Prop/boat matching???

[Dan-Q]

Hello all!

I'm having a problem matching a prop to my "finally" finished runabout project.
It's a 1954 Hunter's Marine 18' mahogany runabout with inboard engine and thru-hull shaft.

I repowered with a 1964 283 Chris Craft engine that I had totally rebuilt. The engine now has about 14 hours on it.
I don't have any specs for the engine and I omited to ask the company that rebuilt it.
I estimate that 4000 RPM is probably real close to the red line.
Weight is around 2700#. Shaft is 1".
I've tried 3 props. The 13"D x 9"P that came with the engine is no good for me. It was pushing a 32' CC Cavalier. But can I say that there ain't no foolin' around when I whant to get on plane!!
Next I tried a 13"D x 12"P prop that gave me 35 MPH (mesured on GPS) at 3600 RPM. That was the max RPM I was able to get that day even though I still had some travel left on the throttle.
About an hour later my mechanical fuel pump blew so I do not know if the 3600 max RPM was due to fuel starvation or was that the max the engine could do with that prop!? (The next morning I left 1/2" of one of the blades of that prop on a damn sidewalk while trailering!#"@?%!)

Then I tried a 12"D x 13"P prop that was too unbalanced to lend a proper conclusion.
The 13" diameter prop tips were passing as close as 5/8 of an inch from the bottom of the boat which I think is too close (they eat away at my bottom paint thrue cavitation!)

I've tried a couple of "prop calculators" on the net but I find that they do not take into account all the parameters of a boat.
The best one so far says I should have a 11.9"D (lets say 12") x 14.8"P (15") prop.
My question is ; can a 283 C.I. (rated apparently at 185 HP) turn such a prop at 4000 RPM?

I sure would like to hear from runabout owners what propeller sizes you have on your crafts.
Just say HP, LOA, #, D and P.
That would give me an idea.

Thanks to all.

[imported_Conrad]

Hi Dan- I've had a number of similar boats. First thing is to drop down to a 12" prop- tip/bottom clearance should be at least 10% of prop diamater, and 20% is better- 12" will just barely meet the criteria from what I can tell. Then, given the stats you quote and asssuming a 1:1 drive, a 14/15" pitch should get you to 4,000 rpm and about 40 knots.

Last boat I had was similar size/weight, had a Ford 289 with 2 barrel and a sport cam, I figured about 200-210hp. 12x14 three blade with cup moved the boat at 43 knots on a cool day at 4300 rpm max, warmer days saw 4100 and about 42 knots by pitot tube speedo.

For better acceleration if skiing, go with the 12x14" and have a shop roll some cup into the blades' trailing edges- at speed this equates to about an inch of additional pitch, while the lower starting number gets you on plane quicker. Only down side is that it narrows and raises the wake a bit, a negative for some hull shapes/skier preferances.

[Dan-Q]

Thanks for the info,
You mentioned a few things I was unaware of. Like the 10 to 20% gap between bottom and tips. Do you think I should consider 11"D? I heard that speed boats look for small diameter and lots of pitch.

Also, what do you think about the fact that I was unable to get past 3600 RPM with the 13"D x 12"P prop?

Thanks

[imported_Conrad]

Hi- the 11" prop may cavitate with the power you have when starting out hard. Could you put a spacer under your strut without making things too complicated- 1/2 to 1/4"? My understanding is that all one gains after 20% is a reduction in noise through the bottom, probably a greater issue in boats/yachts with accommodations. If you use the boat for skiing you'll feel a significant loss in bite out of the hole- blade area is reduced from about 56 to 46 square inches, or 15% and efficiency doesn't go up by much.

The 13" prop could hold rpms down by its diamater alone- nothing absorbs power like diamater, even with large reductions in pitch. Also, in your situation with reduced tip clearance the prop is having to push a lot of water against the bottom. Try a good 12"- I'm pretty sure the results will make you smile!

[Concordia..41]

First you have to know the rated HP @ what RPM of the engine. Then the ratio of the gears. Then answer all the questions asked.
http://www.miwheel.com/MIWheel/PropAnalysis/OutboardAnalysis/OutboFive.asp#dia

Dave

Sun over the foreyard

[Dan-Q]

Thanks Conrad, I'm wearing a smile already!!
You've confirmed what I have been thinking.
I think I'm going to go with a 12 x 15.
I'll let you know the results.

Concordia, thank you for the site but they speak of sterndrive, outboard and "ballistic" analysis ... whatever that is!!. I don't think sterns and outboards are equal to shafts. Maybe i'm wrong!

Regards

[Concordia..41]

Sorry Dan, I sent you the wrong link. I was buying a prop for a 50 HP Honda when I saved that one. Here is the link for inboards and sailboats.
http://www.miwheel.com/MIWheel/PropAnalysis/InboardAnalysis/InboOne.asp

Dave :rolleyes:

Sun over the foreyard.