P1 Puma RIB racing-scratch building

Just wanted to show you how our discussions here in the forum inspired my sons (7 and 9) to build their own rc catamaran with an outboard using Lego te chnic...



They spent hours of playing with the most creative way learning new things... OK it is not fast but they work hard to find the optimum trimming hahaha. Have a nice Sunday guys...

 

New setup but still not satisfied with the sailing behavior, ie porpoising...
Esc and battery were moved forward

This is the max trimming.


New speed record for only 1 kmh.


Accident again... The boat landed to the coast... Fortunately only scratches... The outboard is OK because it was pushed upwards.




A short video will follow

 

The video:

 

Nice run Dim The hull keeps bouncing indeed. It could be the hull design has too much lift. I noticed that with deep-V hulls, the strakes at the bottom don't run all the way to the transom on rc boats.

Best is to have a deep-V expert react on the performance. Jan (Pomplebed) are you reading along?

Too bad about hitting the coast. It is always better to keep some distance, because your boat could also travel quite some distance through the air!

 

This is what I first thought Jan, i mean that is the hull design that causes the problem. I have also noticed that the stakes are also unequal and usually the inner strakes are shorter. Maybe this is the problem...

I have still room to move the battery even more forward and I am also waiting for the trim tabs. If nothing works then it is for sure the hull design. Learning by doing...

 

There you have a point, about the strakes!
Some rc jet-boats have that too for the same reason.

@pompebled , Jan?

 

So you guys think that is worth shortening the stakes? It is a lot of work but since I have to fix the scratches at the bottom of the hull now it's a chance to do it.

 

I did some research and I think that @Dirk Jan is right. The strakes generate too much lift at the transom pushing the boat out of water.
They should be definitely and materially shortened. I am thinking of leaving only a 40% (inner strakes) - 60%(outer strakes) of their total length at the bow OR remove them completely. I think that the shape of the hull generates too much lift by itself already.
This means a lot of sanding again...

 

I am an old-school guy, from the era of Ni-Cad cells and brushed motors.

In those days the strakes up till the transom were mandatory because of the poor powerplant. Models from the 80-ties and 90-ties all featured them like this.

With brushless power things are so different! Check some of those old hulls on youtube retrofitted with brushless motors and lipos. Too hot too handle! Uncontrollable!

You might try some trim options along the waterfront first.
Take your tools and to short runs.

 

Well it wasn't that hard after all. With a light sanding machine I did this in 5 minutes...

 

Much better in terns of porpoising after removing a 30% of the strakes. Now the problem is that the hull tends to turn to the right. Maybe I try turn fins

 

Much better indeed! It is a big change for the good. You are on the right track to tune your boat well. In the end I could see you still have a lot of power reserve. Once the hull is planted, the speed will go up!

 

Just wanted to share with you our small 3,60 meter rib with a 15 HP tohatsu outboard. Tomorrow will sail for the first time for this summer. If the weather is good we may also go fishing.

 

I made this turn fin to prevent rolling from the prop torque and improve the behavior of the boat at turns. First I will try only with one turn fin and see how it goes.

 

Today we went to the pond for the 5th run of the Penarib.
The changes in the setup are summarized below.
1. Homemade turn fin at the right side only,
2. A balsa wedge at the bottom of the hull to cope with prop torque and,
3. The height of the outboard was reduced by 5mm to reduce the overall ride height and make the hull more stable,
4. I reduced the trim also.






Overall the turn fin did amazing job keeping the hull planted at the right turns but a left turn fin is definitely needed to keep the hull straight. So next step is to make a left turn fin and then I think I can increase the height of the outboard again.
Regarding the balsa wedge I am not sure if it is really necessary.

The video:

 

A rule I learned in my professional life and applies to RC development as well: change one property/feature at the time!

1. Changed motor-height
2. Added balsa "contra torque roll" wedge
3. Added a turn-fin at starboard

I count 3

The reason is one can never point out which change is responsible for the changed behaviour.

Having watched your video, it seems indeed that de boat does not want to turn into a single direction (portside), while starboard turns seem to go well now!

What did the change in ride-height bring?

I used to have a Eco-class RC speedboat back in the 90-ties, which did feature such a wedge to counter-balance torque roll. It was all the way to the transom, and quite modest!

It does add drag, as do the turn-fins do of course. Could you not solve that by placing the battery off-centre?

 

Nice run, and the handling is improving every time! The bouncing is much reduced by parrly removing the strakes. As Unusual said, the other changes are hard to trace individually, but cornering right goes much better.

I hear your prop cavitates quite a bit. Did you sharpen the blades? It will help to reduce the cavitation.

 

Thank you both for the comments!

@Unusual RC you are right about the changes, next time I will just add the left turn fin ceteris paribus. I reduced the height of the outboard (ie the prop is deeper now) to increase the wet area of the hull and consequently the stability of the boat. The esc is all the way to the left side. I should try the battery also but it requires more changes and time, so I first try easier solutions.

@Dirk Jan indeed it gets better every time. . As you can see in the photos the bottom is still unpainted and its surface is rough, so I first wait to set up the boat nicely and then paint the boat and sharpen the prop, turn fins etc.
Going back to the strakes and to how usefull they might be I realized that the tubes of the Penarib also play the role of giant stakes contributing to the porpoising effect. This is a particularity of the rib hull design.
Moreover, I read a comment by J. Finch who said that adding stakes to the Wild thing made the hull less stable.

 

Strakes add lift, but also help tracking in corners.
The rails work as a shallow but long fin!
The reason they have to be sharp too!

Presumably the fastest turning boats out there are the NZ, USA and Australian JetSprints!
Check out how many strakes they have! No turn-fins, nor rudder, but up to 7G in the corners.
Not even our Max Verstappen in his F1 car get's such high-G's when racing for his victory!

Deadrise is 22° of this particular NZ design!

There is a big difference here with any prop driven boat: the jet-inlet really sucks the boat down so much, that it will not lift!
One wave...done is the suction, steering is gone.
That is why they feature both a roll-cage and compressed-air scuba-tank and mouth-pieces just in case!



So you need them, and you also do not need them
Compromise is the word here.
I think sanding the last part off is a good solution.
Than you need turn-fins.

A fast moving hull with strakes will lift higher, which can indeed increase unbalance.
Underpowered boats benefit from strakes.
A word not applicable on most RC boats feature here

I just remembered that the Eco-speed class I had also featured a off-centre-axis battery bay.

Good thing about self developing your boats is the insight you get!
Every new build will be better than your previous!
Keep up the good work!

 

I made these trim-tabs-turn-fins (2 in 1). The trim-tabs will be adjusted by adding or removing washers. Another one will be added to the left side.