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Propeller for auxiliary 9.9 outboard

N

Newo

New member
Hi all, I have a 20 foot boat with 200 hp engine approx weight including engine is 1.5 ton. I have lately installed a 9.9 mercury 4 stroke engine year 2007. This is going to be used mainly for trolling, with its standard propeller (f8.5) I do about 5 knots. Is there any other recommended propeller for that weight of boat maybe a high trust propeller or should I increase or decrease propeller pitch? My aim is to increase another knot of speed. Any suggestions would be greatly appreciated
 
Does the motor rev up as it should, or does it labor (at full throttle)? If the latter, you need less pitch on your prop. If the former, you should be okay as is.

Jeff
 
It might take an atomic bomb to move that hull a knot faster.----The biggest concern is full throttle revs of the motor.---If the motor is " lugging " at full throttle you risk damage to the engine. That gets expensive.----------Best find a tachometer to check full throttle revs.
 
Thanks for the reply, I will try to install a small tachometer and let you know the revs at full throttle.
thanks again
 
Today installed a tiny techometer, at wide open throttle engine revs 4000rpm maximum speed 4.8knts If I'm not mistaken engine should be revving between 5000 and 6000 rpm at wot. Should I increase or decrease pitch? Should i go for a four blade propeller since its a heavy boat or remain with three blade? My aim is to increase top end speed if ill manage anything close to 6 knots that would be great. Any suggestions .
 
If you are only making 4000 rpms there is no prop on the market, if you are running an 8.5 pitch that will get you into the WOT range. This is exactly why Merc started making the 9.9 and 15 horse "Pro kicker" motors (portable "bigfoots"). Without a gear change there is no way of getting that little motor running where it should be.

Going down in pitch (and diameter) raises the rpms (easier to turn so revs higher).

The lowest pitch for non-bigfoot 9.9's with a 12 spline prop (which I believe you have as opposed to the earlier 8 spline) is a 5 pitch - that would only raise your current rpms by about 700 - better, but still well short of where you want to be. A 4 blade prop pushes harder so the effect, with the change in pitch, is less than with a 3 blade.

Unfortunately, with your current set-up that's about as good as you can do without replacing your kicker.
 
Thanks for the reply, in your opinion if I decrease pitch to 5 pitch will I be improving abit top end speed or this is not possible?
thanks again
 
You almost never increase speed by "decreasing" pitch. IF you are "under pitched" to start with the motor will not achieve max speed, but if you have too much pitch the best you can hope for is keeping the speed about the same.

Here's how it works (in purest theory - other factors play in but you will the idea).

The gears in your motor are 2:1, so if currently your max rpms are 4000, after the gear case your prop is turning at 2000 rpms max.

Your current pitch is 8 1/2". So, without "slippage, drag and friction" for each rotation of your prop your motor should push you 8.5".

(8.5 x 2000 = 17000 inches per minute = 1416.6 feet per minute = 85000 feet per hour/5280 = 16 mph would be your max theoretical speed)

You note you are making about 5 knots (about 6 mph) - so your slip rate is about 60% of max potential (very heavy boat compared to a very small motor)

Now all else being equal, if you move down to a 5 pitch your rpms should increase to roughly 4700 rpms - so you just do the math again -

5" x 2350 = 11750" per minute = 979 feet per minute = 58750 feet per hour = just over 11 mph. I would expect your slip rate to drop from 60'ish percent to 50'ish percent (because the flatter prop will bite better) which (I believe) would still give you 5-6 mph...
 
Thanks for the great reply, really interesting. Just one final question before I give up, if I fit the engine with a high trust propeller or a four bladed prop do I still get the same slippage? Or this can help a bit?
Thanks again for your reply.
 
The slippage is caused by a number of factors.

Primarily, in your case it will be due to a big boat and little motor.

The motor is pushing as hard as it can but it's not quite enough to attain the hull speed (displacement speed) of your boat - which is a little over 6 knots (there is a bunch of calculations that could determine the required horsepower to achieve hull speed if you plug in all the variables - boat weight, length at waterline, max beam, deadrise etc etc - I used the "quick formula to determine a rough hull speed for your boat).

Anyhow, so nothing you can do about the horsepower. What you can do to mitigate some of the other causes of slippage (and slippage goes by a number of names which essentially mean the same thing):

A 4 blade prop WILL grab more water (and hang onto it better), BUT, it is more difficult to turn than a 3 blade - so it will improve the "bite" but it will "load up" your already loaded up motor. All else being equal I would stick with a 3 blade in this case.

A stainless steel prop will flex less than either an aluminum, composite aluminum or composite (plastic) - again, same issue as above - stainless will cause a greater load on the motor.

Cupped props - whether they be 3, 4 or more blade, will grab better than an uncupped prop of similar dimensions - again, more loading.

What you can do that will help out your little motor:

Run as light as possible - if it's on-board and you don't need it for the trip/day/week etc, leave it on shore.

Keep your 200 horse tilted out of the water (if an outboard), or raise the outdrive as high as possible (if an I/O) when running the 9.9 - that will significantly reduce the drag - if you require the "big motor" to provide steering, keep it tilted as high as possible while still providing steering.

Keep the hull very clean - a smooth clean hull "slips" through the water easier (might sound a little crazy, but your 9.9 could gain 1/2 knot on a "slippery" hull - if using an ablative bottom coat consider a teflon paint next time around).

Travel with the wind at your stern :) (had to add that - hey, might end up in Tahiti and won't care what speed you are making)...
 
Guess that's its time for me to give up :) would like to thank you allot my friend for the replies and great detail.
thanks again
 
My friend has a Party Barge powered by the same tiny motor. He swears it moves if just fine, but I'd hate to have to fight a stiff current with so small a motor.

Jeff
 
If you are relatively satisfied with it's performance I would leave things alone and always run Seafoam (or similar) in the tank for your aux motor.

Overloading it constantly will cause all kinds of nasty deposits - the Seafoam will add to it's short lifespan..
 
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