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TexasWill 04-24-2010 09:21 AM

Gemini 105MC Slow speed under power

I am purchasing a 2001 Gemini 105MC. According to other owners website she should get 7 knots under power, but during our sea trial she maxed out at 4.9 knots.

She has the standard westerbeck 27 hp diesel with around 900 hours.

The centerboards were down during the sea trial.

Both instruments and GPS indicated same speed.

She is in Fresh Water (thought this was a factor, but after speaking with another owner on Lake Michigan his boat does go 7 knots under power)

She has a Generator and 2 AC units. (thought this was a factor but the Gen Set is only 295 pounds, and both ACs are under 100) Even with the 2 of us on board and everything extra I could come up with (water, fuel, battery bank, Kayaks), she still had less than 1500 pounds of additional weight.(1100 stuff +400 for 2 people)

The Engine appears by the tachometer to get to full RPMs (3200RPMS) and there is no audible slipping of the engine or transmission.

The boat was capsized in 2005, but did not suffer any hull damage beyond a hole in the side of one hull.

Any theories anyone has to help explain this would be appreciated. I really love the boat, but a 25% knock in performance is unacceptable. I have no idea how to test if she suffers the same performance hit under sail.

The survey is next Tuesday so any thoughts I will relay to the surveyor. Thank you.


pdqaltair 04-24-2010 10:57 AM

* Dirty bottom. it would take over 1/4" to cause that.
* Dirty prop (worse). Would vibrate a lot, but you might not notice without a baseline.
* Engine RPMs. Did it get to full revs?
* Waterline. Is she lower in the water than other 105mcs?

The first is most likely, and it could be that bad. If it is, require that it be cleaned for the survey.

TexasWill 04-24-2010 11:25 AM

Thanks for the quick reply pdqaltar

The last bottom job was in Jan 2010, and the bottom is currently clean.

Didn't specifically check the prop. We will check during the survey.

The engine did get to full RPM.

I do not know what the typical waterline is on a gemini, but she did have under the recommended wieght loaded.

svHyLyte 04-24-2010 12:25 PM


The Centerboards were down...
Generally, one only uses a single, leeward, centerboard on a Cat so fitted and particularly a Gemini (having sailed one). With both down you're dragging a lot of extra surface area through the sea and you may have no idea what condition the boards are in.


TexasWill 04-24-2010 12:54 PM

I would love that to be the case, The seller did state that he always left the centerboards down, so they could be in rough shape, espicially if they were not cleaned at the same time as the bottom. That is also an easy thing to test and a cheap thing to fix. I didn't think that would create such a large impact on performance.

FarCry 04-24-2010 02:27 PM

Is the prop the original? If it was underpropped it would turn full RPMS but not get to full speed. Did you do your speed test in two opposite directions? Where you towing a large dinghy? Could current, big waves or lots of wind have caused the slow indicated speed? The knot meter should negate that if clean and calibrated properly. My guess is that the centerboards the main culprit.

TexasWill 04-24-2010 02:39 PM

There was very light wind and slight waves the day of the sea trial. There was nothing being towed. We did run the test in both directions, and there was no current being in a relatively small lake.

The instruments and GPS both displayed the same speed, so I do believe them to be properly calabrated.

Thank you all again for your inputs...I am making a list of every possible solution anyone posts to go over with the surveyor, and will post here next week with what we find.

Please keep the ideas coming.


2Gringos 04-24-2010 02:43 PM

you pulled the anchor up, right?

TexasWill 04-24-2010 02:47 PM

Yep, The anchor was up...nothing was being towed ,drug or dangled.

2Gringos 04-24-2010 02:51 PM

Hmm...well, from a logical point of view, it pretty much has to be the prop, then, doesn't it? Smaller diameter or less pitch than normal?

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