Waterline?

[Roy 845]

Hi All,
I have removed the "Garelick" Outboard motor Bracket because the previous installation was incorrect in every respect. The mount was incorrectly assembled and required a stack of wedge shaped plywood pieces to get the mount to sit plumb on the transom and plain steel bolts were used which are now heavily rusted and the wood is rotten. My plan is to repair the previous drillings and secure a 12" square piece of G-10 fiberglass board to the inside of the Transom. My actual question is where is the waterline on my 1977 O'Day 22 ? The Honda 4 stroke manual I have states that the anti cavitation plate needs to be 4" below the waters surface and the Garelick has what I measured as a 8" range of motion up and down. Before I place the new bolts for the motor mount I want to be confident I'm at the right height. There is a Blue stripe painted on the Hull down low that is a couple of inches wide. Is this the actual waterline? Should I do something else to determine the height of my engine?

 

[Ward H]

On my 25 the waterline is about an inch below the boot stripe. You will probably appreciate getting the cavitation plates at least 6" under the surface so it doesn't pop out in chop.
If I was mounting my bracket I would measure from the cavitation plates to the top of the OB brackets. Then the mount at it's highest position and in its lowered position. Using that you should be able to work out the mounting height above the water line.

 

[Roy 845]

Thank you Ward, good comment about the chop, that had never even crossed my mind.

 

[ebsail]

If you have long shaft outboard or even an extra long shaft, try to get the cavitation plate even deeper. The stern bobs up and down quite a bit and the prop will cavitate quite a bit. If you mount too high, you'll have a problem. If too low, you can just raise the bracket one notch when the engines running.

 

[Roy 845]

Deciding to take your advice and not actually knowing where the waterline will be until she is in the water I made the mount ~ 3" lower than the previous installation. The way it looks is that the water intake will be just above the waterline while the mount is raised and the anti cavitation plate about 3-5" below the waterline. The painted waterline is about 2 1/4 " wide. It was suggested to me to drill the bolt hole locations with a 1 1/2" holesaw and fill in the resulting cavities with fiberglass and let it set up solid. Then fiberglass a 12" square piece of G-10 inside the transom as a backer then drill the actual mount holes. Making the holes oversize and refilling removes any wood or foam core and replaces it with solid reinforced resin. Does this procedure sound reasonable?

 

[Brian S]

Does this procedure sound reasonable?

Sealing deck penetrations to prevent rot:

http://www.pbase.com/mainecruising/sealing_the_deck

 

[Scott T-Bird]

The waterline stripe that you describe as 2-1/4" wide is the "boot stripe" that Ward describes. It is typically entirely above the waterline. As Ward says, 1" is probably about right. The boot stripe is probably glossy on your boat, right? Below that you would have the bottom paint, which will typically be a much duller color and probably contrasting. The bottom paint may extend about an inch above the waterline when the boat is light. When the boat is fully loaded, the waterline might rise about an inch. You should be able to distinguish the normal waterline by finding a faint stain on the bottom paint where the boat normally rests.

 

[Ron20324]

On the other hand, some boats have the boot stripe, then about an inch of the actual bottom(white) gel coat, THEN the bottom paint. SOME FOLKS CHOOSE TO BOTTOM PAINT THAT WHITE STRIPE.

 

[Scott T-Bird]

On the other hand, some boats have the boot stripe, then about an inch of the actual bottom(white) gel coat, THEN the bottom paint. SOME FOLKS CHOOSE TO BOTTOM PAINT THAT WHITE STRIPE.

I thought about that, also, but didn't want to confuse the issue. I suppose the more traditional approach is to leave the unfinished gel coat just above the waterline? It seems that it is more common to see the bottom paint bordering the boot stripe.

 

[Ward H]

It was suggested to me to drill the bolt hole locations with a 1 1/2" holesaw and fill in the resulting cavities with fiberglass and let it set up solid. Then fiberglass a 12" square piece of G-10 inside the transom as a backer then drill the actual mount holes. Making the holes oversize and refilling removes any wood or foam core and replaces it with solid reinforced resin. Does this procedure sound reasonable?

That is about what I did when I mounted my boarding ladder but first I drilled a hole to see if any wood came out. None did so I looked carefully at the inside of the transom. I could see where the wood was sandwiched on the other side and where the rudder was mounted. It was obvious that there was no wood on the side I was putting the ladder so I skipped the oversize holes as there was no wood to protect.
You might want to think about another layer of G10 on the outside to sandwich the transom fiberglass for a little more stiffness. If not G10 some board that is easily painted or comes white.

 

[Roy 845]

Thank You Brian, There's a lot of good information here and will be valuable as I go through my repairs.I ran into an actual picture that shows the waterline about 1" below the "Boot stripe" using this information places the mount even farther down the transom. I now wonder how the mount ever worked in the position I originally found it in?

 

[Brian S]

I now wonder how the mount ever worked in the position I originally found it in?

Haha, it maybe didn't. There is a difference between a 22" long shaft, and the 25" extra long shaft found on "Sail Pro" or "Sailwind" models of some outboards. Also, a motor that isn't particularly deep may work well on flat water, but have horrible problems with ventilation (when the prop comes partially out of the water and sucks air) in a chop.

In my opinion, putting the prop (and anti-ventilation plate) well below the water's surface is not a problem on slow displacement hull sailboats. It's insurance against ventilation, as already mentioned, and it's not as if it will prevent water intake or something. The whole shaft housing should be sealed pretty well at least 1/2 way up from the lower unit. However, for a planing hull boat, the manufacturer's recommendation for depth positioning attempts to balance water intake with lower drag, and therefore improved speed. Racing powerboats often have a very low profile lower unit for reduced drag, and a hydraulic lift plate for the transom mount, so that at speed, the motor can be jacked upwards, further reducing wetted drag of the lower unit. Some racing motors are designed with a surface piercing propeller that's designed to ventilate. It's not an outboard, and you won't find one on a sailboat, but one brand that comes to mind is the Arneson Surface Drive: http://new.arneson-industries.com/surface-drives As a kid, I enjoyed pictures (we didn't have internet video then) of the Arneson drive on offshore racing powerboats, because they threw a helluva rooster tail. Also, where my Grandparents had their beach place in an upscale trailer park, the owner's son always had a Donzi Sweet 16 or similar, and when I heard the roar of V8 through straight pipes, I'd run to the bay side, and watch as he got on plane, then adjusted the tilt of the I/O outdrive for speed and a moderate rooster tail. (I hate that guy for being a greedy douche and causing problems for my Grandparents, but how I loved him for having sweet boats. )

 

[Panta Rei]

I think you are missing a major consideration. As your boat speeds up and approaches hull speed the will literally sit in a hole. It's this hole a planing boat powers it's way out of. The boat below has a swim platform that is at the waterline when the boat is at or near hull speed. Note that the waterline below the dark green boot stripe. The swim platform is over 18 inches above the line.
You will want to put this all into consideration. I feel your pain in trying to figure out what the PO had in mind. I had two POs one did good work the other made a mess of things.

 

[Roy 845]

I'm really appreciating all of the input that you fellas are providing. I think that Iv'e spent a lot of time considering my options here and if mechanically all works well I will assume the waterline is about an inch below the boot stripe and make the anticavitation plate sit 6 inchs below that point. It seems now that we will be reaching far over the transom to start the beast. I think Brian's comment "putting the prop (and anti-ventilation plate) well below the water's surface is not a problem on slow displacement hull sailboats. " is well founded and a good point to consider. When the Transom repair is finished I may use Blue bottom paint right up to or maybe even over the boot stripe. Being 2017 will be my first year in the water my goal is to just to make her seaworthy and use her as is this season and evaluate what we want to accomplish next season. If all is well structurally then well make her pretty. There is a Beneteau First 345 that has caught my eye at the club though. Hmmmm....

 

[Ward H]

I think Brian's comment "putting the prop (and anti-ventilation plate) well below the water's surface is not a problem on slow displacement hull sailboats. "

I think Brian's spellchecker got the best of him. I'm sure he mean "anti-cavitation plate".

 

[Brian S]

I think Brian's spellchecker got the best of him. I'm sure he mean "anti-cavitation plate".

Nope. I meant anti-ventilation plate. From what I know, cavitation is the condition where the pressure drops across a propeller blade so low that water boils on the surface of the prop. With really bad cavitation, the collapsing bubbles of the boiling water can erode and damage the surface of the prop. From submarines, I learned that low speeds of the prop reduce the chances of cavitation, but higher speeds are very noisy because of cavitation. Now, ventilation is the condition where the prop sucks air in below the surface, and loses bite on the water. So, how can a plate above the prop, outside the turning radius of the prop, affect low pressure gradients across the surface of a prop blade? As far as I'm concerned, the plate on the lower unit is intended to reduce ventilation, particularly when turning. When I was a 20-something idiot, my friend and I water skied a lot. His boat would ventilate all the time when turning hard to go back to get a skier. He installed a Doel-fin on his motor, which were wings extending out to the sides of the anti-ventilation plate. At the time, there was much to-do about these kinds of fins, in that they would aid hole shot, and get the boat up on plane faster. We found that they allowed the boat to turn much more quickly without ventilating and spinning the prop with high RPMs. (The boat turned so quickly and sharply, it would sometimes hook the spray rail and turn violently sharper. Don't worry, he still feels guilty about the time this caused the boat to run me over. But that's a story for a different time.) It probably didn't do much for top-end speed, as it introduced more drag, but on the other hand, we're talking about a 16' late 60's/early 70's aluminum Crestliner with rounded chines, so there was no inherent speed performance in that hull

Anyway, I'm calling the dang thing an anti-ventilation plate, and I'm sticking to it. If we find a marine hydrodynamic engineer who explains to me why the industry calls it differently, and why I should too, I'll consider it. ('Cos I'm pedantic like that.)

Here's what an installed Doel-fin looks like:

 

[Brian S]

Oh, also, I will whole heartedly agree that putting the motor too low could cause issues with a squatting boat, and swamping the powerhead in a following sea. On the other hand, if your boat squats that much, you really need to reduce power because you are at or above hull speed, which can lead to very squirrelly handling and broaching.

Darn, I feel like a curmudgeon today, all pontificating and admonishing. Sorry.

 

[Ward H]

Anyway, I'm calling the dang thing an anti-ventilation plate, and I'm sticking to it.

And well you should!
After reading your post I went to my Yamaha OB manual. It does identify the "plate" as an "Anti Cavitation Plate" but I also saw a reference to the propeller pulling in air if it is trimmed to close to the surface causing "ventilation".
I did a bit more searching and found writings with the same explanation that you gave for cavitation and ventilation.
So I stand corrected. The plate is actually an anti ventilation plate, despite what the manual says.
Never know when you will learn something new.

 

[Roy 845]

Well I received from Jamestown Distributors today a package with a 12" square x 1/4" thick piece of G-10 fiberglass board for the inside of the Transom and some tubes of "Thixo" Epoxy Adhesive for this project. I'm not looking forward to cramming myself inside the crawl space behind the transom to prep it for this but what will be will be. I also received from McMaster-Carr a 4' square of 1/2" "Starboard" that I'm going to use as the entrance to the Cabin. The "PO" had plywood varnished for that just recently and that was already delaminating. There's also bulkheads that are coming apart that I'll need to address but they're not important for putting her in the water so that will have to wait.

 

[Brian S]

I also received from McMaster-Carr a 4' square of 1/2" "Starboard" that I'm going to use as the entrance to the Cabin. The "PO" had plywood varnished for that just recently and that was already delaminating. There's also bulkheads that are coming apart that I'll need to address but they're not important for putting her in the water so that will have to wait.

Having a 2 or 3 piece drop board can make storage easier. If you choose to do that, make sure you cut the pieces at a 45º angle sloping downward to the outside of the cabin. That way water will not go inside.

Also, some boats such as Precisions have chainplates that attach to the bulkheads. Not sure about the O'day 22, but it's quite common for P-boat owners to have to replace rotted bulkheads because the previous owner did not keep up on resealing the chainplates at the deck, and water leaks down onto the teak-faced plywood of the bulkheads. Not a good thing for sailing...