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Old 09-11-2008, 17:15   #1
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Lagoon 42 (TPI) New Engines ?

I recently purchased a Lagoon 42 TPI cat. She is a great cat and I am currently retrofitting her. The next job on my list is to repower her. She currently has two 30 HP 3cyl Perkins diesels with shaft drive an fixed props. She currently motors at about 7 knots which is a bit slow for her size. I think that the props are too small.

My plan is to repower her with new 30 HP Yanmars again with shaft drive but now folding props.

I am interested to see if anyone has done this or has any comments.

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Old 09-11-2008, 18:44   #2
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1. What are the prop sizes? Fixed props are the most efficient to start.

2. Replacing a 30 HP motor with another 30 HP motor won't give you more speed...

3. A dirty prop will hugely affect speed

4. A dirty bottom could take 1 - 2 knots off top speed

5. What RPM are you getting at full throttle? Is the redline on these engines ~3000 RPM?
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Old 09-11-2008, 18:50   #3
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Reply 1

I will have to measure the props but they are small.

I realize that 30HP = 30 HP I was more interested in comments about Yanmar versus Perkins.

The bottom and the props are very clean.

Max rpm is about 2200 RPM.
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Old 09-11-2008, 19:44   #4
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What will be the home cruising grounds of the boat?

with only 2200rpm achievable you have other issues. 30 per size in a 42 is pushing it. I'm comfortable with 30's in a 38. How much heavier are you in the 42?
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Old 09-11-2008, 21:27   #5
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What's the redline? 2200 RPM is probably indicating your prop is overpitched. I'd be surprised if your redline isn't 2500-3000 RPM.

With an overpitched prop, you will never get close to top engine RPM and you will be slow. If I have saved you buying new engines, you will owe me a donut.
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Old 10-11-2008, 04:54   #6
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I would tend to agree with Evan. If you are not getting to WOT you are overpitched. Maybe do some digging first before yanking the engines.

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What's the redline? 2200 RPM is probably indicating your prop is overpitched. I'd be surprised if your redline isn't 2500-3000 RPM.

With an overpitched prop, you will never get close to top engine RPM and you will be slow. If I have saved you buying new engines, you will owe me a donut.
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Old 10-11-2008, 07:09   #7
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Changing from a Perkins 30 hp to a Yanmar 30 hp won't make any difference, unless the Perkins are worn out and are not running right. When changing from fixed props to folding props, you usually lose some efficiency.
I'm pretty sure those engines are 3600 rpm at WOT, but even if it is 3000 rpm, you are very much overpropped, either in pitch or diameter. I would also check to make sure the throttle cables are opening the throttles on the engines fully, and are the tachs reading correctly? If you can't achieve the specified maximum rpm, you will not be developing the rated horsepower. This all assumes that the props and bottom are clean.
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Old 10-11-2008, 07:50   #8
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I have Yanmar gm30's with 27 hp each. I easily reach 7 knots at 2200rpms. My cat is 46 ft with fixed keels & 3' 9" draft.
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Old 11-11-2008, 17:05   #9
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Originally Posted by tfsilver View Post
I recently purchased a Lagoon 42 TPI cat. She is a great cat and I am currently retrofitting her. The next job on my list is to repower her. She currently has two 30 HP 3cyl Perkins diesels with shaft drive an fixed props. She currently motors at about 7 knots which is a bit slow for her size. I think that the props are too small.

My plan is to repower her with new 30 HP Yanmars again with shaft drive but now folding props.

I am interested to see if anyone has done this or has any comments.

I switched from fixed props to folding props on my Privilege 37 and speed increased from 5.5kts to 7kts under power at 2200 RPMs. The diameter and pitch are the critical factors rather than the type (fixed, folding or feathering). I was able to increase diameter and that made a huge difference. Under sail, the folding props have greatly improved performance and I really like design and durability (Flex-o-fold). A good prop vendor should be able to compute the correct diameter and pitch for your boat based on displacement, engine HP, and shaft RPM.

Are the Perkins engines worn out? If not, it might be worth your while to experiment with props before you spend mega bucks on repowering. Even if you do get new engines, the props should still be OK. If your shaft RPM on new engines/transmissions change, however, you may have to change pitch. Flex-o-fold or other vendors may allow you to swap blades for a different pitch if required which is a simple operation.

I have friends with a 37' TPI lagoon and they seem to like their Perkins diesels just fine and recently repowered one side with another Perkins. Not sure it would be worth the bucks to switch to Yanmars unless the Perkins are really tired.

Happy sailing on your TPI 42. They are great boats.
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