Anyone own a Gemini GRX 420 - Page 2

ThundercatRob · 17 July 2021, 21:48

Cheers mate. Unfortunately, I only tried to unbolt it just before we launched, and I couldn’t get it off. The flat of the Skeg was stopping it sliding off, and I didn’t have a wedge with me to slightly open up the prog guard. I will get it off before the next outing.

But, the throttle adjustment didn’t make any difference to the overall speed. Still only getting 24knts flat out, 2 up with 20l of fuel, and kit. I’m not sure how much speed the prog guard saps, but it’s way down on a zap cat at the moment. So, it needs sorting.

So, any other ideas? Roflhat, hopefully you might have some ideas.

This is the prop:

I’m wondering if the engine is too low. It’s not a very good picture, but the prop shaft looks a fair bit lower than the bottom of the Hijacker. What’s strange though, is if the slow speed is due to the engine height, this boat has always been setup wrong, as there are only the bolt holes in the transom which it’s currently using! The engine is on the lowest setting though, so the engine mount sitting fully on the transom. Any thoughts:

After that, I’m guessing I might need a tinytach to see what rpms I’m getting. Any recommendation on which to get?

Thanks for all the help (especially Roflhat)

69cmw · 17 July 2021, 22:11

I get a lot faster out of my Surfcat and standard 40HP Yam

Roflhat · 18 July 2021, 11:58

That's a decent prop, Powertech semi cleaver B 15 pitch, should be a decent all rounder.
If it were my setup I would
remove prop guard
run string across the bottom of the hijackers, you want the centre of the prop shaft around 10mm above that
keep all the weight towards the back of the boat
play with different trim settings, easiest way to get more speed is to lift more boat out of the water, you only want 10-20cm of the hijackers touching the water at speed
check tube pressures, not sure on the GRX but thundercats are 6psi main tubes and 12 in the hijackers

You should be able to hit the rev limiter, 1up at least. Youll be able to hear it, or get someone to film you and it may be easier to hear, but check the engine height first. If it's too deep in the water you won't get the speed or handling you want, semi cleaver props like to be ventilated

ThundercatRob · 18 July 2021, 20:42

Cheers Seumas, really helpful info, as usual!

I will do what you said and have a look at the engine height, and remove the prop guard! I did manage to have a quick look today, as I was passing where I store the boat. The engine does look to be lower than ideal, although I haven’t measured it yet.

It looks like I can easily move it up about 1 inch or so, by just slackening the engine clamps, and slackening the lower bolts, which will allow the engine to move up. But after that, it’s far more major, as the transom would need to be re-drilled and the old holes sorted.

The boat and engine are both 2002, so it’s really strange if all the previous owners haven’t got it running at design speed.

So, either lifting the engine and inch or so, and removing the prop guard will make a massive difference, the engine isn’t making power / getting the revs up to the limiter top end!

Not sure when my next outing will be. Babies etc!!!

ThundercatRob · 20 July 2021, 21:41

Anyone have any thoughts on the engine trim. I haven’t adjusted it since I’ve had the boat, but to me, it looks like it’s trimmed in to much.

I’m just wandering if this is also adding to the slow speed with the bow trying to be pushed down, once the power is on. Thinking it needs the pin on the bracket moving out. In the picture below, I’m thinking the tiller should be more parallel to the top of the tube?

69cmw · 20 July 2021, 21:48

My Surfcat is in the 1st pin

ThundercatRob · 20 July 2021, 22:14

Is that the first pin as in closest to the transom? Is your surfcat transom fairly square to the deck, or swept back?

The GRX transom is fairly swept back. I would say a fair bit more than my previous Ceasar Thundercat. If you zoom into the top photo, you can see the angle against the tubes.

ThundercatRob · 21 July 2021, 12:03

Quote:

Originally Posted by 69cmw

My Surfcat is in the 1st pin

If you have one, could you put up a side picture like mine above, so I can see how your engine is sitting on the boat when it’s down?

69cmw · 21 July 2021, 14:43

It’s the 1st pin as in closest to the transom

Roflhat · 22 July 2021, 12:11

You want it fairly neutral trim, maybe a touch positive to get the lift. Looks like you’re quite negative there

ThundercatRob · 22 July 2021, 16:49

Cheers Seumas, that was my thinking as well. Cant see the current trim setting helping the speed as it will be constantly trying to bury the bow.

I will bring the pin out a setting or two and see how it handles.

Bren2103 · 20 October 2021, 13:01

Quote:

Originally Posted by ThundercatRob

Just wondering if anyone on here owns a Gemini GRX 420?

I picked a used one up a month or so back. Be interested to speak to any owners, as I know they are a bit niche.

Hi fella I'm curious to where u purchased the 420 from ,I'm desperately looking for one as we had 1 abroad in Africa and I'm trying to repay my dad with a trip on my boat as he did for me , great boats

Roflhat · 21 October 2021, 07:57

Not many about here in the Uk, best bet I’d go direct to Gemini in South Africa. Ceasar also make a similar boat called a surfcat

ThundercatRob · 21 October 2021, 14:53

Quote:

Originally Posted by Bren2103

Hi fella I'm curious to where u purchased the 420 from ,I'm desperately looking for one as we had 1 abroad in Africa and I'm trying to repay my dad with a trip on my boat as he did for me , great boats

They are as rare as rocking horse poo, especially second-hand. I had been looking for a while to see how often they came up, and it was basically never. One came up and I just bought it straight away.

It's a great bit of kit.

I had a quote from Berthon in Lymington for a new one before I saw mine. It was nearly £12k for a basic boat, engine, and trailer. I think there was also about a year's wait for one, at the time!

Bren2103 · 21 October 2021, 14:59

Definitely way out my budget ,may have to go down the avon route or similar ,I have storage now so a permanent trailer option is now obtainable. My zodiac futura failing was a kick in the balls as its the closest to resemble a gemini
This year is done I'll keep looking

ThundercatRob · 21 October 2021, 15:27

I think a Ceasar surfcat is much cheaper. They are PVC I believe, so possibly a bit lighter and faster as well.

You could try and build a package up. Pickup a used Zapcat / Thundercat and reuse the engine and trailer and buy a new surfcat.

Bren2103 · 21 October 2021, 16:07

Really like the look of them to be honest and looks very similar to a grx and has the speed tubes , I'll keep looking and think it's the best option unless a good second hand unit comes up ,my choices are getter more and more or I'm running out of patience and the most important time for my dad to enjoy before he too old ,Thanks again

ThundercatRob · 21 October 2021, 16:38

I think Ceasar can ship directly to the UK. I had a Ceasar Thundercat a few years back and it was great. I would defo have a Surfcat.

Roflhat might have the contact details for the guy at Ceasar to speak to. Think he was called Jacques

69cmw · 21 October 2021, 18:58

The contact details for Ceaser are on the website and yes he ships directly from his factory to the UK.

Roflhat · 24 October 2021, 09:19

A brand new surfcat will be around £2500-3k delivered to your door in the UK. Or at least it was pre-brexit, Jacques Matthee is the owner at Ceasar, good guy and helpful. Say I put you in touch, Seumas Macleod