How deep is deep enough?

[FORTUNATA]

Hi, After all the rain and poor weather, work has been slowed down somewhat but today I did fit my Mariner Outboard to my boat and just maybe it will be O.K. There is one question on my mind, however: I wasn't sure the engine is as submerged to the level I expected. It's fixed on by one of those commercial adaptor brackets and I can say that, yes, the prop goes well into the water. However, the flange that is seen between the gearbox and the upper engine casing is only just about covered with water.

This has all been a bit tricky as I needed to leave room for the pull chord on top of the cowel which is only just about clear of the top of the stern. Any lower and this could be a bit trickier as well as fitting the fuel hoses (also around the top).

I'm hoping my engine is sufficiently deep enough in the water and hope someone can explain whether this is so. I was assuming the water intake to the pump was somewhere around the prop.

Also where is the cavitation llate exactly as I can find no diagrams?

[howardang]

Hi, After all the rain and poor weather, work has been slowed down somewhat but today I did fit my Mariner Outboard to my boat and just maybe it will be O.K. There is one question on my mind, however: I wasn't sure the engine is as submerged to the level I expected. It's fixed on by one of those commercial adaptor brackets and I can say that, yes, the prop goes well into the water. However, the flange that is seen between the gearbox and the upper engine casing is only just about covered with water.

This has all been a bit tricky as I needed to leave room for the pull chord on top of the cowel which is only just about clear of the top of the stern. Any lower and this could be a bit trickier as well as fitting the fuel hoses (also around the top).

I'm hoping my engine is sufficiently deep enough in the water and hope someone can explain whether this is so. I was assuming the water intake to the pump was somewhere around the prop.

Also where is the cavitation llate exactly as I can find no diagrams?

Why not look on the Mariner Outboard site - they have a download facility for all their models with technical drawings, photographs etc. You should be able to find your model there. I think the cavitation plate is what you are describing as tyhe flange between the gearbox and the uper engine casing.

 

Mariner website is

 

http://www.marineroutboards.co.uk/main.php

 

HTH

 

Howard Anguish

Edited August 13, 2007 by howardang

[FORTUNATA]

I may possibly be in a pickle since a shop I phoned for info told me this cavitation plate should be level with the bottom of the hull. I think in my case the propeller itself is the area that's level with the hull and the canalwater line falls just at the point where the gearbox meets the upper casing. It just about laps over it it by millimetres.

The page you forwarded me has a diagram as you scroll right down:

http://www.marineroutboards.co.uk/enginechoice.php

I tried to make some careful measurements when preparing to fit the bracket and had to also provide some space for the fuel hose and starter chord. My suspicion is it may be a bit high unless the water inlet to the cooling system is sowhere near the propeller.

 

Why not look on the Mariner Outboard site - they have a download facility for all their models with technical drawings, photographs etc. You should be able to find your model there. I think the cavitation plate is what you are describing as tyhe flange between the gearbox and the uper engine casing.

 

Mariner website is

 

http://www.marineroutboards.co.uk/main.php

 

HTH

 

Howard Anguish

[DHutch]

It does sound a little high, but it might work ok for you. Your not planning to get it on the plane are you?

- If i where you i would be tempted to 'suck it and see', fire her up and see if it cavitates in normal useage.

- Otherwise you'll have to do something more clever, or maybe ballest the back a little more maybe?

 

Do you have a photo?

 

 

Daniel

[FORTUNATA]

I tend to maybe take these things perhaps a bit too seriously at times but I'm on edge as I put so much work into the fitting of the bracket. I've been working on the diesel engine as well and making some progress with that, of course.The waterline seems to cover about 90 per cent of this flange that lies between the gearbox and the engine body. Of course, this is not in deeper waters and only in the shallows since the boat is still moored up. Perhaps more water would cover the said flange once the boat is drifted out. I do know the prop is virtually level with the bottom of the hull so that's O.K.As I said, if I took the trouble to lower the entire bracket I fitted, this would then create another problem - that of clearance for the starter chord that lies just one inch clear between cowl and fibreglass. Really I need to know exactly where the intake inlet is for the water pump on a Mariner 7.5. If it's down at the lower gearbox end, I can get away with it. If it's near the uppermost flange, I have a problem. I'm still trying to find a Mariner diagram.My brother says I worry too much but I though this was such a good job it would be a pity for the depth to be wrong. He suggested simply loading more weight around the stern to push the engine further into the water.I think I might just get away with this but maybe by the skin of my teeth.

It does sound a little high, but it might work ok for you. Your not planning to get it on the plane are you? - If i where you i would be tempted to 'suck it and see', fire her up and see if it cavitates in normal useage. - Otherwise you'll have to do something more clever, or maybe ballest the back a little more maybe?Do you have a photo?Daniel

I can get you a pic I think. There's a special bracket mounted on thick pieces of ply internally and externally. The structure is so strong it actually supports myself. The engine weights about 6 and a half stone. I even varnished the ply with marine varnish. When fitting it, I measured the same distance from engine clamps to prop and applied that from the bottom of the hull to the bracket. I was surprised my dealer said the cavitation plate should be level with the hull as that would mean on my engine that the prop would be well below the hull, churning up silt. Maybe my cav plate is higher than usual.

It does sound a little high, but it might work ok for you. Your not planning to get it on the plane are you? - If i where you i would be tempted to 'suck it and see', fire her up and see if it cavitates in normal useage. - Otherwise you'll have to do something more clever, or maybe ballest the back a little more maybe?Do you have a photo?Daniel

[fuzzyduck]

the shop is correct

 

the cav plate should be level with the bottom of the hull. If it's not underwater the prop will cavitate, and you'll lose power, this will make matters much worse during a crash stop where 1/2 the reverse thrust will deflect off the transom. There may also be issues with the water cooling system priming.

Edited August 14, 2007 by fuzzyduck

[FORTUNATA]

I can share at this point is I discovered it's no cakewalk to adapt an inboard cruiser to outboard propulsion (in this case as a temporary measure).

For instance, I had to consider things like internal beams of wood that limited where I could bolt my buffer plywood specifically. Then there is the starter pullchord. As things stand this is only clear of the fibreglass by just over an inch. Any lower and I'd possibly have been struggling.

The main point of concern is water cooling. I have a really good beginners book on outboards borrowed from the library and this is what I gleaned: It appears outboard engines vary somewhat. Even boats specifically designed with a transom may need to consider which outboard fits best. Also the water inlet to the pump varies. In my case I suspect the inlet is well submerged and the prop is right at the lowest point of the hull. The water cooling system should work but I'm hoping to be sure before I fire up.

One thing I do know is it's out of the question to remove and refit the bracket. I'd rather somehow modify the block (if possible) or even consider trading my engine for one with an adjustable leg.

However, Daniel's suggestion of using weight is probably the most straightforward. I'm sure with increased water depth (out of the mooring) and some 20 K.G disks I'd get 2 inches above the cavitation plate and I'm hoping to get the diesel engine back to par too.

I just hope I get there in the end.

 

 

the shop is correct

 

the cav plate should be level with the bottom of the hull. If it's not underwater the prop will cavitate, and you'll lose power, this will make matters much worse during a crash stop where 1/2 the reverse thrust will deflect off the transom. There may also be issues with the water cooling system priming.

[HYBRID]

Is your boat a V Hull if so could you not mount the outboard to one side, I had a day boat once with a Mercruiser and stern drive fitted and when the leg was down the cavitation plate was level with the keel of the boat but my second back up engine was to one side and higher up the transom.

[FORTUNATA]

The outboard is indeed mounted to the side. This is because I'm keeping my diesel engine and have been working on this as well. I even have a diesel engineer to help me.

This has been a process of learning and a bit of frustration at times as I go on learning. This outboard is merely intended as a back up engine so I'm aware I may not get the same quality of outboard propulsion as I would with an outboard-designed boat.

So long as it works on outboard for a time, I don't mind. I'm planning to drive the boat to a marina to have all sorts of work done so that's what the extra engine is for.

You know, I figure I've been getting there and continue to learn stuff. I discovered my outboard anodes hadn't been properly bonded to the engine so I rubbed down the gearbox at the appropriate spot and then did a test with a multimeter. It showed zero resistance after that process so this part is O.K.

 

 

Is your boat a V Hull if so could you not mount the outboard to one side, I had a day boat once with a Mercruiser and stern drive fitted and when the leg was down the cavitation plate was level with the keel of the boat but my second back up engine was to one side and higher up the transom.

[chris w]

I'm sure with increased water depth (out of the mooring) and some 20 K.G disks I'd get 2 inches above the cavitation plate....................

 

 

You need to run the maths on this. Remember that every weight you install will displace an equal weight of water so to get 2" increased draft over let's say a 7' x 3' area will take about a 100kg.

 

To get the whole boat lower in the water by 2" will take approx 33kg per foot of length (assuming your boat is narrow beam). So a 30ft boat would require approx 1000kg (ie: 1 tonne !!!) of additional ballast evenly distributed.

 

Chris

Edited August 15, 2007 by chris w

[FORTUNATA]

Of course, I'll be on the boat as well in the cockpit area which is stern end on my boat.

I'm going down today to have a real good look at it as last time I was down we rushed a bit. My brother was down to help me lift the engine on and then we left. I seem to recall the gearbox flange (or cav plate) being under water but only just.

I figure the main thing is to be sure before I start it up. My main concern isn't so much economy of propulsion than poor water cooling. I have to be sure the engine will be getting a steady flow of cooling water. I think mine must have the cav plate fairly high as I've seen outboards in books with very low cav plates. There are also air-cooled outboard engine which are pretty rare but used to be quite common.

Today I'll measure the amount of water I have over the cav plate and let you all know. Maybe it's not as bad if I let it out of the shallow water a bit. I do know the prop itself is dead level with the bottom of the hull.

My gut feeling is I may just get away with this but best not to rush things.

 

 

You need to run the maths on this. Remember that every weight you install will displace an equal weight of water so to get 2" increased draft over let's say a 7' x 3' area will take about a 100kg.

 

Chris

[chris w]

I do know the prop itself is dead level with the bottom of the hull.

 

What do you mean by this exactly? The top of the prop, the bottom of the prop or the middle of the prop.... is "dead level with the bottom of the hull"?

 

Chris

[dccruiser]

The cavitation plate for the prop is the plate immediately above the said prop .... for best results this should be level or just below the bottom of the hull.

The water intakes are usually on either side of the casing just below this plate on the gearbox , just above the propshaft. they are easily recognisable as two black plastic vents with a holding screw in each for easy removal for cleaning.

Unless you have an history of the engine i suggest you remove the lower leg 4 bolts not forgetting to disconect the gear selector rod and change the impellor.

You will find this under the waterpump housing on the driveshaft.

 

Cheers

Rick

[FORTUNATA]

Maybe I'm not quite sure what the cavitation plate is as I've not found a diagram with it clearly marked. I would say my prop would be lower than the hull by maybe half an inch - the bottom of the prop that is.

It will probably be O.K. - let's hope I at least get to the local pub on it!

 

The cavitation plate for the prop is the plate immediately above the said prop .... for best results this should be level or just below the bottom of the hull.

The water intakes are usually on either side of the casing just below this plate on the gearbox , just above the propshaft. they are easily recognisable as two black plastic vents with a holding screw in each for easy removal for cleaning.

Unless you have an history of the engine i suggest you remove the lower leg 4 bolts not forgetting to disconect the gear selector rod and change the impellor.

You will find this under the waterpump housing on the driveshaft.

 

Cheers

Rick

[chris w]

I would say my prop would be lower than the hull by maybe half an inch - the bottom of the prop that is.

 

 

My guess is that this will be an issue. If the flow of water through the propellor is restricted by the fact that a few inches in front of it is your transom, you may find that you cavitate badly due to lack of sufficient water flow. I guess, as you say, you need to suck it and see.

 

Chris

[FORTUNATA]

I have another wee problem to solve and this is definitely nothing I did. It was a fundamental design flaw and I was really annoyed when I discovered it. You know how an outboard engine has clamps? I mean, you turn the big screws (or whatever they call it) and the jaws squeeze tighter and tighter till they clasp your block of wood or transom tight. That's what I call the outboard clamps or jaws. Well, the snag here is that once you clamp the engine on the adaptor bracket, there is no room to turn both of the outboard clamp key ends so they become tight on the wooden block. They're confined within a framework. The keys at the end of the locking device aren't clear of the bracket frame. You can tighten only one of them by moving the outboard either left or right a bit from central on the wooden block.

My brother was with me when we clamped it on. We can tighten one clamp O.K. but not the other clamp. He seems to think I should just saw one of the key-ends off with a hacksaw. But I don't know why the manufacturers of the bracket didn't consider this. I never thought to test the design actually works as I figured the manufacturers know what they're doing.

Of course, it's difficult to describe what an adaptor bracket looks like. Mine has a frame that you bolt directly to your fibreglass and a thick piece of wood fixes to the frame to accommodate the outboard jaws.

The bracket is strong, though. I can stand on it and it supports me. The outboard only weighs 6 and a half stone as I got on the scales with it.

 

 

 

My guess is that this will be an issue. If the flow of water through the propellor is restricted by the fact that a few inches in front of it is your transom, you may find that you cavitate badly due to lack of sufficient water flow. I guess, as you say, you need to suck it and see.

 

Chris

[magnetman]

Mariner 7.5? Is that a new 4 stroke model? I recently bought a mariner 3.5 4 stroke for my dinghy and its given trouble in hot weather, apparently they aren't designed for continuous operation. I've never seen a mariner 7.5.

[FORTUNATA]

It's 2 stroke. I bought it for simplicity, no timing belt, no valves, no fuss. It's also a very powerful engine. I raised this thread mainly because I'm taking care not to damage the engine.

I got it second hand from a local boatyard for about 300 pounds. Personally I like outboards since you can just take them off your boat and then service them as required.

Mine had totally inadequate anodes when I took ownership of it. Not only had they rusted away but they weren't electically bonded to the engine ground.

When I hung the engine on my old boat, it was a great feeling. That boat hasn't chugged in many many years and I can't wait to see it go. It now has a really good alternative engine which I plan to lock on by chain this evening.

Hmmmm, sounds like your engine wasn't cooling too well that day.

 

 

Mariner 7.5? Is that a new 4 stroke model? I recently bought a mariner 3.5 4 stroke for my dinghy and its given trouble in hot weather, apparently they aren't designed for continuous operation. I've never seen a mariner 7.5.