Is 6hp outboard enough power?

[Plumbean]

Originally Posted by rslifkin

With 2 strokes the 6hp model is often a de-rated 8hp. With 4 strokes for most brands, the 6hp is a single cylinder (and available de-rated to 4 or 5hp). The 8 is typically a de-rated 9.8 or 9.9hp 2 cylinder.

Ah, did not know that about the 2 strokes. No longer for sale in the U.S. though.

[poiu]

With a 3.6m dinghy I have a 30 hp engine Fromm Suzuki. It’s just enough.

[Americat3014]

I have the 6 HP Tohatsu with the integral and read about someone that had problems starting the outboard after running out of fuel. After storing the motor, I could not get it to start on the integral tank even though I filled it. I had to loosen the shield on the carb and spray in starting fluid. It fired right up thereafter, but I now am so afraid of running out of fuel that I carry a small fuel can and check the fuel often. For me, it would probably be better to use a small remote tank.

[donradcliffe]

The answer to the original question s one person yes, 2 people no.

[osprey877]

Thanks everyone for your contributions. It looks like it may work if the stars all align, but probably will be a struggle. I’ll see if I can get the dealer to demo a couple of different engines, but not hopeful. In any event will be a couple of weeks before I’m ready to get the motor. I’ll report back when I have an engine and report on its performance. Thanks again for all the help.

[Tin Tin]

No guarantees, but the following may get you on the plane with two aboard:
1. Get weight forward, so remote fuel tank in the bow, plastic anchor etc
2. Stainless prop 1" less pitch than standard
3. Hydrofoil or similar
4. Lift the engine an inch or two higher on the transom (block of wood)
5. Extension on tiller so skipper can sit further forward.
6. Ensure tubes are at maximum air pressure.
7 Higher Octane fuel

[rabbi]

We had a 6HP 4 stroke on a lightweight inflatable (foldable bottom panels with inflatable keel, it it had some shape).
It would plane with me and a school kid. Often also with me and missus on the way to shore. If we only bought a handful of groceries it would _not_ plane on the way back.

8HP 2 stroke would take the two of us on a plane even with lots of groceries.

Kids are now grown up and we have a RIB so 8HP would not make anyone happy. And 10 or 15hp 4stroke are just to heavy for me to handle comfortably.

I found a lightly used 2006 Mercury 15hp 2stroke (some guy sold his camper inflatable used a few weeks every other year).
At 35kg a tad heavier than a modern 6hp 4 stroke, but way more power (and noise).

[GordMay]

IMO: You’ll not likely plane a 9Ft RIB, with a 300# load, using a 6 HP outboard engine, in any but the most favourable [‘glassy’] conditions.

FWIW: 3 US Gallon [2.5 Imp Gal/11.3L] portable fuel tanks are quite common. [Gasoline weighs about six pounds, per gallon]
➥ https://www.westmarine.com/moeller-3...-11882586.html
➥ https://www.stevestonmarine.com/port...Int-Csa-8803I2
➥ https://www.stevestonmarine.com/port...gal-113l-04158

[MicHughV]

I was thinking about this last night.

Engine manufacturers always state some simple number for the horse power of a particular engine, but it's invariable a simple number, ie, 6 hp and not 6.8 hp, etc.
Same goes for the larger engines, which seem to go up in multiples of 50.....ie, 150 hp...200 hp....etc....you don't ever see 156 hp or 197 hp and so on.

The 9.9 engine designation came up about due to some lakes having a 10 hp maximum allowed and so on. It's a contrived number by manipulating the engine rpm to suit a law.

Off course, engine torque is never mentioned.

As a youngster, I had a 50cc motorcycle with a claimed hp output of 6 hp. I simply can't see a 50cc, single piston engine having the power to put two people and a dink on a plane. Not sure if that hp on my bike was measured at the engine or rear wheel. My motorcycle also zipped along at 10,000 rpm or thereabouts, and it had a 5 speed gearbox to boot. I have taken that engine apart, and the piston is about the size of an egg, ie, quite small. I have a hard time imaging that it has the oomph to put a loaded dink on plane.

My 8 hp Yamaha 2-stroke engine has a twin cylinder engine, of 165cc, this is quite a jump up from 50cc, for only a 3 hp gain. Though it's claimed to be an 8 hp engine, it could well be more, I simple don't know.

At the end of the day, the consumer is at the mercy of the engine manufacturer and dealer, where the only number that gets tossed about is the engine hp and the real power of an engine is a mystery.

That an 8 hp engine could likely do the trick for a dink over a 6 hp engine is likely because it is a substantially bigger engine and also is a 2 cylinder vs, a 1 cylinder.

Just some thoughts to ponder.....

[JBP]

Originally Posted by osprey877

I want to avoid the hassle of filling and storing a separate tank, the minimum size of which seems to be about 5 gallons.

FYI, You can get 3 gallon external tanks which are a bit easier to manage.

[Kettlewell]

IMHO, it probably won't plane 2 people. I used to have an 8HP 2-stroke on a 9'6" Achilles that would barely plane 2 light people if all was just right. In general I think you want to go for max HP on almost any small inflatable, RIB or not.

[MicHughV]

For sure, 3 gallon tanks are available. Gas weighs about 6 lbs/gal, so a full tank of gas (including the tank) is likely about 20 lbs, easily carried in one hand.

When you are out cruising, you take the dink to the fuel dock and they hand you a hose, no need to carry anything.

[rslifkin]

Good point about rounding in the ratings. I'm pretty sure the actual rating listed in the detailed specs for my "6 hp" Tohatsu is 5.9 hp at 5500 RPM. That said, with trim tabs on the dinghy, a lower pitch prop, and the mounting height on the outboard raised a little it does perform respectably for such a small outboard.

Even with the small prop and only being a 123cc single cylinder, acceleration isn't the issue. It accelerates quite well lightly loaded. With a heavier load it takes a few seconds to get over the hump and onto plane, but it's not a case of struggling over the hump and then taking off, it doesn't accelerate much more once it's on plane when loaded. It just doesn't have the power to push the thing any faster than "just barely fast enough to plane". At least with this combo, playing around with leaning forward in the dinghy, etc. doesn't make any noticeable difference in ability to get on plane. It's pretty much a hard line of whether or not it'll plane with a given amount of weight (unless you're running downwind where you might be able to surf it onto plane, but even then it sometimes falls off after the wave passes if you've got too much weight).

6hp from a 50cc moped is quite a lot of power from such a tiny engine. I'd expect that was possibly a 2 stroke, and almost certainly quite high revving and probably very short on power at lower RPMs. How high an engine revs, camshaft selection, how the ports in the cylinder head are designed, carb size, etc. all factor into how much power it'll make. Generally more displacement = more power, but it's not a 1:1 relationship. And we see cases like the 4hp and 8hp outboards that are identical to a higher powered model, but have a smaller carb on them to artificially restrict power output.

[MicHughV]

Nope, my bike, a Honda, was a 4 stroke to boot. The valves were about the size of one's finger nail.
It my memory holds Suzuki and Yamaha had 50cc two stroke engines.

https://richardjeaton.medium.com/cla...w-7bf29a84a485

[MicHughV]

It was popular at the time to replace the 50cc piston with a 72cc piston, which also required a new cylinder, but could keep the original head. This way, the law could not tell if your engine was 50cc or more, which was the most allowed for teenagers in my day.

I never did this, but those that did, were clearly faster and more powerful.

Yes, it spooled up to around 10,000 rpm as I recall. Top speed was 45-50 mph, can't remember anymore, might have been 60 mph...going downhill.