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Old 11-07-2016, 09:44 PM   #21
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Peter,
The difference in drag between a FD hull and a SD varies a lot as there are so many variations of SD. Some SD hulls are just about FD and others plane quite nicely at 1.5 to 2 times hull speed. The Monk is a slicer w a narrow easy entry and the whole hull being rather narrow and w soft chines she should or would be about as efficient as a FD but the flat transom sits deep in the water and unlike a canoe the water dosn't flow back to the way it was .... It tumbles out from under the transom/bottom edge. A washing machine wake I call it. FD flows smoothly. SD froths and tumbles. The important part is the aft end of the hull. Not the forefoot.

Re your question "is my SD hull any less efficient at say a knot below HS than a FD?" Yes .... very much so. IMO your SD hull is considerably less efficient a knot below HS than a FD. And your SD hull is more efficient a knot above HS ... and almost certainly at HS. Close to HS there's no speed/drag advantage to FD.

You obviously know more about this than most on TF but you would benefit by looking at the Monk as a SD boat. However most SD hulls are not as efficient as your boat.

Here's a boat similar to yours (but wider) that's diesel powered. She's at LaConner but I don't know how much power. There's another old CC that has been repowered w twin Yanmars. Again I don't know the power or performance. The CC is for sale ($20K) so in that case I could find out about power and performance.

Pics wouldn't come up.
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Old 11-07-2016, 10:05 PM   #22
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Eric,


I know what you mean about the transom churn, mine is only a few inches deep, leaving a relatively nice clean exit. I often wonder if at close to hull speed, the returning bow wake is actually lifting the stern to the point where it exits at sea level. I speculate this because I have no "spilling" of water at the sides into a "trough" created by the boat. It's all wonderfully flat.


I'm going to get an optical tach and experiment in calm seas with various speeds. Check them against my 6.354 curves and see if it really is as easy to push as I think.


I do have a propeller problem though, too much pitch, too much froth.
My feeling is a big 4 blade 24" modern wing style should cavitate less.
Pitch is 16" with a 1.91:1 Velvet drive


Peter
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Old 11-07-2016, 10:09 PM   #23
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Too much pitch?
What's your WOT rpm and your rated rpm?
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Old 11-07-2016, 10:44 PM   #24
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Eric,


Don't really know, no tach, but the curves I have suggest WOT of 2800
I cruise at under 1000 by ear @ 7 knots.


I know the old rule of thumb that if the engine cannot achieve rated WOT RPM it's over pitched.
I cant confirm until I get a tach but I can wind her up pretty damn fast with no real increase in boat speed.
But in my experience an overpitched prop cavitates, or at least just makes a churning frothy mess.


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Old 11-07-2016, 11:21 PM   #25
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Wouldn't it be better to work back from an acceptable prop slip?
What is appropriate for a displacement boat?


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Old 11-08-2016, 09:37 AM   #26
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Peter,
To get the right amount of "slip" most w trawlers choose a prop w about .75 X the dia. Like 24" dia X 18" pitch. Too much dia and too little pitch like 24X12 means there is too much blade area and too much surface area friction drag. On the other hand too much pitch like 19X21 would have too much water at the blade tips spilling over to the other side loosing pressure differential that would otherwise create thrust. And a 4 blade prop should only be used if a 3 blade w an appropriate pitch/dia ratio lacks blade area. The 4 blade is usually used on a trawler if there isn't enough room for a 3 blade w the right blade area.

Re FD my boat has 40hp, 8 tons, 27.5' WLL, 3000rpm rated power, 6.15 knots cruise at 50% load w 2300rpm. A BW gear w 2.57-1 and an 18X13 prop. The prop was 18X14 but couldn't get close to rated rpm. Took out 1" pitch and am still about 25-50rpm short and have called that good enough. At the next haulout I'll cut the leading edge of the prop back perhaps 1/16" to 1/8" and hope for 25-50 rpm above rated.

I once had a wild/crazy idea of cutting the power in half of an overpowered trawler by removing half of the pistons, valves, ect ect as a poor man's way to cut the power to weight ratio in half. A GB36 twin may run at 50-60% load and 6.5 to 7 knot cruise. Just a guess but the sometimes overheating #6 cyl may forever run cool depending on what cylinders were choosen to be closed off. Never heard of it being done and mention it just for laughs.

These are my opinions and there are some here that have different opinions but I think most (close to all .. or all) engine manufacturers would agree. There is lots posted in the past on engine and prop choices so go surfing.
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Old 11-08-2016, 10:12 AM   #27
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Welcome aboard, from up-coast a wee bit
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Old 11-08-2016, 10:25 AM   #28
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Eric,
Thanks for your sage thoughts.
Lots to think about.

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Old 11-08-2016, 10:26 AM   #29
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Murray,

Thanks for the welcome.

Peter
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