Having performance problems? Your answer might be as simple as a prop swap.
It’s been a while since I’ve authored a “propped for performance” article. When the Fisherman’s Managing Editor Matt Broderick reached out to me in October of last year about helping him tweak maximum performance out of his legacy trailerable back bay sportfisher, a 1981 Sea Ox 1700 skiff, I jumped at the chance to dive into one of my favorite subjects. Running the right boat/motor combination with a propeller that allows it to achieve max recommended RPMs is usually a dream to drive. Equip this dream boat with the wrong prop and it quickly turns into a nightmarish experience. Let’s take a deeper dive into this nautical science project to see how things worked out and how you might be able to benefit by understanding this step-by-step process and applying a similar approach to your boat.
The Boat
Matt’s vessel is a Sea Ox 1700 side console skiff that offers a wide-open, utility boat layout. It was built by North American Fiberglass, located in Greenville, NC, and they manufactured coastal fishing and utility skiffs from 1980 through 1994, after which production ended. The Sea Ox 1700 has an overall length of 17 feet with a relatively narrow 6-foot wide beam, featuring two-piece construction consisting of a hull and a fiberglass cap, with a glass over plywood cockpit sole. The floor and wooden stringers of this platform had both rotted over the years, so Matt initiated a rebuilding process. He replaced both the running bottom’s backbone and cockpit sole with fresh sheets of 3/4-inch plywood for both the stringers and deck. The new stringer grip was set into place with fiberglass putty, after which it was totally encapsulated in West System epoxy and fiberglassed into the hull with three applications of 24/15 fabmat. This upgrade repurposed her with a second life as a rock solid, dedicated trailer boat (bottom was never painted) platform that would perform up to expectations and take Matt and his sidekicks wherever the fish were biting in the northeastern coastal arena. With the upgrade, the Sea Ox’s 1,100-pound displacement was now a few hundred pounds heavier than the original spec, with a slightly raised cockpit sole, but she was seaworthy again and still offered her original self-bailing capabilities for safety out on the water.
The Engine
The engine on Matt’s Sea Ox 1700 is a 2000 Yamaha C90 two-stroke outboard. He had changed whatever engine that came with this original 1981 package and replaced it with the Yamaha. The C90 Yamaha is a unicorn of sorts, with an extremely light weight (only 259 pounds), equipped with a 20-inch shaft, 2-stroke powerhead, sporting a 3-cylinder in-line design with a carbureted fuel system that burns 2-stoke oil in a 50/1 mix that’s added directly to the gas.
The Problem
Matt was not achieving max recommended RPMs with his boat/motor setup. Yamaha advises that the ideal max RPM setting for the C90 is somewhere between 5000-to-5500 revs. After a quick sea trial to establish the Sea Ox 1700’s benchmark, we determined that the max RPMs his rig was achieving with trim was only 4600 at wide open throttle (WOT). This condition, where the RPM is below the recommended range, is called “lugging,” a situation where too much stress is being put on the engine, something like trying to go up a hill with a bike in sixth gear, as opposed to an easier-to-pedal second or third gear. The usual rule of thumb for simplified prop math is that for whatever screw you are running, while keeping the diameter, propeller type and blade geometry relatively the same, changing the pitch by one inch up or down, will usually correspondingly increase or decrease your engine’s revolutions by about 150-to-200 RPM.
Matt had purchased his 3-blade aluminum Solas prop from a local prop supplier. He was assured by the sales person that the 15-inch pitch wheel was “the right prop for your boat.” As we subsequently determined from our sea trial, this prop was not ideal for the Sea Ox/Yamaha C90 setup and we were determined to come up with a more acceptable fix. I reached out to Yamaha’s Senior Regional Sales Manager Mike Browne at the Miami Boat Show this past February and we “worked the problem” to generate the right propeller solutions for Matt’s ride. The plan was to test three different wheels, two aluminum and one stainless, that ranged in pitch from 13-to-14 inches.
Yamaha sent me a trio of propeller options that were generated from hypothesized science charts, with the assurance that one or more of them should work. However, one thing I’ve learned about propeller testing, while computer modelling might put you in the ballpark, real world sea trials will place you in the right seat to watch the ballgame. The trio of right-handed props that Mike Browne and the Yamaha tech team sent me was as follows:
- Yamaha 13-5/8 x 14, 3-blade, aluminum (made in Japan)
- Yamaha 13-5/8 x 13, 3-blade, aluminum (made in Japan)
- Yamaha Talon 13-1/8 x 14, 3-blade, stainless steel (made in USA)
Somewhere in this mix, we were hoping to find the answer. We suspected right from the get-go that reducing the blade pitch would increase the Yamaha’s RPM by 200-to-400 revs, but there was even more to observe and learn. Situated side-by-side, the trio of Yamaha props also featured a slight reduction in overall prop diameter compared to the Solas, in addition to Yamaha’s proprietary engineering of the blade shape, cupping, rake and overall geometry of their propellers. All this would increase the Yamaha C90’s RPMs even further. The Yamaha wheels were specifically engineered for their own outboards and the performance data backed up this claim with real world results.
Real World Results
The following table “benchmarks” the original setup, which is always the first place to start before initiating your search for the perfect propeller.
Boat- 1981 Sea Ox 1700 skiff
Outboard- 2000 Yamaha C90 two-stroke
Prop- Solas 15-inch pitch, 3-blade aluminum (part #9411-135-15)
Fuel- 10 gallons (twin portable tanks)
People Onboard- 2 POB (435 pounds)
Gear- 200 pounds
Wind & Sea Conditions- ESE 12-to-15 knots; tight 2-foot chop
Temperature & Humidity- 60 degrees F; 45% Humidity
| RPMs | Speed (knots) | Fuel Consumption* | NMPG |
|---|---|---|---|
| 1000 | 3.7 | 0.9 gph | 4.11 |
| 1500 | 5.2 | ||
| 2000 | 6.4 | 3.0 gph | 2.13 |
| 2500 | 14.6 | ||
| 3000 | 19.2 | 4.9 gph | 3.92 |
| 3500 | 21.6 | ||
| 4000 | 25.5 | 7.7 gph | 3.31 |
| 4500 | 28.7 | ||
| 4600 | 29.0 | 9.5 gph | 3.05 |
Average Time to Plane- 3.0 seconds
*Fuel consumption data was taken from historical Yamaha C90 Performance Bulletins
Analysis
As can be observed from the results, top RPMs were below expectations at only 4600 revs, a total of 400 revs under Yamaha’s minimum guideline for this outboard. While this two-stroke engine planes the hull at only 2500 rpm, achieving an impressive 14.6 knots, the performance data suggests the engine is clearly “lugging”, with this condition being less than ideal for a 25-year-old outboard with an unknown amount of run time on the clock. Going with a lower-pitch prop was clearly the right solution for resolving this problem, but what would be the tradeoffs in pursuing this path?
Comparisons
Before we dive into the comparative statistics, it’s always a great idea to work with your engine manufacturer’s local dealer whenever doing propeller testing or purchasing on your own. Who has the money to buy a variety of different wheels in an effort to determine which one might work best? Your outboard dealer has the experience and the inventory to help you down this path with a minimum of hassles. The dealer might also have some “loaner” wheels to share with you to help make your decision.
The following tables show the performance comparisons for the trio of new Yamaha props we tested on Matt’s Sea Ox 1700 skiff. These tests were conducted on the same day that the benchmark test was performed, all within a three-hour window from start to finish. The boat was taken back to the launching ramp, where the props were quickly swapped out in between test runs in similar wind and wave conditions. Current was not an appreciable factor where we were conducting the test in the back bay.
All three of the new props planed the boat in less than 2.5 seconds, so Matt and his crew will need to hold onto something solid when popping out of the hole on their next fishing trip. The fuel consumption projection profiles were slightly different for these motors compared to the benchmark, since all three props allowed the Yamaha C90 to achieve RPMs within Yamaha’s recommended min/max guidelines:
| Yamaha 13-5/8 x 13 Alum | Yamaha 13-5/8 x 14 Alum | Yamaha 13-1/8 x 14 Talon SS | ||||||||
| RPM | Speed | NMPG | RPM | Speed | NMPG | RPM | Speed | NMPG | ||
| 1000 | 3.2 kts | 3.56 | 1000 | 3.3 kts | 3.67 | 1000 | 3.5 | 3.88 | ||
| 1500 | 4.4 | 1500 | 4.5 | 1500 | 4.6 | |||||
| 2000 | 5.4 | 1.80 | 2000 | 5.6 | 1.87 | 2000 | 5.8 | 1.93 | ||
| 2500 | 7.2 | 2500 | 7.3 | 2500 | 7.7 | |||||
| 3000 | 10.1 | 2.24 | 3000 | 14.2 | 3.16 | 3000 | 14.5 | 3.22 | ||
| 3500 | 16.2 | 3500 | 17.4 | 3500 | 19.2 | |||||
| 4000 | 19.3 | 3.22 | 4000 | 21.1 | 3.52 | 4000 | 23.4 | 3.90 | ||
| 4500 | 22.8 | 4500 | 24.8 | 4500 | 25.7 | |||||
| 5000 | 25.2 | 3.04 | 5000 | 27.6 | 3.33 | 5000 | 28.8 | 3.47 | ||
| 5500 | 27.7 | 2.92 | 5300 | 29.7 | 3.13 | 5300 | 29.9 | 3.15 | ||
Analysis
All three Yamaha supplied props planed Matt’s Sea Ox 1700 skiff in less than two seconds, which is lightning fast. The boat also had a more responsive throttle, especially when changing speeds or punching the control handle when underway at moderate cruising speeds. The 14-K Talon SS propeller was the best of the three Yamaha wheels and also had improved fuel economy numbers when compared to the original Solas from 4000 RPM to WOT. The Talon’s stainless steel construction also offers peace of mind when transiting shallow sandy bays, where one kiss of a sandbar and your aluminum prop is typically rendered toast. What I liked the most about the Talon’s blade geometry and cupping was its ability to really grip the water and push the boat forward with authority. There was no time lag in the throttle response, simply an immediate action and reaction. When Matt punched the throttle, the Sea Ox took off like a scalded rabbit. The two Yamaha aluminum props offered a conspicuously different shape compared to the Talon, with a chopped trailing edge of the blade’s surface, bearing a striking resemblance to Yamaha’s legacy painted steel props of the 80s and 90s that adorned many of their two stroke V-4 and V-6 outboards. Of these two wheels, the 14-inch pitch aluminum version was a close second to the Talon S/S in performance, with a top speed of 29.7 knots, reaching 5300 revs at WOT, which is right in the middle of Yamaha’s recommended 5000-5500 RPM range.
Importance of Correct Props
Why the big fuss over propellers? As we’ve demonstrated here with our sea trial performance numbers, having the right prop on your boat’s engine will enable you to achieve maximum cruising speeds while tweaking the most mileage out of every gallon of fuel, for lower operating expenses. This translates to increased range, which when fishing, allows you to expand your search to more distant grounds to track down your quarry. In coastal and ocean fishing, traveling an extra 10 miles each way can spell the difference between a fantastic catch or no catch at all. And remember, the longer you have your lines in the water, the greater the chances are that you’ll hook-up. When weekend cruising, milking an extra 30-to-40 miles out of each tankful of fuel might allow you to take a trip that otherwise might not have been on the agenda, due to lack of filling stations enroute.
But matching the correct propeller to your boat and motor combination can do even more. It will let your engine breathe easier, achieving factory recommended RPMs for an extended operating life. The right prop can also ensure that your blades will retain their bite when slugging it out in heavy seas when commuting between fishing spots or weekend watering holes. Depending on your choice of prop materials (stainless steel vs. aluminum), your unplanned collision with a sunken wood piling or unseen sand bar can either end your day right there or allow you to keep on plugging along. And for trollers, the right prop will provide the ideal speed to drag either bait or lures in your wake, promoting maximum hookups at minimal fuel expenditures.
