1987 Yamaha 9.9 to 15hp

[Juanvaldez]

Originally Posted by MikeFergie

I still have my Yamaha 15hp 2 stroke on my dinghy and will never swap to a 4 stroke. Be careful with your prop size on my 15 it came with a 91/4 x 11J and with 3 people my dinghy did not plane. I looked at all the enduro 15hp’s I found on the fishing boats and they have a 91/4 x 9J prop. I bought this prop and now I can plane with 3 people very easy. The 11 pitch prop is good for speed but not pick up and go performance.

As far as I know you can change reed valve sizes, jet sizes and the exhaust tube size to get a lot more power. I. Would not bother with a full carb change

Good to know. My 9.9 has a 91/4 9j and I ended up ordering a 10j. I'll experiment and see what works best. I bought a complete carb for 100$ used so will just do a complete swap. Never saw exhaust part differences though?

[MikeFergie]

Not sure on the 9hp but certainly on the 25hp and above. One will need to check the part no’s

[Cheechako]

Originally Posted by MikeFergie

I still have my Yamaha 15hp 2 stroke on my dinghy and will never swap to a 4 stroke. Be careful with your prop size on my 15 it came with a 91/4 x 11J and with 3 people my dinghy did not plane. I looked at all the enduro 15hp’s I found on the fishing boats and they have a 91/4 x 9J prop. I bought this prop and now I can plane with 3 people very easy. The 11 pitch prop is good for speed but not pick up and go performance.

As far as I know you can change reed valve sizes, jet sizes and the exhaust tube size to get a lot more power. I. Would not bother with a full carb change

That's interesting. My Yam 15 was well used and the prop got scarred up in the shallow sands. SO I had this spare new Yam prop I bought at a swap meet, It was a wildly different pitch but I figured it wold be fine just in case I spun a hub out cruising. I did spin the hub after a couple of years, so put it on. I couldn't tell the difference! Plenty of power to plane on my double floor, 130# AB RIB with either prop.

[Adelie]

Throat of the 15hp carb is bigger, not just the jets.

Try putting a larger & flatter prop on the existing motor. That may give you enough thrust for the hole shot with you and your son aboard at the cost of a slightly lower top speed.

[Juanvaldez]

Originally Posted by Adelie

Throat of the 15hp carb is bigger, not just the jets.

Try putting a larger & flatter prop on the existing motor. That may give you enough thrust for the hole shot with you and your son aboard at the cost of a slightly lower top speed.

No room to go up in diameter but was thinking of trying a lower pitch...

[SoonerSailor]

My 4 stroke 9.9 with 8.5x8.5 standard prop woul struggle to get my 180lb hard bottom 10ft inflatable on plane with 2 people, but when it did it made 18 kts. I swapped prop to a 8.5x7.5 and it gets on plane much more easily, and tops out at 16 kts. With only one aboard, it does hit a soft rev limiter just past 16 kts at wot, but I just back it down a hair and it’s fine. The trade off for me was worth it.

[Talbot]

One word of caution. Tohatsu and others sold a 9.8 2 stroke as well as the 9.9. It is a much lighter engine and already at the top of its performance curve.. Don't get them mixed up.

I am not the lightest guy in the world, and my 9.8 had a bot of trouble getting on the plane, so I added 12" flaps at the stern plus doelfins. phew, now she skims.

[OneBoatman]

Why would you need to plane in a dinghy as it is typically but a short distance from your anchored or moored vessel to land. You might be better off buying a longer dinghy and keeping the same outboard. The bigger your motor the harder it is on your back and the more difficut it is to store on the mothership.

[Adelie]

Originally Posted by OneBoatman

Why would you need to plane in a dinghy as it is typically but a short distance from your anchored or moored vessel to land. You might be better off buying a longer dinghy and keeping the same outboard. The bigger your motor the harder it is on your back and the more difficut it is to store on the mothership.

There are several reasons one might want to plane:
A. Anchored in one bay several miles up from the bay with the town and stores.
Assume 8nm each way
-At displacement speeds that s 2.5hr each way, 5hr round trip plus 3hr shopping so an 8+hr day.
-At planing speeds it's 30min each way, 1hr round trip plus 3hr shopping so 4hr or 1/2day.
-Take the mothership at 6kt which means 1h20m each way or 2h40m round trip, plus time to anchor and reanchor plus shopping time. Less than a full day and you can have lunch aboard but more time and more hassle than a planing dinghy.

B. Snorkeling trip 15nm away. That's 4-5hr each way/ 8-10hr round trip at dinghy displacement speeds which really means you need to take the mothership and it's a 2d adventure or you have a planing dinghy and it's 2hr or so round trip plus dive time.

My sense it that folks with smaller motherships that are easier to set and retrieve anchor may be more content about taking the mothership on some of these excursions. With a smaller boat it may be more realistic and cheaper to anchor at the town and stay there a bit than the seek a different anchorage with more room to swing.

[GILow]

Originally Posted by OneBoatman

Why would you need to plane in a dinghy as it is typically but a short distance from your anchored or moored vessel to land. You might be better off buying a longer dinghy and keeping the same outboard. The bigger your motor the harder it is on your back and the more difficut it is to store on the mothership.

Sometimes, as already noted, you have some big distances to cover.

I’ve solved it by having one motor that can get the dinghy up on the plane and another little electric motor for tooling around anchorages and visiting other boats. Attach the motor you need, leave the other on the rail.

But yeah, 90% of the time getting up on the plane isn’t required. It’s the 10% that can be a real problem though.

[SwFlaCats]

Try a lower pitch load prop I bet it will put you on top with 3 adults