8 HP to 9.9?

Hey all,

Rebuilding a 14' flat bottom aluminum jon and I'm starting to look for outboard motors. The Coast Guard plate on the back of my transom says that the boat is rated for an 8hp outboard. I looked at the sticky at the top of the "Motors" page and did the math for the maximum HP rating and got around 8hp also. My question is would the boat suffer if I went with a 9.9? Without modding the boat an 8hp outboard would do alright, but I'm going to have the extra weight of a large deep cycle battery, wood decking, bow mounted trolling motor, gear, cooler, and probably a fishing buddy. I'd like to at least have a 9.9 on it for as much push as I could give it and stay close to the rated HP limit, but I don't want anything too big that would damage the boat. Could I get away with that upgrade?

9.9 would be fine and you will have no problem being over horse powered. Now insurance companies will look at it differently though. But unless you have it insured you will have no worries with a 9.9. Fact is you won't even be able to tell the difference between a 9.9 and an 8hp. I wouldn't hesitate to put a 15hp on there myself lol....

Johny25 said:

9.9 would be fine and you will have no problem being over horse powered. Now insurance companies will look at it differently though. But unless you have it insured you will have no worries with a 9.9. Fact is you won't even be able to tell the difference between a 9.9 and an 8hp. I wouldn't hesitate to put a 15hp on there myself lol....

Click to expand...

15? Really? I forgot to mention that the boat is also a 1430, so rather thin. I'm not too worried about overpowering it, but then again I'm also not worried about going crazy fast. I just didn't want to drop a motor on there that was too heavy for the transom to support. The transom I rebuilt is pretty solid, but I just didn't want to put a motor too heavy for the boat on it. I'm not worried about getting insurance either. I guess I'll have to see if you have to insure a jon boat in my state, but if not I'm not getting it.

Well depending on what year motors we are talking the weight difference between an 8hp and 9.9 is maybe 20 pounds which is nothing. And most 9.9 hp motors weigh exactly the same as a 15hp. The 9.9's are usually de-tuned 15hp motors, meaning that the only difference is the carburetor throat size. Some 9.9's just have restrictor plates on them that doesn;t allow them to make the 15hp.

Now a 4 stroke is going to weigh more than the old 2 strokes. My 1987 Johnson 9.9 weighs 77lbs. But I put a 15hp carb on it and the 15hp exhaust tube. This added no weight at all and it is now a 15hp motor. The 15's of that era weigh the exact same as the 9.9.

15hp may be to much? But it sure sounds like fun on that 1430 :lol: You will have to decide what you are looking for I guess. If you want to go fast then you could run a 15 without a problem most likely. But it will be a scary fast ride most likely :twisted:

Would you suggest getting a 9.9, trying it out and if the boat can handle more coverting it over to a 15 or just go straight to a 15?

Well not all 9.9 can be converted to a 15hp. And it depends on what you are looking for? Do you want to go really fast? And what sort of weight will you be pushing total, that's boat, motor (60-80lbs), you, fuel, equipment, another person.......etc.

Here is a video of a 12ft boat with a 15hp evinrude on it and it flat out hauls with 2 big guys in it. The vid probably doesn't do it justice to how fast it really is going but it will give you a reference : )
https://www.youtube.com/watch?v=3x176tgSzvw

That little boat is hauling tail. The water there looked awful fishy too. Thanks for sharing.

I have an OLD flatbottom (no USCG plate)14' long, 32" wide at bottom. When I got the boat 4 yrs. ago it had a '56 Johnson 10 hp. GPS speed was 17.2 mph with no gear and just myself,gas, and a battery in the boat. Late last season I repowered to an '83 Evinrude 15 hp. First run out I was fully loaded with gear, gas, battery and 2 guys in the boat. GPS speed was a flat 21 mph. Boat handled fine and planed almost instantly. Figure I'll maybe get 25 mph if running alone.(Still have to check!) This was with both motors trimmed to the highest notch. How did I get the Evinrude 15? I bought a 13 ft boat, motor, trailer for for $800. yanked the motor off, sold the boat and trailer for $650. Can't beat a $150 15 hp! Yeah, you could go with a 15.

Mine's an 8 - if I could find a good priced 15 I'd upgrade in a heartbeat. IMO, it doesn't make sense to upgrade the motor unless you're at least doubling the HP provided you're already low in HP to begin with.

I'm still looking for a good deal on an 80's 15hp rude...

Well based off of what you guys suggest I think I'll just go with a 15. If anyone sees a good deal in the Atlanta area let me know!

My boat is a 1434 Delhi with a new 15 hp merc 4 stroke. I have no problems with this motor on this boat.

I have a 1336 (or 1330 haven't measured, don't really care), and I have a 10hp on it with no problems. I had the same 10hp on a 8' Achilles that was only rated for 6hp, and again, had no problems. Fast ride, fun ride, have a fast and fun ride!

IMHO you are asking for potential trouble if overpowering a boat. if a DNR employee is having a bad day, it could end up with an expensive ticket. if you have the misfortune to hit somebody a lawyer will rake you over the coals. I'm wary about putting my 8 hp Yamaha (which is the same size as a 6) on boats rated for 5-6. at Gaston there was an officer in the parking lot checking all rigs for HP. he told one fella if he launched it to get his check book out.
as an aside, anybody out there want to swap a 6hp yamaha cowling for an 8?

bumpy

Modify the transom tag or remove it, fish and game are not that smart to know every boats rated HP. And they usually pay no attention to small boats like these with a few extra HP. If they are looking, they usually only hassle the 100+HP boats that are way overpowered. I still have yet to hear of anyone being sued for running 5-10 extra HP. If you hang a 50hp off a 12ft boat expect to be questioned, but as long as it doesn't look ridiculous they usually pay no mind to ya.

There is a coast guard formula that tells you what your boat can handle HP wise. This calculates transom width, boat length. And there is some formula that I cannot remember. But the HP rating usually calculates higher than manufacturer specs by 5-10hp. Manufactures like to be cautious :roll:

That being said, my engine cowling and engine block tag read 25hp but that is not what my motor is putting out 8) more like 30-35

https://www.tinboats.net/forum/viewtopic.php?f=4&t=12144

HP calculator, right here on tinboats!