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Old 22-02-2012, 00:07   #16
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Re: What Size Engine?

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Originally Posted by delmarrey View Post
Yep, those are fierce currents for sure, but no matter how much HP this chap puts into his Moody 39 it won't power at 10+knots.

It takes no more HP to reach hull speed through the water going against the current than with it. Only the speed over the ground will change!

Being able to power against strong winds and adverse seas will be aided by more power, but once you can reach hull speed in a given set of wind/sea conditions, additional power only digs a bigger hole in the water.

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Old 22-02-2012, 09:30   #17
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Re: What Size Engine?

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Here are some examples of Puget Sound (Salish Sea) currents. And this is why I powered my boat to the max limit.
I feel it's quite rare for people who live outside of those areas to be in a strong current. I've only motored against a > 5 knot current a few times, in passes through atolls and heading North from Bali. And, we always had other options -- like head out of the stream and wait for it to go slack or reverse direction. It was never 'beat the current or death', or anything like that. More like beat the current or wait a few hours.

So my personal preference is to have an engine that fits me in more common conditions. Which, for me, is one without a turbo charger or oil cooler. If I was confronted with a choice between "beat current in a few rare circumstances" or "oil cooler" or "naturally aspirated", I'd without question choose the simpler engine block. But, of course, I do not live in the Pacific Northwest or in a large atoll with a small pass.

Outside of strong currents, I am not sure that having a large amount of HP available always translates into an ability to pound into a short chop, which is probably a more common cruising situation. That also has a lot to do with the prop, lest the prop just sit there cavitating when the bow punches into a short wave.
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Old 22-02-2012, 09:54   #18
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Re: What Size Engine?

There are a lot of places in the world with big tides and strong tidal currents. We have them all around the UK and especially in the English Channel. 4 knot streams are the norm at springs, and in places where the bottom shelfs up or the tidal stream is compressed in some kind of strait, we see 10 knots and more (Alderny Race, for example).

The consequences of these strong tidal streams are many. Besides a current to fight, there can be vicious overfalls, whirlpools, and incredible (for those who, like I did, grew up in an area with smaller tides) sea conditions when the wind blows against the tide. There are places like the infamous Portland Race which can swallow whole container ships, in the right (that is, the wrong) weather.

Some people here seem to imagine that you should be ready to motor against any tide. It don't work that way. As someone else correctly said, hull speed is hull speed, and hardly any sailboat can motor against a 10 knot tidal current. In these waters you more or less strictly avoid sailing against the tide in areas with strong streams -- tidal gates, we call them -- and in general, you would not plan a passage with much time against even a moderate adverse tide -- it just doesn't make any sense.

So tides affect the power you need maybe not at all. Hull speed is hull speed and naturally you would not choose an engine which could not comfortably get you to hull speed with power in reserve. The power in reserve you need not for tides, but to motor against wind and weather, when you have to. Here the power required to make any progress at all goes up steeply with rising chop. My 100 horsepower boat with a 46' foot waterline can motor at 10 knots without pushing the engine too hard, in calm weather, but can hardly maintain 4 knots at full revs motoring against a F8 in a short period, steep chop (don't ask me how I know this). And you might really need to motor against an F8 to get into a safe port someday. That's what you need the extra power for.
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Old 22-02-2012, 10:04   #19
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Re: What Size Engine?

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My 100 horsepower boat with a 46' foot waterline can motor at 10 knots without pushing the engine too hard, in calm weather, but can hardly maintain 4 knots at full revs motoring against a F8 in a short period, steep chop (don't ask me how I know this). And you might really need to motor against an F8 to get into a safe port someday. That's what you need the extra power for.
What kind of prop do you have? I've been on a boat with about the same waterline and engine as yours that was not able to make 2 knots against F6 with a short 2 foot chop. It had a Max-Prop that I feel was just cavitating uselessly -- I don't know if this is from the flat blade shape, that the pitch was adjusted too aggressively, or that this hull shape just can't pound to windward...

My new boat has a MaxProp and I'm curious if this is just how they roll (compared to the Autoprop I had on the previous boat) or not something I can extrapolate out to other boats and circumstances... ?
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Old 22-02-2012, 10:23   #20
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Get the big engine.
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Old 22-02-2012, 12:52   #21
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Quote:
Originally Posted by msponer

What kind of prop do you have? I've been on a boat with about the same waterline and engine as yours that was not able to make 2 knots against F6 with a short 2 foot chop. It had a Max-Prop that I feel was just cavitating uselessly -- I don't know if this is from the flat blade shape, that the pitch was adjusted too aggressively, or that this hull shape just can't pound to windward...

My new boat has a MaxProp and I'm curious if this is just how they roll (compared to the Autoprop I had on the previous boat) or not something I can extrapolate out to other boats and circumstances... ?
Ah, I have the wondrous Brunton Autoprop, which is the worst sailing prop of all the variable geometry ones, but the best device yet invented to turn power into the maximum possible thrust on a sailboat in all conceivable conditions. It is exactly the business for this situation because it will pitch down to suit when the going gets tough. Your Maxprop has a single pitch for all conditions - what you need to cut through flat water is obviously totally different from what you need to power against an F8.

Concerning "cavitating uselessly" - all props, also mine, seem to do this in bad conditions. Max revs will get me 10 knots even against a F6. But the speed falls off sharply beyond F6 because of the sheer tons of water you start moving as you pierce the chop kicked up by that kind of wind. Pay attention to the boat speed - if you're still getting 4 or 5 knots, then it's not actually useless cavitation, notwithstanding what it might sound like.
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Old 25-02-2012, 10:51   #22
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Re: What Size Engine?

See if youu can find a copy of David Gerr's book, "The Nature of Boats." Maybe someone on the morning net will have a copy. There's a lot of useful info there re. sizing engines and props.

As several previous posts point out, hull speed is hull speed. The arguments about powering into a current ignore this fundamental constraint. The ability to reach and maintain hull speed has nothing to do with current, but it has a lot to do with whether you have a headwind and what the waves are doing to you, although as someone else pointed out, at some point when you're smacking into a head sea, the prop will cavitate, and extra power is useless beyond that point.
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Old 25-02-2012, 11:34   #23
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Re: What Size Engine?

40 is ample but 54 is better if you expect to need the engine a lot.

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Old 25-02-2012, 18:49   #24
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Re: What Size Engine?

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Originally Posted by Play Actor View Post
See if youu can find a copy of David Gerr's book, "The Nature of Boats." Maybe someone on the morning net will have a copy. There's a lot of useful info there re. sizing engines and props.

As several previous posts point out, hull speed is hull speed. The arguments about powering into a current ignore this fundamental constraint. The ability to reach and maintain hull speed has nothing to do with current, but it has a lot to do with whether you have a headwind and what the waves are doing to you, although as someone else pointed out, at some point when you're smacking into a head sea, the prop will cavitate, and extra power is useless beyond that point.
Hull speed should not be full throttle! More like 75-80% throttle. e.g. on my vessel hull speed is 7 knots but if I push it I can actually get 8.5 kts. out of it. Which is good for plowing thru rip currents or pulling off a lee shore. But I shouldn't run it at that for more then 15 -30 minutes.
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