Hans Christian 38 reprop

Discussion in 'Sailboats' started by Zappi, Feb 7, 2012.

  1. Zappi
    Joined: Oct 2008
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    Location: Puget Sound

    Zappi Senior Member

    Hi, maybe I don't have enough info in front of me to determine this but...
    We are planning a trip from Tacoma, WA to Homer, AK this summer. The boat is a Hans Christian 38 Traditional. It currently has a two blade propellor in new condition. The previous owner was planning extensive offshore sailing so he propped her with a two blade that was as efficient as two-blade offers I believe. Power is an Isuzu 240 series 58hp. It seems a motorsailor is ideal for the inside passage. We are currently able to cruise at 5 knots but it seems underpropped as I can overrev her at will. (prop and bottom is like new)
    I'm thinking a three blade would be beneficial since we have but three months for the trip and would like to spend some time at ports and anchor. Is it possible switching to a three blade type would increase efficiency and speed. I'm thinking we should be able to consistently hit hull speed with a three blade thats sized appropriately. This would gain us nearly weeks in the big picture. Any thoughts and/or suggestions???
     
  2. kenJ
    Joined: Jul 2005
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    kenJ Senior Member

    I agree you sound under propped. There are several prop calculators, here is an example. I don't know enough specifics about your boat, so plug in the values and see what it gives you.

    http://vicprop.com/displacement_size.php

    Several others show up on Google. Try a couple different ones and come up with an average. How much room do you have between the hull and the shaft. There is a formula that I can't recall that gives you the minimum space between the hull and the prop arc to minimize cavitation and erosion of the hull. If memory serves, it is about 15%of the prop diameter, no less than 10%.
     
  3. Stumble
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    Stumble Senior Member

    Zappi,

    I am going through this, though for different reasons on a Beneteau 381 with a 42hp diesel. I would highly recommend going to a folding/feathering prop for a number of reasons. Primarily is that speed under sail can be increased by more than a knot, and reversing and backing power can almost double.

    I am in the process of working out which one to buy, but there was a great test done by Yachting Monthly two years ago on them available at http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf
     
  4. michael pierzga
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    michael pierzga Senior Member

    Two blade is a cheap way to get a low drag sailing prop when you hide it behind the deadwood.

    Best method to determine maximum prop size is to measure exhaust temp. A good engine, prop shop will have one. High temp means over propped.

    Better to be over revving than to bog a diesel down by being over propped.

    MAX type feathering props are beautiful...and expensive. Many times you can find used Max props. worth the effort
     
  5. Zappi
    Joined: Oct 2008
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    Zappi Senior Member

    Thanks for the responses. I would love a feathering prop but it's not in the budget. It currently is a sailors two blade 22x17. It does hide nicely behind the keel when turned vertical. I recall the previous owner talking about messing with propping a bit and this is the biggest two blade he could make work with out issue. Trans ratio is 2.91 : 1
    I ran the prop calculator you had posted Ken. Thank you, I feel it is much better and gives more info than boatdiesel.com calculator. It tells me a three blade wants to be 20.5x12.5. It also tells me a two blade should be 21.6x 12.5. This seems significantly less pitch than whats on there. Not sure what to make of it. Maybe calculator and real life are that different or maybe the pitch is that far off and it doesnt show because lack of surface area.?
    My thought is we will cruise up the inside and down the outside. I would think sailing would be more prevelant coming back. Would it make sense to throw the two blade on before we leave Homer for more sailing efficiency???
     
  6. Stumble
    Joined: Oct 2008
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    Location: New Orleans

    Stumble Senior Member

    Zappi,

    The primary advantage of a three vs a two is that a three bladed prop will have less vibration than a two bladed prop since it is almost impossible to get a two bladed prop balanced well enough to eliminate it. The practical tolerances with a three bladed are not as exact. Additionally a three bladed prop will generally have more bite in bad weather and against adverse wind and current.

    That being said, I wouldn't think it would be worth the money to replace a reasonable two for a three unless you are willing to go through the process of doing a pretty expensive prop optimization program. This is much more than just ordering the right prop, and includes pitch optimization, balancing, possible grinding, ect to get it exacally right. From what you have described I doubt it would be worth it on your trip.

    On the other hand a folding/feathering prop would probably be less than you think. For the beneteau I have been quoted as low as $1100 plus shipping. I am still waiting on a few quotes, but for the advantage under sail it is a pretty cheap performance gain. Far exceeding going to racing sails, at a fraction of the cost. And no fixed prop can match the performance in reverse.
     
  7. michael pierzga
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    michael pierzga Senior Member

    When they break up a damaged boat the first thing they salvage is the prop. Do some googling for used props. You might be surprised.

    If your boat is out of the water be sure to record the prop shaft taper.

    What Stumble says is true. I have both a two blade and three blade MAX prop. Very little top speed performance between the two.... but reverse thrust is much superior with the three blade.

    If you do re prop...whether MAX or fixed...go three blade if maneuvering around the port is important.
     
  8. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Don't touch it. It's fine just the way it is.
     
  9. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Unless the boat is way overweight you should get a lot more speed than 5 knots. If you can overrev easily with 17" pitch and maximum diameter do not reduce the pitch. A 22" by 17" or 18" three blade of approximately 41%DAR (disc area ratio) should give you around 6.5-7 knots........once you fit the three blade you will never go back to the two, the larger blade area makes a huge diference manuvering at low speed and bucking tide or chop.....

    Look at this using the higher speed.......

    View attachment Copy of Propking.xls
     

  10. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Rereading your original post, you don't have enough power to reliably cruise at 7+ knots. That is max rpm for your motor. If you take the continuous duty rating for that motor as cruising power, you should be able to hit six in nice weather. The hull will likely have a sweet spot around 5.3 knots which is riding the second wave. If you shoot for just a bit more than that, you will have a quiet, steady ride. Can you hit 3000 rpm at 0 knots - as in you shift into forward from reverse and nail it when stalled?
     
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