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Old 29-01-2017, 17:52   #31
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Re: Can you add more hp to a Gemini

You can get the HP upgrade right from the Gemini factory now.

21 knots.......

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Old 30-01-2017, 00:28   #32
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Re: Can you add more hp to a Gemini

linda,

Back to your first post, you said your forehatch was leaking. Don't know if you are aware of this or not, but sometimes, the leak is not between the hatch cover and the frame, but between the frame and the grp of the boat. Repair is simple: remove frame and lid, clean up the grp, and re-bed the frame, using sealant of your choice. Might be a nice thing to do for the possible new owners of your boat, if you decide to keep it. Sometimes the hatch lid, itself needs a new gasket. In that case, you remove the old gasket and take it with you to the gasket store, match it, and get new. Install. Works a treat, and not too expensive.

Ann
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Old 30-01-2017, 12:14   #33
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Re: Can you add more hp to a Gemini

A powercat capable of 20 knots and above and a sailing cat are very different beasts. To make a cat able to get to 20 knots and above you have to immerse the transoms deeply, straighten the aft buttocks (the upsweep of the underwater sections at the stern) and make the hull fine enough to get through displacement mode restrictions.

These design criteria mean that the cat won't sail or motor well at 5-8 knots where the drag of the "fast" shape will be much higher.

The Gemini is a rather fat hulled catamaran. It needs to be to carry enough payload on a relatively short length. Therefore it has a low slenderness ratio and curved aft buttocks. Hopefully the transoms are not immersed heavily or light wind sailing will be compromised.

As many have already stated, adding twice the horsepower will not give twice the speed. This is because wave making resistance increases very quickly for fatter hulled catamarans. In the attached image the resistance curves of different hulls are shown. (Don't worry about the units - they are scaleless units but speed is on the X axis and resistance on the Y axis)

The fatter the hull the steeper the resistance curves. I am guessing the Gemini is about 7 to 1. Its curve shows a very steep increase in resistance. Doubling the power to allow a doubling of resistance only increases speed by a small fraction. This is what people call "hitting the wall" or similar when motoring under power.

If you could put a few hundred horsepower on the Gemini it would eventually get to the less steep part of the graph. The weight of sufficiently powerful motors would severely compromise the whole boat.

So no. Adding more powerful motors will not make the boat much faster unless you want more power in windy or rough conditions when aerodynamic and chop derived drag increase greatly.

cheers

Phil
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Old 30-01-2017, 18:49   #34
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Re: Can you add more hp to a Gemini

Quote:
Originally Posted by catsketcher View Post
A powercat capable of 20 knots and above and a sailing cat are very different beasts. To make a cat able to get to 20 knots and above you have to immerse the transoms deeply, straighten the aft buttocks (the upsweep of the underwater sections at the stern) and make the hull fine enough to get through displacement mode restrictions.

These design criteria mean that the cat won't sail or motor well at 5-8 knots where the drag of the "fast" shape will be much higher.

The Gemini is a rather fat hulled catamaran. It needs to be to carry enough payload on a relatively short length. Therefore it has a low slenderness ratio and curved aft buttocks. Hopefully the transoms are not immersed heavily or light wind sailing will be compromised.

As many have already stated, adding twice the horsepower will not give twice the speed. This is because wave making resistance increases very quickly for fatter hulled catamarans. In the attached image the resistance curves of different hulls are shown. (Don't worry about the units - they are scaleless units but speed is on the X axis and resistance on the Y axis)

The fatter the hull the steeper the resistance curves. I am guessing the Gemini is about 7 to 1. Its curve shows a very steep increase in resistance. Doubling the power to allow a doubling of resistance only increases speed by a small fraction. This is what people call "hitting the wall" or similar when motoring under power.

If you could put a few hundred horsepower on the Gemini it would eventually get to the less steep part of the graph. The weight of sufficiently powerful motors would severely compromise the whole boat.

So no. Adding more powerful motors will not make the boat much faster unless you want more power in windy or rough conditions when aerodynamic and chop derived drag increase greatly.

cheers

Phil
Actually, I believe it's about 9-1 (33' water line and 3.5' beam at waterline)but true for a standard gemini around 7.5-8.0kts you hit hull speed and lots of extra power does very little.

The video recently put out appears to have added stern extensions raising the ratio higher but they don't show the hull shape. My bet is the extensions include flat aft sections to keep the stern from burying, so it acts as a semi-planing hull. Also, it appears they have stripped off much of the cabin lightening the boat significantly which definitely helps with speed.
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Old 06-02-2019, 17:49   #35
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Re: Can you add more hp to a Gemini

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Originally Posted by SV EndlessReach View Post
I installed a 50 HP Honda on my 2000 Gemini Catamaran. The conversion took me and my friend approx. 2 days.

My YouTube link:

https://youtu.be/2uWn8JqBrvs
The utube link isnt working for me right now. Tell us if you got it to go faster and what model of gemini. The m105 seems to have alot of potential for that though the 3400 seems stuck in its ways (wake).
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Old 09-02-2019, 07:00   #36
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Re: Can you add more hp to a Gemini

My Gemini 105MC, 2003 model does 7 knots on the Westerbeke 30B Cruising at 2500 RPM,
8 Knots flat stick,
The Westie motor is 400 Lbs, Plus the Sillette drive, Outboards are around 100 Lbs,

2002 Tony Smith the builder ripped out all the Cherry Wood panelling to make them lighter after his delivery trip to England, and to make them blue water vessels, CE certified,
He stated he got 24 knots on the sails in the north Atlantic, He has a video of it,
I have had mine to 14 knots on sails,
I need 3 knots of wind to make me move on the Genoa,

As for leaking lockers, I use one sided foam tape from a hardware store, My lockers are all bone dry,
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Old 28-02-2019, 05:28   #37
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Re: Can you add more hp to a Gemini

Quote:
Originally Posted by Cotemar View Post
You can get the HP upgrade right from the Gemini factory now.

21 knots.......

Well it's past the date shown in that video. Has there been any more information about it? EDIT: here we go: https://www.geminicatamarans.com/Gem...-Overview.html

Interestingly, I was told a story of a Prout Snowgoose 35 hull built by Ray Walsh that had been turned into a power cat and did 25 knts on the plane.

The Geminis at least the older ones weren't too much different in terms of their shape.
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Old 28-02-2019, 07:10   #38
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Re: Can you add more hp to a Gemini

Quote:
Originally Posted by mikedefieslife View Post
Well it's past the date shown in that video. Has there been any more information about it? EDIT: here we go: https://www.geminicatamarans.com/Gem...-Overview.html

Interestingly, I was told a story of a Prout Snowgoose 35 hull built by Ray Walsh that had been turned into a power cat and did 25 knts on the plane.

The Geminis at least the older ones weren't too much different in terms of their shape.
i went onboard an elite 37 that had been fitted out by prout as a day charter fishing boat using the lower hull moulding,with decks all around a pilot house and large aft deck .
it had twin 70hp shaftdrive inboards, would cruise at 8 knots,with a max speed of about 12 knots.
looked very seaworthy,and stable for fishing in the thames estuary out of canvey island.
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