Mercruiser 3.0 to Mercruiser 5.7

[anesthes]

Hi,<br /><br />Ok.. So I have a boat that had a 3.0 I4 in it. I bought a 5.7 V8.. The mount on the I4 is up front, the V8 appers to mount on the sides.. <br /><br />How do we make this work??

 

[Bondo]

Re: Mercruiser 3.0 to Mercruiser 5.7

Build new mounts......<br />Then regear the outdrive, You've got 1.98/1 ratio, You Need 1.50/1.....

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

Oh I got a new outdrive with the motor. complete pkg. <br /><br />Thansk for the reply!<br /><br />-- Joe

 

[horndog]

Re: Mercruiser 3.0 to Mercruiser 5.7

can your boat handle that power increase? I am sure it will double!

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

Who knows. the boat is a 19 foot cuddy cabin..<br /><br />-- Joe

 

[DJ]

Re: Mercruiser 3.0 to Mercruiser 5.7

Your insurance man and/or the water law will certainly care.<br /><br />If you overpower and you are involved in any incident, you are AUTOMATICALLY guilty.<br /><br />Many boats have different hull layups depending upon the engine package to be insralled.

 

[horndog]

Re: Mercruiser 3.0 to Mercruiser 5.7

djohns is absolutely correct, not only that can your transom and hull handle the increase in power.<br /><br />You will double the torque of the motor which does most of the damage by twisting and could crack the hull. If your boat manufacturer is still around, see if you can contact them and talk to them and see if they had your model with that motor in it. You may want to reinforce the bilge area and the transom just to be "safe" If you don't care I hope you can swim!

 

[Doug Durako]

Re: Mercruiser 3.0 to Mercruiser 5.7

Anesthes---regarding insurance, you should be okay with 250 HP in a 19-foot boat, but you are right at the cutoff point. Most insurance companies accept up to 275 HP under 20-foot before they move you to a "high power" class. You should call your agent and check.<br /><br />Agree with other replies on checking for stress.

 

[Cobotr]

Re: Mercruiser 3.0 to Mercruiser 5.7

Many things come into play in addition to the concerns above. <br /><br />The added weight difference between the 4 vs 8 will be significant. That weight difference will make the boat "sit" different statically in the water.<br /><br />That weight difference will also change the accuracy of the boat capacity in how it relates to persons/weight allowed to be loaded in the boat as stated in the certification sticker. That certificate in essence is the amount of weight the boat can support (with it's designed floatation) when the boat has taken on water (sinking).<br /><br />The question is, is the liability worth it in the long run. Do some research. As mentioned above, maybe the manufacturer offered the same boat with the same power option. If so, find out what was different between the layouts. If not, there may be a reason behind it, such as safety, handling, etc.<br /><br />On a prior post quite some time ago, a guy who had done what you are contemplating gave feedback that included that the boat was nearly undriveable in that the boat wouldn't corner properly and didn't handle the added available speed well. Don't remember all the details such as make of boat or engine swap info but he sure was disappointed with the results. <br /><br />Good luck with the project. If it was just me and my boat I would maybe consider it (after lots of research), but if you have family, that puts alot of extra variables in the situation.

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

Weight shouldn't be an issue.. My boat came with the 3.0, as well as a 5.0. And since the 5.0 and 5.7 weigh the same the handling should be fine.<br /><br />The power rating of the 5.0 that it came with was 200 according to the chris-craft dealer. According to the guy i'm getting this 5.7 off of, he said 260hp. It could be more or less but I don't really have time to dyno it before putting it in.. If it has too much power, then we can reduce it to any of the 5.7's or 5.0's in my shop. (I own a race shop)<br /><br />My real question here was pertaining to the mounting of the motor on the sides.. I don't know what v8 marine mounts look like, I'm just used to an automotive application. One of the marina's I talked to said "I'm betting it will be flush with, if not an inch off of the floor" which makes me visualize the mounts being connected together under the oil pan. <br /><br />Anyone got a pic? an idea??<br /><br />As far as insurance goes, never insured any of my boats.. If I sink it, its sunk.. If I crash into someone, then i've exceeded the speed limit of the river I run in.. (I dont do ocean). And as a part-time police officer, that is NOT an option..<br /><br />Thanks!!!<br /><br />-- Joe

 

[Doug Durako]

Re: Mercruiser 3.0 to Mercruiser 5.7

Cobot makes some very good points.<br /><br />I looked a long time before purchasing my rig because I wanted power in a small boat. I found many boat makers use a "standard" motor that hardly gets the boat on plane and then they add $1500 or more to the price for EACH 50 hp upgrade.<br /><br />That said, no one drives my boat other than the four members of my family. Each of us has more than 10 years experience driving both I/Os and outboards. We all ski and are experienced pulling skiiers. Both my kids took certified safe boating courses years ago before I allowed them behind the wheel.<br /><br />Many, Many, Many people could not handle the rig you are building and it will be scary at full throttle.

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

I can appreicate that. I've been boating, both by sail, and under power of motor; for over 15 years. My "play" boat which I'm selling right now is a 16 foot glastron that does close to 80mph.. Thats scary at full throttle. Thats scary at part throttle. <br /><br />I've had a million outboard combinations. What i'm doing now is building a bigger boat (19 foot, cuddy cabin, 1900lbs boat) that will still pull the skiers, plane off quick enough, and be fun to drive up and down the river.. (big river).. <br /><br />I havn't been out in the ocean in years ( on any of my boats), and don't really plan on it right now. Maybe in the future. <br /><br />Thanks!<br /><br />-- Joe

 

[arks]

Re: Mercruiser 3.0 to Mercruiser 5.7

Since your original question has yet to be answered, let me try to help.<br /><br />Your boat should have stringers (2 parallel "beams") running fore/aft in the engine compartment. Most manufacturers used wood and covered them with fiberglass. Make sure your stringers are not rotted! The fiberglass alone will not hold the weight of an engine and will collapse under the mounts. The engine will be attached to the stringers. I'm guessing the 3.0 had some kind of bracket that mounted to the front and was lagged into the stringers. <br /><br />The 5.7 uses 2 mounts in the front- one on each side just forward and below the exhaust manifolds.<br />They are adjustable up and down (depending on the alignment needed for the outdrive) and lag directly down into the stringers. I've seen "Heavy Duty" mounts made from angle that through-bolt the sides of the stringers. I'd suggest finding another boat with a V-8 and taking a look at it. You can usually find mounts on Ebay. Use the keyword "Mercruiser" - even if you engine is different, it'll work because all the 5.7 blocks are similar.

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

Hi,<br /><br />Thanks for the response.. I figured most of the stern drive parts are interchangleable, so I unbolted the 3.0 by the two bolts in the rear that goto the bellhousing, and the front mount and lifted it out.<br /><br />The driveshaft was frozen pretty good, but a few wacks with the hammer and it went out. may need to replace the bearing in there.<br /><br />Did some measuring. Need to invent some stringers/mounts for the motor on the sides. Shouldn't be too bad.<br /><br />The 5.7 mercruiser I picked up is a fresh water setup, which I think i'm gonna remove and goto standard cooling.. I run in rivers/lakes mostly, and the added complexity of the freshwater setup isn't really needed for my application.<br /><br />-- Joe

 

[DJ]

Re: Mercruiser 3.0 to Mercruiser 5.7

Joe,<br /><br />Regardless of what we think, you're going to do, what you're going to do.<br /><br />I think it is irresponsible, without thoroughly checking it out.<br /><br />You say that you are a Police Officer. Yet, you say you don't insure your boat(s).<br /><br />I hope I don't end up in your jurisdiction-as an HONEST person.

 

[anesthes]

Re: Mercruiser 3.0 to Mercruiser 5.7

<br /><br />Regardless of what we think, you're going to do, what you're going to do.<br /><br />I think it is irresponsible, without thoroughly checking it out.<br />

I did check it out. The boat was offered with a 205hp 5.0, which is the same size and weight of the 260hp 5.7. Yes, the 5.7 is 55hp higher, but prop sizing will dramaticly make the difference in whether or not the boat is overpowered.<br /><br />

<br />You say that you are a Police Officer. Yet, you say you don't insure your boat(s).<br />

Insurance is not required. In 15 years of boating in this area, i've yet to see a accident involving two boats.