is it too big??

[OkcOffRoader]

i have a 1970 Galaxie trihull that one of the PO's fitted an 85 merc OB on...the motor was rebuilt and bored out to a 119...is that too big of a motor for a 15' trihull? the PO said that it keeps up with the big boys cause its all motor and no weight, but im concerned the motor might rip off the back of the boat...any suggestions???

 

[Chinewalker]

Re: is it too big??

There's no way to "bore it out" to a 119, or anything more than an 85 for that matter. Boring the motor oversize merely gets you back to 85hp. Many people thing they are getting more ponies when they rebore a blown motor, but it simply isn't true... An 85 on a 15-foot tri-hull is not uncommon - upper end of the range, but not unsafe and is probably the max recommended for your boat. The only way it's going to rip the transom off is if the transom is poorly constructed, deteriorated with rot, or if you hit something really hard.
- Scott

 

[arks]

Re: is it too big??

Yeah, my first speedboat was a 1976 15' Crestliner with an 85HP Johnson. As I recall that boat was rated for 90. It was quick outta the hole but not all that fast (maybe 35-38 MPH).
Your boat was built pre-1972, so it won't have a capacity plate. IMO you'll be fine with the 85 - if that's what it REALLY is.......

 

[mthieme]

Re: is it too big??

119 is probably the cubic inch displacement.
A couple more CI will give you at least an unnoticeable amount of hp increase.

 

[Chinewalker]

Re: is it too big??

Depending on the year, the 85 might be a 92.6 or 99.6 cubic inch. Boring it out doesn't actually gain you much, if any displacement as they factor that in with the pistons so that you can bore only one cylinder if needed. Otherwise, one cylinder would be "larger" than the others...
- Scott

 

[Chris1956]

Re: is it too big??

The Merc 850s from the seventies were 4 cylinder motors with a 66 cu in displacement. There is no way to bore them to anything close to 119 cu in. The earlier 85HP motors were inline sixes, however, I do not think you could bore an inline six of that era that much.

 

[OkcOffRoader]

Re: is it too big??

Here are some pics of the motor, im gonna go fish for more info from the PO......

 

[Chris1956]

Re: is it too big??

That is clearly an inline 4 from the mid seventies, or at least the cowlind is. It should be a 66 cu in motor

 

[OkcOffRoader]

Re: is it too big??

thanks for the info guys, i thought it sounded kinda fishy on the whole 119 thing, the PO doesnt know crap about motors and probably got confused about something...heck, he told me his 16' VIP had a big block 460 in it, so i went and looked, it had a volvo 4 banger in it...the outdrive was a 460 merc so he assumed that was the motor it had............with all the PO's BS aside, you guys do think that the boat is safe with that motor?? im taking the boat to the fiberglass shop, thats next door to the performance shop i work at, so they can look over things closely before i put it in the water....she isnt the prettiest boat around, but i think she will clean up nice and be fun to putt around in on the lake this summer....

 

[Chris1956]

Re: is it too big??

As long as the hull and transom are sound, and you use good judgment, you should be OK. My friend had a 15.5' Winner with that motor on it. It was real flimsy, especially in the front. It never cracked or got any damage in the dozen years we used it. We did not run it WOT in chop however.

I always felt that as long as you did not hit a good sized wave head on, the boat was fine. On a lake it would be great....I would skip the ocean....