Vibration Affecting Prop Slip

[Fastjeff57]

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

[dave hensel]

Jeff I would say yes. Plus this vibration you are talking about? Could be robbing HP. So you may be running less HP. to the prop with 30 pounds more to the boat? That may add up to your 5 MPH speed lose?

[Fastjeff57]

Thanks for the input. SOMETHING is slowing the thing down. The bigger motor makes a lot more power--it rockets onto plane--but goes no faster as it's vibrating away.

Surely someone out there must have built a conversion (or run an out-of-balance prop) that caused vibration and cost speed.

Jeff

[pdt]

the only difference can be the weight.
if the rpm is the same and lower unit is the same with same prop it can only be weight.
vibration can kill the revs but not the speed !!! if it does say 5000rpm non vibrating and 5000 when its vibrating it will still be same speed.
so it must be the weight difference.

but 30 pound stealing 5mph seems a lot.
what engine is it that loses the speed and what was the other one ?
to be 30 pounds different weight must be totally different engine block.
unless short shaft v longshaft with power trim etc.

vibration can rob horse power, but if the revs are the same it would not be slower.
is it the same tacho being used and the same boat ?

[Fastjeff57]

Should have added more details. Sorry.

The "small" motor is a very strong running (old type) Merc triple with external reeds. The "big" motor is a Merc 850 four cylinder ported to Merc 1500 specs. Both are mounted on Mark 55/ Merc 450 type towers with a non-jet prop LU (1.64:1 gearing, long shaft). Boat is a relatively heavy, 9 footer with a deep vee hull (18 degrees dead rise).

Jeff

[JohnsonM50]

I'll agree, HP is merely a # we hope to achive. When you calculate potential speed its the RPM's that count. A vibrating prop would probably have to vibrate alot to let speed slip away. My 1st gearfoot adapt attempt was slightly off, this caused the foot to loosen [almost fall off] by the time I noticed & it didn't lose noticable speed, just shook hard. The holes became elongated & the nuts unturned. Before that it seemed tight, not more vibration than would be expected from a fish motor hop up. Steering [main pin] loosness might be if you have it another story, one of mine wore excessivly & the vibration unleashed seemed to hold the boat back.. another ?? to address?

[pdt]

are you trying to tell people that you have a 85+ hp engine on a 9ft boat / if so then I don't believe you.

with your mod this must be putting out over 100hp at the crank and around 90 at the prop. on 9ft long deepo v hull ha ha ha ha ha

please post a pic or 2 of this, we realy need to see this.
or do you mean 19 footer ?

[Fastjeff57]

It's a 9 footer. Since it's used as a dingy, hanging off my cruiser, the construction is extra rugged (read, heavy). THe "smaller" motor, te external reeds triple, can be seen in the background (with the candy red hood).

Jeff

[pdt]

that has to be the widest 9 foot boat ive ever seen.
my 30hp twin looked bigger on the back of my 9ft 6in boat than that engine on your boat.
is this 6 feet wide ?
and please show us a side on pic so we can see this mighty midget.
it must be good for 60mph with all that power.
can I ask why you didn't go for a later mid and lower end ?
although this does look pretty cool like this.

[Fastjeff57]

Thanks! It's based on Bob Dillon's Mini-GT design (though I had to shorten it a few feet. Long story.)

Here's a side view with the triple on the back.

Jeff

[JohnsonM50]

It does look cool but I'd guess 60 a tall order as deep as the prop is running. 50 is a big hurdle for non aired out designs w/non surface riding props but wouldn't be surprised it its there & no doubt has acceleration.

[pdt]

I do like that red top cover, and it don't look wrong on the older bottom half.
I see the boat was just cut off and shortend at the back, this explains why the transom looks so wide.
looks a bit longer than 9ft though !!! but this must be a quick little rig.
I would think the bigger block is causing the back end to dig in too much, and this can also cause vibration !!! ive had that before when over powering small boats.

ive seen other pics of this top cover and im amazed just how good it looks, is it fibreglass and did you mould it yourself ?

would be nice to get it all smoothed out, as it does look a very cool engine, and the bottom looks in great condition.
I would say its a weight on the transom issue causing the speed/ vibration problem.

have you tried it by putting say 20 pounds in the bow to counter the extra transom weight / if it stops a bit of vibration you would atleast know your problem.

I had a very tiny boat years ago with a 6hp on the back, I took that off and put on a very good 15hp and had similar problems, I then put ballast up front and the boat was a lot quicker
and it did not vibrate anymore.
just a thought.

[omcstratos]

It's a 9 footer. Since it's used as a dingy, hanging off my cruiser, the construction is extra rugged (read, heavy). THe "smaller" motor, te external reeds triple, can be seen in the background (with the candy red hood).

Jeff

What's the copper thing at the cav plate for??

[pdt]

I bet that's what he uses for running it up without a water butt
sort of water inlet to the prop place that water normally gets pushed up to.

just a guess

[OUTBOARDER]

30 lbs over weight cant air out

[Fastjeff57]

Thanks for all the nice comments!

The hood is a fiberglass one off. I've worked with that stuff for decades. (See photo of the kit cars I developed.)

That copper line goes from the water pickup to the pump inlet. I located the pick up on the left side, so the RH prop flings the water into it. Must work, for that big PH stays dead cool with the small Mark 55 type impeller.

On running attitude...For some reason I can't get the bow of this boat to run up. It'll porpoise a bit with all the motor trim out it will take without cavitation at part throttle, but the bow drops and stays down whern I mash it all the way.


Still learning after 50 years of running little boats!

Jeff

[pdt]

the boat will be bad for running attitude, the running suface has been taken away when shortened.

lots of people have shortened boats by enough to lose the running suface, when looking at the side on pic this looks like that is whats been done.
was this boat about 15ft before it was cut ? or maybe even a 17 footer ?
you have the deep v part but not the rest of it.

I had a model boat and done very similar thing to it, it was brilliant at 5ft long yet at 3ft was waste of time and just would not run right.
probably find a setback might put it right, not jacked up set back just a foot or so to see what happens.

does this also kick up a lot of spray at the transom ?

[MWhite]

It's a 9 footer. Since it's used as a dingy, hanging off my cruiser, the construction is extra rugged (read, heavy). THe "smaller" motor, te external reeds triple, can be seen in the background (with the candy red hood).

Jeff

Jeff I know where there is several of these motors, like the ones in the picture that are for sale they have white cowlings with mercury on them,, just guessing from the 50 and 60s model complete motors, blocks, cranks u name it,,, the owner past away and his wife is wonting to sale all of it. Hes got some ss25 merc with stacks on them ready to run, theres a lot more, any one interested in this kind of stuff let me know.

[Fastjeff57]

The boat was designed (by Bob Dillon) as an 11 foot 11 incher. I shortened the BOW end by making it very blunt so it ended up 9 12/2 foot long. The AFT end is as he designed it.

I'm up to my ears in old Mercs, but the guys at Johns Old Merc site would love to kow about those Mercs

Jeff

[JohnsonM50]

1 Q remains, how fast has it gone so far?

[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

[JohnsonM50]

The automotive driveshaft is likely hollow & has a significant DIA compared to the solid outboard d-shaft & would be more sensitive to balance. The outboard shaft needs to be straight & spin true. The motor, mid & gearcase mount surfaces have to be parallel & aligned as well, there's little tolerance for error there since of course if you weren't looking to run faster you wouldn't do it.

[Fastjeff57]

Too true! And that wasn't the case with the adapter I used. The next one will be far more conventional and should work fine.

Thanks!

Jeff

[pdt]

I do like to read about these types of things, it shows great ingenuity and good old fashioned determination.
plus the end results are nearly always pretty good and turn normal old stock motors into great little engines that go so much better all round compared to just race motors out the box.

keep up the good work guys.