What Happens When You Exceed Maximum Horsepower Ratings of Dinghies?

[Ike]

Thanks to GORDMAY for quoting my web site.

I had this question asked many times when I worked in the Coast Guard Office of Boating Safety. The answer is more complicated than you might think. Many people have answered about what happens to the handling characteristics when you over power a dinghy. But I will try to answer both the practical aspects (handling and performance) and the legal.

first: what type of dinghy is being asked about?
The US Federal hp regulation applies only to hard dinghies. Generally these are rated at 2HP or less because the manufacturer/builder gets a break on flotation and loading (meaning air chambers and more capacity). Rating them for over that means the regulation for outboard monohull boats under 20 feet in length applies. That means no air chambers, more flotation and less weight capacity.

The Coast Guard does not regulate boats that get most of their buoyancy from inflation, except for the requirement for a Hull Identification Number. The capacity label you see on all of these is voluntary based on the ABYC standards for inflatable boats. They don't have to put it on, but if they are a reputable manufacturer they will, especially if they are an NMMA member. Also if the boat is going to be exported just about anywhere in the world then the label and rating is required, by Canada, the EU, UK, Australia/New Zealand and any country that use the ISO rules.

So why do you see inflatable dinghies rated for 4 or 5 HP? Because manufacturers of these use the ABYC, or more often the ISO rules for rating HP. The US rules are based on a set formula which applies to all boats of this type. The ABYC and ISO rules allow a collision avoidance test which was mentioned earlier in this thread. This allows for a higher HP rating. Back in 2003 and 4 the USCG and ABYC conducted comparison tests between the US rules and the ISO rules by running US rated boats through the ISO course. Actually in many cases the USA rules came out at a higher allowed HP.

Does it increase speed? for small increases in HP say 4 to 6. NO. maybe from 7 to 9 mph. For large increase yes but that's why the boat gets unstable. It was not designed for that speed. It increases fuel consumption dramatically. (I had a 2HP Johnson that could run for 2 hours on less than a quart of gas, when I bought a 4 HP the fuel consumption doubled. and my hard dinghy only went maybe one mph faster)

So does this have any legal impact? As some pointed out if you overpower it can make you liable. It can cause your insurance to be void. But more directly in the USA, some states have passed laws making it a state violation to exceed the rating on the label. So, you won't be ticketed by the Coast Guard, unless you are operating in a reckless manner (they won't ticket you for overpowering), but you could be ticketed by a state or local law enforcement for overpowering.

Handling. As some said it can make the boat squirrelly, cause it to kite and flip, Another issue is flotation. The amount of flotation (that is the ability to stay afloat when full of water) takes into account the weight of the outboard. If you put a much heavier outboard on it it will no longer meet the ABYC, or ISO requirements for flotation, which includes being stable when swamped full of water. So you are defeating the flotation requirements for a safe boat.

So to sum up: It changes the handling characteristics (even to the point of being dangerous)
It increases fuel use:
It reduces your carrying capacity (because part of the max load is the engine weight).
It makes you liable in case of an accident.
It voids your insurance
It could be illegal in some state or other countries.
it does not really increase speed other than maybe a MPH or two.

It's not a good Idea

[CarinaPDX]

Many of us use 15hp 2-stroke outboards on RIBs rated for 10hp without problems. IMHO the best 15hp 2-stroke is the Mercury one. In the old Mercury product line the 6, 8, 9.9, 10, and 15 hp engines were essentially the same design: the 6hp weighed 73lbs and the 15hp weighed 75lbs. So there was no significant weight penalty to go up to 15hp. This extra power is very useful for getting up on a plane with a heavy load, but doesn't affect the top speed (about 20mph on my 9ft RIB, assuming it can get on a full plane) as the max rpm and prop size are the same.

Planing with a soft floor is not a great idea. However, the high pressure inflatable floors with an inflatable keel do very well; in this case because of the light weight a less powerful outboard can usually be used. I have seen one small HP inflatable dinghy with a 4hp plane with one person (but not two). RIBs are the best way to go if you can manage to store them. My old 8-1/2' fiberglass RIB worked great with the 15hp. To be fair, there is a learning curve and initially it seemed very unstable and a bit scary but with experience and large fins added to the outboard I felt very comfortable operating it throughout the speed range. My new dinghy is an aluminum RIB and the lighter weight and flatter bottom has me back to learning to adapt - it is more twitchy.

Putting a small outboard on a RIB because it seems unstable is like buying a car that can only go 40mph because a beginner feels uncomfortable going faster. You will learn how to handle the extra power, and you will value it for being able to plane with a heavy load. And no one is holding a gun to your head and forcing you to use all of that power, or travel at top speed - there is no substitute for good judgement.

Personally, I am very happy to have my Merc 15 2-stroke to use on my 9ft RIB, and would never want to trade it for a 10hp. YMMV.

Greg

[Cheechako]

Originally Posted by SomeGuyInaShirt

So I've got a cheapie West Marine roll up dinghy and a 5hp Honda 4 stroke. I top out at 14 knots empty and a slug's pace when loaded with fuel and groceries. I'm thinking of a proper RHIB but something small still and a 15hp 2 stroke engine. But I'm wondering, what happens when you exceed the maximum horsepower rating of a dinghy? Presumably the forces on the dinghy itself are minimal once on a plane and a 15 hp will get on plane easier with groceries etc. The 2 stroke motors are only 20lbs heavier than the much weaker 4 strokes so that seems like a minor concern but I am just surmising. Anyone tried this? I was considering a 3 meter Truekit catamaran dinghy though I heard they aren't good in waves or maybe a proper v-hulled Highfield ultralight 3 meter. I want to go fast and preferably not get beaten to death every time I run over a ripple like I do in my flat-bottomed dinghy. Advice?

If the motor isn't too heavy sinking the stern at rest, more HP is fine IMHO. For instance; The older Yamaha 15 hp was the same basic engine and weight as the 10HP. I had a 15hp and used it on a 10 ft+ heavy double floor RIB. Once on plane it would move quite fast and secure and stable. Later I used it on a replacement 9ft RIB. It was pretty squirrely on the shorter boat. In the end I wish I had got a single floor 10ft RIB.

[colinalleck]

Apart from the dinghy becoming twitchy so might your insurers and statutory authorities in the event of accident or death.

[AGermain]

Well, I had a tinker traveler combination liferaft / dinghy. It was rated for 5 hp, but I used a 6 hp outboard. After about a year the transom cracked. I added a “Partner” board, and kept using it.

[GordMay]

Originally Posted by Ike

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[gregjm]

A couple things you're saying that are incorrect or obvious. 1.) 2 stroke are NOT heavier than a 4 stroke. In the case of a 15hp though you are correct about the weight amount. 2.) These things have a throttle, use less of it if you're concerned about overpowering it. 3.) I read all the other comments...the real concern is engine weight on the stern of a given size boat. Too much weight and getting it flattened out is going to be a real chore unless you start moving all the passengers/freight forward. It becomes a mess. 4.) I run a 10'6' Newport dinghy with a 15hp Mercury 2 stroke and it is perfect for me. It moves above 20 mph and is easy to load and unload. Keep in mind that the 9.9hp and 15hp are the same powerhead and generally speaking the only difference is the carb. In most states anything under 10hp does not require registration. 4.) You talked about flat bottoms and rough rides....the real issue is the diameter of the tubes. I won't own anything less than 18" tubes and that cuts down your available options a LOT. Most are 16" tubes and the difference in ride is substantial, especially in chop. Get a hard floor preferably aluminum, I find them stiffer than wood. 5.) The problem with cats is as tenders they require a lot of fussing around. Here in the PNW I prefer not to be wet all the time. The other issue is becoming airborne, especially when heading into the wind. The use of too much hp will allow headwinds under the hull and can flip the boat. Again, too "heavy" an engine exacerbates this issue.

[gregjm]

The 9.9 and 15 are the same powerhead.....the 5 and 8 hp are the same powerhead.

[Asspload]

We had a 20hp tohatsu on the cheapest $500 Tobin sport costco had. We put a 9.9 sticker so that it would look street legal.

The tender was bought as an impulse buy at Costco and was to hold us over as we hadn't chosen a "good rhib" for our new boat yet.

We never did end up buying a "good rhib" and ended up using our dorky Tobin for a heavy 3 years in the pacific northwest up until we traded boats.

We could leave the mother ship anchored and go exploring on that $500 tobin with a ton range.
Full throttle was doable but not necessary as she would plain at about 1/4 power.

I liked the idea of having a tender powerful enough to push the mothership around if needed.

Out of our whole new boat experience, that overpowered $500 costco roll up dinghy with a 20hp tohatsu disguised as a 9.9 gave us the least amount of drama and was the most reliable part of our journey.

Sounds like others have different experiences and opinions. This was just experience.

Good luck

[Shanachie]

A contrary view: What's the rush?



I have a 2.3HP Honda on a 9-foot inflatable. It does 4.5 knots with two people. That's really about the upper limit if you don't want to offend your neighbors in the anchorage.


The Honda weighs about 30 pounds, which means I can lift it with one hand while replacing it on the stern rail. With Weaver davits and a sugar scoop stern, I can have the dinghy going in less than 10 minutes with no sweat.


People who race through anchorages in dinghies are not much more considerate than jet skiers and small power boaters, IMO. Relax. You're a sailor. Enjoy the scenery and quiet.

[rslifkin]

Originally Posted by Shanachie

A contrary view: What's the rush?



I have a 2.3HP Honda on a 9-foot inflatable. It does 4.5 knots with two people. That's really about the upper limit if you don't want to offend your neighbors in the anchorage.


The Honda weighs about 30 pounds, which means I can lift it with one hand while replacing it on the stern rail. With Weaver davits and a sugar scoop stern, I can have the dinghy going in less than 10 minutes with no sweat.


People who race through anchorages in dinghies are not much more considerate than jet skiers and small power boaters, IMO. Relax. You're a sailor. Enjoy the scenery and quiet.

For short hops or in crowded areas that's about as fast as we ever go in the dinghy. But being able to go faster is handy for exploring or getting to somewhere a bit further away where you're away from the anchored boats. I've run the dinghy 1.5 - 2 miles from the mother ship in one shot. There's a big difference between doing that at 4 kts vs 10 kts.

[Asspload]

No one is talking about being inconsiderate in an anchorage.
I personally like being able to open her up a bit once you are outside your anchorage to cover some ground.
The 4 stroke was also super quiet at low rpm, so good in a crowded anchorage.

Or we could be real purest sailor's and only use a sailing dinghy with ores.

What ever floats your boat.

I was commenting on op asking about an overpowered outboard on a dinghy.
I was sharing my experience only with a cheap $500 dinghy and how the world didn't come to an end.
Or of how to ruin people's evenings in a nice anchorage by being belligerent in a dinghy.

For reference we had a 2.2hp on our previous dinghy so going to a 20hp was a game changer in our case.
I think the tohatsu is 90lb so it is heavier then our 2.2hp.
The tradeoff in weight was worth it in our case as we have a power halyard to assist.

Everyone will have a different take on this so do whatever your comfort level allows.

Good luck

[donradcliffe]

I had Tohatsu 3.5 and 18 hp 2 strokes. I used the 3.5 95% of the time.

The 18 was good for dinghy races, winds over 20 knots, and going over 2 miles round trip. It topped out at 26 knots on the GPS with one person, but it had enough power to get into trouble at full throttle. The Rib was rated for 15 hp, so I took the 18 hp decals off to avoid unpleasantness from the local yokels.

The 3.5 was good for one handed lifting, pulling the dinghy up on the beach, and not getting stolen.

I don't give a fig about insurance companies and lawyers.

[Johan Leopard51]

Horses for courses…

Our vessel is chartered and at times clients take tender to beach or marina while skipper looks after yacht PS. Lessons learnt:
1. Tender combined weight must be as low as possible because charter clients cannot handle anything heavy onto and off beach even with small breakers. A fast heavy tender hits a coral beach harder than a light slow one
2. There are few situations where too little power proves more dangerous than too much power. We don’t have to do for example river entry to ocean at all.
3, Get a semi rigid tender. Inflatable bottoms do not survive rough beaches, especially if at times in hands of less experienced users.
4. Get a prop shield.
5. Get a prop shield.
6. See point 4 and 5.

Unrelated to motor : make yourself a simple woodstep rope ladder for easy boarding from water just off bow attached to the forward davit hooks. The older/weaker swimmers will love you. I have seen enough fat old arses with unplanned G string effect while being hauled into tender to scar me for life!