How Slow Can You Go?

[Bigsfish]

Ted

We were going against the current and you were making a couple of knots (yes, I lied).

I’ll do anything to avoid taking your private cook away from you, anything!

BTW I love the new ice maker, uses very little water plus the water maker and Spot zero is working. Heading for three weeks in the Bahamas in a couple of weeks.

 

[O C Diver]

Ted

We were going against the current and you were making a couple of knots (yes, I lied).

I’ll do anything to avoid taking your private cook away from you, anything!

BTW I love the new ice maker, uses very little water plus the water maker and Spot zero is working. Heading for three weeks in the Bahamas in a couple of weeks.

You can run to the Bahamas, but you can't hide from the cook.

Glad your new ice maker is working to expectations.

Have a good time and tell Watfa I said hello.

Ted

 

[Nomad Willy]

Your dead slow speed often is an indication of how you’re propped. An overpropped boat will be relatively fast at idle.

And there are many trawlers that shake/vibrate at a normal idle so people set the idle speed high. Others aren’t that much bothered w the vibes and keep the idle rpm low.

Some w “Cushy Float” or other soft engine mounts may have a fairly smooth idle and thus normal idle rpm.

Overpowered boats will have a higher speed at idle.

FD boats will be faster at idle because their hull resistance is so low at low speeds. And of course all boats w emersed transoms will be slower. Full planing being the slowest.

As in many things boating .... many variables.

But on average abnormaly high idle speeds is an indication of how overpropped the boat is. When I first had our boat it was overpropped 250rpm. Flip flop’in in and out of gear a lot while trying to go slow in the marinas. Hardly at all since we’ve been propped right. But re the variables I have a high idle speed re other trawlers and very low resistance at very low speeds.
I don’t see having a low idle rpm as good. Many feel having a low rpm across the board is a propper trawler thing. Just as an emotional preference.

Re my idle boat speed .. I don’t know. Just never pay any attention to it.

 

[Bigsfish]

Ted

I’ll take your cook if you join. I know you would regret not being together.

Watfa send her hugs.

 

[dhays]

I checked today. We idle at 600 rpm. That gives about 3.3 knots.

 

[O C Diver]

Ted

I’ll take your cook if you join. I know you would regret not being together.

Watfa send her hugs.

As much as I would like to join you in the Bahamas, I have charter commitments in Hatteras, NC starting next weekend. I greatly appreciate you taking the cook without me.

Tef

 

[Bigsfish]

Dear Ted

You are too good of a friend, I could not possibly take your favorite cook without you also coming. Thanks just the same. Looking forward to February, Watfa has a special meal planned.

 

[MYTraveler]

Your dead slow speed often is an indication of how you’re propped. An overpropped boat will be relatively fast at idle.

Overpowered boats will have a higher speed at idle.

Higher speed than what? Than if the props had less pitch? That is true of a boat that is properly propped as well -- it has a higher speed at idle than if it were under propped. I must be missing something, but I always thought that over propped meant that too much HP (torque) was required to turn the props at a given RPM than the engines could put out. And generally, the greatest demand is at max rated engine rpms, where the props need increasing power and the engines are putting out decreasing torque.

 

[Nomad Willy]

Traveler,
One could conclude higher than a propperly powered or under powered boat.

Hard to talk about things like this and get real specific. Sure as boats go in the direction of the pointy end some guy will say “well what about my boat”. With all the variables there’s always an exception .... at least in somebodies mind.

But yes of course “if the prop had less pitch”....
But torque has nothing to do w it. It’s work over a period of time. Torque is an instantaneous value and does no work. Horsepower pushes the boat and does the work.
Speed is always a product of energy and resistance. With less pitch less water will be pushed aft and the boat will be slower.
As far as I know engines put out the greatest power at their rated rpm. But I don’t assume that. It’s a rating applied by people. I think that power decreases above the rated rpm and also decreases below the rated rpm. But variables complicate the picture. If one plans ahead anticipating eventially there will be a fowled bottom and perhaps a few dirty injectors so the boat propped at the rated rpm will no longer have max rated power. But if the boat was slightly underpropped the boat would have closser to max power over a range of time in service.

My boat is very slightly overpropped (not by choice) so as performance features (like a clean hull) decrease attainable speed decreases more than it would if the boat was underpropped to begin with. The preferable scenario IMO.

Boats are always going up hill (so to speak) and less prop load (as in lower pitch) will allow the engine to pick up a bit of rpm and loose a bit of load. So there will be less “throttle” on and more engine speed. Better for the engine and usually lower loads. Any truck driver knows this. In a big truck you can feel the reduction in load when you downshift. And if you can’t pick up enough rpm to upshift you back off throttle in the lower gear and “cruise” up the grade. Probably (depending on the variables) less fuel burned and less load on the engine to attain the highest speed w/o overloading the engine. And we should do the same on our boats.

Having said that a few minutes after getting up I may have expressed myself not ideally. But I’m looking fwd to my shower and breakfast. So if I said a bad I’ll leave it to you to get us back on an even keel.

 

[MYTraveler]

But torque has nothing to do w it. It’s work over a period of time. Torque is an instantaneous value and does no work. Horsepower pushes the boat and does the work.

As far as I know engines put out the greatest power at their rated rpm.

I am struggling to understand the idea that torque has nothing to do with it. Power, at any given rpm, I believe is rpm x torque. The force required to turn the props at a given prop rpm is measured by torque. (For simplicity, assume that the transmission ratio is 1:1, but it doesn't matter). An engine's peak torque can, and I believe typically does, occur at lower rpms than peak horse power. That result is a mathematical necessity whenever a small increase in rpm produces a smaller decrease in torque, since they product will still be increased. This can be visualized by imagining a torque curve in which torque drops off slowly after hitting peak.

 

[Nomad Willy]

Not trying to argue but the power “drops off slowly” to. A bit faster than torque though .. usually.

Imagine power and torque w a pulley and ropes. If you go from 1-1 to 2-1 you have the same amount of work done minus the extra drag of the additional pulley to achieve the lower “gear ratio”. Same work being done but re lifting weight twice as much weight can be lifted ... but at half the speed. Again same amount of work being done.

On a trawler the same amount of work is done as we switch gear ratios and prop size and pitch. But the lower gear is prefered not because of the increased torque but becausewe can utilize a larger more efficient prop.

I don’t fully understand it either .. I don’t think. But I’m sure the speed of a boat energy wise is the product of horsepower. Torque is involved but I’m not sure how. We don’t care 999% of the time but does torque play a big part in how quickly a boat can accelerate to cruising speed or stopfrom reversing? I think it does but I don’t think it causes our boat to go fwd at it’s top speed .. or any other speed. That is the product of horsepower.

Sorry no edit .. gotta go fix a lawnmower.

 

[FF]

How slow can you go?


The point where the auto pilot has difficulty in steering would seem to be a speed to stop slowing.

 

[dhays]

1.5 knots. Of course the 5.5 knot foul current has something to do with it.