260A/250 Outdrive Prop Selection

J

jeffnick

Senior Chief Petty Officer
Joined
May 24, 2004
Messages
695
My Land N Sea has a Volvo 260A and an Aquamatic 250 outdrive. I'm turning a 15 X 15 LH OEM 3 blade aluminum short hub propeller as witnessed by stampings on it. It has a few pits, but no bends, cracks or other deformations. Took her out today for her first run (fully loaded) as her new owner and I can only spin her to about 4100 rpm and she won't get up on plane, maxing out at less than 15 mph. Literature says she should do over 25mph.

The old motor was a Chrysler 273 but they upgraded to the 260A. I assume the old propeller/drive were kept.

The boat is 28 X 8 X 6500# and has old (somewhat irregular) bottom paint, but no growth.

Nearest I can tell by counting rotations, I have a 1.61:1 ratio outdrive.

Would a 13p solve my problems, or is there something else to consider?

duck.jpg
 
M

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Re: 260A/250 Outdrive Prop Selection

Something is not right with your numbers, or the prop hub is slipping bad.

4100 rpm at 15 mph with 1.61 ratio calcs 58% slip at 15 mph. Normal slippage is 11-12%.

If your numbers (RPM and MPH) are accurate then I think the prop is suspect.

Here is a link to a basic prop calculator, just realize that it is only as accurate as the numbers that are input ....:

http://www.rbbi.com/folders/prop/propcalc.htm

You can post your problem in the "Prop Questions and Topics" section also, lots of prop sizing and prop problem solving help there.
 
M

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Re: 260A/250 Outdrive Prop Selection

Oh yeah, thanks for the pic, very interesting boat!
 
M

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Re: 260A/250 Outdrive Prop Selection

That is an enjoyable blog! We have a daughter that just got diagnosed in the last 2 years with MS, she is 31 now, so we empathize with your trials there.

What I think is happening is, based on that much engine rpm and going so slow, is that the prop itself must be slipping around its' hub.

Here is why I think that...:
I followed a boat story about a nice 25 foot cuddy that just could not stay on plane, and could not get to 25mph on a good day. The owner sold it like that to a friend, this friend was the member of the boating site and was posting the story and progress. The friend knew something about boats and thought the prop was ok, and (like me until then) figured once a hub spun the prop would not get any "traction" and the boat would just not move. After much thought and frustration and posting interaction on the boating site he got a new prop anyway and that fixed the whole problem. It launched like crazy and got up on plane nicely and could cruise at 40mph if I remember correctly.

The only time I spun one the boat would start to move then the engine just revved and the boat quit moving. That was the basis for me thinking once one spun a prop would not do anything but spin under any kind of load. After following the story I just typed in I had to change my view on this.

I think the 15 pitch is right but that the prop itself is not delivering. You can visually check for a spun hub but it is best to mark the prop and do a water test. You can perform the water test somewhat easily by removing the cone then marking an X from one edge to the other acoss the centerline of the prop making sure to get the hub and outside edges, then installing the cone again and try to bring it up on plane, then take it out of the water and see if the X is exactly as you marked it.
 
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