Raised my motor, but questions...
 

So before launch for 2016 I decided to raise my motor up a bit on my 1975 SF18 to see if I can help with my high-planing speed and general drag-ass performance I had last year. (see this thread from last August)

I know it's been mentioned many many times by a bunch of you guys in the know (looking at you Bushwacker. :)), that the motors Anti-Cavitation plate needs to be approx 1-1/2 to 2 inches above the bottom of the keel at the transom for optimal Seacraft performance.

As the boat was set up from last year the Anti-Cav plate was just below the bottom of my keel by about a 1/4 inch, so I raised it up 2 holes.
Now the Anti-Cav plate is about 1-1/4 inches above the bottom of the keel, but it looks kinda weird.
Theres still one hole to go up so its not maxed out, but it just looks kind of odd.

Couple of my co-workers at the boatyard here were assisting me and they all thought that what I was doing couldn't possibly be right.
Here are a couple of pics.
I know she needs a sea trial to really know whats going on but Nantucket Sound has been a lumpy mess all week with winds consistently out of the southwest.
When I finally get out there, I guess I should be looking for prop blow out, and the Anti-cav plate running just about at the surface? Anything else?

Can you guys take a look and let me know if this looks right? Again, this is on an 18 footer with a 115 Mercury 2 stroke.

https://lh3.googleusercontent.com/-U...0/IMG_4303.JPG

https://lh3.googleusercontent.com/-9...4/IMG_4306.JPG

While I have been wrong a lot and occasionally right, I'm thinking you are very close to right on. Whats the the center to center distance on the holes - 1/2"? You probably have all the adjustment range you need. After you dial the height then dial the prop. The prop gurus are way smarter than me on that - I would call Ken Reeves at PropGods.

I second that, looks like a good height for sea trial. Just make sure you can live with the handling up high. My motor is way up and has less drag and better speed but if you get too high you see your boat not do as well making tight turns when trimmed up high. Feels like the boat wants to turn like a flat bottom skiff and gives you that sliding felling if you don't trim it down when up real high (2 inch plus). You can get too high and loose your handling but can still trim down a little and she locks in and will turn find tho when up too high. You got to find that balance point you can be happy at. Just make sure you get the AV plate up out of the water when on plane.

I think you'll be pretty close. As Friz said, what really counts is getting the AV plate up above solid water when on plane. That's sort of step 1 in getting optimum performance.

The other variable is what prop you're running, as some props can run higher than others. Forum member Egg Sucking Dog, who hasn't posted here for some time, used to run a Stilleto on his 20 MA, I believe at 2.5" above the keel with motor on transom and no jackplate or extra setback. Props with a lot of cup in the tips seem to have lots of grip and can typically run very high, so once you get the AV plate above water, the next check is, at your normal cruise speed, you want to check handling as Friz recommends. To do that, trim the motor up to where the prop starts to ventilate, then trim it down a bit, and then try some quick hard turns to see if it ventilates. If it doesn't, you might be able to go up a bit more; if it ventilates, you may want to try a prop with a little more cup, or just live with it, knowing that you may have to trim down a bit before doing any quick turns or running in big swells.

The pics below show what it looks like when motor is too low and just right. 2nd pic is with AV plate at 3.25" above keel, which is as high as it would go without a jack plate. (I had to focus on the black fin I added to AV plate, because it's pretty hard to see the white AV plate in white water!) Since I have a bracket with a lot of setback, I ended up installing a small 3" jackplate because the widely quoted rule of thumb of "raise motor 1" for every foot of setback" appeared to be totally bogus! Last 2 pics were taken after I raised the JP as high as it would go, and AV plate is now at about 6 1/8" above keel with 33" of setback, so the correct "rule of thumb", at least for my boat, is about 2.23"/foot of setback!

BTW, I'm going to move this thread to the Performance section because that's the main reason to adjust motor height.

The person that mounted mine back in '03 has the av plate about 3/4 to1' below the keel. I'm assuming I should raise mine as well. Running a Suzuki df140 on the transom.

Erebus is prrob close. The Suzi 4S runs a larger dia prop and will limit you

That's a good starting point. Mine is 2" above on my 18. The prop I have had no problems at 2 3/4", but being that high caused cooling issues. Pay attention to you temp when experimenting with motor height.

I usually check the E-Tec Owners forum every couple days, where the engine techs often mention the importance of correct mounting height, and it seems like almost all dealers mount the motors too low! I can think of several reasons they might do that:
1. The average boater, especially a newbie, has no idea how much motor height can effect performance.
2. If motor is too high and the prop ventilates during hard turns, even a newbie will notice that, and complain!
3. The typical dealer could care less that you aren't getting optimum performance! He's very busy during the peak boating season, so he doesn't want to be bothered by come-backs! The safe option from his perspective, particularly if he's dealing with a newbie, is to just mount the motor on the low side of normal to minimize complaints. If you find a dealer that pays attention to mounting height, that's a good sign of a much better than average guy that really cares about his customer!

I agree, your motor is mounted way too low if AV plate is BELOW the keel! Just make sure you don't have some oddball combination, like a 25" motor on a 20" transom or vice versa! I think the AV plate set at 1.5 to 2" above keel is about right, but a sea trial to insure the AV plate is above solid water and handling is good is what's important. I sort of like to get it too high initially so I know where the "edge of the cliff" is and then drop it down a hole, but when doing that, it's a good idea to monitor water pressure to make sure you don't create cooling problems! I believe the basic design of the prop and how heavily it's cupped is a more important factor than size in determining running height. For example, some bass boat and race props are designed to be run "semi-surfacing", but that's a very specialized application, not not likely what you'd want on a typical SeaCraft!

I went through this almost to the letter on my 18 last year. My motor is a '92 Evinrude 120 hp and it was mounted in the top set of holes with the ventilation plate 1" below the bottom. It took forever to get on a plane with spray flying everywhere and in general was a dog. I raised it to the bottom set of holes which put the ventilation plate 1" above the bottom. I also put a cheap Apollo 4 blade prop on following advise given on this forum. The difference was really quite amazing! it hops right up on plane now, rpms are where they should be, spray is under control and minimum planing speed is down to around 15 mph. I think I could raise it one more hole but I'm out of adjustment. The boat really came alive with these changes and well worth having to play with the trim a bit more.
Rod

A/V Plate Above Keel
 

I never had to consider where the A/C plate was during the first 37 years of running Mercs on my '76 20' Seafari. Why? probably because both engines were sold & installed by Frank Brown, one of Moesly's race mechanics. Three years ago I bought an ETEC, then I started hearing about about height over the keel from the ETEC owners site. My ETEC dealer raised my ETEC twice and I raised it again. Last week Skippertee was riding in the stern jump seat as I cruised at 30-32 mph. He said the A/V plate was positioned just right and said Moesly recommended 1.5" above the keel, which is what I measured after the last height adjustment. BTW, I have no engine bracket like Bushwacker. As others have mentioned the propeller characteristics play into this too. I have tried a few props, but got my best results for WOT rpm and best wakeboarding speed control from a 5 blade High Five. The other props worked pretty well too, but didn't quite get it to the recommended WOT rpm.

Capt Terry speaks truth.
1-1/4 " is fine. going up 1 more notch will probably be fine. 4-5 blade will help.

Other issues to consider:
1. What is your prop? Make/model (defines shape and what it was designed to do), diameter, and pitch.

2. What is your max RPM at WOT with a full load?

3. Another potential detriment is yer motor - if i'm not mistaken (and please correct me if i'm wrong) , its what Mercury sold as a 2+2 fishing motor. It was designed to save fuel at manatee zone speeds by firing 2 cyls, and then take off as the other 2 cyls fire as you reach high 2k rpm. means your low planing speed is compromised because that 2300-3200 rpm range is virtually non-existent. Thus your putting along or cruising over 20mph.

As Capt Terry noted 4 blade or 5 blade may help.

Post some answers and give the performance guys, something to work with.

Good luck. The 18 is a favorite among its owners and you should be happy!

It was a beautiful day today on Nantucket Sound, light winds out of the north, so pretty flat. Launched the boat and took her for a spin.

https://lh3.googleusercontent.com/-U...2/IMG_0448.jpg

Right off the bat I noticed that she seemed to plane better. Faster.
Felt like I lost some of that drag-ass weight I've been towing around.

Produced 5000 RPMs max, with full throttle. The 2 stroke Merc 115 is indeed a 2+2 and the max rpm range is 4750-5250, so I'm bang on in the middle there.
Unfortunately my prop has absolutely no identifying characteristics whatsover. No stamps, no numbers, no nothin'. So I have no idea what I'm running which is frustrating. What was also frustrating is my garmin apparently lost its mind over the winter and couldnt find a single damn satellite. So speed is unknown, but it felt like somewhere between 27 and 32.

Flyingfrizzle is totally right that the handling is slightly different.
Noticed the sliding feeling in a sharp turn, but didn't bother me in the slightest.
Felt really smooth and controlled the whole time. God I love that Seacraft ride. Every spring I remember how smooth it is. Like riding in a nice creamy pudding. :-)

Otterhound, what were the specs on the cheap Apollo 4 blade you threw on there?
Now I need to start prop shopping.

It was a 13 3/8 x 15 four blade Apollo and you may need a hub kit as well.
Rod

I bet if you take that prop to a good prop shop, they can figure out pitch and diameter, and maybe even assess how much cup it has in it, so that will give you a rough baselne to start from.

Did you happen to get any pics of the AV plate while you were up on plane? Now that you know it handles ok at the current height, I'd be inclined to raise it another hole to see if you can run even higher . . . might pick up another 50-100 rpm at WOT.

How heavily were you loaded today? When testing props, I always test with a full load of fuel, fill the galley seat water jug and live well, and fill my 65 and 70 qt coolers with sea water to simulate the max cruising load I expect to carry. Unless you don't ever run heavily loaded, you run the risk of picking a prop with too much pitch if you don't test with the max load you intend to carry! I'd also check min planing speed on every prop you try. I suspect that's real important for where you're running, because the 18-20' SeaCrafts are pretty light and will start going airborne at about 20 kts when seas get up around 3', so in those conditions, it's nice to be able to slow down and hang on plane at 12-15 mph, which you easily be able to do with that fairly light motor! Any 4B prop will have more stern lift than a 3B and improve low speed planning, and may be more efficient at cruising speed, although it may be a bit slower at WOT.

Merc has some good props, so once you know what you're running, you might contact Ken at Prop Gods, who seems to recommend mostly Merc props. Michigan also makes some very nice reasonably priced props, and their 2-piece hub system is more robust than the Merc Flo-Torque hubs, which have a reputation for shearing the plastic insert. Some PowerTech props also use the Michigan hub system. The 14 1/8 X 20P 4B Michigan Apollo was the first SS prop I tried and it was the most efficient prop I've ever tested, giving me a solid 5.0 mpg at cruise, and with a light load it also gave me 49.8 mph @ WOT, the highest speed I've ever seen! However I could only spin it up to about 5200 with a light load and only 4600 with a very heavy load (this is why I recommend testing @ max load!), and my optimum WOT rpm is 5300-5500, so I needed a lot less pitch!

So the boat was pretty much loaded as if I was going out for the day.
Full fuel (32 gals), but no food, beer or wife. And the bimini wasn't up. All the beach junk, anchors tools, etc. were stowed where they belong.

The AV plate was just visible beneath a scrim of water flowing over the top. The big stainless nut that holds the AV plate zinc was proud of the water flow.
I could probably raise it up another hole, but I think I'm going to stick with it where it is for this season.
Need to move it from my boatyard on Nantucket Sound up to its mooring in Cape Cod Bay in the next day or so.

I ran the merc prop selector tool real quick just now and its telling me I need either a 13x19 4 blade or 3 blade.?
I think the beater prop that's on there might be a 19 but I'm totally guessing. No real prop shops on the Cape. I'd have to send mine to Plymouth to have it checked out. Should've done it over the winter.
Oh well.

I emailed Ken at propgods and gave him the lowdown, and here's what he had to say (this was before I raised the motor):

So, now I have no idea what to do...

That's exactly how it's supposed to feel when you go from having the engine mounted too low to the correct height. I messed around with props on the 18 a little. IMO That hull likes a little bow lift and a smaller diameter prop so it can shake it azz around dancing across the waves. Too much stern lift on the 18 will get you wet and make the ride a bit harsh.

Bigshrimpin's assessment is spot on IMO as that's how my 18 feels after adjustments and the 4 blade prop.
Rod

Stock omc props have diameter/pitch stamped on the end of the hub. The prop nut washer hides the number. Maybe stock merc props are the same.

Erebus

On the numbers on the prop if it is older you may want to take a wire brush to the hub and clean off exhaust and corrosion. If that hasn't been suggested yet.

Also on the Garmin try mashing the Power and Page buttons at the same time as you turn it on. Hold them down until you get to accept or agree screen and that turns yellow. Then release the power and continue to hold the page button until it finds satellites.

I got that from the PO of my boat for the Garmin unit it came with and it works.

I scoured the crap out of that prop. Threw it in the beadblaster, cleaned all the old paint off and looked in every nook and cranny.
No numbers... :-(

Big's experience propping the 18 and Kens advice are concurrent.

My question would be can you get the lower planing speed with the smaller diameter, & significant cup & rake of NRS3 prop relative to the 2+2 midrange rpm. Might ask Ken if the NRS4 blade option would reduce the planing speed and sill get the nose up as Bigshrimpin suggested..:confused:

Boats now on her mooring in Cape Cod Bay, so after work this evening in the dead flat calm I took her on another sea trial with a known prop.
Took off the unknown mystery prop and threw on a Michigan Wheel Vortex 13.25X17P we had kicking around the shop.
Top end RPMS were about ~5600 at 29kts.
The original mystery prop topped out at ~5000.

So from this I can infer that the mystery prop is probably a 19" or a 20"?
200 rpm's per inch of pitch, right? so if the 17p runs 5600 and the mystery runs 5000 then its got somewhere around 3 inches more pitch?
I'm guessing its a 19p.
So, the merc prop calculator with the input from today's tests tells me I should run a 19p prop on my boat/motor combo (actually it said an 18.38").
It recommends the 4 blade spitfire 12.5"x19"p. Or a 13x19 black max.
4 blade gives more stern lift right?
Going to stick with aluminum for now, as sometimes I need to do a little dredging to make it back to my mooring if I spend too long at the beach.
No point ruining a nice stainless. At least not yet.

Yes, the extra blade alone on a 4B will add extra stern lift, although blade rake and how it's cupped also affect stern lift. If your Vortex is a 4B version, you might get more than 200 rpm/inch of pitch with it. The Vortex is an excellent prop; it's comparable SS version is the Apollo, which would probably slightly larger in diameter for the same pitch.

I did some more reading online regarding 4 blade props, and it seems it is recommended that you reduce the pitch by 1 inch or so when switching to a 4 blade from a 3 blade.
With my 17p 3 blade test prop I was turning ~5600 RPMS on a motor that should max out at 4750-5250 (2 stroke merc 115).
For optimal RPMS that prop should be a 19.
So if I get a 4 blade spitfire should I get the 17p? Also the mercury prop selector says I should get a 12.5" diameter 4 blade spitfire.
12.5" seems awfully wee...
Quicksilver Nemesis which is the cheapy version of the 4 blade spitfire comes in a 14x17 and a 14x19.
14 Seems awfully large...

It was just a regular 3 blade.
Have yet to try a four blade. Getting there tho!

4 blade prop update
 

Had a quick run around Cape Cod Bay yesterday with a brand new 4 blade Quicksilver Nemesis 4 blade 12.8" (they call it 13")x17p prop.

Right off the bat she seems to want to plane off faster.
Not sure if the bow rises more initially before the stern comes up but that's what I thought first time up on plane. Just an observation, maybe I imagined it.
Planes good and at a much lower rpm/speed. Can hold plane till about 13 knots on smooth seas at about 3200 RPMS.
Max full throttle was 5000 RPM's on the nose and 31 knots via GPS.
Can cruise happily at 18-20 knots at about 3600 RPM's.
4 blader also grips good in a tight turn, alleviating some of the sliding sensation from moving the motor up two holes I noticed with the old three blade.

So now the question is do I change pitch, or is that pretty good?
I ordered a 4 blade nemesis 13x19p I can try too, but Ill have to pay for it. Or I just run the 17 and call it good. If I go 19 that will lower my RPMS by about 400, correct?
Maybe I should try a 15P?
115 merc likes 4750-5250. So the 17p is smack dab in the middle.
They don't make a 16 which would probably let me get the full 5250.

17P is probably good enough for my needs...

I don't think the 19p offers any benefit. i would cancel that order. Its good to hear you're able to plane at 13 knots. Sounds like you've come down quite a bit in that attribute. I would even consider a 15 pitch. Especially if it has a little more diameter. Or are the nemesis props all the same diameter?

Were you able to borrow the nemesis? If so, the shop probably has the 3 blade. You might prefer the hole shot as it will pop up on plane and level very fast.

You might try the three blade black max/black diamond 13-3/4 x 15, or the 17p variant. I think Beaver is running one of these on his 18 w a tohatsu. Maybe he'll chime in.

Good to hear your making progress.

All the performance changes you've reported are exactly what I would expect from a good 4B prop. Being able to plane at 13 kts indicates boat is fairly well balanced. If the load you were carrying was about what you normally carry, I'd say you've found the right prop. If you normally run heavier, a 16 pitch prop would allow motor to rev a little higher. If it's an aluminum prop, a good shop could probably repitch it.

FYI, as I've posted on here before, if I were running an inboard or a 4-stroke outboard, the first thing I'd do would be add a gage to measure intake manifold vacuum. If you observe a minimum vacuum of 5-7" at cruise, your exhaust valves will live a lot longer! (The exhaust valve is the hottest and typically most life limiting part in any 4-stroke engine.) Basic requirement is for prop to reach max recommended rpm at WOT with normal load and still be within recommended range at max load, but beyond that, I think I'd be inclined to pick whatever prop gave me the max speed at about 7" vacuum, provided that min planing speed was acceptable and it didn't ventilate in hard turns!

The 15p is a little larger in diameter. The 17p is 13 inch and the 15p is 13.4 inch.

I was considering the 15" but going down 2 inches in pitch, wouldn't that raise my rpm's above mercs recommended max for my 115 2 stroke?
Not that I ever run at full throttle...

I did buy the 17p, but I suppose I could cancel the 19p I ordered and buy a 15p instead and see what happens.
About $90 a prop.

Sounds to me like you got it

As Old'sCool suggests, your probably good. But if you want to keep splitting hairs, definitely trade the 19 for 15p.

If the diameter is a little greater on the 15 it might not be as big rpm increase as you think, and even less increase if you were not carrying your max projected load on your test runs.

I was amazed at the difference between a 17 and a 15p black max/diamond.

The Nemisis appears to be a 4 blade clone of the black diamond blade shape. What strikes me is QS's claim of a 25% improvement hole-shot improvement over the Black Diamond. That sounds scary.:cool:

I must try this. Where are you finding this prop for $90?