Trim tab/efficiency

[RTS14]

For an RT 27 OB 2024 model, a trim tab/efficiency question: if the boat will be listing a few degrees due to wind (for example), is it more efficient/economical to deploy a trim tab to correct the list, or is leaving the boat at a list better with regard to speed/efficiency/fuel consumption? I would assume that dragging a trim tab would be a greater parasitic drain than having the hull heeled over a few degrees, but would welcome any thoughts.

 

[scgck]

We have a 2021 R27 LE and have found it somewhat uncomfortable to cruise at 26 kts listing to port.
The reason for the list is of course all the heavy items (batteries, kicker, stove , sink and fridge ) all on the port side.
I have found that a couple seconds of starboard bow down trim is all it takes to bring the boat level.
At this power setting, 4800 rpm, we get 26 kts at 1.6 gph
I don't know what the speed or fuel flow would be without the trim.
Hope this helps
Chris

 

[Submariner]

For an RT 27 OB 2024 model, a trim tab/efficiency question: if the boat will be listing a few degrees due to wind (for example), is it more efficient/economical to deploy a trim tab to correct the list, or is leaving the boat at a list better with regard to speed/efficiency/fuel consumption? I would assume that dragging a trim tab would be a greater parasitic drain than having the hull heeled over a few degrees, but would welcome any thoughts.

It's best to deploy the tabs to balance the boat in my experience.
When the boat is at or near WOT, the amount of trim tabs needed is 56% on the port, and the starboard tab is at 0%.
(30.2 knots, 5550 RPM). (1.3mpg)

Slower speeds, such as 10.3 knots, 3050 RPM, port tab at 85% and starboard tab at 0%. (1.7mpg)

Fuel efficiency will suffer if you need to have both tabs down.
The port tab is needed to counter the standard rotation of the prop (torque of the motor) for a single engine.
When I last did my prop testing on the PowerTech OFS4 15 pitch, I also captured the trim tab positions in pictures throughout various speeds and RPM's from idle up to WOT. The port tab is what's cycled the most.

Here's an example...
25.7 knots, 5100 RPM. Stbd 0% and Port 39% 1.45mpg.
25.1 knots, 5300 RPM. Stbd 26% and Port 84%. 1.25mpg.

I've had sea conditions such that going at or near WOT, I've had 0% on starboard tab, and less than 10% tab on the port side. That's ideal.

I have the Lenco Autoglide system installed so I get % readings on the chartplotter for both trim tabs % deployed as well as pitch angle of the boat (forward to aft). The boat needs to have the correct angle riding into the water to plane efficiency. The tabs lift the stern up which pushes the bow down to get that angle.

(Note, on the trim tab controls, we adjust the starboard "tab", which actually moves the port tab, and vice versus).

Trim Tabs 0% and 25% at 28.6knots (near WOT). 1.4mpg. (OFS4-15 prop).
https://www.letsgochannelsurfing.co...c9c0_f8b458b409524fefbb04becc1dce5acfmv2.jpeg

 

[RTS14]

Thanks Martin, this is very helpful. Do you think the 4 blade propellor induces a greater torque moment on the boat that requires a correspondingly greater trim tab correction?

 

[watson1987]

Martin - with a RT that has an OB. How do engine trim and trim tabs work together or against each other here?

 

[Submariner]

Thanks Martin, this is very helpful. Do you think the 4 blade propellor induces a greater torque moment on the boat that requires a correspondingly greater trim tab correction?

I ran the Yamaha Saltwater Series II 15.75x15 pitch (factory) prop recently from Everett to Bremerton with the Everett Yacht Club (It was the annual Heavy Weather Cruise with the Grand 14 (14 yacht clubs)). (81 nautical miles total).

I've got a screen shot from Feb 17, 2024, that shows:
25.1 knots, 4700 RPM, Stbd 7% and Port 94%. 1.69 mpg.

WOT for me in this configuration is 32.3 knots (37mph). (1.37mpg)

The 3 blade prop is more fuel efficient at higher speeds than the PowerTech OFS4 15pitch. (less blades, less friction, and less slip).

I've got 2 prop's sitting here in my living room from PowerTech to go test. Both are modified prop's. One is my original 4 blade, heavily modified, and the other is a custom modified 5 blade prop. (I'm working directly with PowerTech to see what we can do with a prop for an RT27-OB). I'm hoping for great weather this coming weekend when I head out to do some testing.

I don't think a 4 blade introduces more torque that would otherwise affect the trim tabs. The extra blade improves hole shot a lot and improves reverse thrust (useful for docking), as well as provides fantastic mid-range (3000-4000 RPM operation). The boat is stern heavy, that extra blade helps lift the boat up and keep it out of the hole. My original PowerTech OFS4-15 has about 25% slip. The Factory Yamaha 3 blade prop is about 15%. PowerTech thinks the modifications they made will reduce slip to less than 20% improving performance. I do know what a Sharrow prop can do on an RT27-OB for $5,000. I'd like to find out what's possible on a standard 4 or 5 blade prop for less than $1,000.

 

[Submariner]

Martin - with a RT that has an OB. How do engine trim and trim tabs work together or against each other here?

Last September, 2023, I got lucky with some extremely flat water up at Port Susan, no wind, and slack high tide. I did a bunch of prop testing with the PowerTech OFS4 15pitch. But in addition to getting the numbers I needed, I also took some time to see what happens if I purposely run the engine trim way out of spec. I figured I'd see what the Autoglide would do.

Slowboating
3.83 knots, 1000 RPM. Port 0%, Stbd 0%. 3.33 mpg (boat pitch is -4.0 degrees)
(Autoglide shows me the pitch angle).
The engine is trimmed all the way down to 0% (1 bar).

Midrange
11.2 knots, 3200 RPM. Port 100%, Stbd 0%. 1.69 mpg. (boat pitch 0.0 degrees).
The engine is trimmed to 26% (3 bars)

Cruising Speed
25.7 knots, 5100 RPM. Port 39%, Stbd 0%. 1.45mpg (boat pitch -0.6 degrees)
The engine is trimmed to 17% (3 bars).

WOT
30.7 knots, 5750 RPM. Port 14%, Stbd 0%. 1.3mpg (boat pitch -0.8 degrees).
The engine is trimmed to 18% (3 bars).

Comparison
15.3 knots, 4100 RPM. Port 100%, Stbd 30%. 1.33mpg (boat pitch -0.5 degrees)
The engine is trimmed to 6% (2 bars).

15.5 knots, 4150 RPM, Port 65%, Stbd 0%. 1.41mpg (boat pitch +0.5 degrees)
The engine is trimmed to 20% (3 bars)

The engine trim is necessary as the prop should go straight through the water. As the bow goes up or down the running angle of the boat changes. Engine trim and trim tabs operating together can control the angle of the boat through the water.

I did some more extreme examples, but didn't take pic's of the screen as it was way off. Like, I'd bring the trim up 5, 6 or 7 bars... way high... The auto glide just ran both tabs all the way down to 100% to try and compensate and my fuel efficiency suffered.

I've ran the 3 blade Yamaha prop for 360 engine hours. Then I put on the PowerTech OFS4 15 pitch and have another 400 hours on it. From my experience, the 3 blade is more sensitive to engine trim. (2 or 3 bars, sometimes 4 or 5 if running near WOT). The OFS4 is less sensitive to engine trim. I set it to 3 bars (about 20%). It stays there from 11 knots all the way to WOT at 31 knots. Engine trim with the OFS4 just seems to not make much of a difference in fuel efficiency (as long as it's set around 3 bars).

The engine trim and tabs both should work together to get the boat up and out of the water, on plane. The running angle should be such to minimize the surface area of the boat in contact with the water.

On our SE Alaska trip, Channel Surfing was really heavy. 3 adults, 3 dogs, heading to sea for 2 months. We packed everything but the kitchen sink (thankfully, the boat came with one already, ha!). A lot of the weight was aft as that's where us outboards have the most storage.

At 4.8 knots my pitch angle was -0.7 degrees.
At 10.3 knots, 3500 RPM, on plane, both tabs were down to almost 100%, at 1.3mpg. (boat pitch at 0.5 degrees).

The normal boat pitch for our SE Alaska trip was 1.8 degrees. Port 0%, Stbd 60%. Engine trim at 22% (3 bars), 26.7 knots, 1.26 mpg.

Autoglide worked against me trying to bring the boat pitch down which required both tabs to be fully down. My fuel efficiency suffered. I either needed to re-calibrate Autoglide, throw a bunch of stuff overboard, or use the "favorites" settings on Autoglide to provide an alternative temporary calibration which accounted for the additional weight, or disable Autoglide and operate the tabs manually.

I set the engine trim to 2 or 3 bars when doing 7 knots, deploy both tabs to 100%. Punch the throttles to get the boat up on plane, then retract both tabs. Bring the starboard tab (port switch) all the way up, and use just enough of the port tab (starboard switch) to balance the boat. This is why I like the auto glide so much. It does all this for me and much faster than I ever could. With a 4 blade prop (extra stern lift) and the Autoglide, it doesn't take Channel Surfing long to get up on plane. Hole shot is amazing.

For diesel inboards, the above mostly isn't relevant.
The outboards like 22 to 30 knots, and WOT around 35 knots. They have different hulls than the diesel inboards, more of a planing hull without a keel.

I'm working with PowerTech on testing some custom modified 4 and 5 blade props. The objective is to get into the 10 - 20 knot range (3000 - 4000 RPM) with great fuel efficiency so I can hang back and comfortably cruise with my diesel inboard friends, and extend my range between fuel docks for our trip back to SE Alaska.

 

[RTS14]

Martin-do you have any experience or opinion with respect to a hydrofoil? It would seem to improve planing speed; do you think it may reduce the need for as much trim tab application? Thanks for your thoughts

 

[Submariner]

Martin-do you have any experience or opinion with respect to a hydrofoil? It would seem to improve planing speed; do you think it may reduce the need for as much trim tab application? Thanks for your thoughts

My only experience with a hydrofoil was I got to ride on one years ago on a SE Alaska cruise (Holland America). It was an excursion to see Davidson Glacier (up by Haines Junction) on this hydrofoil passenger ferry. We scooted along pretty quick I remember.

 

[watson1987]

As always, thank you Martin, fascinating stats.

 

[RTS14]

Hi Martin-I think you misunderstood my 'hydrofoil' reference; I was referring to:

https://www.amazon.com/Marine-Sport-Hyd ... SgQAvD_BwE

Any thoughts on this 'hydrofoil' for increasing lift in the stern of an RT 27-OB?

 

[Rebel112r]

Martin. Thanks for all those numbers. Very interesting. Was surprised by the figures for the Alaska trip with the opposite tab doing the work. Also the numbers for 2 bars of engine trim, was interesting. I might try adding some 12 gauge ss to the aft of the port tab just to see if might help, but also have a Sharrow ordered so that might cut down on the amount of tab that is down. Have not seen any Sharrow numbers that reference the tab positions at different RPMs. Seeing those 100% tab numbers makes me think that a bit more tab may be needed, Increasing width of tab would be more helpful
But not as easy in my case with drop fins installed. If I make a change in the tab size, only can hope cylinder can handle increased tab area. Something to mull and play with.

 

[Submariner]

Martin. Thanks for all those numbers. Very interesting. Was surprised by the figures for the Alaska trip with the opposite tab doing the work. Also the numbers for 2 bars of engine trim, was interesting. I might try adding some 12 gauge ss to the aft of the port tab just to see if might help, but also have a Sharrow ordered so that might cut down on the amount of tab that is down. Have not seen any Sharrow numbers that reference the tab positions at different RPMs. Seeing those 100% tab numbers makes me think that a bit more tab may be needed, Increasing width of tab would be more helpful
But not as easy in my case with drop fins installed. If I make a change in the tab size, only can hope cylinder can handle increased tab area. Something to mull and play with.

When I run 3,050 RPM, at 10.2 knots, the port tab is at 100%, starboard tab is at 0%, and I'm at 1.7mpg (using the OFS4 blade prop). Personally, I think the RT27-OB is sized perfect with the tabs installed from the factory. I have no plans to increase them or to even add drop fins to them.

I'm looking at a WOT screenshot from Sept 17 I took. Starboard tab is at 0% and the port tab is 5%. Engine trim is 12% (2 bars) and my boat pitch is -0.9 degrees. Speed is 30 knots at 1.3mpg. (This is using the 4 blade prop). The Sharrow will be similar, 32 knots at 1.4mpg at WOT.

I have numbers on a Sharrow prop on an RT27-OB from two different boats. I know what to expect from it. It shouldn't change the usage of the trim tabs I wouldn't think. I look at the Sharrow and see it as a 6 bladed prop with a 15.8" diameter (3 blades in front, 3 in the back). But that's just my opinion. 🙂

Channel Surfing has 787 engine hours, covered 10,500 statute miles over 3.5 years and I have been collecting data on the RT27-OB throughout our travels. Fuel mileage logs, screen shots, trim tab and engine trim positions. I have excel spreadsheets and graphs of its performance on the 3 blade factory SWS II prop, an after-market PowerTech 4 blade prop, and the Sharrow. I have 2 props sitting here in my living room (a heavily modified 4 blade, and a custom modified 5 blade prop) that I'll be testing out this weekend. This stuff is interesting to me and fun to go play with.

 

[Submariner]

It's also important to note.. The RT27-OB is naturally heavy to the port side.

In the port lazarate you'll find 2 house batteries and a thruster battery, each weighing 67 lbs. The engine battery sits by itself on the starboard side.

If you have an LE, you either have 6 LFP batteries and a thruster battery in the port lazarate, or you have 2 LFP batteries and a thruster battery in the port lazarette.

If you have the T9.9 kicker, it's on the port side, another 100 lbs.
If you have the cockpit refrigerator, it's on the port side for another 50 lbs.

The cabin refrigerator and Force10 stove are on the port side.

It all adds up.

If you have the Dinghy lift, it adds, I'd guess at least another 200 lbs straight aft, directly over the engine, making the boat even more stern heavy.

Then with the standard prop rotation, the torque of the engine twists the boat to the port side.

All this causes the port tab to be cycled more than the starboard tab.

 

[Rebel112r]

I won’t be back home till mid April, and will schedule delivery of new prop then. I have seen numbers on Sharrow from two boats, and their numbers are similar. The layman’s type of testing we do with what we have, is at best ballpark. I expect to see increased stern lift from the new prop midrange and likely will see less tab in that RPM range. I have dinghy lift, kicker and dinghy motor, but have removed all batts from port side. If I see that the port tab is getting to 100%, to do its thing I will play around with adding more tab to that side. I also don’t know if all the Sharrow that I have seen numbers on for the R27 are identical. The one I am getting is supposed to be the same as the fellow from Florida that posted his numbers on The Hull Truth site. Dan

 

[Submariner]

I won’t be back home till mid April, and will schedule delivery of new prop then. I have seen numbers on Sharrow from two boats, and their numbers are similar. The layman’s type of testing we do with what we have, is at best ballpark. I expect to see increased stern lift from the new prop midrange and likely will see less tab in that RPM range. I have dinghy lift, kicker and dinghy motor, but have removed all batts from port side. If I see that the port tab is getting to 100%, to do its thing I will play around with adding more tab to that side. I also don’t know if all the Sharrow that I have seen numbers on for the R27 are identical. The one I am getting is supposed to be the same as the fellow from Florida that posted his numbers on The Hull Truth site. Dan

I've seen two sizes of Sharrow's used on RT27-OB's. MX 15.86 x 15 (3 blade) and a MX 15.68 x 15 (3 blade). I assume it's two sizes and not a type-o, anyway.

On Channel Surfing, when the port tab does hit 100% (usually when I'm cruising slow, like 10-12 knots) the boat is still balanced which is why I've never considered adding more tabs. I just have never felt like I needed more.

 

[Submariner]

Hi Martin-I think you misunderstood my 'hydrofoil' reference; I was referring to:

https://www.amazon.com/Marine-Sport-Hyd ... SgQAvD_BwE

Any thoughts on this 'hydrofoil' for increasing lift in the stern of an RT 27-OB?

I've never even thought of putting one of those on my F300. I've seen them on other boats, not F300's.. usually smaller outboards. I know several have put them on their F200.

 

[Submariner]

My original prop that I sent back to PowerTech is a OFS4 15 pitch. (4 blade propeller). They modified it and sent it back to me.

I tested it out today in sporty conditions. (1 foot seas, and building). We got back to port an hour before they issued the small craft advisory today at 1:15pm.

The results in less than ideal sea conditions:
12-19mph, between 1.7 and 1.9mpg. (2800-3800 RPM).
I'm officially impressed.

Channel Surfing has never seen 1.9mpg before today; And I kept my amazing hole shot, reverse thrust, and docking maneuverability that I've come to love about the 4 blade prop.

We were cruising back to port at 15mph and getting 1.8mpg in 1 foot seas, rock'n and roll'n.
Trim tabs were pretty much Starboard 0%, Port 100% (the boat was balanced port to starboard), with 21% engine trim (3 bars). Boat pitch at 3.6 degrees (high, due to the sea state, keep the bow above the waves).

But I do have a 5 blade prop from PowerTech with some custom work done to it as well to test.
Next weekend's fun. Hopefully with better weather. 😀