Better to Lug or Rev a Nissan 9.8

[pvanv]

One for Elvin:

Last season, I installed a new Nissan 9.8 on my 8,000-pound, 28-foot sailboat, rigged with the 5" pitch x 8.75" diameter, 4-blade HT prop. The factory tach showed max RPM's in excess of 6,000, maybe up to 6,200 -- just below the rev limiter point. The correct WOT range for the motor is 5,000 to 6,000.

This season, I changed to the 7" pitch version of the HT prop, to lower the max RPM's. What's surprising is how MUCH it slowed the motor down. Now the tach only makes it up to barely 4,900 or 5,000 at WOT.

Seems my ideal pitch will be 6", which is not a stock, off-the-shelf prop. I'll see if my local prop shop can re-pitch one of my props to 6". Meantime, is it better to lug slightly with the 7" prop, or run the 5" prop, and be careful not to over-rev?

 

[TOHATSU GURU]

Re: Better to Lug or Rev a Nissan 9.8

Paul,

Run the 5. If you want to try and perfect the propping I would have the 5 pitch altered with a slight cup to drop the RPM 100. Keep in mind that your load really requires a 15 hp in raw hp, so the 5 P 4 blade is really a cheat to compensate for the low hp.

 

[pvanv]

Re: Better to Lug or Rev a Nissan 9.8

Paul,

Run the 5. If you want to try and perfect the propping I would have the 5 pitch altered with a slight cup to drop the RPM 100. Keep in mind that your load really requires a 15 hp in raw hp, so the 5 P 4 blade is really a cheat to compensate for the low hp.

Thanks Elvin. Seems the 5 is the best off-the-shelf pitch for this rig.

Yes, the 4-blade HT prop is a cheat, and works. Rigging small OB's on heavy sailboat applications is always a challenge. In my case, a 15 hp 4-stroke, while better-matched to the load, would not fit through the hatch into the lazarette well -- they're bigger than the old 15 hp 2-strokes. So the 9.8 was the only repowering option, short of installing sweeps ;-)

Of course, I'm not 100% sure of the factory tach accuracy. If it's reading 6200 when I'm actually turning 6000 (accuracy off by 3.3%, plausible), then I was better off with the 5. After all, the rev limiter (I think it's supposed to trigger at 6250) does not activate.

Adding cup, while dropping RPM a little, might hurt my (very important) reverse thrust, though slight re-pitching might be better overall. These heavily-loaded small OB's tend to be very sensitive to minor pitch changes.

I will have to take GPS speed readings to compare whether the 7 actually provides more top end boat speed. There's a possibility that since the 7 causes some lugging, the boat may actually go slower with the 7.

After all, hp is hp, and once you use every last drop of it, there isn't anything left to gain. I don't know any tricks to get more hp out of the 9.8; Are there any?

Thanks again.

 

[TOHATSU GURU]

Re: Better to Lug or Rev a Nissan 9.8

+- 50 RPM on the tach accuracy. I would think about the "light" cup rather than a pitch change. Just enough to drop the RPM down. No tricks on a 9.8 boost that I know of. I'll query a Jap engineer that I'm friends with and see if they have any tricks for this model.