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Message |
    
rendo
Senior Member
Username: rendo
Post Number: 51
Registered: 08-2006
| | Posted on Monday, November 30, 2015 - 05:39 pm: | 
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How does one properly size a prop design and size to motor and boat? Fay & Bowen 25'11" LOA, 5' beam Special; 12hp 4 cylinder, 4 stroke Fay & Bowen LN 40: 7hp @ 800 rpm; 8 1/2 @ 1000 rpm; 12hp @1400 rpm, 365 lbs. Just above an idle, no throttle, 7 mph; lot of throttle, pushing the limit, heating up the motor, really pushing it, 8 mph. |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 1125
Registered: 07-2002
| | Posted on Monday, November 30, 2015 - 08:30 pm: |
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If it's a displacement hull 7 mph is probably hull speed, more power burns more fuel and gives little return. The engine should reach it's rated rpm at wide open throttle, I like to see it able to over run a little, in this case maybe 50 rpm. This helps to assure the engine isn't lugging. I don't think you're going to gain much, run it easy and enjoy it. |
rendo
Senior Member
Username: rendo
Post Number: 52
Registered: 08-2006
| | Posted on Tuesday, December 01, 2015 - 12:45 pm: |
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Thanks for the advice. I read an advertisement on the hull claiming 12 mph. I'd like to get close to that. The prop seems rather small for the space provided for it. |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 1127
Registered: 07-2002
| | Posted on Tuesday, December 01, 2015 - 06:57 pm: |
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On a displacement hull a large prop with less pitch works better than a small high pitch prop, sort of like a larger tire giving more traction. The rule of thumb on these old engines was a prop the diameter of the flywheel. |
miro
Senior Member
Username: miro
Post Number: 831
Registered: 11-2001
| | Posted on Friday, December 04, 2015 - 02:25 pm: |
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Doing the calculation for 25 ft waterline length gives 7.2 mph as the hull speed.
You CAN get to 12 mph, but you'll probably need about 40 HP.
Several months ago I did some tests on a 16 ft displacement hull. It's hull speed was 5.5 mph.
At that speed the thrust needed was about 30 lbs
At just over 6 mph, I could barely hang onto the force measuring guage and at 8 mph, I reckoned I wouldn't be able to hold it.
So for your hull, I would estimate that just below the hull speed you'd need probably just over 34 lbs of thrust. That's what your prop will provide to get that speed ( actually it will be slightly higher since the shaft is not parallel to the water flow). Use the cosine of the shaft angle to get the shaft thrust .
As JB said, relax, and enjoy the ride.
miro |
rendo
Senior Member
Username: rendo
Post Number: 53
Registered: 08-2006
| | Posted on Monday, December 07, 2015 - 11:16 am: |
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Miro, thank you for that information. Very interesting! I have a small diameter three blade prop but since I have the original rudder and prop protector iron under there, it provides much more room for a larger prop. Would it be worth the work to look for a larger prop as suggested by JB? |
miro
Senior Member
Username: miro
Post Number: 833
Registered: 11-2001
| | Posted on Saturday, December 19, 2015 - 05:49 pm: |
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Certainly there would no problem - all that will happen is that your engine will get loaded at a lower RPM, you might get some more speed but for sure, you'll be consuming more fuel.
It will however, make for a nicer ride ( lower RPM)
Miro |
rendo
Senior Member
Username: rendo
Post Number: 54
Registered: 08-2006
| | Posted on Saturday, December 19, 2015 - 10:03 pm: |
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Miro, thanks again. Now how does one pick a proper prop? Ie., diameter, number of blades, and pitch?
Would it help to post a photo of the current prop and its setting? How about the details stamped on the current prop?
Always grateful.
John |
