well - over the weekend we gu

"Well - over the weekend we gutted the entire interior. There is some delamination of the fiberglass that forms the level floor from front to back over the bottom/center inside. Im going to just yank it all out. The backwall to the engine compartment needs replacing as well as the plywood that the aft cabin bedding sits on.

I can guestimate we removed about 700lbs worth of crap - interesting enough - I found a 120gallon aluminum fuel tank hidden under the sink/counter area..... We have 120gallon tank under the aft cabin's bed already ! Whoever owned this previously definitely had a need for LOTS of
fuel....

We will redo the interior but only with the V-Berth and the aft-cabin for sleeping - Will install some lightweight benchseats on port and leave starboard opened up for our fishing gear. We've really lightened this thing up since we started by over 1200lbs ! ! ! ! That should definitely help out top speed from the twin 120s.

A/D vaughn could you find out from your friend what his fuel burn was and his approximate top-speed with those twin 140 I/Os ?

Regards,

JohnnyO"
 
"Find any waterlogged foam? <[

"Find any waterlogged foam?
snow.gif
"
 
No foam in the boat at all - V

No foam in the boat at all - Very thick fiberglass job though - very well built - stringers are all rock solid. The floor delaminated due to humidity from water left in bilge I am suspecting.

JohnnyO
 
"Ayuh,...

Guy,... They Don&

"Ayuh,...

Guy,... They Don't put Foam in Real Boats.....

It's usually in the 20',+ Under boats that get it..."
 
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