Florida boat to Utah Prop sizing?

[Doral 270 Utah]

Hi all, last year I purchased a 1998 Doral 270SC in Florida, took the boat out in the keys a few times, never had any issues. Getting the boat home to Utah and changing from 0-6500' elevation the 7.4 Mercruiser will not get on plane even with help from the trim tabs. I currently have a 22p prop on and was thinking of going to 18 or 19. I'm trying to figure the best option to go with, 3 blade, 3/4 blade of 4 blade? If I go from a 22-18 will it be to much. any and all suggestions are welcome, thanks all.

 

[Doral 270 Utah]

Forgot to add, I am only getting up to 2500 RPM when toped out.

 

[jimmbo]

You have the thinner air working against you and the boat. The less dense air reduces the power output of an engine. A Fuel injected engine may be better able to cope with the thinner air than a carbed engine. Which do you have? The reduced power means that the prop is seriously over loading the engine. You might have to drop a lot lower than 18 to get the revs back into the recommended range, which is probably 4200 - 4600.
I will assume you have a Bravo gearcase? what gear ratio?
Mercruiser probably offers a High Altitude Gear ratio, or Gearcase. That permits usage of the more common props and not some very low pitch model intended for tugboats
A 4 blade... ? Well your engine is having great trouble with a 3 blade, adding another blade isn't going to help matters, unless you drop the pitch even more

 

[Leardriver]

You probably need a 15 pitch. You aren't getting the motor spinning fast enough to even get it into the power band.

 

[Sea Rider]

Is that a carbed or EFI engine ? If carbed will need a high altitude re jet for that given height and a much lower prop pitch size. Check if engine manufacturer has them to start with.

Happy Boating

 

[WesNewell]

your main problem is loss of engine power at high altitude. If there's an air intake restricter, remove it, replace air filter if needed. You need to find some way to get more air into the intake. Years ago we would turn the filter cover upside down to allow better airflow into the carbs. Don't know your setup, but lowering prop pitch won't increase engine power, only compensate for loss of power. Turbo charger maybe. Might try testing without the air filter/breather to see if that helps

 

[Doral 270 Utah]

It is a fuel injected 7.4 I cannot tell what gears the bravo 3 has. Guess I’ll have to split the case and count the teeth unless anyone knows how otherwise

 

[Sea Rider]

You can't possibly get, extract, add more oxygen where there is much less found no matter which filter, device you install. The higher you go the worse it gets. In that case install a much less prop pitch, the bummer is : which size as a high altitude starter prop ?

My cousin used to fly a Fairchild plane, had a manual oxygen system that injected oxygen to the radial engines to cross the high cordillera, worked fantastically well, did so for years.

Happy Boating

 

[Faztbullet]

your main problem is loss of engine power at high altitude. If there's an air intake restricter, remove it, replace air filter if needed. You need to find some way to get more air into the intake. Years ago we would turn the filter cover upside down to allow better airflow into the carbs. Don't know your setup, but lowering prop pitch won't increase engine power, only compensate for loss of power. Turbo charger maybe. Might try testing without the air filter/breather to see if that helps

You could always mount a high velocity CFM 12volt fan in front of air inlet to force more air in....... :eagerness::eagerness::eagerness:

 

[WesNewell]

You can't possibly get, extract, add more oxygen where there is much less found no matter which filter, device you install. The higher you go the worse it gets. In that case install a much less prop pitch, the bummer is : which size as a high altitude starter prop ?

Tell that to people that removed the air restricter from their mercury outboards to achieve more power at higher altitudes, or at sea level for that matter. It's a well documented fact that removing air restricters will increase power in both outboards and and any other ICE if they have a restricter. And while the 12V fan idea is funny, a supercharger which compresses the air in front of the intake would also change the air density, perhaps to that equivalent to sea level.

 

[ahicks]

The fact remains that there is less oxygen in the thinner air found at higher altitudes. Add some warm temps to that and you have a recipe for some pretty dismal performance.

Not knowing any better one way or the other, I'd want to know if the fuel injection system in use is able to calibrate itself for those conditions.

Assuming that it can, you'll need to use your current wide open rpm as a basis to make an educated guess at a prop that will let the engine turn up rated rpm.

 

[jimmbo]

y If there's an air intake restricter, remove it, replace air filter if needed. You need to find some way to get more air into the intake. Years ago we would turn the filter cover upside down to allow better airflow into the carbs. Don't know your setup, but lowering prop pitch won't increase engine power, only compensate for loss of power. Turbo charger maybe. Might try testing without the air filter/breather to see if that helps

I doubt there is any form of Air Restricter on that engine, it was able to pass enough thicker air at sea level. Lowering the pitch will allow the engine to run in its proper rpm range allowing it to make its best power, though reduced compared to sea level power.

Running without the flame arrestor is a recipe for disaster. One backfire and the boat can be on fire, or explode.

A Turbo Charger is a way of restoring performance, even increasing it. However, it just isn't a case of slapping on some parts and turning the key. Any changes do require the use of USCG approved parts and combination of parts.

FWIW, turning over the air cleaner lid did make for more noise, and sometimes a bit more power, but just as often, the lid, upside down interfered with the airflow into the carb and there was no difference or there was a loss of power.

 

[Sea Rider]

Tell that to people that removed the air restricter from their mercury outboards to achieve more power at higher altitudes, or at sea level for that matter. It's a well documented fact that removing air restricters will increase power in both outboards and and any other ICE if they have a restricter. And while the 12V fan idea is funny, a supercharger which compresses the air in front of the intake would also change the air density, perhaps to that equivalent to sea level.

To start with, you don't know if that engine has an air restrictor or not. An air restrictor is used to detune next HP engines having same CC, works along less timing°, carb advance, smaller carbs, whichever. HP restrictions will depend entirely on engine manufacturer. If you remove an air restrictor will need to remove, adjust,, swap whatever needs to be done for a true next size HP increase improvement.

To be 100% true any engine will need to be water tested with proper intruments for that matter, if not, this well documented fact by mostly word of mouth, marina or tea time talk is a total fallacy.

My Chevy MPI 1-8 Ltr turns into a real pig when at 10 K feet, much worse when higher. Diessel engines performs much better at high altitude when dealing with less "Oxygen" present in the "Air" as we all know it...

Happy Boating

 

[WesNewell]

No, I don't know if it ahs a restricter, that's why I said if it does, remove it. If not, suggest he look into water injection to increase the power. That's probably the cheapest fix anyway.
https://en.wikipedia.org/wiki/Water_injection_(engine)

 

[Sea Rider]

Water injection is used mostly on aircrafts, car racing.The only cheap and instant alternative for high elevation boating is to prop engine right with very low pitch props to increase rpm and don't lug the engine which seems is happening right now.

Been reading extensively why internal combustion machines used on high altitudes works crappy. The culprit is Atmospheric Pressure :

Due to the decrease in atmospheric pressure the higher the altitude there’s less temperature, air density, oxygen, nitrogen air molecules available compared to sea level.

The atmospheric decrease affects badly the air density and its oxygen/nitrogen composition entering the combustion chambers consequently affecting the combustion process that employs air as a carburant taking place at high altitude. The higher the elevation, the worse it gets.

No wonder haven't seen any boat powered even with large outboard on plane in Lake Titicaca which is located at 3,820 Mtr (12,600 Ft) above sea level. Only cruising at fast displacement speeds due to not knowing anything about re jetting and propping right an outboard for high altitude uses as currently loaded.

Happy Boating

 

[icwingman]

water is water alt has to do with it, rejet your carbs and all should be fine.

 

[fishrdan]

The EFI should handle leaning out the engine for altitude, so nothing needs to be done to the engine. Propping for high altitude is what's needed, and my SWAG is a 17P 3-blade, but that's a guess, might need a 15P (or 19???). Going 4-blade will help get out of the hole, but you'll need to run 1" less pitch than a 3-blade prop for the same RPM, rule of thumb thing type thing.

You said Bravo3, so that would be dual props, and those props sets are not cheap... It may be best to visit your local prop shop and see what their recommendation is, and see if they will swap props if you're not satisfied with the performance of their recommendation. This may be the cheapest, quickest and and easiest way to get the correct set of props.

If it was a single prop boat, I'd buy a good condition used prop that's close, then see if you're under or over the WOT RPM, and adjust from there. I boat in Utah at times, lakes up to 9000' and have experimented with several props... I drop 4" of pitch going from 1200' to 9000', and the boat performs marginally well, at least gets on plane, but this is with only 140HP.

Changing drive gear ratio is an option too, as mentioned, but an expensive option, and only something I would do to a boat that is going to live at that altitude for the rest of it's life.

 

[Sea Rider]

EFi systems won't cure issues when boating at high altitude, will just lessen them, will work better than carbed engines but not that wow as you can't possible increment the needed correct amount of oxygen along other gases for any engine to work same compared to as when boating at sea level in which those elements are found plenty, period.

Anyway a EFI engine will need to go for a less pitch prop maximization in order to pull wot revs lost at high altitudes, the higher the more rpm will be lost.

Happy Boating