Single engine B31- check it out

[captbone]

https://www.facebook.com/marketplace/it ... _type=post

[Carl]

…and there we go. A single motor in a 31 that will cruise 18 knots at 2200. The kicker is it’s a single 454.


Now the question in my head is why 2200 rpm. Those motors sweet spot is a good deal higher than that rpm. Does performance fall off, wheel cavitation? Or does owner just not want to go faster than 18 knots or motor sips at that speed.

What would performance be with a single diesel.


Oh the questions

[Tony Meola]

Carl

Single diesel. More torque, spinning a bigger wheel, higher cruise, less fuel burn.

[Carl]

Carl

Single diesel. More torque, spinning a bigger wheel, higher cruise, less fuel burn.

More torque-- , Depending on what was installed, no?

spinning a bigger wheel-- , Depending on motor and ratio...certainly, it could be. Heck he could turn a bigger wheel with the 454...just adjust ratio or reduce pitch


higher cruise-- , Again, depends on how its setup...but it certainly could be

less fuel burn--, I'd say your right here



What's frustrating is the 2200 rpm cruise. Why pick that number to cruise at whether it's 18 knots, 8 knots or 28 knots. HAd he said 3200 rpm...I'd say ok he's in the motor's sweeter spot. Then there is the possibility the motor just can't turn up to full rpm and he's running at 2200 as it's all he can get.

If he was so thrilled about the performance, why get rid of it after a major restoration and refit only after a year and using it for a whole NaN mile...as he so eloquently put it. Maybe it's just a major dog. Or it just perfect like he wanted, now he's done with it and wants to move on.

Interesting thought of a single in a 31, wish I had more information to go on...

But I don;t wish enough to actually call and find out...at least not yet.

[Rawleigh]

I cannot get the site to load. Is the hull tunneled?

[Carl]

Yes the hull is tunneled and looks to have a protective skeg with iddy biddy rudder (its a technical term). But running gear shots are lacking as is motor install.

[captbone]

Facebook is down globally temporarily

[Tony Meola]

Carl

I bet it can reach full RPM'S.

Think about trying to get your gas boat on a plane with one engine. Granted, boats been tweeked, while he probably gets it to plane but I bet the engine is struggling.

My boat will plane on one engine with the diesels, but never with the gas engines.

[Tony Meola]

More torque-- , Depending on what was installed, no?

spinning a bigger wheel-- , Depending on motor and ratio...certainly, it could be. Heck he could turn a bigger wheel with the 454...just adjust ratio or reduce pitch


higher cruise-- , Again, depends on how its setup...but it certainly could be

less fuel burn--, I'd say your right here[/b

Carl

I was assuming getting it to work correctly required a proper set up. Who repowers and redesigns a boats power set up without making sure it is going to work? Perhaps someone with too much money and no common sense.

[Bruce]

Who repowers and redesigns a boats power set up without making sure it is going to work? Perhaps someone with too much money and no common sense.

Actualy its three things. Too much money with junior engineer types. Too little money to do a proper job. Or doing just enough to make it a utilitarian vessel. In other words if you look at the boat it looks to be setup as a commercial fishing boat where speed is not the primary concern. Its a Virginia boat? Might be pulling crab traps in the bay.

I saw all kinds of things to save money on dedicated Marine propulsion and modification of boats more for utility purposes.

[Rawleigh]

It might have been one of the old VMRC boats from the look of it. Getting in the cabin looks like it would take gymnastics!!

[PeterPalmieri]

Carl,

Apparently when you sell a boat on FB they ask you for miles like it’s a car. If you leave it blank they fill it in with NaN (not a number). At least that’s what google told me.

[Tony Meola]

It might have been one of the old VMRC boats from the look of it. Getting in the cabin looks like it would take gymnastics!!

Rawleigh

Looks like they put in a short cabin door. You have to be 4 foot tall to get in the door without ducking. The cabin sole is even with the bottom of the door. hell of a drop into the cabin unless there is a raised floor in the cabin.

Crew must be all 3 feet tall.

[Carl]

Carl,

Apparently when you sell a boat on FB they ask you for miles like it’s a car. If you leave it blank they fill it in with NaN (not a number). At least that’s what google told me.

AutoFill, got it. Thanks, that does make sense, I was taking it as a flippant answer.

[Carl]

Carl

I bet it can reach full RPM'S.

Think about trying to get your gas boat on a plane with one engine. Granted, boats been tweeked, while he probably gets it to plane but I bet the engine is struggling.

My boat will plane on one engine with the diesels, but never with the gas engines.

I think that is where I keep circling this topic.
We are...or at least I am guessing as to every aspect of performance on a very different 31 Bertram build. It is something I had thought about many times when various motors were available to me. For me it never made sense as the entire back had to be reconfigured with an unknown end result as I had little to use as a baseline.

Getting a twin screw on plane using only one engine has to be alot tougher than getting a single screw on plane.

My first thought is a twin screw is setup to be utilizing the power of both engines to get over the hump. A single screw with only one of the motors would need more reduction to get over the hump...that could be a higher ratio trans to spin the prop slower with more torque or reducing the pitch in the wheels. What you'd be doing is spinning the motor faster easing its load for the same speed.

My second thought when trying to get a twin screw on plane with a single motor. With one motor shut down its not only NOT producing half the power but that motors running gear is now creating a huge amount of drag.

My 3rd thought about planing a twin-screw boat on a single motor...this I'm not quite sure of...but having the thrust from only one side does not seem beneficial as the rudder has to be turned a good amount to compensate for keeping boat on course. I'm thinking that fight of boat wanting to turn and rudder keeping straight has to be robbing power.


So a single geared down should be able to get boat over the hump, I'd expect it to happen at a higher rpm as its taking more load.

Single would have less running gear which is less drag. Single has less weight with one motor and gear. Single doesn't need the extra battery and fuel??
Remove some more weight using composites, no bridge, small cabin is less weight and less wind drag. Lighten it up enough, remove drag is less load the motor has to deal with...maybe the motor does not need to be geared down as I initially said. Along those lines maybe boat is up on plane, running at 18 knots at only 2200rpm. If that is the case...imagine what the boat could do at top end or cruising along at the 454's sweet spot...considerably higher than 2200 rpm.

Done right I think a single motor can work in a 31 Bertram, even a 454 gas motor.

I guess the question is...is this one done right.

[Tony Meola]

Carl

I plane off with both engines at around 12 knots. I actually can plane off with one engine at around 1800 RPM's. I tested it when we first did the repower, just because one just has to know.

With the gassers, I don't think I could have planned off with one engine.

[Carl]

That's the difference between gas and diesel.

The diesel has tons of low end torque getting you over the hump, even with a single motor. Even still I have a feeling running on one for too long is far from good for the motor. Running on one motor would surely be a huge over-propping / overloading issue. Remember you got on plane at 1800...but could you reach WOT on one motor?

Now pull some pitch out of the wheel or different gear ratio and I'm sure you could run all day on one.


Pull some pitch out of your old gas setup or add reduction with the gearbox and I'm sure the boat would plane off on one motor...it would be a considerably higher rpm...but I'm sure it would plane.

[ktm_2000]

my experience tells me that our hulls are more efficient than we give them credit for. I did a test when I had I/Os and shut down one engine and lifted the drive then tried to plane on one motor, I was able to get the boat up on plane doing 17kts on 135hp, I had more to push out of the motor and probably could have went faster but I didn't try. From the sounds coming from the motor, it wasn't liking it and I think it would be something only done for a short period of time in an emergency but it was do-able. I was running 15x17 pitch props and probably should have been running 15x19 or 15x20s which probably accounted for me to be able to get on plane.

Obviously a 31 is way heavier and would need to be scaled up to work but I bet that it would. I am not going to suggest it would be good for a motor on a long term basis either.

my biggest question is draft. a single engine would need a large diameter prop with low pitch to have the bite to move the boat. That all sitting under the lowest portion of the hull has to add quite a bit, What does the boat now need for draft? 5-6 feet? I thought the area of the Chesapeake was shallow in a lot of places, that is most likely the reason for the sale.

[Tony Meola]

Carl

I did not try to see if I could hit top end. Once she planed off I backed it off. Would I do it for an hour no. If I had to do it coming in Barnegat Inlet I would. But I would not want to make a habit of it.

[Carl]

Carl

I did not try to see if I could hit top end. Once she planed off I backed it off. Would I do it for an hour no. If I had to do it coming in Barnegat Inlet I would. But I would not want to make a habit of it.

An advantage of twin screw is the 2nd is a backup to move if the first has a problem. A slow ride home on one motor is better than no ride home, being able to get up on plane to run an inlet, avoid a collision or whatever the reason you NEED to make time is a good trick to have in your bag.

[Pete Fallon]

Why even bother trying to convert a boat that was designed as a twin screw that runs fine when propped correctly
As far as i am concerned its a complete waste of money and time. If you want a single screw boat buy a down east lobster boat.

[Pete Fallon]

Why even bother trying to convert a boat that was designed as a twin screw that runs fine when propped correctly
As far as i am concerned its a complete waste of money and time. If you want a single screw boat buy a down east lobster boat.

[Carl]

Why even bother trying to convert a boat that was designed as a twin screw that runs fine when propped correctly
As far as i am concerned its a complete waste of money and time. If you want a single screw boat buy a down east lobster boat.

It may have something to do with having a hull with one running motor.