Lake Tahoe Prop Pitch

[RN223]

Hi,

Wondering what motor/props/pitch people are running on their Cobalts here in Lake Tahoe. We just acquired our 2012 Cobalt 273, it comes with sea level props on my Volvo Penta 8.1gi 400 HP, I believe they are F5. The boat dry weight is 5800 lbs.

Sea trial in Lake Coeur d'Alene at 2128 ft was 43 mph at 4700 rpm and 30 mph at 3200 rpm.

She will be a Tahoe only vessel for her life.

Anyone running same boat here in Tahoe? What prop on your Volvo are you running? Suggestions and comments much appreciated!

Thanks all

[AsLan7]

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Hey RN223. So glad you found us!

The high altitude boats are geared lower due to the lack of available HP with the thinner air. What’s your gear ratio on the outdrive? Higher outdrive ratio for higher altitude operated boats. How’s your hole shot?

Btw, the VP 8.1GI motor produces 375 HP. The rarer GXI produces 420.

Any pics?

And welcome aboard mate!

[Krob1114]

A7, where can I find my gear ratio?

[AsLan7]

It’s normally stamped on the side of the drive. Otherwise your drive serial number look up should have it with VP or Merc.

[Big Block Power]

A7, where can I find my gear ratio?

You will have a small silver tag on the drive I think port side with your numbers on it.

[bruceb58]

Typically, there is no gear change done for a high altitude lake like Tahoe. You just change the pitch of the prop. Been boating in Tahoe for over 40 years and know nobody that has other than a stock gear ratio in their drive.

[AsLan7]

Typically, there is no gear change done for a high altitude lake like Tahoe. You just change the pitch of the prop. Been boating in Tahoe for over 40 years and know nobody that has other than a stock gear ratio in their drive.

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A lot of factors in play here Bruce but there are high altitude offerings with different drives and boat model/engine combos. For example, our 496 MAG HO with a Bravo 3X drive has a few outdrive gear ratio options (1.81; 2.00; 2.2 (H)). H=high altitude gear ratio option. These are offered by the manufacturer (Sea Ray shown below) or in the second pic below an option if you want to purchase a 496 MAG HO w/ the high altitude drive ratio (as just one example).

Volvo Penta's high altitude offering is shown below as well. The OP mentioned he has a VP 8.1Gi which does have this high altitude option offered (which is why I mentioned this). The Cobalts I've seen listed for sale which have the higher gear ratios are typically from the Tahoe region or similar.

I've only been boating for 40.00000001 years so I may be wrong on this.


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ratio.jpg

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ratio eng.jpg

ratio again.jpg

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vp 8.1.jpg

vp ratio.jpg

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[bruceb58]

Before I would ever consider changing gear ratios, I would be calling the Cobalt dealer in Rocklin who is down the hill from Tahoe who will then likely send you to call Valley Propellor in Ventura, CA to talk to them.

In the event you ever sell your boat to someone who will be using it at sea level, you will be very glad you didn't do a gear change.

I know 4 people personally, including me, that were running Big Blocks with Volvo Penta DuoProps on Tahoe and none of us changed gear sets.

[AsLan7]

Before I would ever consider changing gear ratios, I would be calling the Cobalt dealer in Rocklin who is down the hill from Tahoe who will then likely send you to call Valley Propellor in Ventura, CA to talk to them.

In the event you ever sell your boat to someone who will be using it at sea level, you will be very glad you didn't do a gear change.

I know 4 people personally, including me, that were running Big Blocks with Volvo Penta DuoProps on Tahoe and none of us changed gear sets.

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Agreed. And in this thread it’s not about switching gear ratios Bruce. It’s about being educated on what’s on your boat so you can make educated prop decisions with the folks mentioned above having all the available information to make that decision.

The speed of your hull can always be calculated using an engineering formula. While some of the variables for the formula may require some experience to apply, the formula works for all hulls, all designs of boats, all power and drive packages, and all speeds.
Example:
V = [RPM x PP x (1-S) x GR] / [1056]

Here’s an example.
RPM = 6000rpm (rev/minute)
PP = 24 (inches)
S = 0.12 (12% slip)
GR = 1.87:1 gear ratio (0.535 reduction at prop)
Then at
Vactual = [6000 x 24 x (1-0.12) x .535] / 1056 = 64.2mph

Hope this helps.
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[Big Block Power]

The good thing of Tahoe is at pretty low altitude. It's pretty close to sea level. Maybe the dealer doesn't recommend it because they are on the bottom of the hill at sea level. Lol

[RN223]

Thanks so much for all the information A7 and Bruce. Everyone here that I talk to in Tahoe is telling me to purchase a "high altitude prop". Then they ask what my WOT rpm's are and cruising rpm's. We had her out yesterday for the first time and I got her up to 4200 rpms going 40-42 mph and at 3000 rpm's I was at 27-29 mph, full tank of gas, my wife and my 5 year old. I definitely need a new prop if I plan on having a few people and gear aboard. Any suggestions on which "pitched down" prop would help me regain some power? I'll be calling around as well but figured this would be the best place to get some great suggestions

[Big Block Power]

Thanks so much for all the information A7 and Bruce. Everyone here that I talk to in Tahoe is telling me to purchase a "high altitude prop". Then they ask what my WOT rpm's are and cruising rpm's. We had her out yesterday for the first time and I got her up to 4200 rpms going 40-42 mph and at 3000 rpm's I was at 27-29 mph, full tank of gas, my wife and my 5 year old. I definitely need a new prop if I plan on having a few people and gear aboard. Any suggestions on which "pitched down" prop would help me regain some power? I'll be calling around as well but figured this would be the best place to get some great suggestions

Prob a F3
Odd I'm surprised you have to change props? Something new everyday

[bruceb58]

Like I said before, call West Coat Boat center...they know what prop you will need since they are the closest Cobalt dealer to Tahoe these days.Many of the new Cobalts in Tahoe come from there.

Call Up Valley propeller in Ventura, CA. They will send you a work sheet and you put in the numbers at various RPMs including WOT. Thery can all also ship you props to try if need be. My guess you will need an F3

Out of curiosity, where are you in Lake Tahoe. We just pulled our boat and won't be up there until December.

[Big Block Power]

Like I said before, call West Coat Boat center...they know what prop you will need since they are the closest Cobalt dealer to Tahoe these days.

Call Up Valley propeller in Ventura, CA. They will send you a work sheet and you put in the numbers at various RPMs including WOT. Thery can all also ship you props to try if need be. My guess you will need an F3

By the way what is the gear ratio on your boat? Did it start out on Tahoe?

[RN223]

Yeah, cool Bruce I'll call them tomorrow, thanks. We are located in South Lake Tahoe, my boat is at Camp Rich. Smokey again today. We are pulling it tomorrow for the season as it looks like weather change is coming Wednesday, snow levels at 8000 ft. So Big Block, I'm looking at the OG invoice for the boat and I believe it is a 1.78 ratio?? I was thinking a F4 or F3, what's the downside if I just buy the F3 and put that on? (besides wasting $1500) That would be really neat if they just send a couple to try, wouldn't be till May however...