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Old 03-17-2014, 01:15 PM   #1
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Hull speed vs. horsepower

Hello All, I am new to this Forum and this will be my first official question. I have waited until Mon. so you all will be calm, fit and fully rested (and, hopefully, patient).

Our first boat (as mentioned in my intro.) was a 30-ft. Clipper Marine sailboat. It was an odd boat in that while it had an endearing clipper bow, it had a rather chopped-off stern and an 8-ft beam. I've only appreciated that design after watching the America's Cup boats - long, narrow, with slab-like sides and soft chines. Like them, ours seemed to sail equally fast either heeled-over or upright. The wetted perimeter did not change significantly in either position.

Anyway, I digress. While puttering around with it at dock, I noticed I could grab a stanchion and easily walk it out of it's slip. In other words, at walking speed (2 to 3 MPH) I could effortlessly tow that boat along. I also noticed, it took equal effort to push it sideways away from the dock, as I always did to check the spring lines and snubbers. My thought at the time was that at walking speed, it wouldn't really matter if it still had the trailer attached - it just didn't take much effort to move it along.

Now before there begins a clamor for more stringent vetting of new members, let me explain where I'm going with this.

Whilst perusing the Seattle WA Craigslist / boats (my favorite site for boat-porn) I came across a twin-engine trawler that some guy actually had installed a Volvo Penta outdrive on. It was fitted on the stern (of course) and apparently had its own small diesel engine inboard. Since that outdrive only swings about a 14-in. prop I later got to thinking ( and FINALLY my question), does a trawler, at under, say 10-MPH, really need all that much HP.? I know you need a lot of HP to get above some threshold, for instance onto plane - but if you are just "no-waking" along, could you in fact, use the equivalent of a small outboard?

I do not know what happened to the aforementioned trawler, it disappeared from Craigslist - but it seemed an intriguing idea.

D.
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Old 03-17-2014, 01:45 PM   #2
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Yep, if you are ok going slow, it takes very little hp.

A case in point: My 38 has a 450hp engine (obviously not a trawler) but I run it as a trawler most of the time. At hull speed of about 7.5kts, it burns 1.9gph. And a good amount of that fuel is wasted just spinning that big engine. My guess is I am putting about 20-30hp to the prop, so a 50hp engine would do fine. At 20kts, it is burning about 11-12gph, probably putting 200hp to the prop.

7.5kts is 3.9nm/gal
20kts is 1.7nm/gal

Speed is EXPENSIVE!!

Most boats follow a hp required curve that rises with an exponent around 2.5 to 3.0, from dead stop to hull speed. That is a steep curve.
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Old 03-17-2014, 04:14 PM   #3
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One good rule of thumb that has been confirmed with the data in Bebe's book is 15 hp per thousand pounds to push a full displacement hull to its hull speed.

But semi-displacement trawlers probably like the triple engine boat that you mentioned use much more, as much as 30 hp per 10,000 lbs. So split the difference and say 23 hp. That 36'er probably weighed 20,000 lbs so it took 46 hp to go to hull speed which is about 7.7 kts.

So a small I/O could certainly push that trawler to a comfortable 7 kts or so. But would it be efficient to leave those big twins sitting there with their props, shafts and struts sitting in the water causing drag. I doubt it.

And whatever efficiency you might gain will be wiped out by the cost of that I/O installation. Unless you wanted to pull the big engines out and use the I/O to only go slow. But then you could never resell it.

David
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Old 03-17-2014, 04:48 PM   #4
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As long as you are on calm water with no wind

When conditions like wind and waves come into play the needed HP goes up. I was surprised to discover that my deep v 28' express cruiser actually used more fuel in following sea conditions then heading into them while my displacement trawler the reverse is true.
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Old 03-17-2014, 05:14 PM   #5
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When conditions like wind and waves come into play the needed HP goes up. I was surprised to discover that my deep v 28' express cruiser actually used more fuel in following sea conditions then heading into them while my displacement trawler the reverse is true.
Seen that too while running at planing speed in following seas. My theory is boat gets hung on the backside of a wave and lingers there. Engine governor adds fuel as rpms drop and boat still lingers, eating fuel. At trawler speed, boat surfs on front of wave, and won't linger on the backside. Surfing is fun!! And cheap!! As long as you don't broach!

Dave are you sure about that 15hp per 1k lbm? That puts my boat at 210hp to run hull spd, that seems high. That's about the hp I run for 20kts.
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Old 03-17-2014, 06:16 PM   #6
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NYCHABIII, welcome to TF from another Tri Cities boater.

You nailed it with your comment about not taking much horsepower to get a boat to idle along. But there are limits to that. On my last boat (330 Sundancer) I installed FloScan fuel monitoring gauges to see where my most efficient operating speeds/rpm's were. I very carefully calculated the mpg at every 200 rpm speed from idle up to about 4500 (WOT) rpm.

The boat got a bit over 1.3mpg at idle, then it dropped a bit until the boat exceeded its hull speed. At that point the stern was digging in as the boat attempted to come up on plane. With the stern buried (before coming up on plane) the boat was only getting about .6mpg.

Once it got on plane it got right at 1mpg and kept that 1mpg all the way up to around 3900 rpm where the secondaries on the carbs were opening. Then it dropped a bit, but not much because of the increased speed. It kept that 1mpg up until around 4000 rpm's where it started to drop off and at WOT it dropped down to about .75mpg.

I don't have the luxury of having FloScan's on my current boat, though I wish I did. I have no idea what my mpg is because we mostly take slow trips up and down the Columbia or Snake Rivers. When we do take longer trips we encounter the dams and there's usually a 30 minute or longer period where the engines are idling but we're not going anywhere (unless you count up and down).

Now, on an unrelated note, where do you keep your boat? We're at the Port of Kennewick on Clover Island, slip 142. Come by and say hello if you're in the 'hood.

Mike
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Old 03-17-2014, 06:47 PM   #7
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Seen that too while running at planing speed in following seas. My theory is boat gets hung on the backside of a wave and lingers there. Engine governor adds fuel as rpms drop and boat still lingers, eating fuel. At trawler speed, boat surfs on front of wave, and won't linger on the backside. Surfing is fun!! And cheap!! As long as you don't broach!

Dave are you sure about that 15hp per 1k lbm? That puts my boat at 210hp to run hull spd, that seems high. That's about the hp I run for 20kts.
That's 15 hp / 10k lbs
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Old 03-17-2014, 08:00 PM   #8
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Quote:
Originally Posted by Ski in NC View Post
Yep, if you are ok going slow, it takes very little hp.

A case in point: My 38 has a 450hp engine (obviously not a trawler) but I run it as a trawler most of the time. At hull speed of about 7.5kts, it burns 1.9gph. And a good amount of that fuel is wasted just spinning that big engine. My guess is I am putting about 20-30hp to the prop, so a 50hp engine would do fine. At 20kts, it is burning about 11-12gph, probably putting 200hp to the prop.

7.5kts is 3.9nm/gal
20kts is 1.7nm/gal

Speed is EXPENSIVE!!
Our boat works pretty much like Ski's. New Moon is 26 feet and about 11,000 lb, fairly deep-V, with a VP KAD44P 260hp diesel and duo-prop sterndrive. At max cruise speed of 18-19 knots, she gets about 1.7 nm/gal. Slow down to 6 knots, and she gets about 4.5 nm/gal. 1350-1400 rpm gives 6 knots, which keeps the engine water temp up at 175 degrees. We've been running her mostly this way for the last 3,500 hours with no ill effects.
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Old 03-17-2014, 08:17 PM   #9
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Thank you all very much for addressing, what may seem to be an idiotic question. Your input is, as always, very informative and thoughtful and confirms my impressions of this HP situation.

GFC, I currently have no trawler, only Phoenix my restored bayliner runabout. I had hoped you would chime in on this. I would love to drop by sometime, thanks for the invite. My sailboat, until we sold it, was berthed at Kelly Pt. / Walla Walla Yacht Club. I was at the time Vice-Admiral. We ( I ) are currently on the trawler search for the Loop trip.

D.
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Old 03-17-2014, 10:37 PM   #10
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D., you've got a PM. Give me a shout if you're around Clover Island.

Mike
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