jpclear

I have noticed that boat performance testers often indicate that their figures would be better if the tests were conducted in saltwater at sea level. Being a freshwater boater at about 800" elevation, I was wondering if anyone out there has a formula for just what kind of performance gains one could expect moving to sea level / saltwater. (all other conditions - temp, humidity, wind and chop - being the same) And if it makes a difference, I'm talking about a displacement (V) hull here. Anyone?? --- Jer

Donzi38ZX

Poker Runs America did an article on this in the last issue, Vol 5 of 2002 I believe. Basically they said Fresh vs. Salt makes no difference despite popular opinion but elevation heat and humidity do. They mentioned that seeing as most fresh water at evelation is cooler with lower humidity they doubt any difference would be seen at sea level.

Steve

jpclear

Thanks for the reply, Steve. I did miss that issue. That goes along with what I kinda thought. Sounds like a test publication trying not to offend an advertiser with a disapointing test result to me. Later! --- Jer

mcollinstn

Saltwater is denser. Your boat will float higher, run higher, and lift more in saltwater. This would seem to point towards a boat running faster in salt - problem is, whatever is still IN the water has to cut thru it which is harder to do since it is denser.

Tradeoff kind of cancels itself out on recreational performance hulls. Big displacement hull boats and semi displacement cruisers DO see a bit of a difference. Nothing earthshaking, but a knot or so.

I have witnessed the difference on a semi displacement Hatteras. The boat in question was "on the edge" of planing with a full load and it was easy to see the differences in salt and fresh. Salt put the boat "over the hump" and it ran 4 knots faster in salt with a full load (planing vs not-quite-planing).

Nordicflame

They mentioned that seeing as most fresh water at evelation is cooler with lower humidity they doubt any difference would be seen at sea level.

Altitude makes a tremndous difference!! Not quite sure what they meant by that statement????
I live and boat at about 4600-5000 feet and when I dop to 1200 feet I spin 3-4" more prop and gain ~6 mph. Based on that I would realistically expect to see an 7-8 mph improvement at sea level. Generally speaking, with a base of 550hp, I lose over 100 hp.

Here's a good spot to calculate your losses or gains.

http://wahiduddin.net/calc/calc_hp.htm

Later,
Dave

formula31

Yea, I dont know about the salt or elevations thing but air temperature makes a big difference in horsepower. Considerable difference in a NA engine between 40 degrees and 80 degrees.

CigDaze

The Saltwater vs Freshwater effects are debatable. mcollinstn, put is best. The density effect decreased displacement is often times negated by the increased frictional drag.

However, altitude can make a huge difference. Below is a chart showing atmospheric pressure at different altitudes.

For instance, if you're used to boating at 4500 ft elevation, and then go down to sea level, your motor will respond as if you put on a blower with 2.25 pounds boost!


Alt._Pressure
(ft)__(PSI)
0000 14.689
0500 14.438
1000 14.188
1500 13.938
2000 13.688
2500 13.438
3000 13.187
3500 12.937
4000 12.686
4500 12.435
5000 12.184
5500 11.932
6000 11.680

Team Gary

When I worked at Kaama, we had a 30 Scarab with 425 Kaama's. The boat would run 85 at sea level, salt water. We took it to Lake Mead Nevada. Higher elevation,not sure how much,and fresh water. We could only get high70's.

Nordicflame

Lake Mead used to be 1229 feet; now it's only 1153 and dropping fast.
Bad news......................

jpclear

Thanks for the input, Guys. I was aware that temp., humidity, and elevation made some real differences; just not how much. The table is nice, Nick. With the saltwater-freshwater thing I just got thinking that since the salty stuff is denser the hull should ride higher, get more benefit from impact lift, the prop might tend to bite better and slip less in the denser medium. Probably too small to be measured, though. Thanks again! --- Jer