Thread: Andrew's Tug
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Old 12-22-2015, 09:29 PM   #19
Nomad Willy
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City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
Andrew,
I came fairly close to getting that same Isuzu but concluded it was too much power for my boat. My boat is 30' long and displaces 8 tons. My 37hp Mitsu gives me 5hp per ton disp ... More than enough power. 4hp would be enough. I also passed up the 47hp Mitsu that was rather heavy duty and developed it's power at 2500rpm. The 37hp Mitsu S4L2 was close enough. A 33hp shorter stroke version was available and I know now I would have been satisfied w that but if I had it to do over again I'd get the S4L2. My cruising rpm (2300) is 700 down from rated rpm (3000) .. nice and smooth and have no noise issues.
My Willard is a FD boat so if your hull is SD the above is not applicatable but I suspect your disp is closer to 6 or 7 tons.


Anyway I had a special damper plate on my Yanmar too w some plastic cam like parts. But the solution to the problem was to get ($700) a bronze gear on gear pump to circulate the oil in the transmission through an oil cooler and back. I utilized the oil drain plug as a return port. I also used several kinds of oil including heavy sticky oil and synthetic oil to keep the viscosity up there .. way up there. It worked. A lot of trouble and no small amount of money for a small boat (25' Albin). Ran the boat for about 4 years that way.

If I were you (and I'm not) I'd find a good 2.5-1 gear that bolts up to your engine. A 17 or 18" prop will work nicly w that gear. You've got a very good engine and probably w the right amount of power.
What problems do you experience w the double reduction set up? The Yanmar has very soft engine mounts so I suspect you could have some alignment problems. Consider British R&D or Austrailian Poly Flex mounts that alow far less movement and very likely will be better at vibration control.

I re-read your post and see you already have the big Isuzu. Since you do I'd run it. It's a very good engine and you could (perhaps) use the 2-1 gear that you already have. Your boat is going to go as fast as you want whether you have a 2-1 or a 3-1 or a 2.5-1 gear. It's going to burn about exactly one gph in any event unless you overdrive your hull. 2-1 will work. I'm not saying it's perfect but you probably won't notice the difference. The smaller prop may run smoother too. Something to consider. You've got plenty of work to do w the cabin.
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Eric

North Western Washington State USA
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