Is my motor/prop too low?

[nwcove]

Re: Is my motor/prop too low?

That seems to be the consensus, but I'm not sure what my options are unless I lock the motor in the correct position, or add a wedge/shim under the motor bracket. It seems to me that there ought to be a simpler solution, unless this boat and motor just weren't made for each other.

your top end speed isnt bad, but imho, you arent getting the full potential out of the motor due to the trim angle....the motor is working hard pushing the hull into the water rather than letting it skim across the water. how you approach the fix ( or enjoy it for what it is) is up to you.

 

[Tim Frank]

Re: Is my motor/prop too low?

FYI, for Tim Frank, here is a photo of the prop. It seems to me to be in pretty decent shape, but I'm open to any suggestions if you can tell anything from this photo. It is a 17 pitch and as I mentioned it gets the boat on plane very quickly and runs smoothly. I don't own a tachometer, so I haven't measured RPMs. If you really think I should, I may replace it down the road, but for right now it's not my highest priority.

View attachment 167033

I'm not lobbying for you to change your prop, just to do the evaluation that will tell you where you are.
What i saw in the video would be fine for me as is, but you did express some concerns about performance so you are getting lots of feedback....that is what happens in here.

 

[jleiii]

Re: Is my motor/prop too low?

Yes, I should have mentioned the trim pin is out as far as it will go. The transom has a pretty steep angle, which makes the motor seem trimmed in too far even with the trim pin set on the outer holes. Someone up above with a similar setup mentioned they put a tapered wedge in between the transom and the motor bracket to push it out further. I would like to see a photo of what that looks like and what material he used.

Here is what my wedge looks like. It's just a 2x4 cut at an angle with the bottom just under 1.5" thick. It is only temporary until I finish my tuning and adjusting. Not sure what the final material will be, but a good chance it will be these wedges wrapped in fiberglass, and sprayed with gelcoat when we finish the hull. I drilled 2 holes in each motor mount leg for SS screws to hold them in place. There are no bolt holes (or sufficient material/location) in the mount to go through the transom, and unless I find a compelling reason, I will not be bolting it. The only flats are where the SS screws are, and it's less than 1/8" thick.

The lower unit developed an oil leak (original 1960 seals), so we're 'out of the water' till I get that fixed. Hoping to have the seal kit tomorrow, and maybe installed on Sunday. If we get it done I plan to do all the tweaks & adjustments and see what final speed we come up with. My father gave me his handheld GPS over the weekend so we can get an accurate measure on speed. He reported 25 knts with the Evinrude 25 he gave me with the boat, and that's all we got in the one test we did.

The prop on the Johnson 40 is a NOS Michigan, made for that motor, 10 3/8 x 14. Compared to my bronze props on my 28' Chris, this prop is more like a rough casting.

Anyone know if a TACH (late 70's Heathkit engine analyzer) designed to read RPM from the points on a coil will work with a magneto? I'm not sure if there is any voltage sensing in the circuit, or if it's purely the count of open/close. The connection is easy - both points have a wire right to the ignition switch.

 

[JRDIII]

Re: Is my motor/prop too low?

Thanks for the view. That looks like a good work-around.

 

[jleiii]

Re: Is my motor/prop too low?

JRDIII,
Have you made any progress on your speed improvement? We were out of the water a couple weeks for lower unit seals, but our previous outing the best we got was about 25 mph, with 2 average adults (~170 lbs each), 12 gal of gas, a battery and some tackle. I did the math on the motor and prop, and I calculated (with no slip) a max speed of 27 mph at 5000 rpm. The prop is 10 3/8 x 14, the OEM spec. The original Evinrude 25 would do that, so I'm a little disappointed with that calculation. I'm hoping that the gear ratio I found for this engine is correct, 2.42:1 (1960 johnson 40), otherwise my calculation is no good.

I've still not figured out how (or tried) to measure the rpm with a magneto. My nephew is a car mechanic and has a nice meter that might have an inductive pickup for RPM, so I'm checking with him.

Hoping to go out tomorrow and do the final adjustments on the engine high speed mixture and tilt (4 fixed positions) with a GPS in hand, and maybe a TACH. Then we can get this all figured out. Still hoping for 30+.

John

 

[steelespike]

Re: Is my motor/prop too low?

Has anyone suggested the Tinytach?Reasonable easy to install and will work on any motor.
There are others available as well.can be installed or hand held. Your ratio is 2.42.
I usually suggest raising the motor to a point just before venting(cavitation) becomes a problem.
Check that the throttle opens all the way and the link and sync is right.( stator cam picks up carb roller at the right point)
While your prop is fairly uniform it appears well worn. I think a Solas Amita 3 blade will help get your goal.
What pitch may depend on your tach results.If you carefully measure the diameter you may actually find the prop
under sized. Measure from center to the tip of a blade and double it.
A prop change may allow more height.
Calculator indicates about 4100 rpm at 25 mph and 10% slip.Of course you may have higher slip numbers accounting for the speed.With a tach it will help pin it down.
Further calculation a 17" prop at 5500,10% slip = 32.9
Near as I can tell the boat weighs about 360 dry.