new prop for altitude

Currently I am running a 13.75 x 15 on my 90hp evinrude vro (2 stroke) (recommended 4500-5500 rpm at WOT). My rig does about 31 mph at 5200 rpm and WOT here in Cache Valley which is 4500 ft or so. I had it to Strawberry (7800 ft) last year, and it did about 27 mph at 4700 rpm and WOT. It gets on plane way pretty reasonable, probably at 4000 rpm or so.
I plan on taking this to the Gorge this year which is what, 6000 or so.

So, is it worth a a 13 pitch prop for the higher elevations, or just live with it as is?

If you are happy with your hole shot, I wouldn't bother changing. prop changes won't effect the top end much if at all going in the direction you are thinking. It has also been my experience that each step up or down is good for a 200 - 300 rpm change. That might get you close to 5,000 at Strawberry and maybe 5,200 at the Gorge. If you have friends on board, it will definitely be worth getting the smaller pitch.

There is a prop guy that can either repair or get a new prop for you right there in your area. I have had excellent results working with him. thumbsup

SuperD, thanks, that is what I was estimating might be the situation regarding performance. It also reminds me that the 'load' at Strawberry was pretty full all the time since it was a family camping and fishing trip.

Bduck, thanks for the info. Hard for me to go the repair route though when you can pick up a new prop on ebay for the cost of the repair.

I always check the the pad to prop measurment first , before spending $$$$. 1/4 of an inch makes alot of difference . Just make sure you have a WP guage in your boat when going up . I will usally go up untill the wp starts to go down 3 1/2 to 4 inch measument usally does it without spending money needed .

agitator said:

I always check the the pad to prop measurment first , before spending $$$$. 1/4 of an inch makes alot of difference . Just make sure you have a WP guage in your boat when going up . I will usally go up untill the wp starts to go down 3 1/2 to 4 inch measument usally does it without spending money needed .

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This is usually a top end factor. When on plane, the cavitation plate should actually be just above the water surface. If all you see is white water at the shaft, you are still too deep.

Check this out:

http://www.veradoclub.com/smf/index.php?topic=137.0

All good stuff to know and I'll check out the cavitation plate position next I get on the water. I have to admit I'm more of a fisherman than a boater, but did notice a difference in performance when going up 3000 ft in elevation.

Overall, I'm happy with the performance of the boat. To me, the set up can't be too bad if getting over 30 mph with a 20 year old 90hp on a 19.5 ft deep V at 4500 ft elevation.

SuperD, I checked that prop calculator and now need to find what my gear ratio is so I can determine the 'slip'. What 'slip' is acceptable/normal and what is indicative poor set up?

Thanks in advance for the info.

Since we are fishermen and not boat racers, I wouldn't be too concerned with slip. My understanding of it relates to power loss due to different prop properties like alum. vs. stainless. But I'm by no means an expert. I posted that link more to go along with agitators post on motor position.

smokepoles said:

SuperD, thanks, that is what I was estimating might be the situation regarding performance. It also reminds me that the 'load' at Strawberry was pretty full all the time since it was a family camping and fishing trip.

Bduck, thanks for the info. Hard for me to go the repair route though when you can pick up a new prop on ebay for the cost of the repair.

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Your so right about prop repair. Its better to get a new prop rather than repair the one you have. However, I had an old damaged prop which this prop guy took towards exchange and saved me some 4moremney on a brand new one... The old prop was fixable but costs were almost the same for a new one. Keep in mind if you if have a damaged prop, the balancing can be off which can lead to & cause problems with your lower unit. I see a lot of units out there with chunks missing out of their props, not good.

I don't think it was said here, but for naturally aspirated engines you lose a percentage of horsepower for every 1000 feet of elevation you go up. I'll look for that figure, but I know that when I went from 2500 foot elevation to 6000 feet I figured my 115 horse outboard dropped Hp to about 85. Makes a lot of difference....especially with a loaded boat. I used to have 2 props, one for lower elevation and one for higher. Sure helped with getting on plane.

I sold that boat with the 115 and bought a boat with a supercharged Verado....elevation is never an issue now.

It's about a 3% HP loss per 1000 feet of elevation from sea level for naturally aspirated engines.

It seems like I fish in the valley about as much as I fish at altitude. I got lazy and just leave my altitude prop on now. I little less top end in the valley and a few more sips of gas but I'm not a hot rodder anyways. With my 150, I just lock in at 4,000 RPM and enjoy the ride.

SuperD said:

It seems like I fish in the valley about as much as I fish at altitude. I got lazy and just leave my altitude prop on now. I little less top end in the valley and a few more sips of gas but I'm not a hot rodder anyways. With my 150, I just lock in at 4,000 RPM and enjoy the ride.

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Yes sir.worthy12 you took the words right out of my big mouth ..

Well, I'm kind of marginal on HP, so decided to get a 13" prop. Got a new one plus prop kit on ebay for $60 to my door. I'll probably use the 15" mostly, but when we're pushin' 8000 ft and a full load of family, the 13" will be put on.

Tight lines to all.

Boy I am glad I found this. Just today I was asking the boat doc about this very problem. I have an old 50 hp mercury and in the valley at about 700 foot elevation she runs like a champ but at higher elevation 5000' or so she is a dog and not like mans best friend kind of dog. How do I know what size my prop is now and how do I know what to go down to?

VGH, pull your existing prop off and stamped either on the side or on the back is the size which would be something like 13 X 15. The first number is the diameter and the second number is the pitch. For higher altitude, you drop the pitch. For 4000' change in elevation, I'd recommend a 3 or 4 number drop from whatever you've got now.

Thanks you very much.

I am an old retired boat racer , I won a couple national championships in
outboard tunnel boats in the mid 70s . I also like to fish .
There is less Oxygen at higher altitudes .
Im also a high time pilot.
You can google sites on the internet that will tell you the % of HP loss at your altitude. Other factors are temps . barametric pressure and humidity .
At 7,500 feet 80 deg , pressure 29.92 ( standard pressure ), 20% humidity my airplane develops just 73.8% of sealevel horspower . On most aircraft you can also lean out the fuel mixture for best power.
If you live and fish only at altitude , you can gain some power back by having your carbs rejetted . I don't own a fuel injected engine but I expect they are self compinsating.
Also on engines that have bolt on heads you can have them milled to raise compression. CAUTION Both of these actions can damage the motor if going down to low altitudes.
I raced out of Albuquerque NM , 5200 ft. I raced at everyplace from 200 ft below sealevel , to Grand Lake CO at 8,000 ft. And everything inbetween .
I had several sets of heads milled between 20 thousands , and .125 thousands .
I also changed 6 high speed and 6 low speed jets before every race if the altitude change was more then 1,000 ft .
I also had about a dozen props . The right prop can get you in the right rpm band for max power , but only some type of supercharging will get your sealevel horspower back .
I do most of my fishing at Lake Navajo NM at 6000 ft . All my engines are jetted for that altitude .

This is the HP calculator site I used . its pretty interesting to check altitude ,temp , and humidity effects.

http://www.csgnetwork.com/relhumhpcalc.html