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Prop choice

FishAddict005

New member
Hey guys I have a 28hp mariner turning 3800rpm was told I can run it up to 4500. I'm clocking 27mph with the boat loaded. Trying to get an idea on what I should go with to gain some more rpm and hopefully a little bit more mph. I'm currently turning a brand new 9.9/13pitch prop right now.

Thanks
 
I don't have a brochure to cover your engine. I do have a '94 Mariner brochure which has a 25 hp and skips to 40 hp engines, the 40 being the old crossflow classic 4 cyl. The 25 is spec'd at 5000 to 6000 WOT operation with 6000 being the preferred number to prop for as 2 stroke engines don't like to lug. If you don't believe that, take a chain saw and set the chain on a log with the engine idling ready to cut. Goose it! What happens? It loads up and if you leave it there the engine will die! Then take the chain off the log and rev up the engine to full rpms and then put the chain on the log and check out the difference in performance......breezes right through it! Same thing with your boat.

4500 seems too low for a max rpm rating for a current production, loop charged engine of any size. The brochure shows the standard prop for that engine to be 3 blade x 10 1/2 dia x 13 pitch. Seems to me your measurement of 9.9 is really 10 1/2 as props don't come in dimensions like that. Usually by the 1/4 inch for dia changes and pitch changes are 1 to 2".

So that says you are running the standard prop and that your conditions apparently WAY over standard for the expected load for that sized engine. So, if your engine is putting out it's rated hp or close, or is trying to, you have a heavy boat, dirty hull, engine setup wrong or something like that.

Assuming it's a heavy boat, then a pitch change would be in order. Read the thread I answered just above this for some ideas.

One other tidbit. If you have been running your engine like this for some time it could be carboned up big time and that could cause poor performance and things like sticking rings and carboned up passages. A good cleanout with something like Sea Foam would be in order to get things right with the engine. Google the words DE CARB and follow the instructions. You basically load up a small amount of gas with a heavy concentration of SF. Run the engine with this mix for awhile and shut it off. Wait for a time; longer the better allowing the naptha in SF to soak on the carbon and then fire it back up staying up wind of the engine. If you live in town the neighbors might call the fire department for all the smoke.

These new loopers usually only use 2 wedge shaped rings that expand on the compression stroke reducing friction but getting a good seal. If you are all carboned up they may not be able to expand as designed and you will be low on power.....your 28 may only be putting out 20 or something like that and that can be your problem if you have a lighter boat that a 28 ought to run. Just for grins, 27 mph from 28 hp is not shabby at all and doesn't support your numbers.

Going to the www.go-fast.com prop slip calculator and plugging your expected 2.25 :)1) gear ratio, your 3800 rpm, and your 13" prop your boat would theoretically only go 21 mph and that is without considering any prop slip. Your numbers are wrong.

Backing into the numbers I plugged in your 27 mph and estimated a very efficient 10% slip, giving a theoretical max speed of 30. I then solved for what engine rpm would be required to do that and the number was 5483. I believe that number as I have proven the calculator to be accurate. Besides 5500 on a 28 hp engine doing 27 mph on a boat designed and loaded for that engine seems perfect!

HTH,
Mark

Ha! I just noticed you posted this back in November. Guess I have been wasting my time here and the identical post you made below. Oh well, maybe someone reading will glean some info out of it.

Mark
 
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