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