1964 Wood 110hp Stern Drive Ventilation/Cavitation Struggles

[dizuster]

I just finished restoring a '64 Chris Craft wood boat. We have had it in the family for over 50 years now. It has the very first '63 Mercruiser I stern drive on it.

After getting it in the water, I've had a ton of trouble with prop ventilation/cavitation. I started with an old mercruiser prop 13" x 15" on it (48-32390A3). Any RPM over ~2500 will take ZERO steering wheel input without revving to the moon. Even while running straight it will start to over rev in slightly choppy water.

I took a hard look at the prop, and it appears it had been hitting the driveway or something during running, because it had a sharp burr on it around the widest circumference. Probably about .050" or so.

I had another old 13" x 17" Michigan Wheel PR-157 to try, so I put that on there. It is significantly better, but still will not make any reasonable turns over about 3000rpm or so. With two people tubing on the boat today, it would not achieve full throttle without over revving either. The 13" x 17" is a dog for sure on this boat too.. 1mph loss in top speed, and killed the bottom end.

This boat has no power trim adjustments. It's currently trimmed full down on the manual plate. The cavitation plate on the drive is roughly even with the bottom of the keel (no adjustments there).

What are my options to improve this boat. Will trimming it up help slightly by getting the nose up/stern down? It was noticeably better with a person sitting at the rear of the boat today, so that gave me the idea.

Since I'd like to put another prop on it, would buying something with some "cup" in it help? The props I have are 1965-85 era, so hardly modern technology!

Please let me know if I can answer any further questions, I'm open for suggestions, and appreciate all the help I can get!

 

[dizuster]

Sorry forgot to add this is an 18ft runabout, Lapstrake wood hull construction..

 

[Texasmark]

I had a 110 in a '65 18' Starcraft alum. Very light hull for the size. I think 35 was tops with just me. It had tilt, no trim so you had to manually set the thrust angle with the tilt pin. It really needed to be the 140. I ran a standard run of the mill alum prop and had no issues (ventilation not a problem) other than CV joint knock (where the outdrive drive line enters the outdrive) in turns, getting progressively worse as the turn was increased, even after having rebuilt it.

The boat had a keel of sorts as most aluminum boats hav, which could have been a source for ventilation in turns but wasn't. I had the outdrive setup roughly 90* off the line of the hull, not tucked in tight as I recall. Not exactly related but a point that I have an association with that engine-outdrive setup.

With what you are saying about the boat and knowing the fact that the I/O mfgr sets up the height of the outdrive (dimensions) before a production run, and knowing how lapstrake boats are constructed and having run that drive setup I see no mechanism for your problem there.

What I do think and you might check for is a "hook" in the hull. Looking from bow to stern, under the boat along the centerline, a hook would have the transom area lower than the midsection of the boat which would cause the water to deflect ahead of the prop (ventilation potential) and cause the bow to run lower in the water than it should.....thought aided by your comment about moving weight aft and improving performance. I doubt a hook would be recoverable. Most develop as a result of an ill fitting trailer upon which the boat rests for long periods of time.....you did say 50 years!

Cupping on a propeller is intended to grasp water better when ventilation is a problem so yes on seeking a cupped prop. Rake on a prop is usually used with bass boats running 60 mph to help in getting the bow up , on the pad, in high trim angles...which you wouldn't be doing with your boat but with a hooked hull and setting your tilt pin farther out, might give you some advantage.

Diddling prop parameters and types, especially as mentioned above, will change the load on your engine, possibly significantly, so you need to keep your WOT rpms in mind when attempting all this. I do best when the rpms are around the max rating at WOT with the usual boat load. Going to a prop shop and doing the "try before you buy" will cost you more, but will give you the advantage of performance testing for the correct answer rather than just guessing.

 

[dizuster]

Thanks for the response. I'll try to throw a string on it to check for "hook" in the hull. Do I check that straight down the keel itself, or the first few boards next to the keel?

The boat was mostly in the water throughout it's life, but had spent time on the trailer from around 1990 until 2016 when I got it. The trailer has bunks that hold the boat, as well as a few keel supports.

I may try moving the trim up one hole just to see if it makes a difference. It's one bolt, so not a big deal to try it.

 

[Bondo]

Ayuh,...... Welcome Aboard,....... The 13" diameter makes me think you had Merc mid-sized gear case Outboard props,.....

I/O props run from 14" to just under 16",......

The hub diameter is 'bout 3/8" wider, 'n fills the space 'tween it, 'n the gear case,.....

The hub length, 'n prop shaft are the same, so in bolts on, nicely,.....

 

[dizuster]

I don't have much to go on, but the late 60's service manual shows a 13" prop for this IO. It's the part number I have on the boat. Obviously technology has come a LONG way since then... but it was a 13" to start. Most prop shops I called today suggested leaving the 13" on it, but going to a 4 blade prop. Apparently that should help the issue quite a bit... but that's coming from guys trying to sell me something too.

 

[Bondo]

Ayuh,..... Post a picture of where the prop meets the lower unit housing,.....

Is there a gap between the 2,..?? How wide is the gap,..??

 

[dizuster]

Looks pretty good, it's got at least some overlap there, which I believe indicates it has the right thrustwasher matched with the right prop design. Radial gap is only around .020"

 

[dizuster]

It has about 5/16" or so of overlap.

 

[dizuster]

Blade seems ok to me, no real signs of cavitation on this 13" x 17" prop. This prop was better then the 13 x 15 that I tried, but still had pretty bad blowout in any moderate turn.

 

[dizuster]

The 13" x 15" I took off of it had been a family favorite prop for years. You can see it's got some heavy pitting. Or has been repaired by a very bad welder!

 

[Texasmark]

Worst hook would be where the new water would hit the lower unit, aka keel area. You could probably eye ball it. Wouldn't use a string to measure. A 1x6, 10' long started at the transom and measured forward till you ran out of board. Should touch the hull all along. Check where the bunks met the hull and look for depressions.

On blow out, Rake and Cupping are made to help you hold the water.

Your prop to gear housing interface looks just right to me. No problem with that.

The casting material removal on the rear side of the blade (in your picture) looks like actual Cavitation to me. Prop usually brings that on by itself due to irregularities in the water flow across the blades....on the back side of the blade, vacuum area, and being near the tip of the blade where you'd loose most of your thrust it may be a clue and cause for your blow out. I'd say, time for a new prop.

I had a new '71 Chrysler 85 with the OEM 3 blade alum and had a screwy reverse hook in the leading edge of the blade. Rather than leaving the root at the barrel and traversing out in a smooth arc like all the other props I ever saw, right after it came off the barrel, it made a reversal for about an inch and then made the smooth arc on out to the tips. I know, the idea was to grab the water better but all it did was to Cavitate a hole in the aluminum....just ate it away...poof.... at the root of each blade where they meet the barrel. Never saw a prop designed like that.

 

[dizuster]

I was going to try a piece of wood, but I figured whatever I got from home depot would be warped worse then the hull... lol But I'll find a way this weekend to check it.

Since the 13 x15 is damaged, and the 13 x 17 was a dog anyway, I ordered a cupped 4 blade 13 x 15 prop (Mercruiser Spitfire), which a lot of prop shops and others have recommended to help the blow out. We'll see this Friday if it helps.

 

[Bondo]

Looks pretty good,

Thank you,.... You've proven me wrong,.....

 

[dizuster]

Just wanted to follow up on this. I ended up trying a new mercury spitfire 4 blade prop, and it fixed 99% of the problem.

The boat will easily make a full throttle turn now, and only really has the slightest slippage of any kind if you try to get the boat back straight too quickly.

One interesting fact though... although mercury suggested that their props are marketed to be the same pitch as a 3 blade, the 13" x 15" pitch I bought would not turn more then 3400rpm. So I had to exchange it for a 13" x 13" (which I've yet to try).

 

[Texasmark]

"One interesting fact though... although mercury suggested that their props are marketed to be the same pitch as a 3 blade."

Where'd you pick up that bunch of gobbledegoop....bs? Way back in the day when Carl was getting his company going, he knew that the fewer the blades, (above one) the higher the vibration and higher the efficiency......that's why back in the day, small engine racing hydroplanes running the Quicksilver racing lower unit they always had tiny, high pitched 2 blade props. It wasn't until the first 100 hp production engine produced came out , the 1000, in Phantom Black with a 3 blade prop he decided that that engine would be pushing big rigs and that 3 will push harder than 2 but at an efficiency loss buttttt.....forget the year but could look it up.

Data available on here from folks who run 3s and 4s, you drop one inch of pitch adding a blade if you keep everything else constant......attesting to the loss of efficiency with the extra blade.