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Evinrude SPL 28 and Jack Plate

J

JasRam

Contributing Member
Hello all,
I have a 1968 Amumacraft semi v with an Evinrude spl 28 long shaft. I've had it for two summers now and I just noticed that the cavitation or anti vent plate is about 2 1/4 inches below my keel. I did some reading and found that it shouldn't be that low. I ordered a mini jacker jack plate to install. My question is..What height should I aim for for the cavitation plate when I install the jack plate? Not sure if this helps, but the prop appears to be about 16' setback from transon right now. Also, I just installed a new maring ply/fiberglass 1 1/2 inch transom.

Other details- I currently top out at 21 mph with one passenger, with two I can reach about 16 mph. Not sure if this motor should push me faster, as this is my first boat.
 
The bottom of the anti-ventilation plate should be even to about 1" above the bottom centerline of the hull. The Mini-Jacker from TH Marine is set up to give you 3" of increased height. I would install the plate so it's at the lowest position and then mount your motor so it's resting directly on top of the plate. That should set your motor 3" higher than your current running position and should be just about right.

Depending on how big and heavy your boat is, I'd think about 25MPH is a reasonable top speed.
 
kevinj said:
The bottom of the anti-ventilation plate should be even to about 1" above the bottom centerline of the hull. The Mini-Jacker from TH Marine is set up to give you 3" of increased height. I would install the plate so it's at the lowest position and then mount your motor so it's resting directly on top of the plate. That should set your motor 3" higher than your current running position and should be just about right.

Depending on how big and heavy your boat is, I'd think about 25MPH is a reasonable top speed.
Click to expand...

Thanks for the response. I went ahead and raised it to be level with bottom of hull. I didn't want to lenghten my steering controls so I just raised it up on the existing transom with 1/4 sheet aluminum and marine ply. I tested it today and the holeshot is about 3x better but my top speed is still about 21. Are there any other things you would recommend to get more speed? I think I have a 9.25 x12 3 blade prop on it now. it does seem like I still have some rpms to go, just judging by the sound.
 
Okay, so the makeshift transom extension sounds like it worked but I don't think I'd trust that long term. It sounds like you really need to raise the engine and something solid like the Jack plate is the best solution.

To really nail this down you need a tach on the engine. Your motor is spec'd to a max RPM of 5,500 and there's no way you can dial this in by the way it sounds. One thing you can do is a get a Tiny Tach https://tinytach.com/gasoline-tinytach and use that to dial in your performance. I'd suggest getting that and run the boat a bit to see what kind of max RPM you get.

The rule of thumb is every 1" of change in pitch has a 200RPM change. So, if you were to go down to a 9X10 prop you'd have 2" of pitch decrease which would increase your RPM by 400.

Also, just to be on the prudent side, do a compression and spark test as well as make sure you have good working spark plugs in the engine. IF you find any issues fix those first, then start chasing prop pitches. Getting the engine height is step #1 and you've done that, now you need to make sure the engine is 100% healthy before you spend money on props. However, if it were me I'd go ahead and get the Tiny Tach anyway because I always like to see exactly how the engine is running.

KJ
 
kevinj said:
Okay, so the makeshift transom extension sounds like it worked but I don't think I'd trust that long term. It sounds like you really need to raise the engine and something solid like the Jack plate is the best solution.

To really nail this down you need a tach on the engine. Your motor is spec'd to a max RPM of 5,500 and there's no way you can dial this in by the way it sounds. One thing you can do is a get a Tiny Tach https://tinytach.com/gasoline-tinytach and use that to dial in your performance. I'd suggest getting that and run the boat a bit to see what kind of max RPM you get.

The rule of thumb is every 1" of change in pitch has a 200RPM change. So, if you were to go down to a 9X10 prop you'd have 2" of pitch decrease which would increase your RPM by 400.

Also, just to be on the prudent side, do a compression and spark test as well as make sure you have good working spark plugs in the engine. IF you find any issues fix those first, then start chasing prop pitches. Getting the engine height is step #1 and you've done that, now you need to make sure the engine is 100% healthy before you spend money on props. However, if it were me I'd go ahead and get the Tiny Tach anyway because I always like to see exactly how the engine is running.




KJ
Click to expand...
Hi Kevin, thanks for the excellent info. I finally found my tinytach, I took the boat out today and here's what I got. At idle, it went anywhere from 650-1050 and never went over 1050. At WOT on the second motor tilt pin, I reached 23mph which I've never done before in this boat. Top RPM for that was 4440. the tach fluctuated a bit between 3900 -4400 but that could have been because of how loosely my wires were wrapped around the plug ( I taped them on to help). When throttling down, I did notice what I would call some blow out?

Then I changed the tilt to the first pin hole closest to the transom. I got a max RPM of 4440 and a max speed of 22.7 .I definitely gained because I could barely reach 20.5 before raising the motor. With the motor in this pin, I had no blow out. I'm pretty happy with this setting. It's amazing how much faster it gets up to speed now, it feels like a different boat..

That said, my prop has some dings and I'm in need of a new one anyway. Would you recommend a different prop? Or would that be pushing it too close to 5000 rpm? Another thing I noticed is that it now cruises really nice at 19 mph at 3990 rpm, it seems to really smooth out righ in that range, not sure why.
I weigh 240 and i could easily see how an average 135-170 lb person could get an even better top speed out of it. All plugs are new, fuel pump is new, rectifier is new, seems to run great.


KJ[/QUOTE]
 
JasRam said:
I finally found my tinytach,....idle, it went anywhere from 650-1050 and never went over 1050. At WOT on the second motor tilt pin, I reached 23mph which I've never done before in this boat. .....When throttling down, I did notice what I would call some blow out?

Then I changed the tilt to the first pin hole closest to the transom. I got a max RPM of 4440 and a max speed of 22.7.....I had no blow out.

That said, my prop has some dings and I'm in need of a new one anyway. Would you recommend a different prop? Or would that be pushing it too close to 5000 rpm? Another thing I noticed is that it now cruises really nice at 19 mph at 3990 rpm.....
Click to expand...
Sounds like you're getting really close.

A dinged up prop will cause drag and decrease efficiency. RPM range for your engine is 4500-5500 so you're right on the edge and a prop with clean edges might get you just above that 4500 range. If you go to a 10 pitch prop you'll likely get very to 5000RPM. Will you go much faster with a 10P? Probably not a lot, maybe 25MPH, but your holeshot performance will get even better and you won't be lugging the engine. Also, with a clean prop you might not experience the 'blowout' which is caused by turbulence around the blades.

Honestly your numbers aren't horrible but if it were me I'd get the 10 pitch so the engine is at least in the middle of the operating RPM range. The 10pitch also lets you operate well if you have more people or load in the boat. With that 12 pitch you'll find performance goes way down as you add weight.

Kevin
 
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kevinj said:
Sounds like you're getting really close.

A dinged up prop will cause drag and decrease efficiency. RPM range for your engine is 4500-5500 so you're right on the edge and a prop with clean edges might get you just above that 4500 range. If you go to a 10 pitch prop you'll likely get very to 5000RPM. Will you go much faster with a 10P? Probably not a lot, maybe 25MPH, but your holeshot performance will get even better and you won't be lugging the engine. Also, with a clean prop you might not experience the 'blowout' which is caused by turbulence around the blades.

Honestly your numbers aren't horrible but if it were me I'd get the 10 pitch so the engine is at least in the middle of the operating RPM range. The 10pitch also lets you operate well if you have more people or load in the boat. With that 12 pitch you'll find performance goes way down as you add weight.

Kevin
Click to expand...
Thanks again for the excellent information. I'm ordering a 10 p prop, will post back once tested.
 
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