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Author Topic: Riva-style speedboat "digging in" at higher speeds?  (Read 2450 times)

clockworks

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Riva-style speedboat "digging in" at higher speeds?
« on: April 08, 2019, 11:06:59 am »

Another secondhand boat that I bought recently. 30" plank on frame Riva Aquarama, not sure if it's from a kit or a scratchbuild. I bought it complete, except for radio gear.


I had to redo some of the wiring, and figure out a way of securing the two 3s 2200 LiPos. It sits level in the bath, going by the painted waterline.
I took it to the lake yesterday, and it was fine at low speed, but the bow doesn't come up at all as the speed increases. If anything, it digs in a little, so the bow wave comes up over the decking and into the driving compartment.


Is it just a case of moving the centre of gravity a bit further back, so that the bow sits higher, or is it something more fundamental?
There's a small amount of lead right at the back, which I can remove if the stern sits too low after moving the batteries rearwards.


Having built quite a few "foamie" planes, centre of gravity is critical, and clearly marked on the plans/instructions. With a secondhand boat though, I'm relying on the builder having painted the waterline in the correct place, as well as fitting the propshafts at the correct angle, etc.
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SailorGreg

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #1 on: April 08, 2019, 05:37:01 pm »

I think this is a case for some experimentation.  I would remove the lead and move the batteries back a short way.  Personally I wouldn't worry too much about the boat sitting on its waterline at rest.  Much more important is keeping the boat dry and looking good on the plane.  If moving the batteries back solves the problem, you can then reintroduce some ballast if the boat looks really weird at rest, but (again, personal view) I am not a fan of putting weight into models that are designed to rush around in a cloud of spray.


On a more general note, I recall reading some discussion about the Riva hull in model form, that it doesn't behave particularly well, especially on the corners. What works at full scale doesn't necessarily transfer directly to a model, and if yours is a scratch build it might faithfully reproduce the original's lines but not be a particularly effective model.  However, try your experiments and see what improvements you can get.


Good luck


Greg

clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #2 on: April 08, 2019, 06:36:42 pm »

Thanks for the info, makes sense. As well as relocating the batteries, I also have the option of moving the ESCs back a bit. They are currently right up against the first bulkhead/frame.


I was worried initially that the stern would be too low, but I guess if the bow is digging in, the stern must be lifting? If that's the case, then moving the CoG rearwards will just affect the stern at rest. At speed, the whole thing will rise up out of the water as it comes on plane?

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grasshopper

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #3 on: April 08, 2019, 08:43:11 pm »

What props are you running on it, certain props create lift as well as thrust.
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #4 on: April 08, 2019, 11:08:13 pm »

Only markings on the props are the size - 32.5mm. 2 blade black plastic.  Motors are 1100kv outrunners, 3s LiPos.
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canabus

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #5 on: April 09, 2019, 11:33:04 am »

Hi clockworks

General start point for the CG is a third the length of the boat from the stern.
Moving the ESCs and battery should get you in that area.

Are the motors 28mm and do they have any markings on them, like 2836-1200 ???

Canabus
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #6 on: April 09, 2019, 11:59:18 am »

Motors are 2836-1100. Mtroniks, which I think are rebadged Turnigy?


Balance point is 32cm from the stern with the current layout, total length is 73cm - quite a lot further forward than a third.

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canabus

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #7 on: April 09, 2019, 01:50:26 pm »

Hi clockworks
The motors are about 350 Watts each on 3S, so once the CG is fix.
The motor will go fairly quick.
Check out Hobbyking on motors around that size.
It's surprising the power difference between them !!!
100Watts to 750Watts !!!!
Banggood have some CNC props which are a good price(mainly dog drive ones).

My Sea Hornet has a 1900kv 750Watt on 3S with a 32mm 2 blade prop.
GPS at 45KPH, so setting up a bit of fun ~!!!

Canabus
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #8 on: April 09, 2019, 02:18:16 pm »

I think the motors came matched with the Mtroniks ESCs, which are rated at 15A - they are sold as a set by Mtroniks, for use at up to 12v.
To me, this means the motors must be rated at a maximum of 180 watts?


Two of them is still a lot of power on a fairly small model though, so I'll probably limit the "travel" of the throttle channel to keep the speed down. My reactions are bit slower than they used to be!
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canabus

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #9 on: April 10, 2019, 01:00:09 pm »

Hi clockworks

I am only 69 , so pushing classic speedboats to the limit is a bit of fun.
With the brushless motors you can go scale speed or to the limit of the boat.
With these 2.4G radios I am back into model boating !!!!
Retirement is fun time and do what you wish to do !!!

Canabus
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #10 on: April 10, 2019, 01:18:07 pm »

I'll probably be happy to go faster once I know the boat isn't going to sink, and I get used to the wind-power guys sharing the same space
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #11 on: April 21, 2019, 12:21:12 pm »

Tested on the lake this morning. Moving the batteries as far back as they would go has fixed the problem. It comes up "on the plane" quite nicely now. No water inside the hull, but I did get a bit inside the seating area. The brass windscreen frame "fits where it touches" the deck, so I think the water is getting through the gap while turning at speed.
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Hotglove

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #12 on: April 21, 2019, 05:25:46 pm »

I took Canabus'advice and fitted a 3846 Motor to my Chris Craft runabout, she jumps on to the plane with great ease and the spray rails fitted by the original builder seem to work perfectly, only get a few drops in the cockpit if the wind lifts them.
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #13 on: April 21, 2019, 06:12:57 pm »

I wonder if spray rails would help on my Riva?

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gra2

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #14 on: April 22, 2019, 10:18:29 am »

They would.

Graham
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clockworks

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #15 on: April 22, 2019, 04:37:03 pm »

Found some pics of other Riva models that had been fitted with spray rails. On the painted section of the hull, so I guess I'll have to strip off the paint to get them to stick on properly. 3mm square basswood strips were recommended, with the top corner rounded off.
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Hotglove

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Re: Riva-style speedboat "digging in" at higher speeds?
« Reply #16 on: April 23, 2019, 05:26:38 pm »

a few pics of my model, there are 2 short videos of her on the Model Boat Mayhem Facebook page, which were taken with her running the standard Dumas prop, which gives the least performance of the variety of props that I have tried,
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