42' Sf Repower

[daniel r morrison]

I Finally Have A Date With The Yard And My Long Awaited Repower Is About To Begin. Just Wondering If There Are Any Other 42sf Owners Out There Who Have Done The Same? Prop Selection , Even With The Pros And The Calculators, Still Bothers Me. The Power I Ended Up With Are 480ce Cummins With Zf 1.75 To 1 Gears. So Far I Have Been Told That My Prop Choices Range Between 24x28x4 To 24x31x4. Differences In Materials And Mgf. Have Me Confused. Any Help Will Be Appreciated.
Bob

[Karl]

When I repowered my 32 Hatt last year with Yanmars, transmission ratio and prop size became major issues and although I was getting a lot of (conflicting) advice from various sources, the final decision had to be mine. I found a few articles on the Internet via Google that helped me understand the details more, but in the long run, I went with the recommendations of the Yanmar factory, which were tuned specifically to my boat. I didn't follow their advice merely because they made it, but because after educating myself about the subjects, I agreed with their logic. Others warned me that following their advice would end up with problems, but to the contrary, the boat has been running very well since January and I am happy with my choices. I know where you're coming from when you say you're confused, because I started from a base of no-knowledge-at-all to finally feeling comfortable about my decisions.

It sounds like you are getting information from sources who have the technology to advise you well. The two main sources I had was a local highly respected prop shop and the Yanmar factory engineers. In each case, I requested what the predicted performance results with their recommendations plus why they disagreed with the other, and it was on that information that I made my decisions. Other than that, I don't think the technical specifics of my decision would help you, but if you have specific questions, I would be glad to try to give you my view.

I'll tell you another issue I unexpectly ran into and that was determining if we had sufficient ventilation in the engine compartment, which we concluded we did not, and how to increase it and by how much. That was an engineering nightmare to me, but I think we finally conquered it. I found out that even though there were air vents in place originally, the temperature in the engine compartment needs to be kept down to reduce wear and tear on the engine and other components. There are formulas to determining this and if you ever need to be educated on this subject, I have a reference I could provide you with. Good luck.

[34Hatt]

I did the same thing last year with the calculator's and then the prop guy. Numbers where close but not perfect which is hard too do the first time out. I found same boat same motors and that was the best data. Just remember you want them UNDERPROP 100-150 over top rpm. If you land up even more you can get by, just don't hammer the pins down all the way. But if you land up short on rpm you have to haul and knock pitch out. I shot for 100 -125 over landed up 85 rpm over so this winter I will have a little cup taking out and get my #s I wanted.
Good luck with finding a similar boat and with repower.

[the kuz]

Dan, a few things I learned on my re-power last year: It would be a good thing if your yard has done similar re-powers with Cummins. The Cummins rep will inspect the installation and conduct instrumented sea trials to insure specs are met and to baseline performance for their warranty period. Talk to your yard about the rep's role, how many Cummins/similar SFs they have done, etc. The Cummins rep won't allow the warranty with an over prop, or inadequate air/fuel/exhaust flow. Cummins is proactive on installs to avoid finger pointing regarding problems durring their long warranty period. Prop size for me was based on yard experience with similar SFs and the same engines (they had done dozens of B series re-powers). I had 2 almost new 3 blade Nibrils, that they thought might be a little strong, so they sea trialed to see if they needed to take out one or two inches of pitch. Their advice to me was that it they could de-prop more precisely than increase. Turns out only inch of pitch came out - ALCON thrilled with performance. If I didn't have props that were in the ballpark, they had a recommendation in mind, but told me we could try several different prop/blade combinations (theirs) until we got it right. Also remember to allow for the weight you will add when you get your boat back. You want a little cushion (40-50 turns WOT) with a pristine bottom and empty boat. I would sea trial with a full bag of fuel and full water tank. Good prop shop essential part of the mix. Good luck, Bob K

[garyd]

Your power sounds low to me.

I would use the slane vents.

And under proping by 100 rpm would be a good idea so when you are fully really loaded everything including the kitchen sink it will easily turns up.

What will be the weight fully loaded?

You can generate a matrix for speed vs rpm that is pretty accurate using all your data and @13% slippage.

Of course the engines have to have the power to be able to hit that RPM.

[jim rosenthal]

Dan, you will probably be fine with that repower. If you want Cummins engines it is either those or the M11 series at much more $$$. You are probably taking out 6-71s, which means your boat will be (literally) a ton lighter, which in turn affects how it performs. They are all correct about propping- you are better underpropped than over, absolutely, and Cummins will not be shy about telling you that. From what I've heard Cummins instruments the boat's engines VERY thoroughly for the sea trial, which they ride along on. You may need additional vent area- if so, the Slane vents are the best out there- who else makes custom side vents for Hatteras yachts that work and look good? Make your engine room look good while you're in there; it's the best time to do it.

As far as what props to use, get the best you can afford- ZF or Veem if you can. 24x28x4 is a good place to start, sounds like.

The 42C is a great boat and one I would seriously look at if I were going to trade up, which is very unlikely. If you have 6-71s now, you will add a lot of speed at cruise without beating up the engines. AND, you'll use less fuel.

[garyd]

Don't guess on how much power to use. Lower HP engines means you have to run them harder and that can actually cause you to burn more fuel. It can also cause you to rebuild sooner than later.

I'm not sure what you will tip the scales at but if your around 40,000 lbs I'd be looking at around 550hp minimum. But the decision is yours. Go for a test ride with a fellow 42 loaded to the gills and see how she feels. How long it takes to get on plane. What happens when your cruising along and a wave tries to knock you off plane.

A large number of boaters on this forum aren't even getting on plane. I think I would go Big or stay at home if ya know what I mean.

It is fun to have a little extra power and it is a drag to be dragging your but around with the ass end stuck in a hole it can barely climb out of. As the engines get older they will not produce quite the same HP. Figure a slightly dirty bottom fully loaded three to four year old engines and now she's a lead sled.

If your around 30,000 lbs then you might be o.k. with that HP.

Have fun boating. garyd

[kwerges]

I also own a 42c, 1975 model, and am very interested in your repower. If you could either keep this thread updated with data and/or photos, or pm me, I'd greatly appreciate it. Currently running VT903M's with twin discs, and I'm thinking of repowering in the next couple of years. Would love to hear how yours works out. For info, weight for mine in the travel lift was right at 40,000lbs. with full fuel and water tanks, and plenty of beer on board. Any other suggestions for engines in this model boat. I would think if the 903's will fit in the bilge, just about anything should....

[daniel r morrison]

Thankyou All For Your Comments, Every Little Bit Helps. I Intend To Post Regular Results On My In Stall. Currently I Run 671ti's 330 Hp , At Wot My Speed Half Loaded Is 20.5k, At 2100 Rpm 16.8-18.5 Depending On The Load. These Are 1977 Engines With Thousands Of Hours On Them, I Have Owned The Boat For 14 Years. Fully Loaded Heading For The Bahamas I Usually Run At 1950 Rpm Approx. 14.5-15 K, Depending On The Seas. Following Seas Do Slow Me Down. The Boat Came With A Full Tower, And An 11' Hard Bottom On The Bow. Windage Was An Issue, And I Never Used The Tower So It Has Been Replaced With A Half Tower. I Dont Intend To Water Ski And If My Calculations Are Somewhere In The Ballpark, By Dropping 1 Ton In Weight And Adding 150 Hp Per Side I Should Cruise Easily At 21k.

Dan

[jim rosenthal]

That sounds about right. What is the cost difference between 480CEs and M11s- is it huge or is it small? I didn't realize that 42Cs were that heavy.