Questions for Boat Handling Guru's....

[Ratickle]

As said, we took the Marlin out for the first time with the new engine and drive.

About the boat;

23'4"
3620 lbs. with the Big Block
Modified Vee of 18 degrees transom
502 modified. I'll have to get the dyno sheets, less than 600 I would guess with the Cutler Fuel injection and semi mild Hydraulic roller.

Used to have a 260/Alpha when I bought it.

Changed to 502 Bravo 1 Standard first

Just changed to XR big shaft with Imco 2" shorty....

 

[Ratickle]

Couple pics;

 

[Ratickle]

The drive is almost exactly 5" center of propshaft from bottom of transom with the shorty.

The prop we are testing is a 28 pitch Bravo I four blade.

 

[Ratickle]

The boat has no tabs.


Comes on plane fine, only a little cavitation, when tucked in just a little, couple of degrees.

Likes to run at 3500RPM, 55 to 56MPH, a couple of degrees maximum uptrim. Any less speed and you can feel the speed drop off as the RPM's come down, even if trimming down a little.

If I trim it up a couple more degrees, the boat steers to the right. If I trim it down a couple of degrees, the boat sets to the left.

WOT, only got just over 4800 at 78.4mph. If I trim up or down from the plus a couple degrees, drops off speed a little. But, if I keep the trim up a little, and really walk it through the goofy chine wiggle, it will come up to 4900RPM or so (did not peek at speed, it was wiggling pretty good in some weird sidewash water).

 

[Ratickle]

Once I get the numbers from the engine for peak RPM and HP, I think I need a 27 pitch or so. But, any input on prop design, style, etc?

Ther is a slight hook in the back of the hull I'll probably take out this winter. Then maybe the 28 is the way to go?

Is it normal with the higher X dimension to have more steering influence from trim? Surprised me the first time with this boat, never did it before.

Best speed we ever saw with the old drive, and a 26" pitch prop, was 82 at 6000 RPM on the chip limiter.

 

[fund razor]

Is this a scan of a 1982 Kodak 110 photo?

But don't answer now. We can talk about it later. Carry on with your drive conversation. Good luck.

 

[MOBILEMERCMAN]

No it shouldn't turn left and right with the trim. It may be because the entire prop is submerged when running. Did you run it with that engine and a stock lower? The x appears{ it is hard to tell for sure by the pic} near standard as was typical in the 80's. You should have external steering at your speeds. That could be contributing to the steering issue as well. In my experience shorties on low X actually tend to hinder performance. Because when you drag the fat part of the gearcase { the part near the mating surface} through the water the increased drag offsets the prop height advantages. In addition to that the gearcase doesn't like to be trimmed much because the crescent drives the stern deeper. Try the stock gearcase if you haven't already with that engine and see if it goes faster. It will like to be trimmed out.

My suggestion is to put external steering at the top of your list. I think you will find a higher X will give you both speed and handling.

 

[Ratickle]

The 82mph number was with the stock gearcase, basically an identical engine before we broke it.... This one is same cam, bore, intake, injection, and heads. Just a new block because we spun a bearing.

This is a 2" shorty though, the old drive was really deep......

Maybe I should watch for a 3" shorty......:sifone:

 

[Ratickle]

The X is standard, I got the layout paper from Merc when we switched from the Alpha. Actually 2 of the holes were identical if I recall correctly.

 

[Ratickle]

I'll have Terri help me with a straightedge on the bottom and take a pic....

 

[MOBILEMERCMAN]

Put a straight edge on the boat and extend it past the geacase. Then look at the side to side thickness of the gearcase where the straight edge crosses. The top few inches of the gearcase is much fatter then section below it and before the bullet. If you are dragging the fat part through the water it won't matter how high the prop shaft is.

 

[MOBILEMERCMAN]

A shorty upper would be better in your case but, I don't think anyone makes one for a stock gimble.

 

[Ratickle]

The part where it increases in thickness is 7/8" above the keel straightline.

Steering, huh????:sifone:

 

[Ratickle]

I guess 3" shortys have clearance problems for the props.....

 

[Tinkerer]

Still looks deep. I would cut the transom and raise it another 2 inches. Bad handling and sensitive to trim has been prop too deep for me.

 

[MOBILEMERCMAN]

The part where it increases in thickness is 7/8" above the keel straightline.

Steering, huh????:sifone:

The water rises and that fat part is likely in the flow.

Yes, Steering. I think anything over 70 steering is a must. 80+ absolutely.

 

[rschap1]

I see trouble that will be easy to fix and provide BIG gains easily...



Getting rid of those aerodynamically troubling neon color splotches may net a mph or 2

AND


adjusting the pitot tube pick up for the speedo will surely show BIG improvments according to the gage.

Just here to help.



BTW...
you are welcome

 

[Ratickle]

That pitot tube kicks up and doesn't work 90% of the time. I use the GPS....


Even when it's down it is amazing how different they read.....


Those splotches add speed, can't you tell that by the aerodynamic layout?????

 

[MOBILEMERCMAN]

Even when it's down it is amazing how different they read.....

Yea, Most speedos are pretty happy.

 

[Ratickle]

Yea, Most speedos are pretty happy.

On this boat the opposite is the case. GPS will be 55, cheapazz speedo setup will be 35.....