Propane outboard in cold weather question
- Thread starter GSBNY
- Start date
Question: what are you measuring as WOT RPM? I only see about 4500 but the specs say 5-6k is normal, if I recall correctly
I have no clue, there’s no tach on the motor.
I’m thinking of hooking up this Sierra Tach / Hourmeter so I keep better track of this.
www.westmarine.com
I’m thinking of hooking up this Sierra Tach / Hourmeter so I keep better track of this.
Small Engine Tach/Hourmeter | West Marine
Check out our Small Engine Tach/Hourmeter and more from West Marine!
www.westmarine.com
My favorite equation that I remember from high school chemistry class is the ideal gas law: PV=nRT
You can do a lot less expensive in a digital tach. I use this $15 one from Amazon: https://www.amazon.com/gp/product/B0049IFX56
Though I will say that I did not install it permanently for hour meter, but rather as needed when I was assessing the differences between 5hp stock prop, and Tohatsu branded "high thrust" prop, and trying to tweak my idle speed low, but not so low the motor conks out.
Yes, I did, and I found it all! Woot! I originally posted over at trailersailers.com. Pertinent info follows, for O'day 192, 18'7" LOA, 16'8" LWL.
I went from the stock "moderate" 7.7" diameter, 8", 3 blade on my Tohatsu/Nissan 5 hp 4 stroke to an 8.375" diameter, 6" pitch, 3 blade "Mickey Mouse ear" "high thrust" prop, also Tohatsu branded, from their prop charts. TOHATSU outboard motors
With stock prop, I would turn 2160 rpm at WOT.
With high thrust prop, 2340 rpm at WOT and hitting max speeds of 6.3 knots with a light breeze on calm fresh water.
Big stern waves follow me, indicating I'm pushing very close to hull speed. My max hull speed, based on the old ship of the line displacement formula (not ideal for small displacement sailboats) is 5.5 knots. My light displacement hull speed, based on the Gerr DL ratio, comes out to a max of between 6.6 and 7.2 knots, depending on what I estimate for my max displacement. Specs for the boat listed at 1400 pounds, but I'm sure the boat weighs more, plus my 200 pounds and junk I carry on board plus motor, so, 6.6 knots at 2000 pounds displacement? I lose approximately .1 knot for each extra 100 pounds over 1400. Also, since I am in fresh water, the boat sits deeper than salt water, so should have a bit more drag slowing down max hull speed. Psychosnail Sailing
Seloc manual says idle should be set to 1100 rpm, which is high. I have my idle set to 750 rpm, which also seems high, but motor doesn't conk out. I travel about 1.7 knots. I need to shift in and out of neutral to scrub speed for docking, and occasional shifts into forward for a bit more steerage. The high thrust prop definitely throws more water and thus has better performance in reverse.
Hope this helps!
Throttle is not proportional to burn rate. It is a mechanical position of a plate.
For 5 hp ~ 2.2 pounds/hr seems reasonable but a little light. Gasoline is closer to 3 pounds per hour. I'd want to allow at least 0.8 pounds/hour. But you never know about wind. pushing into a good breeze could require full power, and you could go through 1/3 tank in just a few miles. Watch your cushion.
For 5 hp ~ 2.2 pounds/hr seems reasonable but a little light. Gasoline is closer to 3 pounds per hour. I'd want to allow at least 0.8 pounds/hour. But you never know about wind. pushing into a good breeze could require full power, and you could go through 1/3 tank in just a few miles. Watch your cushion.