prop question

[cdgn troller]

I've got a 18' lund with 140 merc i/o. Its has 14 1/2" dia 19p prop. I was thinking of getting a lower pitch prop to slow down trolling, it runs on a calm day 2.8-3.2 sog and on a choppy day down to 2.0 -2.6. I have a trolling plate but then I have to run around 1150 rpms to get 2.2 -2.4. I'm thinking about getting a 14 1/2" 17p. Does anybody know how much this will slow me down, or should I get a 15p. I know I'll lose my top end, but trolling speed and fuel economy is much more important!

[rolmops]

If you go from 19 to 17 you would need an extra 400 rpm to stay at the same top speed. If you go down to 15 it would be 800 rpm. Every boat is different because of weight and ways that this weight is stowed in the boat. But is is fairly easy to calculate.

Lets for convience assume that your top speed is 30 mph. Then 3 mph would need 10% of the extra 400 rpm =40 rpm and 1 mph would need 13 rpm

If you would go at 3mph at 900rpm with your current prop, you would use 300 rpm per mile. Dropping down to 17 pitch would mean that in order to go 3mph you would have to go 940 rpm. That would mean 313 rpm per mile.

So you would be able to run 2 mph at 626 rpm with a 17 pitch and with a 15 pitch it would be 652 rpm. This is not entirely accurate ,but I don't want to bore you with calculus.

Anyway, the above calculations are your key. Now just use this for your own special situation.

As for fuel savings,going down in pitch will not save you any fuel. In fact, you would use more. It would be better to use a little 9.9 kicker.That would save fuel and you would still have the right prop for your boat sitting on the main engine.

Cornelis.

[goin deep]

Leave your prop alone and put drift bags out. It's cheaper than a new prop and you won't lose any top end. They also help stabilize the boat when it gets rough.

Brian

[Tim Bromund]

the decision to re-prop should be based primarily on whether you boat can get to the proper recommended rpm range at WOT (Wide Open Throttle), not how slow you can troll down to. If you can reach the proper WOT rpm range with your current prop, you DO NOT want to lower pitch because that will make it possible to over rev the engine (and possibly blow it up).

As was suggested above, if you want to troll slower, the cheapest option is a good set of trolling bags (NOT drift socks, they aren't built to withstand the pressures of trolling) I have a set of 28" Amish Outfitter Beefy Bags for my 24' Thompson and with both deployed, they will slow my boat almost to a dead stop at idle (0.3-0.4 mph down speed). You won't need bags that size for your 18' Lund.

The other option would be a kicker.

Tim

[cdgn troller]

Thank you guys for your responses. I should of just said that my boat at idle 650 rpms runs around 2.8 sog. I want to keep the same rpm but run slower,2.0-2.4. I can this be achieved by going to a 17p prop. I understand about the over rev, I build and drag race high performance engines, and at wot its running 45 mph at 4000 rpms. Anybody think this will work?

[ Post made via Android ]

[L&M]

Thank you guys for your responses. I should of just said that my boat at idle 650 rpms runs around 2.8 sog. I want to keep the same rpm but run slower,2.0-2.4. I can this be achieved by going to a 17p prop. I understand about the over rev, I build and drag race high performance engines, and at wot its running 45 mph at 4000 rpms. Anybody think this will work?

[ Post made via Android ]

As it is now with a 19p x 4000wo rpm's it's over propped.

at wot its running 45 mph at 4000 rpms

That 140 should turn 44-4600 at wide open throttle, trimmed with normal load. To answer your question a 17p will get you in the ball park on the high end & will slow your trolling speed at 650rpms. You should also be able to tweak that carb. down some to 500-550 with no problem & achieve what your after if the 17 doesn't quite get you down to where you want to be.

[High Bidder]

One thing also to consider is that you have to use the same make and model of prop to achieve the 400 RPM change for a 17p vs a 19p prop. If you use a different brand of prop the rpms could vary as much as 1200 rpms. I speak from experience.

High Bidder

[Chas0218]

I second that you need more prop but I would definitely NOT re-prop because you need to troll slower. Has your boat always ran at 4000 rpm? If it just started doing this there is something else wrong. I don't like trolling with my big motor mainly because it loads up a little and like to keep the hours off the motor. Run tolling socks if you can't get down slow enough. I would start with a larger sock and work your way down but do not get the biggest sock you can find. When I trolled with a friend he made his own socks using 5 gallon buckets and running a threaded rod thru the bucket (handles won't hold up to trolling)

[ray koziatek]

i drag a 18" kayak sea anchor about 15 bucks on my 22 ftr when needed,dont know how long they will last but mine is 5 years old and has around a thousand hours on it.

And if Hank says its so (prop pitch) ya might be a little foolish not to follow his Idea..

One of the (oldest) er I mean best boat mechanic on lake ontario....(or NY state) trust me....P.S. I ant no slouch on mechanical thingys either yer motor will be happier with smaller pitch and you will to when the boat is more than empty...ya ant gonna get 10 more miles per gallon ,but a better matched prop and motor for the load being used (boat weight) will give ya better performance and use a tad less gas doing so,,,but dont look for fuel savings to offset the cost of prop,but a overstressed lower unit and motor might cost more than a prop in the long run......

[DtD]

Well, don't have the data , only the results.......I have a 40 Mariner on my sea nymph 17.

Wanted to get to 1 mph or so going down wind, and found a Happy Troller plate for the cavitation plate bolt-on. A very easy attachment but...... in the down position it was too much of a baffle, so I put in a pair of slots midway between full up and full down... it works to slow me down with a very acceptable range depending on the wind direction. They may have a unit that will fit the larger motor. Good luck. Dave

[spoonfed-1]

Got to tell you Mr. Hank knows his business and as already mentioned, it might be a good idea to take his advice.

Good luck

Glen

[cdgn troller]

thanks again for the info guys! My cost savings I was comparing to was when I need to use the trolling plate to slow down I have to increase my rpms to about 1000 to get to desired speed and that burns alot of gas! So I'm going to try a 17p prop, will let you guys know in a few weeks how it works out.