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Author Topic: MFA Spearfish Mk2 build advice  (Read 1743 times)

Akamini

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MFA Spearfish Mk2 build advice
« on: July 15, 2020, 09:22:53 am »

I recently did a modest refurbishment of my long abandoned childhood favourite Mk1 spearfish with new electrics and an MFA Torpedo 850 motor (from the old MFA Marlin) and 3s lipo, which runs nicely. Then by chance, a neighbour of a friend inherited someones RC vehicle collection and among them was an unbuilt MFA spearfish Mk2, which I picked up from him as I fancied building it up as a faster brushless setup (can't face cutting up the Mk1 deck to get access to do major changes) and these boats have a nostalgic appeal to me compared to modern RTR options.


I am not an experienced boat builder nor familiar with modern methods so I had a few questions before I got my hands dirty:


1. I have sourced a surpass hobby 3674 1580kv motor which I intend to run on either 3s or 4s lipo. Could I use this on the stock brass tube and shaft, or it is worth putting something else in? Any pros/cons with taking shaft out and lubricating regularly vs some sort of oiler you can squirt a syringe into?
2. Should I stick with threaded props, or is it wise to install a shaft which can take a drive dog?
3. Will GRP props work, or is metal the way to go?
4. Best options for mounting the motor - my default approach would be to use an angled metal mount, but looking at options there are various watercooled mounts which also offer ability to change motor angle. They look like a good idea, but they have vertical sides and no base plate so I wondered what the best way to fix these into the hull would be?


Thanks in advance for any answers, any tips appreciated. Also, if anyone has a scanned copy of plans for a Mk2 spearfish (not much to it, but there are various bits of ply which I'm not even sure were for this kit to sort out) they would be helpful!
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canabus

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Re: MFA Spearfish Mk2 build advice
« Reply #1 on: July 15, 2020, 10:43:46 am »

Hi
Firstly, you lucky duck, what a boat !!!

The motor will suit the boat, good on 3S but on 4S a dam rocket !!!

Prop shaft upgrade to one with an oiler and 4mm shaft.

Banggood have some nice CNC props(35mm to 40mm 2 blade) dog drive.

ESC a Surpass car 120Amp with program card.

Lipo battery 4S 5000mah 65-130C.

Add two good stringers in the bottom of the hull for strength and to mount the battery in the stern.

As for the motor mount a solid alloy angle one .

I will get the balance point setup for the boat later, but they run bow up.

 
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Akamini

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Re: MFA Spearfish Mk2 build advice
« Reply #2 on: July 15, 2020, 10:56:45 am »

Thanks for the tips, much appreciated. Nice photos, hope mine looks something like when it runs! I'll look for a 4mm tube/shaft setup with a dog drive and an oiler. Couple more questions:


What pitch prop do you use at those diameters?


Any chance you could you measure the distance from the transom where your propshaft goes through the hull?
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canabus

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Re: MFA Spearfish Mk2 build advice
« Reply #3 on: July 15, 2020, 11:26:10 am »

Hi Akamini

The centre of the shaft is 290mm from the stern.
The prop is a 38mm 1.4 pitch 2 blade CNC alloy.

By the way the Precedent Huntsman 31 is the same hull !!!

Canabus
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Akamini

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Re: MFA Spearfish Mk2 build advice
« Reply #4 on: July 15, 2020, 02:11:27 pm »

Wow, 290mm from the stern...I just measured my Mk1 and its only 190mm from the stern so you have a much shallower angle - is a shallower angle better for a faster setup?


Been looking at shafts/props/drive dogs...confusing! It seems that most drive dogs and shafts are 3/16"/4.76mm diameter but are flex shafts, I did find one rigid M4 shaft/tube with a drive dog on it on Banggood but that's the only one I could see and the max length available (35cm) might not be enough. Am I looking for the right parts?! Any links to what I should be aiming for appreciated if someone has the time.


Thanks again.
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mogurnda

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Re: MFA Spearfish Mk2 build advice
« Reply #5 on: July 15, 2020, 06:09:10 pm »

There is currently one  for sale on E&&y   item no 254651628112..... for any one interested.

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Akamini

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Re: MFA Spearfish Mk2 build advice
« Reply #6 on: February 21, 2021, 05:00:35 pm »

Just to finished off this thread in case anyone else ever seeks advice for the same setup. I went with the 3674 1580kv on 4S and then 6S with a 35mm 1.4 prop - it was a bit too much, revved high and boat became unstable at high speeds plus a lot of prop walk. I just changed it to a 3680 980kv surpass rocket, and on 6S with a 40mm 1.4 prop - runs great now, smooth and lower revs very little prop walk and full speed is just about the limit on flat water for my setup.
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