Mercruiser Upgrade

ljs22

Seaman Apprentice
Joined
Aug 20, 2008
Messages
46
I am thinking of replacing my 120 mercruiser in my 22 Ft. Starcraft Chieftain with a 302 and outdrive from a 70's Sea Ray. There are tons of those old boats around here with the 188 HP set up that are being practically given away. I am quite familiar with the mechanics of the operation but has anyone actually done this upgrade? I am looking for ideas on how to attach new motor mounts to the sides of the engine area since it has the aluminum stringers. Does anyone have any pictures of their mounts for a V-8 or even V-6 in one of these boats? I can have almost anything fabbed up out of aluminum, but I don't want to go into it blind. I plan to buy an old boat with a complete functional motor and drive and strip the rest.

a 15 MPH cruising speed is just not enough on the great lakes!
 

ljs22

Seaman Apprentice
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Aug 20, 2008
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46
Re: Mercruiser Upgrade

Finding a 70's vintage Starcraft with anything but a 4 Cyl or the L-6 is not an easy task. They really only put the 8's in the 25 Footers. I was hoping someone that had actually done this conversion has some pictures. I can "engineer" the mounts if need be, but I would like to do it safely. I did not see anything on the stringers that would indicate this boat was set up for anything but a "L" engine.
 

jtybt

Senior Chief Petty Officer
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Aug 5, 2009
Messages
730
Re: Mercruiser Upgrade

What are the distance between stringers? Measure the distance between the motor mounts of one of the many 888/302 motors and go from there.

There may be a reason only smaller motors have been put in the smaller aluminum boats...torque!!! and the vibration may fatigue the aluminum quickly. Even if you beef up the stringers, their attachment to the thin aluminum hull won't change.

Fiberglass can be reinforced at the stringer/hull attachment. I wouldn't trust even epoxy to aluminum attachment because a ding to aluminum would probably pop off the epoxy bond.
 

ljs22

Seaman Apprentice
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Aug 20, 2008
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46
Re: Mercruiser Upgrade

Torque I could buy, but I challenge you to find a properly running v-8 that vibrates as much as even a properly running 120 or 140. I am pretty sure (not 100%) that my hull is the same as the the 25 foot hull, with just 3 less feet. My hull is also rated for 190 HP on the plate if I recall corectly. It is more a matter of the engine mounts as these boats are very open in nature as far as stringers and floor.

Someone has to have upgraded one of these things!
 

CheapboatKev

Vice Admiral
Joined
Oct 4, 2008
Messages
5,813
Re: Mercruiser Upgrade

Your saying you can only get 15mph outta your current 120?
 

ljs22

Seaman Apprentice
Joined
Aug 20, 2008
Messages
46
Re: Mercruiser Upgrade

no, I can get about 25 out of the 120. But I don't really want to run it at 3800 RPMS all day. That tends to use a lot of gas. At 15 MPH it runs about 3000 RPMS. Then engine is fine mechanically, it is just not enough grunt for a boat this big. The Chieftain has a larger cabin than the Islander. I use this as a cruiser, not a fishing boat.
 

CheapboatKev

Vice Admiral
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Oct 4, 2008
Messages
5,813
Re: Mercruiser Upgrade

I see...
It would have been cool if SC woulda made a twin 140 version for these bigger boats..or twin 165 straight 6's...

Maybe you & I need to trade boats! lol
 

Bondo

Moderator
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Apr 17, 2002
Messages
70,468
Re: Mercruiser Upgrade

I am looking for ideas on how to attach new motor mounts to the sides of the engine area since it has the aluminum stringers. Does anyone have any pictures of their mounts for a V-8 or even V-6 in one of these boats?

Ayuh,.... I yanked the 140hp/ mod.1 driveline outa my 74 221 Islander,+ dropped in a 4.3LX/ Alpha 1...

My motor mounts are .090 sheet aluminum coming off the top of the stringers flat, then angled down, then bent to match the ribs at the hull...
The top flats are about 4" wide, for the wood to sit on,,+ there's about 4" between them on the bottom's ribs...
All pop riveted(3/16") in place with wooden blocks to match the motor mounts on the motor...
And,...
Unfortunately,.. I Don't have photos....
 

ljs22

Seaman Apprentice
Joined
Aug 20, 2008
Messages
46
Re: Mercruiser Upgrade

Well that does give me hope. All I need to do now is find the right donor with an Alpha 1 or older and at least a V-6. I guess this would be a great one to document with some pictures. I can now visualize how you did those mounts. It doesn't sound that complicated. I am guessing I will have to build a wider dog house too.
 
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