Prop size

Discussion in 'Main Morgan 38 Sailboat Forum' started by Coy McDonald, Mar 6, 2020.

  1. Coy McDonald

    Coy McDonald New Member

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    I have owned Delia for about four years now and have been disappointed with her speed while under power. All I have been able to get out of her is a little over 5 kts at 2100 rpm. Hull speed should be around 7.2 I believe. I suspect the previous owner may have under propped her. She has a Perkins 4.108, a gearbox ratio of 1.9:1, and a 16" featherstream with an 11" pitch setting. What size props are the rest of you running?
     
  2. Travis

    Travis Member

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    I have the same power plant with a 16 x 13 two-blade propeller. I experience the same problem you describe - nothing over 5.5kts at 2400rpm.
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    Two separate boaters that I trust have advised trying a prop with an additional 2” of pitch. Very interested to hear others’ opinions about this.
     
  3. dave_a

    dave_a Dave Ahlers

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    My 383 would gps 6.5k at 3000rpm. It would max out at 3500, go a bit faster but bury the stern and smoke. This is with the OE 3 blade 16x11 and a Hurth 150 trans.
    The boat seemed to like 25-2700rpm, burned 3/4 gal/hr, 5 to 5.5k depending on sea state. Motor sailed nicely at 2000rpm, burned 1/2 gal/hr.
    Your engine is 50hp at 4000 rpm. My opinion is your boat should be turning more rpm, so I think its over propped as is - or something else is going on.
    More pitch equals more load. So your max rpm will be lower if you add pitch. Not good for the diesel. Quite listening to your buddies. They have it backwards.

    So you have gone "off the reservation" by changing the trans gear ratio. Here's the specs on a 383/4 from the owners manual on this site. So your shaft is spinning faster than OEM (more load) and the engine can't reach the rpm Morgan wanted the engine to turn with one less blade on the prop! You need less pitch to unload the engine. Contact a good prop shop or Michigan propeller. They have simulator software. I'd target your max rpm (3-3600) based on the Perkins manual snip below.
    Good luck
    Dave

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  4. Travis

    Travis Member

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    Dave,

    Great info, thanks. Lots to digest. I did not realize the 383 had a 3-blade prop. In the 382 Morgan owners manual available for download, it says 2-bladed, but doesn't give any info about the prop specifically as far as I can tell. At the top of the page, it says "Effective with Hull 113 on". Perhaps it changed more than once even? I notice the prop specs seem to be printed in your manual, but not in the Perkins owners manual or the 382 Morgan owners manual linked in the references section. Here is all I could find.
    Morgan propulsion.png
    Perkins 4108 specs1.png
    Also, now that I think carefully about your 383's reported performance at various rpms, we really aren't far off of that at all. I rarely motor above 24-2500, and we see a steady 5.5kts at that engine speed. I imagine at 3000, I would hit 6+kts. I was under the impression that the Perkins motor is only to be driven at higher rpms for a short period of time, but the manual says you can run at peak rpm for an hour before it needs a rest. Wow!

    Anyhow, if we have the logic backwards as you say, I’ve got some more learning to do. The logic is not very intuitive. I would have guessed that with the assumption that there is available torque, more pitch = more water passing the prop at a given rpm = higher boat speed. When you say “max rpm” do you mean the maximum effective rpm at the prop will be lower? Or do you mean that with a larger prop, the engine’s max rpm would be lowered? It has to be the former right?

    I agree that having it modeled by a pro is the right way to go about this. Maybe I’ll give those guys in Michigan a call. Do you recall how much they charged for that sort of advice?
     
    Last edited: Mar 8, 2020
  5. terry_thatcher

    terry_thatcher Terence Thatcher

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    Two bladed props are much less effective than 3 for powering. Only advantage is when sailing--less drag. I have the 16x11 3 blade standard prop (because I haven't spring for a MaxProp as have some members.) I cruise at about 6 knots or slightly under when powering at about 2000 rpm with a clean bottom and no appreciable wind.
     
  6. dave_a

    dave_a Dave Ahlers

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    Coy, I believe Morgan purposely over pitched the 3 blade slightly to reduce max engine speed to 3-3500. Its a common practice with diesels to make them work hard, it's good for them to run hot. But overloading it so it can't turn proper revs can shorten engine life.

    So - I enjoyed the 3 blade's great power. By freewheeling the prop I'd pick up 1/2 knot under sail. Which is ok with a Hurth gearbox. It would plow thru any sea. I did a fair bit of "A to B" cruising, not always under sail. Perhaps you might find a prop shop who can help find a 3blade to work with your trans ratio. Get you up the St. John on an outgoing!
     
  7. terry_thatcher

    terry_thatcher Terence Thatcher

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    Dave, I used to have a shaft alternator, running off a free wheeling pro. My ZF Hurth manual not to do that and I removed the alternator, which meant I could no longer generate power that way. Is there something, with proof, that demonstrates that following the manual is unnecessary? Love to get the amps back.
     
  8. dave_a

    dave_a Dave Ahlers

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    I'll look for my documentation on that Terry. It was in a H/Z-F document if I recall correctly. It also said if you wanted to lock the shaft you could put the trans in one gear...but not the other. Can't remember which one tho'.
    I'll look.
     
  9. royaltern

    royaltern Bert Willett

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    I believe the manual says to put it in reverse to lock it,and I also remember that it was ok to free wheel.
     
  10. Travis

    Travis Member

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    Terry,
    This is the thread I trust the most on the issue of the HBW-10 transmission gear position under sail: https://www.morgan38.org/morgan38/index.php?threads/hurth-transmission-freewheeling.9949/#post-94677

    Turns out it was started by you almost 19 years ago!

    Key bits I relied upon:
    Given the above, I don't see why an alternator on the prop shaft would be an issue. I might have to look into that idea myself...very attractive idea. Terry, do you happen to have any photos of your installation? How much power were you able to generate?
     
    Last edited: Mar 11, 2020
  11. dave_a

    dave_a Dave Ahlers

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    Here you are Terry, file attached. Page 30 to save the trouble
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    Attached Files:

  12. terry_thatcher

    terry_thatcher Terence Thatcher

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    I am properly chastened. But I took the alternator off, having somehow concluded it was a risk. Maybe after my transmission seized up while under power.
     
  13. dave_a

    dave_a Dave Ahlers

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    You are wearing the cutlass bearing, but who knows how much?
    My concern with Hurth has been the small oil capacity. My 150 holds 1/2 quart. Pretty small capacity in my opinion, so I used a Mobil 1 synthetic and changed it every few years. There is an accesory oil cooler that bolts to to the trans side, and runs coolant thru it.
     
  14. Coy McDonald

    Coy McDonald New Member

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    The prop that I have is a fully feathering prop so freewheeling should n
    The previous owner suggested cruising at about 1800 rpm but she had a cooling issue when I bought her so that may have been his reason. I have since serviced the heat exchanger and changed some hoses and seem to have solved the problem. I think that I may still be wary of higher rpms because of all that. It is a fully feathering prop so I shouldn't be dealing with any freewheeling. Based on your input I will push up the power next time I have her out and see how she responds. Thanks.
     
  15. dave_a

    dave_a Dave Ahlers

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    Is your pitch adjustable on your prop?
    With a clean heat exchanger, exh. elbow and a good impeller you should be able to run 3000 w/o a big temp increase.
    20 degs increase, maybe less.
     
  16. Coy McDonald

    Coy McDonald New Member

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    I strongly expect that is is adjustable but I am not in town to confirm that right now. I am looking into changing the prop and have one quote already.

    Optimums - Variprop GP–16” x 12 x 4-blade RH – 7.6Kts, Blade loading 92%, Thrust 360lb - List $5570.00 Winter Boatshow special $4150.00 for order placed by March 30th.


    Variprop GP–17” x 12 x 3-blade RH – 7.3Kts, Blade Loading 104%, Thrust 334lb - List $5121.00 Winter Boatshow special $3770.00 for order placed by March 30th.

    That is based on 3000 rpm.

    That is a lot of money but I am really tired of going so slow. I am still trying to educate myself and I have noticed that there are many differing opinions out there. When I get home I will read up on the engine.
     
  17. john english

    john english Member

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    I've' run a 4 blade Variprop for 15 years. with a Yanmar. It cavitates twice as bad as a 2 blade behind the deadwood. So noisy that we now run at at 1920 rpm making 5-1/2 to 6 mph (not knots) Good fuel economy. More throttle just makes more noise and buries the stern even though you go faster. They claimed it would power a tugboat but they didn't mention cavitation. Not really a fan of a flat bladed prop but when I hit a log a few years ago one of the blades suffered a distortion but i was able to restore it using clamps and wood.blocks.
    I played around a little with the pitch but to no avail. When sailing at hull speed there is a noticeable vibration if you put your hand on the prop shaft
     
  18. toph3r

    toph3r New Member

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    The 382 that I looked at had a 16" MaxProp 3 blade, pitched at 24. It was a little over-pitched and should probably be a little lower, it was a couple hundred RPM below listed max while full throttle. Prev. owner said 18 was too low, so probably 20 or 22 would be the right pitch.

    Edit: This boat has the 27hp Yanmar
     
    Last edited: Mar 18, 2020

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