Dave Gerr H.M. Pope iii

Good eye, TollyWally - I'v often thought that when I extend my boat, I should extend it in a step and exit the exhaust just aft of the step tho am afraid to experiment and can't afford an engineer...
I have been up to my ears in epoxy and glass all day - can't really cope with the math you guys are considering right now but Gerr is not immune to mistakes. If you were to see mine, you'd know why I am partial to this boat - It is a keel-less, delicate version of my boat!
What Marshmat says makes perfect sense but I would look at this design more in terms of moving the nine tons at 30 knots and made of plywood (cheaper and lighter than foam-cored epoxy/glass construction). It is so much in the realm of financial mortals that I am considering writing the designer for his input.

 

Matt,
Your explanation makes perfect sense. I couldn't see the forest for all of the trees. Sure the drawing was vague, but my interpretation wasn't very logical.

I agree with your analysis of outboards but for some strange reason I am a single screw inboard man. Outboards and their stern drive derivitives mildly offend me when I consider them as a solution for my personal marine power needs. Strange and irrational, I won't pretend to defend the logic.

I want to once again express admiration for bringing up that bit about effiency. My copy of that book is filled with scribbling in the margins, different colored highlighting, etc. LOL, thats why it was so easy to cite it page and verse in a post. I didn't mention the fact that I have it highlighted and attached to a question mark and a WTF in the margins. I am fascinated by all this stuff but getting to where I understand it at a gut reflex level takes me a lot of careful study and analysis.

What I need is a copy of "Hydrodynamics for Dummies"

 

You made my day with "Outboards and their stern drive derivitives mildly offend me"! At one time, I thought surface drives were my answer but I thank God I saw them struggle on other's boats in mean seas - but maybe on a boat like this, which cannot see much weather anyway...perfect for doodling around Kachemak bay. If I had the money, I'd build it on speculation and I KNOW someone here would buy it.

 

For the non-commercial boat owners in Homer, that's really all they want -- to do the bay and the inlet in fair weather. The HP on this design I find stunningly excessive.

 

Thanks Matt, Finally some one noticed this big boo boo. I wrote to Gerr when the book came out about it but received no answer. The mystery is how this error got repeated over and over for a whole chapter and never caught.

Along with others, I have used the form you give and called it transport efficiency. Using this form, H M Pope comes out at 707. A good but not exceptional rating. Will Allison's Graphite rates higher at 850. In truth, these numbers are not so difficult to beat and show nothing about how a boat performs at less than top speed.

What H M Pope offers is high speed in rough offshore water with good efficiency and that combination is hard to beat. For most of us, feeding the hungry maw of 1600 horsepower is out of the question.

 

I, likewise, find the HP excessive, Sheetwise.
"high speed in rough offshore water" - This vessel has no business out of easy reach of shelter.

 

Mark, the quote is from Dave's book. " She's a salty, seaworthy cruiser/fisherman capable of well over 30 knots in rough going"

 

"Gerr is not immune to mistakes" - post #16.

 

Interesting

Wasn't part of Gerr's discussion about hp/weight ratio of the motors. Thats why he used 800hp Manns. However don't the newer diesels have much better hp/w ratios. As well as the advancements of in materials, drive trains etc. I also wouldn't want 1600hp or 9 tonnes, but a fuel efficient 42 footer I could use. 800hp and 4.5 tonnes and 40mph... now we're talking.

 

Moving 4.5 tonnes at 40 knots, with a hull design similar to the Gerr boat, would require closer to 550 hp.

That's still a lot of engine to feed, but certainly more appealing than twin 800s.

 

A boat dragging that much transom will never be as fuel efficient as it could be.
A friend has been running his yanmar diesel for several years now with cooking oil forced thru a filtre. Free , no problems so far. Smells nice too.

 

Yes Brent, burning biodiesel is spot on when one is discussing a high performance hydroplane that would undoubtedly be more fuel efficient if only it wasn't dragging that nasty old after plane along for the ride.

 

I think there's no doubt that there's an error in the transport efficiency calculation in The Nature of Boats. Tom brought this discussion to my attention a few days ago, so I took a quick look at my copy of the book, which I hadn't read for some years, and sure enough, like Tolly there was a mark in the margin... though in my case it was a more respectable "?" Quite how an error like this got past the peer review process that I know Dave undertakes with all his books I don't know.

As Tom noted the TE of Graphite (by the more widely accepted displ/hp*speed) is between 850 and 900 depending on the load (it goes up as load is increased). But in reality, the TE of any vessel running flat out is of little use - how many of us run our boats at full revs? Having said that, power curves can be hard to come by, so making calcs at other speeds can be difficult.
Fortunately, I have all that info for Graphite, so for interests sake I plotted the TE for a variety of the speeds that I would consider to be within her realistic cruising speed range, based on a fairly light cruising load of 3200kg (7000#)
10 knots, TE = 1330
14 knots, TE = 1225
17 knots, TE = 1190
20 knots, TE = 1170
23 knots, TE = 1070

Now what's interesting to me is that in spite of the fact that the fuel consumption, in terms of nmpg, is virtually constant at 10 to 23 knots the TE gradually decreases as speed increases. Those consumption figures were taken when the boat (& engine) was quite new, so I must record them again, now that both the engine has freed up, and the boat has more stores aboard. But at least it gives a more realistic figure for the TE.

 

Will,
What units are you using for that comparison?

The reason I ask is that "transport efficiency" is not a dimensionless quantity- if stated in the form (displacement/power)*speed, it has dimensions of time squared over length (ie, 1 / acceleration), not to mention that inconsistent units- often kilograms, knots, horsepower- are commonly used. To use the TE for comparative purposes requires that we state the units of each input quantity, eg. one should state " XXX (kg/hp)*kt ".

It would not surprise me at all if it turns out that, even though your fuel consumption per mile stays fairly flat, the efficiency of the engine itself is varying considerably over such a wide range of conditions- the variation in engine efficiency with load flattening out the fuel-per-mile curve compared to what one would expect from the TE alone.

 

Yes - apologies - I was just using the same ones used by Tom above - pounds, hp, knots