The mystery of a proper prop and terrible performance

Thanks for answer. As soon as you have said 30% more lift, it occurred to me that it probably directs all the water flow on the (bottom) surface of tab to go downwards, preventing part of it to escape on sides, interesting. Though I am not an expert, but knowing real efficiencies at different "relative" speeds and fin sizes is probably the most interesting. Regarding keeping steering more straight at displacement speeds (flat stern issue), it occured to me that attaching something like that on tabs is more convenient and less evasive than attching something on stern drive, but I assume fins would have to be much deeper than what is required for lift at planing speed. Then again, if fins are big to achieve that, question is how thay would affect planing, no two things at the same cost in engineering, well rarely.

 

It's been over a year since I posted anything here. I had a couple of health issues that slowed me down but fortunately I've recovered. So hopefully the 2025/26 boat season will see me finally get my little cruiser down to Florida for the winter. I realized that I never posted any photos of my boat actually planing after all the work. I know it probably doesn't matter but here goes anyway. This is just a nice easy cruise at about 11+ knots. I won't win any races but that was never the point.

 

Fantastic news and great to hear you're on the mends MIA. Is that 11 knots at 3000 rpms from the look of the tach in the pic?

 

Thanks @missinginaction . We love a good update.

 

Thanks for the replys guys. She runs at 20 knots wide open and like you noticed 7728, about 13 knots at 3,000 rpm. I was running against a little current when I snapped that picture. Those trim tabs made a big difference. It looks like I'm a little overpropped though. The 302 Ford will only reach 3,900 RPM. It's rated for 4,400, so I'm about 500 RPM short. I talked with Precision Propeller down in Newburgh NY. They said they could take an inch or two out of that prop.

I'm probably going to just have the prop adjusted. I have thought about putting a pair of aluminum cylinder heads on her. Based on some research I've done, just a head swap, and a set of roller rocker arms would add perhaps 40 or 50 horsepower and a corresponding increase in torque. I'd keep the same cam and stay with the original 1.6:1 ratio rockers, just change out the lifters and pushrods. The only issue with all this is that the existing set up runs fine as is. No leaks, no oil consumption, good compression. My logical brain keeps saying "It's not broken so don;t fix it!". We'll see. I have to ponder this, but I have all winter.

I spent some time adding an anchor winch this year and a cabinet to store some kitchen supplies. My boat is a real challenge for storage. Take it easy guys. If anyone has any opinion about my cylinder head idea I'd be interested to hear it.


MIA

 

I was thinking similarly. And maybe some large trim tabs would help a lot

 

I had to repair my 1970 tabs as the 304 SS rotted.
I bought 60 thousands 316 SS sheet and made some

I also added drop fins, bending the edge down
That help hold more lift
Since my transom is curved, I had to bend them at an angle


I also painted them silver Por-15


These use the very old Bennet red pump, surprisingly it all works, after I repaired all the electrics.



Album
Bennett Trim Tab repairs - Google Photos

 

Nice job on the tabs Sdowney. You may not have read through this long thread. Here is the condensed version.

Bought boat in 2000 for a couple of thousand.
Boat wouldn't plane.
One motor mount broken and engine stringers in need of replacement. I figured that was the cause. I was wrong.
After a major restoration boat STILL would not plane.
Much research done. Received a lot of help from Walter Gear (v-drive), Michigan Wheel, Olympic Propeller and a number of people who contributed to this thread!
Replaced V-drive (changed ratio from 1.5:1 to 2.05:1.
Replaced propeller (which was the primary source of the problem).

Added 24x12 hydraulic trim tabs.
Problem solved.
Issues were....
1. Incorrect V-drive installed by builder in 1973 (wrong ratio).
2. Incorrect propeller originally installed.
3. Trim tabs were necessary.

Boats been performing well with a Michigan Wheel DQ486 17" propeller.

Regards,
MIA

 

Glad you solved it.
Tabs can make a huge difference in how a boat runs.
I have had my tabs off the last few years as I had to make new ones, I am hauled out and they will be back on.
After I took them off, boat was more of a slow barge, threw up huge stern wave. Very speed limited.

Shocked me how much of a difference they can make. So these tabs I made are 12" deep and 42" wide and have the drop wings.
My original tabs were same width but only 6" deep.

The new ones should get a good grip and push the stern up easier and at less of an angle than the smaller tabs.

My boat is a 37 Egg Harbor, with twin v8 inboards.