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Old 14-08-2020, 14:34   #16
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Re: exceeding inflatable outboard hp rating

I recommend the 20hp as well, but to match itwith the smallest dinghy that supports it which I think is the Highfield 340. Love the combo!
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Old 14-08-2020, 14:44   #17
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Re: exceeding inflatable outboard hp rating

Well, as somebody said, you don't have to use all the power available. But if you're not going to use it, why do you need it?

Personal experience, at the low end: our 8 foot Highfield ultralight with a 6 hp motor (all purchase decisions by the previous owner, came with the boat) will plane. I've done it once to prove I could, it was unstable as hell, I have no interest in doing it again. I'm about to switch to a 3 hp equivalent electric. Appropriate for our needs, probably not yours.
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Old 14-08-2020, 17:08   #18
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Re: exceeding inflatable outboard hp rating

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Originally Posted by CassidyNZ View Post
An outboard motor is as powerful as the operator wants it to be. If the operator has a 30hp motor on a 10 foot dingy and wants to use all the power, he will soon discover his lack of wisdom.

I have a 15hp on a 10ft dingy simply because my new dingy is smaller than the old one and the motor is in too good a condition to spend money downsizing it. My new dink jumps up on a plane with two adults and once there, I can throttle back to a “slow run” and it stays on the plane while sipping fuel. If I try to exceed the speed limitations of the dingy, it soon tells me not to be silly.

But when I need to push the mothership off a sandbank or tow it into a marina when the diesel won’t start, the small “recommended” outboard just wouldn’t cut it.

So don’t ask if the outboard is too powerful, ask whether you have discipline and good manners to handle it intelligently.

Oh, and if you do handle it intelligently, the chances of having to explain the power anomaly to your insurers is vastly diminished.
Couldn’t agree more. The idea that a dingy “ gets squirrelly” after getting on a plane makes no sense to me. 300 MPH dragsters are very stable at 50 MPH, at 300 not so much.
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Old 14-08-2020, 17:42   #19
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Re: exceeding inflatable outboard hp rating

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Originally Posted by s/v Jedi View Post
I recommend the 20hp as well, but to match itwith the smallest dinghy that supports it which I think is the Highfield 340. Love the combo!
My 10’6” AB is rated for a 20, as was my old Zodiac 3.10, but I can’t recommend the Zodiac.
The best prop for the AB is an 11” in SS, the Zodiac even though it was heavier was much faster and could take a 12” SS prop, the AB is much lighter, but has a much deeper V, and handles chop better, and is a drier boat, but is slower and isn’t as stable at speed as the Zodiac was.
By slower I mean 10 kts slower, the Zodiac was a 30 kt boat and stable at speed, the AB is I think in the low 20’s?
To carry 3 adults and plane with the AB requires a 10” prop, the 11 just isn’t quite up to it, but the 10” with just two people will tap the rev limiter at full throttle.

If I had 5 more HP, over the rated max, then it could get onto plane with three adults without having to slightly underprop it.

That’s one reason to have a higher HP motor.

By the way, it’s rated for 5 people or 1519 lbs, I seriously doubt it would plane with the max HP motor at half the max weight, maybe if seriously underpropped, say a 9” prop it might.

For the way we use our dinghy, I wouldn’t have use of one that won’t plane. Go to George Town in the Bahama’s, watch the people in the small dinghies coming back or going to get water or fuel, food etc, they usually arrive soaking wet, many wear foulies on just a normal day, there are many, many anchorages that you have to cross open water to get to the dock, or a snorkel site etc.

Been a few years ago but in Key West I pulled two different small sailboats off of a grounding, I had to seriously work at it too, several different pulls in different directions until I got them off, no way I could have done that with say a 9.9.

I don’t know who normally gets them off as everyone else was just passing by.

However that link I linked to on how the max HP is computed, is a CFR, which is law if your a US boat. I’m pretty certain that no one cares, but if an LEO really has it out for you, that’s another ticket they could write, I’ve never heard of it happening, but it could.
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Old 14-08-2020, 17:57   #20
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Re: exceeding inflatable outboard hp rating

My husband put a 40hp two stroke on our Highfield 380 and we couldn’t be more pleased with the result. The dinghy planes easily with 700 pounds of human and all of their stuff even at slower speeds. The insurance company knows about it and hasn’t recommended any changes.

The dinghy actually handles much better with more horse power and is in fact less squirrely than our Highfield 360 with a 15hp two stroke. We are now looking for another 30-40hp outboard for the 360 Highfield.
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Old 14-08-2020, 19:14   #21
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Re: exceeding inflatable outboard hp rating

I don't think it is a good idea to go over the HP rating. I put a 15 HP on an inflatable that was rated for a 9.9 HP. The two engines are basically the same and are the same weigh. As has been pointed out, the 15 made the inflatable squirrelly. It was fun, but not what I'd want for the long term. Trying to throttle back to get a lessor amount of power never seemed to work well.
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Old 14-08-2020, 20:00   #22
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Re: exceeding inflatable outboard hp rating

Obviously there are two issues: Legal and safety.

Legal: You may be considered negligent if you have an accident when operating a boat with horsepower in excess of the plate on the boat. And you could get a ticket in some jurisdictions, and your insurance may use that as justification to deny a claim.

Safety: Excess horsepower and weight may be dangerous, handling problems, etc.

All that being said I have often had bigger motors than recommended. I had a Merc 25hp 2 stroke on my Avon 3.41 soft bottom. It would literally leap out of the water from a standstill. I flipped it twice. But it was a fun boat and I felt I knew what it could take and what the dangers were, and I pushed the limits. Flipping it was not unexpected, nor did it teach me any lessons. But Judy wouldn't drive that boat with the 25 so I sold that motor and bought a 15 hp which we now use on a Zodiac 360 soft bottom. Much more mellow of a boat, but less fun.

My friend had a very light 15ft boat with a 150 merc on it with jackplate, power trim and a foot throttle which would go in excess of 70mph. That too was fun, very wild at full speed and quite a handful in a turn in choppy water, but since it wasn't mine I did not flip it.
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Old 15-08-2020, 22:44   #23
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Re: exceeding inflatable outboard hp rating

I had a 18Hp on a 9Hp rated Aquapro 2.4 for years. I mirror calm water the GPS showed 45knots flat out. I could pass a smaller jetski, but needed to be well braced in, sitting on the floor.

I could plane easily with four adults and gear (say 250kg), and could go hard with 100kg of gear and me.

After a few years of my amateur transom repairs it ripped off completely about 2km offshore. I'd had the foresight to tie a bouy, just in case.

Overall that tender gave me 20 years of service.

PS Wot's "insurance" for a tender?
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Old 17-08-2020, 14:31   #24
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Re: exceeding inflatable outboard hp rating

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PS Wot's "insurance" for a tender?
My boat insurance includes itemised comprehensive cover for both the tender and the outboard motor. I have declared the size of both the tender and the outboard on my policy docs so if my insurers intend to refute a claim due to an “over-sized” outboard, they’ll meet some resistance.
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Old 17-08-2020, 15:10   #25
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Re: exceeding inflatable outboard hp rating

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I have declared the size of both the tender and the outboard on my policy docs so if my insurers intend to refute a claim due to an “over-sized” outboard, they’ll meet some resistance.

You must be new to this whole insurance claims thing.
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Old 17-08-2020, 15:23   #26
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Re: exceeding inflatable outboard hp rating

Stolen from a UK forum. 25hp on a 2.1m dinghy. The guy says it does nearly 9 knts at just over tickover and over 24 knts with half throttle.

Obviously this is extreme but does look like fun.

As to insurance, normally in the UK possibly the EU, insurance would have to pay up to a third party even if you breach their terms. They may or may not try to get some of that back from you. However, they would only likely be successful if your breach or negligence was the reason for the accident.

There are hundreds of cases like this with modified cars and bikes in the UK. If the modification wasn't the cause or a contributable factor in the accident then you are off the hook.
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Old 24-08-2020, 07:29   #27
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Re: exceeding inflatable outboard hp rating

The ratings on the dingy are there for a reason. Transoms are only so strong and are designed to certain limits. Not saying that a 20hp on a dink rated at 15hp will definitely cause damage, but I know of at least one person that did this and ended up with a stress crack where there transom meets the floor (aluminum dink). It did go fast tho.....
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Old 24-08-2020, 08:58   #28
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Re: exceeding inflatable outboard hp rating

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The ratings on the dingy are there for a reason. Transoms are only so strong and are designed to certain limits. Not saying that a 20hp on a dink rated at 15hp will definitely cause damage, but I know of at least one person that did this and ended up with a stress crack where there transom meets the floor (aluminum dink). It did go fast tho.....
The transoms are only so strong, that is correct. In terms of transom strength then, what are they rated for? HP, weight, thrust? The trim angle would also have an affect.

I have a 2.60m 3D Tender. It is rated at 10hp and has welded gussets supporting the transom. A friend of mine bought the newer 2.8m version. Same thickness aluminium, same attachment to the tubes, but doesn't have any support gussets. It's rated at 15hp.
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Old 24-08-2020, 09:00   #29
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Re: exceeding inflatable outboard hp rating

I used to do survey work for insurance claims. One of the first things they wanted to know was the information on the rating plate. Next was engine hp AND if there were any engine mods like an upgraded carb.

Saw several claims shot down in flames with no recourse because of this. One for a carb that added enough extra hp to exceed the rating.

If there is personal injury as part of the claim it will be very ugly as you will face it on your own.

Do what you will but understand what you do!
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Old 24-08-2020, 09:03   #30
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Re: exceeding inflatable outboard hp rating

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I used to do survey work for insurance claims. One of the first things they wanted to know was the information on the rating plate. Next was engine hp AND if there were any engine mods like an upgraded carb.

Saw several claims shot down in flames with no recourse because of this. One for a carb that added enough extra hp to exceed the rating.

If there is personal injury as part of the claim it will be very ugly as you will face it on your own.

Do what you will but understand what you do!
In which country? Where in the world? As I mentioned earlier, in the UK modifications wouldn't have any bearing on insurance claims unless the modification itself could be shown to be the cause of the accident. Insurers don't necessarily like it, but that is the case law.
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