View Full Version : altitude

10-03-2002, 05:57 PM
i boat at about 48000 ft i know the altitude affects top speed. so my question is when some one says they are running 80 mph at say havasu with the same motor and boat as mine im running 70mph and only turning 6000rpm will i always run slower ?

10-03-2002, 11:16 PM
My mikuni pocket slide rule isn't calibrated for 48000 ft. If you mean 4800 ft the general rule of thumb for horsepower loss is 2.5%/1000ft elevation increase above sea level. Therefore at 4800ft you are down 12% in power to somebody running at sea level.

10-04-2002, 04:32 PM
thanks for the correction. so when i hear about boats doing 90 mph plus i would have to assume they are running lower altitude than i am .if high altitude is going to affect my rpms would it help to go to a lower pitch prop for more rpms ? im running a 22 mach now. thanks for the input.

10-14-2002, 09:12 AM
Every 2-stroke I heard of runs smaller jets for high altitude, check factory recommendations. The air is thinner so the motor gets less and needs less gas for the proper mixture ratio. Then they normally say more rpm to help makeup for the power loss, but less than redline of course. A snowmobile can be clutched for more rpm at part throttle for better response for example. I am guessing on a boat you would run less rpm due to power loss, so you would just run less prop to get back up to max rpm recommended (compared to running sea level).

I think the motor will still make its max power at the same rpm (anyone correct me if not) as at lower altitude, just a bit less power because it can't get as much air. You may also be able to run more compression, or mod the intake for more flow to make up for this. Note that running it at lower altitude after these mods will cause it to run lean, and may cause damage. Not sure how much altitude will require changes.

10-14-2002, 06:26 PM
thanks for the reply. so if i run a smaller prop i can get more rpm but the bigger prop runs less rpm but moves the boat thru the water faster woudnt i be running the same speed with a smaller prop at higher rpm ?

10-15-2002, 09:36 AM
Yes, I think you have it right. Bigger/smaller meaning pitch of prop. Think of it as gearing for the motor. You want to 'gear' or pitch the prop so the engine run at the rpm it makes the most power so you will go the fastest.

Too much and the rpms will be lower where the engine makes less power, too low of pitch and the engine over-revs and typically makes less power as well as it can go bang. Most engines are rated at a peak power rpm, and a max rpm maybe 500rpm higher--or a range 500 higher and 500 lower than the peak. Some have a high/low range like older Mercs where they run best in the center(or maybe+200rpm) of the range.

Generally; most engines go fastest at the peak or maybe closer to the max rating. A very few go fastest at or even above the max if you want to push durability, and you have one that does. This assumes no mods to the motor of course. Make sure your carbs are all the way open at full throttle too, as well as the normal tuneup stuff. I only run weekends and change LU lube in the fall, so I use synthetic lube for the couple of bucks a year it costs extra. On some older motors you can advance the timing a bit and run premium for another 1-2mph, on some it don't work.

You can gain more free speed by doing this: Get as good of a SS prop as you can afford/find that works good on your boat and sharpen the forward edge only with a file. Vary your angle of filing so it is rounded some, and don't worry about cutting paper with it. Should only take 15-20min and don't worry about the edge close to the hub. Don't touch the cup or ends of blades. It must be in perfect condition w/no damage. Round off the front edges of your lower unit/skeg below the cav plate. Do not sharpen to a point here, think airplane wing or it can steer funny, and round off any dings. Remove a minimum amout of material in all of this. Doing this stuff might give you up to 200rpm if not done before.

Eliminate extra weight from boat, move weight around for better plane/side to side balance, some even sand the pad on the hull(supposed to make more air bubbles there). I'm not getting into the whole jackplate/setback thing here, but make sure your cav plate is above the bottom at speed unless you have a slow boat. If you run lots of trim, try to put the weight in the back as you'll go faster with a more level motor providing your hull is at the right attitude. Note the fastest boats have the motor running level, but not all of us have 300hp either...

10-15-2002, 02:29 PM
i am running a 260 hp merc on a 21 daytona eliminator stock as far as i know . it is efi. any thing i need to know about the efi. does it need to be changed to run high altitude ?

10-16-2002, 09:12 AM
I am not sure if the Merc EFI adjusts for altitude or not. I think the Optimax does as well as the Fitch or whatever it is. Check out some of the performance gurus here. I have heard people recommend using the Merc carbon cleaner on these though. Not seeming to be a big problem though as I have not heard of it. Could also call a dealer and ask.

10-17-2002, 08:30 AM
thanks for your help. i will call the manufacter and talk to them on it

10-21-2002, 01:24 PM
I live and boat at 2200 ft. I don't have a super performance rig like a lot of you guys run, but have a high 70's Bullet Bass boat with a 225 efi Merc. I can tell you that I notice a big diff in performance when I go to lower altitude lakes. By leaving the mtns of western NC and going to Westpoint lake in Lagrange, Ga I will easily pickup 2mph w/out changing anything in weight or setup. An old lakeracer veteran from the 60's told me one time that at lower altitudes the engines are able to get more-denser air into them for a more compacted fuel charge. Plus, one other thing which affects speed on diff bodies of water is the hardness or softness of the water which you are running on. Here in the mtns the water is very soft, which tends to make the boat sit "heavier" in the water. On lakes that have hard water or a high mineral content the boat doesn't sit as low in the water, thus is easier to push. This info may not be as technically advanced as you want, but the basics are very true.

RB in NM
12-17-2002, 02:56 PM
I run at lakes out here in New Mexico at 4500 up to near 6000 feet in elevation. Talk about keep your prop guy busy,,,,,I have tried no telling how many props, playing with setback, changing lower unit gear ratios,, you name it. It sucks.....

The EFI Mercs do compensate for different altitudes every time you turn the key on. No problems there.

I'm from South Louisiana originally, sea level is difinately where it's at... More oxygen, equels better combustion, equels better horsepower gains.

I was yapping over a couple of beers with some other hi performance boaters, gravity came into the conversation. It was said that the higher in elevation a body of water is located, the higher ratio of specific gravity the water has. Kinda makes sense to me. So the more denser the water becomes, thus more drag or friction it generates.

I think I need a cuple of more beers with theses guys and get to the bottom this this therory, heck, it was making good sense to me...

I have a hell of a time getting big numbers out of my Allison 2003T w/225 Pro Max SS, up here. Need to take this rig back down to Gods' country,(La.), and let her rip....

Later, Russ

12-17-2002, 03:30 PM
Well, traditionaly a given motor makes less power at high altitude. The air is not as dense so you get less in the motor every revolution. A turbo is good for helping here. This may or may not screw up your mixture, and does not on newer electronic engines like efi, new cars, etc. Snowmobiles used to have problems with this while racing mountain courses. Even if the efi changes your mixture, you still are mechanically getting less air.

Tempurature does a similiar trick, in that cold air is more dense. Cold water is too, but changes less than air because air is so compressable. I found a 5mph difference on my 1500 from 60 to 90 degrees. Cold air is like having more compression, more air is in there in the same engine size as that does not change. I would guess the water is softer too, but I bet the air change is the greatest effect to performance. You guys need ram-air! Problem is, if you modify it to work better up there you will have problems at lower levels. In the old days people changed jets in the carbs all the time for altitude, tempurature, and I bet they still do. Used to have to be careful when I had a beefed up sled, and could not hold it wot for long if it got to zero or colder. It would puke a piston, but went faster:D