To much power!!!!

I have a somewhat odd question. Every boat has a max hp rating that you aren’t supposed to go over. Well what is the reasoning behind that? I understand that you can’t put a 150hp on the back of a 12ft boat, but my questions is, how and why do they place the limits for certain boats.

A older 9.9 Johnson is the same exact motor as a 15hp Johnson, just larger carb on the 15. My 12ft boat is rated for a 9.9hp, but why couldn’t i slap the 15hp on my 12ft?

Are the manufactures concerned about the transom being able to take the torque of the larger motor? I understand the larger motor would greatly increase the speed of the boat, therefore possibly effecting the handling of the boat.

Are the reasons i listed, the only reasons, or are there others i’m overlooking?

Thanks!

I thinking its more of coast guard standards issue for safty reasons ,, so people DONT over power certain boats . I could be wrong thoo,but it makes sence to me

While I was in the Navy I asked this same question to a Coastie Officer. It was explained to me that its based on the amount of weight the boat can maintain level floatation with while swamped. It actually has nothing ot do with the HP, but that most outboards of certain HP ratings generally weigh about the same. For example a 25 hp motor weighs 40 lbs regardless of the Mfg. Theirfor, a boat rated for a 25 hp motor will maintain its level floatation with a merc, jonnyrude or yammie.

this came off the uscg web site.
Can I use a bigger motor on my boat than what it’s rated for?
It is not a violation of Coast Guard regulations to install or use an engine larger than specified on the capacity label, but there may be state regulations prohibiting it, and restrictions from your own insurance company regarding this.
There are no Coast Guard regulations against exceeding the safe loading capacity, however, there may be State regulations or restrictions from your insurance company which prohibit this. There is a Coast Guard regulation that gives Coast Guard Boarding Officers the power to terminate the use of a boat (send it back to shore) if, in the judgment of the Boarding Officer, the boat is overloaded. There is no fine for this, unless the operator refuses the Boarding Officer’s order. We certainly hope that you will abide by the rating, as overloading may lead to capsizing or swamping of the boat.

NOTE: The Coast Guard Capacity Information label is required only on monohull boats less than 20′ in length. The label is not required on multi-hull boats, pontoon boats (catamarans), or on any sailboats, canoes, kayaks, or inflatable boats, regardless of length.

It is liability. Boat manufacturers decide what rating the boats get, not the CG. I asked this Q to 2 boat manufacturers, and got the same answer. My reason for asking it was in regard to the low HP ratings a tiller 17 foot boat has. You take that same 17 foot boat, and put a console in it…125 HP! It has been proven that tiller boats are more “Dangerous” so a lower HP rating is tagged on it.

Tiller Guys are almost as “wild” as Bass Guys Chris!

Makes sense to me…

You guys are talking out of your trees Obviously you have not seen my tiller

Tiller boats are more dangerous… to the fish.

Ride in a boat that is overpowered, it’s not fun, not in my opinion anyways. I’ve been in some little “duck” boats with a motor way too big, and it’s a bit scary, atleast the driver knew it and never pushed the boat, so it was fine. I highly doubt a jump from a 9.9 to 15 would be too much.

In addition to the HP rating, I saw something that it went by weight or HP, you make the choice.

Well i’m going to keep my eyes open for a 9.9 or 15hp, and swap carbs if i get a 9.9. Thanks for all the replys!

I had a 12′ boat for many years that was rated for 10hp but I had a 15hp on it. It was fine with 2 people, but if I was alone, the bow of the boat would bounce up and down pretty severely w/ at full speed.

For a 12′ boat, it definately flew! I wouldnt hestitate to do it again. Like you said, the added weight of the motor was negotiable.

I have a 12ft John Boat rated for a 10HP motor. The boat planes great with 1 or 2 people and a 10HP Johnson. When I trioed my 15 HP Evinrude on it, it just did not perform well, regardless of how the motor was set on the boat or what prop I used. I can’t explain it, but it actually works better with the 10HP.