Outboard Motor Height ?'s

[barr5150]

http://i1000.photobucket.com/albums/af125/barr5150/91.jpg?t=1261140512


I'm building a 14' Dory and I am not sure on the motor placement. Maybe someone here can help? The way I have it set up now the cavitation plate is about 3" lower than the bottom of the boat. I would like to keep the motor as high as possible to allow me to fish shallow areas. The water intake on this motor is on the under side of the lower cavitation plate. I have seen guys mount the motor higher where the cavitation plate is level with the bottom of the hull but I would think those motors to have water intakes on the sides of the lower unit. The motor looks too low to me but I'm worried if I raise it any the motor would starve for water when it's on plane. Any help is greatly appreciated. Thanks.

[jmann652401]

This link may help you out.

http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

[donsmarine]

mount where cavitation plate is 1" below bottom.allow for any keel that may be there also.

[Stan D]

First thing I'd suggest is finding the owners manual. Many manufacurers today post old manuals online, so it's probably available. It will tell you how to mount the motor.

What you are referring to as the "lower cavitation plate" is the only cavitation plate you have. What concerns me is the apparent separation at the joint about 6" above the cav plate. It could just be a reflection in the pic you posted. The entire lower unit should be sealed and the only openings should be the water intake(you found this) and the exhaust. There are a few other capped ports, 2 for filling and draining lower unit oil, and possibly a 3rd for hooking a garden hose for running the motor out of the water.

To the question at hand. It has been my experience that the cav plate should be slightly below the hull, maybe 1/2". There is no harm in having it mounted as you have, but to get the least draft, you should raise it up some.

[Bill Huson]

Quick & dirty - the OB engine shaft lengths available are 15", 20", and 25". Small outboards are usually 15" and that number is the transom height, as in 15" from top of transom to the keel.

That said, boat will go faster with less lower unit drag. If you have a keel, shave the aft end down to zero, then use shims under the clamp brackets, such as opposed cedar shingle strips or paint stirrers, and jack that engine up! I use to jack my 25hp Merc until cranking it into a hard turn would cause cavitation. then back it down about 1/8".

Highest I've run an outboard is prop shaft 1/8" ABOVE the bottom. Don't try that with yours - mine was a racing hydroplane and the water pickup was lower than a standard gearshift outboard.

[barr5150]

Thanks for the help guys. Bill I think your right on the shaft lenght - 15". My transom height right now is at 13" and I know it's too low. If I raise it 2" then the cavitation plate will be 1" below the bottom of the boat and that matches what donsmarine said. Perfect. Thanks again!

[rbgarr]

The term is ventilation, not cavitation. Cavitation occurs on the propeller blades. The problem of ventilation has to do with air getting into the water pickups.

AFAIK most outboard mfrs. recommend that the anti-ventilation plate be two inches below the bottom of the transom, but check your manual or with the mfr's rep.

[Lewisboats]

The term is ventilation plate, not cavitation. Cavitation occurs on the propeller blades. The problem of ventilation has to do with air getting into the water pickups.

Amazing how this keeps being perpetuated. I don't know how many times I have had to try and explain this to people and they still insist on calling it a cavitation plate and/or cavitation when it is really ventilation. True cavitation is the reduction of pressure to the point of pretty much boiling the water at certain points on the surface of a prop...it can cause pitting and other nasty stuff and is usually caused by over speed.

[rbgarr]

... how many times I have had to try and explain this....

:D. I've (almost, but not yet) given up correcting the phrase 'try and explain'.

It's try to explain.

[Stan D]

While we're being pedantic, let me fix this.


:D. I've almost, (but not yet) given up correcting the phrase 'try and explain'.

Back on topic. The manufacturers use terms that, while technically not exactly correct, will still be understood by the masses. And the difference between true cavitation and ventilation is virtually nil. The end result is the same. Air in the prop means prop has no bite. Even the word "ventilation" sounds like it's something you would want. You certainly wouldn't want "cavitation".

[rbgarr]

If mfrs use the term cavitation plate in their manuals I'd be very surprised. (Anyone got one?)

And you definitely don't want ventilation of the water inlets. Why would that be desirable?

Cavitation isn't adequately described as 'air in the prop'. It occurs in liquid when bubbles form and implode in pump systems or around propellers. Pumps/propellers put liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to vaporize, just like boiling water. Yet in such a small, sensitive system, the bubbles can't escape so they implode, causing physical damage to parts of the propeller. It's an entirely different phenomenon than pumps not getting sufficient water in the first place.

Even the masses can understand it!

[Stan D]

The manual to my Honda refers to the "anti cavitation plate", and to answer the original question, it says to mount the motor with the "anti cavitation plate" 2 -5 CMs (.8 to 2.0 inches) below the bottom of the boat.

And the comment regarding "understood by the masses". I misspoke. What I meant to say was understood by the lowest of the masses. Try to(:D) explain the difference between cavitation and ventilation to the (insert disparaging phrase for "IQ impaired here).

[rbgarr]

I'm truly surprised about that manual. :eek:

[donsmarine]

short shaft=15"-17",long shaft=20"-22",xlong=25"-27",xxlong=30"-32",mount on the long side,you can always shorten on self built if you plan.1" lower is best place to start.use a straight edge off bottom at deepest point with motor perpendicular to water line.measure from inside transom clamp[where it hangs on boat]to cavitation plate or whatever you want to call"the biggest plate above the prop" to get a starting measure.to be extreme keep raising while testing till it cavitates[make sure you turn hard ]then drop it down again.all motors of the same model group,i.e. merc 9.9's are the same dimension waterline down length changes from water line up for transom height generally.

[Stan D]

Yamaha says that 1"(25MM) is the proper distance for the "anti cavitation plate"to be from the bottom of the boat. They start out by saying that improper installation will allow cavitation to occur. They obviously mean ventilation, as a 9.9hp outboard could never cavitate.

[donsmarine]

in any propeller installation you must be careful of cavitation erosion corrosion,even on a 9.9hp outboard

[rbgarr]

Cavitation can arise from propeller pitch, cup, clearance and engine rpm problems no matter what the HP.

[Stan D]

True cavitation? I always thought that cavitation (not ventilation) was a result of high prop speed, like in a racing situation.

[donsmarine]

i beleive a 1 bladed prop might be the best for cavitation/ventilation issues,althou it may shake just a little

[rbgarr]

True cavitation? I always thought that cavitation (not ventilation) was a result of high prop speed, like in a racing situation.

That would be the rpm issue.

[Breakaway]

Hey Buddy,

Just to confirm ....mount your engine so the plate is between a half and 1-inch below the bottom with the motor vertical. (Your transom angle is probably 12-15 degrees). Use a straightedge. I cant see the top of your transom/motor cutout, but the mounting bracket need not be in contact with the transom. Adjust using one of the lower mounting hole pairs and you'll be fine.

As installed, you'll increase drag, decrease speed, and probably have a gusher of water shooting vertically up at the transom as well.

The water intake is the black square seen on the forward edge of the gearcase several inches below the plate --black plastic grate.

Finally, its an ANTI-ventialtion plate. It prevents prop from from sucking air through surface of the water, which is ventilation, i.e, "an opening to the outside." Cavitation results from too large a pressure difference between the leading and railing edge of the blade. When the relative pressure becomes low enough on the leading edge the water there boils, creating, naturally, bubbles. Not only is this bad performance, but when theses bubbles "pop" they do so violently enough to take teeny bits of the prop off. Thats the erosion the other poster referred to.

Cheers

[donsmarine]

i doubt the size motor that is being installed has mounting holes,other then 2 on the bottom,therefore the wood where the clampscrews[mounting brkt] are is important for a secure mount.once the motor is confirmed to be at the correct height,then drill and use them,as long as the transom construction allows for this.i'd test 1st before drilling holes.i'd also put some type of mounting pad under the clamp buttons so you don't damage the wooden transom when you tighten them.in closing you can still call the "big plate on the bottom"anything you want.what matters is location,location,location.some things in life you could refer to it as position.

[Breakaway]

Doh! Donsmarine is absolutely right. No mounting hole adjustment on a small ob. I'm too V-6 oriented for my own good. My bad.

Have a happy.

[MiddleAgesMan]

An old Johnson 28hp motor I purchased for use in my Simmons has the water intake ports above the cav plate. If the OP has such a motor mounting it so the plate is 3 inches below the bottom would not be unreasonable. I believe my cav plate will be more like 2 or 2 1/2 inches below the bottom but I won't know for sure until i get the hull outside and stick the motor in the well.

[Breakaway]

Thanks Middle Ages. Went through some old clymer manuals and found some engines that had water intakes above and below the 'plate. (I cant figure the reasoning, though?) Anyway you helped me learn something and that's why I'm here.

I also google this, which, is a pretty good primer on OB installs, including heights, speeds and boat type considerations. I found it informative and thought others might as well.
http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html
Cheers

[donsmarine]

most motors have 2 water intakes incase 1 gets plugged you don't toast the motor?

[MiddleAgesMan]

I read somewhere--on here, maybe a post by Erster?--that my particular motor was designed for operation in shallow water. The intake is above the cav plate to minimize the silt or sand or mud that gets sucked into the cooling water ports. This means that cav plate has to be lower and the shaft length longer. I had been baffled by the shaft length I measured--roughly 22 1/2 inches--and thought I had to build a 22 1/2 inch mounting height into the motor well. Not so--the motor was designed for a 20 incher so that's what I've got.

[donsmarine]

middleagesman:on the older 28 look at the skeg,there should be a intake screen there.look at my #14 post reguarding shaft lenght.other than jets i don't beleive any motor was designed for just shallow water,but i haven't seen everything yet.

[Willin']

It's impossible for a high RPM propellor such as we have on all of our outboard motors to not cavitate. This I learned in US Navy submarine sonar operator school.

Cavitation is simply the dissolved gases in water coming out of solution on the trailing side of a propellor due to the pressure differential between the in situ water and that behind the blade. In most cases the gas bubbles immediately collapse as soon as the differential disappears, causing a good deal of sound and energy loss. This energy loss is what causes pitting on the blades.

Submarine and warship props are designed to reduce or minimize this, but it will occur during high speed maneuvers and rapid acceleration. Outboard prop rpms are so high that it will always occur at normal operating speeds.

I'm not sure how ventilating a prop affects cavitation, and would be interested to learn what the designers know about this.

[donsmarine]

isn't prop cavitation/ventalation a result of slippage which is a feature of every prop driven boat?

[MiddleAgesMan]

middleagesman:on the older 28 look at the skeg,there should be a intake screen there.look at my #14 post reguarding shaft lenght.other than jets i don't beleive any motor was designed for just shallow water,but i haven't seen everything yet.

The only things below the cav plate on my old motor are: the prop, exhaust port, and a flush screw I assume is the drain plug for the gearcase lub. The water intake is on the port side of the foot just above the cav plate. It's too close to it to use one of the muffs for running the motor out of the water. I'll have to rig something special for that or stick it in a bucket of water.

That lub oil drain plug got me thinking about preparing the motor for winter (no freeze here yet). That's something I'll need to address but I will have to do some research since I have no experience with such things.

[donsmarine]

what year or model# is the 28

[MiddleAgesMan]

what year or model# is the 28

It's an '87, just like this one:

http://www.smalloutboards.com/j2887.htm

Model is J28ESLCUC

[donsmarine]

when you said old i thought 60's[28,33,40hp series]nice looking boat!!!!

[rbgarr]

when you said old i thought 60's[28,33,40hp series]

Me too.

[barr5150]

Wow! Thanks for the replies! I moved the transom motor mount height up 3". That puts the "plate" ( carefull not to call it anything specific ) right at 1" below the bottom of the boat. All the info I received said this was the right place for it to be. We will see when it's in the water. Thanks again for all the help.

- Bryan

[MiddleAgesMan]

1987 is OLD to me and I'm probably 10 years older than any of you young whippersnappers! The motor in that for-sale listing looks much nicer than mine but my powerhead looks much better. Under the cowl, it looks brand new, leading me to suspect it might have been rebuilt not too long before I bought it.

As for what to call that plate thingy--"cav plate" works for me. :)