Gemini 105MC Slow speed under power - Page 2 - SailNet Community

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  #11  
Old 04-24-2010
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Well, I'm glad you checked the obvious. You asked the right questions.

I don't have a Gemini, but my boat is very similar and I have been around them. 4.9 knots is very slow. That is only about 1/4 horsepower (I can reach that with one engine at ~ 1/2) if everything is right. My top speed is about the same - 7.2 knots with 2 x 9.9 hp.

Under propping seems absurd on a late-model boat... but I don't blame the poster one bit, because those are the things you check when nothing else makes sense. Odd things happen.

Do check the boards carefully. You should be able to hit 7.2 knots boards down - mine don't come up! Additionally, his reason for leaving them down was very lame. The reason folks leave them down is generally because they are jammed and possibly laminated. The factory recommends leaving them up EVERY TIME. They may also be wood and ruined. Check the Gemini site (Yahoo) for opinions, but this is real $$$ or really big DIY effort.

See the picture of me pulling a Seabrake drogue at 4.2 knots!!!!
Sail Delmarva: Drogue and Parachute Sea Anchor Testing: A Summary for Small to Medium Cruising Catamarans. Sail Delmarva: The Merits of Learning to Sail on a Small Boat[/url]. It's GOT to be prop/engine issues.
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Last edited by pdqaltair; 04-24-2010 at 03:56 PM.
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  #12  
Old 04-24-2010
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Please let us know what you find. I am very interested, as we are also planning to buy a Gemini 105Mc later this year. In our case it will likely be around October. Third Quarter.
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  #13  
Old 04-24-2010
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Owner of a Gemini..

As a previous owner of a Gemini 105 MC, and reading your posts, I would say to look at these things first: (1) Make sure the prop is the factory prop from Sonic (in the UK). (2) Make sure the engine is maxing at its proper RPM, and if it is not start with the fuel system (Raycor filters, etc). (3) Then depending on the weight of the gear aboard could be causing more drag. If you find out the problem I would love to hear what is was.

On another note, how did the Gemini capsize in 2005? Would love to hear about that as well.
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Old 04-24-2010
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Thank you all for your input and ideas so far. In summary these are the ideas I have gathered from myself, the seller, the surveyor, and all of you;

Things that could cause the slower speed

1. Hulls not parallel as a result of the accident
2. Centerboards dirty or damaged
3. Transmission slipping
4. Prop smaller diameter or less pitch (non-factory)
5. Something on the prop causing it to wobble or damage to the prop
6. Dirty/old fuel causing less HP

Things that have been ruled out

1. Boat is over weight (It is carrying about 800 pounds of extras, but only 2 of us were on board)
2. Slower speed due to less buoyancy in fresh water (verified with Gemini owner in Lake Michigan this is a non-factor)
3. Engine not reaching full RPMs (Tac read 3200 RPMs)
4. Instruments being faulty (GPS Confirmed)
5. Bottom Dirty (owner stated bottom was recently cleaned however will be double checked with Haul Out)

This is not hypothetical, nor a trick question...it is a real world example of a problem that I hope together we can solve.

Last edited by TexasWill; 04-24-2010 at 05:34 PM.
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Old 04-24-2010
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Any chance someone upgraded the alternator or added some belts to drive something to suck down HP?
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Old 04-24-2010
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If the engine is reaching full rpm then the horsepower is adequate. With the boards up, if the speed is still lacking, it pretty much has to be a propellor problem or trans slipping since you say the bottom is clean and you are discounting weight as a factor. You might consider having a mechanic use a test tach to verify that yours is accurate, but that's grasping at straws. I have to admit that I would be tempted to offload everything I and a few strong friends could carry and then see what happens.
Good luck.
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  #17  
Old 04-24-2010
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Is the tach accurate? I know that some Yanmars, for instance, will read up to 500 rpm high. Did it FEEL like 3200 rpm?
If it's making the revs, then fuel is not the problem.
Prop, clutch or a lying tach are the possibilities that come to mind.
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Old 04-25-2010
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Have you tried raising the boards at all? If they are able to be raised up into their cases, then a flattened or splayed board is probably not a culprit.

Did the boat "pull" to one side at all, or does it feel balanced and symmetrical tracking straight, etc?
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Old 04-25-2010
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Quote:
Originally Posted by 2Gringos View Post
Have you tried raising the boards at all? If they are able to be raised up into their cases, then a flattened or splayed board is probably not a culprit.

Did the boat "pull" to one side at all, or does it feel balanced and symmetrical tracking straight, etc?
While I expressed concern about why the boards were left down and the possible results, I doubt they could slow the boat so much. As I indicated, I could make 4.2 knots with a big drogue.

It is also possible (probable?) that it is several of the suggested causes and one more that we have not thought of.
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  #20  
Old 04-25-2010
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flotation chambers

You might want to check the flotation chambers. I believe there are four. These have been known to fill up and it could add hundreds if not thousands of pounds to the boat. They can fill up from with rain or lake water depending where the leak might be. It is known problem with the gemini. I think the 105 has leaks from the rudder compartment that can go into the floation chambers. The gemini yahoo website is a good source of info for the known problems and fixes of all of the Gemini saiboats
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