first time out - low RPM

M

moabarch

New member
Hi all,
So we finally got our new (used) 22 out on the water. She's got a 90hp BF90 with 13.5 x 15 prop. The previous owner couldn't remember if he'd run the fuel out when last used in Sept or so of '08 - I didn't get back east to pick her up until March of this year. I ran the moter and she started fine and ran smoothly sitting on the trailer. Out on the water, she still started fine and ran smoothly, but with 4 folks on board, full fuel and no other extras, I could only get up to 4300 RPM WOT and a speed of 16.3 knots. I believe from my reading that 5500-6000 is normal. Is this likely a throttle cable problem, or a carb problem? I don't mind running her over to the nearest Honda guy (140 miles from here), but I want to go in at least with some prior knowledge.

It was fun even at that speed - Lake Powell is next on the schedule, followed by Yellowstone lake.

thanks!
 
Tom and Nancy,

Congrats on the new (to you) boat!

IMHO it sounds like a prop problem. We had the same experience when we first got our boat. We were "overpropped" on our Suzuki, which means the pitch was too high. We attained similar numbers to yours at WOT. Once we went down in pitch, the motor handled like a champ at the recommended RPM range.

Also, is the boat located at high altitude? And was it at high altitude with the former owners? That would make a difference on how it's propped.

Do a search on props, there's lots of info on this subject.
 
Tom - Peter is almost certainly right, it is an altitude problem. Our CD25 just wallowed and could not get on a plane with our sea level prop at Lake Powell. Fortunately, the original prop was no good at sea level but adequate for the elevation at Lake Powell. I don't know what prop is right for your CD22, but somebody who boats at high elevations no doubt can steer you right!
 
Thanks - I did some looking into props and it would seem that mine is pretty standard - but you did bring up a very good point about altitude. I hadn't thought of that, but when I looked up the altitude of the lake we were on, I found it is close to 7000 ft. The boat came from sea level. Could the altitude have made that much difference?
 
Pat - hi! I had just answered Peter when your comment came in - I will definitely do the research on a prop for higher altitude - it's not going to keep me off Lake Powell in the meantime, though! I guess I'll meet you there this September?
 
In addition to the prop, I believe there is an adjustment that can be made to the carbs to help compensate for the higher altitude. I don't remember the details, but it should be in the engine manual...

Steve

Just found it online.... From the manual

－
12V 70AH
Carburetor Modif ication f or
High Altitude Operation Battery
Minimum Requirements
TECHNICAL AND CONSUMER INFORMATION
When the carburetors have been
modif ied f or high altitude operation,
the air-f uel mixture will be too lean
f or low altitude use. Operation at
altitudes below 5,000 f eet (1,500
meters) with modif ied carburetors
may cause the engine to overheat
and result in serious engine damage.
For use at low altitudes, have an
authorized Honda Marine dealer
return the carburetors to original
f actory specif ications.
At high altitude, the standard
carburetor air-f uel mixture will be
too rich. Perf ormance will decrease,
and f uel consumption will increase.
A very rich mixture will also f oul the
spark plugs and cause hard starting.
High altitude perf ormance can be
improved by specif ic modif ications
to the carburetors. If you always
operate your outboard motor at
altitudes above 5,000 f eet (1,500
meters), have an authorized Honda
marine dealer perf orm this carburetor
modif ication.
Even with carburetor modif ication,
engine horsepower will decrease
about 3.5% f or each 1,000-f oot (300-
meter) increase in altitude. The ef f ect
of altitude on horsepower will be
greater than this if no carburetor
modif ication is made.
For complete inf ormation, ref er to
the battery manuf acturer’s
instructions.
 
Can respond only obliquely, but this may suggest a solution:

We run the J.C.Lately CD-22 with BF90A turning a 13 x 15 Honda 4-blade prop and typically get a fat 15 kts at 4500, crew of two + a golden retriever, 44 gal gas, 16 gal water all of the above a sea level.
A run to Port Townsend with an additional two BIG folks required closer to 5000 RPM to get similar performance.

FWIW years ago when we owned a small airplane and flew over the CO prairies at 7500 MSL, I usually figured that the 90 nominal HP that little engine claimed was likely delivering no more than about 60 HP at the prop on a nice hot summer day.

I wouldn't be surprised if a better prop for use in UT would be about 13 x 13.

Paul Priest
Sequim, WA
 
We've run from sea level to over 7,000'... the difference in performance is SIGNIFICANT. No doubt that is the problem you are experiencing. A change of prop will help some, but the motor is just making less power at that elevation. At sea level, I can push Wild Blue into the upper 20s; at Lake Powell I'm luck to see upper teens; at Yellowstone (7,800 feet or so), it can barely get on plane.

Haul the boat down where it can catch its breath and you'll see a marked difference.

Best wishes,
Jim B.
 
I see that Wandering Sagebrush just touched on the other item I just recalled - mixture. My BF90A was built for the '06 model year and is the last of the carburetted 90s where the carbs are very simple but non-compensating for air density.
I presume that such compensation came along with the fuel injection of the later versions of the engine, but that could require investigation.

What model year is the engine in your boat?

If you expect the bulk of your boating to be on the high altitude lakes, I'd expect that a conversion of the carbs to smaller jets would be in order.

Good luck,

Paul Priest
Sequim
 
I think you guys have all nailed it - the engine is a 2006, so guess I'll be setting it up for high altitude. I'll also look into the prop a bit more. I should have thought of it - but my mind was on that gas that might have been sitting there for a very long time.
Thanks!
 
Well now you have a whole lot of stuff to think about! We'll be leaving here September 18th, probably on Lake Powell some time on the 21st and stay until October 2nd. Look forward to hooking up! BTW, we went boating this past weekend with Lydia and Conor, say hello to Jen!

moabarch":29k7e24d said:
Pat - hi! I had just answered Peter when your comment came in - I will definitely do the research on a prop for higher altitude - it's not going to keep me off Lake Powell in the meantime, though! I guess I'll meet you there this September?
Click to expand...
 
Tom & Nancy - Your experience was mine 13 years ago when I went from sea level to a higher elevation (Yellowstone Lake at around 7,800 feet). With four aboard I couldn't get on plane to help increase the rpm until I had several fellows crawl into the berth area. Same set up. 90 Honda on a 22' cruiser. So I went on line and started to educate myself about what was happening. As this is what I discovered. Besides weight of gear, gas, water, people, etc., there are five important variables that interrelate as to efficiency with a carb (naturally aspirated) outboard hooked to a C-Dory(assumption that gas is fresh and not diluted with some kind of mixture) These are:
1. Appropriate pitch of prop
2. Air temperature
3. Relative humidity
4. barometric pressure
5. altitude

Of course overall weight is a hugh variable and an extra 100 lbs can really affect performance. There is a point where weight can only be compensated by additional horsepower, but assuming that excess weight isn't the problem, what is causing your drop in rpm and hence less horse power?

I'll venture that variable number five - altitude -is robbing you of over 20% of your horse power. For each 1,000 feet the air density is reduced by 4.5 %, which translates into less oxygen to help create combustion - about 3% less oxygen per thousand feet. Temperature, barometric pressure, and relative humidity also play a role in air density - hence more or less oxygen which of course varies from day to day.

Now you have to consider that a C-Dory comes on plane at around 9 mph, but getting it on plane takes some real punch. If you do not have trim tabs, you are really putting the boat so to speak, "behind the power curve." Once on plane, the drag diminishes and the boat picks up speed.
as does the rpm.

I made an independent decision in 1996 to put on Bennett trim tabs because I was boating at high elevation much of the time, but the tabs create other benefits (and negatives, too).

I also decided to increase horsepower to 115 and choose an EFI fuel delivery system which helps somewhat at elevation.

Had an interesting experience at Yellowstone Lake last week. The starboard tab wouldn't retract and finally it was fully extended. Normally, I use 60% of tab to go on plane and readjust until the sweet spot (4,200 rpm) is reached, water conditions permitting. But coming on plane with the port tab at 60% and the starboard at !00% left the boat with a pronounced list. So I put the port tab at 100% and the maximum speed reached was 21 mph (measured by GPS). The day before it was 28 mph wot. So full extension had a negative effect on efficiency.

Evidently, running at full extension put enough pressure on the hydraulic system that the starboard tab retraction started working again.

Your Honda 90 is an excellent motor, and with some smaller jets and trim tabs, it should get you on plane. But you have to accept the reality of less horse power at higher elevations.

John
 
A lot of times it is hard to really tell if the motor is "running smoothly". You may still have a carb that is plugged, or a plug that is fouled and not be getting full power from all cylinders. It can be checked by running the motor and pulling plug wires one at a time to see if it changes the rpm's. If it doesn't change the rpm's, that plug or cylinder is not doing its share of the work. To safely pull the plug caps loose, I normally pull all of them free and just let them rest on the plug top. Then under idle, use an insulated tool to just pull the cap away from the plug and observe the engine rpm. If it changes, put it back in place -- that cylinder is firing correctly. If no change, you've found the problem plug, or carb, that may have a fouled jet. Ron
 
Nice treatment by Yellowstone of what all flyers in the West learn about as "density altitude", the "thinning" of the air due to both increase in elevation and increased temperature. Flying small airplanes out of a suburban Denver airport on a hot summer day was akin to taking off from a 10,000 foot elevation airport. Loaded to Gross Weight and with a full fuel load some of them simply cannot get out of ground effect or greater than the wingspan above the ground.

And speaking of trim tabs, I've discovered that periodically extending the cylinders fully while on the trailer, wiping them clean and covering the shafts with a light coat of silicone grease seems to facilitate their smooth operation and allows them to retract fully into the cylinders, something which was becoming problematical prior to treatment.
Incidentally, does anyone have a proven procedure for finding the optimal combination of trim tab position vs motor tilt position?

Paul Priest
Sequim
 
Paul - Thanks for the tip on keeping the tab shafts lubricated. I haven't done that.
Optimum position of tabs and motor for greatest efficiency?These are my thoughts. First, keeping the engine perpendicular to the surface should maximize thrust. Using the engine's trim and tilt to raise or depress the bow will decrease thrust. Of course getting on plane may require using that feature for a few seconds. I use the tabs to get on plane set at 60% and then begin tweaking them (usually less) watching my speed.

Second, I reply on the GPS speed reading along with rpm as indicators during this process trying to keep the engine perpendicular. Varying the throttle and tweaking the tabs will quickly indicate which speeds are most efficient.

The 22' cruiser likes to ride bow high, and once on plane the sweet spot (Merc 115 EFI) seems to be around 4,200 rpm. Lowering the bow to decrease jarring from chop will take more power (increase fuel consumption).

Of course the water conditions dictate how one uses tabs and engine. I like to travel at 1,000 rpm whenever time permits, and rarely use WOT.For really nasty waves when some distance must be covered, I go bow high just on plane, but that is not fuel efficient.

Have come to believe that onboard weight robs efficiency more than is recognized, and for that reason carry only a few gallons of water. Why carry full water when that equates to another passenger?

As fuel consumption increases, weight must drop. A couple of hundred pounds less of gas translates into less hull in the water. For that reason when in an area where gas is readily available, I rarely carry full fuel.

Guess the most abbreviated answer to your question on tab and engine position is this: it depends on many things. No two boats carry the same weight. The only real constant is the hull design. Trial & error is my MO.
John
 
Yellowstone":c2bll981 said:
....

Your Honda 90 is an excellent motor, and with some smaller jets and trim tabs, it should get you on plane. But you have to accept the reality of less horse power at higher elevations.

John
Click to expand...

I agree with John, the BF90 is an excellent engine. With respect to the recommendation for trim tabs, there is a less expensive alternative, Permatrim. We went with the Permatrim for fore and aft trim, and found that it works quite well. You can get on a plane easier, and it brings the nose down to help handle chop. What you don't get is the lateral trim that the tabs can provide.

Steve
 
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