Inflatable horsepower maximum

[Ten1951]

Perhaps someone can provide me with some additional information on the maximum horsepower posting on the transom. Bought a new (still-in-the-box) 12ft. inflatable. It states that the maximum horsepower rating is 20hp. Here's my question, ie: A tohatsu 4-stroke short shaft shows the same weight of 114lbs. for the 15hp,18hp and 20hp models. Can I use an older Merc 25hp 2-stroke that weighs perhaps 95-100lbs. ie: it's lighter, but has 5hp more than the posted maximum? Also, is it the weight of the motor alone that is the determining factor, or the power of the motor that places additional stress on the inflatable, or both? Bottom line, can I use this Merc 25hp when clearly stated that 20hp is max. Thanks in advance for your comments.

 

[Drowned Rat]

Re: Inflatable horsepower maximum

Also, is it the weight of the motor alone that is the determining factor, or the power of the motor that places additional stress on the inflatable, or both?

It is both in addition to the potential speed of the craft. However, I would not hesitate to put the 25 on there. Just know that it will be easier to exceed the capabilities of the boat and/or the operator.

 

[Sea Rider]

Re: Inflatable horsepower maximum

It is both in addition to the potential speed of the craft. However, I would not hesitate to put the 25 on there. Just know that it will be easier to exceed the capabilities of the boat and/or the operator.

Agree with that, infalte your sib to it's max factory recommended presure that should be 3.5 PSI, buy a presure gauge, a must have article, balance mates properly towards bow. And don't make full throttle, rather throttle gradually untill on plane.

Happy Boating

 

[trendsetter240]

Re: Inflatable horsepower maximum

Perhaps someone can provide me with some additional information on the maximum horsepower posting on the transom. Bought a new (still-in-the-box) 12ft. inflatable. It states that the maximum horsepower rating is 20hp. Here's my question, ie: A tohatsu 4-stroke short shaft shows the same weight of 114lbs. for the 15hp,18hp and 20hp models. Can I use an older Merc 25hp 2-stroke that weighs perhaps 95-100lbs. ie: it's lighter, but has 5hp more than the posted maximum? Also, is it the weight of the motor alone that is the determining factor, or the power of the motor that places additional stress on the inflatable, or both? Bottom line, can I use this Merc 25hp when clearly stated that 20hp is max. Thanks in advance for your comments.

No you should not put at 25hp motor on a boat that is rated for 20hp max. 5hp may not sound like a lot but in your case you are overpowering the boat by 25%.

The ratings are there for safety purposes. The boat is designed for a specific amount of stress on the transom and a specific speed at which it can maintain control. You risk losing your motor off the back or worse losing lives in the process.

Weight is not as important a fact as HP. If you want a lighter motor then by all means go with a 2-stroke. Just get yourself a 20hp and have a safe and happy boating experience.

Cheers

 

[Sea Rider]

Re: Inflatable horsepower maximum

You risk losing your motor off the back or worse losing lives in the process.

This statement is very exagerated, the only way to loose an engine is by not adjusting engine clamps to transom properly or a defective glued transom. To make it short, seems by the 1 post on this forum that you are a complete boating newbie. No body is born with boating skills, you get them along by experimentation.

So to have 25% more power is not a big issue, you will have the extra 5 HP at the end of the trottle, What you can do is : don't make hole shot outings at full throttle, throtle gradually to be on plane or put a mechanical throttle limiter to limit max throttle for peace of mind.

Throttle with prudence untill you are self confident and skilled boating a 12" sib with a 25 HP engine, other issues to consider, number of mates, cargo, sea conditions, etc when cruising. Besides who runs at full throttle 100% of the boating time ? Nobody!! The nice part of counting with the next HP model engine is that you can upgrade to next larger sib with same model engine.

Don't want to give newbies bad boating examples, but we use 12.5" sibs with Tohatsu's 30 HP engines with no problems at all, correctly inflated and prudently driven, a breeze to handle...

Happy Boating

 

[trendsetter240]

Re: Inflatable horsepower maximum

This statement is very exagerated, the only way to loose an engine is by not adjusting engine clamps to transom properly or a defective glued transom.

No not by a "defective glued transom" but by applying 25% more force than the transom joints (glued hypalon and plastic) are safely capable of handling. If the boat was safe to operate at 25hp then the manufacturer would have rated it as such. Maybe it is an overstatement but it's certainly possible.

Not all ribs are the same. Some 12.5 ribs are rated for 30hp! Just because you have a 30hp motor on your rib doesn't mean it is safe for the OP to have one.

 

[Sea Rider]

Re: Inflatable horsepower maximum

Sorry, are sibs not ribs. I understand plainly you worries for the extra 25% power on transom, to get the idea right, nobody maintains 100% full throttle speed to achieve this free stress on transom ? As an example, our sibs are rated to 20 HP also, are being used on daily basis for 8 hours with 30 HP engines for the last 2 years, + have to cross 8-10 feet waves in a row to reach surfers field, so make your own conclusions. The sibs are same as when taken out of the box, including transoms. To make a sib with slight bigger engine dangerous or not will depend entirely on the boater experience.

Happy Boating (With prudence)

 

[smokeonthewater]

Re: Inflatable horsepower maximum

my thoughts are: No you should not under any circumstances exceed the manufacturers rating........ would I?... you bet your uhuh I would

 

[kenmyfam]

Re: Inflatable horsepower maximum

OK so I am going to be the "fly in the ointment" here. I do not know where the original poster is from so will make some general comments.

Max HP as stated by the manufacturer should be the max fitted to the boat. The manufacturer can and most likely will refuse any warranty issues you have due to you nullifying the warranty by not following their guidelines.
Do you have insurance on the outfit ?? If so and you need to use it due to a claim made against you the insurance company have every right to refuse the claim as you had "overpowered" your boat.
More power will put more stress on the transom. I don't believe that can even be argued. Will it take the additional stress ?? can be argued as it may or may not be able to.
Would you put 25% more weight in the boat than it's maximum weight rating and still think you were safe ?? or would you only go out in 25% better conditions ??

Please note that I am NOT saying it can not be done but for the reasons above I personally would not consider it.
Make your own mind up as to your decision but remember what happens when you drive 25% over the speed limit !!!

 

[smokeonthewater]

Re: Inflatable horsepower maximum

very well stated..... it is a calculated risk..... the manufacturers rating considers many things.... they figure tradeoffs when the say 20 hp they know full well that some people will hang an extra long shaft motor on there which will greatly add stress to the transom.... they also know that some folks will dig up some old 1950's 20 hp outboard that will be VERY heavy... It is also very common and perfectly legal for a manufacturer to rate an engine lower than what it actually makes within reason..... Years ago I worked in a lawnmower plant and the only difference between a certain brand 18 hp and 22 hp engine was the sticker...... really we took them out of the same box.
Yes you do take some risk and YES it all falls on your lap if something goes wrong..... so no you shouldn't do it but then I've been pulled over for speeding more than once and I would in a heartbeat

 

[Drowned Rat]

Re: Inflatable horsepower maximum

I hung an 18hp Johnson on the back of a canoe rated for 5hp once. I wish I had taken some pictures of it. Needless to say, I was the fastest canoe on the lake.

Just be careful. Those inflatable boats can take alot more abuse than most people give them credit for. The only thing I would be worried about is the operator exceeding his limits with the extra speed.

 

[kenmyfam]

Re: Inflatable horsepower maximum

I hung an 18hp Johnson on the back of a canoe rated for 5hp once. I wish I had taken some pictures of it. Needless to say, I was the fastest canoe on the lake.

Just be careful. Those inflatable boats can take alot more abuse than most people give them credit for. The only thing I would be worried about is the operator exceeding his limits with the extra speed.

Not saying it can't be done, just the potential for being on the wrong side of a lawsuit should something go wrong.

 

[Sea Rider]

Re: Inflatable horsepower maximum

Well, if it's all about insurance and legal issues don't put a slight larger engine on your sib. Have not seen any sib rated for 30 HP using 40 HP engines have any sort of problems specially on transoms. In some markets as ours you cannot find 20 HP engines, you have 18/25/30 HP, so a boat rated for 20, next inevitable option would be a 25 HP engine. Besides we don't have legal issues regarding exceeding max HP rated for a given boat. Can place a 40 and fly to the moon...

The weight a transom can hold, is far superior to it's rated HP, that is, a sib rated for 20 HP engine can hold the weight of next 25/30 HP with no problem, specially if using 2 strokes engines, as they weight 10-12 kilos less than their 4 strokes counterparts. Modern sibs are rated to hold 4 strokes much heavier engines on their transoms.

Bottomline: 25% more engine power will not give you 25% more final speed, will give just some mph more, will handle more weight more efficiently, will depend also on the engine prop pitch, boat weight & hull type, number of mates on board and sea conditions. Max HP "stress" is reached at the end of full throttle at wot, and not at any other throttle position. So boat with prudence untill you dominate the beast at wot if opting for next slight larger engine.

Happy Boating

 

[kenmyfam]

Re: Inflatable horsepower maximum

I do not disagree with what you are saying. It is just that the "legal issues" could cost someone for the rest of their life.

 

[TOHATSU GURU]

Re: Inflatable horsepower maximum

U.S. courts have held that merely having an engine on your boat, that exceeds the Coast Guard RECOMMENDATION, makes you inherently liable for reckless behavior. We live in a world where you have to have a "Coffee is hot" warning sign

 

[Drowned Rat]

Re: Inflatable horsepower maximum

U.S. courts have held that merely having an engine on your boat, that exceeds the Coast Guard RECOMMENDATION, makes you inherently liable for reckless behavior. We live in a world where you have to have a "Coffee is hot" warning sign

I believe you are inherently liable for reckless behavior regardless of what size engine you have on your boat.

 

[TOHATSU GURU]

Re: Inflatable horsepower maximum

You misunderstood...Exceeding the hp recommendation is looked at as being reckless in and of itself regardless of any other behavior. For example: A boat at rest, legally anchored, engine off, but hp overpowered, is hit by another vessel that violates some regulation. The courts will automatically find that the boat, at anchor, has contributory negligence in the accident and assign them a percentage of responsibility for the accident.

 

[Drowned Rat]

Re: Inflatable horsepower maximum

You misunderstood...Exceeding the hp recommendation is looked at as being reckless in and of itself regardless of any other behavior. For example: A boat at rest, legally anchored, engine off, but hp overpowered, is hit by another vessel that violates some regulation. The courts will automatically find that the boat, at anchor, has contributory negligence in the accident and assign them a percentage of responsibility for the accident.

Sorry, TG, but that is not the case. Even if that were so, who would view it as negligent operation? No federal jurisdiction, that's for sure. Not even the Coast Guard enforces their own recommendation because it's a recommendation, not a law. At best it's a local issue and who's to say it would be a problem in the OP's area? Just sayin'...

 

[smokeonthewater]

Re: Inflatable horsepower maximum

civil suits are not based on laws but rather causes or at least perceived causes

I think we have very thouroughly answered this question

no it is not technically advisable
yes many folks feel that it would still be reasonable
some folks feel that it would in no way be ok
There is absolutely 0% chance that we will all agree on this.

I believe that the risk factors do indeed go up but I personally would go for it and at the same time advise against it

 

[TOHATSU GURU]

Re: Inflatable horsepower maximum

D R,

Unfortunately the recommendation has now become law in some states and the courts have always looked to assign blame where anything looks out of place. Its a fact that the courts now look at exceeding the recommended hp as reckless and if your reckless your negligent and that makes you liable. Your completely right about the CG not being able to cite for it. Personally, I think that regulating hp size on a boat is yet another intrusion in to our rights. You can not legislate good sense and responsibility...There is no limit on the size of the engine in your car, at least not yet. But, I'm sure the gooberment will around to that soon enough