Vibration Affecting Prop Slip

[Fastjeff57]

I was afraid someone would ask that! With the triple it hits 45 mph;; with the Merc 850 I'm seeing only 40 mph.

I know....it's a slug.

Jeff

[pdt]

45mph !!!!!!!!!
a slug !!!!!!!!!!!!!
that aint a slug....how fast do you want to go in a 9ft boat ? ha ha

I think its going to be a weight issue on the transom to get more speed.
45mph is a good speed for the engine and boat. its not like its a racing design built light.
must be a good ride when flat out

[Fastjeff57]

Thanks for being so charitable! Actually, the boat handles rough water remarkably well for its size, and is a stone blast to run around in. I have thoroughly enjoyed it.

That said, I want to "crack the 50 mph" barrier with this it before I'm too old to keep boating--I just hit 70. So...off came the deep vee bottom this week, to be replaced with a flat bottom design that's 36 inch wide in the back. Up forward the deep vee is being retained for a measure of decent rough water running. And I'm on a weight reduction kick, thinning areas that have proven over-built and eliminating anything unnecessary. The goal is a substantial weight reduction, a faster hull design, and hopefully a lot more speed.

Thanks for all the inputs!

Jeff

[Yellowjacket]

You probably should have kept the padded V hull, they are pretty darn efficient and with the power available easily could have gotten you over 50 MPH. A big flat bottom is fine, but it will pound like crazy in anything over 25 mph in a heavy chop with hull that isn't quiet 10 feet long. Don't ask me how I know....

The Dillon's are running around 50 mph with only 35 hp on a well set up rig and a good prop. Yes you are heavier, but you have a lot more power to work with, and the faster you go the better a padded V works.

I would have looked first at raising up the motor up on the rig you had. You were running really deep and that was causing a lot of drag. After raising it up you would have needed to change props, but I'm sure the speed would have been there, and it would have been a lot better ride than what is essentially a D class runabout with the nose cut back.

JMHO, but padded V's are a very good compromise. Good efficiency and a pretty good ride. Flat bottoms are fast, but ride like he!!, and eventually they pound themselves into pieces (again speaking from experience here). The rig you had wasn't bad, it just needed some tuning and it could have delivered what you wanted.

[Fastjeff57]

You might just be right! Rrrrr.

The shortened, padded, Vee hull was a death trap, chine walking like crazy unless aimed directly into the waves. (That's why I removed the pad and went to a true deep Vee.) My shortening of Bob's design was MY fault, and that's what caused the instability. (Yes, he warned me, but I had no choice.)

Guess I'll have to leave her hanging from the davits when the water is rough. It'll be worth t if she hauls butt!

Jeff

[JohnsonM50]

You probably can exceed 50 some but when your nearly there every 2mph's is party worthy. Try more props & if you dare raise the motor a bit.

[champ20B]

Has anyone experienced a situation where a motor conversion deal (motor adapted to some racing lower) had a drive shaft vibration (due to misalignment or ?). If so, was the bite of the prop affected, causing excessive slip?

I have a mystery problem where the same boat with the same prop and lower unit, but running a motor with a lot more power, makes 5 mph LESS speed at the same rpms. The more powerful motor weighs 30 pounds more, but other than that everything is the same.

Thanks for any input.


Jeff

Jeff, This sounds like a serious problem to me. If all else is the same with no visible or noticable compromising issues, you must have a problem with your new powerhead. Something is modified, or loose and/or out of balance in your engine or on it (flywheel maybe?)....no telling what. I would seriously inspect that powerhead if all else passes, being that power-loss at vibration is at hand. This can be a problem that CAN DESTROY THE ENGINE and can be VERY DANGEROUS TO OPERATE....

[Fastjeff57]

It appears that the drive shaft adapter I made is the problem. I'm building a different one that will have the connector at the bottom (vs the top). That's how guys running racing lowers do it.

Jeff

[pdt]

has the proshaft been balanced if its had any modifications ?
might sound a bit daft but it don't take much to put everything squiffy if the balance is a touch out.

ive had a small tag come of a car prop shaft and upset the whole thing, it only weighed about 2 ounces, but made the car shudder until rebalanced by a auto propshaft specialist.

just maybe something us upsetting the balcance, a small bearing not fitting exactly true could also be the cause, only need a slight bit of dirt under a bearing edge.

just a few thoughts.

+

[Fastjeff57]

Thanks for the input.

I balance my homemade props to a reasonable standard, and run them on another motor without vibration problems. It's the Merc 850's driveshaft arrangement that's causing the problem--for the moment. A cure is in the wind.

Thanks again.

Jeff