R31 CB not planning with dinghy on Davit

[OdyseaII]

I have installed a dinghy on a Davit on the back of our R31 CB and now I am struggling to get on plane.

Even if I have a couple of persons on the bow the boat will not go over 9.5 knts at 3,000 RPM

Any ideas are appreciated. Is adding weight to the bow an option?

Thank you
Keith

 

[scross]

When was the last time the bottom was cleaned? A dirty bottom adds a lot of drag!

 

[OdyseaII]

That was one of my first thoughts. It’s been 4 months but here in Texas things grow fast.

Even minor growth plus the dinghy may be working together against me.

I am going splurge this week and pull it to clean rather than rely on a diver.

Thank you,
Keith

 

[dbsea]

How heavy is your dinghy / davit setup? The setup that comes from the factory doesn’t weigh much more than 150lbs… A lot more than that and you’ll be stern heavy.

 

[Red Raven]

Take the dinghy off and test again. Sounds like something else is going on unless you have a very heavy dinghy. What size is the dinghy motor and is it attached to the dinghy? What dinghy mount are you using? A large motor hanging back far could create the problem you describe.

Curt

 

[OdyseaII]

We have a 100 lb dinghy hanging on a Kato Islander Davit system. My motor is only a 2.5 hrs that is mounted on the swim platform rail.

My plan is to pull clean the bottom first and then if I still have the problem pull the dinghy and see what the situation is.

Thank you for all of the feedback.

Keith

 

[srhawk454]

I have RossKelly Olsson davit which is 81lbs according to specs. The dinghy adds another 130lbs.Alltogether with the weight of propane tanks the total weight is around 250lbs. Never had any problems with speed or getting on plane.

 

[dbsea]

We have a 100 lb dinghy hanging on a Kato Islander Davit system. My motor is only a 2.5 hrs that is mounted on the swim platform rail.

My plan is to pull clean the bottom first and then if I still have the problem pull the dinghy and see what the situation is.

Thank you for all of the feedback.

Keith

I'd love to see photos of your Davit Installation. I have my Dighy on the Factory Weaver Davits, and I'd love to do something different. The existing setup acts like a giant drag parachute and blocks the usage of the swimstep while it's raised up. I saw a photo of the Islander setup on a RT31 on their website, very intrigued by it and would like to learn more / see photos!

 

[OdyseaII]

A bit of an update on our planning issue.

The issue was a VERY dirty bottom with growth and barnacles. It seems the diver that was cleaning the bottom was taking off paint, which accelerated the growth.

I also found a problem with my starboard trim tab not contracting, which may have added to the problem.

A few days on the hard for a new paint and new actuators on both trim tabs fixed the problem.

Boat is planning well at 10 - 11 knots and runs well at 12-13 knots, top speed 16+ depending on conditions.

I still plan to run some tests with and without the dinghy but really do not want to go without it.

Thank you to everyone for their comments.

Keith

 

[Happy Place]

Crazy question but....is the R31 a planing hull? I've always believed it was a displacement hull.

I have NEVER gotten our R31 CB up on a plane....

 

[watson1987]

The R31 is a semi-displacement hull, it does plane.

The CB has a lot more windage and weight than the S.

At 3000-3100rpm, my R31-S comfortably cruises at 16 knots STW, even with the dingy on the back.

At 3550rpm I push 20 knots

 

[Robertsinger]

I have a new 31-S and I can’t get it on plane.

 

[watson1987]

The following is the data published by RT for the new 320hp R31-S.

https://rangertugs.blob.core.windows.ne ... d4-320.pdf

How much off is the performance folks are seeing on their boats? I have a 300hp, slightly less, but reasonably in line with these published numbers.

 

[OdyseaII]

We have done over a thousand miles on our R31CB since we fixed our planning problem and while most of these miles have been at slower speed I have no problem planning at about 11-12 knots.

The key seems to be using the trim tabs to get the bow down at about 3,000 RPMs.

Keeping the bottom clean is also an important factor.
At 3,000 RPMs we cruise nicely at 14 knots.

 

[RJM1953]

Both the R-31 S and the R-31 CB share the same hull, and they are both semi-displacement that should easily get up on plane. The major deterrent to getting on plane is usually a fouled bottom. Under water growth and barnacles will significantly slow the boat down and could ultimately prevent it from planning. My R-31 CB is serviced by Winter Island Yacht Yard in Salem Massachusetts and when they commission it in the Spring, I always instruct them to repaint the bottom with anti-fouling paint as they see necessary. It is money well spent. Additionally, I run the boat every week and when I take it out I usually run her at 10-12 knots but I always run her wide open for a minute or two just to keep the Volvo D-4 limber as using the higher speed to help keep the bottom cleaner. As an aside, when I am up on plane, I always fiddle with the trim tabs, going up and down while watching the boat speed on the Garmin until I reach the 'sweet spot' of maximum speed. I also ordered my tug with the dingy and the Weaver quick release davits. What I did notice was that when on plane, not matter how I adjusted the davits, the dingy would always collect water in the bottom from spray. This of course created a drag as well as requiring me to clean the dingy every time I went out. To circumvent the problem, I had a canvas cover made for the dingy which solved the problem.

 

[Sandro]

It should plane. And with clean bottom 15 knots is normal. My r29 Cb planes from 12-20 knots with dinghy and engine on swim platform. I give it throttle then once it’s over it’s bow wave back off. I cruised 3 hrs last week at 3050-3100 rpm at 15- 16 knots. This is the Volvo 300. Clean bottom. Trim tabs down

. https://share.icloud.com/photos/03feIEP ... XT9OQ2fxZQ

 

[SJI Sailor]

I would add that Bennett drop fins help a lot, too. On our C30 they made the plane much flatter, the dinghy drier, and added a bit of speed (+1-2 kts) and fuel efficiency (+0.1 nmpg), depending on conditions of course.

 

[watson1987]

I would add that Bennett drop fins help a lot, too. On our C30 they made the plane much flatter, the dinghy drier, and added a bit of speed (+1-2 kts) and fuel efficiency (+0.1 nmpg), depending on conditions of course.

I've heard several reports of this. I have my fins on order, but my diver just said I don't need a haul out to paint for at least 6-9 months (12 months if I want to push it). I'm trying to justify painting early or hauling out to add the fins alone. Bleh, can't decide which

 

[Bobby P.]

My 2017 R31 CB planes easily. I do have the trim tabs with fins, but it planned easily before I added the fins.

I have always carried the factory fiberglass bottom rib, a 9.9 Suzuki, 5 gallon gas tank, 3 propane tanks and a Gaugher lifting crane. We also pack plenty of stuff, so i am sure we run heavy.

Something is out of sync with your set up or performance ability.

Make sure your trim tabs are actually going down. Have someone operate them while you have your face almost in the water at the back of the swim platform.

Make sure you are running at an appropriate temperature. Most of these Volvos run at 185 degrees.

When trying to get up on plane, make sure your dink is not catching water. You'd be surprised at the resistance that can cause.

If this has been the case since you bought the boat, you should make sure your are correctly propped. It's not impossible that the wrong prop could have been installed.

I also like to make sure the engine is appropriately tuned. This includes fuel flow (replace your racor filter.

I also run barnacle bust through the water jacket of the engine. You can Youtube how to do this or hit me up (PM) and I'll walk you through it.

In short, the R31CB should easily plane.

Best of luck on your hunt. You will find something...