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Old 21-01-2016, 13:50   #1
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Can you add more hp to a Gemini

I am thinking about a Catamaran, but don't know much about them.

Unlike a displacement monohull, the stability of a catamaran is generated from the width which allows cats to hydroplane, thus cats can go 10-12 knots or more in 20-30kts of breeze where a monohull is limited by hull speed.

So, couldn't you replace the engine/engines on a catamaran with more powerful engines. For example, I see many Gemini 105MC cats with about 30 hp motors. Couldn't this be replaced with a 150 hp motor? Shouldn't a cat theoretically be able to motor at 15kts with a bigger motor?

Since I have never seen this nor heard of this, what am I missing?
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Old 21-01-2016, 14:34   #2
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Re: Can you add more hp to a Gemini

Cats don't hydroplane, each individual hull is so narrow they are not as limited by hull speed as monos. This is part of how the Navy maximizes speed on some surface vessels, the old cruiser designs and earlier destroyer designs did this, length to beam ratio was as high as 10:1.

On the other hand planing is how some monos significantly exceed hull speed.


The problem with adding power to increase motoring speed is that you are also adding weight, not just the weight of the engine but the added weight to carry the increased weight of the engine and even more importantly the added thrust of the engine. This weight works against increasing speed. Keep in mind the increasing speed will vastly increase your fuel use, if you double your speed you could be increasing fuel use by 4x to 8x or worse per mile.


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Old 21-01-2016, 15:01   #3
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Re: Can you add more hp to a Gemini

I know of 2 gems with no rig, one has twin 40hp hondas and modified hulls its for sale on Craigslist in key west, the other is a charter snorkel boat with larger 4 strokes.

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Old 21-01-2016, 15:35   #4
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Re: Can you add more hp to a Gemini

y'all do realize this is a troll, right?
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Old 21-01-2016, 15:53   #5
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Re: Can you add more hp to a Gemini

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Originally Posted by Dulcesuenos View Post
I know of 2 gems with no rig, one has twin 40hp hondas and modified hulls its for sale on Craigslist in key west, the other is a charter snorkel boat with larger 4 strokes.

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of all places for a Gemini to find its true self, the charger one has twin 60s. I couldn't help but think "this is what a Gemini was meant to be"

Meant to take a picture for my blog.
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Old 21-01-2016, 18:19   #6
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Re: Can you add more hp to a Gemini

This is not a troll. I am coming from a Hunter 27 and looking for a coastal cruiser. I have found the Hunter too slow. At best, I could motorsail and achieve 5 to 5.5 knots. I also found it too small, mostly because the front hatch leaked into the v-berth so that it was frequently unusable because once the cushions got wet, they started to stink, therefore the boat was effectively about a 16 footer many times.

So my two interests are speed and space. It seems a Gemini 105M or MC would be a good fit.

I think the argument previously made about fuel consumption is the main reason you don't see larger motors. I have seen in videos where cat owners say they frequently motor with just one motor because it is only about a knot slower than using both, but they use half the fuel.

So the maximum hp for speed in a Gemini might be 80-120 hp. And the optimum for distance is the factory motor?
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Old 21-01-2016, 18:26   #7
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Re: Can you add more hp to a Gemini

ok, I'll bite.

In the case of adding HP to the Gemini (or anything else not designed for it) you'll have to figure out how to support the extra weight as well as the extra torque and stresses that you'll be applying to the structure that is NOT designed for those loads. And if you were a naval architect, frankly, you'd not be at an Internet forum asking this question.

Honestly, you'd be better served to buy either a designed powerboat catamaran or else a SeaRay or its ilk. I think you'll be happiest with the SeaRay.
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Old 21-01-2016, 18:30   #8
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Re: Can you add more hp to a Gemini

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Originally Posted by lindabarzini View Post
I also found it too small, mostly because the front hatch leaked into the v-berth so that it was frequently unusable because once the cushions got wet, they started to stink, therefore the boat was effectively about a 16 footer many times.
If only someone would invent a way to fix leaking hatches!
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Old 21-01-2016, 18:35   #9
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Re: Can you add more hp to a Gemini

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If only someone would invent a way to fix leaking hatches!

Those pesky leaks.....just ignore them and they'll go away!!


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Old 21-01-2016, 19:49   #10
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Re: Can you add more hp to a Gemini

Geez!

I have two engines on my boat and neither one has done anything to reduce the leaking of the infernal hatches!

Do I need another engine? Maybe a larger one? I've heard that I should be using them 120% of the time(the engines, not the hatches), but I'm afraid I'd run out of petrol. Since I only filled up once in 2015, I feel bad because gas is so cheap these days.

I'm sorry if this did not help the OP at all.

Warning: Some of what appears above is not expected to be interpreted literally! If you choose to understand it in some illiterate manner, there is nothing the USCG can do to rescue you. Sincere apologies to everyone...

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Old 22-01-2016, 06:44   #11
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Re: Can you add more hp to a Gemini

There are a few custom cat builds that sail ok but still use higher hp obs. I think twin 70 or 115, they run 20 knots under power but fuel burn and economy is equal to a powerboat at those speeds. We had a 30ft endeavourcat that came stock with a 20hp ob, we put on a newer fuel injected 40 (same weight and we got a deal) the boat went no faster under full throttle, the engine just couldn't push it faster, the sails could but no pitch change or trim would do it.

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Old 22-01-2016, 09:58   #12
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Re: Can you add more hp to a Gemini

See the unveiling of the new Gemini open power cat at the Miami Boat Show.
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Old 22-01-2016, 10:10   #13
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Re: Can you add more hp to a Gemini

After Sailcraft, Brightlingsea, closed, the Iroquois moulds were used in the USA with extended transoms to produce a cat sold with very large outboards and claimed to reach high speeds under sail or power. This was about 30 years ago.
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Old 22-01-2016, 10:12   #14
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Re: Can you add more hp to a Gemini

I'm pretty sure the Gemini uses the Silllette outdrive. The outdrive cannot support anything larger than 30HP (I'm pretty sure, but check with Sillette)
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Old 22-01-2016, 10:15   #15
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Re: Can you add more hp to a Gemini

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Originally Posted by Dulcesuenos View Post
There are a few custom cat builds that sail ok but still use higher hp obs. I think twin 70 or 115, they run 20 knots under power but fuel burn and economy is equal to a powerboat at those speeds. We had a 30ft endeavourcat that came stock with a 20hp ob, we put on a newer fuel injected 40 (same weight and we got a deal) the boat went no faster under full throttle, the engine just couldn't push it faster, the sails could but no pitch change or trim would do it.

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Look at simple old lake pontoon boats, people will put 150 HP motors on those things, and they will just about sink the stern and roll a huge wake and go 2 kts faster than one with 50 HP, just not planing hulls.

Some newer pontoon boats, have planing hulls and will run 30 kts with that same motor, its all in the hull
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