4.7m Gemini 1991 model very unstable at speed

Skeltonsc

Member
Hi guys I've been trolling around your site and other sites for some time now so I think its time to post.

I bought a 4.7m Gemini Semi which is in a fair condition its rated for 60HP and came with a 60HP Johnson motor that besides needing to be rewired was in an excellent condition.

Over Easter weekend I decided to take a chance and launch it in the Vaal dam.

First few tries  I had the bow right up in the air and after playing with the trim I got it to plane quickly without too much lift.

Anyways the wind was quite strong and there was lots of chop on the water I found that the boat would seem to dig in the bow and lurch to the side, I think they call it bow steering or chine walking. I would have to throttle right back and almost drop off the plane for this to stop.

My previous 3.6m Duck never did this but then it only had a 15hp motor.

Any ideas what I can do about this? or is a characteristic of this boat and hull design?
 

Leon Brits

New member
I might be wrong, think you have to trim your motor up so your bow runs a bit higher out the water.

Gemini is one of the best semi rigid hull designs ever built.
 

trier

Senior Member
Is it a cat hull? What was your weight distribution like? Try and load at the COG. Check the engine height for too high I personally think you got the trim too far up/out and raised the bow too much, trim down
 

Skeltonsc

Member
Good day,

Its a mono hull

Yes the trim was way out I had to take the trim rod out completely to get the bow down and get it on the plane quickly.

However once it gets up to any decent speed it seems to bow steer, and it feels as if its going to buck you overboard.

The water conditions were very choppy but what I cannot understand is that these boats are pretty seaworthy and if its acting up on a small chop of about 20 to 30cm how will it preform offshore?

Thanx
 

IWyk

Sealiner
Sounds strange.

Have you checked that there is no water in the hull sloshing around? This can cause balance problems and the hull wanting to go left or right especially in windy/choppy conditions. Much the same as two okes on the boat rocking from side to side.

Other than that could be the motor is not set-up correctly. You mention initially that the trim was all the way out/up and that you removed the rod. This may now have had the trim too far down, causing the bow to dig in too much and wanting to veer left or right especially in windy short chop conditions.
 

yacoob

Senior Member
As some previous posters said, your bow is too low in the water. It's called ploughing. You have to find the sweet spot with the trim & tilt, it varies as the water conditions and weight on the boat varies. A good practice is to tilt the motor up till the bow starts bouncing, then down little by little till it stops bouncing. That is the point at which the boat should run smoothly.
 

Edgy

New member
Those 4.7m Geminis are super stable! I had one with a 60 Bigfoot on and only used it surf launching and deepsea....most awesome capable little boat! I would imagine as the other guys have said that your trim/pontoons/water in the hull etc?
 

Skeltonsc

Member
Hi thanx, now I wish I had a trim and tilt on that motor :)

on my previous rubber duck and speed boat I would have never even considered taking them out in that chop and wind that weekend.

I just need to find a balance between trim settings, however I find if the trim is a little out the bow goes right up and takes ages to come down onto the plane, would those whale fins help in this regard?

Thanx guys
 

yacoob

Senior Member
Hey bud, I thought you had trim & tilt. Ok, the next best thing is to distribute the weight a bit. Get some weight into the nose and leave the trim where it is (planing so easy) then to assist in getting the nose down just ease off the accelerator for a second and as soon as she come down to about level you can accelerate all you want.
 

Skeltonsc

Member
Thanx guys, will try this out and see what it does :)

have to most importantly sort my Skippers and COF

So it will have to wait a month or three

::slr::
 

Marcello

New member
I have a gemini 4.3m with a 40hp on the back I set my trim pin second last hole, it always comes out the hole high (you actually need to hold on or you fall out)but it eventually comes down. I put the beers up front which usually helps.

Nose riding to me sounds like something in the hull is throwing you out (weight wise) or engine set up. taking your trim out def not right for a gemini, they are fantastic little inflatables.Good luck.

I'm interested to hear what the cause is.
 

Skeltonsc

Member
Hi guys I took the boat out last weekend had strong winds and same short chop and swells of around 30 to 50cm  and pumped the pontoons a little harder, and set the trim to the 2nd hole from the transom, then I played with the weight distribution.

I found if I left all the weight at the front, toolbox, anchor Etc it made all the difference in the world, could get on the plane quite easily.
 
However I did find I could replicate the problem if I was running at full tilt behind the waves, crossing them at a 30 degree angle, so the problem was ME, the idiot between the steering wheel and the outboard _seal1_

Also found when I had 3 large adults on board I could not get it to plane at all, but the boat flies when I'm alone.

Thank you for your assistance ::slr::
 

Marcello

New member
HI there
I would not take all the credit there, you have a 60hp
And can't get on the plane with 4 up, there is still
Something wrong there sorry. I have 40hp and easily
Plain with 4 to 5 up and fully loaded. And trust me I'm
Not the lightest of chaps.
What prop is on ur engine?
 

Skeltonsc

Member
Hi its got the standard prop on, the motor is a 1981 model Johnson 60hp 2 cylinder electric start.

The motor needed a bit of work as it hadn't been run in years, I changed the water pump as the old impeller was shot. along with the primer bulb and fuel line

I have cleaned both carbs and set the needle and seats, have not however changed the spark plugs, the wiring was shot when I got her if you touched any of the wires the insulation would just fall off. so I spent a day rewiring everything from controls to coils.

I also have not checked the compression on this motor but she starts and runs fine without excessive smoke and noises.

I need to possible check the plugs and timing on this motor but she doesn't splutter or misfire at all, engine is nice and smooth of takeoff but seems to labor if there is too much weight.

I personally thing that it was trimmed wrong with the extra weight? I left the trim rod in the 2nd hole.

Lots to learn :fishn
 

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Marcello

New member
Hi Skeltonsc

You obviously know a lot more about mechanics than I do
Sorry can't help you with all the tech stuff. I would suggest
You take it in too a pro and see what they say.

All I know is that boat should fly with that 60 on it.

Good luck
 

IWyk

Sealiner
Agreed, the motor should rev about 5500 rpm at full throttle. The air screws are very sensitive on these engines and should be adjusted at 1/12 intervals. The sweet spot should be between 1 1/2 to 2 turns out.

Standard props can also be wrong. The prop should be determined by the boat motor combination performance and not just the motor.

Check on the prop it should have all the sizes on there. Not sure what the maximum rated Hp is for this boat but the closer you get to max Hp the bigger the prop and pitch you can use.

I have 2 x 50 Hp Evinrudes on my Ski Vee 500. This is the maximum rated Hp for this boat. I have the biggest highest pitch props on and it works very well no getting on the plane problems and the boat flies. 12 1/4 x 15 pitch.

I would double check the prop sizes and then make sure the motor is running sweet. You mention it seems to labor when loaded, this can indicate that the engine is not getting enough fuel when under load. Back the two air screws out 1/12 and give it about 3 minutes and keep fine tuning. Normally the timing should not go out unless somebody has tampered with it.

On these motors also make sure that your earth wires are properly clean and connected.

Good luck.
 

IWyk

Sealiner
Be very careful who you take the motor to if you want to have it checked. Very few competent people who can still work on these older engines.

I have spent lots of money in the past and in the end found and fixed( a small) problem myself. 3 different boat shops and 8 grand later! They love to charge you but don't know how to trouble shoot!
 
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