27 Flybridge conversion from I/O to Outboard

[cdavisdb]

I know zip about newer outboards, but based on my experience with duoprop 1/0s I'd be a little concerned about longevity of a 300 hp outboard. The 27 is a much bigger and heavier boat than mine. I run a 5.7, about 300 hp, fuel injected V8 with a volvo duoprop in a 25 Seafari, an extremely torquey combination. It's perfect with a light to moderate load. When you load it real heavy, the engine is working very hard, easy to exceed the safe vacuum range, which would seriously hurt engine longevity. A very big, low pitch prop would be a good idea.

Anybody know if you can install a vacuum guage on a big outboard, and what readings would be normal?

[Bushwacker]

Originally Posted by cdavisdb

Anybody know if you can install a vacuum guage on a big outboard, and what readings would be normal?

This is a good question, indicating Connor and Blue Heron have come to understand the importance of a vacuum gage on a 4-stroke engine from their personal experience and maybe from some PM’s/e-mails I've sent them. Perhaps some background info on the subject would be of interest to other folks running 4-stroke motors (Inboard, I/O and OB versions).

The piston aircraft engines (Continental, Lycoming, etc.) were designed to cruise at about 75% power, typically called the “Maximum Continuous Power Rating”. The Takeoff Power rating is 100% power and typically only used for a short time during takeoff and initial climb out. Similar ratings are used on other industrial engines as well. Since the exhaust valve is the hottest and most limiting part in any 4-stroke engine, these heavy duty engines are built with some very special (= expensive!) parts to address that weak link. They use sodium-filled exhaust valves, positive valve rotators, and Stellite (a very hard high temperature Cobalt alloy) valve seats to increase the life of those parts.

Thermodynamic calculations show that an intake manifold vacuum reading of 5 inches of mercury (" Hg) works out to be about 75% of max power on any naturally aspirated Otto cycle engine. (100% power = 0” Hg reading on a vacuum gage or an absolute pressure of 29.97 “ Hg/14.7 psi). Doesn’t matter whether it’s a 2 of 4 cycle engine; the calculations are the same. Since aircraft engines are built to give a reasonable service life at this 75% power setting, using the corresponding 5” Hg manifold vacuum level is a logical starting point for trying to establish the “Max Continuous/Cruise” power setting on our marine engines, and I understand this is the factory recommendation for a cruise power setting on MerCruiser engines. Since they don’t have all the exhaust valve goodies of the aircraft engines however, you’d get more valve life if you cruised at a higher vacuum level. Frank Brown, who was an ace mechanic that used to crew for Carl Moesly on the race boats, told his customers their engines would last longer if they cruised at about 7” instead of 5" Hg! I’m really amazed that this info is not widely published by the factories, especially on the 4-stroke outboards. I hate to think what it’d cost to do a valve job on those fancy 4-valve cylinder heads they use! If I was running a 4-stroke outboard, I’d definitely have a vacuum gage on it and would observe a cruise limit of about 7” Hg!

Since 2 strokes don’t have any exhaust valves to burn out, there is no benefit to using a vacuum gage on them. The limiting 2-stroke parameter seems to be piston temperature, since BRP went to a higher temperature piston alloy on the E-Tec motors. Even if it doesn't melt or come apart however, a piston that gets too hot, either from detonation or too lean a mixture, can expand enough to start scuffing the bore. You’d need a knock sensor and an exhaust gas temperature or oxygen sensor to prevent that. I believe the 300 E-Tec does have knock sensors but haven't heard about any O2 sensors on them. I think the Yami HPDI O2 sensors are a good idea. Does anyone know if they pop a warning flag if the EMM cannot correct a lean condition, like from a plugged injector or fuel filter? Denny

[Ryan]

Brian if you are going keep her forever you should consider diesel power or twin outboards. I think you will wear a single outboard out quickly pushing anything more than a light load. Gas is gonna be expensive no matter what.

[3rdday]

Thanks all.
I plan to keep her forever. I'm not looking for speed.
Been reading up on large boats with single 300 etecs.

I'm going to experiment with an older single evinrude v8 4 liter 300 hp. I know it's a fuel pig, the 454 was a fuel pig.

If I don't like it I can bracket twin 175 ho or 200 etec.

[Islandtrader]

I guess what I would do is track down the guy who bought Carl's boat and see what his take is on the out board situation.

[cdavisdb]

Thanks Denny. You keep adding to my vac knowledge.

[captbone]

Great project and I am looking forward to pictures.

I have a 25ft Bertram FBSF with a single 225hp Optimax. I cruise at 25mph and top out with a light load at 40mph. I think that you will be able to match those with your hull and a 300hp easily. Its all about set up and propping.

I have fished with 6 adults, 80 gallons of fuel and full gear without problem and that is on a 25ft boat with 10ft beam and a flybridge.

The engine will be fine and last just as long as any other engine. An outboard does not know if it is on a 40ft barge swinging a 11pitch prop at 4000rpm or a 20ft speedboat swinging a 25pitch prop at 4000rpm. As long as your treat your engine correctly it will last just as long. If 4000rpm = 24mph then so be it. The only reason that engine dies quicker on a heavier single boat is because the owner tries to go faster and cruises at higher rpm to make up for the lack in power.

I have seen a water taxi in the carribean, a 28ft Bertram flybridge with a single 250hp Yamaha that ran just fine, day in and day out.

The only downside that you need to be aware of is low speed handling. It will not back down well and turning takes more room.

[3rdday]

captbone,

Thank you for your input and valuable insight and experience..
I hope it works ok, like I said the 300 v8 will be for experimenting and short term, if it does well i know the etec 300 will. In any case it wont be a big deal to reset for twins.
Im going to be adding knees to the stringers and transom and a longitudinal beam across the transom. I re cored the transom last year with composite, eglass biax and epoxy......
Its easy work as long as its not in the dead of summer- my only complaint is grinding glass in the summer.

EDIT+=Sorry for the thumbs down at the top of the post - meant for it to go at the end of"grinding glass in the summer"

[gofastsandman]

Originally Posted by 3rdday

captbone,

Thank you for your input and valuable insight and experience..
I hope it works ok, like I said the 300 v8 will be for experimenting and short term, if it does well i know the etec 300 will. In any case it wont be a big deal to reset for twins.
Im going to be adding knees to the stringers and transom and a longitudinal beam across the transom. I re cored the transom last year with composite, eglass biax and epoxy......
Its easy work as long as its not in the dead of summer- my only complaint is grinding glass in the summer.

EDIT+=Sorry for the thumbs down at the top of the post - meant for it to go at the end of"grinding glass in the summer"

Always thinking outside the curve...good for you. Called Monty and Gordon said you will be happy with the big E tec. Twins are a bit heavier depending on choices, but in the middle of the panel, may be happy, balanced, and neutral through the controls.

Cheers
Us

[captbone]

I always loved the 3.6 liter and 4.0 liter OMC V8 outboards. Please post pictures when you hang it. It is smart starting off with the $2k engine as you can really get the feel. That was my plan and I am still on my test engine as baby and home put it on the back burner.

My long term goal is a 31 Bertram with a single 350hp Yamaha.

Start searching ebay for a 15pitch Mirage Plus prop as that will most likely be your prop.