Does a R31 get up on plane

R

rshoop

Active member
Joined
Jan 30, 2019
Messages
25
Fluid Motion Model
C-302 C
Vessel Name
Roxy
I've recently purchased a 2019 RT31S. The diesel engine and generator sit at the back of the boat. I've had her out three times on the Allegheny river, there's been lots of rain and so the river has a pretty good current. So far I like everything about the boat, but when I try to increase the throttle and lower the trim tabs in order to get up on plane I've not been able to get her up on plane. With the increased throttle the bow rides very high in the water.

After the first time I went out, I went back to dock and checked if the trim tabs were actuating. They take about 13-14 seconds to go from straight to a little less than 90 degrees. Check, the trim tabs work.

Does anyone have any suggestions?

Thanks, Robin
 
That boat should get up on a plane fairly easily with or without the trim tabs. At any speed over about 6-7 knots (hull speed) your boat is planing. Not sure what the problem is, but others on here will have lots of ideas. Good luck!
 
Hmmm. The bow rides very high in the water then.
 
I'm not sure it really planes. The inboard Tugs and Cuts have a semi-displacement hull. The boats are very efficient at hull speed but require every bit of the Horse Power to push the boat through the water at higher speeds. If you are use to a planing hull boat, throttle up boat, gets on plane, back off throttle and adjust tabs and on plane. Your not going to experience that with a Tug. When we purchased our Cutwater we did it without a Sea trial. When we took delivery at first run I had the same question as you. The boat performed as advertised but the bow was riding very high. It felt like it could not break plane. Well it can't! The hulls really are not designed to. The hulls are designed to run very efficient at hull speed.The hull design has more water line length then a comparable boat of it's size which increases the hull speed and efficiency. Fluid Motion puts high speed, high horse power engines in the boats to give you the ability to cruise faster but unfortunately this is not with economy. When cruising at high speed you are pushing a lot of water to maintain speed. If the bow high ride is not comfortable for you adding Bennett drop fins will help with that. They also help lift the stern when additional weight is aft. I was told that Bennett drop fin's were originally designed for the Dave Livingston Bayliner designed hulls of the 80's. The Bayliner Motor Yacht line eventually came standard with them and so did many other Bayliners to assist with the high riding bow. I'm not saying its a bad design. Do a search on TugNuts, Drop Fins, Bow riding high, you will see this is common to the Tugs and Cuts. Many say it isn't a problem for them, awesome! Many say it is a problem, the drop fins will help. I have used them for 3 years and can't find one issue (except hard to rack store the boat) and experienced better performance and lower bow attitude . This year I installed larger planes ( to rack store the boat)and experienced the same effect as the drop fins. The drop fins are much easier to install.

My advise is use the boat for a while get use to it and then determine if it is an issue. Its not going to change they all ride bow high.
 
Thank you Brian,
I searched the term "on plane" but couldn't find any good advice.

I will run the boat for a full season and then make a decision whether to purchase the Bennett Drop Fins.

According to the gauges, I am getting really good fuel consumption, I just wasn't used to the bow riding so high.

Thanks again, Robin
 
I suggest getting up to speed then applying trim tabs in small increments. Too much trim can adversely affect attitude. Our R29 classic performs best if we add just enough trim to make the bow wave start just forward of the pilot house door. The bow is a little high, but not that bad. We don't have the weight of a generator in the stern, and that may make a difference. You may want to play with the trim tabs a little, first one side then the other, to change the side to side angle of the boat. That way you'll know for sure that they are doing something.
 
Our former R-31 went through a steep fuel burn climb between about 8 knots (hull speed) and about 13 knots (semi -planing). Then it would flatten out up to 15 or so depending on conditions. We'd trim the nose down for visibility and watch fuel efficiency (mpg) sweet spot. We'd also watch our bow wake as Brad mentioned.
 
Bruce and Brad,

Thanks for the advice. I wasn't able to get out this weekend, but plan to try your suggestions and will let you know how things went with a reply to this message.

Thanks! Robin
 
rshoop,

As BB Marine noted, the R31S does not have a planing hull; it is semi-displacement. In 2015, I posted graphs showing "Speed vs Fuel Burn", along with other parameters that give you a visual picture of what is happening as speed increases.

Click on this link for that data:

viewtopic.php?f=5&t=5868&p=42866&hilit=R31+Sedan+graphs#p42866

As you can see in the first graph, the fuel burn rate climbs steeply until about 10 Knots, then it reverses to slightly improved fuel burn rate as the R31S begins to semi-plane from about 10 Knots to 15Knots. Above 15Knots, the fuel burn rate starts slightly climbing again, since the bow is still somewhat high and too much water is being pushed aside.

I have found that if I have ANY passengers in the cockpit, I cannot even achieve what the graph shows. If it is only my wife and me at the navigators seat and the helm, we are able to almost plane, but never completely get there. Certainly, the bow is down somewhat from what it is with passengers in the cockpit.
 
Our R 31-S (Hull #1 /2019) is equipped with an enclosed Camperback and running with 2 onboard, full watertank and 50% fuel, the sweet spot is around 3000 RPM.
That gives us a speed of 15 - 18 NM per hour, depending on wind and waves, and approx. 1.75 gallons per mile.
We usually have the trim tabs fully down for better visibility in busy Puget Sound. We also keep both Radar and our Vesper AIS going for the same purpose.
 
That gives us a speed of 15 - 18 NM per hour, depending on wind and waves, and a fuel burn of approx. 1.75 gallons per mile.

Wow...I feel much better about my 1.5 miles per gallon, now.
I assume Kaptajnen has a typo... 🙂

Rocky
 
Agree with Bruce, NG from 8-13. Seems same from 13 to 15 which is about WOT.
Fuel about 1.5 MPG at high cruise, over 3 at slow speed.
Cannot imagine 1.75 gal/m maybe 1.75 MPG with tailwind?
 
Rocky":20hl7dip said:
That gives us a speed of 15 - 18 NM per hour, depending on wind and waves, and a fuel burn of approx. 1.75 gallons per mile.

Wow...I feel much better about my 1.5 miles per gallon, now.
I assume Kaptajnen has a typo... 🙂

Rocky
Click to expand...

Yes, I stand corrected. 1.75 miles per gallon.
 
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