Missing horsepower, resolved

C

Ctarmigan

Member
For some time, I've had a nagging concern that the max rpm on my 2004 Honda BF90A has been lower than it should be. It hasn't made much practical difference, as I cruise around 4200rpm and almost never go to maximum throttle.

I was recently testing the difference between two props, though, and the results just didn't make sense; for both props I was maxing out at 4800rpm, with a lightly loaded boat on a flat calm lake. Something was amiss in a way I couldn't ignore.

Back home, I popped off the cowling and checked on starboard throttle cam range of motion while operating the throttle at the helm. Sure enough, full throttle at the helm translated to less-than-full throttle at the outboard.

Resolution was simple; I used the upper throttle cable adjustment screws to ensure I was just barely hitting the throttle stop (behind the cam) when I got to full throttle at the helm. I checked against the port side throttle arm to ensure all three (helm, throttle cam, throttle arm) were hitting full range of motion simultaneously, and I think I'm done.

I've yet to try this out on the water again, but my expectation is that I've picked up considerable power I hadn't previously had access to, all through 5 minutes of adjustment (and 5 years of procrastination).

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The engine should reach between 5,500 and 6,000 at wide open throttle, Boat speed should be close to 30 MPH.

What speed were you reaching at 4800? What is the difference in the two props? What is the elevation where you are testing and using the boat?
 
Ha - I did consider asking the 'which prop' question, but thought I had not quite enough information at hand to properly inform folks. Happy to provide it as a challenge to the technically savvy!

2003 Honda BF90a (carbureted) - I originally misstated 2004
22 Cruiser
2.5 passengers, relatively light load
Flat calm conditions
2300ft elevation

Prop 1: 13.5 x 15 3-blade aluminum (Solas; Rubex hub - slight ding in one blade)
Speed at 4200rpm: 30km/h (18.6mph)
Speed at 4800rpm: 39km/h (24.2mph)

Prop 2: 13.25 x 16 3-blade stainless steel (Honda Turbo; standard hub - new condition)
Speed at 4200rpm: 28km/h (17.4mph)
Speed at 4800rpm: 35km/h (21.7mph)

My most relevant use case, however, will be different. My longest trips, where I'm most interested in balancing fuel economy and speed, are in SE Alaska (0ft elevation), with 3-5 passengers and a heavy load.

I'm departing from Skagway on exactly such a trip tomorrow (meaning I may see Hunky Dory again this year). Destination Elfin Cove and Hoonah for the week.

My current plan is to run with a (new, undinged) 13.5 x 15 3-blade aluminum, with the stainless steel prop as a spare. At some point in the trip I'll swap the props for a better head-to-head test under matched conditions and full range of rpm now available from the throttle.
 
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