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
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
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)
Mike
from what I've read, prop size and pitch is more art than science.....
I wish you luck
Bill
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 .
Mike, just curious - did you know those figures off the top of your head?Originally Posted by Mike Goodwin
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)
You need a prop puller , looks like a gear puller .
You can make your own if you cant find one to borrow.
2 pieces of 1/2" all thread rod about a foot long , 4 nuts and washers .
2 pieces of heavy metal , one with a U shaped hole cut out of the side and holes drilled for the all thread near the ends .
I draw it for you after dinner .
The size and pitch of the prop are embossd on the hub , something like 9x9 or 8x7 , look for it .
Figures aren't on top of my head , nothing is anymore , but the math is simple enough and I'm good at simple math and I know a knot is 6080' , but where are my car keys?
Last edited by Mike Goodwin; 12-05-2005 at 12:51 PM.
OK, quickie sketch .
Put all thread through small holes , notch goes behind prop and plate with no notch goes against end of shaft. Tighten evenly on nuts until you have a good strain then whack it with a wood/leather or rubber mallet ,tighten some more and whack , repeat till it comes off .
any ?
Whack the puller , not the prop !
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.
Prop: 13x11Originally Posted by bill@ariel231
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)
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 .
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.
Mike
Totoro (Sea Sprite 23 #626)
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 .
BTW , the Vetus coupling doesn't add any length to the drive train .
Mike, thanks as always!
I'll get back to you in the spring on this one.Originally Posted by Mike Goodwin
This is where I started. My local boatyard said the same thing and I was planning on downsizing until I ran the figures through the prop calculators.Originally Posted by Mike Goodwin
I suppose one route to go, after rewiring the tachometer, is to find a smaller cheap prop somewhere and see how that does next season. I'd hate to muck it up so bad that it'd force a haulout to replace the prop - the season is short enough as it is.
Thanks again.
Mike
Totoro (Sea Sprite 23 #626)