Outboard a little weak?

[sailingharry]

We have a Mercury 8.5 ft aluminum bottom rib. The first year we had it, we had an absolutely ancient Evinrude eight horse two-stroke. The spring of 22, we scored a 9.8 horse 2018 tohatsu 9.8, and it was a noticeable improvement. But I've constantly thought that I really should be doing better. With two of us and a dog, total cargo weight under 400 pounds, we can't make 12 knots. As an added data point, sometimes we barely make seven.

Today at WOT I was decidedly smoked. The offender was a high-field, maybe 9 and 1/2. They had two adults and two kids. And they had a 9.9, I didn't catch the make.

The engine runs very well. Starts on first or second pull every time. Last summer we probably made 300 trips in it, we have a dog and he goes ashore at least twice a day. I run a compression test, I don't remember the numbers but the two cylinders were comparable. Both plugs look good. Pulling either spark plug wire makes a noticeable change in engine speed. I have disassembled and sprayed carb cleaner all over the carb, although it looked spotless to begin with.

I am willing to pay an expert to look at this. But only if I get value for it. My problem is that I am fiercely independent, and do nearly all of my own work on everything. As a result, I have no idea how to hire professionals. We have an excellent Tohatsu repair shop in Annapolis, Annapolis Inflatables. If I take it to them and tell them that it runs beautifully, but just not a lot of oomph, will they charge me $200 and say that everything looks good, or do they have a way to compare actual output to expected output? The marine equivalent of a dyno test on a car. Or perhaps they have ways of looking at a smooth running engine and knowing what's wrong. Perhaps it's just easy stuff that they know how to test and adjust, like timing or advance or high speed mixture screw.

I'm kind of lost and frustrated and hoping for guidance.

[team karst]

Motor trimmed out? With wings?

[sailingharry]

Yes, wings. No thoughts on trim -- I do have it set all the way forward/down. If I lift it up a notch or two, I would think that would make the bow rise higher. But perhaps that might make it plain faster.

[team karst]

Yes it will

[cr180]

Suspect nothing wrong with your outboard. Suspect your Mercury dinghy holding you back. Think approx 400 lbs in 8.5’ dinghy a lot. The Highfield likely more efficient hull shape.

So would encourage thinking about dinghy hull shape and efficiency. Also agree with your trepidation about paying an expert. Will be Hard to find a good small outboard mechanic that could do more than you have already.

Our experience is that a 10’ (or 10’ and some change) rib provides the most efficient hull to weight trade off. And newer 20 hp outboards weigh the same and cost not much more than a 9.9 hp (same block). We are able to plane with four adults (maybe 600 lbs) with an AB 10 and 20 hp outboard which we carry on Kato Marine davits.

[Don C L]

Any chance that outboard had been previously used as a kicker for a sailboat and has a prop with a pitch for power not speed?

[team karst]

Good advice above.
More data: I run a couple of 10’ inflatables. One is a rib. The rib is paired with a 9.9 4-stroke. The rollup has a 15 2-stroke. The 9.9 goes 20 kts with 3 persons. The 15 does 20 kts with 4 persons. When marginally too much weight, half the crew needs to lean forwards until we get over the hump. Motors trim position is out.

[CSY Man]

The 8.5 may indeed be too short for speed.
My last Mercury dink was a 310 with an inflatable floor, with a 4hp Yamaha 4-Stroke I would see 10.9 knots a full bore with just me, a 230 lbs Norwegian and a few items in the boat.

[Montanan]

Recommend the use of lifters to dramatically enhance one's ability to get to plane on a dinghy.

Reference link and videos.

https://www.maximarine.com/products/lifters/index.html

Bon voyages.

[Franziska]

Might be a wrong prop. We reduced pitch on our 4hp Suzuki and suddenly we get on a plane with our Zodiac Cadet Rib.
Carb revision helped too.

[donradcliffe]

Most people with short dinghies are disappointed with their performance, getting on a plane and planing..

[sailingharry]

The boat is quite light. Boat is 140lb, engine is 54lb (if I recall correctly), all in under 200lb (well, with 3 gallons fuel, maybe busts 200). Admiral, me, and dog total to under 400.

I bought it from original owner. He used it on a dink in the Bahamas, but couldn't plane with crew and full load of SCUBA gear, so went bigger. Presumably original prop.

Might have a chance to swap with a 9.9 4-stroke next week. It will be double the motor weight (a 9.9 4 is close to 100lb) but would really answer some questions.

[Tedd]

If the compression is good, the plugs read normal, and the engine runs smoothly, then it probably is producing something close to rated power. Perhaps ask the shop you're thinking of if they have a leak-down tester (instead of or in addition to a compression tester). A leak-down tester will sometimes pick up cylinder leakage that a compression test misses.

Since you're a hands-on person, perhaps consider getting your own leak-down tester. They're handy to have since you can use it on virtually any internal combustion engine. Note that you need an air compressor and you'll also need a means of preventing the crankshaft from rotating during the test. I haven't done this on an outboard but a common method is to make a fixture that engages teeth on the flywheel and bolts or securely clamps to the engine block.

[Lee Jerry]

Originally Posted by sailingharry

Yes, wings. No thoughts on trim -- I do have it set all the way forward/down. If I lift it up a notch or two, I would think that would make the bow rise higher. But perhaps that might make it plain faster.

Trimming the engine out (up) should almost certainly help with top speed a little bit, maybe a couple of knots. Picture that you are riding a little more on top of the water rather than pushing through it. It's also probably the easiest change to try. However, it will actually make you plane slower (and perhaps ride a bit rougher / bouncier).

It wouldn't hurt to check the engine speed at WOT, especially since you're handy. I expect you will be at rated RPM. If so, you can then try bumping the pitch up one notch and check again. (If you have "access" to the appropriate prop. The problem with just checking the PRM with the first prop is that you don't know how close to rated power you are; you have to increase the prop load until you can't reach rated speed to really know where you are on the map.)

Finally, w.r.t. the wings. Did you have trouble getting on plane without them? They do add drag (as well as the lift), so you could try without them if you're not sure (about the planing). The lifters (or similar) referenced above might give you similar lift at lower drag. Probably don't want both.

[Cheechako]

1) is it possible the rubber hub in your prop is slipping? I had that problem.
2) How many hours on the engine? Main bearings went bad in my Yamaha and it would not rev properly. Luckily I ran into a guy who knew what it was quickly.
3) with the same Yamaha 15 I had a spare prop that was vastly different specs than the original. (I bought it new as a bargain for backup.) After a couple years of wearing the original in sand etc I put the oddball prop on. I have to say, There seemed to be no difference in getting out of the hole, speed etc. Was it just because the Yam had plenty of power or...?

2 stroke?
I assume you are getting on a plane but suffering for speed? That sounds like lack of power: main bearings or maybe valve issues to me. Your tank vent isn't clogged is it?