Is 6hp outboard enough power?

[rslifkin]

Originally Posted by 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

Just did some research. Looks like that thing revved to 11k RPM to get 6 hp out of that tiny engine. That's quite an impressive little engine and certainly pushed pretty much to its limit.

Seeing that Honda could manage that with an air cooled engine in the 60s makes me wonder why none of the outboard manufacturers seem to want to cross the 6000 RPM mark in a small outboard. And none of them seem willing to go bigger than 140 - 150cc in a single cylinder either, despite (lower revving) air cooled lawn equipment and such going much bigger (I have a 420cc single cylinder snowblower, for example, although max rated RPM on that engine is only 3700).

Seems like if the noise and vibration could be kept tolerable and it wasn't so weak at low RPM that it wouldn't push a dinghy up on plane, a slightly bigger single cylinder that revs to 8000 or so and makes 8 - 10 hp at the weight of the current 6 hp 4 strokes would sell like crazy, although it might gain a pound or 2 for deeper reduction gears and the bigger prop used on the 8/10 hp outboards. I think plenty of people would gladly sacrifice the smoother running of a 2 cylinder for having the power/weight ratio of the Tohatsu 9.8 2 stroke (which is basically unmatched by any other outboard).

[MicHughV]

Yep, 11,000 rpm is probably right on the money for that bike. Gotta hand it to Honda, for sure.

You could put two teenagers on that bike, and it would still go at a good speed.

[MicHughV]

A lot of motorcycles have rpm ranges north of 6,000 rpm, many in the 10,000 rpm plus range.
The Honda 750cc 4 cylinder, which came out in 1969, spooled up to to 9,000-10,000 rpm or thereabouts.
I think some of the Kawasaki's went even higher....real screamers.

I quit riding bikes after a while, as the speeds and power they had were insane.

In my later years, I had a Harley, quite the opposite track, very large displacement, mine was 1,600cc, but also very low revving. Tremendous power, but lacking in top speed somewhat.

[rslifkin]

Originally Posted by MicHughV

A lot of motorcycles have rpm ranges north of 6,000 rpm, many in the 10,000 rpm plus range.
The Honda 750cc 4 cylinder, which came out in 1969, spooled up to to 9,000-10,000 rpm or thereabouts.
I think some of the Kawasaki's went even higher....real screamers.

I quit riding bikes after a while, as the speeds and power they had were insane.

In my later years, I had a Harley, quite the opposite track, very large displacement, mine was 1,600cc, but also very low revving. Tremendous power, but lacking in top speed somewhat.

The existence of all of those high revving bike engines that hold up well enough over time is exactly what leaves me wondering why the outboard builders seem so conservative. As long as it doesn't leave the thing gutless at low RPM where it would never get over the hump, it should be possible to open up the engine on a small outboard a bit more, rev it higher, and get a couple extra hp out of it. With the demand for light weight, it doesn't quite make sense why none of the builders are doing that.

If I could sort out raising the rev limiter and could trust the flywheel to not turn into a frag grenade, it would be tempting to find another 6 hp, pull it apart, port and polish the head, maybe find a way to get a bigger cam in it, and adapt the bigger carb from a 9.8 onto it. Let it rev out to 7000+ and see how much faster it'll push the dinghy to get an idea of how much extra power it's making.

[MicHughV]

Honda came out with a twin cylinder 50cc engine...in their race bikes....

13.7 hp at 21,500 rpm...

https://www.motorcycleclassics.com/c...-zm0z22ndzols/

probably not practical for the marine environment though...

[Lee Jerry]

The primary reason, IMHO, that bikes can have those small displacement, high-rev engines and that they won't / don't work in small outboards is that the bikes have transmissions with multiple gear ratios (and outboards don't).

[rslifkin]

Originally Posted by Lee Jerry

The primary reason, IMHO, that bikes can have those small displacement, high-rev engines and that they won't / don't work in small outboards is that the bikes have transmissions with multiple gear ratios (and outboards don't).

Agreed, lack of multiple gears to make up for lost low rpm torque is a limiting factor in a marine application. As is durability at sustained high RPM and load. But I think things could still be pushed a little bit further than the current outboard builders do in terms of RPM and power density (power relative to displacement). Plus they definitely have headroom to go a bit bigger with a single cylinder as long as it can be done without unmanageable vibration issues.

Apparently there are a few bigger single cylinders on the market, but it's not available in the US as far as I can tell.

Selva makes an 8 hp, 165cc, rated for 6000 rpm it looks like, and about 59 lbs: https://www.selvamarine.com/en/produ.../black-bass-8/

They also make a 9.9 hp, 181cc, 6000 rpm single cylinder. It's heavier at 67 lbs though: https://www.selvamarine.com/en/produ...s/piranha-9-9/

[MicHughV]

So many variables involved with engines, hard to say where the truth lies.
Even gas octane can have an effect on the hp an engine can produce...

Some years ago, I had an opportunity to drive a Ferrarri, with the proviso, that I fill the tank first, which I gladly agreed to do.

I discovered that when you put your foot into it, you can suck a tank dry in under an hour.

Ah well, had to do it once.....

[David Mathis]

We have a 9.9 on a 10' aluminum bottom inflatable which, with two people in the boat (total about 300 pounds), pulls my 100 pound grandson on a kneeboard. You will have about 1/3 less power.

[MicHughV]

I'm not an automotive engineer, but it would seem that the 2 stroke engine would be a more likely engine to attain high rpm, as it does not have so many moving parts, ie, the valve train.
Race car engines of various makes, especially those of the Formula 1 variety, can also attain very high rpm, albeit with the use of gears.

Packaging all this in an " outboard" package, ie, something that can be easily removed from the mother ship, is another hurdle, which engine manufactures must grapple with.

Still, if one looks back the outboard engine as we know it has been transformed by leaps and bounds into the current crop of engines we are familiar with.

I can still remember the old British Seagull outboard engine being the mainstay of many a small dink.
My first dink engine was a 2 hp Suzuki 2-stroke....2 hp !....not much at all, but it did an admirable job for it's diminutive size.

Who knows what tomorrow will bring........

[GordMay]

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The standard coefficient of gasoline's expansion/contraction equals 0.069 % per degree Fahrenheit.

The energy content of fuel [gas/diesel], however, is directly related to its weight, not its volume.

If your retailer does not temperature compensate the volume sold, you get slightly less gas [by weight], for warmer gas, than for cooler gas.

In jurisdictions, which do not require temperature compensation for retail gasoline sales, purchase fuel, when it’s cooler.


See ➥ https://consumerwatchdog.org/resourc...lUSAJune07.pdf

[himnher]

Surely the answer is in what use you want to put the dinghy to and so whether you will ever have a need to plane? If you want to run to/from the shore in an anchorage, will you need more than 6hp? And you must consider how you stow and mount the engine to the dinghy. If you have a davit, then you can go larger (and heavier), but if you expect to install by hand, smaller is going to be better for your back and even safety during fitting/removing in any kind of swell

[Schapera]

Originally Posted by osprey877

I have just bought a new 9' RIB. It is fairly light at 80 lbs and is rated for 10hp.
I would like to power it with a new 4 stroke 6 hp outboard (2 stroke is not available in US/Canada) as this is relatively lightweight to hang off my pushpit and for my wife and I to manhandle. It is also the largest outboard that has a an integral gas tank. I want to avoid the hassle of filling and storing a separate tank, the minimum size of which seems to be about 5 gallons. I currently have an integral tank 2.5hp and fill it from a couple of 1 gallon jerry cans that fit in my vented cockpit storage bin. I am hoping to keep using the same system.
My question is whether or not a 6hp outboard is powerful enough to get the RIB onto a plane at WOT when carrying the 300 odd lbs that constitute my wife and I. The dealer says yes, but I'm hoping someone with actual experience can chime in. Most people seem to use a 9.9, but all seem to agree that is way more powerful than necessary if there's only 2 in the boat, and the 9.9 is considerably heavier than the 6. Will the 6 work, or do I need to bite the bullet and get an 8 or 9.9?l Thanks

Wouldn’t it make most sense to ask the dinghy manufacturer rather than those of us here?

[OneBoatman]

I would say yes but not because of a lack of power. It will be capable of planing only in relatively flat water if it is of the newer lightweight RIBs having an aluminum hull, but you would still have to limit the total added displacement to ~350 lbs. For all other RIBs its too big an ask to be on plane in anything but ideal conditions due to their short water line length and high drag when loaded. Attitude matters, depending on your wt. when alone with minimal cargo with anything more than 6 you may need a tiller extension and have sit more forward or have cargo in bow. First ask yourself is it OK if your butt is soaking wet and then decide if you really need to be on plane when using your dinghy, if the answer is yes, best to go up one size on hull length and consider an 8 if you want to be on plane and not be close to WOT.

[mcon12000]

My personal experience says that you alone with groceries will be fine in calm weather. However I would go with the maximum your dink is rated for in mine for example I’m switching to a 20hp from 9.9 Honda. Why? Bc in wind on the nose and chop a 9.9 will often not plane with two adults or 330lbs. This is dual floor 10’ zodiac so it’s much heavier than yours but you might experience more problems with wind lifting the bow in the lighter rib.

It also depends on where your cruising bc your 6hp will seem ok if most of your time is spent on the ICW and away from open bays.