What No Speed?

[Fire99]

Hi People,

I know i posted some info on this before but i now have some more information.

I've got a Freeman 30 (aft cockpit) '66 ish.
She has two virtually new (38 hours each) Lombardini 2004M (50hp) engines.
They are rated to rev to 3000rpm.

3000 revs gave me 7.1 knots around the top of the tide, but on the way back with a tide 1.5 - 2 hours past high tide i was just getting 5.8 - 6 knots flat out.
At 2400 (much more comfortable) she was just doing 4.8 - 5.0 knots..

If i push the throttles forward, she revs up very quickly. Infact it seems she is not governed by either prop size or engine limiter but by the throttle. It appears that the engine fitter has set the throttles so the leavers are fully extended (on their stops) at 3000...

Anyway, surely i must be able to push a bit more than this on the speed department......
I'm no expert on marine engines but i take it the revs climb quickly and the props catch up some how??
anyway, the revs do rise very quickly when you push the throttles up..
Assistance is appreciated as always (and desparately needed before my boat gets overtaken by driftwood..

cheers,

Nick

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[longjohnsilver]

7.1 kts sounds about right, with twin 50s you certainly won't get more than maximum displacement speed. Are you taking the speed over the ground from a gps or a log and how much tide were you pushing?

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[Fire99]

That was on the GPS...(Garmin 230)..
Not sure on how much tide i was pushing but 4.8 Knots???? (tears flood down my face!!)
and 7.1 was at 3000, drop her down to 2400 or 2600 and she was doing about 5.8 on the GPS...
I thought the displacement speed for a 30 was higher than 7 knots..

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[Chris_d]

Your displacement speed is: square root of the waterline length(28ft?) times 1.7....i think so about 8.9knots.
But are you sure your measuring the speed right, try 2 runs in opposite directions to cancel out the tide, and take the average. Does sound like your achieving full revs a bit too easily though, so could need coarser props.

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[Robin]

Displacement speed is 1.34 x square root of waterline length, so say 28' is 7.1kts.
Methinks he is also comparing GPS SOG figures which of course include tide.

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[Fire99]

Thanks Gents for the replies...

The speed i was reading was from my GPS.. Is that not the correct way to obtain speed readings??

regards,

Nick


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[hlb]

Well, I,ve been known to do over 90 knots on GPS!! So it depends really.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[Fire99]

90 knots??
I take it the GPS readings are not the most accurate then..

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[Robin]

GPS is accurate but gives only speed over the ground, SOG. Your displacement hull is limited to hull speed (kts) of 1.34 x square root of waterline length in feet but this is through the water, if the water is going backwards at your hull speed you will be standing still, if going your way, your SOG would be doubled! Welcome to the reality of displacement speeds, you can see why us yotties work the tides.



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[duncan]

or put another way - you aren't going to go faster than 7.1 knots throught the water (although you might do it at lower revs if the props are too fine at the moment (can't really say - there are prop calculators on the web).
If the water is going the other way to you intended course at 2 knots you will only 'make' 5 knots in the direction you want to go - conversely of course you should be able to get (subject to engines and props) 9 knots going with the water!
From the number you quote above I would guess that you are not actually getting 7.1 without a little help from the tide/wind given the lower end figures you quote.


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[milltech]

Gentlemen that reminds me

that I once missed the tidal gate round the east end of the IOW and spent 2.5 hours at 3 knots ish, too painful to contemplate. Always think of the tide first and where to go second!

<hr width=100% size=1>John
<A target="_blank" HREF=http://www.allgadgets.co.uk>http://www.allgadgets.co.uk</A>

 

[Fire99]

Yeah, its very interesting regarding the tides.
When i had my planing boat, all i considered the tide for was whether there was enough water for me to get back into the marina..

Now it seems just a bit more complicated...


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