Reply
 
Thread Tools Search this Thread
 
Old 12 August 2015, 16:39   #1
Member
 
Country: UK - England
Town: Huddersfield
Make: Yamaha
Length: 3m +
Engine: Yam 8hp Petrol
Join Date: Aug 2015
Posts: 34
Outboard for a Yamaha 310STI

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!
__________________

__________________
Flamair is offline   Reply With Quote
Old 12 August 2015, 17:07   #2
Member
 
Country: UK - England
Town: East Anglia
Boat name: Nimrod II
Make: Aerotec 380
Length: 3m +
Engine: Suzuki DF20 EFI
Join Date: Nov 2007
Posts: 3,279
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.
__________________

__________________
Fenlander is offline   Reply With Quote
Old 12 August 2015, 17:14   #3
Member
 
Country: UK - England
Town: Huddersfield
Make: Yamaha
Length: 3m +
Engine: Yam 8hp Petrol
Join Date: Aug 2015
Posts: 34
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
__________________
Flamair is offline   Reply With Quote
Old 12 August 2015, 17:45   #4
Member
 
Country: UK - England
Town: East Anglia
Boat name: Nimrod II
Make: Aerotec 380
Length: 3m +
Engine: Suzuki DF20 EFI
Join Date: Nov 2007
Posts: 3,279
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?
__________________
Fenlander is offline   Reply With Quote
Old 12 August 2015, 18:28   #5
Member
 
Landlockedpirate's Avatar
 
Country: UK - England
Town: St Helens
Boat name: Wine Down
Make: Maxum
Length: 8m +
Engine: Inboard
Join Date: Apr 2012
Posts: 934
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
__________________
Landlockedpirate is offline   Reply With Quote
Old 12 August 2015, 19:06   #6
Member
 
Country: UK - England
Town: East Anglia
Boat name: Nimrod II
Make: Aerotec 380
Length: 3m +
Engine: Suzuki DF20 EFI
Join Date: Nov 2007
Posts: 3,279
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.
__________________
Fenlander is offline   Reply With Quote
Old 13 August 2015, 02:03   #7
Member
 
Country: UK - England
Town: Horley
Make: Yamaha 3.1 STI
Length: 3m +
Engine: 9.8 Tohatsu 2 stroke
Join Date: Sep 2013
Posts: 246
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 is offline   Reply With Quote
Old 13 August 2015, 02:16   #8
Member
 
Country: UK - England
Town: Horley
Make: Yamaha 3.1 STI
Length: 3m +
Engine: 9.8 Tohatsu 2 stroke
Join Date: Sep 2013
Posts: 246
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
__________________
Mickhitchuk is offline   Reply With Quote
Old 13 August 2015, 03:18   #9
Member
 
Country: UK - England
Town: Huddersfield
Make: Yamaha
Length: 3m +
Engine: Yam 8hp Petrol
Join Date: Aug 2015
Posts: 34
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..!
__________________
Flamair is offline   Reply With Quote
Old 13 August 2015, 03:41   #10
Member
 
Country: UK - England
Town: Horley
Make: Yamaha 3.1 STI
Length: 3m +
Engine: 9.8 Tohatsu 2 stroke
Join Date: Sep 2013
Posts: 246
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 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 06:27.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.