TOPIC: 1961 Merc 800 - video from shore - different prop

Well, I coerced a friend to take a video of me using a rather crappy camera yesterday.

The prop used is a 14" x 21" Mercury 2 blade bronze (uncupped). Just touching 50mph GPS at 5600rpm as I come into the frame. Then I got out of it.

I was curious to see that angle of the motor and how much boat was out of the water.

I was able to get to almost 6000rpm, by trimming out further, but saw no increase in speed and quickly backed down.

Pete

Looks good to me but ya sure are getting a lot of side spray until planed out? Take a pic out the water from the rear. I can't figure what would cause that? Little ok, but thats weird.

Yeah, frankly I'm a bit embarrassed by it. It happens with the motor trimmed in. I wonder if it is the cav plate deflecting water. Maybe the motor is too low still? I'll get some photos over the back next time out.

Pete

I have no idea what the side spray means but I do find it amusing I heard some proclaim that there side spray is embarrassing. Not a statement I saw in my future. Love the boat side spray or not great job!

First, don't run uncupped props. Your speeds and height make cupping mandatory, which is why you saw no change in speed with trim. The spray may be height related. With a straightedge on the pad, how high/low is the cav plate? If your long term goal is fastest top speed possible, a setback jackplate will be in your future. With your current setup, consider anything over 50mph to be a victory worth celebrating- with extra gas and sunny days.

Seymour wrote:

I have no idea what the side spray means but I do find it amusing I heard some proclaim that there side spray is embarrassing. Not a statement I saw in my future. Love the boat side spray or not great job!

You never know how bad it is until you see yourself from a different perspective! Boat's running above 5K now, at least I know the tach works.

Pete

Hi Kerry,

Thanks for the advice. Currrently, with CMC PT/T and a 5 deg wedge on transom, there is about 7" set back. The cav plate is currently 2 3/8" above pad. Have good water pressure. I suspect the "side spray" is caused by cav plate deflecting the water when it's trimmed in.

I may be able go higher on the CMC w/out re-bolting the unit to the transom. The motor mount hole locations (port side) on the Merc will interfere with the hydr. sump on the CMC if I do decide to go higher, leaving only about 5/32" of thread or so to put the nut on, before interference. Not too keen on that idea though. That's with the unit trimmed all the way in. Or, I could put a hard stop, keeping the unit from trimming all the way in, leaving enough room for the bolt.

I have tried a cupped 13" x 24" two blade, 48-49614 A4, with this set up and was only able to get 5000 rpm's and 51 mph. Don't know if that rpm is too low for this old motor or not.

One prop I'd like to try (if I can find one) is the 13" x 22" cupped two blade 48-49612 A4. I think this prop would get the rpm's up to 5200 - 5300. Not a lot of these on Ebay, though!

In any event, 50mph is fine by me, I'm the only one who likes to go that fast, all my passengers prefer a max of 30! I just like fiddling with the set up now that the boat is done.

Pete

Pete, Im with Kerry on this one, you need a cupped prop and something with a high rake. Engine height and bow lift will be your recipe for more speed, having said that, 50 is very respectable. A side on pic of the stern with the engine in the running position would help folks understand your side spray issue.

Hi Dave, thanks for the suggestion. Here are two photos showing the CMC unit and cav plate. I'll try to get an overall side view showing everything at once.

In this photo, cav plate is parallel to keel/pad. The upper edge of the level is touching the keel/pad. It shows the cav plate 2 3/8" or so above keel/pad.



This next photo shows the set up from the side, including wedge and CMC unit.



Pete

Hi, Pete, just a couple of questions/comments, does your motor have the water intake holes in the trim tab?

If so, it might be difficult to fit a prop with more rake, since that fitment normally requires a special trim tab to clear the back-swept prop blades.

If you do have the trim-tab intake, you may ultimately need to switch to a later-model lower unit, or add a low-water pickup to the existing L/U.

And do you have a water pressure gage? You're knocking at the upper limits of your rig's performance, and need to know if you've gone too far; otherwise there's a high risk of running out of water and frying your motor.

Cheers...........ed

Hi Ed,

I do have a water pressure gauge, and have been keeping a close eye on it

My lower unit specs are:
Through hub exhaust
4.25" dia. hub
7.5" from center of prop shaft to cav plate
15 spline hub

Here are side and back views of the lower unit



Pete

Looks good Pete, you have enough clearance with that trim tab to take a higher rake prop. Ed is right on with the water pressure.

Your current height (as Ed says in his post) is pushing the limit for water intake. Your top holes on the lower unit may be out of the water at trim so keep an eye on that water pressure. Nice set up, wish I lived closer to see it run in person.

Thanks Dave, glad I put that water pressure gauge on. I'm leaving on a trip in a couple of days so I can't run it for a while; going through withdrawals already!

Pete, looks like you could use a prop with more rake, since you have the water intakes on the side of the L/U.

Merc makes a special trim tab which will accomodate a prop with more rake. You'll get some stern lift, which may be what your rig needs.

BTW the Doc showed me what he's done on lower units such as yours; he's either tapped some of the upper intake holes & installed allen plugs, or had the holes welded up.

This ensures that, if you're running a drastic engine height, the upper holes don't suck air.

Your L/U would be a perfect candidate for a nosecone and low-water intake, maybe that'll be something for down the road, as it were!

Edit: didn't notice before, but I see you already have the Special Modified Hi-Rake Trim tab installed!!! :0 Pretty funky there!

Just my 2 cents..(probably not worth a penny!)Spray starts .38 in the vid, gone at .42. Best I can think looking the "level" pic, its simple the V of the hull displacing water, and returning to fill the void created and hitting above the cav plate until you get more lift back end and gone. Understand what I'm trying to convey? Its not a big deal and don't worry it.

And looking the level pic, I would at least fill the top water intake hole as looks like it would be out of the water at speed.

But you are getting damn good speed out of it! Wish you were closer as let ya try my SS12x24 cupped. And yep, damn hard to find. I found one ebay and bought it on the spot. Lucky find!

Hi Robby, just saw your post here after your SF post. Man, I'm getting no work done today!

Your analysis is probably very accurate. You've put my mind at ease, too.

Interesting props don't come along too often do they?

I'd really like to try a 13"x22" cupped 2 blade bronze 48-49612A4. I have the 13"x24" cupped and can only turn 5000 rpm at 51mph.

It'd be nice to settle on a prop that doesn't lug the engine or over rev it and still have good speed. Sounds like a never ending quest!!

Pete

ed-mc wrote:

Pete, looks like you could use a prop with more rake, since you have the water intakes on the side of the L/U.

Merc makes a special trim tab which will accomodate a prop with more rake. You'll get some stern lift, which may be what your rig needs.

BTW the Doc showed me what he's done on lower units such as yours; he's either tapped some of the upper intake holes & installed allen plugs, or had the holes welded up.

This ensures that, if you're running a drastic engine height, the upper holes don't suck air.

Your L/U would be a perfect candidate for a nosecone and low-water intake, maybe that'll be something for down the road, as it were!

Hi Ed, thanks for the good advice. I bet putting a short shaft on it with low water p/u might be the ticket. I'm gonna start saving my pennies!

Pete

Here's another style of hi-rake trim tab that I've used before:

www.boatzincs.com/MER46399.html

Had never seen the one you have on your 80hp, but I reckon it must be an improved design, cause I found one online just like it!

www.propmd.com/propeller-accessories/trim-tab-anode-high-clearance.html

You will be surprised how high you can run that motor up and still get water pressure. Water kicks up from the hull and goes higher than you think. I ran my stock 1500 with side holes up around 4 inches without any modifications. I did have to add intake scoops to deflect water into the water intake when I ran closer to 5 inches up.

IMO you can raise that motor up at least another inch and have no problems. But don't take your eyes of the water pressure gauge the first time you do it. Try it a 1/4 inch at a time. When you sense water pressure issues drop it back down a 1/8th.

The boat in the video looks good. It looks clean and a lot of it is out of the water without a ton of positive trim.

I would look for a 20 or 22 chopper. Or a Laser II prop. The newer stainless props are more efficient.

I bet with raising the motor and testing some good stainless props that boat will be up to 55 in no time.

here are some Laser II's on eBay:
21:

22:http://www.ebay.com/itm/Quicksilver-Laser-II-Stainless-Steel-Propeller-15-spline-48-16992A40-13x22-Pitch-/311117617347?pt=Boat_Parts_Accessories_Gear&hash=item48700e18c3&vxp=mtr

Non of these are mine, just examples of whats out there. If you look for a Chopper prop. Make sure its a "small ear inline" prop. The early ones were smaller, made for the older motors before V6's

The one thing I like about the LaserII is the diameter is smaller . Its hard to lug a 14 or 14.5 inch diameter speed prop with a 800 motor.

Looks great!

ed-mc wrote:

Here's another style of hi-rake trim tab that I've used before:

www.boatzincs.com/MER46399.html

Had never seen the one you have on your 80hp, but I reckon it must be an improved design, cause I found one online just like it!

www.propmd.com/propeller-accessories/trim-tab-anode-high-clearance.html

Ed, the second one (the one that Pete has on his boat) is aluminum as opposed to zinc. On a wooden boat the aluminum is often preferred. It is still sacrificial since the alloy is less noble than the alloy used in the engine.

Hi Conrad,
Thanks for the advice about the props and motor height.

Sounds like raising the motor is the first thing to do, but I don't think it'll be a trivial job (when is anything easy!) I've maxed out the height with the current bolt positions and upon closer inspection, it turns out that if I drill new holes in the CMC to re-position the bolts, the bolt in the lower port-side slot in the motor bracket would interfere with the hydr. pump, I think there would be about 5/32" clearance there - not many threads available!

Also, the upper bolt holes in motor bracket and the turn screws would be above the clamp plate.

Some options I could try include putting a short shaft on with low water p/u and lower the motor; integrate a later model Merc tilt/trim bracket and exchange the CMC PT/T for some sort of jack plate; or remount the CMC higher on the transom (don't want to put more hole in the boat).

Might just try a few more cupped props at this motor height and save the big work for next year.

Again, thanks for the thoughtful advice.

Pete

Daveswaves wrote:

Your current height (as Ed says in his post) is pushing the limit for water intake. Your top holes on the lower unit may be out of the water at trim so keep an eye on that water pressure. Nice set up, wish I lived closer to see it run in person.

I don't mean to step on toes... but,
Have you actually tried to run a FGS lower unit higher than this? I have done it quite a bit. You do not have to have the water inlets below the bottom of the boat. You can easily have half the water inlets above the bottom of the boat. I ran ALL of my inlets above the boat before I had to do any modifications to direct water. And I ended up running my prop shaft 1 inch below the bottom of the boat.

Water does not just come straight off the back of the bottom of the boat. Water naturally lifts leaving the back of the boat under speed. Also having the motor set back off the transom lets you raise the motor even further. Its also surprising how much spray the lower unit bullet (round portion containing the gears) throws water up.

But do nothing without a water pressure gauge.

Peter, at some point you might as well have a short shaft mid section if one is available to you. It will bring things back down to a normal looking height. But until then your doing really good. The boat looks great and looks like its riding well once under speed.

This is my 1500 on a Carlson Contender. Standard side inlets on the lower unit.

At this height I resorted to making scoops to direct water spray into the inlets. But if I remember correctly about an inch lower I was able to get water pressure without the scoops. And that is still having all he intakes above the bottom the boat.

I believe later on I went about 1-1.5 inches higher than this photo.

Every boat will throw water differently off the back and each lower unit is slightly different. But this give you a general idea what you can do.

Conrad

I don't mean to step on toes... but,
Have you actually tried to run a FGS lower unit higher than this?
If you are referring to me, then the answer is yes Conrad I have, my propshaft was even with the bottom of the boat.
Just trying to save Pete some grief.

Daveswaves wrote:

I don't mean to step on toes... but,
Have you actually tried to run a FGS lower unit higher than this?
If you are referring to me, then the answer is yes Conrad I have, my propshaft was even with the bottom of the boat.
Just trying to save Pete some grief.

Dave, then please explain to me why you think it will not pickup water pressure? I assume Peter likes playing with eeeeking out as much speed as he humanly can..... The only way to do this is get the prop and lower unit out of the water. The more prop that is out of the water the less drag. you and I have been around each other for a long time, so don't want pick at this too hard. But Im surprised at your level of caution.


This is an excellent video to show you the dynamics of water hitting a lower unit. This man has a full tunnel hull so the water "cleans up" so you can see the lower unit at work. He also has a low water pick up on the boat for powerhead cooling. The thing I want you to watch is every time the bullet peeks below the surface of the water. Every time the bullet center is below the water level water is force fed hard up all across the gear case. Now when he runs the bullet above (his jack plate is adjustable on the fly) and you can see the water disappear going over the gear case.



respectfully conrad

Good video Conrad, I am being cautious because Im on the other side of the country and cannot see the actual hull and set up. I also think Pete needs to find the right prop before he takes it higher. I agree with you on the Laser 11 or something similar.
You have the undisputed speed runs and experience with building Inline 6,s. I might have more experience blowing them up LOL perhaps thats why Im cautious suggesting engine heights.

Any way, no harm no foul.
Respectfully Dave

10-4 Dave,

Good point you and Kerry make. Raising any higher will probably just slip the props he is currently using. Getting a newer designed cupped prop like a laser II or inline chopper will help. Or at the very least have his current brass prop cupped. But 8 out of 10 times the newer designed props will run faster than even a cupped brass prop.

I bet there is 5mph hiding in there still.

Thanks for the video's Peter. Keep up the good work.
Conrad

Was looking at the spray 21 32 seconds in the video and that is when he is throttling down coming off. more boat in the water. normal to me.

On IL6 water pressure gauge. Curious as never used one. This was on S+F asking..

Inline xs What kinda water pressure by gauge should an xs show.
Answer
An inch above the pad with a stock xs case on my Vixen at 7000rpm is about 12 psi. Cruising around 6-8 psi at the top of the block. Idle, not enough to move the gauge.

Thoughts some the masters?

dave bernard wrote:

Was looking at the spray 21 32 seconds in the video and that is when he is throttling down coming off. more boat in the water. normal to me.

Hi Dave, I've been watching more YT videos of some boats and that spray seems to happen not infrequently, I've just never noticed it before. Happens when I slow down and trim in. Funny.

Pete

Hi Conrad,

This thread has been a real education, glad I put on the H2O gauge, I watch it like a hawk. I know now that at a minimum cupped is the way to go, gonna figure out a way to raise the motor at some point.

I'm getting a new camera and will create more videos of each set up along the way

Pete

Robby321 wrote:

On IL6 water pressure gauge. Curious as never used one. This was on S+F asking..

Inline xs What kinda water pressure by gauge should an xs show.
Answer
An inch above the pad with a stock xs case on my Vixen at 7000rpm is about 12 psi. Cruising around 6-8 psi at the top of the block. Idle, not enough to move the gauge.

Thoughts some the masters?

Not a master, but my gauge read just above 10 at 5600, and basically no reading (that I can distinguish) at idle.

Pete

Daveswaves wrote:

Good video Conrad, I am being cautious because Im on the other side of the country and cannot see the actual hull and set up. I also think Pete needs to find the right prop before he takes it higher. I agree with you on the Laser 11 or something similar.

Respectfully Dave

Hi Dave, really appreciate your input on this, I'm feeling fairly conservative at this point, as I still can't believe the package actually works! I value all advice and criticisms in getting the boat to realize its potential. Looking forward to reporting any and all performance milestones

Pete

Sounds about right to me.

Pete...PARK THE Boat and go on the overseas vacation! When ya get back I'll send that 13x23 2B to ya. Might just be the ticket, but like said PM at the moment I have no use for it. If it works, good. Keep it. If not leave it in your stash. One can never have enough props! Just never the one ya need! Have fun on vacation!

Pete, prop is boxed and waiting yer return to send ya. Heres a couple pix it 13x23 and next my 12x24.

Whoops, pix to big and had downsize. Hope this works now.

That might be a perfect prop for him Robby. At some point to really utilize that prop Im guessing the motor will have to go up at least an inch.



Peter, Vance Manufacturing makes a thin 3 inch jack plate you probably could attach to your CMC tilt . It would give you a total of 9 inches of set back, which you may or may not need, but it would give you easy ability to raise and lower the motor. The Vance would give you 4 inches of travel up.

Just an idea.

Conrad

Yeah, that 2 blade looks absolutely wicked, I just wonder about its ability to "hold" the water/air mix at speed. It will, up to a point, then it will fall off. There is a reason for higher speed boats having 3 or more blades. The more air you introduce to the prop, the less "grip" it has. Adding a third blade gives more blade area in the mix, and adds grip. I do believe the best prop, especially with more height, would be the 1970's 22p inline Chopper already mentioned. However, if the 2 blade is close, and just needs a tweak, I've heard lots of good about these guys. www.dahpropellers.com/