HP and Thrust

[Rob Ripley]

Has anyone checked the bollard thrust of the different Seagulls ?

Leaning forward does help ![attachment=0]RIMG14701.jpg[/attachment]

[Charles uk]

Yes Seagull did, but not a static bollard pull, they had in the late 1980's a transom bracket with a set of scales built into it, so they could measure the thrust at the higher revs when under way.

Which was higher than the static bollard pull & more representative of the results that a customer could expect.

I unsuccessfully built something similar some years ago, using electric bathroom scales parts, but you couldn't read the push numbers, as the engine vibrations caused the numbers to change faster than I could read, & incorporating a damper system would confirm everyone's view that I am a Cathedral builder!

[Sandro Picchio]

Post edited. See next post.

[Sandro Picchio]

Quote: "Which was higher than the static bollard pull".

So between readings done against a static bollard or in a boat underway there is difference.
Is this difference large?
I expected that the value given by the bollard pull test would have been higher than that on a moving boat.
If one start pushing a huge barge that hardly moves (forget inertia) one gets te bollard pull value. if one gradually decreases the hull resistence by using more and more streamlined shapes the speed increases. When the speed corresponding to the propeller pitch is reached the propeller screws itself freely in the water and push is zero. Thus the push on a boat moving at an halfway speed should be lower than the bollard pull value.

Nonetheless Seagull tests results give an opposite outcome. So where is the flaw in my reckoning? (As I am convinced that any correctly worked out phisical measure is more reliable than all theoretical speculation)

[Charles uk]

Measure the revs at full throttle of a static bollard & full throttle in open water.

You'll find your getting more revs going free, more revs means more power strokes = higher push!

[Sandro Picchio]

I would say that more revs means less torque load on the motor from the prop that is less push.
Usually at same throttle opening easing the load (torque) revs increase.
I may be wrong.

[dandrews]

https://www.icloud.com/sharedalbum/#B0pGqkRUiGPLHOT

Here is SJM555R6 being tested on the dock at Woodbridge Tide Mill in 2016. Judge for yourself by ear how many more revs and therefore how much more push might be available if she was running free. (She is a bit high in the water so maybe not a perfect example)

Dave Gerr's Propellor Handbook is the bible in this area.

[Sandro Picchio]

I fully agree that, at same throttle opening, revs are higher when the motor is free to move forward in the water than when it is kept fast on a dock.
But I think that just this increase in revs means a lower torque load on the motor.
What can be argued is: Is propeller torque proportional to propeller push?
Of this I think yes but am not sure.

[Charles uk]

Doesn't 10% more firing strokes, imply your putting 10% more power into the water that's used to propel your boat forward?
(How much push your getting on the back of your boat).

As measuring the power output at less than maximum revs in it's normal working environment, would give you a lower inaccurate answer.

[Charles uk]

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Perhaps the above might explain better what I'm trying to say, it was taken from a Seagull technical report S87 dated (01.09.88) unfortunately Doug Hele passed away some time ago, so it's no longer possible to pick his brain on the subject.

He was the gentleman responsible for the initial design of the 170/125 family & if you look him up on the net you can check his pedigree!

[Sandro Picchio]

Thank you for sharing the interesting old document.
It says that there is a difference between results of bollard pull and of moving boat test. Unfortunately it does not say in which direction is dhe difference.

I wrote a post yesterday and thought to have sent it but I don't find it in the thread. Lost perhaps or my memory growing week.

[Charles uk]

No comparison figures in that tech report only push in newtons!

You can't help wondering why they didn't try a height adjusting bracket for the long shaft Kingfisher, if they could measure a short shaft Kingfisher without major problems?