Repowering a 37

[MC1]

I take the point that truly good seamanship, navigation skills, local knowledge, use of up-to-date charts / current tables, etc. should generally keep one out of trouble to where reserve power shouldn't be needed, and certainly I think in your own case, you're making a valid and well reasoned choice. In my first post in this thread, I was just attempting to add the affect on resale value into consideration, seamanship and safety preferences notwithstanding.

As far as my own preference for having a power reserve, here's my line of thought on the matter . . . I don't have extensive offshore experience, there are many unknowns when sailing in unfamiliar areas, and human error is inevitable. So, reserve power is one asset that could be brought into play in situations where perhaps I've miscalculated. To cite a few possibilities (although you could probably cite better ones, and you're right, they all involve less than stellar seamanship): clawing off a lee shore, fighting an adverse current, getting the boat moving quickly when needing to in a hurry for whatever reason, assisting with challenging docking maneuvers. True 'dat - knowledge and experience should minimize the need for a power reserve to bail one out of these situations - but we're not all advanced sailors and sometimes having a boat that can take care of us in spite of ones miscalculations isn't a bad thing.

Given my own preferences and (lack of) experience, I'd prioritize having reserve power over extending the range of a sailboat under power (if I had a trawler however, I might choose differently). Perhaps I'm in the minority, but this preference has been reinforced through extensive reading about offshore sailing from authors including (in no particular order) Calder, Roth, Rousmaniere, Leonard, Coles, Dumas, Moitessier, Cornell, Payson, Henerson, the Pardeys, and many others. In all this reading, I don't recall any preference being expressed for downsizing one's HP except perhaps for the Pardeys who preferred to sail with no engine at all. I've seen it expressed fairly consistently that for long range offshore sailing, sailors ought to plan on sailing and not rely on motoring to get them where they need to be (due to limited fuel supplies, the possibility of breakdowns at sea, etc. etc..). Maybe this is a nice concept, but you feel differently from your own experience in the North Atlantic. I'm certainly interested in hearing what you and others here have to say about it (and other matters, hence you'll notice I've attempted to "pick your brain" in other threads here in the PSC forum).

[billbalme]

I have an old Universal - I think it's got 35hp (perhaps when new)! Like MC1, I don't have extensive experience and wish I had more power occasionally - especially having been turned back by adverse current a couple of times. However, I'm not about to re-power - unless the current unit dies.

As to fuel consumption, I'm not sure it's really true that a 50hp engine uses any more fuel per hour than a 30hp unit, if the boat is being pushed along at say, 5kts... Surely, 50HP just allows one to use more fuel by going faster than would be possible with the smaller engine.


Bill
s.v. Toodle-oo!
PSC37 #148

[SimonV]

Most 50hp engines are not run at max reves say 4000 revs = 50hp @4lts/h but about 2300 revs = 35hp @2.5lts an hour or less. But a 35hp run at 4000revs = 35hp@3lts/h (numbers are not fact but used for example only)

[delivered]

SimonV. Thanks good point, off to scratch my head at power/fuel consumption curves.

[SVArgo]

Originally Posted by delivered

SimonV. Thanks good point, off to scratch my head at power/fuel consumption curves.

I recently looked at the power and SFC curves, and I think that the 50 does give a little better consumption at something around 80% MCR, ~35 HP or so, compared to running the smaller one at 100%.

[delivered]

Yes I agree, however I hope to be using the 39 at around 70% for duration. Yanmar used to publish "specific" fuel consumption curves which gave fuel consumption per horsepower plotted against output. To my eye this showed that most engines were most efficient at just over half power. Unfortunately all they show now is a fuel consumption curve which I believe is based on the prop power curve.

[SVArgo]

Originally Posted by delivered

Yes I agree, however I hope to be using the 39 at around 70% for duration. Yanmar used to publish "specific" fuel consumption curves which gave fuel consumption per horsepower plotted against output. To my eye this showed that most engines were most efficient at just over half power. Unfortunately all they show now is a fuel consumption curve which I believe is based on the prop power curve.

I got into that a couple of months ago while doing a launch design. The SFC curves are plotted against RPM, so you have to look at a power curve to determine RPM at the point of interest, then plot it out on the SFC curve. The power curves show power at the crankshaft, along the shaft, and then the propeller power curve. I've always been a little skeptical of these since the prop delivered power (Pd) is a function of the torque and shaft RPM, which aren't the same given the different reduction ratios of the gears, but I guess Yanmar assumes that the error is small enough.

I have the 3JH5E spec sheet with the above mentioned plots. We can't post .pdf's so if you don't already have it I can email it to you or direct you to it on Yanmar's website.
Ryan

[delivered]

Thanks Ryan I have that curve from the Yanmar site.