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  #1  
Old 10-17-2008
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Cruise speed with Volvo MD6A diesel

I have recently purchased a Dufour 31 with MD6A 10HP diesel. In the next month I need to decide whether to remove and repair the motor or replace it with a Yanmar 18hp. The repair will be a small portion of replacement expense, but don't want to throw good good money at a bad or underpowered motor. So my first question is to ask anybody with an MD6A and similar sized boat if they feel they have enough "horses under the hood" to cruise at a decent speed under power. Second, if properly repaired / rebuilt, will I get a few good years out of the MD6A or is it likely to give me problems?
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Old 10-17-2008
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First off I know nothing about the Volvo, however I have a bendy toy about the same size as your Dufour. My boat has 2gm20 yanmar in her and 3 blade prop. I cannot image my boat with just 10 horse power. I was once anchored and a storm rolled in and the tide was going out. I fired up the engine to take the strain off the anchor rode. In mid storm I was using every one of those 18 horses. I can hit 7+ at full throttle but her sweet spot is 5.2 to 5.4 kts she burns a 1/4 gallon a hour.
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Old 10-18-2008
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Repair or Replace? Always a difficult decision. And one for which generalities are of no help. The small Volvo diesels have a good reputation but probabilities mean nothing about the particular case at hand. Best option is to get a good mechanic, other than the one that would do the work, to give you an opinion. This will cost, but is probably worth paying.
Before making a decision to go to higher power, make sure you are using the current one to best advantage. Can you use the current engine (you did not say what is wrong with it)?
As far as I can determine the MD6A is rated at 2400 rpm. This determines a couple of things.
1) At wide open throttle (WOT) you should just hit 2400 rpm. If you cannot or if it gets there before WOT you need to worry about the prop first.
Now if you have the right prop.
2) Your normal cruise rpm should be about 1950 rpm. Use an optical tach to check your boat tach before believing it too closely. If you cruise at less than this, you will pay the price in more maintanence and shorter lifetimes.
If 1950 rpm brings the boat close to hull speed and if you can increase rpm (up to a max of 2400) in order to keep the speed up when motoring into head winds and seas, then you have the right size engine and prop for the boat.
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i have a md6a from '76 that pulls my 5000lb. westerly just fine. small repair it I*d say for now.
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Old 10-18-2008
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We have a 13.5 Hp Beta on a C&C 35 about 10000# runs 7 WOT and and easy 6 cruise
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Check the cost of some of the spare parts for the Volvo first. You are in for one collosal shock when you do.
Avoid, avoid, avoid.
Example : exhaust manifold for a 20 year motor, the MD17c.
The cost, in 1997? for the manifold alone....
.... £1300.... close to $2000.
Avoid, avoid, avoid.
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I have a 27 foot albin vega with the MD6A. The vega is considered slightly underpowered at 10 hp. I think it's like 5000 lbs. Not sure why they put that in a 31 foot boat. Is your boat very light?
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Old 10-26-2009
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Thanks to all for your input. I pulled the engine to have it rebuilt. A good repair shop estimated the repairs at higher cost then a new comparable 10 HP Volvo MD2010 that they had in stock. So I decided to stick with a similar sized two cylinder engine.
I think it was the right decision as we can cruise at 6 knots without pushing the engine too hard. It does rev high and a larger engine would have less vibration we've heard. But the extra space to service and ridiculously low fuel consumption more then offsets.
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