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Old 01 March 2019, 14:24   #1
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9.9hp 4-stroke or 15hp 2-stroke for Aerotec?

I need to make my mind up if I want to buy a 15hp 2-stroke or a 9.9hp 4-stroke for my Aerotec. This is partly sparked by a number of 4-strokes and no 2-strokes available now.

The limiting factor is weight, I'm not sure I'll be able to handle 20hp 4-stroke without an occasional struggle (but then I've not checked that either).
Usual load will be 1 adult, 1-2 kids, fuel, maybe 10kg of kit. Max load will be 2+2 with fuel and 15kg of kit.

I'm assuming 15hp will have no problem with full load, but how much of a struggle will this be for a 9.9?

I know Fenlander made a thread with engine performance comparison, but I spent 15 minutes and didn't manage to find it ;-(

PS: At the moment I have a 6hp 2-stroke that came with the boat. It's good for Great Ouse, but I can't get this on the plane even solo.
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Old 01 March 2019, 14:32   #2
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15 2-stroke all day long dude. Itís still early in in the season yet, they will appear. I guarantee you will regret buying 9.9 four shortly after you buy it as I did. Bang for buck a 15 will put a grin on your face like a Cheshire Cat, all the 9.9 will have you doing is going further up the boat to get it to plane. Heed the advice and hold out!!!
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Old 01 March 2019, 15:39   #3
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What he said. If you want a four stroke get the lightweight Suzuki 20hp that Fen runs. Otherwise stick with a 2 smoke Merc/Mariner 15. Be patient, they appear all the time.
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Old 01 March 2019, 16:02   #4
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15hp 2 stroke
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Old 01 March 2019, 16:43   #5
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>>> 9.9 will have you doing is going further up the boat to get it to plane.

Not quite so... 8hp yes that's true... 9.9 is OK... as follows at the bottom...


Max you've forgotten... I happily have a 2018 9.9 Mariner 4-stroke now.

I can give accurate comparison Jerry having run an Aerotec with both motors you're considering. BTW this is the thread you were looking for...

Mariner 9.9 - New 10hp outboard choice.

When we considered the downgrade I happened to use the Aerotec with an 8hp Mariner and was taken aback how well it went for 40% of the power compared to the Suzuki 20hp I had before... so made the educated guess a 10hp would do the trick... and it has.

I started off with the Mercury 15 2-stroke on the Aerotec… suited it nicely, easy to handle ashore and made 18kts flat out.

Then upgraded to the Suzuki 20efi 4-stroke. Biggest advantage was the mid range grunt, top speed up to just over 20kts and a good fuel saving over the Mercury 15. But soon realised it was both a little heavy and bulky for daily setup.

Used a loan 8hp Mariner 4-stroke on holiday. Slow to plane and only made 10.5kt and would fall off the plane easily. Impressed for its size but never a long term solution.

Downsized my 20 to the new Mariner 9.9 4-stroke. Top speed down to 15kts. Goes on plane easily and while having far less mid-range grunt than 20hp not terrible compared to the 15hp. 30% less fuel used than the 20hp.

What is at least or more important than the specific OB HP was the weight carried. We are now... 12l tank full and a fair bit of day out/safety kit... plus 27 stone of 2 adults. That is about the load for all the speed figures above. What's your crew load 1+2 or 2+2?

When our girls grew to older teens and we had ourselves plus those two and a medium dog then the 15hp was pulled down to the performance of the 9.9 with just us two. It was mainly being 4 frequently up that made me upgrade from 15-20hp.

Re a long wait with the bow up using a 9.9.... This is not the case with a properly trimmed Aerotec. Their hull shape negates much of the excessive bow lift period so helps the smaller OBs achieve an easy plane attitude.

But really I guess the safe advice is to get a decent 15hp 2-stroke... however if you found a used 9.9 4-stroke at a good price then I'd not rule it out.
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Old 01 March 2019, 17:24   #6
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I had, getting old!
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Old 01 March 2019, 18:16   #7
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2 stroke every time for me.
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Old 02 March 2019, 04:53   #8
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Go for the biggest engine with the most horses that you can physically manhandle. I had a 9.9 yam on the back of a 4.2 futura, lasted 2 weeks before I sold it in favour of a nice merc 15 2 stroke. Iím 2 years in with this set up and still grinning.
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Old 02 March 2019, 05:20   #9
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Remember your Futura is 60% heavier than the Aerotec, longer, broader... and designed to be driven by a large OB so it skims up on the speed tubes. So not comparable to how a 9.9 would be on an Aerotec.

Perhaps I'm lucky... after a lifetime going on the sea my grin starts when I open the car door at the coast... it isn't dependent on those last 3kts.
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Old 02 March 2019, 05:53   #10
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To be honest what's important is that the engine has enough grunt to take me on the plane with max load. Top speed is just a derivative.
At the same time it would be beyond silly to buy a motor only to change it later because it's missing half a horse...
15 is clearly better, but then 4 strokes have their positives as well (less smell, quiet, reusable fuel, etc).
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Old 02 March 2019, 06:19   #11
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Be worth you working out your total crew weight inc yourself Jerry to give the max load. With that info and experience of 9.9-20hp on the same SIB I can give a good indication how it will go.
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Old 02 March 2019, 06:25   #12
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Max crew would be 200kg (2+2). 25L of fuel, some kit. I'm not counting the dog, it's 1.5kg.
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Old 02 March 2019, 09:52   #13
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Hmmm OK.... thought you might be closer to our load... we are 378lbs with 12l fuel and quite heavy anchor/kit bags.

You will be 62lbs more... plus probably roughly the same kit.

What I do know is adding 112lbs to our normal load blunts performance with the 9.9.

So unless you bought a used 9.9 at a price you could sell on if needed with no/minimal loss then the 15hp 2-stroke is a safer bet.

OBs can be bargains this time of year but the flipside is there are not many on offer. Come early April I bet some decent 15s will come to the market.
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Old 02 March 2019, 10:18   #14
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Have you checked out this one... https://www.gumtree.com/p/boats-kaya...ine/1329913417
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Old 02 March 2019, 11:24   #15
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On Boats and Outboards merc , mariner offers if £1749 for a 15 four stroke merc suits Cambridge outboards
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Old 02 March 2019, 12:45   #16
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Quote:
Originally Posted by Jerry View Post
Max crew would be 200kg (2+2). 25L of fuel, some kit. I'm not counting the dog, it's 1.5kg.
You need at *least* a 15!

The Aerotec is a very draggy boat comparable to a Deep V RIB in some respects in the way it drops off the plane easily if you drop power too much.

IME a regular 3.4m sausage keel SIB will do with a 9.9 Tohatsu 2 stroke what an Aerotec needs a 15 to do.

Our Aerotec with the Mercury 15 smoke and around 150kg load will pop up on the plane very nicely but will top out around 16.5 kts. Me on my own and dumping the load it will get 2 kts more.

If you're contemplating a 200kg load then it has to be a 15 hp 2 smoke or a lightweight 20hp 4 stroke.

That's *if* you want to use it in typical UK F3/4 sea conditions and be safe and want to have fun. If you just want to pootle on rivers or 1/2 mile out in calm sea to fish then a 9.8 Tohatsu or say a four stroke 8 hp would be fine.

As Maverick says and it's something that is often repeated on here.

The rule is simple (to expand it a little) - *if* you want to use the boat to its maximum potential with a given crew then:

Rule No. 1: Buy the most powerful engine the boat is rated for that you can manhandle from the car to the launch point.

Simple as that. If there are *always* helping hands then that comes into the equation but it always can only apply to YOU and *your* situation.

ie, one, two, three helping people? kids? strong blokes? hernia prone?, steep shingle? concrete slip? 20 yds? 1/4 mile? All situations are different but just apply rule 1.
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Old 03 March 2019, 03:28   #17
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Looks tidy!

There's a mint short shaft Yammie here (pick from Nottingham area). Bidding closes, March 6.
https://www.ebay.co.uk/itm/Yamaha-15...MAAOSw5XJcdvUJ
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Old 03 March 2019, 04:49   #18
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Already checked the engine from Gumtree, seller is away for 3 weeks, we've agreed he'll drop me a line when he's back - we'll see, the pictures look good.
On the Yam, I'm more unlucky - I'll be in the air when the bidding ends.
There's one Merc with replacement hood on eBay, but on that one I'd have to very, very carefully inspect it, and also don't like the black hood.
A quick eBay/Gumtree browse is becoming my morning coffee routine
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Old 03 March 2019, 05:07   #19
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Max, completely agree with Rule 1!
I would go for 20 4stroke if I knew handling wouldn't be an issue. My two sets of crew would be either I with Daniel (9yo, so can't help manhandle the engine just yet) or all of us - the wife could then help with 20hp.

Having both 20 and 9.9 4-strokes to cover both situations would be too much (she'll have a fit if I buy 2 engines!), so this looks like 15hp is a compromise.
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Old 03 March 2019, 05:40   #20
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Jerry take massive care with the seller of the black hood one... their reputation on here is terrible.

And note the Yamaha is on remotes which you don't want and if there's no tiller that's a right faff.

Also beware if you were tempted by a 20hp... having an OB where you need help is OK in theory but there are many reasons at the setup/launch point and back at home when that help might not be available where you end up risking your back.

Hence us just having the 20hp a couple of years before sense prevailed. Much better overall experience with the 9.9... as it would be with a 15 2-stroke as they weigh about the same.
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