Monday, September 26, 2016

Learning about horsepower and props at G10

One of the great things about amateur boatbuilder gatherings is the wealth of accumulated knowledge and experience that is present. People from all walks of life gather together over their shared interest in building boats, large and small. There is much to learn from this great group of people, and this year's 10th Annual Gathering of Glen-L Boatbuilders was as rewarding as ever.

This year, I got the opportunity to ride in another builder's Utility. He had constructed his to an extended length of 12', and powered it with a 15hp Mercury motor.

Glen-L Utility extended to 12 feet LOA
12' Glen-L Utility with 15hp Mercury motor.

With two adults aboard, the 15 hp motor provided plenty of power for the boat to perform with confidence... notably better than my 8hp Nissan. This reaffirms my belief that 15hp is about right for regularly carrying two adults in this boat design. For one person alone, 8hp is just about perfect in my opinion.

With a third person in the boat, however, even the 15hp struggled considerably. The added weight was just too much for anything more than displacement speed. 

As I've mentioned before, the Utility is such a small boat that I don't think it's very safe to carry more than two people. However, this particular boat was extended an extra foot, which provided a surprising amount of extra interior space.... and we were only making a short trip from the loading ramp to the dock at the marina.

Another factor that contributed to the comfortable amount of interior space in his boat was his use of 7" wide thwarts. Comparatively, the thwarts in my boat are 12" at the back and 11" at the front. I'd say the wider thwarts are slightly more comfortable, though the difference is not drastic, in my opinion. If you are building a Glen-L Utility, and want to maximize the interior space, the 7" thwarts are certainly worth considering.

Glen-L Utility extended to 12 feet LOA

Glen-L Utility extended to 12 feet LOA
The 7" thwarts add to more usable interior space. Also, the plywood floor (which I omitted on my boat), was certainly more comfortable to stand on... particularly with the non-skid coating.

of Tiny Tachs and Prop Pitch...

Another builder, who has built a Glen-L Zip, and who also purchased a used Glen-L Utility, was kind enough help me determine some options for improving my motor's performance with a new prop or two.

We took the cowling off of my motor, and attached his Tiny Tach to one of the spark plug wires, to check the RPM. With both of us aboard, the motor only reached 4,650 RPM. With just me aboard, the boat performed much better, with the motor reaching 5,880 RPM.

The maximum operating range for the Nissan NSF-8A3 is 5,000 – 6,000 RPM. Ideally, as I'm learning, you want the motor to be running near its maximum operating RPM. Using a lower-pitch prop should increase the RPM, as well as increase thrust. This should hopefully result in better performance with two adults aboard.

However, there's a catch. A lower-pitch prop would probably also allow the motor to over-rev if the boat was not loaded with two people. So basically, I'd need to change props, depending on whether I was carrying one person aboard, or two.

Since the motor reaches 5,880 RPM with just me aboard, there is an additional possibility. I could also use a higher-pitch prop to attain more speed. I'm not sure yet if I want to do that, or not. We'll see. I really have no complaints about the boat's speed when it's just me aboard.

The next step is to find out exactly what prop came with the motor in the first place. I haven't seen any tell-tale markings on the outside of it. I'll be removing it before long. Hopefully there will be visible markings on it, somewhere.


  1. Should be a 9x9 pitch stock prop.

    1. I'm sure you're right, Chris! I'm planning to remove the prop when I do regular maintenance on the motor this spring.