Outboard for a Yamaha 310STI

[Flamair]

Hi All

I have recently purchased a (bargain) like new 310 STI from eBay..

I will be running mostly either 2 adults with a little kit (day out stuff) and sometimes with 2 adults and 2 kids (10 and 11 yr old)

I'm thinking 9.9 hp (which is the recommended max. However, do I go 2 stroke (preferred, due to weight), or a newer 4 strokes?

Or, any other suggestions??

All ideas gratefully received!

[Fenlander]

Well most of us will say 2-stroke due to hp/weight balance.

A 9.9 will be fine but bear in mind it weighs the same as a 15 in pretty well all makes so you are not getting the best hp for weight.

The ideal and many folks ultimate in this class is the Tohatsu 9.8 at 26kg... but they hold very firm prices.

Another favourite of mine is the Yamaha 8hp which will almost match the Tohatsu for performance on a small SIB but usually at a price several hundred pounds less... and they're lovely quality.

[Flamair]

Thanks Fenlander..

Your comments on the Yamaha interest me as I know of an 8hp for sale (2 stroke)

What would you say the difference would be in performance, compared to the 9.8hp Tohatsu?

I was concerned the 8hp motors would struggle with 4 onboard in a 3.1m boat

I think the Yam is plated at 10.1hp. If the motor weight is within the limits of the boat, would fitting a 15hp unit be cause for concern? (apart from the insurance implication)

Many thanks

[Fenlander]

Re overpowering... just me but I don't do it. I've seen transom seam issues on older boats and suffered transom wood delaminating at the point of max stress when powered hard in choppy waters. So happy to keep within limits.

Different folks with different outboard/SIB combinations report very varying performances with regard to speed vs load carried so any personal prediction is just a guess really. It's weight that's crucial. It's possible on one hand to get a surprisingly lively performance from an 8hp with two light adults and ideal conditions... or it's possible to bog down the performance of a 15hp with heavy crew and lots of kit.

Regarding the Tohatsu 9.8hp vs Yamaha 8hp I was told by a dealer I trust that the extra performance of the Tohatsu 9.8hp over the otherwise identical Tohatsu 8hp was at the very top end of the rev range so in the crucial "getting on the plane" speed range you were only at half revs so at that point the extra 1.8hp wasn't helping.

If it were me I'd be happy to take a chance on an 8hp Yamaha 2-stroke at say £695 over a Tohatsu 9.8 which may be £1000+ due to their popularity.

The important thing is to buy any used outboard at/below the right price then if a swap is needed in the future you'll lose nothing.

How do you mean the Yamaha is plated at 10.1hp?

[Landlockedpirate]

Fenlanders assessment is spot on. The Tohatsu 9.8hp is the holy grail for your size of boat, failing that I would go for the 8hp Yamaha 2 stroke, its a better engine in general than the Tohatsu, but does lack the power to weight ratio.

While the Yamaha 9.9 is identical to the 15, overpowering by 50% is a big step. It will probably be fine, but it could also rip off your transom

[Fenlander]

Just thinking more about speed vs load...

From my experience with 10hp and below you often have to accept that you will have a certain lighter load that will allow you to plane... and a higher load that may mean you will be held to displacement speed and need to plan your trip accordingly.

A 15hp nearly takes you out of this issue with the ability to plane most sensible loads in a 3-3.5m SIB.

To be totally certain of getting on the plane with a full boat around 3.5m though 20/25hp is preferable... but too heavy for easy lifting.

[Mickhitchuk]

I also have a 310 sti, mine is a a 2013 version and there is some confusion with this model as on the Yamaha web site they recommend a 9.9 hp but it's plated as max 8 hp, the guy I bought it from had a yam 8 hp 4s and told me it would plane with that motor but only if one person was on it, which was why he was selling it as he wanted a sib with a larger outboard rating, I put this problem to the ribnet members and was told that zodiac have now down scaled the engine sizes because of the extra weight of 4 Stoke motors, I have just purchased a new tohatsu 9.8 2s for mine but I've got to run it in so can't help you with performance yet,as I understand the law you can have a new 2 stroke but are not allowed to sell it, as I won't be selling my motor I decided to buy a new one,

[Mickhitchuk]

And just to add
When I had a honwave rated at 6hp, I tried the mariner 4s and a Suzuki 4s, and both were very heavy and did not seem to have anywhere near the performance of the tohatsu 5hp 2 stroke I eventually settled on, I could not recommend a 4 s of any kind for your sti

[Flamair]

Thanks for your replies, very helpful!

Well, I feel from that, I probably need the Tohatsu 9.8 2 stroke. I guess it's just going to be finding one...! I will often run two up or 2+1, and I don't want to be unable to plane if I can help it.

The fella I've purchased the boat from says the pressure in the air floor is absolutely critical for the performance to be right. I've owned SIBs before ,but never an air deck. Would you say this is the case? Floor air pressure is critical for the best in performance? (It would make sense!)

Edit to say:

I've found a Tohatsu 2 stroke 9.8hp (2006) model for sale. It's up for £1000. Does that sound about right? It's hardly been used by all accounts! (Looks pretty good, just a few scratches and scrapes here and there) Trouble is, it's 330 miles away and he wants it collecting so don't want to be overpaying for the motor..!

[Mickhitchuk]

It is vital to keep it at the correct pressure, (are you using a bravo electric pump?) a bit naughty of me but I used to take my pump with me then top up when underway, now I just overinflate it a bit before I go out, I don't find the zodiacs lose much air pressure as the air floor is separate and inside the boat, where as a honwave floor is in direct contact with the water, you will know if the floors a bit under pressure as at wot you will get kind of ripples coming down the floor, I say vital at the start of this reply, but only vital if you want the boat to perform at its best when flat out, if just poodling around or its underpowered a bit under won't hurt, it won't sink anyway, forgot my pump once and only managed to foot pump it up to 3 bar not great but going slow on calm water it was ok

[Mickhitchuk]

Saw that one myself, looks good spare prop as well, the money is right, finding these motors for sale is the problem, was going to buy that one myself but dug deep £500 more for a new one

[Flamair]

Thanks for that speedy reply! As I thought, it sounds as if it's critical for optimum performance. Am I correct in thinking the Yam SIBs are just re-branded Zodiacs?

No I don't have a Bravo pump, I've always used the standard foot pump in the past. I've had it recommended now by several folk so that looks the way to go I'd think!

I'm beginning to think I maybe should have gone for a 3.8m boat and with either a 15hp or even 20hp. Oh well, I think we'll see how this one performs! I can always upgrade next year!

I'm vey tempted by the Tohatsu.. That weight v performance is a belter! That said, I've found a cracking Yam 8hp for around half the cost of the Tohatsu..
Goodness! It's a complicated business! Is the extra grunt of the 9.8 worth almost double the money of the 8hp..?

[Fenlander]

Remembering what I said about the revs at which the Tohatsu develops its power compared to an 8hp... personally as long as that Yamaha 8hp isn't a really old one (is it post 2000?) I'd buy it and see how you get on. If OK you've done well.

Then if you feel you need a 9.8/10/15 buy at winter prices from Oct onwards then sell the 8hp next May/June for maximum money.

[Mickhitchuk]

Grab the yam, enjoy the rest of the summer, then it sounds like an aerotec upgrade is on the cards at some point, that's my tuppence worth,

[Flamair]

Thanks so much chaps for that!

I'm very tempted with the Yam.. I will use the boat a fair bit on my own, when I'm sure the 8hp will be fine as I only weigh 11 stone!

I'm not sure on age, but I imagine late 90s. The owner says he's had it in the family from new.

I'll try and post a pic up of it..

[Flamair]

I think outboard pic should be below somewhere..

[phantom 19]

1998 or later. I still think of them as new engines, but then again it doesn't seem that long ago to me that Yamaha did major revisions to their 2 stroke engine range.... And that was in 1984!

[Flamair]

Thanks Phantom.. I thought about then. ALmost bought a Yam 4hp outboard around that time but ended up with a Mariner in the end. Thought it looked familiar.

Fella is wanting about 600 for it I believe. I'm pretty clueless about the value of outboards as it's a good few years since I had one!

What would you think to pay? It's always been flushed out after use and seems to have been well looked after.

Many thanks

[Flamair]

Excuse my rusty ignorance... is that a short shaft or standard shaft engine?

I understand I will require a shirt shaft with the Yam 310.. Would you agree chaps?

Thanks again..

[Mickhitchuk]

Agree