Under propped?

[Nomad Willy]

Diver,
Your hull speed is 7.6 knots. 5.7 X 5.7 X1.34 = 7.63...

Best speed for economy w/o getting ridiculously slow is 3/4 to one knot slower than hull speed so a little less than 7 knots would be a good place to start.

You might consider a speed less than hull speed but faster than the point where your bow starts to rise. That may be what I've described above.

Some here have run quite comprehensive tests on their boats. Those tests could be more relevant to you w your fast SD hull. Search "fuel consumption" ect. SeaHorse for one did that on a 34' IG hull and a 220hp Cummins. His turbocharger may alter the results relative to applying them to your boat. I think you'll find others though.

 

[Ski in NC]

Diver- Another way to look at is to consider the hull speed a sort of "wall". If you try to push boat faster than that speed, it is about as effective as pushing against a wall. Beyond hull speed any small increase in speed takes a large amount of power.

But all boats are different. Hull speed calc's out at around 7.6, but different boats feel better a little above or a little below that. One way I look for hull speed is to gradually increase speed and watch the bow relative to the horizon. If you increase speed and see the bow start to rise, you are probably getting into that inefficient speed zone. Back off half a knot and you will be at a pretty efficient speed.

If you run below hull speed, your nmpg will get better, even down to absurdly low speed. Since most of us want to actually get somewhere, a good compromise is hull speed or a half knot below. These things are slow enough!!

Also good for motors to have some load on them. Usually hull speed has them loaded enough to be happy.

 

[psneeld]

yes....simply put....

 

[superdiver]

wait, that math doesnt make sense to me...

5.7 x 5.7 x 1.34 = 43.5 not 7.63

what am i missing?

 

[Ski in NC]

Calc is sq root of water line length in feet, multiplied by 1.34.

 

[superdiver]

OK, so it should be 5.7 (which is the sq rt of 32) x 1.34 = 7.6

ok that makes more sense..

 

[psneeld]

OK, so it should be 5.7 (which is the sq rt of 32) x 1.34 = 7.6

ok that makes more sense..

and 1.34 is sort of a "general constant"...add in other boat design feature and most of the SD trawlers are much more efficient running down around 1.0 times the sq rt of the LWL instead of 1.34 which is more in the realm of full displacement.

 

[markpierce]

Running at hull speed isn't efficient. A knot or so less is.

 

[Al]

On this subject of props. the thread has swung from under prop to over prop as thread creep tends to do. This is fine in this instant as I’d like to submit a over prop presentation.

To day I took our 18 foot Poulsbo out for a halibut fishing effort. As I was leaving the harbor the subject of over prop came to mind and I took the opportunity to go WOT on our 6 hp Yanmar one cylinder diesel Hrumph! Max RPM was 1100. (Rated for 3000 RPM) Has been this case as long as we have had the boat/engine some 30 years. Never gave it a thought as the boat runs fine in all aspects. Yes, at anything over 900 RPM the exaust becomes very black!
We operate this engine at 700 RPM which would equal the 3/4 of available RPMs were the 1100 RPM to be that operating range not the 3000 RPM Yanmar rates as WOT

Okay. Now we know we are over propped. Upon return and the boat out of water an inspection showed the wheel is 11 inch in diameter and 13 in pitch. So off to Vicprop formula with this information. Here we go:
WLL=16 feet, beam 4’.6” draft, 1 foot. Wt: 1500# engine-6 hp Yanmar diesel,RPM rated; 3000 RPM 2:1 reduction plus the remaining data required. End result by Vicprop is wheel 11’ diameter X 8.6” Pitch. for a hull speed of 5.3 knots As a result we are 4.4 inches over propped for the boat. Extrapolate that difference out to our larger boats and it is a huge amount which would fit the broader discussion on this thread as to the potential outcome of over propping.

What is the running situation with this over prop? At 700 RPM the hull speed by GPS is 5 + not over 5.4 knots. Our GPH fuel burn is 20 NMPG per gallon. Engine temp must be within range as the indicator is a red light if over temp. Not happening. Salt water cooled which may affect that measurement, exhaust is cool to the touch and good volume. As to idle, the engine idles at 400 RPM and yes, we are moving at salmon trolling speed no doubt. About 2 knots per hour, however manuvering is quick and acturate making a very responsive boat during and leaving the moorage!

Annual usage of this boat is light with no more than 25 hour per year so the subjective damage of over prop has, will, take more time than normally allocated to that see the effects of wearing.

One has to consider that the engine is 30 plus years of age in a 78 year old boat.

I suppose the outcome of this post is that the little Yanmar over propped as it is, seems happy to be under the extra stress the over prop is pressing the engine . I can add to this that when I hook this boat to a fire wood log off the beach, of some length say 30 foot by 18 inches in diameter, the towing speed is closer to 4 knots vs. the 5 + knots running free, no change in engine operation, which represents the plus factor of having a over prop. issue.
Would I consider changing to a more compatible wheel measurement to Vicprop formula? I think not.
Cheers,
Al-Ketchikan (Bridge to Nowhere) Alaska

 

[FF]

The Sq rt of the LWL is known as SL.

1.34 x SL is usually inefficient.

SL times .9 to 1.15 is usually the best compromise in terns of fuel burn and miles made good,

 

[bayview]

Hull speed is an obsolete and misleading term boaters use to tell each other how fast they should go to waste fuel. At that speed to length ration FD hulls will have a wave at the bow and stern with the a trough amidships. Climbing out of that trough is what makes trying to go faster difficult. SD hulls will be starting to climb onto plane but operating at a speed of great resistance. Making that wave is what uses a lot of fuel. Even SD hulls would be better off to go slowe or faster than a S/L of 1.3.
As others have said keep the bow down and as little stern wake as possible is what is needed.
A S/L ratio of 1 would be much more efficient but don't try to tell that to the boating community who take 1.34 as gospel.