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Old 04-09-2015, 14:57   #1
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165 hp cummins for 50ft roberts ketch

Hi everyone,
Our drawn-out search for a family Pacific cruiser has us now looking at a Roberts 50. She looks tidy but one issue I'm pondering is that she looks to be over-powered with a 165hp Cummins fitted. The Bruce Roberts design suggests 60- 100hp. What are the consequences of being overpowered?
She's a fibreglass boat with pilot house design, 500lt diesel tank.
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Old 04-09-2015, 15:28   #2
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Re: 165 hp cummins for 50ft roberts ketch

in theory if coupled with a great reduction gearbox and a relative big propeller with long pitch is...bigger engine...running slower, lasting longer, maybe bit more expensive to mantain, pratically the same fuel consumption if not less

p.s. think in case of bad wheater, wind on the nose...a bit of more power is never useless
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Old 04-09-2015, 15:36   #3
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Re: 165 hp cummins for 50ft roberts ketch

A previous owner put an overly powerful engine in our boat...175 hp turbo in a 55 ft boat...Occasionally when getting out of inlets agains wind, current and waves we have been very grateful but now we are making long passages, 2,000+ nm we have to carry yellow jugs on the deck to give us a reasonable motoring range.
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Old 04-09-2015, 15:39   #4
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Re: 165 hp cummins for 50ft roberts ketch

If the engine doesn't work as hard, won't there be more tendancy to coke up?
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Old 04-09-2015, 16:02   #5
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Re: 165 hp cummins for 50ft roberts ketch

coke up means? sorry i'm not english...if you mean that going always slow may ruin the exhaust manifold or similar i guess it's not a problem...maybe sometimes, every month a 5 min full throttle to clean everything with pressure...
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Old 04-09-2015, 17:30   #6
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Re: 165 hp cummins for 50ft roberts ketch

I imagine it is a Cummins 6BT, a very reliable workhorse of an engine, 90% of the time you will only use a small percentage of available HP, but on occaision you will be thankful for its size, just give it a good blast every so often to blow the cobwebs out and it will run forever
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Old 04-09-2015, 17:50   #7
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165 hp cummins for 50ft roberts ketch

Can't see the problem in the extra power. We have an engine that is nominally 60% more HP than theoretically needed for our boat and it has not suffered from being run at under 50% output for over 30 years. Very nice to have in a crisis too, we backed off a sandbar into a raging current on western port bay after running aground. Would not have been able do that with the original 65 hp engine. Events like these give you the chance to give it a good blast too.

Biggest downside I can see is that if it has a fixed prop like ours you will be dragging a great big bucket through the water, a lot bigger than you would for a smaller engine. I am looking forward to making our fixed prop into our spare prop.


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Old 04-09-2015, 17:56   #8
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Re: 165 hp cummins for 50ft roberts ketch

As others have said, running in the lower part of the power curve could make the engine last longer than you.
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Old 04-09-2015, 17:56   #9
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Re: 165 hp cummins for 50ft roberts ketch

Also, I am sure Mr Bruce Roberts knows his stuff, but we have a 100hp engine on our 42 footer and there's no way I can imagine that it would be powerful enough for a great big thing like some of the Roberts 50s I have seen. They were all serious boats with high freeboard, big cabin sides and tall deck structures. Get that lot in a big wind and I would think 100 hp would be helpless.


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Old 04-09-2015, 19:40   #10
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Re: 165 hp cummins for 50ft roberts ketch

Have a Cummins 150hp in my 60' yacht which sails well. Burns 200 l in 24 hrs at 6 knts.

After that experience i am installing a Perkins 215hp commercial rating in my 66' new build.

Useful to have a bit in reserve on occasion. I don't get stopped so easily when pounding into a choppy sea and headwind - happens often enough. All the time if you live on east coast of Oz and want to go North. Or South.😃

Really appreciated the reserve pushing into tide a few times - saved waiting hours on each event. No big deal to wait i suppose but was appreciated at the time.


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Old 04-09-2015, 20:59   #11
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Re: 165 hp cummins for 50ft roberts ketch

Well, I'm hardly a purist, but this seems grossly overpowered to me, considering that it is supposed to be a sail boat. The sailing qualities of the boat are going to be adversely affected by the additional weight of the huge engine, tranny, shaft, tanks and fuel, and the huge fixed prop (folding or feathering props of this size are rare beasts). Once you degrade the sailing qualities, then it is easy to justify the "need" for lots of horsepower. A sort of circular argument IMO.

If the motoring abilities are so important to the owner, perhaps just building/buying a trawler type motor yacht makes more sense.

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Old 04-09-2015, 21:22   #12
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Re: 165 hp cummins for 50ft roberts ketch

All very true Jim, but I think you'd agree that your current boat needs a fraction of the HP of a Swanson 42 to make it safe and workable, despite being a good 4 feet longer?


Well we looked at a Roberts 50 early on in our dream-boat search and it made our Swanson 42 seem a little flimsy and under-built, a bit trivial in fact.


I had a quick look at the adverts for them here in Oz and the first one I found was fitted with 120HP, which suggests someone else thought 60 - 100HP was inadequate.


Aside from the prop drag issue, how much extra weight are we talking I wonder? My 100HP with transmission weighs in at 450kg, maybe another 40 for the shaft and prop. The only vaguely convincing figures I could find for this engine put the engine at 550kg, add 100kg for the transmission, so at worst another 250kg in a 50 foot boat? All below the water line at least.


Does not seem unreasonable, maybe not ideal, but not crazy, it would not stop me from buying the boat in and of itself if the rest of the boat was good.


But I may well be a closet motor-boater who is yet to "come-out" of the sail locker.


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Old 04-09-2015, 22:39   #13
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Re: 165 hp cummins for 50ft roberts ketch

Well, historically I have been a supporter of the engine as a true auxiliary of lesser power but more recently been happy to have a bit more in reserve. Tom Colvin's philosophy of just the minimal amount of HP to move the boat has some virtue.

Nevertheless, I have been caught several times with just enough or not enough power to get out of tight spots and don't relish any more of that.

Both of my boats do have feathering props and the difference in weight of the machinery and transmission say between 100 hp and 200hp is bugger all in the scheme of things in yachts of this size.

I reckon more important is the volume of fuel carried and the cost to motor. Try not to motor across the Pacific too often. 😃

I do agree with Jim in the general philosophy of leaving a small footprint. Probably makes long term cruising more affordable and practicable. Just have to be thinking further ahead sometimes.




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Old 04-09-2015, 23:00   #14
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Re: 165 hp cummins for 50ft roberts ketch

We've got a Cummins 4BT (150HP) in our 50' ketch. It was originally powered by a 100HP Perkins. Although it would be nice if we didn't have a 1.75" shaft from a prop cost & options perspective, and the 6 cylinder is heavier than our 4 cylinder, I wouldn't consider it a deal breaker. I'd rather have an overpowered cummins than an old tired perkins or lehman. You can always strap a couple of big alternators on it to chew up some of the extra HP.

We motor as little as possible.
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Old 05-09-2015, 19:55   #15
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Re: 165 hp cummins for 50ft roberts ketch

Twin 500hp engines in a 24ft. Donzi-type powerboat. Yes, entirely different type of boat, but they still handle the weight and horsepower. 5.9 Cummins is a great engine and not that heavy.
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