9.9 High Thrust Operating RPM

micmor64

Seaman Apprentice
Joined
Dec 9, 2015
Messages
46
The outboard is a new 1999 9.9 High Thrust Yamaha. The boat is a 26' St. Pierre Dory displacement hull with a displacement of roughly 3600 lbs. The prop is a factory 11 3/4 x 9. At 850/900rpm my gps shows 3.5 mph and I am attaining hull speed at a measured 1950 rpm and wot at 2200 rpm.

At the recommendation of my local Yamaha dealer I tried to increase the wot to 4K plus rpm. To do that, I purchased a Solas Anita 10x5. My speed dropped to just over 2mph at idle, hull speed was reached at just over 2150 rpm and wot increased to just over 2600 rpm. RPM is being measured with a Tiny Tach.

Question: Will operating this outboard at a low rpm hurt the motor overall? I did not like the performance of the Solas prop, I do like the factory Yamaha prop but don't like the 3.5 mph at idle. I am considering a Yamaha 11 3/4 x 7 but am afraid of running the engine at such low rpm under load.

I realize as I try to figure out my prop options I should probably be in that forum, but for now, am I going to hurt the engine running it at these speeds, ( my local dealer says not to do this but can't tell me what harm I'm doing). Any suggestions/recommendations.

Thanks,

Mike
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
not sure how you found a new 1999 motor at this time and date, but lugging a motor is not good for it
 

micmor64

Seaman Apprentice
Joined
Dec 9, 2015
Messages
46
I found a gentleman who had it installed on his fishing boat as a kicker and due to life's' other priorities, and then some health problems, never completed the install or ran the engine. I had a local mechanic go through it before I fired it up and it's running like a top. I now have the break in done and almost 20 hours on it (break in was done on a smaller aluminum boat, no Tach).

The motor runs well and sounds good throughout it's "limited" rpm range. Does not appear to be lugging/straining in the least (but hey, my experience is with old '50's/60's Johnson/Evinrude 2 Strokes), so I'm not sure what a lugging Yamaha 4 Stroke would run/sound like.

All I have read suggests roughly 2HP per displacement Ton is required to drive a hull, so 4HP plus some extra for tides/winds suggests that the 9.9 is more than adequate for the application. I have come across a couple of posts suggesting that this has been done successfully on similar craft by others (though I have no specific details). But, how do I achieve engine speed if I am limited in prop selection?

The engine when running, doesn't sound like it's reaching 4k+ rpm, but let's assume the tach is off, without purchasing a new tach, what's the best way to check speed? Can I run the engine in gear with no prop for a few moments (cooling water supplied through muff's) without harm to see if what rpm the Tach will display?

Thanks,

mike
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
Messages
47,295
get an optical tach. they are $11 on amazon

not advised to rev an outboard when on muffs. there is no rev limiter

if you are truly only getting 2600 RPM, the motor is only at half its rated speed. that is severe lugging.

then again, your carb could be plugged or you could be running on 1 cylinder
 

River Explorer

Seaman Apprentice
Joined
Sep 5, 2016
Messages
44
Check where the throttle link is on your carb. Make sure it's opening properly. Had this happen on my 15hp yamaha 4 stroke. It always wants to hang up even after I put grease on the slide for the neutral stop.
 

River Explorer

Seaman Apprentice
Joined
Sep 5, 2016
Messages
44
get an optical tach. they are $11 on amazon

not advised to rev an outboard when on muffs. there is no rev limiter

if you are truly only getting 2600 RPM, the motor is only at half its rated speed. that is severe lugging.

then again, your carb could be plugged or you could be running on 1 cylinder

These don't have a revlimit? They just float the valves?
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,295
no, the rev limiter is the prop load

there is a reason that the manual says do not rev in neutral

its not that they float the valves, the motor may simply come apart
 
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