Need help on motor/prop setup

[nphilbro]

I need a lot of help here so hopefully all this makes enough sense to prompt some replies for you good folks.

I highly suspect my motor and prop set up are off. Both the boat and motor are new to me this year and I did a complete restoration over the winter but didn't make and structural changes. And my sincerest apologies that my old tachometer flew apart for some reason a week earlier so I don't have definitive readings - which I know are very important. I hope there is enough information here that I can at least get pointed in the right direction to make macro adjustments before I take it back out AND have a new tach.

The motor is a 1980 Johnson 140hp and it's hanging on a 1980 19 1/2 ft Glasply. I think the dry weight on the boat is roughly 2000lbs, 2450lbs with the two motors and has a 50 gallon tank that I'm not using using until I install new hoses in the next day or two. This weekend (and for all discussion here) the hull tank was empty and I was using a 6 gallon tank in the stern storage.

As you can see from one of the pics I've got it mounted about 1" above the transom as I've heard it makes the motor more efficient but I've never run it set down. The two props came with the motor.



Prop 13.75 x 15 aluminum: I just can't make it "feel right." When I run it on calm open water the wake looks a bit "shallow" but this could also be normal for the boat since it's not a ski boat. I also can't tell if it is getting up on a good plane since because my other (much smaller an lighter) boat had a very noticeable transition when it got up on plane and the water let go. When it is trimmed where top speed is reached at WOT (32mph on GPS) the bow holds quite high and I sense bow drag. This weekend we took it out on the Puget Sound from Pt. Defiance to Blake Island through Colvos Passage on very calm water and I was trying to find a good cruising speed which seemed to be about 11-13 mph based on healthy engine sound (that's just arbitrary) and the bow held quite high regardless of trim with the only change being a small window in the trim position where the rpms would audibly increase by about 200 but without noticeable change in speed (my GPS only managed to flip between 11.5 and 13 but didn't settle). I made it 12 miles on 6 gallons of gas and ran the first 5 miles at 18mph before dropping back to 11-13 to conserve fuel. Based on fuel usage, there was only a slight fuel savings at the slower speed which makes me suspect issues getting on plane.

I'm not trying to show off my daughter here but it's the only pic I have from last weekend of the wake. I'm running at 18mph here

Prop 13.75 x 19 stainless: It loses grip on fast starts with normal hole shot trim set but the prop holds fine trimmed farther in but with an obviously slower hole shot (than the other prop) with about a 3 second push to full throttle. The boat settles in nicely at top speed with trim adjustment (36mps) and the bow comes back down as I slowly trim our during acceleration but I can tell it's laboring a bit and not reaching optimum RPM because it will surge and bog with small changes in water surface wouldn't impact a properly tuned setup. These pics are screen shots from video taken on my first run out. My guess is that is was about 4200-4800 (based on sound compared to the other prop) at WOT and settled down.

The prop is slipping in this pic.

I'm running WOT at 36mph

One other thing I'll throw out there: I have a 12" steel extension plate pulled from an I/O to outboard conversion I can mount directly to my transom. The biggest drawback is that is weighs about 50 lbs. Do any of you think the pros would outweigh the cons if I mounted this? I've read in numerous places that motors run much better in the calmer water farther behind the boat. I also see many boats similar to mine with various versions of this set-up- especially those built for big water.

Really looking forward to your input.

 

[steelespike]

Re: Need help on motor/prop setup

First thing I notice is the hydrofoil They can be very helpful getting on plane and staying on plane but they can steel speed if they a dragging in the water at speed.Staying on plane at 11-13 mph I think is exceptional I'm sure the foil helps.
By the way the most ideal cruising speed can be from just on plane to about 1000 rpm higher.About the only way to get actual gph and mpg is with a flow meter.And you would need a flowmeter to tell the difference between a well setup boat and a perfectly setup boat. The deep V hull will behave differently than those with a shallower V or flat bottom longer to plane harder to keep on plane. Again the foil will mask some of that. The deep V will ride smoother and have more wetted surface and more drag.
I think for the initial run the boat behaves really well.
It does appear that your rpm may be lower than ideal.We do need the wot rpm and gps speed.
a Tinytach $47 is easy to install and will work on any motor .Can be hand held or installed.
Be sure to read all the info.
If the foil isn't out of water at speed the boat won't reach its true wot speed.

 

[nphilbro]

Re: Need help on motor/prop setup

First thing I notice is the hydrofoil They can be very helpful getting on plane and staying on plane but they can steel speed if they a dragging in the water at speed.

If the foil isn't out of water at speed the boat won't reach its true wot speed.

From my pic how would you gauge the current prop height? I just went back to the garage and there is 2" between the ventilation plate and bottom of transom. I'm aware the foils can kill a slight amount of top-end but considering how little I time I'll spend there they are well worth it to me for fuel savings and ride. I'll pull it during micro-tuning to see what actual top-end at WOT is for a true baseline.

To be clear, are you saying if I'm on plane should I clearly see the foil above the wake? I don't recall ever hearing this but that's an easily observable performance metric.

Staying on plane at 11-13 mph I think is exceptional I'm sure the foil helps.

I'm pretty sure I wasn't, but as I said, there was just no clear indication that I was or wasn't and as much as I love my wife freaked out every time I leaned over the splashwell or over the size so the I'm hoping to take it out today with a buddy.

It does appear that your rpm may be lower than ideal.We do need the wot rpm and gps speed.
a Tinytach $47 is easy to install and will work on any motor .Can be hand held or installed.

Agreed, I was looking at Tinytachs on ebay last night. I think Schucks/O'Reilly sells dashmount tachs that read off a plug so I'll pick one of those up today. It's extremely critical information and I hate not having that. I'm a systems engineer so I almost didn't post this but there are so many other potentially obvious things I can change before my next sea run so I'm hoping to save an extra data collection trip.