{"id":719,"date":"2011-05-28T19:42:55","date_gmt":"2011-05-29T03:42:55","guid":{"rendered":"http:\/\/svrainshadow.com\/?p=719"},"modified":"2024-06-14T09:21:09","modified_gmt":"2024-06-14T17:21:09","slug":"prop-analysis-and-replacement-part-i","status":"publish","type":"post","link":"https:\/\/svrainshadow.com\/?p=719","title":{"rendered":"Prop Analysis and Replacement – Part I"},"content":{"rendered":"

Rainshadow has a 16 inch diameter, 12 inch pitch propeller that is specified by Camper and Nicholson for the Nic 38.\u00a0 It sits in an aperture, which is about 20 inches in diameter.\u00a0 She also has the original Perkins 4-108 with a 2:1 reduction transmission.\u00a0 We find there are three major problems.Firstly, at about 2000 RPM cavitation begins.\u00a0 It sounds just\u00a0like gravel getting sucked though the prop.\u00a0 Horrible.\u00a0 Secondly, the propeller noise is loud \u2013 unpleasantly loud.\u00a0 And finally, her wide open throttle under load (WOT) RPM is just 2600.\u00a0 WOT out of gear is >3600 (we didn\u2019t go above that).\u00a0\u00a0 The manufacture\u2019s design specs for \u201cintermittent duty\u201d load (which means running at that level for no more than an hour at a time) is 3600 RPM, and \u201ccontinuous duty\u201d load is 3000 RPM.\u00a0 So, Rainshadow is over propped and it\u2019s time to fix that.<\/p>\n

What does over propped mean?\u00a0 Basically, an engine can produce a certain number of HP at any given RPM.\u00a0 A prop requires a certain HP to turn it at any given RPM.\u00a0 When, at a given RPM, the HP required by a prop exceeds the HP that the engine can produce, you get a stalemate \u2013 you cannot raise the RPM any higher because the engine does not have the \u201coomph\u201d to push the prop any faster.\u00a0 Worse, the engine cannot reach its full strength because to do so requires running at a higher RPM.\u00a0 So you are running a nominally 50 BHP engine and getting much less out \u2013\u00a0 at 2600 RPM, the engine can only produce about 37 HP.\u00a0 This is not good if you are trying to power off a lee shore, or are motoring into a headwind and waves.\u00a0 And it\u2019s bad for diesel engines to run at lower than rated RPM.<\/p>\n

Below is a chart for Speed vs RPM for Rainshadow and also for Far Out, Arild Jaeger\u2019s Nic 38, also with a 16×12 prop.\u00a0 Arild posted his data to the Nic38 forum at www.nicholson38.org.\u00a0 We also calibrated our tachometer, and found it consistently low by 5%.\u00a0 As you can see, hull speed is somewhere in the region of 7.5\u00a0– 7.8 knots.\u00a0 Note, we have sailed faster than we were able to achieve while motoring with this prop.<\/p>\n

\"\"<\/p>\n

 <\/p>\n

Prop Sizing<\/h3>\n

So, I set about trying to understand how props are sized.\u00a0 There is a ton of superficial information on the Internet, with many places having a \u201cprop selection tool\u201d.\u00a0 None were very convincing or satisfying.\u00a0 The best source of information has been David Gerr\u2019s book: Propeller Handbook : The Complete Reference for Choosing, Installing, and Understanding Boat Propellers<\/a>\"\".\u00a0 This is a wonderfully detailed book, with lots of data and empirical formulae to help figure things out. If you really care about understanding props, buy it. Of course, if you \u201cdon\u2019t need no stinkin\u2019 formula\u201d (to quote Bob Perry on Cruiser\u2019s Forum), or don\u2019t believe in them, then it\u2019s not for you. (It only uses high school level math and most equations are also presented as graphs where you can just look up the relevant values).<\/p>\n

I liked this book so much I bought another of his books sight unseen \u2013 Boat Mechanical Systems Handbook: How to Design, Install, and Recognize Proper Systems in Boats<\/a>\"\". This is also really good, and practical, and focuses on the mechanical systems of boats.<\/p>\n

There are some people who claim that prop selection is an \u201cArt\u201d and you have to get a professional to help you.\u00a0 So, I called up several of the \u201cprofessionals\u201d who were recommended by various knowledgeable people (including Max Prop in Seattle, the Prop Shop in Everett, and some others), and funny thing \u2013 I got different responses from all of them.\u00a0 One recommended a 15×9, one a 16×12, and another a 17×12. It seems more trial and error \u2013 you try a prop and if it doesn\u2019t work, you try a different one\u2026.. at your own cost.\u00a0 Just make sure the prop shop will exchange the old new one for a \u201cnew\u201d new one.<\/p>\n

So, I took Gerr\u2019s book, and ported relevant parts to a spreadsheet (specifically Crouch\u2019s method).\u00a0 I now have a fairly simple spreadsheet that incorporates most of what\u2019s needed to do a complete prop analysis for Rainshadow.\u00a0 And the results are interesting.\u00a0 Firstly, it clearly depends on the characteristics of the boat \u2013 hull shape, etc.\u00a0 These are hard to quantify.\u00a0 Some factors you can measure (such as hull speed).\u00a0 Some you have to guess.\u00a0 That\u2019s why there does not seem to be a simple answer.<\/p>\n

I\u2019ll describe the calculations here, but if you want to understand where they came from and the justification for them, you\u2019ll need to refer to Gerr\u2019s book.\u00a0 Importantly, Gerr\u2019s book will help you apply the right numbers to get better results.<\/p>\n

Input parameters<\/h3>\n

The engine is the key factor in the analysis.\u00a0 Here is data from the Perkins 4-108 manual that came with the boat:<\/p>\n\n\n\n\n\n\n
Perkins 4-108 Manual<\/td>\nHP<\/td>\n@RPM<\/td>\n<\/tr>\n
Max Brake HP<\/td>\n50<\/td>\n4000<\/td>\n<\/tr>\n
Max Intermittent Duty Rating<\/td>\n45<\/td>\n3600<\/td>\n<\/tr>\n
Max Continuous Duty Rating<\/td>\n37<\/td>\n3000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The reduction ratio of the transmission is 2:1.<\/p>\n

For the analysis, I assumed some HP were lost to the alternator, transmission, etc. giving a Shaft HP (SHP) of 43.5 HP at the Intermittent Duty Rating.<\/p>\n

Next, several physical characteristics of the boat are needed:<\/p>\n\n\n\n\n\n\n\n\n\n
Nicholson 38<\/td>\n\u00a0<\/td>\n<\/tr>\n
Displacement *<\/td>\n16,000 lbs<\/td>\n<\/tr>\n
LWL**<\/td>\n27 feet<\/td>\n<\/tr>\n
Beam at waterline<\/td>\n9 feet<\/td>\n<\/tr>\n
Draft excluding keel<\/td>\n1.5 feet<\/td>\n<\/tr>\n
Depth of prop shaft<\/td>\n2.5 feet<\/td>\n<\/tr>\n
Theoretical hull speed ***<\/td>\n7.8\u00a0knots<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

* Jeremy Lines, one of the C&N designers, says the quoted 16,000 lbs displacement for the Nic 38 is for a moderately loaded vessel, with half full tanks.\u00a0\u00a0 Using 18,000 lbs for fully loaded reduces theoretical hull speed to 7.5.<\/p>\n

** The LWL underway is significantly increased by the large overhangs at the bow and stern.\u00a0 Underway, the LWL extends all the way back to the end of the transom, and forward some distance as well.\u00a0 29 feet is probably a reasonable value underway, which would increase the hull speed to about 8.1 knots.<\/p>\n

Gerr\u2019s calculation for Hull Speed uses LWL, Displacement and Shaft HP to calculate hull speed.\u00a0 The old 1.34 x \u221aLWL is just an approximation used for \u201caverage\u201d boats.<\/p>\n

Hull Speed = 10.665 \u221aLWL*((SHP at Max Int)\/Displacement)^(-0.333)<\/p>\n

For Rainshadow, this gives a Hull Speed =\u00a07.8 knots.<\/p>\n

Gerr\u2019s equation can of course be inverted to calculate the power required for any given speed, as shown below (calculated assuming LWL = 30 feet).<\/p>\n

Required SHP=Displacement*(Speed\/((10.665*\u221aLWL))^3<\/p>\n

\"\"<\/p>\n

Calculating the Diameter<\/h3>\n

The next set of calculations use the actual shaft HP (SHP of 43.5),\u00a0 and shaft RPM (SRPM) at the Max Intermittent Duty Rating to calculate prop diameter:<\/p>\n

Prop Diameter= 632.7*(SHP^0.2)\/(SRPM^0.6 )<\/p>\n

For Rainshadow, this works out to 15\u201d.<\/p>\n

Gerr says to make the diameter calculation using the intermittent duty rating for the engine. Gerr\u2019s method works by calculating where the engine power and the prop power curves cross on a power vs RPM chart. The method chooses a prop so that the prop power curve crosses at the intermittent duty HP and RPM (line 2 and line 3 in the diagram below, taken from the Perkins 4-108 sales pamphlet ca 1984).<\/p>\n

\"Perkins<\/p>\n

I copied the engine power curve off the chart and worked out the equation SHP as a function of RPM:<\/p>\n

SHP (RPM) = -3 x 10-6 x RPM2 + 0.0262 x RPM \u2013 12.652<\/p>\n

Calculating Pitch<\/h3>\n

Pitch is calculated based on the continuous duty rating for the engine.\u00a0 One of the input parameters is the \u201cslip\u201d of the prop.\u00a0 If the prop were a screw in wood, it would have zero slip.\u00a0 Because water is not solid, the prop \u201cslips\u201d, and the proportion is called the slip.\u00a0 Using data for Far Out and Rainshadow, I calculate the slip to be 45% to 50%.\u00a0 Gerr also gives an empirical formula:<\/p>\n

Slip=1.4\/(Speed at Max Cont Duty)^0.57<\/p>\n

which works out to be 44% for the Nic 38.<\/p>\n

The pitch calculation starts by calculating the zero slip pitch from the speed and shaft RPM at the Max Continuous Duty rating.\u00a0 This works out to be 6.3 inches.\u00a0\u00a0 Then, dividing by the slip gives you the actual pitch required \u2013 this works out to be 12.5 to 13.9 inches (for slip = 0.45 and 0.5 respectively).<\/p>\n

Gerr has a rule of thumb that 1\u201d of diameter absorbs 2\u201d of pitch.\u00a0 So a 15×12 prop is nominally equivalent to a 16×10.\u00a0 This suggests the 16×12 may be too large for the 4-108 with the 2:1 reduction ratio.<\/p>\n

Propagation of Errors<\/h3>\n

Next, I did something called \u201cpropagation of errors\u201d to the analysis.\u00a0 This is a standard technique in science where you account for uncertainties in the input parameters.\u00a0 For instance, in the pitch calculation, you use the slip factor.\u00a0 You can find this by measuring the boat speed through the water vs RPM.\u00a0 Then, it\u2019s simple math.\u00a0 The measurement works out to be about 50% at max RPM.\u00a0 However, the experimental error (uncertainty) is about +\/- 5%.\u00a0 So really, it might be 45% or it might be 55%.\u00a0 In a propagation of errors calculation, you work out values for both extremes.\u00a0\u00a0 With this, you get a pitch ranging from 11.4 to 13.9 for that 15\u201d diameter prop.\u00a0\u00a0 The range for the prop diameter using this method is from 13.5\u201d to 16.4\u201d.<\/p>\n

What this shows is that although these formula are helpful in general, they are not helpful in specifying the exact prop you need.<\/em><\/span>\u00a0 I think this also shows why there is no such thing as a prop expert \u2013 there are just guidelines, and then you just need to try one out, and if it doesn\u2019t work, try another. (Caveat \u2013 this is a little pessimistic \u2013 in fact there are ways of improving the odds if you have experimental data from the boat in question).<\/p>\n

Cavitation<\/h3>\n

Gerr warns that the cavitation calculation is very approximate, but at least it is a guideline.\u00a0 It depends on calculating the effective pressure in front and behind the prop blades.\u00a0 When the negative pressure in front of the blade exceeds the vapor pressure of water, you get cavitation. In summary, the calculation shows that a three blade prop is highly likely to cavitate.\u00a0 The solution is to use a prop with a larger surface area \u2013 namely a four blade prop.<\/p>\n

The Dilemma<\/h3>\n

The calculation concludes that a correctly sized prop for Rainshadow is a 15×12 pitch prop or perhaps a 16×10.\u00a0 However, although we should be able to reach hull speed at the right Max RPM, it also predicts that this will not solve the cavitation problem.\u00a0 To do that, we would need a 4 blade prop.\u00a0 However, no one makes a 15\u201d four-blade prop.\u00a0 One option is a 16\u201d 4-blade (actually a 17\u201d that is cut down to 16\u201d diameter) with a pitch of 10\u201d.\u00a0 We got a quote for one for about $700 including shipping.<\/p>\n

But what about drag under sail for three vs four blades?<\/p>\n

I\u2019ve not found any good analysis or empirical data on this.\u00a0 One can argue that a four blade prop won\u2019t be that much different to a three blade prop (free to spin) because under sail, on average, a four blade prop has two blades sticking out into the main water flow, with two hidden behind the deadwood.\u00a0 For a three blade prop, on average, two blades are sticking out into the main water flow, with one hidden behind the deadwood.\u00a0 So, perhaps the difference would not be great.\u00a0 An in any case, we\u2019re cruisers, not racers and if we really cared about drag under sail, we\u2019d get a FeatherStream feathering prop.<\/p>\n

There is also some fear that a four blade prop in an aperture would have worse noise because two blades are passing the deadwood at the same time.\u00a0 Dunno about that.<\/p>\n

What we did<\/h3>\n

The calculations in Gerr\u2019s book are based on empirical data taken for traditional screw props.\u00a0 A UK company called Axiom makes a prop that departs from that form factor, and instead uses what can be described as S shaped wings instead of screw shaped blades.\u00a0 They claim (and it\u2019s somewhat backed up by independent testing by Yachting and Boating World) that compared to screw props, their blades provide equal or better thrust in forward, with dramatically better thrust in reverse, with less prop walk,\u00a0 and lastly, that they produce far less cavitation than screw props.<\/p>\n

The better thrust in reverse is what initially attracted Marilyn to the prop \u2013 she\u2019s at the helm and confirms that Nic 38s have a mind of their own in reverse.\u00a0 They just do what they want, and all you as the helmsman can do is give it power or not.\u00a0 Reports are that with Axiom props, unruly long keeled boats become relatively well behaved.<\/p>\n

So, that what we did \u2013 we ordered an Axiom 16\u201d x 19.5 degrees (Axiom calibrate their props by the angle of the blades, not the pitch of the prop).<\/p>\n

Next: Part II – The Axiom Prop<\/a>, where we report on the Axiom prop as installed<\/p>\n","protected":false},"excerpt":{"rendered":"

Prop analysis for the Nic 38<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,16],"tags":[20,49,15],"_links":{"self":[{"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/posts\/719"}],"collection":[{"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=719"}],"version-history":[{"count":32,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/posts\/719\/revisions"}],"predecessor-version":[{"id":1590,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=\/wp\/v2\/posts\/719\/revisions\/1590"}],"wp:attachment":[{"href":"https:\/\/svrainshadow.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=719"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=719"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/svrainshadow.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=719"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}