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Old 27-02-2020, 15:21   #1
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Yamaha 6/8hp vs Tohatsu 9.8hp

Seems my Yamaha 6hp 2 stroke was actually an 8hp or close enough. Inspection of the exhaust revealed the washer had been removed, and the carb had a larger main jet than original specified on the 6hp.

It would propel my 2.6m air floor dinghy to the dizzying heights of 15-17knts one up and 11-12knts two up.

I since changed to a 2.6m aluminium rib, weighing the same as the old air floor dinghy. On this rib the motor will push me along at 15knts, but will not plane at all two up.

Dropping down to an 8.5" x 6.5" prop may help a little (currently running 8.5" x 7.5"), but I'm not convinced. The aluminium rib can only carry 300kg vs 500kg of the same sized air floor dinghy, so there's a lot less buoyancy.

I'm wondering if the small additional power of the Tohatsu 9.8 2 stroke is worth the outlay. The Yamaha 15hp would be nice, but the weight is too much. 26kg is about all I can manage moving around.
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Old 27-02-2020, 15:44   #2
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

I recently went from a Tohatsu 6hp to the 9.8hp.

I performance isn't as much as i would have expected. The biggest change is that it is quieter and smoother.
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Old 27-02-2020, 15:49   #3
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

Quote:
Originally Posted by mikedefieslife View Post
Seems my Yamaha 6hp 2 stroke was actually an 8hp or close enough. Inspection of the exhaust revealed the washer had been removed, and the carb had a larger main jet than original specified on the 6hp.

It would propel my 2.6m air floor dinghy to the dizzying heights of 15-17knts one up and 11-12knts two up.

I since changed to a 2.6m aluminium rib, weighing the same as the old air floor dinghy. On this rib the motor will push me along at 15knts, but will not plane at all two up.

Dropping down to an 8.5" x 6.5" prop may help a little (currently running 8.5" x 7.5"), but I'm not convinced. The aluminium rib can only carry 300kg vs 500kg of the same sized air floor dinghy, so there's a lot less buoyancy.

I'm wondering if the small additional power of the Tohatsu 9.8 2 stroke is worth the outlay. The Yamaha 15hp would be nice, but the weight is too much. 26kg is about all I can manage moving around.
Yah I have an 8 hp two stroke on a bare 3.10. rib

That baby hauls ass.

I can terrorize the harbour , women and children screaming and diving for cover

Seagulls take fright, lift off and poop all over the neighboring boats

That 8 hp is about max for manpower lifting and storage
Good choice
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Old 27-02-2020, 15:50   #4
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

My 2.9m fibreglass rib will plane two up with a Tohatsu 9.8, but stops planing if a bit more than a couple of jerry cans of water are thrown in as cargo on top. Having said that, a 9.8 on this size of dinghy makes it very twitchy one up. Possibly twitchy enough to be a little dangerous.



The problem with short waterlines is that they can be more difficult to get up on the plane. I'd see if moving forward while trying to dig out of the hole will pop the dinghy up onto the plane as once you're planing you will be fine.


An alternative solution may be to try those fin things they fit to outboard legs to help boats get up on the plane. These work really good for boats heavy in the bum, but they can be downright dangerous and cause broaching if the boat is too light.
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Old 28-02-2020, 13:57   #5
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

Goodness, I love my 13 foot two seater kayak and powered by arm and chest.
I also have a porta-bote 10 footer with oars. 4 hp 2 stroke Johnson has been rebuilt but not running correctly. Another day.

Although I love my kayak I understand trying to carry fuel and water any distance would be miserable. I’m just a coastal sailor and have no big need to cross large bodies of water.
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Old 05-03-2020, 11:43   #6
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

IF you are interested in purchasing a used 9.8 hp 2 cycl Tohatsu, please contact me.

Outboard engine is located in NJ
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Old 05-03-2020, 16:25   #7
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

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Originally Posted by Caravela1986 View Post
IF you are interested in purchasing a used 9.8 hp 2 cycl Tohatsu, please contact me.

Outboard engine is located in NJ
I'm in Guadeloupe. I need to update my profile, but NJ is a long way away. The shipping alone would probably make it better off buying a brand new one from St Martin at $1,600 export price.

As it is, I found out that although the exhaust restriction on mine has been removed. The carb is a 6H rather than the 6G model in the 8hp versions. The carbs are almost identical, same jets even, but the 6G has a larger throat.

I've ordered a genunie Yamaha one ($200), since none of the aftermarket suppliers can be trusted to supply the right item. They can't differentiate between the 6H and 6H.

I've also order a new 8.5 x 6.5 prop ($80), so an inch less pitch should help given the reduced buoyancy i.e. the dinghy sitting deeper in the water. In any case I wanted a spare of a different pitch.

I will report back my findings in the coming months.
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Old 22-04-2020, 01:48   #8
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

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Originally Posted by mikedefieslife View Post
I'm in Guadeloupe. I need to update my profile, but NJ is a long way away. The shipping alone would probably make it better off buying a brand new one from St Martin at $1,600 export price.



As it is, I found out that although the exhaust restriction on mine has been removed. The carb is a 6H rather than the 6G model in the 8hp versions. The carbs are almost identical, same jets even, but the 6G has a larger throat.



I've ordered a genunie Yamaha one ($200), since none of the aftermarket suppliers can be trusted to supply the right item. They can't differentiate between the 6H and 6H.



I've also order a new 8.5 x 6.5 prop ($80), so an inch less pitch should help given the reduced buoyancy i.e. the dinghy sitting deeper in the water. In any case I wanted a spare of a different pitch.



I will report back my findings in the coming months.


... had a chance to test the new set up yet?
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Old 22-04-2020, 20:52   #9
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

Yep.

I put the 8hp carb on and the engine felt a little punchier. Maybe only 0.25-0.5 difference on my runs though.

I also put the 8.5 x6 1/2 prop on for when two up. Again a tiny difference, but not enough to get up on the plane with two people. Very very close. It goes 7knts 2 up but still in a semi displacement mode.

Adding tabs, or fins to the engine would likely do it if I really wanted to go that rode.
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Old 22-04-2020, 23:20   #10
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

Mike, I just get a kick out of your id. Mike defies life. That is very fun.
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Old 22-04-2020, 23:24   #11
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

We replaced 6hp yamaha with 9.8 Tohatsu 2 stroke with next pitch size down on standard prop on the back of a 2.9m Highfield alloy rib - planed two adults easily (160Kg).


Later on another bigger fibreglass RIB with 20HP 4 stroke we found adding stingray fins also makes a big difference as it lifts the rear out of the water and planes easier forcing the nose down
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Old 23-04-2020, 00:56   #12
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

Quote:
Originally Posted by mikedefieslife View Post
Yep.

I put the 8hp carb on and the engine felt a little punchier. Maybe only 0.25-0.5 difference on my runs though.

I also put the 8.5 x6 1/2 prop on for when two up. Again a tiny difference, but not enough to get up on the plane with two people. Very very close. It goes 7knts 2 up but still in a semi displacement mode.

Adding tabs, or fins to the engine would likely do it if I really wanted to go that rode.
Go to a local store that carries cookware, buy an appropriately sized plastic cutting board, saw off two corners, fasten securely to the skeg? little horizontal plate above the prop. Several short 10/32 bolts worked for me.
Test it out if it gets you to plane. My experiment helped a lot - and it.ís still on my 5.5 Tohatsu.
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Old 23-04-2020, 01:03   #13
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Re: Yamaha 6/8hp vs Tohatsu 9.8hp

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Originally Posted by slug View Post
Yah I have an 8 hp two stroke on a bare 3.10. rib

That baby hauls ass.

I can terrorize the harbour , women and children screaming and diving for cover

Seagulls take fright, lift off and poop all over the neighboring boats

That 8 hp is about max for manpower lifting and storage
Good choice
I can do the same with my 5.5 Tohatsu.

My technique is to ride my dinghy standing through the Mogote anchored fleet, and wearing a hat and sunglasses. Nothing else.

Water has to be flat to plane though, and only one up.

This is after going swimming off the point because the water in La Paz harbor itself is getting more and more polluted by the sewage of 250,000 townspeople.
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