Will an outboard motor that is too big for my boat hurt my transom on my zodiac

[Cmcguffs]

Hi, I was wondering what would happen if I put a Johnson 15 horsepower outboard on a zodiac fish n hunt rated for a 10 horsepower motor. Do any of you guys know if anything bad would happen in a 5hp difference or if zodiac just says it can only go up to 10hp when you can actually put a bigger motor on and be ok.
Thanks!

 

[smokeonthewater]

Sure... Might damage the boat or possibly even kill someone....

That said, some 15's are the same block as 9.9's and weigh the same...

IF that is what you are looking at then it would only be the hp that was in question...

You say only 5 hp but it's a 50% increase.
Would be like putting a 225 on a boat rated for 150 or a 300 on a boat rated for 200.

It's a choice you have to make... You will get answers both ways on a forum from sure it'll be fine to no way in heck...

Just remember that people who know more about your boat than anyone else in the world specifically told you NOT to run a motor bigger than 10 hp...
They even wrote it on a plaque in the boat.

 

[robert graham]

And if you had a boating accident and anyone got injured or killed, you're outside the law with the oversized motor....just not a real good idea...

 

[MTboatguy]

It is sure not worth the risk, here where I live, if something happens and somebody gets killed in a boat that is overpowered, you will be looking at a negligent homicide situation.

 

[cptrick3]

Looks like more of a legal problem than a physical one. I have no doubt that when the boat was engineered they put a fudge factor in the design.

 

[jbcurt00]

Yep, USCG is all about fudge factors for ratings so disregard capacity plates as you see fit :facepalm:

As noted by 4/5ths of the replies, overpower your boat at your own peril, but please noted there may be others affected if you encounter problems, those that are aboard and those at the incident scene.

 

[rallyart]

With a 5 hp engine that boat will plane with two adults in it. With a 9.9 it is very fast and starting to get squirrelly. The transom can take the weight of a 15 and probably handle the power. You won't have a very pleasant boat to drive with the 15 on it though, and someone who is inexperienced could mess up bad.

 

[H20Rat]

And if you had a boating accident and anyone got injured or killed, you're outside the law with the oversized motor....just not a real good idea...

Please show me the law that says I can't do that. (hint, that little plate in the boat isn't a legal document, and in many states, not even required. My boat doesn't have one.) Next, please show me a single legal case where the overpowering of a boat was the sole and direct cause of the lawsuit. Thanks!


At least in my opinion, a 10 to 15 is fine. As smokeonthewater mentioned above, in many cases, it is the exact same motor. Especially on a zodiac, there is also fairly large safety cushion in capacities built in, as they are often 'rough service' boats.

 

[smokeonthewater]

Eh... I DIDN'T say that.... I said some are the same block..... When HP increases it isn't the exact same motor.....

 

[BWR1953]

Please show me the law that says I can't do that...

Depends on where you live. The OP hasn't provided his location.

But here's the regulation for Florida.

http://myfwc.com/boating/regulations/#nogo

[h=3]Maximum Loading and Horsepower[/h]

• No person may operate a monohull boat of less than 20 feet in length while exceeding the maximum weight, persons, or horsepower capacity as displayed on the manufacturer's capacity plate.

 

[UncleWillie]

The question was, "Will it damage the Transom?" Not likely!

If you already own the engine, and are concerned about the overpower question; Limit the rpms to 87% (2/3 Power) by not throttling all the way up, Limiting the
throttle range mechanically, or placing an air flow restriction under the carburetor.

 

[NYBo]

Limiting the throttle range mechanically, or placing an air flow restriction under the carburetor.

I remember reading that in some engine lines, the carb is the major difference between the 9.9 and 15 HP models.

 

[smokeonthewater]

The question was, "Will it damage the Transom?"

Actually that was the title.
The question asked if anything bad would happen and if "zodiac just says it can only go up to 10hp when you can actually put a bigger motor on and be ok."

All sorts of bad things COULD happen and he might well NOT be ok.

If he were to modify a 15 hp engine to only make 10 hp and it didnt weigh any more than a 10 hp engine he wouldn't actually BE putting a 15 hp on the boat BUT in the eyes of the law he could still be in trouble.

BTW 2/3 is 66.6...% not 87% BUT since you referred to throttle that is totally worthless... Hp output and throttle travel percentage are not directly linear.
It would have to be dyno'd to accurately detune the engine.

 

[smokeonthewater]

Whoops.. Meant to say rpm...

Either way... 66.6%, 87%, or any other number is worthless here be it rpm or throttle.

Would need dyno chart info for the exact engines in question....

In the end I think we have covered both ends of the spectrum just like I told him we would. Lol

 

[UncleWillie]

The 87% RPM is based on the Fluid density and the Propeller being fixed, and that HP is proportional to the cube of the velocity of the flow.
.874 is the cube root of .666.
All other things being equal, 87% RPM, NOT 87% Throttle, should produce very close to 2/3 power of the reference RPM in a Fluid/Propeller scenario.
The actual engine is immaterial. It is the prop pushing a mass of water at a velocity that determines the power.

 

[jbcurt00]

I think we've covered the topic and then some. If Cmc has further questions about either the for or against arguments let's wait for him to respond before discussing technical data. I'm fairly confident the technical aspects of how much more a 15 vs a 10 wasn't the question.

 

[Watermann]

[h=1]Zodiac Fish N Hunt 310S[/h]
Overall Length:10?2?
Overall Width:5?5?
Buoyancy Tube Diameter:1?5?
Passengers:4
Air Tight Compartments:3+1
Shaft:Short
Maximum Power Allowed (hp):10
Maximum Speed (mph):24

I would like to know who here wants to go over 24 MPH in a 10 foot rubber raft? :lol:

 

[MTboatguy]

I would like to know who here wants to go over 24 MPH in a 10 foot rubber raft? :lol:

Not on the top of my Hit parade! Been there done that, didn't have fun!

 

[Boaty_McBoatFace]

There are a lot of liability issues involved with this... a big personal injury case just got settled in Ontario after someone made a similar "enhancement" to their boat motor. Don't have the link but just Google Oatley Boating Accident Toronto. Again, not sure where you live but if you're under similar laws, I wouldn't take the gamble.

 

[southkogs]

Welcome aboard Boaty ... please read our forum rules about posting in old topics.

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