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Old 28 February 2020, 03:22   #1
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Outboard Weight vs Power

I'm currently looking at upgrading my 5hp outboard for a 10 or 15hp. I have seen a second hand 2 stroke Yamaha 8hp. It looks mint and has recently been serviced. Only weighs 28kg.

The other option would be a 4 stroke 9.9hp or 15hp which are both nearly double the weight if not more.

Do you reckon the 8hp will feel noticeably weaker than a 9.9 that is double the weight? I'm thinking an extra 20kg will offset the extra 2hp.

https://www.ebay.co.uk/itm/yamaha-8h...gAAOSwoQdeT~c7

The boat is a 3.4m Mercury Quicksilver Sport SIB rated to max 15hp.
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Old 28 February 2020, 05:12   #2
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Welcome to the forum.

Unless you have £2000+ for one of the latest model lighter Efi 4-strokes then the sweet spot for a 3.4m SIB is a 15hp 2-stroke weighing around 36kg.

A Yamaha, Mercury or Mariner (possibly Suzuki but check the weight on the data plate) but ideally not Tohatsu as their 15hp 2-stroke, apart from the engine bore, is 99% the same motor as their 18 and a bit heavier than most 15s.

With these 2-stroke motors the 9.9 is mostly the same motor as the 15 with a lower state of tune and weighs the same so not such a good power for weight to carry.

The Mercury/Mariners 9.9s can be upgraded to a 15 but it needs a new carb at the very least which is a fair cost. The 9.9 Yamaha can be easily upgraded to a 15 at no cost.

Those 8hp Yamaha 2-strokes are great motors and give a decent turn of speed if you are lightly loaded say one up with minimal kit... but will feel sluggish with a bit more weight such as two heavier folks and a bit of kit.

For me an earlier 15hp 4-stroke that may weigh the same as a 20hp around or over 50kg is totally the wrong motor for your SIB.

As a matter of interest I've used my 3.8m SIB with 8hp 36kg 4-stroke, 9.9hp 36kg 4-stroke, 9.9hp 36kg 2-stroke, 15hp 36kg 2-stroke and 20hp 44kg Efi 4-stroke.

The 8hp was OK one up or just OK adult plus light teen but bogged down by two adults. 10.5kts max

The 9.9hp 4-stroke was much better but at the speed I wanted to cruise was flat out. 15kts max.

The 9.9hp 2-stroke was a bit better than the 9.9 4-stroke perhaps because it was based on the 15hp but with smaller carb. Bit more mid range grunt and felt less stressed. 16kts max.

The 15hp 2-strokes... had a Mercury first which was the perfect motor unless 4-up when that blunted its performance. Now have a Yamaha which is equally perfect as we never travel 4-up any more. 18kts max.

The 20hp 4-stroke was only bought during the period we were often 4-up. It had the grunt for that OK. But even as one of the new lighter 44kg models added to its large dimensions was too much of a faff for me to move about and lift on & off etc. 20kts max.

Max speeds are all with same load... two adults and a full tank of fuel plus some kit.
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Old 28 February 2020, 05:21   #3
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Something like this recently sold motor is what I'd be saying would suit...

https://www.ebay.co.uk/itm/Mariner-1...p2047675.l2557
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Old 28 February 2020, 05:22   #4
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Originally Posted by Fenlander View Post
Welcome to the forum.

Unless you have £2000+ for one of the latest model lighter Efi 4-strokes then the sweet spot for a 3.4m SIB is a 15hp 2-stroke weighing around 36kg.

A Yamaha, Mercury or Mariner (possibly Suzuki but check the weight on the data plate) but not Tohatsu.

With these 2-stroke motors the 9.9 is the same motor with a lower state of tune and weighs the same so not such a good power for weight to carry.

The Mercury/Mariners 9.9s can be upgraded to a 15 but it needs a new carb at the very least which is a fair cost. The 9.9 Yamaha can be easily upgraded to a 15 at no cost.

Those 8hp Yamaha 2-strokes are great motors and give a decent turn of speed if you are lightly loaded say one up with minimal kit... but will feel sluggish with a bit more weight such as two heavier folks and a bit of kit.

For me an earlier 15hp 4-stroke that may weigh the same as a 20hp around or over 50kg is totally the wrong motor for your SIB.

As a matter of interest I've used my 3.8m SIB with 8hp 36kg 4-stroke, 9.9hp 36kg 4-stroke, 9.9hp 36kg 2-stroke, 15hp 36kg 2-stroke and 20hp 44kg Efi 4-stroke.

The 8hp was OK one up or just OK adult plus light teen but bogged down by two adults. 10.5kts max

The 9.9hp 4-stroke was much better but at the speed I wanted to cruise was flat out. 15kts max.

The 9.9hp 2-stroke was a bit better than the 9.9 4-stroke perhaps because it was based on the 15hp but with smaller carb. Bit more mid range grunt and felt less stressed. 16kts max.

The 15hp 2-strokes... had a Mercury first which was the perfect motor unless 4-up when that blunted its performance. Now have a Yamaha which is equally perfect as we never travel 4-up any more. 18kts max.

The 20hp 4-stroke was only bought during the period we were often 4-up. It had the grunt for that OK. But even as one of the new lighter 44kg models added to its large dimensions was too much of a faff for me to move about and lift on & off etc. 20kts max.

Max speeds are all with same load... two adults and a full tank of fuel plus some kit.
Thank you very much for this information. Will be more than useful when deciding which motor to choose.
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Old 28 February 2020, 06:24   #5
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sound advice from fenlander personally i always look at the max hp for the boat it gives more scope then and dont forget the engine won't work as hard
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Old 08 March 2020, 16:06   #6
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Hi, not my intention to divert the post, but I'm in a similar position and was interested in the comments from Fenlander regarding optimum engine choice - interesting because I have an aerotec 380 and looking for a replacement for my Johnson 2 stroke. Its a 9.9 short shaft running with a 15hp carb - its a 'just about ok' lift for one person out of the back of the car at around 35kg.

I was looking for an easier / cleaner engine - the 'lightweight' 4 stroke efi tohatsu /mariner/mercury engine but the impression i get from the post from Fenlander is that its a heavy lift onto the boat and perhaps doesn't add any more power?

As an alternative I saw some more recent 15HP 2 strokes advertised by Ivan Burdfield in Warsash which might be suitable, but then maybe a marginal improvement on what I have.

Finding someone to cast a professional eye over the motor is a challenge, everyone seems to want to work on more modern engines, I seem to spend more time servicing and trying to get a solid tick over on mine than I do using it! The other 2nd hand engines from dealers aren't cheap at around £1300.

I never seem to find clean low hours engines like the Yamaha 15hp featured in the posts last year!
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Old 08 March 2020, 18:38   #7
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>>> the 'lightweight' 4 stroke efi tohatsu /mariner/mercury engine but the impression i get from the post from Fenlander is that its a heavy lift onto the boat and perhaps doesn't add any more power?

I would never argue with anyone who preferred the feel of a 20 or 25hp on the Aerotec but for me when just self, wife and dog then the 15hp is the perfect sweet spot combo... but add more folks and the the 20hp was beneficial.

For me of average strength, but with a 64yr old back that needs care, the extra 10kg or so and far more bulky powerhead/leg cowl of the Suzuki efi over a 15hp 2-stroke just started to spoil the portability aspect.

Ivan isn't cheap but he does have a very broad selection of clean 15hp 2-stroke motors. He is a little unusual/old fashioned to deal with and has been quite insulting to me on here in years gone past but if he had the engine I wanted I'd buy from him.... actually I did ha ha.

I have no experience with your Johnson 15hp model but note your problems with the carb. If it ran OK as a 9.9 and you are 100% sure the jets are all spotless, the float height correct and no gasket air leaks then I'd wonder if the 9.9hp to 15hp conversion needs something else to be changed or adjusted that's not yet done??
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Old 08 March 2020, 19:07   #8
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Thanks for the insight, did the extra weight spoil the trim of the boat? I figured the extra weight to be similar to a full 12l fuel tank, so maybe not too much difference?

I need to strip the carb and see if there's something wrong, it sounds like a blocked airway to the idle screw, but each time it's stripped it requires some new gaskets and if anything actually breaks spares need to come from the US if they're available at all.
Checking through the parts list line by line there aren't any other differences - it would be reassuring to get it checked but so few places want to look at them.
I wonder if using the Quicksilver Premium Plus 2 stroke oil would have made any difference over the quicksilver Premium - need to test both.

It runs great at WoT, your analysis of the engine power vs top speed and planing capability was really helpful in seeing the differences - convinced not to go for the 9.9 options even though the weight would be better even in 4 stroke. Many people have encouraged going to 18/20hp when there isn't a weight penalty but I'm cautious about the weight to carry and the impact on the boat trim.

So I'm in a loop like the OP, 15hp is the right HP, need to decide to keep mine, get a later 2 Stroke or a newer 4 stroke efi. That should keep me busy...
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Old 08 March 2020, 19:27   #9
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No the trim was fine with the 20hp... as you say about the weight of a full 12l tank extra... or the difference between a 12st or 13.5st pilot! After all the 380 is rated to a 72kg OB!

Well if you'd sussed out my full SIB OB history my loop went 3.5hp-9.8hp-15hp-4hp-10hp-15hp-20hp-9.9hp-15hp.... somewhat erratic.
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Old 08 March 2020, 19:35   #10
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There cant be many Aerotec owners that can speak from experience about such a wide range of engines and have had the chance to test with different loads.

The only other outboard i've used on an inflatable was a seagull, an oil slick followed us wherever we moored, the noise was close to unbearable after a while. Better as an ornament than on an inflatable.
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Old 09 March 2020, 04:57   #11
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I ended up pressing go on a new Tohatsu 15hp EFI short shaft at the weekend. My 5hp Yamaha 2 stroke decided to seize up over winter which speeded up the decision. The weight is stated as 43KG which I think is the lightest on the market. Still a big jump up from my 21kg 5hp 2 stroke, but should last me many years. The smoke and oil slick coming out of my oil two stroke also battled with my environmental conscience.

Ive not told the missus yet! Working out how to break the news lol
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Old 09 March 2020, 05:50   #12
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Well done... those efi Tohatsu motors look good. Let us know how you get on with it.

Good luck with the Mrs!
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Old 09 March 2020, 12:32   #13
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Quote:
Originally Posted by benhben View Post
I ended up pressing go on a new Tohatsu 15hp EFI short shaft at the weekend. My 5hp Yamaha 2 stroke decided to seize up over winter which speeded up the decision. The weight is stated as 43KG which I think is the lightest on the market. Still a big jump up from my 21kg 5hp 2 stroke, but should last me many years. The smoke and oil slick coming out of my oil two stroke also battled with my environmental conscience.

Ive not told the missus yet! Working out how to break the news lol
Can I ask what your doing with the 5hp? I have one with a cracked gearbox
Id be interested in buying the complete engine if your selling it
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Old 09 March 2020, 14:33   #14
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Originally Posted by Sailpower View Post
Thanks for the insight, did the extra weight spoil the trim of the boat? I figured the extra weight to be similar to a full 12l fuel tank, so maybe not too much difference?

I need to strip the carb and see if there's something wrong, it sounds like a blocked airway to the idle screw, but each time it's stripped it requires some new gaskets and if anything actually breaks spares need to come from the US if they're available at all.
Checking through the parts list line by line there aren't any other differences - it would be reassuring to get it checked but so few places want to look at them.
I wonder if using the Quicksilver Premium Plus 2 stroke oil would have made any difference over the quicksilver Premium - need to test both.

It runs great at WoT, your analysis of the engine power vs top speed and planing capability was really helpful in seeing the differences - convinced not to go for the 9.9 options even though the weight would be better even in 4 stroke. Many people have encouraged going to 18/20hp when there isn't a weight penalty but I'm cautious about the weight to carry and the impact on the boat trim.

So I'm in a loop like the OP, 15hp is the right HP, need to decide to keep mine, get a later 2 Stroke or a newer 4 stroke efi. That should keep me busy...
For info.

I have an Aerotec and started off with a vgc 15hp mariner. It was a great engine, light weight, easy to maintain, start etc.. but I felt I needed a bit more power and went for a 25hp tohatsu @52kg.

Not the easiest to lug around but loved the acceleration and instant planing. Biggest problem apart from the weight was it was a thirsty beast and I eventually swapped it out for a new 20hp tohatsu efi.

I really like this engine on the aerotec. Nice easy starter, hasn't got the instant smack of the 25hp but not too far behind. Cruises nicely and is very frugal. I think it's about 43kg so not as light as the 15hp but it has a nice handle and the weight is ok to manage. It also has just about zero transom splash if that concerns you.

All different but all great in their own right. Depends what you plan to do with your boat really..
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Old 09 March 2020, 14:51   #15
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For info.

I have an Aerotec and started off with a vgc 15hp mariner. It was a great engine, light weight, easy to maintain, start etc.. but I felt I needed a bit more power and went for a 25hp tohatsu @52kg.

Not the easiest to lug around but loved the acceleration and instant planing. Biggest problem apart from the weight was it was a thirsty beast and I eventually swapped it out for a new 20hp tohatsu efi.

I really like this engine on the aerotec. Nice easy starter, hasn't got the instant smack of the 25hp but not too far behind. Cruises nicely and is very frugal. I think it's about 43kg so not as light as the 15hp but it has a nice handle and the weight is ok to manage. It also has just about zero transom splash if that concerns you.

All different but all great in their own right. Depends what you plan to do with your boat really..
Thanks, as I understand the 15 and 20hp tohatsu Efi are the same weight, so itís very tempting, but around £300 more - as itís a bit of a stretch for the 15 I donít think I can justify the 20 on our use. Itís good to know itís a good match for the Aerotec though, I was offered a px deal at Bill Higham so although itís a bit of a drive I might well just go for it.

Are there dealers you would recommend in the NE? Iím a fairly frequent visitor so it would be interesting to know, the thing holding me back from a new motor is the need to spend cash (£150 - £200) a year on servicing to validate the warranty.
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Old 09 March 2020, 15:01   #16
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Originally Posted by benhben View Post
I ended up pressing go on a new Tohatsu 15hp EFI short shaft at the weekend. My 5hp Yamaha 2 stroke decided to seize up over winter which speeded up the decision. The weight is stated as 43KG which I think is the lightest on the market. Still a big jump up from my 21kg 5hp 2 stroke, but should last me many years. The smoke and oil slick coming out of my oil two stroke also battled with my environmental conscience.

Ive not told the missus yet! Working out how to break the news lol
Good luck with that conversation! Where did you go for the motor?.
I was told by one dealer that the weights are fairly random, in so much as theyíre weighed without oil and prop to achieve the claim Ďlightest in classí but theyíre all pretty close.

Iím dithering between a late model 2 stroke or the 15hp tohatsu, held back by the ongoing service costs. Bill Higham seemed to have good prices but would need to get the servicing somewhere else, maybe midlands or SW. Would be very interested in how you get on.
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Old 09 March 2020, 15:28   #17
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Thanks, as I understand the 15 and 20hp tohatsu Efi are the same weight, so itís very tempting, but around £300 more - as itís a bit of a stretch for the 15 I donít think I can justify the 20 on our use. Itís good to know itís a good match for the Aerotec though, I was offered a px deal at Bill Higham so although itís a bit of a drive I might well just go for it.

Are there dealers you would recommend in the NE? Iím a fairly frequent visitor so it would be interesting to know, the thing holding me back from a new motor is the need to spend cash (£150 - £200) a year on servicing to validate the warranty.
I bought mine from Amble Marina who were comparable to the best I could find at the time. Valid point about the servicing costs ref. warranty. They have you over a barrel..
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Old 09 March 2020, 15:53   #18
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That’s a very nice part of the world. A bit further than I could travel for servicing but good to know. I’ll give the 20hp some thought!
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Old 09 March 2020, 16:09   #19
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I bought mine from Cambridge Outboards ebay shop. They had the cheapest offer for the same motor (£1930). Theres a slightly worrying thread about them on this forum but they rang today to say delivery will be on weds or Thursday so fingers crossed. Will let you all know how I get on.

If anyone is interested there are a couple of dealers on ebay offering a scrappage scheme on tohatsus.

Was toying for ages about a second hand 2 stroke but thought this would hopefully be an investment for many years.
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Old 09 March 2020, 17:50   #20
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I bought mine from Cambridge Outboards ebay shop. They had the cheapest offer for the same motor (£1930). Theres a slightly worrying thread about them on this forum but they rang today to say delivery will be on weds or Thursday so fingers crossed. Will let you all know how I get on.

If anyone is interested there are a couple of dealers on ebay offering a scrappage scheme on tohatsus.

Was toying for ages about a second hand 2 stroke but thought this would hopefully be an investment for many years.
Glad to hear theyíve confirmed delivery, I enquired about the scrappage via eBay with Ďanother dealerí and found a maze of confusing conditions, not what it first appeared to be. Maybe better to strike a straightforward deal as you have.
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