Topic: Sea Hunter at high altitude

[ENDERUS]

Finally realized the dream and took my 2000 Sea Hunter to lake Tahoe, 6000' altitude.
I have 70 hp Suzuki 4 stroke. My max speed was 10 mph! Max rpm 3000 !
The boat would not plane, except a few times got a good kick in the butt from wakes and
planed and hit 20 mph, 4000 rpm. I expected some performance loss but 10 mph?
In the Pacific ocean it sees 5000 rpm, 28 mph. I have a 3 blade Solas 13 1/4 X 17 prop.
This is not something I can experiment with due to the distance and lack of time.
Does anybody have the magic answer; what prop I should use at 6000'.

 

[Dbobby23]

Even on lake chelan which is 1100 ft elevation I noticed a big difference in performance with my Yamaha f60
My old prop was a Yamaha 11-1/4 x 14 I could barely get on a plane when fishing chelan.
My new prop is a mercury cupped 10-5/8 x 12 which made a night and day difference.
My top speed is not great but the revs are there when I need them especially negotiating bad weather and ocean swells. One thing I do need to be careful of because of the undersized prop is over reving. I know I have a rev limiter but I don't even want to go there. With this prop I can easily get to high revs quickly when on a plane. Both props are 3 blade old and new

[ATGEP]

Finally realized the dream and took my 2000 Sea Hunter to lake Tahoe, 6000' altitude.
I have 70 hp Suzuki 4 stroke. My max speed was 10 mph! Max rpm 3000 !
The boat would not plane, except a few times got a good kick in the butt from wakes and
planed and hit 20 mph, 4000 rpm. I expected some performance loss but 10 mph?
In the Pacific ocean it sees 5000 rpm, 28 mph. I have a 3 blade Solas 13 1/4 X 17 prop.
This is not something I can experiment with due to the distance and lack of time.
Does anybody have the magic answer; what prop I should use at 6000'.

 

The first question is if you can hit max rpm at home during normal conditions. If you are over propped at sea level, it will be way worse at altitude. 

[ENDERUS]

I may be over, WOT spec is 5400. I can go 5000 max.
I found an altitude prop chart that showed -4 pitch for 6000'.
West Marine prop wizard came up with 14x13 for heavy loads or 13.75x15 for speed
at sea level. I have 13.25x17. That is a big difference, I do not think I am that much over.

[SeaPacer]

So you must not be fuel injected?


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[ATGEP]

HP loss is about 3% per 100 feet. 82% x 70 =57 Possible HP available.   If you are stuck at 3k RPM max power at that RPM could be as low as 30-40 HP and not enough to get out of the hole. You need to drop pitch to get more RPM's and more potential HP. 

[AndyC]

I regularly boat at 5,000' around Denver, and 8,000 plus at Lake Granby. I originally had a 11X15 prop on my Honda 60HP - 16 foot Sea Explorer.  I got a new prop with a pitch of 10, and this upped my RPM's a lot. I lost about 8-10 MPH off my top speed, but now I can get on plane with 4 people on the boat at 5,000' and cruise at 21-24MPH

I only know Honda and they have a 14X11 prop that seems in the ballpark for your setup. When I had to make this same kind of decision I spent a lot of time trying different scenarios with the Honda Propellor tool they have on their website: https://marine.honda.com/parts/propeller-calculator

[Yachter Yat]

   Andy:  Just for general info......... I'm running Honda's 11.1x14 aluminum 3 blade with the 2016 Honda 60 on my 16 SC.  "Standing" on the throttle gets it to 6100 RPM's dead on.  Seems to be the perfect pitch.  That said, it's generally only the wife and myself on board with a limited amount of gear and fuel. 

   I've always felt these motors should be allowed to run a hair above max RPM's; especially on these Arimas.  It's almost like being slightly "downshifted".  I find running in this range not only makes hole shots a little quicker, but also helps keep the boat on plane at slower speeds without having to play throttle jockey.   Makes the boat feel "lighter on its feet", if you will.  I tried a 15 when we first got this motor.  Top speed was a little higher, but seemed a bit "sluggish" everywhere else. 

Yat

[AndyC]

Yachter, that is good information. I wasn't happy with the 11x15 performance when I was using it at sea level.  I've been wanting to buy one of those 11x14 props for when I'm out on the west coast at sea level, your experience might push me over the edge to actually purchase one.

I've got that 11x15 prop sitting around if anyone needs one, I imagine I'll never use it again.

[Yachter Yat]

   Hahaha........Andy, this is kinda funny.  Maybe we can make it a "2 for the price of one" deal.   You see, I've got an 11x15 "trial prop" as well.  That 11.1x14 seems to be "the" prop for a BF60 on a 16 SC.  That said, (as I was posting for Fisher) if your running the BFP60, I think the 13.5x14 Solas Amita is the way to go.  A bit more torque out of that lower end, I suspect. 

Yat

[AndyC]

Yachter, I'm hoping to try out some props when I'm back in Oregon.  I'll definitely give that 13.5x14 a try.

[troll4fish]

Living, working and boating here in the Rockies around Denver and into the high country for decades we have learned that power loss for a normally aspirated engine is 4% per 1000' in elevation not 3% per 100' as ATGEP has stated. I run a 12.5 x 15 pitch on my 17 Sea Chaser with a Mercury 90HP 4 stroke EFI and get about 5500rpm and 27MPH at 8500' with 2 guys and full fuel and gear. prop experimentation is required when changing altitude. I recently went to Lake Fort Peck in Montana and at 2250' elevation I left my 15 pitch on and was running at 5500 rpm at less than 3/4 throttle and doing 28mph with only me in the boat (no partner for that trip). When I go to Vancouver Island I put on a 12.5 x 17 pitch and run at 5000rpm at 28mph (still breaking in the motor so I don't run WOT yet and probably will do little of that since I like conserving fuel.