Interesting Technical Question

I'm a bit late and the thread has moved on but I had to go and do some work !! Isn’t it amazing how differently people see things.

At the sure risk of another slap I answer your post from earlier today. (I'm 4.5hrs ahead of you)

"I can understand that because the force they have to apply to the shaft to stop it spinning IS to counter the drag it is creating - it was so big they could not stop it"

Don’t agree. I think the force to stop the shaft is due to the extra drag being created by trying to stop it.

“This force on the shaft is not there helping the prop to propel the boat forward it is the force exerted by the prop dragging the boat”
Prop dragging the boat ??
The boat is surely dragging the prop. This makes the prop turn. Drag is indeed still being created by the turning prop but if you try to stop it you create even more drag.

“You are right ... the captain is wrong. The force you are applying to the shaft to stop it equates to the drag being exerted by the rotating prop - if this was not true then the force was the other way then we would have this miracle of a feely rotating prop produces large quantities of forward propulsion!”

No, don’t think so. Again the force being applied to the shaft has to overcome the EXTRA drag being created by trying to stop the prop while it is being dragged through the water.

“You are mixing things up - things obey the laws of physics and in many situations drag is created - a drag that can be reduced. “
I agree. Drag that can be reduced by allowing the prop to spin.

“So part of what you said is right - there is a large force in that rotating shaft - you need to apply and equal and opposite force to counter it - that force in the shaft has to be drag... can you see that now?”

No. The rotating prop isn’t creating drag. Dragging the prop is making it rotate.

May have to agree to differ. I’m supposed to be working you know. /forums/images/graemlins/smile.gif
 
But there is a massive difference between a trawler and a Squadron capable of over 30 knots (I'd dispute that is an SD hull at all).

At the end of the day if you're that concerned about saving the odd bit of fuel here and there at the risk of your gearboxes then it would make more sense to have a single engine boat in the first place.
 
"Don’t agree. I think the force to stop the shaft is due to the extra drag being created by trying to stop it."

That does not make sense to me.

"The boat is surely dragging the prop. This makes the prop turn. Drag is indeed still being created by the turning prop but if you try to stop it you create even more drag."
Nope and you give no explanation as to why.

"No, don’t think so. Again the force being applied to the shaft has to overcome the EXTRA drag being created by trying to stop the prop while it is being dragged through the water."

No again - you are not making any sense withj the extra drag bit.

"I agree. Drag that can be reduced by allowing the prop to spin."

No - the energy in the shaft on that ship is a measure of the wasted energy - there is no doubt that just like a plane prop a still prop offers less reistance.

"No. The rotating prop isn’t creating drag. Dragging the prop is making it rotate."

Yes - the dragging prop is making the shaft rotate ... so what?
 
I am not taling about the Squadron - as already stated I am talking about my next boat whcih has an Sd hull.

The single engine running really does not apply to planing boats. The Squadron is a planing boat with a planing hull.
 
I thought of something simpler. Maybe ?

If a raggy is sailing along with the prop stopped but he then sticks it in neutral what will happen ? I pretty sure it would start spinning.
This being the case, if the minimum drag condition on the prop is when it is stopped why would it start spinning ?
 
My rational might not make sense to you as yours doesn't to me but the trawler yacht data would seem to agree with me. Wouldn't it ?

Really have enjoyed this thread but now it's time to go to bed.

Paul
 
Ok, but 25 knots seems very fast for a true semi-displacement form.

I still don't see the point though. Why not just have the boat designed with a single engine in the first place and with a lot less power. You'll save thousands in fuel that way. 1400hp is excessive for an SD boat in my opinion, unless it's over 70ft.
 
The actual measurements of the actual drag by the university confirm it to be the case - the only info against is the trwler yacht and I am coming back on that. I repeat the simple science I have stated is correct.
 
Give over, I've posted build piccies of the Elling, and Magnum did for his stunning P67. Make with the piccies in a new thread (only don't ask any questions in the same thread /forums/images/graemlins/wink.gif

Rick
 
I understand your points.

The fuel consumption on twin 500hp runnig at say 15 knpts is anout the same as the 700hp running at 15 knots - it nice to have the turn of speed when needed but claiming that 1400hp is a bit big for a sd hull is a valid, understandable point.

I have speced this boat from top to bottom - its the highesst spec one they have ever built to date - I have spent a lot of time making sure it what I want including redesign of inside on amny points etc.
 
Now that is all pretty much sorted (BTW I have agreed with you all along), what about if one has water jet drives?

Several instances to consider which are common - 2 drives, 3 drives and 4 drives - for each instance what is the most economical combination fuel wise in terms of number of engines run? /forums/images/graemlins/smirk.gif

John
 
Does the reduction in engine hours by splitting the effort between engines have a beneficial effect on resale value which outweighs any slight increase in fuel consumption?
 
Gludy, you never responded to my comment very early on in thread that research shows that props vary in whether they cause more or less drag when freewheeling or stopped, depending on many variables. A PhD student did this as research. So I still maintain, it depends, not absolute.

Haven't got the link to hand, but have posted it many times before on similar threads if you want to have a look at the research data first hand.
 
smilie_hang6.gif
 
It's an interesting site :ignoring the reported advantage of freewheeling - does anyone know why the starboard engine figures give much better mpg than the port?
 
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