I have a boat rated at 45HP max on the coast guard specs. I'm currently using a 30 hp motor. I have added a built in gas tank/casting deck since purchasing the boat which has added some weight. 45's are hard to come by and I would really like to put a 50 HP on there. What are the repercussions for having an overpowered boat? Is it possible to get a new rating for the boat? Any knowledge is appreciated.

I think the difference in a 45-50 is minimal at best. I would not recommend overhorsepowering, but in this case, I think you would be fine with a 50.


The only thing I would be worried about is some sort liability thing if anyone ever got hurt.

I can see no reason to not put any size motor on that boat you want to...as long as you are not concerned about insurance. The coast guard tag is easily removed and the boat registered as home built as there is no title on boats in many states. I guess the only other consideration is your and your passengers safety. (Can the hull physically handle the added weight and speed the larger engine will supply.) The short answer is no the boat cannot be re-rated and with the added weight in all likelyhood it would probably drop due to the fact that the boat now weighs more.

I personally think you would be better off going with a 40 hp motor than going higher than the manufacture's rating states. I know you want to go as fast as possible, but those tags are there for a reason. Generally a boats transom can hang a larger engine than what the tag says, but the hull and boat as a whole is most likely not able to take the extra forces. There is a lot of engineering that goes into hull and boat design and safety factors are a part of engineering for a reason. I don't really think it is legal to simply take the tag off and register it as a home made boat if it was factory produced. I will search through some of my books and find that tidbit of info this weekend.

Agree with dtro. Should be fine being your not going over board on the amount of hp added and you have the wieght to offset. And your bennifit maybe torque added to youe performance. May not see much added to top end. Out of the hole will be better.

Some boats are rated for speed and weight. If you add excessive horsepower you stand a chance at exceeding the limits of the boat. Porpoising that leads to chiming(chiming is when boat dances to one side of hull to the other) is a factor at higher speeds. The boat may become out of control. Some hull designs are not meant to go over a set speed. Ive been in boats that were overpowerd and it gets scary and dangerous.

Also remember that a lot times added horsepower (5-10hp difference) is merely a larger carb. 35 and 40 are essentially the same outboard in most cases, just a different carb.

dtro-I'm not following your logic here
"35 and 40 are essentially the same outboard in most cases, just a different carb"

This is the case with the Merc Optimax 135, 150 and 175. Essentially the same just different sized carbs/injectors; does that mean I can put the 175 on a 1700 explorer? I think not!

The manufacturer plate is there for safety not to increase someones motor sales.

Horsepower ratings sometimes seem a bit fishy to me. Take for instance the Ranger 1750 Reata. It's rated up to 130hp. But wait a minute! Nobody makes a 130hp motor....so how'd they come up with that! Probably some computer and engineering calcs and not field testing. If a boat's hull isn't designed to take on an added 5hp, then that boat has some serious problems! I wouldn't want to be in a boat like that.

Going 5hp over the limit from a practical standpoint probably won't be a problem at all. I mean really...put a 5hp motor on the back of a 16' or 17.5' boat and how much added weight and torque does it produce? Not much I'd guess. However, as mentioned, the insurance and liability is another thing. If it waives your insurance, that aint good! Better to stay under than go over IMO, but some of the hp ratings like the 1750 Reata don't make sense.

Apples/Oranges

Your trying to compare a 35/40 to a 135/175.......come on now

I think with every situation an individual needs to [PoorWordUsage] what the risks might be. Nobody went ahead and said, "yeah sure hang that 50 on there". There could be possible repercussions to slapping a bigger motor on there.
IMO, I wouldn't have a problem with doing it to my boat.

Do we know if that rating was based upon the horsepower at the engine or the prop, or merely a weight issue?

I am sure you put some of the bigger motors on the dyno, there would a + or - of at least 5 hp between them. Heck, a 45hp motor maybe really putting out 48hp or even 50hp. You might have a 50 running at 43hp or 55hp.

I have also heard about what dtro said. I think it has to something with the 9.9hp to a 10hp. What a wopper that is

I guess I would do it if I got the 50hp at a deal. Take it easy and see what it does.

For about 10 years my fishing buddy has had a 60 on a Lund Alaskan that is rated for a 50 and has had no negative effects on performance or the boat.

I've heard from a couple of old timers that the 9.9 came about when restrictions were put on lakes in some national parks that banned motors 10 hp and up.
The manufacturers just slapped 9.9 decals on thier 10 horses to make them legal.

The 9.9 and the 15 are the same motor with different carburation. I dont think that the couple horse power over that the 50 would give you would do any harm. You can get the same performance change on some motors with a prop change. Just use your common sense when you run it and you should be good to go.

Not to highjack this topic, but what about if you wanted to increase HP? I have a 15 HP mariner, would there be a chance I could get a carb and have it be a 20HP??

Your boat will handle the small difference with no problem, but one of the main concerns of this is liability in case of accident. Insurance might not cover if they see boat was overpowered, even if by few HPs.

Regarding motors with different HP many have same block and powerhead, just carburation or fuel injection are modified.
In the case of Opti 135, 150, and 175, yes they are the same motor with different fuel injection setups.