Dunphy X-55 fine tuning

Hi folks,

I noticed there is are some Dunphy X-55 owners on this site, more than anywhere else, so I'm hoping I can get some info about this boat I've had for a couple years.

I am currently getting about 43 mph top speed with a bit of porpoising, what feels like a plowing porpoise rather than a bow up porpoise. My quest is to get it up to the 50-55mph advertised top speed.

It was restored about 15 years ago, and while is mostly stock running gear, it's hard to know what is, and is not original.

Here are the specs.
Prop 12 x 13 not original. Original prop was a Dyna Jet 12x15 cupped. Currently sourcing a new prop. Sent a request to michigan wheel. Thinking about the DJX 12x16 cupped.

Drive angle 16 degrees. Strut part number is D-160 cast into the strut, so guessing this is original, but who knows.

Engine appears to be in the original location based off the engine mounts. There is room to mount the engine lower, So with a new strut the drive angle could be reduced. There is only 1.5" of prop to hull clearance already at this steep angle, so that makes me think 16 degrees is original.

Other issue I have is poor maneuvering control at idle. Rudder turns to an angle of just shy of 40 degrees either direction. I can move the cable to the inner hole on the rudder shaft and get probably 5 more degrees out of it. But the rudder seems maybe a bit small, and doesn't extend down as far as I would think it should. It also extends beyond the transom and so kicks up a bit of a spray during high speed turns and will even ventilate and lose some control in sharper high speed turns. Rudder has 2600-26D stamped into it.

Thank you for any input, and I'll keep you posted on how things go fine tuning this boat.

Thanks for the reply. My boat is far from original, but it does have the YH carbs and original trailer, so that's a plus. I have the original wheels and tires for the trailer as well, but they aren't made for Texas highway travel speeds.

Thanks for the reply Pete. I appreciate your input. Boat started in WA and moved to ID where I got it. There are 50 hours on the meter which appears to have been added during the resto. I found it around 2013.   My guess is resto around early 2000’s.

Regarding the RPM. Somewhere between 3000 and 4500 at WOT. I love old gauges. Except for temp which is way too accurate usually on most old stuff, and will drive you crazy. I’ll take an idiot gauge for temp any day.

I’m installing a new tach where an aftermarket pitot tube Speedo is currently. I use my phone for speed.

I’m having fun seeing what this beauty will do. I’ve driven plenty of inboards small to large, and this is definitely the worst. I’m more confident maneuvering a 50’ sailboat than this.

Based on the restoration, which is top notch wood work but lacking mechanically, I question any of the geometry of the work. Not that I’ve found anything terribly out of sorts, but a few things have made me want to start from scratch rather than analyzing top down. It’s a lovely machine, and quite the hotrod of a ski boat. I look forward to perfecting it.

If someone wouldn’t mind checking the casting numbers on their strut and rudder, I would be ever so appreciative.

8122pbrainard wrote: Jon, One note regarding the original 12x15 Michigan's is they were modified by grinding a concave relief in the hub. I still run the original and there is a noticeable difference from the stock 12x15 Michigan.

A picture is worth a thousand words right here Pete

It’s like the rudder isn’t in the prop wash at all. There is no scooting the rear of the boat left and right. Basically requires a few + mph to start turning. The tightest 180 I could manage to pull was about 20-30 ft radius with lots of fwd and reverse. Pretty much the same radius as if I go full rudder and idle it.

Regarding the tach, it’s very bouncy. Should just say fast and slow, as that’s how accurate it is.

I too want to see a picture of this prop!

I'll get down to the boathouse later today and see if I can get a better picture that shows the modification but the concave ground out area is on the hub forward and into the blade just below my thumb. My understanding is Dunphy had a local prop shop in Oshkosh Wi. modify their props. The area ground out is were under some conditions there's cavitation that the modification eliminates. I have seen evidence of it on a couple props where the hub is eroded in the area.



With the stock 12x15 Michigan, speed and RPM (by a couple hundred) is reduced. When I first noticed the reduced performance, I took both props to a prop shop thinking there was a slight difference in pitch between the two but, both matched up with the 15" pitch block. He was the person who told me Dunphy had their prop shop do the modification. Asking him if he would grind out the area, he said no feeling the grind would reduce the strength. The unmodified prop went on one year and it ended up throwing one of it's blades. Concerned with the modified prop doing the same, I had it X-rayed. Not knowing what a good or bad prop should look like, I sent the Xray to Michigan Wheel. Then I called and talked with Michigan's chief engineer asking him what he thought. His response was he didn't know because he had never seen an Xray of one of his props before!!!!

Ken,
The ground out area is behind the Photobucket banner.

Pete's probably out swimming with his phone to get a picture of his prop but if you click on the link, you'll find some pictures of his strut   

link

I've taken some pics and will post here. Your rudder may not be OEM. Where your measured 8.25" deep, original is 9.0", and your width of 5.75" s/b closer to 6.5".
Confirming the 16 degree strut angle. ( D 160)

Gary S wrote: Your funny , Pete with a phone with a camera. He’s out trying to find film for his Polorid

That is funny.
I pictured him swimming with a pencil and some tracing paper...

Thanks for the photos guys! Definitely looks like my running gear is not original. Did you see any stamping on your strut?

Nose isn't planting so much as it's not hovering at speed. Feels that the porpoising is more from the water pushing the nose up, rather than nose too high and gravity pulling it back down. On an I/O it would be more like too much down trim rather than too much up trim. Definitely not to the point of danger, or digging in. No speed loss when it comes down. Just an annoyance.

I am going to track down my angle finder and measure the pictures. Should give an idea of the original shaft angle. Tonight I will photograph my engine mounts and maybe we can compare that as well.

Regarding the rudder, that explains alot! Mine is too small.

Also, I noticed the stock rudder has a good bit of surface in front of the shaft, where I have very little, which is definitely affecting my low speed maneuvering, as it isn't catching enough prop wash.

Extratx wrote: Thanks for the photos guys! Definitely looks like my running gear is not original. Did you see any stamping on your strut?

Yes, barely legible "D 160" on port side.

Jon,
Taking another look at my strut angle, I'd say Brian's 16 degree measurement is correct. Go by his and not the angle I came up with.

Wow !! Didn't know that they were that fast .