engine height df140 on SF20

[TMala]

All,

I recently did a bunch of work to my 1969 SF20 including new transom at 25 in height and a new to me Johnson/Suzuki Df140. Batteries mounted under console, gas tank under floor in front of console and penske board used in transom.

I built the transom to 26.75 inch height, I originally mounted the engine to the third or second hole which put me at 1.5 in above keel which I read on here is a good place to start. On first trip bow came up a bit much before planing and would propoise too easily. Today I raised it up another hole and it seems to have helped a bit with getting on plane and the porpoising was slightly eliminated but still there when engine tilted up. I am going to go to final hole up to see if that helps.

My question: is it normal for cavitation plate to be mounted that high above keel on these boats? I will be at almost 3 inches above keel on the final hole. I'm trying to eliminate the porpoising by mounting engine higher, but maybe it needs to be mounted lower? I'm at a cross roads on which way to go.

I took a few pictures along the way and hopefully I can post a build thread soon so others can see what I did!

Thanks for the help all!

[strick]

Sounds like you did everything right in the build as far as weight distribution..you might want to consider talking to someone who knows something about props.. I dont. When all else fails a small anti-cavitation plate may be the answer. Looking forward to your pictures.

strick

[TMala]

Thanks Strick. I will consider that. I read of some people that swear by those fins you bolt up to your cavitation plate.

I have the 14X20 suzuki SS prop, which seems to be the go to for this set up.

After looking at pictures of other seacrafts with 25in transoms I see engines mounted as far high as "prop shaft 5/8in above the keel" on ones built for high performance to boats with the engine mounted on the lowest holes.

I'm not looking for a speed boat, but trying to find the balance between ease of planing, lowest planing speed, and comfortable ride (eliminate porpoise). I may have to add a livewell or some seating in front of the console to try and shift more weight forward, but would like to tune boat best I can before adding weight which is probably unnecessary.

I read this comment by Fr Frank from here on another thread "You need to be careful about height when mounting behind a 20'. One problem can be excessive porpoising caused by thrust distance from the hull rocker point. If you increase height, you will possibly need to increase mounting angle as well. Stock transom angle on the 20' is 14 degrees, I believe. Consider wedges if necessary. There's a 20' SF near me with a Merc 250 on a B Bracket, he had to go to 17 degrees, and has trimming difficulties still below 35 mph."

This makes me believe from the get go my mounting was too high although other places I researched stated porpoise can be caused by mounting too low, so I don't even know what my next move will be haha!

Anyway, it's blowing 50 mph out of the west here today so no test runs anyway, just a whole lot of contemplating.

[strick]

Forgot to mention trim tabs of course you already know that. My 20 scepter has a bracket so the motor is mounted really high...no porpoising have a 4 blade prop...I’d try experimenting with some weight forward first then talk to a prop shop guru to see what they have to say. The dole fins work great but you will loose some top end speed. Good luck.

Strick

[Capt Terry]

Tmala-
The advice I have heard and read on the ETECowners site is the first thing is get the engine height so at cruise the AV plate is skimming on the top of the water (you can search for example photos). Next choose a prop the gets you to the recommended rpm at WOT to avoid lugging the engine. Some props will run higher w/o cavitation. Last year When I redid my transom on my Seafari I raised it to 22” to continue to run with my 20” ETEC 150 and the bottom of the AV plate is 2 3/8” above keel, running a 5b High Five (found this to be the best choice for steadier watersport speeds). The engine has been raised 4 times since original dealer install

[TMala]

Thanks strick and capt Terry. That is the info I was looking for. My prop currently gets me to the lower range of the engines max, I can hit 5500 the range is 5500-6000. I'm on the fourth hole now and appears I can come up some more but will experiment a bit more.

I did mount my go pro on the swim platform looking down on the engine while on plane. It appears the cavitation plate is still a bit low by the looks of the video but i'm as high as I would want to go on my holes. I may buy a jackplate in the future but will use it like this for a bit to get a feel for it.

[Capt Terry]

My ETEC was as high as it would go (on holes probably drilled by the ETEC dealer).. it was pretty good at this position. When I redid the transom I raised the engine another 5/8” - 3/4”, where ironically it was about dead nuts with earlier holes for my Merc, drilled by Frank Brown. Frank was Moesly’s race mechanic snd my dealer for my SeaCraft and first two Mercs. At the new position I am where I need to be RPM-wise. Haven’t run it too much, think I am happier at this position, but left room to drop it one hole.

[TMala]

Makes sense Terry, I feel like I can come up a bit too.

I took this video before reading about the electric tape trick, so it is still tough to see the cav plate but in my opinion it is still buried. I am still getting porpoising when I try to trim for speed.

It looks like I can raise the engine one more hole, would that be advisable to use that set of holes, the bottom hole would then be out of the slot, not sure if i'm supposed to go that high?!

Otherwise I think i'm looking at a manual jack plate, which I would like to avoid. I think they are around 40lbs i'm trying to limit as much weight as possible.

Here is the video of the boat getting up on plane for a bit. Let me know if the link doesn't work

https://www.youtube.com/watch?v=BJN1...ature=youtu.be

[DonV]

I always wondered about that trimming for speed. I tried that and seemed to just get a nice rooster tail, then I noticed the speed go down and figured that was not the secret. My 23'er seems to like the cav plate parallel with the water coming off the keel/transom. It's not a speed boat. Now on my Pathfinder I had the Bob's jack plate and if I kept the cav plate parallel with the water and raised the plate up to where it was just above the water I did pick up speed and still had a nice rooster tail. Anything above that I had to keep any eye on water pressure!!

[Capt Terry]

Tom-
Once again your boat and work looks terrific! And you did this all in a tent during the winter?

My Seafari has more weight forward than yours. Another difference is I am running a 5 blade prop and using a Dolphin. When I raised my ETEC to 2 3/8” from keel to bottom of AV plate (about ¾” higher than the last position), the primary objective was to dial in the WOT RPM better, which I did and picked up about 1 MPH and easier steering at higher trim settings. I haven’t run the boat much since that time, but noticed maybe a tendency to porpoise at one condition where a throttle or trim change eliminated it. So at this point I am happy with the change, but even though I increased the transom 2”, I left room to put it back down another hole if needed. My operation is on relatively smooth SC lakes, not sure how it would be in the rough stuff. (As I previously remarked think I am about where Frank Brown, Moesly’s race mechanic set up my original Merc).

The photos show the back tape on my AV plate (parallel to the keel) at 0 degree trim and 30 MPH.

I don’t recall if this boat is new to you or if you used it before your project. Is the porpoising a new problem? See my example of Estimating CG shift of 9-24-16. Your removal of the casting platform and smaller fuel tank probably reduced forward weight. And did you increase aft weight? As mentioned by others a 4B prop will help lift the stern.