HP rating for boats

[Rosscoe]

What is the opinion (or facts) here concerning how they rate a particular boat for X amount of horse power? My guess is: hull design (stability) and transom strength?
My Surflite is rated for 75 max and I'll be installing and 85. Might be unstable? Might be too much for a 50 YO transom?
Thoughts?

[Rich_V174SS]

I think you also have to take into account prop hp vs. power-head hp. Back then horse power was rated at the engine, not the prop (from what I hear) and what passed for a vintage 75 horse power engine might equate to one of today's 50 horse power engines.

[Rosscoe]

Yes, I do remember that the ratings changed..at least for Mercs. I thought they rated them different then the others at one time.
So if I put a vintage motor on a vintage boat with more HP, then its still too much. Hmm....

[Hyperacme]

HP changed about 1985 to the prop.
Which 85 hp motor are you going to install ?
In the 60's Johnson/Evinrude's were all V4's from 50HP up so weight would be the same or very close.
Remember Jerry telling me that the old flat bottom boats (like your 1961) don't handle speed, turning and waves to well, so you might want to be careful and take it easy.

[Rosscoe]

Its actually GTJeff's old motor. The one he had on his GT150 He said it's a 69...Evinrude
Yeah I do not find it hard to believe that this boat would not handle well. Quite flat in the back but there is a keel going up the center as well as a couple of other "thingys" Dont know what you call them, technically. IF I ever get this on on the water before selling it, I'd be hesitant to open it up. It would probably outrun my 163 if it didnt do something nasty.

[David CVX-16]

Mercury changed their horse power rating in 1982. My V-153 is rated for a 90 HP and I am putting a 140 HP old rated, 115 HP new rated engine on it. The transom on the V-153 seems to be holding up. Secure the motor by putting in four bolts through the transom tightly, clamp screws tightly, and big washers or plates to distribute the weight of the bolts on the end of the nuts. Accerate gently, no sudden turns, no sharp turns, and watch the waves ahead.

Deep V's can handle waves better than tri-hulls, and some hull brands were not engineered correctly and were subject to cracks, flexing, and breaking.  

There is a design flaw on the V-153's that cause the hull to roll over if turning sharply at low speeds. Really scary the first time it happened with a 80 HP Mercury on it, and would be really really scary with a 115 HP Mercury. I mentally am aware of the problem and avoid that set of conditions. 

[Rich_V174SS]

I experienced one of those torque rolls in my V174, scared the livin' $#!^ out of me! Took a sharp left turn at about 30 mph and the boat rolled 90* to port, I thought my windshield frame scraped the water. I yanked right on the wheel and simultaneously pulled back the throttle, good thing I have power steering or it could have been more difficult to right the boat.

[David CVX-16]

This from iboats forums as to the year of the change in horsepower ratings:

inline 6s were around 81-82, v6s about 83-84

1985 was the first year all outboards had to meet propshaft specs

[Rosscoe]

Torque roll? Roll over? 
Seriously, what did that fell like? You were turning to port and it rolled up on its side? Did it "feel" like it was sliding sidways or were you still making the turn? I'm trying to imagine what it was like. I dont think I've experienced that with the 163 but the hulls are probably different and I generally do not take sharp turns too fast. Scary!

[Rosscoe]

Mercury changed their horse power rating in 1982. My V-153 is rated for a 90 HP and I am putting a 140 HP old rated, 115 HP new rated engine on it. The transom on the V-153 seems to be holding up. Secure the motor by putting in four bolts through the transom tightly, clamp screws tightly, and big washers or plates to distribute the weight of the bolts on the end of the nuts. Accerate gently, no sudden turns, no sharp turns, and watch the waves ahead.

Deep V's can handle waves better than tri-hulls, and some hull brands were not engineered correctly and were subject to cracks, flexing, and breaking.  

There is a design flaw on the V-153's that cause the hull to roll over if turning sharply at low speeds. Really scary the first time it happened with a 80 HP Mercury on it, and would be really really scary with a 115 HP Mercury. I mentally am aware of the problem and avoid that set of conditions. 

I may even consider a transom stiffner or some type of reinforcement but I need to see how good the transom is and depending on all that, I may never have this floater in the water. If I'm going to have 3 boats, one is going to be a GT, CV or CVX

[Rich_V174SS]

Torque roll? Roll over?  
Seriously, what did that fell like? You were turning to port and it rolled up on its side? Did it "feel" like it was sliding sidways or were you still making the turn? I'm trying to imagine what it was like. I dont think I've experienced that with the 163 but the hulls are probably different and I generally do not take sharp turns too fast. Scary!

To kinda give you an idea of what it felt like - picture yourself tipping back on a wooden chair just at that point where it starts to fall backward and at the last instant you catch yourself to right the chair and you get that brief adrenalin rush, that kind of scary.

The boat wasn't sliding sideways, I was just in a tight left-hand turn and it felt like something snagged the boat like it went off balance and just rolled into an almost 90* bank. It could have been attributed to the hefty passenger I had in the left-hand seat at the time, that's the only time it has done it. See video below to give you an idea of the handling of the boat:

http://www.youtube.com/watch?v=Ry0y-h3xjpk

[Rosscoe]

OK I understand it more now.
I'm trying to be patient and watch the video. I have dial up and it takes a long time.