How do I upgrade my propellor ?

I own #127, Kathrian, hull #127 Corbin 39 Special Edition. I am thinking of upgrading my propeller and wondered if any of our other Corbin owners had any advice for me. I have a VW 16D engine (from Pathfinder out of California in the late 70’s). It has a 52bhp rating at 4800 rmp rating. It’s hooked up with a Hurth V drive transmission which has a reduction ratio of 2.13 in forward and 2.22 in reverse. Currently I have a 3 blade fixed prop with these markings on it 17D 13 ( I’m assuming 17″ diameter, 13″ pitch). I’m thinking of changing to a feathering prop such as a MAX prop, or one of the other manufacturers. How does one determine the right size of prop, let alone the pitch, for their boat? I only want to do this once. Oh yes, I did see questions & also Marius Corbin’s answer about how to determine pitch. It didn’t talk about diameter though. What I was hoping to find out was what Corbin owners with similar engines were running. Also what the max diameter of propeller people are putting on as well. I’m sure some of our owners have upgraded to feathering props and I’m wondering what parameters they used to determine diameter and pitch. Jack Verheyden (#127, Kathrian).

a. Hi Jack, here’s my $ .02 ( CDN ) you may find it of some value. We originally had a 18*13 3 blade fixed prop @ 33 HP 3600 RPM engine and 3:1 gearbox , that set up was not quite right. I looked around for a variable pitch prop and settled on the 20″ dia. J Prop from bomon marine www.bomon.com and re pitched down. Since then, we re powered to a 50 HP @ 2600 RPM Beta Engine, same gearbox as yours and re pitched up. I like this prop as it can be CW or CCW rotation and can be re pitched easily ( even in the water ) and the blades have the same profile in forward or reverse and the hub is relatively small compared to others that I have seen. I have had it 4 yrs now and I love this product. As the pitch is not an exact science, there is a table that comes with the prop that gives the initial set up ( pitch ) but I’m sure one will have to adjust 1 or 2 clicks up / down from that initial setting. Of note: Bear in mind the idle speed of your engine ( ours is high @ 850 RPM ) as when you adjust for optimum motoring, you may find that your creep speed @ idle may not be what you want ( ours is a bit high ) Regards, Frank Bryant, s/v #186, Visitant (186)

b. Hi Jack My Corbin has a very similar power plant to yours. but my numbers are quite different. I have the Pathfinder, marine rated at 42 HP @ 4000 RPM. (Volks Wagon rates it at 50 HP @ 5000 RPM) My Hurth Gear (HBW 10) has a 2.7 to 1 reduction in forward and 1.5 to 1 in reverse. I had an 18 inch — fixed 3 blade prop, with a 12 inch pitch. I replaced it with a 19 inch, 3 blade Max Prop. The different reductions of the Hurth Gear (2.7 and 1.5) played a big part in the final solution, and I would like to share with you what I learned. First of all, it was my understanding that Hurth designed the ratios of the gear box to turn the wheel faster in reverse than in forward to off-set the inefficiencies of a fixed prop. The ratios that you offered for your transmission suggest that your wheel turns slower in reverse than in forward. I’m thinking that your numbers are probably wrong. If your transmission is installed in the boat, you can check the ratios as follows. Mark the prop shaft and turn the engine over by hand, counting the crankshaft revolutions required to complete 1 shaft revolution. Perform this test in both forward and reverse. This will yield the actual reductions. Feathering propellers: — The feathering propeller is great. It enabled me to convert more of my horsepower to thrust, increasing my motoring speed and efficiency, it stopped my shaft from turning while sailing, and increased my sailing speed and efficiency.

But it didn’t go off without a hitch. My old style Max prop does not have external pitch adjustments — it does not have provisions for obtaining different pitches in forward and reverse — it can not be removed or installed like a regular fixed prop. (For all intents and purposes, the boat must be hauled and the prop built on the shaft) There are feathering props on the market today that address and solve all these problems. For my application it was a bit of trial and error. Here’s why the different reductions in forward and reverse are important. The feathering propeller has the same efficiency in reverse that it has in forward. Therefore, the logic of Hearth increasing propeller speed in reverse does not apply. I found that the best pitch for my Max Prop was 24 degrees. (That’s degrees, not inches of pitch. For some reason the world may never know, Max Prop decided to depart from the standard) 24 degrees converts to 15.8 inches of pitch. Try to imagine my surprise the first time I threw the old girl in reverse. With almost twice the propeller RPMs as in forward, the engine was overwhelmed and bogged right down. I had no power in reverse. This was totally unacceptable, and I had to haul the boat again. I sent the prop to PYI and they modified the prop so that it had 6 degrees less pitch in reverse than it had in forward. This solved the problem and my prop was now optimized with 15.8 inches of pitch in forward and 11.5 inches of pitch in reverse. These numbers should help you establish a starting point once you are certain what your reduction ratios are. The greater the reduction is, the greater your pitch will need to be. Conversely, less reduction will require less pitch. The good thing about today’s feathering propellers is that they feature external pitch adjustment. Instead of hauling the boat, you just take a little dip into the refreshing waters of Canada. Hope I’ve helped. Best from Bob Cox (s/v Dorisea) (Hull 70)

c. We used to install 18″ 3 bladed Max Props, but 17″ is OK too. For the pitch, he should start setting it to the equivalent of 13″ and try it. He should be very close. Remember to allow the engine to turn its maximum RPM or 100 RPM less. Marius Corbin.

d. Hi there Jack, Nice to hear that you are coming along. We met last year when you kindly showed us your “baby”. We have repowered Necessity’s Westerbeke 33hp @ 3000 RPM with a Westerbeke 44Bfour (also 3000 RPM) last year. We retained the 3:1 ratio V-drive transmission. The 18″RH 16″Pitch 3 blade prop NecessityStrut.JPG was going to be too small for the new engine so we decided to bite the bullet and upgrade to a 20″ 3 blade feathering prop. (our old prop is for sale if you are interested.) After looking at the Max Prop & J Prop we decided on the Variprop because of it’s robust hub and the damping action as it switches from forward to reverse. Also, we can set a different pitch in forward and reverse and adjust them easily. I had a Max prop on an earlier boat and found that there was often quire a clunk on switching gears. The variprop seems quite easy on the shaft. We had a slight problem installing it as there was a very slight ridge on our shaft which prevented the new prop from gong on until it was removed. We found that a 20″ prop was all we could fit in the opening and it seems to work fine. With our low gear ratio our prop turns slower than yours will for a given RPM and you could likely get away with a smaller one. Adjustable pitch allows some latitude in fine tuning. We relied on Jesco Afheldt at Nautilus Propeller in Toronto jesco and nautilusvariprop.ca/ for sizing and initial setting. I think it is a type of black art. Engine manufacturers sometimes will offer a recommendation but in any event, if the prop is too large or too small you risk running afoul of the warranty. Here’s another site with a fair bit of info westbynorth.com/ Hope this helps. Let me know if you have more questions. Brian Hall (#135, Necessity).

e. Hi Jack, I am Lou Lieto, owner of #193, “Impresa” a PH Corbin 39. We are from the upstate NY area and sailed for many years in Lake Ontario. We loved Kingston; it’s a great small city! We currently sail in the Chesapeake and along the Atlantic coast. Impresa has a Yanmar 50 HP engine configured thru a Hurth V drive. As I recall the ratios are the same as the ones you quoted so it is likely the same model transmission. The engine & shaft are canted slightly to starboard to allow the shaft to be removed past the strut when needed. We have a MaxProp installed and have been very pleased with its performance. The boat moves in the lightest of air so, although I don’t have a fixed prop to compare it with, it seems that the feathering is performing as expected. The folks at Max Prop plus the yard that installed the engine selected the proper sizes. My prop is the model that may have the pitch adjusted, without disassembly, by a ring on the rear of the shaft. Some set screws need to be removed first. When the prop was first installed, I found that the engine would not get above about 2600 rpm. It is rated at a maximum of 3900 rpm. The experts I consulted advised me that the pitch was incorrect so I reset it and that cured the problem. My Yanmar runs smoothly and powering is no problem even in some rough conditions. Unfortunately, I cannot give you the prop dimensions right now. The information is on the boat which is about 360 miles from my home. I am planning to make a visit to the boat in the next few weeks so if that time frame meets your needs, I will let you know then. I can provide Max Prop model number, diameter, and final pitch setting. Is there anything else you would like? I could make you a copy of the owner’s manual if you like. Incidentally, the owner & designer of the Max Prop is usually at the US Boat show in Annapolis in October and I have found him to be very anxious to help his customers. I am sure that he would also respond to an email. I do not remember his name but the web site should be able to get you there. Unfortunately for us, the shaft angle and the natural prop walk caused by the prop rotation cause the boat to swing significantly to starboard in reverse until there is significant way built up. This makes backing into a slip very interesting – either done slowly with much pushing off and other comedies or done at 3 knots in reverse with attendant trepidation. I wonder if you have a similar problem and what you have done to manage it. What direction does your prop rotate? Good luck, Lou & Marilyn Lieto, #193 “Impresa” [add : Jack, The prop diameter is 17″ and it is set at a blade angle of 18 degrees which turns out to be just right for my engine to reach its maximum RPM of 3900. Cheers, Lou ]

f. We repowered #144, “Cormorant” our Corbin 39 Special Edition with a Yanmar 4JH2E (50hp) and then added a 3 blade Max prop. Our surveyor and Fred Hutchinson at PYI (Max prop guru) both calculated the diameter and pitch which worked out to 18 inch diameter and 12.5 inch pitch. If you are planning on installing a Max prop, be sure to speak directly to Fred Hutchinson at PYI and DO NOT trust anyone else, as someone or some people at PYI don’t have a clue. We have found the diameter and pitch to serve us very well in eight years and around 28 thousand NM –Annapolis to New Zealand and many side trips later. Do yourself a favor and COUNT the shaft revolutions from the engine to the prop to make double sure that your ratios are correct, as any error here will affect the pitch calculations. The prop maximum radius (not diameter) should be no greater than 90 per cent of the clearance between the center line of the prop shaft and the hull. In other words, prop tip clearance to hull should be ten percent of the distance between the shaft center line and the hull to avoid “pounding” and the prop tips pass the hull. Hope this helps. Our Corbin had a two blade fixed prop when we bought her, and was an absolute bear to handle in reverse. With the Max prop (and a bit of practice) we can now pretty much put her where we want her. It has made that much of a difference. More free advice: If and when you get your Max prop, take it out of the box a day or two before you intend to install it. Read the instruction book at least twice (maybe three times if you are slow learner like me!) and practice setting the pitch and assembling the prop at least twice or more. This will pay off big time when installation time comes. Two more points: You will need a good metal cutting blade in your hacksaw to cut off the end of your prop shaft to clear the end cap on the Max prop. The instruction book mentions this, but only in a sentence or so, and it’s easily overlooked. Second and last point: Think about spending a few more bucks and get the externally adjustable Max prop, because if you get the pitch wrong the first time, the haulout fee will exceed the cost of the external adjustment. And if you get the pitch wrong the second time….you get the picture? Happy Sailing. Harry and Jane Hungate (#144, Cormorant), Whangarei, New Zealand.