Outboard Motor HP

Stef · Post by **Stef** » Thu Oct 11, 2007 12:19 pm

I can get a good deal on a Merc. 4 HP 4 stroke, only 1 year old. Anyone have experience with a motor this small?

Post by **Tom Elsen** » Thu Oct 11, 2007 1:03 pm

The power should be sufficient in most conditions, provided that you don't have big waves and/or really tight conditions at the dock. Make sure your motor mount will keep the prop submerged though. A standard shaft length would likely be problematic.

Stef · Post by **Stef** » Thu Oct 11, 2007 3:10 pm

Oops, forget to mention it has a long shaft.

LarryE-old · Post by **LarryE-old** » Thu Oct 11, 2007 7:54 pm

I bought a 6 HP, 4 Stroke Tohatsu, last year.
The standard long shaft is 20 in. For some reason the only long shaft, in stock, was 23 in. The extra 3 in. is an extra option. Got it for the standard long shaft price.
This is a GREAT package. Motors along almost as good as an inboard, very happy with it.

tpf · Post by **tpf** » Mon Oct 15, 2007 1:42 pm

Stef,
Check the PHRF requirement, I believe the PHRF specifies that the motor needs to be able to push the boat as some minimum speed. I am not sure a 4HP will meet the PHRF requirements.

Post by **fleck** » Mon Oct 15, 2007 2:01 pm

I'd bet that the 4hp would meet any speed requirements. If not, here's even better news. The Merc/Tohatsu 4hp, 5hp and 6hp all are the same size motors (123cc). I'm pretty sure the jets in the carb are the only difference. I'm using the 5hp ( I wanted the external tank) with the 'power' prop and it works great, hull speed or better. Be sure the keep the mount as low as possible, the 'extra' long shaft would be nice. The power prop has a lower pitch and flatter blades. With it we were able to TOW an S2 10.3 doing 4-4.25 knots. Mercury part number is: 812951A 1 .

Dale Eager · Post by **Dale Eager** » Thu Oct 18, 2007 5:47 pm

I have a Nissan 5hp 2-stroke, long shaft. Switching to the Merc high thrust prop recommended by Fleck made a world of difference. Compare it to switching from a 3.0 rear end and bicycle tires on your car to a 4.56 and racing slicks. Before the switch, the prop would spin any time you hit the throttle, and would spin before you hit WOT. And every time you bounced the prop out of the water, it took forever to get traction upon re-entry. Now it digs in and lets me run the engine at a higher RPM so I can get the full HP out of it. So if you have a Nissan/Tohatsu/Merc in the 4-6 hp series (2 or 4 stroke), get this prop!

If you order it online from Mercury, they give you a list of dealers. Check the price with multiple dealers. The first place I checked wanted over $100, the one I ordered from wanted something like $66.

From what I've read, all outboards under 10hp come with a standard, middle of the road prop. If you really want the best performance pushing around a keelboat, try to find a high thrust prop.

Post by **Tom Elsen** » Thu Oct 18, 2007 7:04 pm

Hey Dale and Bob

Great info. Thanks.
I had no idea you could get a power-prop for such a low hp outboard. Makes great sense. Thanks again.

BarryE · Post by **BarryE** » Thu Oct 18, 2007 7:47 pm

I switched to the 6 hp Tohatsu/Nissan 4 stroke this last spring, no complaints, but if I can get better performance, especially pushing upstream under the Bluewater bridge, I'm all for it.
Is there a part number for a better prop?

BarryE · Post by **BarryE** » Thu Oct 18, 2007 7:49 pm

Oops! I just re-read Flecks post; the part numer is there, thanks.

richrand · Post by **richrand** » Fri Oct 19, 2007 5:41 pm

I've got a question. What's the performance difference between 2 and 4 strokes? I read an article where the author said that his two options were a 25HP 2-stroke or a 15HP 4-stroke. That's quite a rating difference for the same application!

-Rich

dave · Post by **dave** » Sat Oct 20, 2007 11:43 pm

That sounds screwy. HP is supposed to be rated at the prop. 2 strokes are lighter for a given HP, simply because every revolution has a power stroke and there are way fewer parts involved. 4 strokes have a power stroke every other revolution, per cylinder on both, of course.

grandillusion · Post by **grandillusion** » Sun Oct 21, 2007 5:55 pm

then there's always the Megregor 26X, with it's masive 65 HP honda, you could make a race where the up wind leg had to be motored then set the chute downwind and not even have to own a head sail, LOL

tpf · Post by **tpf** » Mon Oct 22, 2007 7:47 am

They are probably comparing them based on weight.

The 2 stroke engines do not have cams, pushrods, valves and the head weighs about 1/10 the weight of the heads on a 4 stroke. Also since there are double the number of power strokes the crank, connecting rod and piston are lighter.

Two strokes make much more power per lb than the 4 stroke engines. Four strokes make it up by being more efficient, quieter and lower maintenance especially when run below full throttle. Because of the low compression ration 2 strokes make peak power at high RPM, the four stroke will have a lot more low end torque and will push a boat better at 1/ or 1/2 throttle.

For a sail boat the lower weight is much more desirable especially hanging off the stern of the boat. Oil injection has made 2 strokes much more reliable.

rogerssp · Post by **rogerssp** » Mon Oct 22, 2007 7:58 pm

I've used a 2-shoke 9.9 Evinrude and replaced it last year with a 4-stroke 9.9 Honda. Both move the me at well at hull speed and thru ocean inlets with big waves. I would not consider less HP for my conditions. Both were 25-inch shafts, necessary to avoid cavitation or flooding in big waves. The Honda is heavy and would prefer lighter if I could fine it for the HP and shaft length, but I am so for happy with it.

Spencer Rogers
288 AVATAR

S2 7.9 Meter Class Association

Outboard Motor HP

Outboard Motor HP

outboard

PHRF

2 vs 4 stroke

HP