3.5cc tunnel hull setup

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Jeremy Bodley

Member
Joined
Jan 2, 2020
Messages
13
Hi Guys, after some advice setting up a tunnel. Have a few 3.5cc's (OS outboards and Prather lapcats) that I have built and have been for recreational use, would like to get some pointers on getting a bit more speed.

Current prather lap cat hull and OS 21xm. Standard plastic prop. 10% nitro. Seems to run happily at 50kph on the GPS.

I see there are guys getting upwards on 50mph on the standard OS outboards

Other props I have
Prather 215
Prather 220
Octura X442
Octua X440/3

After some advice to get a good gain here. I am assuming one of the above (the 215 is a popular prop?) or any other recommendations, maybe professionally balanced etc? I mix my fuel so can increase the nitro %. Is there any other pointers or things I can change to get the most out of the boat setup?

Photo attached of boats

Cheers!
 

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Nice looking boats!

There will be a few options, but easiest and most common are:

- Remove head shim from motor
- Use 50-60% nitro
- then try the following props
M440
Abc/grimracer 40x52/3
Abc/grimracer 40x53/2
Abc 1514-17-45

Then test-test-test :)
 
Hi Kris,

Cheers, the gold boat was the first one I did, the black one I spent a fair bit of time getting the stickers and layout to match Tom Percivals tunnel hull which I have modeled the stickers from. They're clear coated over the top to not come off.

No worries I will mix up some fuel and get a few other prop

The ABC props I assume that's 52 - 53mm of pitch, the 1514 looks to be 53ish. Have I got this right? I thought everyone was after more pitch. The Prather 215 if I recall was something like 63mm pitch (Understood that was used a fair bit). I guess things like blade shape, area and diameter come into it all and it's just trial and error. Just want to make sure I am working with the tried and tested props :)
 
Your numbers sound right. Although I am not sure which will work best on your hull, but generally speaking, those props work well on alot of boats. Along with a reduced diameter x442.

Just pick up 1 or 2 pf the above, sharpen and balance and see how they go for you. You have a good motor, so you should be seeing high 60km/h speeds, but not sure on the hull weight and area of lake you run on (ie small with no time to wind out wont help top end speeds).

I think the head shim and going to 50-60% nitro and testing some props you already habe is a good stsrting point also.
 
No worries, I will sharpen and balance those props and give them along with some more fuel a bit of a go. Wouldn't mind getting a professionally sharpener prop, is there a few guys on here that do them?

I run the OS boats at Taupo, very large lake but far away. I also have an old lap cat with a K&B 3.5. (The old lapcat hull handles poor, and the K&B sometimes seems like hard work). Run this in the sea as live by it. Give it a good wash and drown with crc after. So lots of space for running anyway
 
Thanks Kris & Brad. I had been trying to balance a few props but hadn't had much success, is there any on here that would be able to give advice around this? I'd like to try master this art. It seems possible to get for example a two blade prop balanced with a blade either side (horizontally), but unable to get them to be turned 360 degrees and sit in a particular spot, I can't establish a pattern or logic that one blade is heavier than the other, so I assume there are a few more factors than just removing material until the blades can sit anywhere when spun on the balancer. I have the dubro balancer and a magnetic one. I did watch a few videos but yet to find a video that goes into detail and explains the problems encountered and how to get a fair result.

Appreciate the help fellas

Jeremy
 
If the blades always want to sit horizontal, then you need to shave it off the hub on the lower side...or anytbing below the shaft centerline...
This means sometimes I take a lottle.off the tips and leading edge of blades on the lower side. But mainly the hub needs material off it.

The blade does not need to sit perfect in any location, the main thing (especially on a magnetic balancer) is that it only moves slow and does not drop fast and rock back and forth...this means a heavy spot.

Have you got links to the videos you watched? I could be keen make a short video with explainations on what I have learnt to do from Aaron from OZSPEC racing.
 
Hi Kris

Thanks for the reply that is really helpful. I looked at a variety of videos on youtube, quite a few but didn't find too many that told more than trim a bit off until the blade spins freely and doesnt sit in one spot.

I will have a go trying to remove some material off the hub. Happy to wreck a few props in the spirit of learning

A video would be above and beyond. If you have time will not go wasted that would be awsome

Jeremy
 
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