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  1. #1
    Join Date
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    Inboard Issues

    A couple of inboard related questions:

    1. Shaft alignment: I had my shaft log redone last summer and a new prop shaft installed. Can I assume that it was aligned properly at that point?

    Quote Originally Posted by Mike Goodwin
    If you cant get it just right , get a Vetus shaft coupler . It allows alignment to be several degrees off... Problem solved , but not cheap!
    2. Also, I'm a little concerned about engine vibration adversely affecting the new shaft log. Is this "Vetus shaft coupler" the answer? Will it add to the length of the new shaft and necessitate shortening my $$ new shaft $$ to accomodate the coupler?

    3. My boat has an old raw water cooled Yanmar 2GM. I believe it is overpropped because I can only run the RPMs up to about 2100 under load and the engine is rated at 3600 RPMs. I'm planning on contacting a local boat shop to check into having the prop repitched. Is there anything else I should do about this?

    This stuff is all new to me. I'm gathering all the info I can this winter so I can figure out where best to allocate my boating $$ for next year.

    Quote Originally Posted by Mike Goodwin
    BTW , I have a small Vetus coupler tucked away in the far reaches of my shop . I could be talked out of it. The one I have is good for minor mis-alignment and vibration control . I'll post the spec's when I find it and dig it out of the parts locker .
    Mike, let me know - I may be interested if it makes sense for my set up.

    Thanks!
    Mike
    Totoro (Sea Sprite 23 #626)

  2. #2
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    Mike

    do you know the pitch of your current prop (and maybe the transmission ratio or type?)

    The prop's current pitch may be stamped on the hub.

    bill@ariel231

  3. #3
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    Bill, I have no clue. If it's not too frozen this weekend I'm going to try and get out there, take some measurements, and remove the prop. I spoke with a local boatyard today, and he said it sounds like I have way too much propeller. So I'll probably take it there and have them take a look.
    Mike
    Totoro (Sea Sprite 23 #626)

  4. #4
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    Mike

    from what I've read, prop size and pitch is more art than science.....

    I wish you luck

    Bill

  5. #5
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    Sizing a prop isn't that hard to do . If it isn't turning up the rpm's then it is too big or too much pitch or both . It is probably too much pitch .
    In a perfect world a 12" prop pitch would try to advance the boat one foot per shaft rpm ( not engine ) .
    So if you are trying to hit 6 knots ( 36480' per hour ) with a 12" prop , you need to make 608 shaft rpm's ( with no slip ) .

    I had that motor, I think, 10hp Yanmar and it would not swing a 9x9 prop .

  6. #6
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    Quote Originally Posted by Mike Goodwin
    So if you are trying to hit 6 knots ( 36480' per hour ) with a 12" prop , you need to make 608 shaft rpm's ( with no slip ) .
    Mike, just curious - did you know those figures off the top of your head?

    You're talking to a guy who can't even get the gd prop off! Having had a new shaft installed this summer, I figured it would be a simple thing. But I tried this weekend, and tried again, and tried some more... I removed the nuts on the shaft, then tried hitting it with a rubber mallet, rotating, whacking it, rotating, whacking it.. I tired putting a block of wood against the prop by the shaft and whacking that, ad nauseam. I ended by spraying the thing down with WD40. I hope that's not bad.

    Got any tricks? A quick web search tells me I could use a "prop puller", whatever that is.

    I'm trying to remove it so I can take it a boatyard and have it looked at.
    Mike
    Totoro (Sea Sprite 23 #626)

  7. #7
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    Yikes! Mike, thanks for digging it up. Would this coupler add to the length of the shaft and necessitate shortening my $$ brand new $$ shaft? Frankly, it sounds nice, but I'm thinking I should probably have a local yard take a look at the alignment and see if it is even an issue, and spend my dwindling boat funds on necessities right now.

    Quote Originally Posted by bill@ariel231
    Mike do you know the pitch of your current prop (and maybe the transmission ratio or type?)
    Prop: 13x11
    Transmission ratio: 2.62

    I was able to borrow a prop puller, and the whole process was anti-climactic really. It just kind of came loose once I had tightened the nut down. After I cleaned off some of the bottom paint on the prop, I could read the above stamp.

    Curious thing: after entering my data into a couple of prop calculators, they came back with a LARGER prop recommendation! Also, a very helpful guy at Michigan Wheel said that this prop is about the theoretical size for my given parameters.

    The only thing I can think is the tachometer is malfunctioning. Is it possible to have a "wrong" tachometer that doesn't read the RPMs correctly?

    Could it be something else? Am I missing something?
    Mike
    Totoro (Sea Sprite 23 #626)

  8. #8
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    You want to reach hull speed @80% of max rpm {1065.6rpm shaft}( for many reasons) and you want the boat to advance at the same rate as the prop .
    So with a 13x11 prop at 1065.6 shaft rpms you are trying to push the boat at 11.1mph ( twice as fast as it should be ).

    With a 13x11 prop turning at 1332 rpms (shaft) , you would be doing close to 15 mph or 14 knots + or - OR in order for you engine to max @ 3600 rpm with a reduction of 2.62 giving a shaft speed of 1332 rpms the boat would have to be going 14 to 15 mph .

    I would say you need a 13x6 or 12x6 or 10x7 . You have too much diameter and too much pitch for 13.? HP which you only reach at 3600rpm , at 2100 it is much less. Your peak torque curve is near 2880 rpms ( engine ) and a 13x6 would put you in that ballpark .

    Too much diameter is hard to turn over and too much pitch is just as bad.
    I bet your rpm's drop radically when you put her in gear at idle , almost to a stall .

    Will your tach read above 2100rpm when not in gear?

    This is a 2 blade prop , right?

    When you talk with anyone , make sure they understand it is 5000lbs you are trying to push to no more than 6 knots .

    If your prop is too big and there is not enough 'tip' clearence , you will get vibration as the prop passes the hull . You need aprox. 18% dia. in clearence, 20% is better, so your tip should not come within 2.5" of the hull .

    It is a big ballancing act to get the prop right .

  9. #9
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    It's a Yanmar 2GM. 13HP, 3400 RPM, and 15HP at 3600 RPM max.

    I also have a three bladed prop. (See pictures)

    "Will your tach read above 2100rpm when not in gear?"

    This is the million dollar question for which I am ashamed to admit, I have no answer.

    I didn't realize I was in trouble until after the season and I was reading the manual while winterizing the engine. I'm pretty sure it does - at least the engine revs pretty good when in neutral. I just never looked at the tachometer while doing it.
    Attached Images    
    Mike
    Totoro (Sea Sprite 23 #626)

  10. #10
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    Say no more, I solved the problem

    You have one blade too many on that prop . No way you can turn a 13x11- 3 blade with 13 hp unless it was in a 400lb skiff . At 2100 rpms your are not even getting 13hp. That is a huge prop for a small boat .

    My figures were for a 2 blade 13x11 , that extra blade needs more hp to spin it.

    It is probably a motor boat prop too , and has huge surface area .

    Get a 12x6 sailor prop (2 blade ) and you will be happy for the rest of your life .
    You will sail faster too (less drag )
    1216 shaft rpms should give you 6+or- knots with a 12x6 ( 6" pitch ) prop , so if you throttle back a little 5.5 knots is easy .
    You know you can have props re-pitched , so get a 2 blade that is close and try it and if it is not exact either pitch it up or down until you can get 5.5knots at 2800 rpms .
    There are a lot of other factors involved , but this gets you in the ball park .
    I have re-proped dozens of boats , half were right on the 1st try and the others took one more pitch change to get close enough . planing motorboats are a lot harder to get right .

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