RT27-OB Outboard Fuel Logs and Prop Mount

[Submariner]

I've now got 3 full seasons on Channel Surfing.
Looking over my fuel logs provides some great info on the 3 blade Yamaha 15.75 x 15 Saltwater Series II prop vs the PowerTech 4 blade 15.25 x 15 prop.

2020 season shows great fuel efficiency. We'd just got the boat so it was loaded minimally as we were figuring out how we would be using the boat.

2021 season shows us fully moved in, figured out how we use the boat (our second home, waterfront property). We bring a change of clothes, food, and the boat keys and take off for a week.

2022 season shows the same as 2021, but with the 4 blade prop. (about 13% less fuel efficient).

I would also note that 2020 was a clean hull with bottom paint. 2022 had a slimy bottom until I had the bottom cleaned and bottom paint redone at the end of 2022.

Out of 535 engine hours, 109 hours were spent below 2,000 RPM (slow boating). The other 426 hours spent above 2,000 RPM. Most of the time I'm running between 4600 and 5200 RPM which is 23-28mph. (about 25 knots).

2023 will be an interesting year as we plan on putting about 125 hours on the engine during our 6 weeks in SE Alaska. Then we still have the rest of July, August, September and October, for additional engine hours. We're going to be, borderline, liveaboards this season. 🙂

I keep great fuel records in part because I file with the state of Washington for a refund of the gas tax (boats don't use roads, and thus are exempt from the gas tax). Unlike diesel, gasoline is only sold with all applicable taxes included in the price. I've received back anywhere from $250 - $400 dollars, each year.

3 years of fuel logs on Channel Surfing
https://www.letsgochannelsurfing.co...kxcs0cum-c18e4dda-0944-4fe4-a1ae-6f85feffa509

I also found this really cool spare propeller mount for the boat. I keep the Yamaha 3 blade prop onboard.
https://www.letsgochannelsurfing.co...kxcs0cum-779cc2df-0523-499e-9650-7d1f80ca79dd

Spare Propeller Mount
https://www.letsgochannelsurfing.co...kxcs0cum-8bb9c721-bed7-48e2-87db-f0d6fb59a390
https://www.letsgochannelsurfing.co...kxcs0cum-6bf39444-baf3-4781-b828-21b8df7b3097

 

[Tippetknot]

Thanks for the summary of your results. Looking forward to your Alaska trip report and any new fuel economy observations.


Sent from my iPhone using Tapatalk

 

[BB marine]

Great job on the fuel Logs! I am surprised at the fuel economy numbers though. I expected them to be a bit higher when averaged. We have been doing the Loop since last August. I keep good Fuel logs and do my calculations almost every day. I have mechanical engines. I don't have a fuel burn read out. I have a tank dip stick which is real time fuel gauge. We cruise about 50% at 8 kts and 50% at 16 kts. 1500 rpm or 2600 rpm (2) 240 hp Yanmar diesels. At 1500 rpm I average 2.4 mpg or 4 gph. At 2600 rpm I burn 11 GPH 1.70 mpg. At 2900 I start to burn the fuel at 24mph and WOT just sucks the fuel and I'm only gaining a few more mph at 28mph but still 1.1 mpg. The boat as loaded is between 18000 lbs an 20000 lbs depending on fuel and water. As of now over, 4000 miles we have averaged just over 2 mpg. I guess a smaller boat does not equate to better fuel economy.

I do know some folks from the PNW that have a 27 Ranger Tug Diesel power that we cruised with off and on for a couple thousand miles this winter. I say off and on because they cruised at 6.5 mph all day long. So we would get ahead of them stop and a few days later they would catch up. A few times we fueled at the same time. It would make me sick when we compared fuel usage. They were getting twice the fuel economy as us. A few miles per hour faster comes with a price!!!

In your case 28 mph comes with a price.

I get the same fuel economy with my 34' Mainship two 240 hp engines as I did with my 26' Cutwater single 220 hp Volvo. The cruising speeds are comparable. But slightly slower with the Cutwater. The Cutwater was WOT at my Mainship 2600 RPM cruise speed of 16kts when loaded. We cruised at 13 kts with the Cutwater.

 

[Submariner]

We cruise about 50% at 8 kts and 50% at 16 kts. 1500 rpm or 2600 rpm (2) 240 hp Yanmar diesels. At 1500 rpm I average 2.4 mpg or 4 gph. At 2600 rpm I burn 11 GPH 1.70 mpg. At 2900 I start to burn the fuel at 24mph and WOT just sucks the fuel and I'm only gaining a few more mph at 28mph but still 1.1 mpg. The boat as loaded is between 18000 lbs an 20000 lbs depending on fuel and water. As of now over, 4000 miles we have averaged just over 2 mpg. I guess a smaller boat does not equate to better fuel economy.

In your case 28 mph comes with a price.

At 7 mph, I'm over 3 mpg around 2.2gph.
At WOT, I'm doing 34mph at 26.7gph for 1.27mpg.

Mid-range cruising speeds:
11mph at 7.2gph for 1.53mpg.
13mph at 9.4gph for 1.38mpg.

Once I get up to about 15mph, my fuel efficiency doesn't really change much all the way up to WOT. Anywhere from 1.2 to 1.4mpg usually. This is what I see on the 4 blade PowerTech prop.

There is definitely a cost to speed paid at the fuel dock. But then again, I can get from Everett to Roche Harbor (65nm) in less than 3 hours at a comfortable cruising speed of around 22-25 knots. Speaking of which... I did do this experiment:

Thistledown, a brand new 2022 RT27-OB with 10 hours on her.. factory stock boat. Dinghy lift on the back, Yamaha 3 blade prop. Channel Surfing, a 2021 RT27-OB, no dinghy lift, and a PowerTech 4 blade prop. Each boat had 2 adults on board. Full tank of gas and water. Empty SAN tank.

We traveled together from Everett to Roche harbor for the Rendezvous (year 2021). 153 miles round trip.

Thistledown with a 3 blade Yamaha prop burned 100 gallons of fuel. (1.53mpg)
Channel Surfing with a 4 blade PowerTech prop burned 111 gallons of fuel. (1.38mpg).

We leave in a week for Southeast Alaska. I'm counting on the speed to aide in crossing the Dixon Entrance and Queen Charlotte Strait (both exposed to open ocean) by taking advantage of good weather.

 

[Cutwater28GG]

id also note that bottom paint has a bigger impact than most people realize.

even a few weeks in the summer can cause a difference.

I'm a cut28. clean and new paint at cruise of 18 knots I will get 1.8NMPG
older paint even if recently cleaned by the diver, I will get 1.6NMPG at the same speed.

 

[Submariner]

id also note that bottom paint has a bigger impact than most people realize.

even a few weeks in the summer can cause a difference.

I'm a cut28. clean and new paint at cruise of 18 knots I will get 1.8NMPG
older paint even if recently cleaned by the diver, I will get 1.6NMPG at the same speed.

I will admit and classify this as a partial fail on my behalf...
Bottom paint was applied to my boat new, in May 2020.
New bottom paint applied in October 2022.

The fail... I never once cleaned the bottom from May 2020 through Oct 2022. I pressured washed the bottom in preparation for new bottom paint. Visually, the bottom looked free of growth. It was slimy as I ran my fingers across the bottom. I will pressure wash the bottom of the boat at least annually from here on out. Our water is cold (about 43 degrees year round), and I've got a covered slip, both of which reduce growth. If I ever see green, it's on my trim tabs as they see the late afternoon/evening sunshine sticking out the back of my slip.

I'm hoping for some better fuel efficiency this season now that I have fresh/new bottom paint. But then again, my first big trip out will be heading to SE Alaska, where I'm loaded down with gear, spare parts, 3 adults, 3 dogs... Any savings I might have seen may very well be spent on carrying extra weight.